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The  Project  Physics  Course 


Reader 


6 


The  Nucleus 


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The  Project  Physics  Course 


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Q  The  Nucleus 


A  Component  of  the 
Project  Physics  Course 


Published  by 

HOLT,  RINEHART  and  WINSTON,  Inc. 

New  York,  Toronto 


This  publication  is  one  of  the  many 
Instructional  materials  developed  for  the 
Project  Physics  Course.  These  materials 
include  Texts,  Handbooks,  Teacher  Resource 
Books,  Readers,  Programmed  Instruction 
Booklets,  Film  Loops,  Transparencies,  16mm 
films  and  laboratory  equipment.  Development 
of  the  course  has  profited  from  the  help  of 
many  colleagues  listed  in  the  text  units. 


Directors  of  Harvard  Project  Physics 

Gerald  Holton,  Department  of  Physics, 

Harvard  University 
F.  James  Rutherford,  Capuchino  High  School, 

San  Bruno,  California,  and  Harvard  University 
Fletcher  G.  Watson,  Harvard  Graduate  School 

of  Education 


Copyright  ©  1 971 ,  Project  Physics 

All  Rights  Reserved 

SBN  03-084563-7 

1234  039  98765432 

Project  Physics  is  a  registered  trademark 


Picture  Credits 

Cover  picture:  Ink  drawing  by  Pablo  Picasso 
from  Le  Chef-d'oeuvre  inconnu,  by  Honore  de 
Balzac,  Ambroise  Vollard,  Paris,  1931. 


2  4 

5  I 

3  * 


Picture  Credits  for  frontispiece. 

(1)  Photograph  by  Glen  J.  Pearcy. 

(2)  Jeune  fille  au  corsage  rouge  lisant  by  Jean 
Baptiste  Camille  Corot.  Painting.  Collection 
BiJhrle,  Zurich. 

Harvard  Project  Physics  staff  photo. 
Femme  lisant  by  Georges  Seurat.  Conte  crayon 
drawing.  Collection  C.  F.  Stoop,  London. 


(3) 
(4) 


(5)  Portrait  of  Pierre  Reverdy  by  Pablo  Picasso. 
Etching.  Museum  of  Modern  Art,  N.Y.C. 

(6)  Lecture  au  lit  by  Paul  Klee.  Drawing.  Paul  Klee 
Foundation,  Museum  of  Fine  Arts,  Berne. 


Sources  and  Aclcnowledgments 
Project  Physics  Reader  6 

1.  Rutherford  from  Variety  of  Men,  pp.  3-30,  by 
C.  P.  Snow,  copyright  ©  1967  by  C.  P.  Snow. 
Reprinted  with  the  permission  of  Charles 
Scribner's  Sons. 

2.  The  Nature  of  the  Alpha  Particle  from  Radioactive 
Substances  by  E.  Rutherford  and  T.  Royds  from 
the  Philosophical  Magazine,  Chapter  6,  Volume 
17,  1909,  pp.  281-286.  Reproduced  by  permission 
of  Taylor  and  Francis  Ltd.,  London. 

3.  Some  Personal  Notes  on  the  Search  for  the 
Neutron  by  Sir  James  Chadwick  from  Actes  du 
Xeme  Congres  International  d'Histoire  des 
Sciences,  Hermann,  Paris  and  from  Ithaca, 

26  VIII-2  IX  1962.  Reprinted  with  permission. 

4.  Anti-Protons  by  O.  Chamberlain,  E.  Segre, 
C.  Wiegand,  and  T.  Ypsiiantis  from  Nature, 
Volume  177,  January  7,  1956,  pages  11-12. 
Reprinted  with  permission. 

5.  The  Tracks  of  Nuclear  Particles  by  Herman 
Yagoda  from  Scientific  American,  May  1956, 
copyright  ©  1956  by  Scientific  American,  Inc. 
Reprinted  with  permission.  All  rights  reserved. 
Available  separately  at  250  each  as  Offprint  No. 
252  from  W.  H.  Freeman  and  Company,  660 
Market  Street,  San  Francisco,  California  94104. 

6.  The  Spark  Chamber  by  Gerard  K.  O'Neill  from 
Scientific  American,  August  .1962,  copyright  © 
1962  by  Scientific  American,  Inc.  Reprinted  with 
permission.  All  rights  reserved.  Available 
separately  at  250  each  as  Offprint  No.  282  from 
W.  H.  Freeman  and  Company,  660  Market  Street, 
San  Francisco,  California  94104. 

7.  The  Evolution  of  the  Cyclotron  by  E.  O.  Lawrence. 
(Nobel  Lecture,  December  11, 1951.)  Copyright 
The  Nobel  Foundation,  1952.  Elsevier  Publishing 
Company,  Amsterdam.  From  The  Development  of 
High-Energy  Accelerators,  edited  by  M.  Stanley 
Livingston,  Classics  of  Science,  Volume  3, 
Dover  Publications  Inc.,  New  York,  1966. 

8.  Particle  Accelerators  by  Robert  R.  Wilson  from 

Scientific  American,  March  1958.  Reprinted  with 
permission.  Copyright  ©  1958  by  Scientific 
American,  Inc.  All  rights  reserved. 

9.  The  Cyclotron  as  Seen  by  .  .  .  Cartoons  by 

David  L.  Judd  and  Ronald  G.  MacKenzie,  prepared 
for  the  International  Conference  on  Isochronous 
Cyclotrons,  Gatlinburg,  Tenn.,  May,  1966. 
Reprinted  from  the  proceedings  of  the  conference 
(IEEE  Transactions  in  Nuclear  Science,  vol. 


NS-13,  No.  4,  August  1966)  with  the  permission 
of  the  IEEE. 

10.  CERN  by  Jeremy  Bernstein  from  A  Comprehen- 
sible World:  Essays  on  Science,  copyright  ©  1964 
by  Jeremy  Bernstein.  Reprinted  with  permission 
of  Random  House,  Inc.  This  article  originally 
appeared  in  The  New  Yorker. 

11.  New  World  of  Nuclear  Power  from  Introduction 
To  Natural  Science,  Part  I:  The  Physical  Sciences 
by  V.  L.  Parsegian,  pages  633-641,  copyright  © 
1968  by  Academic  Press.  Reprinted  with 
permission. 

12.  The  Atomic  Nucleus  by  R.  E.  Peieris  from 
Scientific  American,  January  1959.  Reprinted 
with  permission.  Copyright  ©  1959  by  Scientific 
American,  Inc.  All  rights  reserved. 

13.  Power  from  the  Stars  by  Ralph  E.  Lapp  from 
Roads  to  Discovery,  pages  159-170,  copyright  © 
1960  by  Ralph  E.  Lapp.  Harper  &  Row,  Pub- 
lishers, New  York. 

14.  Success  by  Laura  Fermi  from  Atoms  In  The 
Family,  copyright  1954  by  the  University  of 
Chicago  Press,  pages  190-199.  Reprinted  with 
permission. 

15.  The  Nuclear  Energy  Revolution — 1966,  by  Alvin 
M.  Weinberg  and  Gale  Young.  Proceedings  of  the 
National  Academy  of  Science,  Vol.  57,  No.  1 , 

pp.  1-15,  January  1967.  Research  sponsored  by 
the  U.S.  Atomic  Energy  Commission  under 
contract  with  the  Union  Carbide  Corporation. 
Reprinted  with  permission. 

16.  Conservation  Laws  by  Kenneth  W.  Ford  from 
The  World  of  Elementary  Particles,  copyright  © 
1963  by  Blaisdell  Publishing  Company,  a  division 
of  Ginn  and  Company,  Watham,  Massachusetts, 
pages  81-112.  Reprinted  with  permission. 

17.  The  Fall  of  Parity  by  Martin  Gardner  from  The 
Ambidextrous  Universe,  copyright  ©  1964  by 
Martin  Gardner.  Reprinted  with  permission  of 
Basic  Books,  Inc.,  New  York  and  Penguin 
Books  Ltd. 

18.  Can  Time  Go  Backward?  by  Martin  Gardner 
from  Scientific  American,  January  1967,  copyright 
©  1967  by  Scientific  American,  Inc.  Reprinted 


with  permission.  All  rights  reserved.  Available 
separately  at  250  each  as  Offprint  No.  309  from 
W.  H.  Freeman  and  Company,  660  Market  Street, 
San  Francisco,  California  94104. 

19.  A  Report  to  the  Secretary  of  War,  by  J.  Franck, 
D.  J.  Hughes,  J.  J.  Nickson,  E.  Rabinowitch, 

G.  T.  Seaborg,  J.  C.  Stearns,  L.  Szilard,  June 
1945,  Chapter  3  of  The  Atomic  Age,  edited  by 
Morton  Grodzins  and  Eugene  Rabinowitch, 
copyright  ©  1963  by  Basic  Books,  Inc.,  New 
York.  Reprinted  with  permission. 

20.  The  Privilege  of  Being  a  Physicist  by  Victor  F. 
Weisskopf  from  Physics  Today,  August  1969, 
pages  39-43,  copyright  ©  1969.  Reprinted  with 
permission. 

21.  Calling  All  Stars  by  Leo  Szilard  from  Voice  of  the 
Dolphins,  pages  105-111,  copyright  ©  1961  by 
Leo  Szilard.  Reprinted  by  permission  of  Simon 
and  Schuster,  New  York. 

22.  Tasks  for  a  World  Without  War  by  Harrison  Brown 
from  Daedalus,  Fall  1960,  Journal  of  the  American 
Academy  of  Arts  and  Sciences,  Boston,  pages 
1029-1038.  Reprinted  with  permission. 

23.  One  Scientist  and  His  View  of  Science  by  Leopold 
Infeld  from  Quest,  copyright  1941  by  Leopold 
Infeld.  Reprinted  by  permission  of  Russell  & 
Volkening,  New  York. 

24.  Development  of  the  Space-Time  View  of  Quantum 
Electrodynamics  by  Richard  P.  Feynman  (Nobel 
Lecture,  December  11,  1965),  copyright  ©  The 
Nobel  Foundation  1966,  Elsevier  Publishing 
Company,  Amsterdam.  Reprinted  with  permission 
from  Science,  August  12,  1966,  Volume  153, 
Number  3737,  pages  609-708. 

25.  The  Relation  of  Mathematics  to  Physics,  by 
Richard  P.  Feynman  from  The  Character  of 
Physical  Law,  pages  55-57,  British  Broadcasting 
Corporation,  London,  copyright  ©  1965  by 
Richard  P.  Feynman.  Reprinted  with  permission 
of  the  author  and  the  M.I.T.  Press,  Cambridge, 
Massachusetts. 

26.  Where  Do  We  Go  From  Here  by  Arthur  E.  Ruark 
from  Physics  Today,  September  1969,  pages 
25-28,  copyright  1969.  Reprinted  with  permission. 


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IV 


This  is  not  a  physics  textbook.    Rather,  it  is  a  physics 
reader,  a  collection  of  some  of  the  best  articles  and 
book  passages  on  physics.    A  few  are  on  historic  events 
in  science,  others  contain  some  particularly  memorable 
description  of  what  physicists  do;  still  others  deal  with 
philosophy  of  science,  or  with  the  impact  of  scientific 
thought  on  the  imagination  of  the  artist. 

There  are  old  and  new  classics,  and  also  some  little- 
known  publications;  many  have  been  suggested  for  in- 
clusion because  some  teacher  or  physicist  remembered 
an  article  with  particular  fondness.    The  majority  of 
articles  is  not  drawn  from  scientific  papers  of  historic 
importance  themselves,  because  material  from  many  of 
these  is  readily  available,  either  as  quotations  in  the 
Project  Physics  text  or  in  special  collections. 

This  collection  is  meant  for  your  browsing.    If  you  follow 
your  own  reading  interests,  chances  are  good  that  you 
will  find  here  many  pages  that  convey  the  joy  these 
authors  have  in  their  work  and  the  excitement  of  their 
ideas.     If  you  want  to  follow  up  on  interesting  excerpts, 
the  source  list  at  the  end  of  the  reader  will  guide  you 
for  further  reading. 


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Reader  6 
Table  of  Contents 

1  Rutherford  1 

Charles  P.  Snow 

2  The  Nature  of  the  Alpha  Particle  19 

Ernest  Rutherford  and  T.  Royds 

3  Some  Personal  Notes  on  the  Search  for  the  Neutron  25 

Sir  James  Chadwick 

4  Antiprotons  32 

Owen  Chamberlain,  Emilio  Segr6,  Clyde  E.  Wiegand,  and  Thomas  Ypsilantis 

5  The  Tracks  of  Nuclear  Particles  35 

Herman  Yagoda 

6  The  Spark  Chamber  43 

Gerard  K.  O'Neill 

7  The  Evolution  of  the  Cyclotron  51 

Ernest  O.  Lawrence 

8  Particle  Accelerators  65 

Robert  K.  Wilson 

9  The  Cyclotron  As  Seen  By  . . .  77 

David  C.  Judd  and  Ronald  MacKenzie 

10  CERN  83 

Jeremy  Bernstein 

1 1  The  World  of  New  Atoms  and  of  Ionizing  Radiations  95 

v.  Lawrence  Parsegian,  Alan  S.  Meltzer,  Abraham  S.  Luchins,  K.  Scott  Kinerson 

12  The  Atomic  Nucleus  103 

Rudolf  E.  Peierls 

1 3  Power  from  the  Stars  1 09 

Ralph  E.  Lapp 


VI 


1 4  Success  1 23 

Laura  Fermi 

1 5  The  Nuclear  Energy  Revolution  1 35 

Alvin  M.  Weinberg  and  Gale  Young 

16  Conservation  Laws  141 

Kenneth  W.  Ford 

1 7  The  Fall  of  Parity  1 75 

Martin  Gardner 

18  Can  Time  Go  Backward?  193 

Martin  Gardner 

19  A  Report  to  the  Secretary  of  War  201 

James  Franck,  Donald  J.  Hughes,  J.  I.  Nickson,  Eugene  Rabinowitch, 
Glenn  T.  Seaborg,  Joyce  C.  Stearns,  Leo  Szilard 

20  The  Privilege  of  Being  a  Physicist  21 1 

Victor  F.  Weisskopf 

21  Calling  All  Stars  221 

Leo  Szilard 

22  Tasks  for  a  World  Without  War  227 

Harrison  Brown 

23  One  Scientist  and  His  View  of  Science  237 

Leopold  Infeld 

24  The  Development  of  the  Space-Time  View  of  Quantum  Electrodynamics       241 

Richard  P.  Feynman 

25  The  Relation  of  Mathematics  to  Physics  251 

Richard  P.  Feynman 

26  Where  Do  We  Go  From  Here?  253 

Arthur  E.  Ruark 


C.  p.  Snow's  highly  personal  account  of  Ernest  Ruther- 
ford IS  based  partly  on  Snow's  research  work  In  the 
Cavendish  Laboratory  while  Rutherford  was  director. 


1         Rutherford 

Charles  P.  Snow 

Chapter  from  his  book.  Variety  of  Men,  published  in  1967. 


IN  1923,  at  the  meeting  of  the  British  Association  for 
the  Advancement  of  Science  in  Liverpool,  Rutherford 
announced,  at  the  top  of  his  enormous  voice:  "We  are 
living  in  the  heroic  age  of  physics."  He  went  on  saying  the 
same  thing,  loudly  and  exuberantly,  until  he  died,  fourteen 
years  later. 

The  curious  thing  was,  all  he  said  was  absolutely  true. 
There  had  never  been  such  a  time.  The  year  1932  was  the 
most  spectacular  year  in  the  history  of  science.  Living  in 
Cambridge,  one  could  not  help  picking  up  the  human,  as 
well  as  the  intellectual,  excitement  in  the  air.  James  Chad- 
wick,  grey-faced  after  a  fortnight  of  work  with  three 
hours'  sleep  a  night,  telling  the  Kapitsa  Club  (to  which 
any  young  man  was  so  proud  to  belong)  how  he  had  dis- 
covered the  neutron;  P.  M.  S.  Blackett,  the  most  hand- 
some of  men,  not  quite  so  authoritative  as  usual,  because 
it  seemed  too  good  to  be  true,  showing  plates  which 
demonstrated  the  existence  of  the  positive  electron;  John 
Cockcroft,  normally  about  as  much  given  to  emotional 


display  as  the  Duke  of  Wellington,  skimming  down 
King's  Parade  and  saying  to  anyone  whose  face  he  recog- 
nized: "We've  split  the  atom!  We've  split  the  atom!" 

It  meant  an  intellectual  climate  different  in  kind 
from  anything  else  in  England  at  the  time.  The  tone  of 
science  was  the  tone  of  Rutherford:  magniloquently 
boastful — boastful  because  the  major  discoveries  were 
being  made — creatively  confident,  generous,  argumenta- 
tive, lavish,  and  full  of  hope.  The  tone  differed  from  the 
tone  of  literary  England  as  much  as  Rutherford's  person- 
ality differed  from  that  of  T.  S.  Eliot.  During  the  twenties 
and  thirties  Cambridge  was  the  metropolis  of  experimen- 
tal physics  for  the  entire  world.  Even  in  the  late  nine- 
teenth century,  during  the  professorships  of  Clerk  Max- 
well and  J.  J.  Thomson,  it  had  never  quite  been  that. 
"You're  always  at  the  crest  of  the  wave,"  someone  said  to 
Rutherford.  "Well,  after  all,  I  made  the  wave,  didn't  I?" 
Rutherford  replied. 

I  remember  seeing  him  a  good  many  times  before  I 
first  spoke  to  him.  I  was  working  on  the  periphery  of 
physics  at  the  time,  and  so  didn't  come  directly  under 
him.  I  already  knew  that  I  wanted  to  write  novels,  and 
that  was  how  I  should  finish,  and  this  gave  me  a  kind  of 
ambivalent  attitude  to  the  scientific  world;  but,  even  so,  I 
could  not  avoid  feeling  some  sort  of  excitement,  or  en- 
hancement of  interest,  whenever  I  saw  Rutherford  walk- 
ing down  Free  School  Lane. 

He  was  a  big,  rather  clumsy  man,  with  a  substantial 
bay-window  that  started  in  the  middle  of  the  chest.  I 
should  guess  that  he  was  less  muscular  than  at  first  sight 
he  looked.  He  had  large  staring  blue  eyes  and  a  damp  and 


Rutherford 


pendulous  lower  lip.  He  didn't  look  in  the  least  like  an  in- 
tellectual. Creative  people  of  his  abundant  kind  never  do, 
of  course,  but  all  the  talk  of  Rutherford  looking  like  a 
farmer  was  unperceptive  nonsense.  His  was  really  the 
kind  of  face  and  physique  that  often  goes  with  great 
weight  of  character  and  gifts.  It  could  easily  have  been 
the  soma  of  a  great  writer.  As  he  talked  to  his  companions 
in  the  street,  his  voice  was  three  times  as  loud  as  any  of 
theirs,  and  his  accent  was  bizarre.  In  fact,  he  came  from 
the  very  poor:  his  father  was  an  odd-job  man  in  New  Zea- 
land and  the  son  of  a  Scottish  emigrant.  But  there  was 
nothing  Antipodean  or  Scottish  about  Rutherford's  ac- 
cent; it  sounded  more  like  a  mixture  of  West  Country 
and  Cockney. 

In  my  first  actual  meeting  with  him,  perhaps  I  could 
be  excused  for  not  observing  with  precision.  It  was  early 
in  1930;  I  had  not  yet  been  elected  a  Fellow  of  my  own 
college,  and  so  had  put  in  for  the  Stokes  studentship  at 
Pembroke.  One  Saturday  afternoon  I  was  summoned  to 
an  interview.  When  I  arrived  at  Pembroke,  I  found  that 
the  short  list  contained  only  two,  Philip  Dee  and  me.  Dee 
was  called  in  first;  as  he  was  being  interviewed,  I  was  re- 
flecting without  pleasure  that  he  was  one  of  the  brightest 
of  Rutherford's  bright  young  men. 

Then  came  my  turn.  As  I  went  in,  the  first  person  I 
saw,  sitting  on  the  right  hand  of  the  Master,  was  Ruther- 
ford himself.  While  the  Master  was  taking  me  through  my 
career,  Rutherford  drew  at  his  pipe,  not  displaying  any 
excessive  interest  in  the  proceedings.  The  Master  came  to 
the  end  of  his  questions,  and  said:  "Professor  Ruther- 
ford?" 


Rutherford  took  out  his  pipe  and  turned  on  to  me  an 
eye  which  was  blue,  cold  and  bored.  He  was  the  most 
spontaneous  of  men;  when  he  felt  bored  he  showed  it. 
That  afternoon  he  felt  distinctly  bored.  Wasn't  his  man, 
and  a  very  good  man,  in  for  this  job?  What  was  this  other 
fellow  doing  there?  Why  were  we  all  wasting  our  time? 

He  asked  me  one  or  two  indifferent  questions  in  an 
irritated,  impatient  voice.  What  was  my  present  piece  of 
work?  What  could  spectroscopy  tell  us  anyway?  Wasn't  it 
just  "putting  things  into  boxes?" 

I  thought  that  was  a  bit  rough.  Perhaps  I  realized 
that  I  had  nothing  to  lose.  Anyway,  as  cheerfully  as  I 
could  manage,  I  asked  if  he  couldn't  put  up  with  a  few  of 
us  not  doing  nuclear  physics.  I  went  on,  putting  a  case  for 
my  kind  of  subject. 

A  note  was  brought  round  to  my  lodgings  that  eve- 
ning. Dee  had  got  the  job.  The  electors  wished  to  say  that 
either  candidate  could  properly  have  been  elected.  That 
sounded  like  a  touch  of  Cambridge  politeness,  and  I  felt 
depressed.  I  cheered  up  a  day  or  two  later  when  I  heard 
that  Rutherford  was  trumpeting  that  I  was  a  young  man 
of  spirit.  Within  a  few  months  he  backed  me  for  another 
studentship.  Incidentally,  Dee  was  a  far  better  scientist 
than  I  was  or  could  have  been,  and  neither  Rutherford 
nor  anyone  else  had  been  unjust. 

From  that  time  until  he  died,  I  had  some  opportuni- 
ties of  watching  Rutherford  at  close  quarters.  Several  of 
my  friends  knew  him  intimately,  which  I  never  did.  It  is  a 
great  pity  that  Tizard  or  Kapitsa,  both  acute  psychologi- 
cal observers,  did  not  write  about  him  at  length.  But  I  be- 
longed to  a  dining  club  which  he  attended,  and  I  think  I 


Rutherford 


had  serious  conversations  with  him  three  times,  the  two  of 
us  alone  together. 

The  difficulty  is  to  separate  the  inner  man  from  the 
Rutherfordiana,  much  of  which  is  quite  genuine.  From 
behind  a  screen  in  a  Cambridge  tailor's,  a  friend  and  I 
heard  a  reverberating  voice:  "That  shirt's  too  tight  round 
the  neck.  Every  day  I  grow  in  girth.  And  in  mentality." 
Yet  his  physical  make-up  was  more  nervous  than  it 
seemed.  In  the  same  way,  his  temperament,  which  seemed 
exuberantly  powerful,  massively  simple,  rejoicing  with 
childish  satisfaction  in  creation  and  fame,  was  not  quite  so 
simple  as  all  that.  His  was  a  personality  of  Johnsonian 
scale.  As  with  Johnson,  the  fagade  was  overbearing  and 
unbroken.  But  there  were  fissures  within. 

No  one  could  have  enjoyed  himself  more,  either  in 
creative  work  or  the  honors  it  brought  him.  He  worked 
hard,  but  with  immense  gusto;  he  got  pleasure  not  only 
from  the  high  moments,  but  also  from  the  hours  of  what 
to  others  would  be  drudgery,  sitting  in  the  dark  counting 
the  alpha  particle  scintillations  on  the  screen.  His  insight 
was  direct,  his  intuition,  with  one  curious  exception,  in- 
fallible. No  scientist  has  made  fewer  mistakes.  In  the  corpus 
of  his  published  work,  one  of  the  largest  in  scientific  his- 
tory, there  was  nothing  he  had  to  correct  afterwards.  By 
thirty  he  had  already  set  going  the  science  of  nuclear 
physics — single-handed,  as  a  professor  on  five  hundred 
pounds  a  year,  in  the  isolation  of  late-Victorian  Montreal. 
By  forty,  now  in  Manchester,  he  had  found  the  structure 
of  the  atom — on  which  all  modern  nuclear  physics  depends. 

It  was  an  astonishing  career,  creatively  active  until 
the  month  he  died.  He  was  born  very  poor,  as  I  have  said. 


New  Zealand  was,  in  the  i88o's,  the  most  remote  of 
provinces,  but  he  managed  to  get  a  good  education; 
enough  of  the  old  Scottish  tradition  had  percolated  there, 
and  he  won  all  the  prizes.  He  was  as  original  as  Einstein, 
but  unlike  Einstein  he  did  not  revolt  against  formal  in- 
struction; he  was  top  in  classics  as  well  as  in  everything 
else.  He  started  research — on  the  subject  of  wireless  waves 
— with  equipment  such  as  one  might  rustle  up  today  in  an 
African  laboratory.  That  did  not  deter  him:  "I  could  do 
research  at  the  North  Pole,"  he  once  proclaimed,  and  it 
was  true.  Then  he  was  awarded  one  of  the  1 8  5 1  overseas 
scholarships  (which  later  brought  to  England  Florey, 
Oliphant,  Philip  Bowden,  a  whole  series  of  gifted  An- 
tipodeans).  In  fact,  he  got  the  scholarship  only  because 
another  man,  placed  above  him,  chose  to  get  married: 
with  the  curious  humility  that  was  interwoven  with  his 
boastfulness,  he  was  grateful  all  of  his  life.  There  was  a  pro- 
posal, when  he  was  Lord  Rutherford,  President  of  the 
Royal  Society,  the  greatest  of  living  experimental  scien- 
tists, to  cut  down  these  scholarships.  Rutherford  was  on  the 
committee.  He  was  too  upset  to  speak:  at  last  he  blurted 
out: 

"If  it  had  not  been  for  them,  I  shouldn't  have  been." 
That  was  nonsense.  Nothing  could  have  stopped  him.  He 
brought  his  wireless  work  to  Cambridge,  anticipated  Mar- 
coni, and  then  dropped  it  because  he  saw  a  field — radio- 
activity— more  scientifically  interesting. 

If  he  had  pushed  on  with  wireless,  incidentally,  he 
couldn't  have  avoided  becoming  rich.  But  for  that  he 
never  had  time  to  spare.  He  provided  for  his  wife  and 
daughter,  they  lived  in  comfortable  middle-class  houses. 


Rutherford 


and  that  was  all.  His  work  led  directly  to  the  atomic 
energy  industry  spending,  within  ten  years  of  his  death, 
thousands  of  millions  of  pounds.  He  himself  never  earned, 
or  wanted  to  earn,  more  than  a  professor's  salary — about 
£i,6oo  a  year  at  the  Cavendish  in  the  thirties.  In  his  will 
he  left  precisely  the  value  of  his  Nobel  Prize,  then  worth 
£7,000.  Of  the  people  I  am  writing  about,  he  died  much 
the  poorest  '*" :  even  G.  H.  Hardy,  who  by  Rutherford's  side 
looked  so  ascetic  and  unworldly,  happened  not  to  be  above 
taking  an  interest  in  his  investments. 

As  soon  as  Rutherford  got  on  to  radioactivity,  he 
was  set  on  his  life's  work.  His  ideas  were  simple,  rugged, 
material:  he  kept  them  so.  He  thought  of  atoms  as  though 
they  were  tennis  balls.  He  discovered  particles  smaller 
than  atoms,  and  discovered  how  they  moved  or  bounced. 
Sometimes  the  particles  bounced  the  wrong  way.  Then  he 
inspected  the  facts  and  made  a  new  but  always  simple  pic- 
ture. In  that  way  he  moved,  as  certainly  as  a  sleepwalker, 
from  unstable  radioactive  atoms  to  the  discovery  of  the 
nucleus  and  the  structure  of  the  atom. 

In  19 1 9  he  made  one  of  the  significant  discoveries  of 
all  time:  he  broke  up  a  nucleus  of  nitrogen  by  a  direct  hit 
from  an  alpha  particle.  That  is,  man  could  get  inside  the 
atomic  nucleus  and  play  with  it  if  he  could  find  the  right 
projectiles.  These  projectiles  could  either  be  provided  by 
radioactive  atoms  or  by  ordinary  atoms  speeded  up  by 
electrical  machines. 

The  rest  of  that  story  leads  to  the  technical  and  mili- 
tary history  of  our  time.  Rutherford  himself  never  built 
the  great  machines  which  have  dominated  modern  parti- 

"■  One  has  to  leave  Stalin  out  of  this  comparison. 


cle  physics,  though  some  of  his  pupils,  notably  Cockcrof  t, 
started  them.  Rutherford  himself  worked  with  bizarrely 
simple  apparatus:  but  in  fact  he  carried  the  use  of  such 
apparatus  as  far  as  it  would  go.  His  researches  remain  the 
last  supreme  single-handed  achievement  in  fundamental 
physics.  No  one  else  can  ever  work  there  again — in  the  old 
Cavendish  phrase — with  seahng  wax  and  string. 

It  was  not  done  without  noise:  it  was  done  with 
anger  and  storms — but  also  with  an  overflow  of  creative 
energy,  with  abundance  and  generosity,  as  though  re- 
search were  the  easiest  and  most  natural  avocation  in  the 
world.  He  had  deep  sympathy  with  the  creative  arts,  par- 
ticularly literature;  he  read  more  novels  than  most  liter- 
ary people  manage  to  do.  He  had  no  use  for  critics  of  any 
kind.  He  felt  both  suspicion  and  dislike  of  the  people  who 
invested  scientific  research  or  any  other  branch  of  crea- 
tion with  an  aura  of  difficulty,  who  used  long,  methodo- 
logical words  to  explain  things  which  he  did  perfectly  by 
instinct.  "Those  fellows,"  he  used  to  call  them.  "Those  fel- 
lows" were  the  logicians,  the  critics,  the  metaphysicians. 
They  were  clever;  they  were  usually  more  lucid  than  he 
was;  in  argument  against  them  he  often  felt  at  a  dis- 
advantage. Yet  somehow  they  never  produced  a  serious 
piece  of  work,  whereas  he  was  the  greatest  experimental 
scientist  of  the  age. 

I  have  heard  larger  claims  made  for  him.  I  remember 
one  discussion  in  particular,  a  year  or  two  after  his  death, 
by  half-a-dozen  men,  all  of  whom  had  international  repu- 
tations in  science.  Darwin  was  there:  G.  I.  Taylor:  Fowler 
and  some  others.  Was  Rutherford  the  greatest  experimen- 
tal scientist  since  Michael  Faraday?  Without  any  doubt. 


Rutherford 


Greater  than  Faraday?  Possibly  so.  And  then — it  is  inter- 
esting, as  it  shows  the  anonymous  Tolstoyan  nature  of 
organized  science — how  many  years'  difference  would  it 
have  made  if  he  had  never  lived?  How  much  longer  be- 
fore the  nucleus  would  have  been  understood  as  we  now 
understand  it?  Perhaps  ten  years.  More  likely  only  five. 

Rutherford's  intellect  was  so  strong  that  he  would,  in 
the  long  run,  have  accepted  that  judgment.  But  he  would 
not  have  liked  it.  His  estimate  of  his  own  powers  was 
realistic,  but  if  it  erred  at  all,  it  did  not  err  on  the  modest 
side.  "There  is  no  room  for  this  particle  in  the  atom  as  de- 
signed by  w^/*  I  once  heard  him  assure  a  large  audience. 
It  was  part  of  his  nature  that,  stupendous  as  his  work  was, 
he  should  consider  it  lo  per  cent  more  so.  It  was  also  part 
of  his  nature  that,  quite  without  acting,  he  should  behave 
constantly  as  though  he  were  lo  per  cent  larger  than  life. 
Worldly  success?  He  loved  every  minute  of  it:  flattery, 
titles,  the  company  of  the  high  official  world.  He  said  in  a 
speech:  "As  I  was  standing  in  the  drawing-room  at  Trin- 
ity, a  clergyman  came  in.  And  I  said  to  him:  T'm  Lord 
Rutherford.'  And  he  said  to  me:  T'm  the  Archbishop  of 
York.'  And  I  don't  suppose  either  of  us  believed  the  other." 

He  was  a  great  man,  a  very  great  man,  by  any  stand- 
ards which  we  can  apply.  He  was  not  subtle:  but  he  was 
clever  as  well  as  creatively  gifted,  magnanimous  (within 
the  human  limits)  as  well  as  hearty.  He  was  also  superbly 
and  magnificently  vain  as  well  as  wise — the  combination 
is  commoner  than  we  think  when  we  are  young.  He  en- 
joyed a  life  of  miraculous  success.  On  the  whole  he  en- 
joyed his  own  personality.  But  I  am  sure  that,  even  quite 
late  in  his  life,  he  felt  stabs  of  a  sickening  insecurity. 


Somewhere  at  the  roots  of  that  abundant  and  crea- 
tive nature  there  was  a  painful,  shrinking  nerve.  One  has 
only  to  read  his  letters  as  a  young  man  to  discern  it.  There 
are  passages  of  self-doubt  which  are  not  to  be  explained 
completely  by  a  humble  colonial  childhood  and  youth.  He 
was  uncertain  in  secret,  abnormally  so  for  a  young  man  of 
his  gifts.  He  kept  the  secret  as  his  personality  flowered  and 
hid  it.  But  there  was  a  mysterious  diffidence  behind  it  all. 
He  hated  the  faintest  suspicion  of  being  patronized,  even 
when  he  was  a  world  figure.  Archbishop  Lang  was  once 
tactless  enough  to  suggest  that  he  supposed  a  famous  scien- 
tist had  no  time  for  reading.  Rutherford  immediately  felt 
that  he  was  being  regarded  as  an  ignorant  roughneck.  He 
produced  a  formidable  list  of  his  last  month's  reading.  Then, 
half  innocently,  half  malevolently:  "And  what  do  you 
manage  to  read,  your  Grice?"  "I  am  afraid,"  said  the  Arch- 
bishop, somewhat  out  of  his  depth,  "that  a  man  in  my  posi- 
tion really  doesn't  have  the  leisure.  .  .  ."  "Ah,  yes,  your 
Grice,"  said  Rutherford  in  triumph,  "it  must  be  a  dog's 
life!  It  must  be  a  dog's  life!" 

Once  I  had  an  opportunity  of  seeing  that  diffidence 
face  to  face.  In  the  autumn  of  1934  I  published  my  first 
novel,  which  was  called  The  Search  and  the  background 
of  which  was  the  scientific  world.  Not  long  after  it  came 
out,  Rutherford  met  me  in  King's  Parade.  "What  have 
you  been  doing  to  us,  young  man?"  he  asked  vociferously. 
I  began  to  describe  the  novel,  but  it  was  not  necessary;  he 
announced  that  he  had  read  it  with  care.  He  went  on  to 
invite,  or  rather  command,  me  to  take  a  stroll  with  him 
round  the  Backs.  Like  most  of  my  scientific  friends,  he 


10 


Rutherford 


was  good-natured  about  the  book,  which  has  some  de- 
scriptions of  the  scientific  experience  which  are  probably 
somewhere  near  the  truth.  He  praised  it.  I  was  gratified.  It 
was  a  sunny  October  afternoon.  Suddenly  he  said:  "I 
didn't  like  the  erotic  bits.  I  suppose  it's  because  we  belong 
to  different  generations." 

The  book,  I  thought,  was  reticent  enough.  I  did  not 
know  how  to  reply. 

In  complete  seriousness  and  simplicity,  he  made  an- 
other suggestion.  He  hoped  that  I  was  not  going  to  write 
all  my  novels  about  scientists.  I  assured  him  that  I  was 
not — certainly  not  another  for  a  long  time. 

He  nodded.  He  was  looking  gentler  than  usual,  and 
thoughtful.  "It's  a  small  world,  you  know,"  he  said.  He 
meant  the  world  of  science.  "Keep  off  us  as  much  as  you 
can.  People  are  bound  to  think  that  you  are  getting  at 
some  of  us.  And  I  suppose  we've  all  got  things  that  we 
don't  want  anyone  to  see." 

I  mentioned  that  his  intuitive  foresight  went  wrong 
just  once.  As  a  rule,  he  was  dead  right  about  the  practical 
applications  of  science,  just  as  much  as  about  the  nucleus. 
But  his  single  boss  shot  sounds  ironic  now.  In  1933  he  said, 
in  another  address  to  the  British  Association,  "These  trans- 
formations of  the  atom  are  of  extraordinary  interest  to 
scientists,  but  we  cannot  control  atomic  energy  to  an 
extent  which  would  be  of  any  value  commercially,  and  I 
believe  we  are  not  likely  ever  to  be  able  to  do  so.  A  lot  of 
nonsense  has  been  talked  about  transmutations.  Our  inter- 
est in  the  matter  is  purely  scientific." 

That  statement,  which  was  made  only  nine  years  be- 


ll 


fore  the  first  pile  worked,  was  not  intended  to  be  either 
optimistic  or  pessimistic.  It  was  just  a  forecast,  and  it  was 
wrong. 

That  judgment  apart,  people  outside  the  scientific 
world  often  felt  that  Rutherford  and  his  kind  were 
optimistic — optimistic  right  against  the  current  of  the 
twentieth  century  literary-intellectual  mood,  offensively 
and  brazenly  optimistic.  This  feeling  was  not  quite  un- 
justified, but  the  difference  between  the  scientists  and  the 
non-scientists  was  more  complex  than  that.  When  the 
scientists  talked  of  the  individual  human  condition,  they 
did  not  find  it  any  more  hopeful  than  the  rest  of  us.  Does 
anyone  really  imagine  that  Bertrand  Russell,  G.  H.  Hardy, 
Rutherford,  Blackett  and  the  rest  were  bemused  by 
cheerfulness  as  they  faced  their  own  individual  state? 
Very  few  of  them  had  any  of  the  consolations  of  religion: 
they  believed,  with  the  same  certainty  that  they  believed 
in  Rutherford's  atom,  that  they  were  going,  after  this 
mortal  life,  into  annihilation.  Several  of  them  were  men 
of  deep  introspective  insight.  They  did  not  need  teaching 
anything  at  all  about  the  existential  absurdity. 

Nevertheless  it  is  true  that,  of  the  kinds  of  people  I 
have  lived  among,  the  scientists  were  much  the  happiest. 
Somehow  scientists  were  buoyant  at  a  time  when  other  in- 
tellectuals could  not  keep  away  despair.  The  reasons  for 
this  are  not  simple.  Partly,  the  nature  of  scientific  activ- 
ity, its  complete  success  on  its  own  terms,  is  itself  a  source 
of  happiness;  partly,  people  who  are  drawn  to  scientific 
activity  tend  to  be  happier  in  temperament  than  other 
clever  men.  By  the  nature  of  their  vocation  and  also  by 
the  nature  of  their  own  temperament,  the  scientists  did 


12 


Rutherford 


not  think  constantly  of  the  individual  human  predica- 
ment. Since  they  could  not  alter  it,  they  let  it  alone.  When 
they  thought  about  people,  they  thought  most  of  what 
could  be  altered,  not  what  couldn't.  So  they  gave  their 
minds  not  to  the  individual  condition  but  to  the  social 
one. 

There,  science  itself  was  the  greatest  single  force  for 
change.  The  scientists  were  themselves  part  of  the  deepest 
revolution  in  human  affairs  since  the  discovery  of  agricul- 
ture. They  could  accept  what  was  happening,  while  other 
intellectuals  shrank  away.  They  not  only  accepted  it,  they 
rejoiced  in  it.  It  was  difficult  to  find  a  scientist  who  did 
not  believe  that  the  scientific-technical-industrial  revolu- 
tion, accelerating  under  his  eyes,  was  not  doing  incom- 
parably more  good  than  harm. 

This  was  the  characteristic  optimism  of  scientists  in 
the  twenties  and  thirties.  Is  it  still?  In  part,  I  think  so.  But 
there  has  been  a  change. 

In  the  Hitler  war,  physicists  became  the  most  essen- 
tial of  military  resources:  radar,  which  occupied  thou- 
sands of  physicists  on  both  sides,  altered  the  shape  of  the 
war,  and  the  nuclear  bomb  finished  large  scale  "conven- 
tional" war  for  ever.  To  an  extent,  it  had  been  foreseen  by 
the  mid-thirties  that  if  it  came  to  war  (which  a  good 
many  of  us  expected)  physicists  would  be  called  on  from 
the  start.  Tizard  was  a  close  friend  of  Rutherford's,  and 
kept  him  informed  about  the  prospects  of  RDF  (as  radar 
was  then  called).  By  1938  a  number  of  the  Cavendish 
physicists  had  been  secretly  indoctrinated.  But  no  one,  no 
one  at  all,  had  a  glimmering  of  how,  for  a  generation 
afterwards,    a   high    percentage   of    all    physicists   in   the 


13 


United  States,  the  Soviet  Union,  this  country,  would  re- 
main soldiers-not-in-uniform.  Mark  Oliphant  said  sadly, 
when  the  first  atomic  bomb  was  dropped:  "This  has  killed 
a  beautiful  subject."  Intellectually  that  has  turned  out  not 
to  be  true:  but  morally  there  is  something  in  it.  Secrecy, 
national  demands,  military  influence,  have  sapped  the 
moral  nerve  of  physics.  It  will  be  a  long  time  before  the 
climate  of  Cambridge,  Copenhagen,  Gottingen  in  the 
twenties  is  restored:  or  before  any  single  physicist  can  speak 
to  all  men  with  the  calm  authority  of  Einstein  or  Bohr. 
That  kind  of  leadership  has  now  passed  to  the  biologists, 
who  have  so  far  not  been  so  essential  to  governments.  It 
will  be  they,  I  think,  who  are  likely  to  throw  up  the  great 
scientific  spokesmen  of  the  next  decades.  If  someone  now 
repeated  Gorki's  famous  question,  "Masters  of  culture, 
which  side  are  you  on?"  it  would  probably  be  a  biologist 
who  spoke  out  for  his  fellow  human  beings. 

In  Rutherford's  scientific  world,  the  difficult  choices 
had  not  yet  formed  themselves.  The  liberal  decencies  were 
taken  for  granted.  It  was  a  society  singularly  free  from 
class  or  national  or  racial  prejudice.  Rutherford  called 
himself  alternatively  conservative  or  non-political,  but 
the  men  he  wanted  to  have  jobs  were  those  who  could  do 
physics.  Niels  Bohr,  Otto  Hahn,  Georg  von  Hevesy,  Hans 
Geiger  were  men  and  brothers,  whether  they  were  Jews, 
Germans,  Hungarians — men  and  brothers  whom  he  would 
much  rather  have  near  him  than  the  Archbishop  of 
Canterbury  or  one  of  "those  fellows"  or  any  damned  Eng- 
lish philosopher.  It  was  Rutherford  who,  after  1933,  took 
the  lead  in  opening  English  academic  life  to  Jewish  refu- 
gees. In  fact,  scientific  society  was  wide  open,  as  it  may 


14 


Rutherford 


not  be  again  for  many  years.  There  was  coming  and  going 
among  laboratories  all  over  the  world,  including  Russia. 
Peter  Kapitsa,  Rutherford's  favorite  pupil,  contrived  to 
be  in  good  grace  with  the  Soviet  authorities  and  at  the  same 
time  a  star  of  the  Cavendish. 

He  had  a  touch  of  genius:  in  those  days,  before  life 
sobered  him,  he  had  also  a  touch  of  the  inspired  Russian 
clown.  He  loved  his  own  country,  but  he  distinctly  en- 
joyed backing  both  horses,  working  in  Cambridge  and 
taking  his  holidays  in  the  Caucasus.  He  once  asked  a 
friend  of  mine  if  a  foreigner  could  become  an  English 
peer;  we  strongly  suspected  that  his  ideal  career  would  see 
him  established  simultaneously  in  the  Soviet  Academy  of 
Sciences  and  as  Rutherford's  successor  in  the  House  of 
Lords. 

At  that  time  Kapitsa  attracted  a  good  deal  of  envy, 
partly  because  he  could  do  anything  with  Rutherford.  He 
called  Rutherford  "the  Crocodile,"  explaining  the  crocodile 
means  "father"  in  Russian,  which  it  doesn't,  quite:  he  had 
Eric  Gill  carve  a  crocodile  on  his  new  laboratory.  He  flat- 
tered Rutherford  outrageously,  and  Rutherford  loved  it. 
Kapitsa  could  be  as  impertinent  as  a  Dostoevskian  come- 
dian: but  he  had  great  daring  and  scientific  insight.  He  es- 
tablished the  club  named  after  him  (which  again  inspired 
some  envy)  :  it  met  every  Tuesday  night,  in  Kapitsa's  rooms 
in  Trinity,  and  was  deliberately  kept  small,  about  thirty, 
apparently  because  Kapitsa  wanted  to  irritate  people  doing 
physical  subjects  he  disapproved  of.  We  used  to  drink  large 
cups  of  milky  coffee  immediately  after  hall  (living  was 
fairly  simple,  and  surprisingly  non-alcoholic,  in  scientific 
Cambridge),  and  someone  gave  a  talk — often  a  dramatic 


15 


one,  like  Chadwick*s  on  the  neutron.  Several  of  the  major 
discoveries  of  the  thirties  were  first  heard  in  confidence  in 
that  room.  I  don't  think  that  the  confidence  was  ever 
broken. 

I  myself  enjoyed  the  one  tiny  scientific  triumph  of 
my  life  there.  At  the  time  Kapitsa  barely  tolerated  me, 
since  I  did  spectroscopy,  a  subject  he  thought  fit  only  for 
bank  clerks:  in  fact  I  had  never  discovered  why  he  let  me 
join.  One  night  I  offered  to  give  a  paper  outside  my  own 
subject,  on  nuclear  spin,  in  which  I  had  been  getting  in- 
terested: I  didn't  know  much  about  it,  but  I  reckoned 
that  most  of  the  Cavendish  knew  less.  The  offer  was  un- 
enthusiastically accepted.  I  duly  gave  the  paper.  Kapitsa 
looked  at  me  with  his  large  blue  eyes,  with  a  somewhat 
unflattering  astonishment,  as  at  a  person  of  low  intelli- 
gence who  had  contrived  inadvertently  to  say  something 
interesting.  He  turned  to  Chadwick,  and  said  incredu- 
lously, "Jimmy,  I  believe  there  is  something  in  this." 

It  was  a  personal  loss  to  Rutherford  when  Kapitsa,  on 
one  of  his  holiday  trips  to  Russia,  was  told  by  the  Soviet 
bosses,  politely  but  unyieldingly,  that  he  must  stay:  he 
was  too  valuable,  they  wanted  his  services  full-time.  After 
a  while  Kapitsa  made  the  best  of  it.  He  had  always  been  a 
patriotic  Russian:  though  both  he  and  his  wife  came  from 
the  upper  middle-class,  if  there  was  such  a  class  in  old 
Russia  (his  father  was  a  general  in  the  Tsarist  engineering 
corps),  he  took  a  friendly  attitude  to  the  revolution.  All 
that  remained  steady,  though  I  don't  think  he  would  mind 
my  saying  that  his  enthusiasm  for  Stalin  was  not  unquali- 
fied. Still,  Kapitsa  threw  all  his  gifts  into  his  new  work  in 
the  cause  of  Soviet  science.  It  was  only  then  that  we,  who 


16 


Rutherford 


had  known  him  in  Cambridge,  reaUzed  how  strong  a  char- 
acter he  was:  how  brave  he  was:  and  fundamentally  what 
a  good  man.  His  friendship  with  Cockcroft  and  others 
meant  that  the  link  between  Soviet  and  English  science 
was  never  quite  broken,  even  in  the  worst  days.  Only 
great  scientists  like  Lev  Landau  can  say  in  full  what  he 
has  done  for  science  in  his  own  country.  If  he  hadn't  ex- 
isted, the  world  would  have  been  worse:  that  is  an  epitaph 
that  most  of  us  would  like  and  don't  deserve. 

Between  Leningrad  and  Cambridge,  Kapitsa  oscil- 
lated. Between  Copenhagen  and  Cambridge  there  was  a 
stream  of  travellers,  all  the  nuclear  physicists  of  the 
world.  Copenhagen  had  become  the  second  scientific  me- 
tropolis on  account  of  the  personal  influence  of  one  man, 
Niels  Bohr,  who  was  complementary  to  Rutherford  as  a 
person — patient,  reflective,  any  thought  hedged  with 
Proustian  qualifications — just  as  the  theoretical  quantum 
physics  of  which  he  was  the  master  was  complementary  to 
Rutherford's  experimental  physics.  He  had  been  a  pupil 
of  Rutherford's,  and  they  loved  and  esteemed  each  other 
like  father  and  son.  (Rutherford  was  a  paterfamilias  born, 
and  the  death  of  his  only  daughter  seems  to  have  been  the 
greatest  sorrow  of  his  personal  life.  In  his  relations  with 
Bohr  and  Kapitsa  and  others,  there  was  a  strong  vein  of 
paternal  emotion  diverted  from  the  son  he  never  had.) 
But,  strong  as  Rutherford's  liking  for  Bohr  was,  it  was 
not  strong  enough  to  put  up  with  Bohr's  idea  of  a  suitable 
length  for  a  lecture.  In  the  Cavendish  lecture  room,  Bohr 
went  past  the  hour;  Rutherford  began  to  stir.  Bohr  went 
past  the  hour  and  a  half;  Rutherford  began  plucking  at 
his  sleeve  and  muttering  in  a  stage  whisper  about  "another 


17 


five  minutes."  Blandly,  patiently,  determined  not  to  leave 
a  qualification  unsaid,  as  indefatigable  as  Henry  James  in 
his  last  period,  Bohr  went  past  the  two  hours;  Rutherford 
was  beginning  to  trumpet  about  "bringing  the  lecture  to 
a  close."  Soon  they  were  both  on  their  feet  at  once. 

Rutherford  died  suddenly  when  he  was  age  sixty-six, 
still  in  full  vigor.  He  died  not  only  suddenly,  but  of 
something  like  a  medical  accident:  he  had  a  strangulated 
hernia.  There  was  no  discernible  reason  why  he  should  not 
have  lived  into  old  age. 

It  was  a  sunny,  tranquil  October  morning,  the  kind 
of  day  on  which  Cambridge  looks  so  beautiful.  I  had  just 
arrived  at  the  crystallographic  laboratory,  one  of  the  build- 
ings in  the  old  Cavendish  muddle;  why  I  was  there  I  don't 
remember,  nor  whom  I  was  talking  to,  except  that  it  hap- 
pened not  to  be  Bernal.  Someone  put  his  head  round  the 
door  and  said:  "The  Professor's  dead." 

I  don't  think  anyone  said  much  more.  We  were 
stupefied  rather  than  miserable.  It  did  not  seem  in  the 
nature  of  things. 


18 


Rutherford  reports  on  his  ingenious  experiments  proving 
that  the  alpha  particle  is  a  charged  helium  atom. 


The  Nature  of  the  Alpha  Particle 

Ernest  Rutherford  and  T.  Royds 

A  paper  in  Philosophical  Magazine,  published  in  1909. 

rr^HE  experimental  evidence  collected  during  the  last 
_L  few  years  has  strongly  supported  the  view  that  the 
a  particle  is  a  charged  helium  atom,  but  it  has  been  found 
exceedingly  difficult  to  give  a  decisive  proof  of  the  relation. 
In  recent  papers,  Rutherford  and  Geiger  f  have  supplied  still 
further  evidence  of  the  correctness  of  this  point  of  view. 
The  number  of  a  particles  from  one  gram  of  radium  have 
been  counted,  and  the  charge  carried  by  each  determined. 
The  values  of  several  radioactive  quantities,  calculated  on  the 
assumption  that  the  a  particle  is  a  helium  atom  carrying  two 
unit  charges,  have  been  shown  to  be  in  good  agreement  with 
the  experimental  numbers.  In  particular,  the  good  agree- 
ment between  the  calculated  rate  of  production  of  helium  by 
radium  and  the  rate  experimentally  determined  by  Sir  James 
Dewarl,  is  strong  evidence  in  favour  of  the  identity  of  the 
a  particle  with  the  helium  atom. 

The  methods  of  attack  on  this  problem  have  been  largely 
indirect,  involving  considerations  of  the  charge  carried  by 
the  helium  atom  and  the  value  of  ejm  of  the  a  particle. 
The  proof  of  the  identity  of  the  a  particle  with  the  helium 
atom  is  incomplete  until  it  can  be  shown  that  the  a  particles, 
accumulated  quite  independently  of  the  matter  from  which 
they  are  expelled,  consist  of  helium.  For  example,  it  might  be 
argued  that  the  appearance  of  helium  in  the  radium  emana- 
tion was  a  result  of  the  expulsion  of  the  a  particle,  in  the 
same  way  that  the  appearance  of  radium  A  is  a  consequence 
of  the  expulsion  of  an  a  particle  from  the  emanation.  If 
one  atom  of  helium  appeared  for  each  a  particle  expelled, 
calculation  and  experiment  might  still  agree,  and  yet  the 
a  particle  itself  might  be  an  atom  of  hydrogen  or  of  some 
other  substance. 

We  have  recently  made  experiments  to  test  whether  helium 
appears  in  a  vessel  into  which  the  a  particles  have  been  fired, 
the  active  matter  itself  being  enclosed  in  a  vessel  sufficiently 
thin  to  allow  the  a  particles  to  escape,  but  impervious  to  the 
passage  of  helium  or  other  radioactive  products. 

*  Communicated  by  the  Authors. 

t  Proc.  Roy.  Soc.  A.  Ixxxi.  pp.  141-173  (1908). 

t  Proc.  Roy.  Soc.  A.  Ixxxi.  p.  280  (1908). 


19 


The  experimental  arrangement  is  clearly  seen  in  the  figure 
The  equilibrium  quantity  of  emanation  from  about  140  milli- 
grams of  radium  was  purified  and  compressed  by  means  of  a 


mercury-column  into  a  fine  glass  tube  A  about  1-5  cms.  long. 
This  fine  tube,  which  was  sealed  on  a  larger  capillary  tube  B, 
\yas  sufficiently  thin  to  allow  the  a  particles  from  the  emana- 
tion and  its  products  to  escape,  but  sufficiently  strong  to 


20 


The  Nature  of  the  Alpha  Particle 


■withstand  atmospheric  pressure.  After  some  trials,  Mr. 
Baumbach  succeeded  in  blowing  such  fine  tubes  very  uniform 
in  thickness.  The  thickness  of  the  wall  of  the  tube  employed 
in  most  of  the  experiments  was  less  than  jJq  mm.,  and  was 
equivalent  in  stopping  power  of  the  a  particle  to  about 
2  cms.  of  air.  Since  the  ranges  of  the  a  particles  from  the 
emanation  and  its  products  radium  A  and  radium  C  are  4*3, 
4*8,  and  7  cms.  respectively,  it  is  seen  that  the  great 
majority*  of  the  a  particles  expelled  by  the  active  matter 
escape  through  the  walls  of  the  tube.  The  ranges  of  the 
a.  particles  after  passing  through  the  glass  were  determined 
with  the  aid  of  a  zinc-sulphide  screen.  Immediately  after 
the  introduction  of  the  emanation  the  phosphorescence  showed 
brilliantly  when  the  screen  was  close  to  the  tube,  but  practi- 
cally disappeared  at  a  distance  of  3  cms.  After  an  hour, 
bright  phosphorescence  was  observable  at  a  distance  of 
5  cms.  Such  a  result  is  to  be  expected.  The  phosphorescence 
initially  observed  was  due  mainly  to  the  «  particles  of  the 
emanation  and  its  product  radium  A  (period  3  mins.).  In 
the  course  of  time  the  amount  of  radium  C,  initially  zero, 
gradually  increased,  and  the  a  radiations  from  it  of  range 
7  eras,  were  able  to  cause  phosphorescence  at  a  greater 
distance. 

The  glass  tube  A  was  surrounded  by  a  cylindrical  glass 
tube  T,  7*5  cms.  long  and  1'5  cms.  diameter,  by  means  of  a 
ground-glass  joint  C.  A  small  vacuum-tube  V  was  attached 
to  the  upper  end  of  T.  The  outer  glass  tube  T  was  exhausted 
by  a  pump  through  the  stopcock  D,  and  the  exhaustion 
completed  with  the  aid  of  the  charcoal  tube  F  cooled  by 
liquid  air.  By  means  of  a  mercury  column  H  attached  to  a 
reservoir,  mercury  was  forced  into  the  tube  T  until  it  reached 
the  bottom  of  the  tube  A. 

Part  of  the  a  particles  which  escaped  through  the  walls  of 
the  fine  tube  were  stopped  by  the  outer  glass  tube  and  part 
by  the  mercury  surface.  If  the  a  particle  is  a  helium  atom, 
helium  should  gradually  diffuse  from  the  glass  and  mercury 
into  the  exhausted  space,  and  its  presence  could  then  be 
detected  spectroscopically  by  raising  the  mercury  and  com- 
pressing the  gases  into  the  vacuum-tube. 

In  order  to  avoid  any  possible  contamination  of  the 
apparatus  with  helium,  freshly  distilled  mercury  and  entirely 
new  glass  apparatus  were  used.  Before  introducing  the 
emanation   into    A,   the  absence  of  helium    was    confirmed 

*  The  a  particles  fired  at  a  \ery  oblique  angle  to  the  tube  would  be 
stopped  in  the  glass.  The  fraction  stopped  in  this  way  would  be  small 
under  the  experimental  conditions. 


21 


experimentally.  At  intervals  after  the  introduction  of  ihe 
emanation  the  mercury  was  raised,  and  the  gases  in  the  outer 
tube  spectroscopically  examined.  After  24  hours  no  trace 
of  the  helium  yellow  line  was  seen  ;  after  2  days  the  helium 
yellow  was  faintly  visible ;  after  4  days  the  helium  yellow 
and  green  lines  were  bright ;  and  after  6  days  all  the  stronger 
lines  of  the  helium  spectrum  were  observed.  The  absence 
of  the  neon  spectrum  shows  that  the  helium  present  was  not 
due  to  a  leakage  of  air  into  the  apparatus. 

There  is,  however,  one  possible  source  of  error  in  this 
experiment.  The  helium  may  not  be  due  to  the  a  particles 
themselves,  but  may  have  diffused  from  the  emanation 
through  the  thin  walls  of  the  glass  tube.  In  order  to  test 
this  point  the  emanation  was  completely  pumped  out  of  A,. 
and  after  some  hours  a  quantity  of  helium,  about  10  times 
the  previous  volume  of  the  emanation,  was  compressed  into- 
the  same  tube  A. 

The  outer  tube  T  and  the  vacuum-tube  were  removed  and 
a  fresh  apparatus  substituted.  Observations  to  detect  helium 
in  the  tube  T  were  made  at  intervals,  in  the  same  way  as 
before,  but  no  trace  of  the  helium  spectrum  was  observed 
over  a  period  of  eight  days. 

The  helium  in  the  tube  A  was  then  pumped  out  and  a 
fresh  supply  of  emanation  substituted.  Results  similar  to 
the  first  experiment  were  observed.  The  helium  yellow 
and  green  lines  showed  brightly  after  four  ciays. 

These  experiments  thus  show  conclusively  that  the  helium 
could  not  have  diffused  through  the  glass  walls,  but  must 
have  been  derived  from  the  a  particles  which  were  fired 
through  them.  In  other  words,  the  experiments  give  a 
decisive  proof  that  the  a  particle  after  losing  its  charge  is  an 
atom  of  helium. 

Other  Experiments. 

We  have  seen  that  in  the  experiments  above  described 
helium  was  not  observed  in  the  outer  tube  in  sufficient 
quantity  to  show  the  characteristic  yellow  line  until  two  days 
had  elapsed.  Now  the  equilibrium  amount  of  emanation 
from  100  milligrams  of  radium  should  produce  helium  at  the 
rate  of  about  'OS  c.mm.  per  day.  The  amount  produced  in 
one  day,  if  present  in  the  outer  tube,  should  produce  a  bright 
spectrum  of  helium  under  the  experimental  conditions.  It 
thus  appeared  probable  that  the  helium  fired  into  the  glass 
must  escape  very  slowly  into  the  exhausted  space,  for  if  the 
helium  escaped  at  once,  the  presence  of  helium  should  have 


22 


The  Nature  of  the  Alpha  Particle 


been  detected  a  few  hours  after  the  introduction  o£  the 
emanation. 

In  order  to  examine  this  point  more  closely  the  experiments 
were  repeated,  with  the  addition  that  a  cylinder  of  thin  sheet 
lead  of  sufficient  thickness  to  stop  the  a  particles  was  placed 
over  the  fine  emanation  tube.  Preliminary  experiments,  in 
the  manner  described  later,  showed  that  the  lead-foil  did  not 
initially  contain  a  detectable  amount  of  helium.  Twenty-four 
hours  after  the  introduction  into  the  tube  A  of  about  the 
Fame  amount  of  emanation  as  before,  the  yellow  and  green 
lines  of  helium  showed  brightly  in  the  vacuum-tube,  and 
after  two  days  the  whole  helium  spectrum  was  observed-.  The 
spectrum  of  helium  in  this  case  after  one  duy  was  of  about 
the  same  intensity  as  that  after  the  fourth  day  in  the  experi- 
ments without  the  lead  scret-n.  It  was  thus  clear  that  the 
lead-foil  gave  up  the  helium  fired  into  it  far  more  readily 
than  the  glass. 

In  order  to  form  an  idea  of  the  rapidity  of  escape  of  the 
helium  from  the  lead  some  further  experiments  were  made. 
The  outer  cylinder  T  was  removed  and  a  small  cylinder  of 
lead-foil  placed  round  the  thin  emanation-tube  surrounded 
the  air  at  atmospheric  pressure.  After  exposure  for  a  definite 
time  to  the  emanation,  the  lead  screen  was  removed  and 
gested  for  helium  as  follows.  The  lead-foil  was  placed  in  a 
glass  tube  between  two  stopcocks.  In  order  to  avoid  a 
possible  release  of  the  helium  present  in  the  lead  by  pumping 
out  the  air,  the  air  was  displaced  by  a  current  of  pure  elec- 
trolytic oxygen*.  The  stopcocks  were  closed  and  the  tube 
attached  to  a  subsidiary  apparatus  similar  to  that  employed 
for  testing  for  the  presence  of  neon  and  helium  in  the  gases 
produced  by  the  action  of  the  radium  emanation  on  water 
(Phil.  Mag.  Nov.  1908).  The  oxygen  was  absorbed  by 
charcoal  and  the  tube  then  heated  beyond  the  melting-point 
of  lead  to  allow  the  helium  to  escape.  The  presence  of 
helium  was  then  spectroscopically  looked  for  in  the  usual 
way.  Using  this  method,  it  was  found  possible  to  detect 
the  presence  of  helium  in  the  lead  which  had  been  exposed 
for  only  four  hours  to  the  a  rays  from  the  emanation.  After 
an  exposure  of  24  hours  the  helium  yellow  and  green  lines 
came  out  brightly.  These  experiments  were  repeated  several 
times  with  similar  results. 

A  number  of  blank  experiments  were  made,  using  samples 
of  the  lead-foil  which  had  not  been  exposed  to  the  a  rays, 
but  in  no  case  was  any  helium  detected.     In  a  similar  way, 

*  That  the  air  was  completely  displaced  was  shown  by  the  absence  of 
neon  in  the  final  spectrum. 


23 


the  presence  of  helium  was  detected  in  a  cylinder  o£  tinfoil 
exposed  for  a  few  hours  over  the  emanation-tube. 

These  experiments  show  that  the  helium  does  not  escape 
at  once  from  the  lead,  but  there  is  on  the  average  a  period 
of  retardation  of  several  hours  and  possibly  longer. 

The  detection  of  helium  in  the  lead  and  tin  foil,  as  well  as 
in  the  glass,  removes  a  possible  objection  that  the  helium 
might  have  been  in  some  way  present  in  the  glass  initially, 
and  wjis  liberated  as  a  consequence  of  its  bombardment  by 
the  a  particles. 

The  use  of  such  thin  glass  tubes  containing  emanation 
affords  a  simple  and  convenient  method  of  examining  the 
effect  on  substances  of  an  intense  a.  radiation  quite  inde- 
pendently of  the  radioactive  material  contained  in  the  tube. 

We  can  conclude  with  certainty  from  these  experiment'^ 
that  the  a  particle  after  losing  its  charge  is  a  helium  atou). 
Other  evidence  indicates  that  the  charge  is  twice  the  unit 
charge  carried  by  the  hydrogen  atom  set  free  in  the  electrolysis 
of  water. 

Univfirsity  of  Manchester, 
I^ov.  13,  1908. 


24 


Chadwick  reminisces  on  the  period  when  he,  as  Ruther- 
ford's collaborator,  searched  for  evidence  of  the  neutron 
in  the  seal ing-wax-and -string  tradition  of  experimenta- 
tion. 


Some  Personal  Notes  on  the  Search  for  the  Neutron 


Sir  James  Chadwick 


Speech  delivered  before  the  10th  International  Congress  of  History 
of  Science  at  Cornel!  University,  New  York,  in  1962. 


•t/LiA'C^H'CC      ^    A     yytut^uU'      jja/y^^^      /rT^»H^    /y    Tkc    cl^>^ 

t/tx/irtdttrn    •      /OP     \/it€t6cnU    t*    y^c  .     n-yyumA    fCc^tu.     hra^    'T/iiv/ 
1^        d/H/lAyKCi/O  . 


25 


AtCiAin^^     J      IJU,U.      i(^ee^.     idkt/uo ^^STcU^/fu't^^ .      ^ 

Uc    M   ^*n^  /ictUU^   .t^fi^    Kit/  4/n^  .z^^r^^U^u^    ^   fC^ 


26 


Some  Personal  Notes  on  the  Search  for  the  Neutron 


m^4m^     %c    A^^t^tt    ej-'^kt,    £/n/ni^U'  /i.aAUiUlryy  .         lOUt. 

/?-      h^ucko       ^      ^     k//}u,    /^     j't/r%>iUn^     y'O^fytu.    .!a41^c<.     J^-^3Ci> 
1uUi4A^     •         IvC-     eU^     U<i/**i/ttU^      A%    %4.    jOA^nU    t^r*^     ^-€^>**'C. 
/^ylL.     A<AO     fit/i4a  *W     i^      /KA^     M^^HU^     /#<     C^ldJ^ 

A     ^//yu^i/*t'     hr>iy(d      l^-ociiM^x      /C-OHC      hH'^i^      f4,>iiCL     i^n^M^ 
Z      A<C/»^     Xj>     /OcJu^nU      t^ixA/tA      ^^i**     iUA^    hm4L    -  -  .   ,         /- 


27 


fir    /fh ,      4W     iV»    fKi.     U^virxlo     A    4ilui,    urrtk     I    ^ui^ 

/Ul     Ui/M4XAWL     UMTUi^    ff-    k</Uui^    ^     /a«^/,^J*^^       iC^  h444u^ 
JiA>iUALCwry       un^       turf       0<   h/uiZuUo     M^tiAi    ^     '>uf«/^«<>t    • 

hud     t*    (U/rU^    /U444aMc    xluTi^cd/  '^*u^tnC<i    ^  cruyfji^  . 


28 


Some  Personal  Notes  on  the  Search  for  the  Neutron 


LU     Ut^    iU/t<i/ll^    Ui    ^l^ci^AyeMa-        m^  fui/f-  ln/h    tia^ 

iA\/u^M/i/Ui^<   .      Wiu/i\   0ic    'tC^iA'  c<^\cfiI2^     h«,4yttu.    cunu/Ci^-ic 

IsK/U      -W^      XA^     /J^WTt'H^*^^       — '       t      WX^    yl^OtM      C/il4^      (4- 

aj(y^iuvn^<X<r    ^    Ih  ■  C.  F.  S  uA.^yv*^»*^    a^    J^ .  F.  V^c^    ^    ^ 

uj)    Uio    Au-^Z^      AhV     ^^      hK^U     PU     ^l-fA^    t*   joi4H<^    AM**^ , 


29 


tJu^    4   nuu^    fi«/i^iUi    SSwU    ki^riUi^      l^utnlc        tC%^Jl^ 
/5tU    AljinC  I   *tf   a/UfKi4/kU    *^    I  i^rvo .      A  U^CU    U/U,    fLtJ" 


30 


Some  Personal  Notes  on  the  Search  for  the  Neutron 


31 


The  authors  establish  the  existence  of  antlprotons  and 
explain  their  belief  that  there  must  be  antineutrons. 


4      Antiprotons 

Owen  Chamberlain,  Emilio  Segre,  Clyde  E.  Wiegand, 
and  Thomas  J.  Ypsilantis 

From  the  periodical  A/aft/re, published  in  1956. 


SINCE  the  development  of  Dirac's  theory  of  the 
electron  and  the  brilliant  confirmation  of  one  of 
its  most  startling  predictions  by  the  discovery  of  the 
positron  by  Anderson,  it  has  been  assumed  most 
likely  that  the  proton  would  also  have  its  charge 
conjugate,  the  antiproton.  The  properties  that  define 
the  antiproton  are  :  (1)  charge  equal  to  the  electron 
charge  (also  in  sign)  ;  (2)  mass  equal  to  the  proton 
mass ;  (3)  stability  against  spontaneous  decay ; 
(4)  ability  to  become  annihilated  by  interaction  with 
a  proton  or  neutron,  probably  generating  pions  and 
releasing  in  some  manner  the  energy  2  mc^  ;  (5) 
generation  in  pairs  with  ordinary  nucleons ;  (6) 
magnetic  moment  equal  but  opposite  to  that  of  the 
proton;  (7)  fermion  of  spin  §.  Not  all  these  properties 
are  independent,  but  all  might  ultimately  be  sub- 
jected to  experiment. 

In  cosmic  rays,  where  such  antiprotons  could 
appear,  some  events  have  been  observed  which  could 
be  due  to  antiprotons  ;  but  their  interpretation  is 
uncertain. 

In  order  to  generate  antiprotons  in  the  laboratory, 
an  absolute  lower  limit  of  the  necessary  energy  is 
2  mc^  =  I  -88  JBeV.-i  but  the  mechanism  of  the 
collision  and  the  conservation  of  momentum  influence 
this  lower  limit,  which  becomes  5-6  BeV.  if  the 
process  is  a  nucleon-nucleon  collision,  or  4-4  BeV.  if 
the  process  is  a  two-step  one  with  the  formation  of  a 
pion   in  a  nucleon-nucleon  collision   followed  by   a 


pion-nucleon  collision  in  which  the  nucleon-anti- 
nucleon  pair  is  generated.  These  thresholds  can  be 
lowered  appreciably  by  internal  motions  of  nucleons 
in  the  nucleus.  (Energies  are  quoted  in  the  laboratory 
system.) 

When  the  Berkeley  bevatron  was  planned,  the 
goal  of  6  BeV.  was  set,  in  the  hope  that  this  energy 
would  be  sufficient  to  create  antiprotons. 

The  methods  of  detection  of  the  antiproton  can 
make  use  of  any  of  the  seven  properties  listed  above. 
It  seemed  that  (1),  (2)  and  (3)  might  be  the  esisiest 
to  ascertain  ;  (4)  would  also  be  highly  desirable  ; 
whereas  (5)-(7)  are  at  present  very  difficult  to 
observe. 

There  are  classical  methods  of  measuring  charge 
and  mass  of  a  particle  that  go  back  in  their  origin 
to  J.  J.  Thomson.  They  entail  the  simultaneous 
measurement  on  the  same  particle  of  any  two  of  the 
quantities  momentum,  velocity  or  energy,  which  in 
turn  can  be  obtained  from  the  observation  of  electric 
or  magnetic  deflexions,  time  of  flight,  range,  scattering 
in  photographic  emulsions,  etc.  As  for  the  charge,  it 
is  sufficient  to  measure  its  sign  and  its  absolute  value 
in  a  rough  way  only,  because  it  is  assumed  that  it 
is  an  integral  multiple  of  the  electronic  charge. 

After  a  detailed  discussion,  it  was  decided  that 
momentum  p.  and  velocity  v  constituted  the  most 
promising  combination  for  ascertaining  the  mass. 
The  first   successful   experiment*  was   performed  at 


32 


Antiprotons 


the  end  of  September  1955,  aa  follows.  The  momentum 
WE«  measured  by  passing  the  particles  generated  by 
bombardment  of  a  copper  target  with  6-2  BeV. 
protons  through  two  deflecting  magnetic  fields  and 
two  magnetic  lenses.  This  ensemble  let  through 
only  particles  for  which  p  =  1-19  BeV./c,  if  their 
charge  is  equal  to  that  of  the  electron,  including  sign. 
The  velocity  was  measured  by  a  time-of -flight 
measurement  between  two  scintillation  counters 
40  ft.  apart.  The  pulse-size  in  the  scintillators  showed 
that  the  particles  were  singly  charged. 

The  chief  difficulty  of  the  experiment  rests  with 
the  fact  that  the  antiprotons  are  accomptuiied  by 
many  pions — 44,000  pions  per  antiproton  in  the  most 
favourable  conditions.  For  this  reason  provision 
must  be  made  for  eliminating  spurious  background 
effects.  One  of  the  most  important  steps  is  the 
insertion  in  the  beam  of  two  Cerenkov  coiuiters  : 
one  that  is  activated  by  particles  with  u/c  =  P  >  0-79, 
and  one  of  a  special  type  that  is  activated  by  par- 
ticles with  0-75  <  P  <  0-78.  Pions  with  p  = 
1-19  BeV./c  have  p  =  0-99,  while  antiprotons  of  the 
same  value  of  p  have  p  =  0-78,  and  their  respective 
times  of  flight  for  an  interval  of  40  ft.  are  40  X 
10"'  sec.  and  51  x  10-»  sec.  Particles  with  p  in  the 
interval  between  0-75  and  0-78  trigger  the  sweep  of 
an  oscilloscope  in  which  the  time  of  flight  between 
two  scintillation  counters  40  ft.  apart  is  displayed. 
This  time  of  flight  appears  as  the  distance  between 
the  two  'pips'  due  to  the  traversal  of  the  counters. 
From  this  time  of  flight  the  mass  is  determined  with 
an  accuracy  of  10  per  cent  for  each  particle.  Up  to 
now,  about  250  particles  have  been  observed  and 
the  average  mass  is  known  to  about  5  per  cent.  It 
is  1,840  ±  90  electron  masses. 

The  functioning  of  the  whole  apparatus  is  checked 
by  sending  through  it  positive  protons  in  a  separate 
run.  These  are  obtained  from  a  subsidiary  target, 
and  their  orbits  are  selected  in  such  a  way  that  they 
have  the  same  momentum  as  the  antiproton. 

The  particles  are  observable  after  a  time  of  flight 
of  10-'  sec.,  which  rules  out  particles  with  a  mean 
life  much  shorter  than  10"'  sec,  in  particular  the 
known  hyperons.  These  measvirements  are  thus  in 
agreement  with  points  (1),  (2)  and  (3)  mentioned 
above,  and  the  identification  of  the  new  particle  with 
the  antiproton  is  a  natural  one,  although  not 
absolutely  established. 

There  are  also  some  indications  on  the  fourth 
point  mentioned  above,  namely,  the  terminal  process 
of  the  particle.  Particles  selected  as  antiprotons  by 
the  apparatus  of  ref.  1  were  sent  into  a  block  of  heavy 
glass  and  the  Cerenkov  radiation  generated  in  it  was 


measured'.  This  radiation  does  not  correspond,  of 
course,  to  the  entirety  of  the-energy  released  ;  actually 
it  is  oiily  a  small  part  of  it.  However,  a  calibration 
was  performed,  and  from  the  pulse  size  the  visible 
energy  was  estimated.  Values  up  to  800  MeV.  were 
found.  This  is  consistent  with  the  expected  modes 
of  ajinihilation  for  an  antiproton,  and  with  the 
energy  it  would  throw  into  Cerenkov  radiation  in  a 
detectable  form  ;  but  it  is  not  sufficient  yet  for 
positive  identification  on  that  score  only. 

Another  type  of  observation  on  the  terminal 
phenomenon  accompanying  the  absorption  of  the 
antiproton  was  also  performed*  with  the  photo- 
graphic plate  technique.  Particles  of  selected 
momentum  obtained  with  an  arrangement  similar  to 
that  described  in  ref.  1  were  slowed  down  by  a 
copper  absorber  and  finally  stopped  in  a  stack  of 
photographic  emulsions.  Among  a  background  of 
many  pions  one  particle  was  found  which  has  pro- 
tonic  mass,  comes  to  rest  and  produces  a  star  con- 
taining six  black  tr«W5ks,  one  grey  proton,  one  pion 
of  58  MeV.  and  one  minimum  ionization  track.  The 
visible  energy  released  is  Itirger  than  830  MeV.  The 
total  energy  released  cannot  be  known,  because  there 
are  neutral  particles  emitted ;  bu''-  this  amount  of 
visible  energy  is  also  consistent  with  the  annihilation 
of  an  antiproton. 

Clearly  mtiny  questions  are  raised  by  the  new 
particle.  Its  identification  should  be  fvu-ther  cor- 
roborated ;  it  is  important  to  study  in  detail  its 
annihilation  properties  for  complex  nuclei  and, 
possibly  even  more  interesting,  the  annihilation  with 
hydrogen  and  deuterium.  In  addition,  the  cross- 
section  for  nuclear  interaction  and  the  mechanism  of 
production  are  clearly  to  be  investigated. 

The  existence  of  the  emtiproton  entails  with  virtual 
certainty  the  existence  of  the  antineutron.  Its 
experimental  demonstration  is  a  most  interesting 
problem.  Probably  the  neutron  beam  of  the  Berkeley 
bevatron  contains  an  appreciable  numbet  of  them, 
but  their  disentanglement  from  the  ordinary  neutrons 
appears  a  formidable  task.  It  is  likely  that  the  best 
approach  will  be  either  :  (1)  to  transform  an  anti- 
proton into  an  antineutron  by  a  collision  with  a 
proton  ;  or  (2)  to  convert  an  antineutron  into  an 
antiproton  by  collision  with  an  ordinary  neutron  and 
detect  either  the  final  antineutron  in  (1)  or  the  final 
tintiproton  in  (2). 
>  Chamberlain,  Segrfe,  Wiegand  and  Ypsilantis,  Phyt.  Rev.,  100,  947 

(1955). 
•  Brabant,  Cork,  Horwitz,  Moyer,  Murray,  Wallace  and  Wenzel,  Phyt. 

Rev.  (in  the  press). 
•Chamberlain     Chupp.    Ooldhaber,    Segrft.    Wieirand,   and    Amaldl, 
Baroni,  Castagnoli,  Franzinetti  and  Manfredini  (to  be  published). 


33 


GIANT  SHOWER  OF  MESONS  is  recorded  in  this  photomicro- 
graph of  a  small  section  of  nuclear  emulsion  carried  to  a  height  of 
106,000  feet  by  a  Navy  "Skyhook"  balloon.  At  the  top  of  the  photo- 
micrograph is  the  heavy  track  of  an  enormously  energetic  iron  nu- 


cleus in  the  primary  cosmir  radiation.  Above  the  nucleus  is  a  "star" 
resulting  from  the  collision  of  the  iron  nucleus  and  a  nucleus  in  the 
emulsion.  Below  the  star  is  a  jet  of  about  40  pi  mesons.  To  the  left 
and  right  of  the  star  are  heavier  fragments  of  the  target  nucleus. 


34 


Elementary  particles  can  be  studied  by  the  traces  they 
leave  in  photographic  plates. 


5    The  Tracks  of  Nuclear  Particles 


Herman  Yagoda 


Article  published  in  1956  in  the  Scientific  American. 


A  nuclear  physicist  studying  the 
elementary  particles  of  nature  is 
in  much  the  same  position  as  an 
explorer  trying  to  picture  unknown  ani- 
mals from  their  tracks.  The  physicist 
never  can  see  the  particles  themselves— 
only  their  footprints  in  a  cloud  chamber 
or  a  photographic  plate.  But  from  these 
tracks  he  deduces  a  particle's  mass, 
movements,  speed,  lifetime  and  social 
impact  on  its  fellows.  By  now  the  tracks 
of  some  members  of  the  nuclear  family 
are  almost  as  familiar  and  readable  as 
the  footprints  of  a  domestic  animal.  In- 
teresting new  tracks  keep  turning  up, 
some  strange,  some  predictable— the  lat- 
est to  make  its  appearance  is  that  of  the 
long-sought  antiproton.  It  seems  a  time- 
ly moment  to  survey  the  scene  and  re- 
view the  gallery  of  footprints  that  iden- 
tify the  members  of  the  strange  popula- 
tion in  the  nucleus  of  the  atom. 

We  shall  consider  the  tracks  as  they 
are  recorded  in  photographic  emulsions. 
It  was  in  this  medium  that  the  existence 
of  particles  in  the  nucleus  of  the  atom 
was  first  detected— through  the  fact  that 
Henri  Becquerel  left  some  uranium  near 
photographic  film  in  a  drawer.  Becquerel 
noted  simply  that  radioactive  emana- 
tions from  the  uranium  had  fogged  his 
film.  That  the  "fog"  might  consist  of  a 
network  of  tracks  was  not  discovered  un- 
til 13  years  later.  In  1909  Otto  Mugge  of 
Germany  expo.sed  some  film  to  tiny  crys- 
tals of  zircon,  a  feebly  radioactive  miner- 
al. To  study  the  faintly  developed  image 
he  had  to  use  a  microscope,  and  he  then 
noticed  that  there  were  fine  linear  tracks 
radiating  from  the  crystals.  Not  long 
afterward  the  tracks  of  alpha  particles 
emitted  by  radium  were  recorded  in  fine- 
grained emulsions  at  Lord  Rutherford's 
famous  laboratory  in  England. 

When  a  charged  particle  travels 
through    a    photographic    emulsion,    it 


forms  a  latent  image  in  the  silver  bro- 
mide grains,  just  as  light  does.  In  the  case 
of  the  moving  particle,  the  latent  image 
results  from  ionization  by  the  particle 
along  its  path.  This  image,  marking  the 
track  of  the  particle,  is  then  made  visi- 
ble by  development  of  the  emulsion  in 
the  usual  way.  So  that  fast  particles  may 
be  brought  to  a  stop  within  the  emulsion, 
it  is  usually  made  as  thick  as  possible. 
Emulsions  used  to  track  cosmic  rays 
and  high-energy  particles  from  accelera- 
tors are  often  more  than  one  millimeter 
thick— about  100  times  thicker  than  in 
ordinary  photographic  film.  The  length 
of  a  particle's  track  in  the  emulsion  must 
be  measured  precisely  to  determine  its 
kinetic  energy.  Since  the  path  slants  into 
the  emulsion,  its  length  cannot  be  meas- 
ured directly:  it  is  computed  by  means 
of  the  Pythagorean  theorem  from  the 
two  measurable  distances— the  depth  at 
which  the  particle  comes  to  rest  in  the 
emulsion  and  the  horizontal  distance 
along  the  emulsion  surface  from  the 
point  of  entry  to  the  point  directly 
above  the  end  of  the  track. 

At  best  the  search  for  particle  tracks 
in  emulsions  is  slow,  tedious  work.  It 
takes  many  hours  or  days  of  poring  over 
the  photographic  plate  with  a  micro- 
scope to  find  and  trace  the  faint  lines  of 
silver  grains.  For  this  reason  physicists 
long  preferred  to  use  cloud  chambers 
for  particle  detection  work.  But  the  pho- 
tographic plate  has  an  obvious  advan- 
tage over  a  cloud  chamber.  Particles 
traveling  through  this  denser  medium 
are  more  likely  to  collide  with  atomic 
nuclei  and  produce  interesting  develop- 
ments. A  great  deal  of  work  has  been 
done  to  improve  nuclear  emulsions.  In 
1947  Pierre  Demers  of  the  University  of 
Montreal  found  a  way  to  prepare  stable 
emulsions  containing  90  per  cent  silver 
bromide,  instead  of  the  usual  30  per 


cent,  and  in  these  more  concentrated 
emulsions  particles  produce  more  robust 
tracks. 

Jet  us  proceed  to  examine  some  of  the 
^— '  identifying  tracks.  We  shall  begin 
by  immersing  a  photographic  plate  in  a 
very  dilute  solution  of  a  soluble  com- 
pound of  the  radioactive  element  radi- 
um. After  leaving  it  for  a  time  (days, 
weeks  or  months)  in  a  dark  place,  we 
remove  the  plate,  develop  it  and  inspect 
it  under  a  microscope.  Here  and  there 
on  the  plate  we  see  starlike  sets  of  short 
heavy  tracks,  each  set  radiating  like 
spokes  from  a  hub  point.  The  tracks 
identify  the  particles  as  slow  alpha  par- 
ticles, and  the  formation  is  known  as  an 
alpha  star.  At  the  center  of  the  star  a 
radium  atom  has  emitted  a  series  of  al- 
pha particles.  The  radium  atom  decays 
first  to  radon,  then  to  other  unstable  de- 
scendants and  finally  to  lead.  In  this 
spontaneous  transmutation  from  radium 
to  lead  a  total  of  five  alpha  particles 
(plus  several  beta  particles)  is  emitted. 
Each  in  the  series  comes  out  with  a 
characteristic  kinetic  energy,  and  the 
different  energies  (ranging  up  to  7.7 
million  electron  volts)  cause  the  tracks 
in  a  star  to  be  of  different  lengths. 

Occasionally  the  star  seen  in  a  pho- 
tographic plate  may  represent  the  disin- 
tegration of  not  one  but  many  radium 
atoms.  This  was  made  clear  by  an  exper- 
iment performed  by  Mile.  C.  Chamie  at 
the  Curie  Institute  in  Paris.  She  exposed 
a  plate  in  an  extremely  dilute  solution  of 
polonium,  the  last  alpha-emitting  de- 
scendant of  radium  in  the  transition  to 
lead.  It  was  supposed  that  single  tracks 
of  alpha  particles,  from  separate  atoms 
of  polonium,  would  appear  in  the  emul- 
sion. Instead  Mile.  Chamie  found  stars 
consisting  of  several  hundred  alpha 
tracks  from  a  common  center.  All  the 


35 


tracks  were  of  the  same  length,  corre- 
sponding to  the  energy  of  alpha-emis- 
sion from  polonium.  Evidently  even  in 
an  extremely  dilute  solution  the  po- 
lonium atoms  are  not  completely  disso- 
ciated into  individual  ions  but  may 
cluster  in  groups  of  several  thousand 
atoms.  The  collections  have  been  named 
radiocolloids. 

All  matter  contains  traces  of  radio- 
active substances,  and  their  energy  fields 
have  been  pulsating  in  minerals  since 
the  earth's  crust  soUdified  eons  ago.  Na- 
ture strews  the  investigator's  path  with 
clues— if  we  could  only  see.  Long  before 
the  discovery  of  radioactivity,  geologists 
had  observed  that  grains  in  certain  min- 
erals, such  as  mica,  were  sometimes  sur- 
rounded with  halos  of  colored  material. 
They  could  find  no  way  to  explain  how 
these  colored  bands  might  be  formed. 
In  1907,  when  radioactivity  was  a  topic 
of  growing  interest,  John  Joly  in  Ireland 
noted  that  the  distance  from  the  center 
of  each  tiny  sphere  to  the  halo  around 
it  was  about  the  same  as  the  range  of  an 
alpha  particle  emitted  by  radium  or  tho- 
rium. He  suggested  what  is  now  taken  to 
be  the  correct  solution  of  the  mystery: 
that  alpha  particles  radiating  from  radio- 
active atoms  at  the  center  ionize  iron 
atoms  in  the  mica  near  the  end  of  their 
path,  cause  the  iron  to  become  oxidizled 
and  thereby  produce  the  colored  bands. 

Just  as  familiar,  and  as  ubiquitous,  as 


the  footprints  of  alpha  particles  are  the 
footprints  of  beta  particles,  or  electrons. 
These  light  particles  make  very  faint, 
highly  scattered  tracks  in  an  emulsion. 
Originating  from  radioactive  substances 
and  from  cosmic  ray  showers,  flying  elec- 
trons record  their  presence  in  emulsions 
wherever  placed  or  however  carefully 
shielded.  Even  at  great  depths  under- 
ground a  photographic  plate  will  show 
about  one  million  electron  tracks  per 
cubic  centimeter  for  each  day  of  its 
underground  exposure. 

IVTo  footprints  are  more  fascinating 
^  than  those  of  the  strange  particles 
known  as  mesons.  Had  present  emul- 
sions been  in  use  in  the  1920s,  their 
tracks  would  have  been  discovered  first 
and  "explained"  afterward;  as  it  was, 
the  particles  were  predicted  by  the  theo- 
retician Hideki  Yukawa  two  years  before 
they  were  actually  found.  Yukawa  in- 
vented the  meson  to  account  for  the 
binding  force  that  holds  particles  to- 
gether in  the  atomic  nucleus.  Tracks  of 
a  particle  such  as  he  had  predicted— 
about  200  times  heavier  than  the  elec- 
tron—were first  discovered  in  1937  in 
cloud  chambers  monitoring  the  products 
of  cosmic  rays.  A  mystery  soon  devel- 
oped: the  theory  said  that  these  parti- 
cles should  interact  strongly  with  atomic 
nuclei,  but  experiments  proved  that  they 
were  rarely  absorbed  by  nuclei. 


SPECIAL  MICROSCOPE  ii  used  to  (can  nuclear  emulsions.  The  large  stage  enables  the 
viewer  to  follow  long  tracks.  Here  the  emulsion  is  a  disk  embedded  in  a  rectangular  Lucite 
frame  fitted  with  a  cover  glass.  The  depth  of  the  track  is  read  on  the  wheel  at  upper  right. 


While  the  theoreticians  were  ponder- 
ing this  hiatus  between  theory  and  ex- 
permient,  the  younger  physicists  were 
busy  climbing  mountains  and  exposing 
photographic  plates  to  the  intense  cos- 
mic radiation  high  in  the  atmosphere. 
By  1947  they  had  discovered  a  second, 
heavier  meson  which  did  react  strongly 
with  matter  [see  "The  Multiplicity  of 
Particles,"  by  Robert  E.  Marshak;  Sci- 
entific American,  January,  1952].  A 
Bristol  University  team  of  investigators 
headed  by  C.  F.  Powell  obtained  photo- 
graphs showing  that  when  the  heavy  pi 
meson  came  to  rest  it  promptly  decayed 
into  the  lighter  mu  meson. 

A  year  later  the  young  Brazilian  C.  M. 
G.  Lattes,  a  member  of  the  Bristol  cos- 
mic ray  group,  came  to  the  University  of 
California  and  in  cooperation  with  Eu- 
gene Gardner  succeeded  in  detecting 
mesons  from  nuclei  attacked  by  a  400- 
million-electron-volt  beam  of  alpha  par- 
ticles from  the  Berkeley  cyclotron.  Two 
types  of  pi  meson  tracks  were  then 
identified.  Positively  charged  pi  mesons 
decayed  into  mu  mesons.  Negatively 
charged  pi  mesons  reacted  with  atomic 
nuclei,  and  the  disintegration  of  the 
capturing  nucleus  produced  a  star. 

Meanwhile  the  European  investiga- 
tors, lacking  funds  for  the  construction 
of  expensive  accelerators,  continued  to 
study  mesons  in  the  cosmic  radiation— 
the  poor  man's  cyclotron.  These  simple 
experiments  gave  birth  to  a  perplexing 
number  of  new  particles. 

Their  first  addition  to  the  growing 
fraternity  of  Greek-lettered  mesons  was 
the  tau  particle.  The  Bristol  University 
investigators  found  its  track  in  an  elec- 
tron-sensitive plate  exposed  beneath  a 
12-inch-thick  block  of  lead  at  the  Jung- 
fraujoch  High  Altitude  Research  Station. 
The  particle,  heavier  than  a  pi  meson, 
produced  an  unusual  three-pronged  star 
on  coming  to  rest.  All  three  prongs 
could  be  identified  as  the  tracks  of  pi 
mesons.  From  the  available  evidence 
Powell  came  to  the  conclusion  that  the 
tau  meson  was  an  unstable,  singly 
charged  particle  about  1,000  times 
heavier  than  the  electron.  Powell's  bril- 
liant deductions  tempt  one  to  finish  oflF 
his  description  with  the  admiring  excla- 
mation: "A  new  particle— elementary, 
my  dear  WatsonI" 

The  heavy  tau  meson  is  very  rare, 
but  an  extensive  vigil  has  now  detected 
a  number  of  these  events  and  established 
the  particle's  properties.  Recent  con- 
trolled experiments  with  the  six-billion- 
electron-volt  Bevatron  at  Berkeley  indi- 
cate that  the  tau  particle  and  certain 
other  heavy  mesons  (known  as  K  mes- 


36 


The  Tracks  of  Nuclear  Particles 


ALPHA  PARTICLES  made  the  image  in  this  dark-field  photomi- 
crograph. The  emulsion  itself  contains  tiny  colloid  particles  of  radi- 


um, one  of  which  is  at  the  center  of  the  image.  The  tracks  were  made 
by  alpha  particles  emitted  by  radium  and  its  daughter  elements. 


ALPHA  STARS  emerged  from  thorium  atoms  in  this  emulsion. 
The  stars  at  left  and  right  represent  the  serial  decay  of  single  thor- 


ium atoms.  First 'the  thorium  atom  emitted  an  alpha  particle,  then 
the  daughter  isotope  emitted  another  alpha  particle,  and  so  on. 


37 


ons)  probably  are  all  the  same  particle 
showing  alternate  modes  of  decay. 

TVTeutral  particles  unfortunately  leave 
-'■  ^  no  footprints  in  an  emulsion  or 
cloud  chamber.  They  may,  however,  sig- 
nal their  presence  indirectly.  For  exam- 
ple, a  fast  neutron  charging  through  an 


emulsion  may  coUide  head  on  with  a 
hydrogen  atom,  rip  away  the  latter's 
electron  and  cause  the  proton  to  recoil 
and  make  a  track  that  tells  the  story  of 
the  collision. 

At  Berkeley  all  eyes  are  focused  just 
now  on  the  footprints  of  the  antiproton, 
which  at  long  last  was  generated  by  the 


Bevatron  a  few  months  ago.  The  anti- 
proton— the  negatively  charged  counter- 
part of  the  positive  proton— has  only  a 
fleeting  life,  but  it  makes  its  existence 
unmistakably  known  by  the  spectacular 
manner  of  its  death.  When  the  particle 
comes  to  rest  in  an  emulsion,  there  is  an 
explosion  which  generates  a  large  star. 


GROUP 

MEMBERS 

SYMBOL 

REST  MASS 

(ELECTRON  MASSES) 

MEAN  LIFE 

(SECONDS) 

PROTON 

P+ 

1836.13 

STABLE 

NUCLEONS 

ANTIPROTON 

P~ 

1840  ±  90 

—  5x10-8 

NEUTRON 

nO 

1-838.65 

750 

ELECTRON 

e- 

1 

STABLE 

LEPTONS 

POSITRON 

e  + 

1 

ANNIHILATES 

NEUTRINO 

V 

0 

NEGATIVE  PI  MESON 

ll~ 

272.8  ±  0.3 

2.44x10-® 

POSITIVE  PI  MESON 

71  + 

273.3  i  0.2 

2.53x10-8 

LIGHT  MESONS 

NEUTRAL  PI  MESON 

n" 

263.7  ±  0.7 

5x10 -'5 

NEGATIVE  MU  MESON 

^" 

207  ±  0.5 

POSITIVE  MU  MESON 

t** 

206.9  1  0.4 

2.15x10^6 

TAU  MESON 

T  + 

965.5  1  0.7 

-5x10-8 

THETA  MESON 

e° 

965  ±  10 

I.6x10-'C 

HEAVY  MESONS 

CHI  MESON 

X(Kn2) 

963  ±  9 

1x10-8 

{Kll2) 

960  -  7 

1x10-8 

KAPPA  MESON 

K(Kii3) 

955  ±9 

1x10-8 

(Ke3) 

~  960 

LAMBDA  PARTICLE 

A° 

2182  ±  2 

3.7xl0-'0 

HYPERONS 

POSITIVE  SIGMA  PARTICLE 

Z  + 

2327  ±  4 

-lO-'O 

NEGATIVE  SIGMA  PARTICLE 

z- 

2325 

—  lO-'O 

CASCADE  PARTICLE 

~ 

2582  ±  10 

-  lO-'C 

I 


FUNDAMENTAL   PARTICLES   are   li«ted,  together   with  their  mesons  are  called  L  particles;  the  heavy  mesons,  K  particles;  the 

characteristic  tracks  in  nuclear  emulsions.  The  photon  and  gravi-  hyperons,  Y  particles.  The  clii  and  kappa  mesons  have  dual  sym- 

ton  are  omitted  to  simplify  the  organization  of  the  chart.  The  light  hols,  the  second  of  which  segregates  them  according  to  their  mode 


38 


The  Tracks  of  Nuclear  Particles 


The  particles  emerging  from  the  explo- 
sion, among  which  are  several  pi  mesons, 
have  a  large  kinetic  energy;  the  total 
energy  released  is  about  that  predicted 
by  the  theory  that  the  antiproton  and  a 
proton  combine  and  annihilate  each 
other,  converting  mass  into  energy. 
The   Bevatron  produces   antiprotons 


when  a  beam  of  high-energy  protons  (at 
6.2  billion  electron  volts)  hits  a  copper 
target.  The  fast  protons  attacking  the 
nuclei  of  the  copper  atoms  generate 
large  numbers  of  heavy  mesons  and  an 
occasional  antiproton:  the  yield  is  about 
one  antiproton  per  62,000  mesons.  The 
theory  suggests  that  a  high-energy  pro- 


DECAY    SCHEME 


N. 


yn- 


H+ZV 


"■%/-  e 


e  + 


n* 


,....-^  n- 


^ 


^'i 


rr-< 


S-         c-     - 


e+.- 


f\N^ 

—  - li* 


of  decay.  K7r2,  for  example,  indicates  that  this  K  (not  kappa)  particle  decays  into  two  pi 
mesons.  The  decay  schemes  may  be  followed  by  beginning  with  the  particle  in  that  group. 
The  wavy  lines  (gamma  rays),  circles  and  arrows  denote  particles  that  do  not  make  tracks. 


ton  interacts  with  a  neutron  to  form  an 
antiproton-proton  pair. 

The  antiproton  has  the  same  mass  as 
a  proton.  One  would  therefore  expect 
that  it  should  have  about  the  same  prob- 
ability of  collision  with  atomic  nuclei  as 
it  travels  through  matter  But  experi- 
ments with  the  new  particle  show 
that  the  antiproton  actually  has  about 
twice  as  great  a  collision  probability, 
or  cross  section,  as  the  proton.  This 
surprising  property  has  presented 
nuclear  physicists  with  an  intriguing 
problem. 

Enlightening  as  the  work  with  atom- 
smashing  machines  has  been,  tlie 
investigators  of  particles  have  not  by 
any  means  lost  interest  in  the  wild  as- 
sortment of  nuclei  and  nuclear  debris 
that  rains  into  our  atmosphere  from  the 
bombardment  of  the  cosmic  radiation. 
Of  the  primary  cosmic  radiation  itself, 
little  reaches  ground  level,  for  the  at- 
mosphere absorbs  it  as  efiFectively  as 
would  a  three-foot-thick  layer  of  lead 
completely  surrounding  the  earth.  But 
physicists  are  capturing  the  footprints  of 
primary  particles  coming  in  from  space 
by  floating  their  instruments  and  photo- 
graphic plates  to  the  top  of  the  air  ocean 
in  balloons.  Great  impetus  was  given  to 
this  work  by  the  U.  S.  Navy's  develop- 
ment of  the  plastic  "Skyhook"  balloon. 
Unlike  nibber  balloons,  the  plastic  vehi- 
cles can  be  held  at  a  fixed,  preset  eleva- 
tion. Stacks  of  emulsions  have  been 
flown  to  100,000  feet-almost  at  the  bor- 
ders of  empty  space,  for  the  weight  of 
the  overlying  air  there  is  only  13  grams 
per  square  centimeter,  as  against  1,030 
grams  at  sea  level. 

As  the  primary  cosmic  rays  smash  ni- 
trogen and  oxygen  atoms  in  the  air,  they 
generate  a  fallout  of  secondary  and  ter- 
tiary particles.  The  footprints  of  these 
fragments  are  being  recorded  at  moun- 
taintop  stations  all  over  the  world.  Men 
who  risk  their  lives  to  climb  a  mountain 
simply  "because  it  is  there"  are  usually 
very  cooperative  with  the  cosmic  ray 
physicists.  A  light  package  of  photo- 
graphic plates  does  not  add  appreciably 
to  the  burden  of  the  climb,  and  it  may 
add  incentive  as  a  form  of  applied  moun- 
taineering. In  the  ascent  of  Mt.  Everest 
Sir  Edmund  Hillary  took  a  small  pack- 
age of  plates  (given  him  by  Professor 
Eugster  of  Zurich  University)  to  the 
25,850-foot  camp  site.  Unfortunacely,  in 
the  excitement  of  the  triumphant  de- 
scent from  the  peak  the  plates  were 
overlooked.  Sir  John  Hunt,  the  leader  of 
the  expedition,  apologized  in  his  book. 
The  Conquest  of  Everest:  "1  very  much 


39 


t* 


•  • 


SLOW  NEUTRON  gave  rise  to  this  track  in  an  emulsion  contain- 
ing lithium  borate.  The  neutron  encountered  a  lithium  atom  at  the 
lower  end  of  the  short,  heavy  line  at  the  top.  The  track  was  then 
made  by  two  fragments  of  the  nucleus  recoiling  from  each  other. 


ELECTRONS  made  the  faint,  wavy  tracks  in  this  emulsion,  which 
was  aged  for  50  days  before  it  was  developed.  The  heavy  track  at 
the  bottom  was  made  by  an  oxygen  nucleus  in  primary  cosmic  radi- 
ation. The  electron  tracks  along  this  image  are  called  delta  rays. 


regret  to  say  that  the  plates  have  re- 
mained on  the  South  Col,  where  they 
must  by  now  have  made  a  very  definite 
recording  of  . . .  cosmic  ray  phenomena." 

Among  the  first  to  get  a  recording  of 
-'*-  these  phenomena  was  Marietta  Blau 
of  the  University  of  Vienna.  Nineteen 
years  ago  she  exposed  a  series  of  photo- 
graphic plates  for  four  months  on  a 
mountaintop  at  Innsbruck.  When  she 
developed  them,  she  found  not  only  the 
familiar  alpha  stars  from  radioactive 
substances  but  also  a  number  of  bigger 
stars  with  much  longer,  less  dense 
prongs.  The  tracks  evidently  were  pro- 
duced chiefly  by  protons.  Dr.  Blau  sur- 


mised correctly  that  they  were  the  de- 
bris of  nuclei  disrupted  by  cosmic  rays; 
she  followed  up  this  finding  and  today 
is  studying  nuclear  disruptions  produced 
by  the  Cosmotron  at  the  Brookhaven 
National  Laboratory. 

The  smashing  of  nuclei  by  cosmic  rays 
increases  rapidly  with  altitude.  At  sea 
level  in  northern  latitudes  the  rate  of 
star  production  in  photographic  plates 
is  about  one  per  cubic  centimeter  of 
emulsion  per  day  of  exposure;  at  14,260 
feet  on  Mt.  Evans  in  Colorado  the  rate 
is  20  times  that;  and  in  balloons  near  the 
top  of  the  atmosphere,  2,500  times. 

The  tracks  of  the  primary  cosmic  par- 
ticles that  arrive  there  from  space  are 


often  extremely  robust.  These  thick 
tracks  are  made  by  heavy  nuclei,  much 
larger  than  the  nuclei  of  hydrogen 
atoms.  The  track  is  covered  with  a  fur 
of  spurs  projecting  from  its  sides— sec- 
ondary ionizations  which  are  known  as 
delta  rays.  Since  the  amount  of  ioniza- 
tion by  a  particle  along  its  path  is  pro- 
portional to  the  square  of  its  charge,  the 
amount  of  delta-ray  ionization  identifies 
the  particle.  The  primary  cosmic  parti- 
cles have  been  found  to  include  the 
nuclei  of  almost  all  the  elements  from 
hydrogen  to  nickel.  Iron  nuclei  often 
produce  tracks  heavy  enough  to  be  seen 
with  the  naked  eye. 

Sometimes  the  incoming  heavy  nu- 


IRON  NUCLEUS  in  primary  cosmic  radiation  entered  this  picture 
from    the    left.    Escaping    catastrophic    collision    with    nuclei    in 


ine  emulsion,  it  hnally  came  to  rest  at  the  right.  Its  energy  wag  dis- 
sipated by  a  series  of  encounters  in  which  it  removed  electrons 


40 


The  Tracks  of  Nuclear  Particles 


-"«;?" 


■%*. 


t 


I  'v  • 


NEGATIVE  PI  MESON  made  the  track  between  these  two  stars.  At 
the  top  is  a  nucleus  disrupted  by  a  primary  cosmic  ray.  At  the  bot- 
tom is  a  second  nucleus  disrupted  by  the  pi  meson.  Negative  mesons 
are  readily  absorbed  by  nuclei  because  of  their  opposite  charge. 


: 


PROTON  in  primary  cosmic  radiation  made  the  nearly  vertical 
track  at  the  top  of  this  emulsion.  The  tracks  produced  by  its  en- 
counter with  a  nucleus  in  the  center  of  the  emulsion  are  character- 
istic of  fragments  and/or  particles  with  a  single  electric  charge. 


cleus  is  partly  sheared  ofiF  by  a  glancing 
collision  in  the  air,  and  the  separated 
bundles  of  nucleons  diverge  from  the 
point  of  collision.  Sometimes  the  cosmic 
primary  hits  an  atom  head  on  and  dis- 
integrates it,  emitting  a  shower  of  heavy 
mesons:  as  many  as  200  charged  mesons 
have  been  seen  in  a  single  star.  Many  of 
the  pi  mesons  decay  during  flight  into 
mu  mesons;  the  latter,  nearly  immune  to 
capture  by  atoms,  zip  through  the  at- 
mosphere and  often  plunge  deep  into  the 
earth. 

A  small  proportion  of  the  heavy  nu- 
clei from  space  escape  catastrophic  col- 
lisions and  are  eventually  slowed  down 
by   ionization  processes  in  the  atmos- 


phere. When  these  particles  are  caught 
in  an  emulsion,  they  produce  very  spec- 
tacular tracks.  The  track  is  first  thick  and 
furry;  then  as  the  heavy  nucleus  slows 
down  and  begins  to  pick  up  electrons, 
the  reduction  of  its  positive  charge  di- 
minishes the  ionization  it  produces,  so 
that  its  track  tapers  down  to  a  needle 
point  at  the  end  of  its  flight. 

The  last  grain  at  the  rest  point  of  a 
heavy  primary  cosmic  particle  is  a 
thing  to  mai-vel  at.  Embedded  within 
the  grain  of  silver  in  the  emulsion  is  an 
atom  with  a  history  unlike  that  of  its 
neighbors.  It  is  an  atom  which  may  have 
been  blown  out  of  a  star  in  our  galaxy 


millions  of  years  ago.  It  was  accelerated 
through  interstellar  space  by  magneto- 
hydrodynamic  fields.  For  millions  of 
years  it  escaped  collision  with  cosmic 
dust.  Finally  it  plowed  into  the  earth's 
atmosphere,  and  in  a  single  moment  lost 
its  store  of  energy  accumulated  since 
birth.  Such  is  the  ever-increasing  en- 
tropy of  the  universe,  of  which  Swin- 
burne wrote: 

We  thank  with  brief  thanksgiving 

Whatever  gods  may  be 
That  no  man  lives  forever. 
That  dead  men  rise  up  never; 
That  even  the  weariest  river 

Winds  somewhere  safe  to  sea. 


from  atoms  in  the  emulsion.  These  electrons  made  the  wavy  tracks 
along  the  path  of  the  iron  nucleus.  The  track  is  about  a  16th  of  an 


inch  in  length,  too  long  to  be  shown  in  a  single  photomicrograph. 
It  has  accordingly  been  depicted  in  a  mosaic  of  photomicrographs 


41 


Our  knowledge  of  elementary  particles  depends  on  the 
spark  chamber  and  similar  devices  which  make  visible 
the  tracks  of  these  subatomic  particles. 


The  Spark  Chamber 


Gerard  K.  O'Neill 


Scientific  American  article, published  in  1962. 


The  present  understanding,  imper- 
fect but  growing,  of  the  funda- 
mental nature  of  matter  has  come 
largely  from  observation  of  the  elemen- 
tary particles.  The  protons,  neutrons, 
electrons,  mesons  and  other  particles  re- 
veal the  most  when  they  can  be  studied 
one  at  a  time  or  when  only  two  or  three 
of  them  interact.  When  larger  numbers 
are  present,  the  sheer  mathematical  com- 
plexity of  their  interaction  hides  the  fun- 
damental simpHcities.  For  this  reason 
the  efforts  of  many  experimental  phys- 
icists over  several  decades  have  gone 
into  the  development  of  sensitive  meth- 
ods for  detecting  single  particles. 

There  is  no  single  best  design  for 
a  particle  detector.  To  obtain  certain 
characteristics  it  is  usually  necessary  to 
sacrifice  others,  and  the  choice  depends 
on  the  nature  of  the  experimental 
"events"  one  wishes  to  observe.  Physi- 
cists working  with  the  large  particle-ac- 
celerating machines  have  increasingly 
been  concerned  with  extremely  rare 
events,  epitomized  by  the  recent  discov- 
ery at  the  Brookhaven  National  Labora- 
tory that  there  are  two  kinds  of  neutrino 
rather  than  one  [see  "Science  and  the 
Citizen,"  page  52].  To  obtain  the  evi- 
dence for  this  discovery  the  30-billion- 
electron-volt  proton  accelerator  at 
Brookhaven  was  operated  for  six  months. 
Over  this  period  the  number  of  recorded 
events  caused  by  neutrinos  averaged 
fewer  than  one  every  three  days.  The 
particle  detector  used  in  the  experiment 
is  of  an  entirely  new  type:  it  is  called 
a  spark  chamber.  Before  explaining  its 
operation  I  shall  describe  the  general 
nature  of  the  particle-detection  problem. 
The  problem  is  far  from  easy,  because 
an  elementary  particle  can  pass  freely 
through  many  atoms  of  any  substance 
without  leaving  a  trace.  Even  at  pres- 
ent there  is  no  practical  device  that 
can    detect    electrically    neutral    parti- 


cles without  destroying  or  deflecting 
them.  Charged  particles,  however,  exert 
a  strong  electrostatic  force  on  the  elec- 
trons of  the  atoms  through  which  they 
pass.  Usually  the  electrostatic  force  be- 
tween the  negative  electron  and  the 
positive  nucleus  is  enough  to  keep  the 
electrons  from  breaking  free,  but  occa- 
sionally—roughly once  in  every  1,000 
atoms  through  which  a  charged  particle 
passes— an  electron  is  jolted  loose.  In 
air,  for  example,  about  100  electrons  are 
freed  along  each  centimeter  of  the  path 
of  a  charged  particle,  and  for  each  free 
electron  a  corresponding  positive  ion  is 
formed.  If  the  small  amount  of  energy 
contained  in  this  "ionization  trail"  can  be 
made  to  produce  some  visible  effect,  the 
physicist  can  find  out  where  the  particle 
went.  He  can  also  measure  the  momen- 
tum of  a  particle  by  observing  the  radius 
of  curvature  of  its  track  in  a  magnetic 
field,  and  he  can  obtain  information 
about  the  way  it  interacts  with  other  par- 
ticles by  observing  sudden  changes  in 
direction  of  its  track. 

In  one  of  the  first  of  all  elementary- 
particle  experiments  Hans  Geiger  and 
Ernest  Marsden,  working  in  the  Caven- 
dish Laboratory  at  the  University  of 
Cambridge,  detected  the  small  energy  of 
an  ionization  trail  without  amplification 
by  using  the  extreme  sensitivity  of  the 
dark-adapted  human  eye.  They  observed 
the  small  flashes  of  light  made  when 
alpha  particles  went  through  certain 
crystalline  materials  called  scintillators. 
From  Geiger  and  Marsden's  observa- 
tiotis  of  the  angles  at  which  alpha  par- 
ticles scattered  from  a  target  into  the 
scintillator,  Ernest  Rutherford  conclud- 
ed by  1913  that  the  positive  charge  of 
the  atom  was  concentrated  in  a  nucleus. 

A  fast,  singly  charged  particle— a  cos- 
mic ray  meson,  for  example— produces 
only  about  a  thousandth  as  many  free 
electrons  per  millimeter  of  track  as  a 


slow,  doubly  charged  alpha  particle 
does.  The  detection  of  fast  particles 
therefore  requires  some  kind  of  ampli- 
fication of  the  energy  of  the  ionization 
trail.  Since  Rutherford's  time  the  de- 
vices used  to  detect  elementary  particles 
have  divided  into  two  broad  classes, 
both  of  which  amplify.  One  class  consists 
of  "counters."  Every  counter  includes  a 
sensitive  volume  of  gas,  liquid  or  solid 
with  well-defined  dimensions  in  space. 
When  a  charged  particle  passes  through 
the  sensitive  volume,  the  counter  pro- 
duces a  brief  electric  pulse,  or  signal. 
The  pulses  can  be  tallied  electronically; 
hence  the  name  "coimter." 

The  other  class  does  not  have  a  well- 
recognized  generic  name,  but  it  can  be 
called  the  class  of  "track  detectors."  A 
track  detector  shows  where  a  charged 
particle  went  by  indicating  many  points 
in  space  along  the  particle's  ionization 
trail.  Usually  the  information  provided 
by  a  track  detector  is  recorded  by 
photography.  In  fact,  for  certain  pur- 
poses stacks  of  photographic  film  or  a 
single  block  of  photographic  emulsion 
can  be  used  directly  as  a  track  detector. 
A  charged  particle  sensitizes  emulsion 
grains  along  its  track  and  amplification 
is  achieved  by  means  of  a  chemical  de- 
veloper. In  the  next  few  years  some  ad- 
vanced track  detectors  may  be  built  that 
will  put  out  information  in  the  form  of 
electrical  signals. 

If  one  compares  the  two  classes,  it  is 
apparent  th^t  the  counter  gives  only  a 
limited  amount  of  information,  but  it 
gives  it  immediately  in  a  simple  form 
suitable  for  direct  use  in  electronic  cir- 
cuits. In  modern  counters  the  informa- 
tion is  often  available  in  less  than  10 
nanoseconds  ( 10  billionths  of  a  second) . 
The  track  detector  gives  much  more  in- 
formation, but  the  information  goes  into 
photographic  emulsion,  where  it  is  un- 
available until  the  emulsion  is  developed 


43 


COUNTER 


3 


-> 


AMPLIFIER 


mzmnmmmnmmiMii^m 


GLASS 

WINDOW 


VAPOR 
AND  GAS 


PISTON 


V77)?m777m7777i/////////////////////7IA 


V 


MECHANICAL 

DRIVE 

SYSTEM 


PARTICLE 
PATH 


CLOUD  CHAMBER,  invented  in  1911  by  C.  T.  R.  Wilson,  was  the  first  of  the  particle-track 
detectors.  A  counter,  which  simply  senses  the  arrival  of  a  particle,  triggers  the  movement 
of  a  piston  that  expands  the  gas  and  vapor  inside  the  chamber.  This  makes  the  vapor  super- 
saturated, and  fog  droplets  rapidly  grow  along  the  ionization  trail  left  by  passage  of  the  par- 
ticle. The  droplets  form  clear  tracks,  which  are  photographed  stereoscopically  for  analysis. 


CIRCULATING 
PROTON  BEAM 
IN  PARTICLE 
ACCELERATOR 


GLASS 
WINDOW 


BUBBLE  CHAMBER,  a  track  detector  invented  by  Donald  A.  Glaser,  contains  a  liquid  near 
its  boiling  point.  When  the  chamber  pressure  is  lowered,  the  liquid  becomes  superheated 
and  babbles  of  vapor  grow  along  the  ionization  trail  left  by  a  charged  particle.  A  timing 
mechanism  moves  a  target  into  the  beam  of  circulating  protons  in  an  accelerator,  thereby 
din>rting  particles  into  the  chamber  at  the  instant  it  is  most  sensitive  to  bubble  growth. 


and  analyzed.  A  counter  with  a  sensitive 
volume  of  a  cubic  foot  can  only  signal 
that  a  charged  particle  has  passed  some- 
where within  that  cubic  foot.  Some  track 
detectors  with  the  same  sensitive  volume 
can  indicate  each  point  of  the  particle's 
path  within  a  thousandth  of  a  centi- 
meter. The  space  resolution  of  the  track 
detector  balances  against  the  reporting 
speed  of  the  counter. 

In  modern  elementary-particle  experi- 
ments the  experimenter  often  wants  to 
trace  all  or  part  of  the  life  histories 
of  particles  entering  his  detectors.  He 
wants  to  identify  the  mass,  charge  and 
frequently  the  energy  of  each  particle 
that  enters.  In  addition  he  wants  to  ob- 
serve if  and  in  what  way  the  entering 
particles  react  with  the  atoms  in  his  de- 
tector. If  new  particles  are  produced  by 
reactions,  he  wants  to  measure  the  prop- 
erties of  these  product  particles  and 
to  see  if  they  decay  spontaneously  into 
combinations  of  other  particles.  In  most 
cases,  the  rarer  the  reaction,  the  greater 
its  significance.  Typically  only  one  in 
many  thousands  of  particles  entering 
a  detector  will  produce  an  interesting 
event.  If  the  experimenter's  apparatus 
includes  track  detectors,  it  is  much  to  his 
advantage  to  use  counters  to  select  those 
events  that  are  worth  recording  in  the 
track  detector.  Otherwise  he  may  have 
to  search  through  hundreds  of  thou- 
sands of  pictures  to  find  the  rare  events 
of  interest. 

^  I  ''he  first  successful  track  detector  was 
^  the  cloud  chamber,  invented  by  C. 
T.  R.  Wilson  in  1911.  Wilson  recog- 
nized that  a  supersaturated  vapor  is 
unstable  and  that  the  vapor  will  con- 
dense into  droplets  around  any  available 
free  ions.  In  cloud  chambers  (which 
are  still  used)  a  saturated  vapor  is 
maintained  in  a  closed  volume  under 
well-controlled  conditions  of  tempera- 
ture and  pressure.  When  a  charged  par- 
ticle passes  through  the  chamber,  the 
ionization  trail  it  leaves  persists  for  a 
fraction  of  a  second.  Either  before  or 
directly  after  passing  through  the  cloud 
chamber  the  particle  traverses  counters, 
which  produce  an  electric  pulse.  The 
pulse,  signaling  the  passage  of  a  particle, 
is  made  to  initiate  the  outward  motion 
of  a  piston;  this  allows  the  gas  inside  the 
chamber  to  expand  and  renders  the 
vapor  in  the  gas  supersaturated  \see  top 
illustration  at  left].  The  vapor  then  be- 
gins to  form  droplets  of  fog,  which 
condense  around  the  ions  of  the  charged- 
particle  track.  Droplets  also  tend  to  form 
around  dust  particles  or  droplets  left 
over  from  a  previous  expansion.  But 
under  the  right  conditions    (achieving 


44 


The  Spark  Chamber 


them  is  rather  tricky)  there  forms  in  the 
chamber,  in  a  fraction  of  a  second,  a 
clear  trail  of  vapor  droplets,  which  shows 
with  good  fidelity  the  path  of  the  particle 
that  triggered  the  counters.  The  advan- 
tage of  the  cloud  chamber  is  that  it  can 
be  triggered.  A  chamber  may  remain  idle 
for  hours  waiting  for  a  rare  cosmic  ray 
event,  but  when  the  event  occurs  and  is 
recognized  by  the  counters,  the  chamber 
operates  on  demand  to  record  it. 

Unfortunately  cloud  chambers  have 
two  rather  serious  drawbacks.  First,  the 
device  is  slow  to  set  in  operation,  and  the 
ionization  trails  persist  for  a  large  frac- 
tion of  a  second.  As  a  result  the  number 
of  incoming  particles  must  be  limited 
to  prevent  chamber  pictures  from  being 
cluttered  with  more  tracks  than  one  can 
"read."  The  second  drawback  is  the  dif- 
ficulty of  putting  into  the  chamber  ma- 
terials with  which  one  might  like  to  see 
particles  interact.  If  material  is  intro- 
duced in  the  form  of  plates,  the  plates 
must  be  relatively  few  and  widely 
spaced;  otherwise  the  chamber  will  not 
work.  If  much  material  is  needed,  it  must 
therefore  be  in  the  form  of  thick  plates, 
with  the  result  that  interactions  tend  to 
occur  deep  in  the  plates,  where  the  tracks 
cannot  be  seen.  It  is  rather  like  Greek 


tragedy,  in  which  all  the  mayhem  occurs 
offstage  and  the  audience  is  treated  only 
to  a  secondhand  account  of  it. 

In  the  early  1950's  Donald  A.  Glaser, 
then  at  the  University  of  Michigan, 
developed  a  new  type  of  track  detector, 
the  bubble  chamber,  for  which  he  re- 
ceived a  Nobel  prize  in  1960.  This  de- 
tector is  also  based  on  an  amplification 
principle— the  growth  of  bubbles  in  a 
superheated  liquid.  Some  of  the  energy 
from  an  ionization  trail  goes  into  a  few 
fast  electrons,  which  can  give  up  1,000 
or  2,000  volts  of  energy  in  a  small  vol- 
ume to  produce  rapid  local  heating.  If 
the  trail  is  in  a  liquid  that  has  suddenly 
been  superheated  by  expansion,  the 
bubbles  will  tend  to  grow  fastest  along 
the  "heat  track"  and  only  slowly  in  other 
parts  of  the  liquid.  Glaser's  invention 
was  soon  in  use  in  many  laboratories 
throughout  the  world,  and  it  is  safe  to 
say  that  by  1959  more  than  half  of  all 
experimental  research  in  elementary  par- 
ticle physics  employed  the  bubble 
chamber. 

An  important  virtue  of  Glaser's  device 
is  that  one  can  fill  the  chamber  with  a 
wide  variety  of  liquids,  choosing  the  one 
that  provides  interactions  of  particular 
interest.  For  many  purposes  liquid  hy- 


drogen is  ideal  because  it  presents  as  a 
target  for  incoming  particles  only  elec- 
trons and  protons.  In  all  other  substances 
neutrons  are  also  present.  Other  useful 
liquids  are  propane— in  which  the  target 
atoms  are  carbon  and  hydrogen— and 
xenon,  whose  massive  nucleus  (54  pro- 
tons and  77  neutrons)  provides  high 
stopping  power.  In  addition  the  bubble 
chamber  produces  particle  tracks  of 
higher  definition  than  those  made  by 
any  other  track  detector,  except  for 
tracks  made  directly  in  photographic 
emulsion. 

The  bubble  chamber  shares  with  the 
emulsion  method  one  serious  disad- 
vantage: it  cannot  be  triggered.  Since 
there  is  no  way  to  select  rare  events  one 
has  no  choice  but  to  photograph  the 
chamber  at  every  expansion  cycle,  de- 
velop the  films  and  examine  hundreds 
or  thousands  of  exposures  looking  for 
events  of  interest.  Triggering  is  impos- 
sible because  the  heat  track  produced  by 
a  charged  particle  cools  down  in  much 
less  than  a  millionth  of  a  second.  This 
is  far  too  short  a  time  for  the  mechani- 
cal expansion  system  to  set  the  chamber 
in  operation.  As  a  result  bubble  cham- 
bers are  used  almost  exclusively  with 
large  accelerators,  where  a  timing  se- 


M  PROTON 

n'^  POSITIVE  PI  MESON 

JT~  NEGATIVE  PI  MESON 

A  NEUTRAL  LAMBDA  PARTICLE 

K  NEUTRAL  K  MESON 

e"^  POSITRON 

e~  ELECTRON 

y  GAMMA  RAY 


h 

\    -       p    > 

1 

K°p 

-^ 

\^ 

/ 

n 
\ 

\ 

BUBBLE  CHAMBER  TRACKS  (right)  were  photographed  in  the 
72-inch  liquid-hydrogen  bubble  chamber  at  the  Lawrence  Radia- 


tion Laboratory  of  the  University  of  California.  The  map  and  key 
at  left  identify  the  particles  taking  part  in  the  event  recorded. 


45 


METAL  TUBE 


OUTPUT 
SIGNAL 


{ 


JL 


ARGON  and/alcohol  VAPOR 

/ 

/ 


/   PARTICLE 
/    PATH 


D.C. 
POWER 
SUPPLY 


X 


GEIGER-MULLER  COUNTER,  invented  in  1928,  was  the  first  device  to  use  the  ampHfica- 
lion  process  available  in  an  electric  spark  to  detect  the  passage  of  a  charged  particle.  A  cen- 
tral wire  inside  a  tube  is  placed  at  high  voltage.  Electrons  set  free  from  gas  atoms  by  the  pas- 
sage of  a  particle  are  accelerated  by  the  strong  electric  field  and  free  other  electrons  in  a 
chain  reaction.  The  result  is  a  large  output  pulse  that  needs  no  amplification  to  be  detectable. 


SPARK  COUNTER  was  a  nontriggered  forerunner  of  the  spark  chamber.  A  high  constant 
voltage  is  maintained  on  a  metal  plate  placed  between  two  grounded  plates.  Passage  of  a 
charged  particle  provides  free  electrons  that  initiate  sparks  in  the  gas  between  the  plates. 


COUNTER 


AMPLIFIER 


HIGH-VOLTAGE 

PULSE 

GENERATOR 


/    PARTICLE 
/     PATH 

HODOSCOPE  CHAMBER,  another  forerunner  of  the  spark  chamber,  utilizes  the  trigger- 
ing scheme  usually  employed  with  cloud  chambers.  The  chamber  consists  of  neon-filled 
glass  tubes  stacked  between  two  metal  plates.  When  a  charged  particle  trips  the  counter, 
a  high-voltage  pulse  is  s(;nt  to  the  plates,  placing  the  tubes  in  a  strong  electric  field.  Tubes 
through  which  the  particle  passed   contain   ions  and   free  electrons  and   therefore   glow. 


quence  first  expands  the  chamber,  then 
sends  in  a  burst  of  particles  to  be  an- 
alyzed [see  bottom  illustration  on  page 
38].  The  chamber  must  then  be  given 
about  a  second  in  which  to  recover. 

Unlike  the  cloud  chamber  and  the 
bubble  chamber,  the  spark  cham- 
ber was  the  work  of  many  hands.  Its  de- 
velopment was  based  on  one  of  the 
most  spectacular  methods  known  for 
making,  ionization  trails  visible— the  elec- 
tiic  spark.  The  generation  of  an  electric 
spark  is  an  extremelv  complicated  proc- 
ess, but  it  is  clear  that  under  some  con- 
ditions a  spark  can  develop  from  a  type 
of  chain  reaction.  The  reaction  starts 
when  an  electron  from  an  ionized  atom, 
accelerated  by  a  strong  electric  field, 
bumps  into  and  ionizes  other  atoms.  The 
electrons  from  these  atoms  cause  further 
ionizations,  leading  in  a  very  brief  time 
to  a  brilliant  electric  spark.  In  1928  the 
amplification  process  available  in  the 
electric  spark  was  used  in  the  first  of  all 
electrical  detectors  for  single  charged 
particles,  the  Geiger-Miiller  counter.  In 
this  simple  device,  named  for  Hans  Gei- 
ger  and  Walther  Miiller,  a  central  wire 
inside  a  tube  is  charged  to  high  voltage. 
When  a  particle  goes  through  the  count- 
er, the  electrons  of  its  ionization  track 
are  swept  toward  the  wire.  Accelerating 
as  they  approach  the  wire's  strong  field, 
they  ionize  more  atoms.  The  ionized 
atoms  emit  photons  (light  cjuanta), 
which  release  additional  electrons  from 
the  gas,  spreading  the  discharge.  Within 
millionths  of  a  second  the  gas  all  along 
the  center  wire  serves  as  the  path  for  an 
electric  spark.  Geiger  counters  make 
tremendous  pulses,  which  was  a  great 
virtue  when  sensitive  electronic  ampli- 
fiers were  still  diflRcult  to  build. 

In  the  1930's  the  standard  equipment 
of  the  elementary-particle  physicist  con- 
sisted of  a  cloud  chamber  triggered  by 
Geiger  counters.  In  the  late  1940's,  when 
Geiger  counters  had  been  generally 
superseded  by  the  development  of  scin- 
tillation counters  (faster  and  capable  of 
giving  more  information),  a  few  physi- 
cists began  trying  to  use  the  mecha- 
nism of  the  electric  spark  in  a  detector 
that  would  make  visible  the  track— not 
just  the  presence— of  a  charged  particle. 
J.  W.  Keuffel,  working  at  the  California 
Institute  of  Technology  and  later  at 
Princeton  University,  built  several  spark 
counters,  consisting  of  well-polished 
condenser  plates  kept  at  high  voltage. 
If  the  plates  were  carefully  aligned, 
clean  and  dust-free,  and  maintained  just 
below  the  potential  needed  for  a  spark  to 
jump  between  them,  thev  would  some- 
times spark  preferentially  along  the  trail 


46 


The  Spark  Chamber 


of  an  incoming  cosmic  ray  particle.  Keuf- 
fel  suggested  the  use  of  arrays  of  his 
parallel-plate  spark  counters  to  obtain 
tracks  of  the  passage  of  a  charged  parti- 
cle, but  these  counters  were  so  difficult 
to  build  and  to  operate  that  it  was  not 
easy  to  follow  up  the  suggestion. 

In  1955  M.  Conversi  and  A.  Gozzini 
described  in  the  Italian  physics  journal 
Nuovo  Cimento  an  intermediate  type 
of  track  chamber  somewhat  similar  to 
the  Keuffel  spark  counter.  Their  device, 
called  a  hodoscope  chamber,  consisted 
of  many  neon-filled  glass  tubes  stacked 
between  two  parallel  metal  plates  [see 
bottom  illustration  on  opposite  page]. 
Within  a  few  millionths  of  a  second  after 
the  passage  of  a  charged  particle  through 
the  stack  of  tubes,  a  set  of  counters  out- 
side the  stack  triggered  an  electronic 
circuit  that  placed  a  strong  electric 
field  on  the  tubes.  Those  through  which 
the  particle  had  passed  then  glowed, 
much  as  a  neon  sign  glows.  Other  tubes 
remained  dark  if  the  applied  pulse  was 
on  for  only  a  short  time.  The  hodoscope 
chamber  was  fairly  easy  to  build,  and  its 
inventors  had  introduced  a  technique 
that  was  essential  for  the  development  of 
spark  chambers:  the  use  of  counters  to 
pulse  the  electric  field.  In  their  chamber 
the  high  voltage  was  on  only  when  they 
were  sure  a  particle  track  was  there  to  be 
photographed.  If  the  high  voltage  had 
been  left  on  continuously,  as  it  was  in  the 
earlier  spark  counters,  some  neon  tubes 
would  eventually  have  fired  even  in  the 
absence  of  an  entering  track.  The  chief 
defect  of  the  hodoscope  was  that  it  re- 
vealed only  two  dimensions  of  a  parti- 
cle's three-dimensional  path. 

In  1957  two  British  physicists,  T.  E. 
Cranshaw  and  J.  F.  de  Beer,  reported  in 
Nuovo  Cimento  the  next  step  toward  a 
practical  spark  chamber.  They  combined 
the  parallel-plate  geometry  of  the  spark 
counter  with  the  pulse-triggering  tech- 
nique of  the  hodoscope  chamber  to  make 
an  efficient  spark  chamber  with  six  one- 
millimeter  gaps.  They  also  introduced 
the  use  of  a  continuous  electric  clearing 
field  to  remove  from  the  chamber  ioniza- 
tion trails  older  than  a  few  microseconds. 
This  electric  field,  well  below  the  thresh- 
old needed  to  make  a  spark,  caused  a 
slow  continuous  drift  to  the  plates  of 
all  free  electrons  and  ions  released  in 
the  chamber  gas.  In  this  way  it  "erased" 
ionization  trails  in  a  few  microseconds.  A 
similar  clearing  field  had  long  been  used 
in  cloud  chambers  to  sweep  out  the  slow- 
moving  positive  ions. 

It  happened  that  Cranshaw  and  de 
Beer  chose  to  use  air  rather  than  neon  in 
their  chamber,  and  this  small  difference 
made  it  impossible  for  their  chamber  to 


detect  two  or  more  simultaneous  tracks. 
Still,  their  work  was  so  successful  that 
several  other  groups— in  Germany,  Japan, 
the  U.S.S.R.  and  the  U.S.-continued  to 
work  along  similar  lines. 

T^he  final  step— substitution  of  neon  for 
air-was  taken  by  S.  Fukui  and  S.  Mi- 
yamoto of  Osaka  University  and  report- 
ed in  1959.  The  two  Japanese  physicists 
were  interested  in  developing  a  track  de- 
tector that  could  be  used  for  cosmic  rays. 
Bubble  chambers  are  not  useful  for  such 
work,  since  they  cannot  be  triggered. 
Fukui  and  Miyamoto  found  that  in  a 
chamber  containing  neon  rather  than  air 
several  simultaneous  particle  tracks 
could  be  seen. 

One  big  difference  between  the  be- 
havior of  air  and  of  neon  in  spark  cham- 
bers is  that  oxygen  molecules  ( Oj )  in  air 
can  combine  with  the  free  electrons  of 
the  ionization  trail,  whereas  neon  atoms 
cannot.  The  inertness  of  neon— and  of 
other  "noble"  gases— is  explained  by  the 
fact  that  it  has  a  full  complement  of 
eight  electrons  in  its  outer  electron  shell. 
In  contrast  an  oxygen  molecule  can  ac- 
quire one  electron  and  thereby  become 
a  negative  ion  (02).  The  electrons  are 
well  anchored  to  the  oxygen  molecules, 
some  60,000  times  more  massive  than 
themselves,  and  cannot  be  freed  except 
by  application  of  a  strong  electric  field. 


Consequently  an  air-filled  spark  chamber 
requires  an  operating  pulse  of  7,000  to 
10,000  volts  for  each  millimeter  of  space 
between  its  plates.  This  is  about  three 
times  the  voltage  needed  for  a  neon 
spark  chamber. 

The  formation  of  oxygen  ions  also  ex- 
plains other  characteristics  of  an  air 
spark  chamber.  If  the  electron  in  an 
ionization  trail  can  migrate  freely  to  the 
plates  of  the  chamber,  its  travel  time  is 
brief.  But  if  it  is  attached  to  an  oxygen 
molecule  along  the  way,  the  velocity  of 
the  resulting  ion  is  much  slower  than 
that  of  the  electron.  In  fact,  if  the  mass 
of  a  particle  is  suddenly  increased  by 
60,000  times,  its  velocity  must  decrease 
by  the  square  root  of  60,000,  or  by  a 
factor  of  about  250.  Because  most  of  the 
electrons  liberated  in  an  air  spark  cham- 
ber are  slowed  down  in  this  fashion,  they 
require  many  microseconds  to  migrate 
to  the  plates  of  the  chamber.  Such  a 
chamber  therefore  remains  sensitive  for 
a  long  time,  and  in  it  old  tracks  cannot 
be  quickly  erased. 

It  is  not  so  clear  why  air  chambers 
show  only  one  spark  per  gap  even  though 
several  ionization  trails  may  be  present. 
It  may  be  that  at  the  high  electric  fields 
needed  to  operate  such  chambers  the 
spark  produced  by  the  first  electron 
freed  from  an  oxygen  ion  occurs  so  rap- 
idly  that   the   plates   are   quickly   dis- 


IV.   UNSEEN 

'\  NEUTRAL  PARTICLE 

COUNTER 


COUNTERS 


NEON 


PARTICLE 
PATH 


GROUNDED 
PLATES 


TO  PULSED 
PLATES 


HIGH-VOLTAGE 

PULSE 

GENERATOR 


SPARK  CHAMBER,  which  became  practical  with  the  work  of  S.  Fukui  and  S.  Miyamoto 
in  1959,  consists  of  an  array  of  thin  metal  plates  surrounded  by  neon.  It  is  also  provided 
with  counters  and  a  "logic"  circuit  for  determining  when  a  particle  meeting  certain  criteria 
has  appeared.  When  it  appears,  a  high-voltage  pulse  is  sent  to  ahernate  plates  and  sparks 
occur  along  the  ionization  trails  left  by  each  charged  particle.  In  the  example  shown,  a 
charged  particle  interacts  in  counter  A,  yielding  one  neutral  and  one  charged  secondary. 
The  secondary  decays  in  the  chamber,  producing  two  charged  particles  and  a  neutral  one. 


47 


charged  below  the  threshold  field,  pre- 
venting any  other  attached  electrons 
from  getting  free  to  start  other  sparks. 
This  is  consistent  with  an  observation 
by  Cranshaw  and  de  Beer  that  only  one 
electron  is  needed  to  start  the  spark. 

TT'ollowing  the  announcement  of  a  prac- 
•*•  tical  spark  chamber  by  Fukui  and 
Miyamoto  in  1959,  the  idea  was  imme- 
diately taken  up  by  physicists  in  the  U.S. 
and  elsewhere.  Within  a  matter  of 
months  Bruce  Cork  of  the  University  of 
California    had    built    a    six-gap    spark 


chamber  and  had  operated  it  in  a  beam 
of  particles  from  the  six-billion-electron- 
volt  accelerator  of  the  Lawrence  Radia- 
tion Laboratory.  Almost  simultaneously 
James  L.  Cronin  of  Princeton  University 
built  and  operated  a  large  18-gap  spark 
chamber,  which  yielded  high-quality 
pictures  of  the  tracks  made  by  cosmic 
rays  and  by  accelerator-produced  parti- 
cles. Both  of  these  chambers  used  noble 
gases  (neon  or  argon)  and  employed 
clearing  fields  to  erase  the  ionization 
trails.  Cork  and  Cronin  were  also  the 
first  to  conduct  actual  experiments  using 


a  spark  chamber  as  a  particle  detector. 
In  their  work,  as  in  most  subsequent 
experiments  using  spark  chambers,  the 
occurrence  of  an  interesting  event 'was 
recognized  by  a  system  of  conventional 
counters,  which  then  triggered  the  oper- 
ation of  the  chamber.  Typically  particles 
arrived  at  the  spark  chamber  at  intervals 
of  a  few  microseconds  and  their  tracks 
were  swept  to  the  plates  by  the  continu- 
ous clearing  field  after  only  one  micro- 
second. .Consequently  the  pulsing  of  the 
spark  chamber  had  to  be  carried  out  in 
much  less  than  one  microsecond  so  that 


7 


SPARK  CHAMBER  PICTURES  show  the  appearance  of  particle 
tracks  when  the  particles  are  curved  by  a  magnetic  field  (top  right) 
and  when  they  are  not   (top  left).  The  maps  below  each  picture 


identify  the  charged  particles,  which  leave  tracks,  and  the  neutral 
ones,  whose  presence  is  inferred.  The  reaction  at  the  left  was  seen 
in  a  spark  chamber  operated  at  Brookhaven  by  James  L.  Cronin 


48 


The  Spark  Chamber 


the  interesting  track  would  still  be  there 
to  be  detected  bv  spark  amplification. 

Within  the  past  three  years  a  wide 
variety  of  spark  chambers  have  been 
built,  each  designed  to  exploit  certain  de- 
sirable features.  Some  have  been  made 
with  thick  carbon  plates  to  allow  in- 
teractions of  the  incoming  particles  with 
carbon.  Others  have  been  built  in  the 
form  of  a  cylinder,  to  study  the  scattering 
of  particles  by  a  target  located  on  the 
axis  of  the  cylinder. 

Along  with  several  other  physicists,  I 
have  been  particularly  interested  in  the 


of  Princeton.  The  picture  at  right  was  made 
in  author's  two-cubic-foot  spark  cnamber 
at  Brookhaven,  shown  at  bottom  of  page  36. 


design  and  use  of  thin-plate  spark  cham- 
bers that  can  be  operated  in  a  magnetic 
field.  In  a  uniform  magnetic  field  the 
path  of  a  charged  particle  of  constant  en- 
ergy is  a  circle  whose  radius  is  propor- 
tional to  the  momentum  of  the  particle. 
The  idea  of  using  a  magnetic  field  to  ob- 
tain momentum  information  goes  back 
to  the  early  days  of  the  cloud  chamber, 
and  bubble  chambers  are  nearly  always 
operated  in  such  a  field.  The  measure- 
ment of  the  momentum  of  each  charged 
particle  in  a  reaction  is  alwavs  useful, 
and  frequently  essential,  for  identifving 
the  particles  and  learning  the  details  of 
their  interactions. 

When  a  magnetic  field  is  used  in  a 
spark  chamber,  the  sparks  trace  the  ion- 
ization trails  more  closely  if  the  spacing 
between  the  chamber  plates  is  small.  As 
the  spacing  is  reduced,  however,  it  be- 
comes increasingly  important  for  the 
plates  to  be  flat  and  uniformly  spaced, 
and  the  triggering  pulse  has  to  rise  from 
zero  to  the  peak  voltage  at  higher  speed. 
Fukui  and  Miyamoto  had  used  spacings 
of  10  millimeters.  Cork's  chamber  had  a 
six-millimeter  spacing.  Within  a  few 
months  we  found  in  our  laboratory  at 
Princeton  University  that  the  spacing  be- 
tween spark-chamber  plates  operated  in 
neon  could  be  as  small  as  two  millimeters. 

Unless  very  close  plate-spacing  is 
wanted,  the  construction  of  a  spark 
chamber  is  not  too  difficult  and  might 
make  a  feasible  project  for  an  amateur 
scientist.  A  chamber  with  an  adjustable 
plate  spacing  of  two  to  10  millimeters, 
the  first  model  built  by  our  group,  was 
largely  the  work  of  college  sophomores 
majoring  in  physics.  Our  second  instru- 
ment was  small  but  operated  in  a  mag- 
netic field.  It  contained  50  gaps  of  three 
millimeters  each,  separated  by  alumi- 
num foil  a  thousandth  of  an  inch  thick.  A 
third  chamber,  with  128  gaps  of  three- 
millimeter  spacing  and  a  volume  of  two 
cubic  feet,  can  measure  the  momentum 
of  particles  with  good  accuracy.  When 
the  tracks  cross  100  or  more  gaps,  the 
accuracy  of  momentum  measurement 
approaches  that  obtainable  in  a  good 
bubble  chamber. 

At  present  the  advantages  the  bubble 
chamber  retains  over  the  spark 
chamber  are  two.  First,  pure  licjuid  hy- 
drogen can  be  used  as  the  only  mate- 
rial in  the  bubble  chamber,  thereby  lim- 
iting nuclear  reactions  to  those  between 
elementary  particles  and  hydrogen  nu- 
clei (protons).  In  1960  we  studied  the 
possibility  of  imitating  a  hydrogen  bub- 
ble chamber  by  using  liquid-hydrogen- 
filled  hollow  plates  in  an  atmosphere 
of  gaseous  helium.  We  established  that 


such  a  chamber  would  work  but  so  far 
no  one  has  needed  its  properties  badly 
enough  to  build  one.  The  second  advan- 
tage of  the  bubble  chamber  is  that  it 
yields  very  fine  ionization  trails,  and  it 
produces  them  no  matter  which  way  the 
particle  is  moving.  The  bubbles  trace  a 
particle's  path  with  an  uncertainty  of 
less  than  a  thousandth  of  an  inch.  Even 
in  narrow-gap  spark  chambers  the  sparks 
scatter  in  a  region  15  or  20  thousandths 
of  an  inch  wide.  Moreover,  in  a  spark 
chamber  the  path  uncertainty  increases 
as  the  particle  approaches  a  course 
parallel  to  the  plates. 

In  spite  of  these  drawbacks  the  spark 
chamber  has  two  big  advantages  over 
the  bubble  chamber.  First,  the  decision 
to  photograph  a  given  event  can  be 
made  after  the  event  has  occurred.  Sec- 
ond, because  old  ionization  trails  are 
swept  to  the  walls  after  only  one  or  two 
microseconds  the  spark  chamber  picture 
shows  only  the  tracks  produced  during 
the  last  microsecond  before  the  chamber 
was  pulsed.  Because  of  these  two  fea- 
tures one  can  select  and  photograph  an 
interesting  event  caused  by  a  single  en- 
tering particle  out  of  many  thousands, 
all  arriving  over  a  few  thousandths  of 
a  second.  Each  ionization  trail  of  the  un- 
interesting majority  of  tracks  is  swept 
away  and  does  not  remain  to  confuse 
the  picture. 

The  decision  as  to  which  events  to 
photograph  is  made  by  "logic"  circuits 
that  analyze  the  output  of  counters, 
which  may  be  located  outside  or  inside 
the  spark  chamber  itself.  Frequently  the 
logic  requirements  are  severe  and  the 
pulses  from  many  counters  must  be  di- 
gested and  analyzed  before  a  decision 
is  made  whether  to  pulse  the  chamber  or 
not.  Ordinarily  a  time  of  about  100  nano- 
seconds (100  billion ths  of  a  second)  is 
available  for  the  decision.  This  is  not 
uncomfortably  short  with  present-day 
circuitry.  For  the  past  10  years  it  has 
been  practical  to  use  circuits  that  operate 
in  20  nanoseconds  or  less. 

Those  of  us  who  have  jumped  on  the 
spark  chamber  bandwagon  are  naturally 
enthusiastic  about  future  prospects  for 
the  instrument.  We  have  found  that 
physicists  who  formerly  used  bubble 
chambers  are  delighted  to  have  a  de- 
vice that  eliminates  great  masses  of  un- 
interesting pictures.  And  former  counter 
physicists  are  happy  to  see  the  tracks 
they  knew  were  going  through  their 
counters.  We  all  know  that  neither  bub- 
ble chambers  nor  counters  are  going  to 
be  put  out  of  business  by  the  new  track 
detectors,  but  to  a  remarkable  degree 
spark  chambers  allow  us  some  of  the  best 
of  both  worlds. 


49 


This  speech  is  a  lucid  historical  introduction  to  the 
cyclotron,  with  frank  references  to  missed  opportunities. 


f        The  Evolution  of  the  Cyclotron 

Ernest  0.  Lawrence 

Nobel  Prize  lecture  given  in  December  1951. 

The  development  of  the  cyclotron  was  begun  more  than  twenty  years  ago 
and  perhaps  it  is  appropriate  on  this  occasion  to  give  something  of  an  historical 
account.  The  story  goes  back  to  1928  when  I  had  the  good  fortune  of  becoming 
a  member  of  the  faculty  of  the  University  of  California.  At  that  time  it  seemed 
opportune  to  review  my  plans  for  research,  to  see  whether  I  might  not  pro- 
fitably go  into  nuclear  research,  for  the  pioneer  work  of  Rutherford  and  his 
school  had  clearly  indicated  that  the  next  great  frontier  for  the  experimental 
physicist  was  surely  the  atomic  nucleus. 

It  seemed  equally  obvious  also  at  that  time  that  a  prerequisite  to  a  successful 
experimental  attack  on  the  nucleus  was  the  development  of  means  of  acce- 
lerating charged  particles  to  high  velocities  —  to  energies  measured  in  millions 
of  electron  volts,  a  task  which  appeared  formidable  indeed!  Accordingly,  I 
devoted  considerable  time  and  thought  to  the  technical  problem  of  ways  and 
means  of  reaching  millions  of  electron  volts  in  the  laboratory.  The  problem 
seemed  to  reduce  itself  to  two  parts,  A  the  production  of  high  voltages  and  B 
the  development  of  accelerating  tubes  capable  of  withstanding  such  high 
voltages. 

Since  transformers  and  rectifiers  for  such  high  voltages  seemed  rather  out 
of  the  question  for  various  reasons,  not  the  least  of  which  were  connected  with 
financial  limitations,  I  naturally  looked  for  alternative  means  of  producing 
high  voltages  —  the  surge  generator  which  was  used  by  Brasch  and  Lange  — 
the  electrostatic  generator  which  Professor  W.  F.  G.  Swann  was  working  on 
when  I  was  a  student  under  him  at  the  University  of  Minnesota  in  1924  and 
which  was  later  brought  to  practical  development  by  Van  de  Graaff,  and  the 
Tesla  coil  source  of  high  voltage  which  Tuve,  Breit  and  Hafstad  brought 
to  a  fruitful  stage  of  development. 

One  evening  early  in  1929  as  I  was  glancing  over  current  periodicals  in  the 
University  library,  I  came  across  an  article  in  a  German  electrical  engineering 
journal  by  Wideroe  on  the  multiple  acceleration  of  positive  ions.  Not  being 


51 


t  <. 


EntladnngB- 
ranm 


Erde 


Fig.  I.  Diagram  of  linear  accelerator  from  Professor  G.  Ising's  pioneer  publication  (1924) 
of  the  principle  of  multiple  acceleration  of  ions. 

able  to  read  German  easily,  I  merely  looked  at  the  diagrams  and  photographs 
of  Wideroe's  apparatus  and  from  the  various  figures  in  the  article  was  able  to 
determine  his  general  approach  to  the  problem  —  i.  e.  the  multiple  acceleration 
of  the  positive  ions  by  appropriate  application  of  radio  frequency  oscillating 
voltages  to  a  series  of  cylindrical  electrodes  in  line.  This  new  idea  immediately 
impressed  me  as  the  real  answer  which  I  had  been  looking  for  to  the  technical 
problem  of  accelerating  positive  ions,  and  without  looking  at  the  article  further 
I  then  and  there  made  estimates  of  the  general  features  of  a  linear  accelerator 
for  protons  in  the  energy  range  above  one  million  volt  electrons.  Simple  cal- 
culations showed  that  the  accelerator  tube  would  be  some  meters  in  length 
which  at  that  time  seemed  rather  awkwardly  long  for  laboratory  purposes. 
And  accordingly,  I  asked  myself  the  question,  instead  of  using  a  large  number 
of  cylindrical  electrodes  in  line,  might  it  not  be  possible  to  use  two  electrodes 
over  and  over  again  by  bending  the  positive  ions  back  and  forth  through  the 
electrodes  by  some  sort  of  appropriate  magnetic  field  arrangement.  Again  a 
little  analysis  of  the  problem  showed  that  a  uniform  magnetic  field  had  just 
the  right  properties  —  that  the  angular  velocity  of  the  ions  circulating  in  the 
field  would  be  independent  of  their  energy  so  that  they  would  circulate  back 
and  forth  between  suitable  hollow  electrodes  in  resonance  with  an  oscillating 
electrical  field  of  a  certain  frequency  which  now  has  come  to  be  known  as  the 
"cyclotron  frequency". 


52 


The  Evolution  of  the  Cyclotron 


Fig.  2.  First  crude  models  of  the  cyclotron  constructed  by  Edlefsen  in  1930. 

Now  this  occasion  affords  me  a  felicitous  opportunity  in  some  measure  to 
correct  an  error  and  an  injustice.  For  at  that  time  I  did  not  carefully  read 
Wideroe's  article  and  note  that  he  had  gotten  the  idea  of  multiple  acceleration 
of  ions  from  one  of  your  distinguished  colleagues,  Professor  G.  Ising,  who  in 
1924  published  this  important  principle.  It  was  only  after  several  years  had 
passed  that  I  became  aware  of  Professor  Ising's  prime  contribution.  I  should 
Uke  to  take  this  opportunity  to  pay  tribute  to  his  work  for  he  surely  is  the 
father  of  the  developments  of  the  methods  of  multiple  acceleration. 

Perhaps  you  will  permit  me  first  of  all  to  show  a  slide  of  the  diagram  of  the 
linear  accelerator  in  his  original  publication.  Fig.  i. 

I  hope  I  have  not  belabored  excessively  these  early  incidents  of  history  and 
now  I  should  like  to  trace  rapidly  the  evolution  of  the  cyclotron  by  showing 


53 


Fig.  3.  Working  model  of  cyclotron  constructed  by  M.  Stanley  Livingston  which  pointed 

the  way  to  later  developments. 

examples  of  the  apparatus  in  our  laboratory  as  it  was  developed  in  the  course 
of  time.  In  doing  so,  I  am  afraid  I  shall  not  be  able  to  mention  all  those  who 
deserve  great  credit  for  the  developments  —  as  from  the  beginning  the  work 
has  been  a  team  effort  involving  many  able  and  devoted  co-workers  in  many 
laboratories.  As  I  am  sure  you  well  appreciate,  a  great  many  diverse  talents 
are  involved  in  such  developments  and  whatever  measure  of  success  is  achieved 
is  dependent  on  close  and  effective  collaboration. 

Although  the  cyclotron  was,  so  to  speak,  invented  early  in  1929,  actual 
experimental  work  on  its  development  was  begun  in  the  spring  of  1930  when 
one  of  my  students,  Nels  Edlefsen,  constructed  two  crude  models  shown  in 
Fig.  2.  One  of  the  models  which  gave  slight  evidence  of  working  consisted  of 
two  copper  duants  waxed  together  on  a  glass  plate  with  a  filament  source  along 
the  diameter  at  the  center  much  like  later  models. 

In  the  fall  another  student,  M.  Stanley  Livingston,  continued  the  devel- 
opment and  quickly  constructed  the  model  shown  in  Fig.  3  which,  as  you  see, 
had  all  the  features  of  early  cyclotrons  and  which  worked  very  well  indeed  as 


54 


The  Evolution  of  the  Cyclotron 


Fig.  4.  General  view  of  first  cyclotron  used  in  nuclear  transformations. 


Fig.  5.  Vacuum  chamber  of  cyclotron  (Fig.  4)  which  produced  i  million  volt  protons. 


55 


Fig.  6.  General  view  of  27"  cyclotron  built  by  young  physicists  including  M.  S.  Livingston 
(left)  and  E.  O.  Lawrence  (right).  The  lack  of  good  engineering  design  is  quite  evident! 

80,000  volt  protons  were  produced  with  less  than  1,000  volts  on  the  semi- 
circular accelerating  electrode  —  now  called  the  "dee". 

The  next  milestone  in  the  development  was  the  construction  of  a  larger 
model  Figs.  4  and  5  which  produced  protons  of  the  desired  energies  —  in  the 
region  of  one  million  electron  volts.  Livingston  and  I  had  the  remarkable  good 
fortune  of  observing  that  this  apparatus  was  rather  more  successful  than  we 
had  expected.  For,  as  you  can  well  imagine,  we  were  concerned  about  how 
many  of  the  protons  would  succeed  in  spiralling  around  a  great  many  times 
without  getting  lost  on  the  way.  We  soon  recognized  that  the  focussing  actions 
of  the  electric  and  magnetic  fields  were  responsible  for  the  relatively  large 
currents  of  protons  that  reached  the  periphery  of  the  apparatus;  but  we  must 
acknowledge  that  here  again  experiment  preceded  theory ! 

We  were  busy  with  further  improvements  of  the  apparatus  to  produce 
larger  currents  at  higher  voltages  when  we  received  word  of  the  discovery  by 
CocKCROFT  and  Walton,  which  this  year  has  been  recognized  by  the  Nobel  Prize 
in  physics.  We  were  overjoyed  with  this  news  for  it  constituted  definite  assur- 


56 


The  Evolution  of  the  Cyclotron 


Fig.   7.  The  chamber  of  the  27"  cyclotron  showing  two  dees. 

ance  that  the  acceleration  of  charged  particles  to  high  speeds  was  a  worth- 
while endeavor.  As  you  can  imagine,  we  went  ahead  with  all  speed,  and  it  was 
not  long  before  the  disintegration  of  lithium  by  protons  had  been  observed  with 
the  apparatus. 

Now  we  may  proceed  rapidly  with  examples  of  later  developments.  Figs. 
6  and  7  show  the  first  two  dee  27"  cyclotron  which  produced  protons  and  deu- 
terons  of  several  million  volts  and  was  used  extensively  in  early  investigations 
of  nuclear  reactions  involving  neutrons  and  artificial  radioactivity. 

Again,  with  this  apparatus  the  discoveries  of  Chadwick  and  the  Curie- 
JOLIOTS  were  promptly  confirmed.  Indeed,  looking  back  it  is  remarkable  that 
we  managed  to  avoid  the  discovery  of  artificial  radioactivity  prior  to  their 
epoch-making  announcement:  for  we  tried  at  first  to  use  Geiger  counters  in 
observing  nuclear  radiations  produced  by  the  cyclotron  and  observed  that 
their  background  was  always  variable  and  large.  In  those  days  Geiger  counters 
had  the  reputation  of  being  unreliable  and,  rather  than  looking  into  the  matter 
of  their  apparent  misbehavior,  we  turned  to  ion  chambers  and  linear  amplifiers 


57 


Fig.  8.  Early  photograph  of  60"  cyclotron  showing  first  evidence  of  good  engineering 
practice  introduced  into  our  laboratory  by  W.  M.  Brobeck  (right)  and  Donald  Cooksey  (left), 

to  observe  heavy  particle  nuclear  reactions.  Of  course,  the  Geiger  counters  were 
simply  being  faithful  to  duty  and  recording  the  radiations  from  the  artificial 
radioactive  substances  and  this  became  immediately  apparent  after  the  Curie- 
JOLIOT  announcement.  Again,  we  were  overjoyed  at  the  richness  of  the  domain 
in  the  nucleus  accessible  to  particles  of  several  million  electron  volts  energy 
and  there  followed  a  happy  period  of  intensive  experimental  investigations, 
-which  indeed  through  the  years  has  gained  ever-increasing  tempo  in  laboratories 
the  world  over. 

The  next  milestone  in  our  laboratory  was  the  construction  of  the  60"  cyclo- 
tron, and  this  undertaking  was  greatly  strengthened  by  the  joining  of  our  team 
of  William  Brobeck,  a  truly  outstanding  young  engineer.  Brobeck  brought 
to  our  laboratory  sound  engineering  practice  which  from  the  day  he  joined 
us  has  had  a  profound  effect  on  developments.  To  him,  more  than  to  any  other 
one  individual,  goes  the  credit  for  the  success  of  the  60"  cyclotron  and  all  sub- 
sequent developments.  As  you  can  see  in  Fig.  8,  the  cyclotron  for  the  first  time 
began  to  look  like  a  well  engineered  machine.  It  was  with  this  machine  that 
the  discoveries  of  the  transuranium  elements  were  made  which  have  been 


58 


The  Evolution  of  the  Cyclotron 


Fig.  9.  Artist's  sketch  of  184"  cyclotron  designed  by  Brobeck  before  the  war  to  produce 
100  million  electron  volt  protons. 

rewarded  this  year  by  the  award  of  the  Nobel  Prize  in  chemistry  to  McMillan 
and  Seaborg.  Perhaps  the  finest  example  of  a  60"  cyclotron  is  now  in  operation 
at  the  Nobel  Institute  here  in  Stockholm. 

Soon  our  objective  was  the  production  of  protons  and  deuterons  of  much 
higher  energies  and  Bethe  pointed  out  the  difficulty  introduced  by  the  relativity 
increase  in  mass  of  the  particles  as  they  increase  in  energy  in  the  course  of 
acceleration  which  causes  them  to  get  out  of  resonance  with  an  oscillating  electric 
field  in  a  uniform  magnetic  field. 

However,  Thomas  devised  a  magnetic  field  that  avoided  the  limitation 
discussed  by  Bethe,  and  also,  of  course,  it  was  recognized  that  one  might 
modulate  the  frequency  in  step  with  the  changing  angular  frequency  of  the 
accelerated  particles.  These  two  solutions  of  the  technical  problem  of  yet 
higher  energies  —  the  region  of  100  miUion  volts  —  seemed  impractical;  at  least 
much  less  practicable  than  simply  so  designing  the  cyclotron  that  a  million 
volts  or  more  could  be  applied  to  the  dees,  so  that  the  particles  would  need  to 
circulate  around  relatively  few  times  in  reaching  the  desired  high  energies. 

Accordingly,  just  before  the  war  Brobeck  and  co-workers  designed  the  great 
184"  cyclotron  shown  in  Fig.  9. 


59 


Fig.   lo.  General  view  of  184"  synchrocyclotron  which  produces  340  Mev  protons.  The 
concrete  shielding,  partially  removed  in  this  photograph,  is  15'  in  thickness. 


As  is  well  known  the  war  prevented  the  building  of  this  machine  and  imme- 
diately afterwards  McMillan,  and  Veksler  independently  a  few  months  earUer, 
came  forward  with  the  principle  of  phase  stabiUty  which  transformed  the  con- 
ventional cyclotron  to  a  much  more  powerful  imstrument  for  higher  energies 
—  the  synchrocyclotron.  Fig.  10  shows  the  main  features  of  the  Berkeley  184" 
synchrocyclotron  which  produces  340  Mev  protons,  while  there  are  later  and 
more  modem  installations,  notably  at  Columbia  University  and  University  of 
Chicago,  which  produce  somewhat  higher  energies.  As  I  am  sure  this  audience 
is  well  aware,  a  beautifully  engineered  synchrocyclotron  is  nearing  completion, 
at  Upsala. 

On  completion  of  the  184"  synchrocyclotron,  it  was  natural  that  Brobeck 
should  turn  his  attention  to  the  engineering  problem  of  applying  the  synchro- 
tron principle  to  the  acceleration  of  heavy  ions,  particularly  protons,  to  much 
higher  energies  —  in  the  range  of  billions  of  electron  volts.  It  was  not  long 
before  his  engineering  studies  indicated  the  practicability  of  producing  protons 
in  the  energy  range  well  above  one  billion  electron  volts. 


60 


The  Evolution  of  the  Cyclotron 


Fig.   II.  One-quarter  scale  operating  model  of  6  Bev  proton  synchrotron. 

With  the  extensive  developments  in  the  atomic  energy  field,  large  funds 
became  available  for  research  purposes  —  much  larger  than  seemed  possible 
before  the  war  —  and  indeed,  as  soon  as  all  concerned  were  convinced  of  the 
practicaUty  of  building  a  proton  synchrotron  for  several  bilhon  electron  volts, 
the  construction  of  two  installations  was  begun,  one  at  Brookhaven  for  about 
3  bilhon  electron  volts  and  a  second  at  Berkeley  for  about  twice  this  energy. 

The  first  step  in  these  large  undertakings  was  to  build  a  substantial  operating 
model  to  test  out  the  theory  of  the  proton  synchrotron,  as  well  as  the  engineering 
principles  of  design.  Accordingly,  a  quarter  scale  operating  model  was  con- 
structed and  is  shown  in  Fig.  ii.  A  small  cyclotron  was  designed  to  produce 
large  current  pulses  of  i  Mev  protons  which  were  injected  into  the  "race  track" 
of  the  S5mchrotron  by  an  appropriate  magnetic  and  electrostatic  deflecting 
system  which  can  be  seen  in  the  foreground  of  Fig.  ii.  This  model  worked  as 
expected  and  provided  a  great  deal  of  practical  data  giving  confidence  that  the 
full  scale  machines  will  function  successfully  and  satisfactorily. 

It  is  hardly  appropriate  here  to  describe  either  the  Brookhaven  or  Berkeley 
proton  synchrotrons  (the  former  is  called  the  cosmotron  and  the  latter  is  called 


61 


Fig.  12.  General  view  of  "race  track"  magnet  in  process  of  assembly  for  6.3  Bev  proton 

synchrotron  or  "bevatron". 


Fig.   13.  Showing  coil  winding  of  bevatron  magnet. 


62 


The  Evolution  of  the  Cyclotron 


Fig.  14.  The  size  of  the  bevatron  magnet  is  here  indicated.  Left  to  right  (E.  O.  Lawrence, 
W.  M.  Brobeck,  H.  A.  Fidler  and  D.  Cooksey). 


Fig.   15.  Bevatron  motor  generator  equipment. 


63 


Fig.  1 6.  Ignitrons  and  associated  switchgear  for  bevatron  motor  generator. 

the  bevatron)  but  perhaps  it  is  of  interest  to  show  a  number  of  photographs 
which  display  the  general  features  of  this  great  machine.  Figs.  12,  13,  14,  15 
and  16. 

Now  that  we  shall  soon  have  5  or  10  Bev  particles  in  the  laboratory,  what 
possibilities  are  there  for  going  on  higher  to  50  or  100  Bev?  One  answer  is  that 
the  limitation  of  the  bevatron  is  largely  a  financial  one.  With  a  correspondingly 
larger  expenditure  higher  energies  surely  can  be  reached. 

But  I  should  like  to  close  by  emphasizing  that  a  more  feasible,  if  not  more 
interesting,  approach  to  the  problem  of  higher  energy  nuclear  projectiles  is  the 
acceleration  of  multiply  charged  heavier  ions  such  as  C*+,  or  Ne^"\  Already 
extraordinarily  interesting  nuclear  reactions  have  been  produced  by  the  acce- 
leration of  C*"^  ions  to  120  Mev  in  the  60"  cyclotron  and  such  particles  in  the 
Berkeley  bevatron  would  be  accelerated  to  more  than  36  Bev.  Since  in  the 
cosmic  radiation  such  heavy  particles  play  an  important  role,  they  will  surely 
be  produced  in  the  bevatron  some  day,  contributing  to  further  progress  in  our 
understanding  of  nature. 


64 


These  "machines"  are  used  for  two  purposes:  to  "see" 
fundamental  particles  of  matter,  and  to  produce  new  ones. 
Though  published  in  1958,  this  article  is  still  an  excellent 
Introduction  to  the  basic  design  used  to  build  many  current 
accelerators. 


8   Particle  Accelerators 

Robert  R.  Wilson 

Article  in  Scientific  American,  1958. 


From  time  to  time  in  the  course  of 
history  men  have  been  swept  up 
by  intense  currents  of  creative  ac- 
tivity. In  the  pyramids  of  Egypt,  in 
Greek  sculpture  and  in  Florentine  paint- 
ing we  find  monuments  to  such  bursts  of 
expression.  My  favorite  example  is  the 
Gothic  cathedrals  that  so  magically 
sprang  up  in  12th-  and  13th-century 
France,  for  I  like  to  relate  that  magnifi- 
cent preoccupation  with  construction  to 
an  obsession  of  our  own  time— the  build- 
ing of  nuclear  accelerators. 

Like  nuclear  physics  today,  religion 
at  that  time  was  an  intense  intellectual 
activity.  It  seems  to  me  that  the  designer 
of  an  accelerator  is  moved  by  much  the 
same  spirit  which  motivated  the  design- 
er of  a  cathedral.  The  esthetic  appeal  of 
both  structures  is  primarily  technological. 
In  the  Gothic  cathedral  the  appeal  is  pri- 
marily in  the  functionality  of  the  ogival 
construction— the  thrust  and  counter- 
thrust  that  is  so  vividly  evident.  So,  too, 
in  the  accelerator  we  feel  a  technological 
esthetic— the  spirality  of  the  orbits  of  the 
particles,  the  balance  of  electrical  and 
mechanical  motion,  the  upward  surge 
of  forces  and  events  until  an  ultimate  of 
height  is  reached,  this  time  in  the  energy 
of  the  particles.  In  both  cases  we  find 
the  architects  working  at  the  very  limit 
of  technical  knowledge.  In  both  there 
is  intense  competition  between  localities, 
regional  and  national.  Both  structures 
are  expensive:  a  really  large  accelerator 
can  cost  $100  million;  the  cost  of  a 
cathedral,  in  terms  of  medieval  econom- 
ics, was  possibly  higher. 

But  where  a  cathedral  was  a  commu- 
nity enterprise,  with  many  people  in  the 
region  participating  in  its  financing  and 
construction,  and  nearly  everyone  in  its 
enjoyment,  an  accelerator  is  esoteric.  Its 
presence  in  a  community  is  usually  un- 
known and  unsung.  Few  are  the  workers 


who  help  to  build  it,  and  fewer  still  are 
those  who  use  it. 

So  the  accelerator  building  boom  goes 
on  largely  unnoticed,  but  at  a  quicken- 
ing pace.  Cyclotions,  the  original  "atom 
smashers,"  are  now  dotted  almost  all 
over  the  globe.  They  have  evolved  into 


synchro-cyclotrons,  and  have  reached 
their  culmination  in  three  giant  ma- 
chines, one  at  the  University  of  Cali- 
fornia in  Berkeley,  another  at  the  Euro- 
pean Organization  for  Nuclear  Research 
(CERN)  in  Switzerland  and  another  in 
the  US.S.R.  These  machines  accelerate 


PROTON  SYNCHROTRON  IN  GENEVA  is  designed  to  yield  25  bev.  Shown  here  is  a  sec- 
tion of  the  interior  of  its  ring  building.  This  structure  is  approximately  660  feet  in  diameter. 


65 


/N 


^ 


/ 


A 


^ 


>'4 


A 


4\    A    A    A    A    A    A 


/ 


y 


A 


A 


> 


/ 


^. 


A 


^' 


^y^ 


MAGNETIC  FORCE  on  moving  charged 
particles  {black  dots)  is  indicated  by  arrows 
pointing  down  and  to  right.  Upward  arrows 
show  the  speed  of  the  particles  and  colored 
arrows  the  direction  of  the  field.  Large  dot 
at  the  bottom  represents  a  heavier  particle. 


protons  to  energies  of  between  600  and 
700  million  electron  volts  (mev).  Syn- 
chrotrons, another  development,  are 
even  bigger  and  more  powerful.  The 
Cosmotron,  a  2,200-ton  monster  at 
Brookhaven  National  Laboratory  which 
emits  3-billion-electron-volt  (bev)  pro- 
tons, is  small  compared  to  the  6-bev, 
10,000-ton  Bevatron  at  Berkeley.  This 
in  turn  is  topped  by  the  10-bev,  36,000- 
ton  Phasotron  in  the  U.S.S.R.  Two  even 
larger  machines  are  under  construction 
at  Brookhaven  and  CERN;  they  are  de- 
signed to  produce  protons  of  25  to  30 
bev.  And  still  bigger  accelerators  are 
being  planned. 

Nuclear  Microscopes 

Why?  What  is  the  purpose  behind 
this  almost  feverish  effort  to  build  more 
and  bigger  machines?  Perhaps  the  sim- 
plest answer  is  that  accelerators  are  the 
microscopes  of  nuclear  physics.  We  usu- 
ally think  of  an  accelerator  as  a  sort  of 
gun,  producing  high-speed  particles 
which  bombard  the  nucleus  of  the  atom. 
But  since  particles  are  known  to  have 
wave  properties,  it  is  equally  appropriate 
to  say  that  the  accelerator  shines  "light" 
on  the  nuclei,  enabling  us  to  "see"  them. 

Now  the  resolving  power  of  a  micro- 
scope, i.e.,  its  ability  to  distinguish  small 
objects,  depends  on  the  wavelength  of 
the  light  it  employs.  The  shortest  wave- 
length of  visible  light  is  about  four 
100,000ths  (4  X  10-5)  of  a  centimeter; 
with  these  waves  one  can  perceive  a 
microbe,  of  about  the  same  length. 

To  examine  smaller  things,  biologists 
now  use  the  electron  microscope.  The 
wavelength  of  a  particle  depends  on  its 
mass  and  its  energy.  At  a  few  thousand 
electron  volts— the  energy  at  which  elec- 
tron microscopes  operate-an  electron 
has  a  wavelength  some  10,000  times 
shorter  than  that  of  visible  light  (about 
lO"'*  centimeter).  With  these  waves  one 
can  begin  to  see  the  details  of  molecules. 

The  nucleus  of  an  atom  is  about  10"^- 
centimeter  in  diameter.  This  is  the  wave- 
length of  a  proton  with  an  energy  of  1 
mev.  To  "see"  the  nucleus  we  therefore 
need  a  1-mev  proton  "microscope,"  and 
to  make  out  some  of  its  internal  details 
we  need  some  10  to  20  times  as  much 
energy.  Thus  a  laboratory  interested  in 
classical  nuclear  physics  will  invariably 
have  a  Van  de  Graaff  accelerator  or  a 
cyclotron  operating  in  the  range  of  1  to 
20  mev. 

But  physics  has  pushed  beyond  this 
point.  At  present  many  of  us  are  inter- 
ested not  in  the  nucleus  as  a  whole  but 
in  the  structure  of  the  protons  and  neu- 


trons (nucleons)  of  which  it  is  com- 
posed. It  is  the  old  problem  of  worlds 
within  worlds,  for  the  proton  itself  turns 
out  to  have  a  rich  structure.  It  is  per- 
haps 10-13  centimeter  in  diameter,  and 
to  resolve  it  requires  an  energy  of  sev- 
eral hundred  mev.  To  see  it  in  as  fine 
detail  as  we  can  see  the  structure  of  the 
nucleus  we  must  have  still  higher  energy. 
It  is  for  this  reason  that  the  25-  to  30-bev 
machines  are  under  construction.  If  and 
when  the  structure  of  the  proton  is 
known,  will  its  component  parts  turn  out 
to  have  their  own  structure?  Very  pos- 
sibly so,  and  if  they  do,  machines  of 
higher  energy  will  be  built  to  explore 
that  structure. 

The  microscope  analogy  does  not  tell 
the  whole  story.  When  we  get  to  suffi- 
ciently short  wavelengths  (i.e.,  when 
the  bombarding  particles  in  our  accel- 
erators reach  sufficiently  high  energy), 
we  not  only  see  particles,  but  we  also 
make  new  ones.  These  new  particles  are 
created  out  of  energy.  At  1  mev  an 
electron  has  enough  energy  to  create  a 
pair  of  particles— an  electron  and  a  posi- 
tron. At  150  mev  it  makes  pi  mesons 
(pions)  when  it  collides  with  a  nucleon. 
Our  1-bev  electron  accelerator  at  Cor- 
nell University  produces  more  massive 
particles:  K  and  lambda  mesons.  The 
Bevatron,  which  produces  6-bev  pro- 
tons, is  able  to  create  antiprotons,  anti- 
neutrons  and  still  heavier  particles  such 
as  xi  and  sigma  mesons. 

Thus  as  the  energy  of  the  machines 
has  increased  it  has  become  possible  to 
create  more  and  heavier  new  particles. 
Obviously  the  exciting  next  step  is  to 
attain  even  higher  energies,  and  then  to 
see  what  sort  of  monster  particles  are 
created.  One  has  the  very  strong  feeling 
that  new  particles  will  indeed  show  up. 
It  may  well  turn  out  that  they  will 
prove  to  be  only  complexes  of  particles 
which  we  already  understand;  however, 
it  is  exactly  to  answer  such  questions 
that  we  are  building  the  machines. 

Originally  we  constructed  our  accel- 
erators in  order  to  search  for  the  ultimate 
in  elementary  particles.  We  expected 
these  particles  to  be  fragments  and 
hence  to  be  successively  smaller;  it  was 
to  improve  our  definition  of  them  that 
we  went  to  higher  energies.  Ironically 
the  fragments  now  seem  to  get  larger. 
One  has  the  uneasy  feeUng  that  new  ma- 
chines make  new  particles  which  lead 
to  the  construction  of  new  machines,  and 
so  on  ad  infinitum.  In  fact,  there  may  be 
lurking  here  a  new  kind  of  indetermin- 
acy principle  which  will  inherently  limit 
our  knowledge  of  the  very  small. 

So  much  for  the  reasons  why  accelera- 


66 


Particle  Accelerators 


CYCLOTRON'S  OPERATION  is  like  that  of  a  circular  pendulum 
Heft)  in  which  the  weight  is  pushed  repeatedly  to  give  an  ever- 
widening  swing.  The  schematic  diagram  at  the  right  shows  a  par- 
ticle (dot)  spiraling  within  two  D-shaped  electrodes.  The  magnetic 


pole  pieces  which  provide  the  guiding  field  (colored  lines)  are 
outlined  in  light  broken  lines.  The  particles  are  accelerated  by  an 
oscillating  electric  field  between  the  dees.  The  generator  which 
produces  the  field  is  shown  as  a  wavy  line  within  a  rectangle  (fopi . 


tors  are  built.  Let  us  turn  to  the  ma- 
chines themselves.  All  of  them  operate 
on  the  same  fundamental  principle: 
charged  particles  (electrons  or  positive 
ions  usually  protons)  are  put  into  an 
electric  field  which  exerts  a  force  on 
them,  pushing  them  to  high  speeds  and 
energies.  (The  electron  volt,  in  which 
the  energy  is  usually  measured,  is  the 
energy  acquired  by  a  particle  with  one 
electronic  unit  of  charge  accelerated  by 
a  potential  difference  of  one  volt.)  The 
simplest  form  of  accelerator  is  a  pipe 
along  which  a  steady  electric  field  ac- 
celerates the  particles.  This  is  the  well- 
known  Van  de  Graaff  machine.  To  ob- 
tain higher  energies  a  long  pipe  may  be 
used  with  several  accelerating  electrodes 
which  kick  the  particles  to  higher  and 
higher  speeds  as  they  travel  down  the 
tube  [see  "The  Linear  Accelerator,"  by 
Wolfgang  Panofsky;  Scientific  Ameri- 
can, October,  1954].  But  to  attain  a 
really  high  energy  by  this  method  would 
require  an  extremely  long  pipe.  To  get 
around  this  difficulty  the  particles  can 
be  made  to  travel  in  a  circular  or  spiral 


path  which  brings  them  back  through 
the  same  electrodes  where  the  accelerat- 
ing voltage  is  applied  again  and  again. 

It  is  with  such  circular  machines  that 
we  are  chiefly  concerned  in  this  article. 
In  these  machines  the  circular  motion  is 
brought  about  by  magnetic  fields.  A 
magnetic  field  exerts  a  force  on  all  elec- 
tric charges  that  move  through  it;  the 
force  is  always  at  right  angles  to  the  di- 
rection of  the  charges'  travel.  It  is  the 
same  kind  of  force  that  acts  on  a  stone 
whirled  at  the  end  of  a  string.  The  mag- 
netic field,  like  the  string,  forces  the  par- 
ticles, to  move  in  a  circular  path.  The 
stronger  the  field,  the  sharper  the  curva- 
ture of  the  path;  on  the  other  hand,  the 
faster  or  heavier  the  particle,  the  less  it 
is  curved  by  a  given  field  [see  diagrams 
on  opposite  page]. 

The  simplest  and  oldest  type  of  accel- 
erator to  make  use  of  magnetic  bending 
is  the  cyclotron.  The  operation  of  this 
machine  can  be  most  easily  visualized 
by  imagining  a  weight  suspended  by  a 
string  and  pushed  so  as  to  describe  a  cir- 
cular motion.  As  with  any  pendulum  the 


time  required  to  complete  a  full  circular 
swing  is  the  same  whether  the  circle  is 
small  or  large.  Thus  if  the  weight  is 
pushed  rhythmically  it  will  move  out- 
ward in  an  ever-widening  circle,  return- 
ing to  the  pushing  point  in  the  same  time, 
on  each  revolution  [see  diagram  above]. 
So  it  is  in  the  cyclotron:  each  ion  whirls 
inside  of  two  semicircular  electrodes  or 
"dees,"  getting  an  electrical  push  when 
it  passes  from  one  to  the  other.  A  ver- 
tical magnetic  field  provides  a  constant 
inward  push  and,  like  the  string,  holds 
the  ion  in  a  circular  path  and  guides  it 
back  to  the  gap  between  the  dees,  where 
it  is  given  another  electrical  push.  The 
velocity  of  the  ion  then  becomes  greater 
and,  as  a  result  of  its  inertia,  the  curva- 
ture of  the  circular  path  caused  by  the 
magnetic  field  becomes  larger.  The  time 
taken  to  traverse  a  full  circle  is  the  same 
no  matter  how  big  the  radius,  because 
the  increase  in  speed  just  compensates 
for  the  increase  in  path-length  per  turn. 
Now  if  the  voltage  across  the  dees  is 
made  to  oscillate  rapidly,  and  if  its  pe- 
riod is  adjusted  so  that  it  exactly  matches 


67 


the  period  of  revolution  of  the  ions,  then 
the  ions  will  be  pushed  in  the  right  di- 
rection at  the  right  time  at  each  cross- 
ing of  the  gap  between  dees;  the  energy 
of  the  ions  will  build  up  until  their  path 
takes  them  to  the  edge  of  the  magnetic 
field,  where  they  can  be  used  or  extract- 
ed in  the  form  of  a  beam. 

If  the  cathedrals  had  great  designers 
such  as  Suger  of  St.  Denis  and  Sully  of 
Notre  Dame,  the  accelerators  have  their 
Cockcroft  of  Cambridge  and  Lawrence 
of  Berkeley.  In  1928  J.  D.  Cockcroft  and 
E.  T.  S.  Walton  built  a  device  in  which 
a  voltage  generated  between  two  elec- 
trodes accelerated  ions  to  a  high  enough 
speed  to  cause  the  disintegration  of  a 
bombarded  nucleus.  They  were  still 
working  in  the  magnificently  simple  tra- 
dition of  Ernest  Rutherford's  laboratory 
at  the  University  of  Cambridge.  A  quite 
different  tradition  was  established  with 
the  building  of  the  first  cyclotron  by 
Ernest  O.  Lawrence  in  1930.  It  has 
spread  from  his  laboratory  at  the  Uni- 
versity of  California  and  has  come  to 
dominate  experimental  nuclear  physics 
in  this  country.  Indeed,  one  can  begin 
now  to  trace  this  spirit  abroad,  particu- 
larly to  the  U.S.S.R.,  where  it  may  flour- 
ish even  more  vigorously  than  it  does  in 
the  U.  S. 

This  tradition,  called  "berkelitis"  by 
its  detractors,  is  a  true  departure  in  ex- 
perimental physics.  Previously  experi- 
mental equipment  had  been  constructed 
to  test  a  particular  surmise  or  idea.  But 
building  a  large  accelerator  is  more  anal- 
ogous to  outfitting  a  ship  for  an  expedi- 
tion of  exploration,  or  to  the  construction 
of  a  huge  telescope  to  study  a  variety  of 
astronomical  objects.  After  several  cyclo- 
trons had  been  built  at  Berkeley,  the 


SYNCHROTRON  restriris  parlirles  to  a  nearly  circular  path  by 
means  of  a  magnetic  field  (colored  lines)  which  grows  stronger  as 
ihe  particle  energy  increases.  At  top  an  electron  (hutched  circle) 
is  in  an  orbit  that  brings  it  to  the  accelerating  gap  (riffhl)  just  as 
ihe  voltage  changes  from  accelerating  to  retarding   (curve  at  bot- 


tom). In  the  center  drawing  the  field  is  made  stronger  and  the 
electron  (black  circle)  is  bent  more  strongly,  following  a  shorter 
path  and  arriving  at  the  gap  in  time  to  get  a  push.  After  a  number 
of  pushes  it  spirals  out  to  the  original  path.  The  cross  section 
at  bottom  right  shows  magnetic  pole  pieces  around  the  doughnut. 


68 


Particle  Accelerators 


students  and  associates  of  Lawrence 
traveled  far  and  wide  to  spread  the  gos- 
pel. By  World  War  II  they  had  helped 
to  build  cyclotrons  not  only  at  universi- 
ties in  the  U.  S.,  but  also  in  several  other 
countries.  The  biggest  of  these  machines 
produced  protons  of  about  10  mev.  As 
we  have  seen,  this  is  an  appropriate  en- 
ergy for  exploring  the  nucleus  as  a  whole, 
but  not  for  examining  its  parts.  Just  be- 
fore the  war  Lawrence  had  begun  to 
build  a  giant  cyclotron,  to  enter  the  en- 
ergy region  above  100  mev,  with  which 
he  could  start  to  probe  nucleons. 

The  Synchrotron 

It  was  characteristic  of  Lawrence  that 
he  went  ahead  despite  a  prevalent  con- 
viction that  the  energy  limit  of  the  cy- 
clotron was  about  20  mev.  This  convic- 
tion was  based  on  an  effect  predicted  by 
Albert  Einstein's  theory  of  relativity: 
particles  traveling  at  nearly  the  speed  of 
light  will  increase  in  mass.  At  20  mev  a 
proton  has  entered  this  "relativistic"  re- 
gion, and  further  increases  in  energy 
will  result  not  so  much  in  greater  speed 
as  in  greater  mass.  When  this  happens, 
the  particle  in  a  cyclotron  begins  to  fall 
behind  schedule  as  it  spirals  farther  out- 
ward, and  it  no  longer  arrives  between 
the  dees  at  the  right  time  to  get  a  push 
from  the  oscillating  voltage. 

The  war  interrupted  work  on  Law- 
rence's big  machine.  Its  huge  magnet 
was  used  to  separate  isotopes  of  uranium 
for  the  atomic-bomb  program.  At  the 
end  of  the  war  V.  I.  Veksler  of  the 
U.S.S.R.  and  E.  M.  McMillan  of  the 
University  of  California  independently 
and  almost  simultaneously  enunciated 
the  so-called  synchrotron  principle.  This 
principle  showed  the  way  to  accelerat- 
ing particles  into  the  completely  relati- 
vistic region.  It  was  exactly  the  sort  of 
deus  ex  machina  that  Lawrence  had  en- 
visioned when  he  gkmbled  some  $1  mil- 
lion in  starting  his  big  cyclotron.  The 
principle  was  immediately  adopted.  A 
successful  synchro-cyclotron  was  built 
which  produced  protons  in  the  region  of 
100  mev  (eventually  730  mev).  In  the 
next  few  months  a  number  of  important 
features  of  the  proton  were  discovered. 

To  understand  the  synchrotron  prin- 
ciple, it  is  easier  to  consider  its  applica- 
tion in  the  electron  synchrotron  rather 
than  in  the  more  complicated  synchro- 
cyclotron. Some  half-dozen  of  these  elec- 
tron accelerators,  with  maximum  ener- 
gies of  about  300  mev,  were  also  built 
just  after  the  war. 

In  a  synchrotron  electrons  travel  on  a 
circular  orbit  inside  a  narrow  doughnut- 


shaped  vacuum  vessel.  At  one  point  in 
the  doughnut  is  a  pair  of  accelerating 
electrodes  across  which  there  is  an  oscil- 
lating voltage  like  that  in  the  cyclotron. 
A  ring-shaped  magnet  surrounding  the 
doughnut  produces  a  field  which  forces 
the  particle  to  travel  on  orbits  close  to 
the  center  of  the  tube  [see  diagram  on 
opposite  page].  The  electrons  are  inject- 
ed into  the  doughnut  from  a  small  linear 
accelerator  at  an  energy  of  about  2  mev. 
At  this  energy  their  speed  is  some  98 
per  cent  of  the  speed  of  light;  hence  they 
cannot  travel  much  faster.  To  make  mat- 
ters simpler  let  us  assume  that  the  speed 
is  exactly  the  speed  of  light  and  that  the 
whole  increase  in  energy  goes  into  mass. 
Now  imagine  an  electron  in  a  circular 
orbit  at  the  center  of  the  doughnut.  The 
electron  is  held  there  by  a  constant  mag- 
netic field.  Also  imagine  that  our  oscil- 
lating voltage  is  applied,  but  that  the 
electron  crosses  the  accelerating  gap  just 
at  the  time  when  the  voltage  falls 
through  its  zero  value.  The  frequency  of 
the  voltage  is  made  the  same  as  that  of 
the  electron  traveling  around  its  orbit  at 
the  constant  speed  of  light.  The  electron 
now  passes  the  gap  on  all  subsequent 
turns  just  as  the  voltage  becomes  zero. 
Thus  nothing  happens;  the  electron  re- 
mains on  its  orbit  and  keeps  the  same 
energy.  Now  we  increase  the  magnetic 
field  slightly.  Since  the  energy  (mass)  is 
still  the  same,  the  particle  is  forced  into 
a  sharper  curve,  i.e.,  its  orbit  gets  small- 
er. But  because  the  orbit  is  smaller  and 
the  speed  is  constant,  the  time  it  takes 
the  electron  to  return  to  the  accelerating 
gap  is  shorter.  Hence  the  electron  ar- 
rives slightly  before  the  voltage  has  fall- 
en to  zero;  it  is  accelerated  slightly.  On 
the  next  turn,  if  the  energy  is  still  not 
large  enough,  the  orbit  will  still  be  too 
small:  the  electron  will  arrive  still  earlier 
and  be  accelerated  even  more.  Eventual- 
ly the  energy  will  increase  enough  (that 
is,  the  electron  will  get  heavy  enough) 
so  that  it  is  bent  less  sharply  and  edges 
out  to  its  original  orbit.  If  the  energy 
should  become  too  great,  the  orbit  will 
be  too  big  and  the  time  it  takes  the  elec- 
tron to  make  each  turn  will  be  too  long. 
This  will  cause  the  electron  to  drop  be- 
hind the  accelerating  voltage  and  be 
pushed  backward  so  that  it  will  lose  en- 
ergy. Thus  we  have  a  beautiful  auto- 
matic device  for  keeping  the  electron  on 
the  right  orbit,  or  at  least  oscillating 
around  the  right  orbit.  That  is  all  there 
is  to  the  synchrotron  principle  or,  as  it 
is  sometimes  called,  phase  focusing. 

Now  we  can  see  that,  if  the  magnetic 
field  of  the  synchrotron  is  increased  con- 
tinuously, the  energy  of  the  electrons 


STRONG  FOCUSING  is  produced  by  mag- 
netic fields  which  are  alternately  bowed  out 
and  in.  Horizontal  arrows  show  radial  forces 
on  the  particles  at  inner  and  outer  edges  of 
the  field.  Slanted  arrows  represent  forces 
which  focus  or  defocus  particles  vertically. 


69 


SYNCHRO-CYCLOTRON  ai  tlie  Berkeley  Radiation   Laboratory 
of  the  University  of  California  is  now  the  most  powerful  machine 


of  its  kind.  A  modification  of  its  design  last  year  increased  the 
energy  of  its  proton  beam  to  730  million  electron  volts   (mev). 


ELECTRON  S\NCHROTRON  was  photographed  in  the  author's 
laboratory  at  Cornell  University  while  its  guiding  magnet  was  un- 


der construction.  Machine,  which  produces  an  energy  of  1  bev,  is 
the  first  to  use  strong  focusing.  Accelerating  electrodes  are  at  right. 


70 


Particle  Accelerators 


will  also  increase  continuously;  the  elec- 
trons will  receive  energy  at  just  the  right 
rate  to  keep  them  on  the  central,  or  syn- 
chronous, orbit.  In  practice  electrons 
can  be  injected  into  the  doughnut  when 
the  magnetic  field  is  rather  weak  ( about 
10  gauss)  and  ejected  when  the  field  is 
quite  strong  (more  than  10,000  gauss). 
A  synchrotron  with  a  large  enough  radius 
can  accelerate  electrons  up  to  energies 
of  about  10  bev.  There  are  now  about 
six  machines,  built  or  being  built,  which 
are  designed  to  yield  electron  energies 
between  1  and  1.5  bev.  At  Cambridge, 
Mass.,  a  6-bev  electron  synchrotron  is 
being  constructed  by  a  joint  Harvard 
University-Massachusetts  Institute  of 
Technology  group. 

Let  us  return  to  the  synchro-cyclotron. 
It  works  in  essentially  the  same  way  as 
a  synchrotron  but  it  is  shaped  like  a  cy- 
clotron. Instead  of  a  varying  magnetic 
field  it  has  a  constant  field,  but  the  fre- 
quency of  the  accelerating  voltage  ap- 
plied to  the  dees  is  varied.  This  means 
that  the  synchronous  orbit  of  the  protons 
is  not  a  fixed  circle  but  a  growing  spiral. 

In  another  class  of  accelerators,  the 
proton  synchrotrons,  both  the  magnetic 
field  and  the  frequency  of  the  accelerat- 
ing voltage  are  varied.  The  increasing 
field  counteracts  the  protons'  tendency 
to  spiral  outward  as  they  get  up  to  rela- 
tivistic  energies,  and  the  orbit  is  again 
a  fixed  circle.  Above  about  5  bev  the 
protons  are  traveling  practically  at  the 
speed  of  light,  and  from  here  on  the  pro- 
ton synchrotron  works  just  like  an  elec- 
tron synchrotron. 

If  I  may  extend  the  figure  of  speech 
with  which  I  began  this  article,  each 
kind  of  accelerator  has  its  own  architec- 
tural style.  To  me  synchro-cyclotrons  are 
baroque.  Proton  synchrotrons  are  defi- 
nitely Romanesque,  although  their 
rounded  arches  are  horizontal.  Electron 
synchrotrons  have  a  lightness  and  grace 
that  could  only  be  Gothic. 

The  Newer  Machines 

This  brings  us  more  or  less  up  to  date 
in  the  evolution  of  accelerators.  We  may 
now  ask  whether  we  are  near  the  end  of 
this  movement  in  physics  or  still  at  its 
beginning.  The  field  still  has  tremendous 
vigor,  and  it  is  my  guess  that  we  are  at 
about  the  same  stage  as  the  cathedral 
builders  were  after  they  had  completed 
Notre  Dame  of  Paris.  The  significant  in- 
novations were  behind  them,  but  most 
of  their  masterpieces  were  yet  to  come. 

Early  in  this  article  I  mentioned  that 
two  machines  now  under  construction, 
one  at  Brookhaven  National  Laboratory 


COSMOTRON,  the  3-bev  proton  synchrotron  at  Brookhaven  National  Laboratory, 
first  one  of  the  muhi-bev  accelerators.  Its  2,200-ton  magnet  has  an  inside  diameter  of 


w  as  the 
60  feet. 


PHASOTRON  is  a  10-bev  proton  synchrotron  in  the  U.S.S.R.  Its  magnet,  of  which  a  portion 
appears  in  this  photograph,  weighs  36,000  tons  and  is  approximately  200  feet  in  diameter. 


71 


FFAG  (fixed-field  alternating-gradient)  design  is  embodied  in  an 
electron  accelerator  built  as  a  model  for  a  larger  proton  machine 


at  the  laboratory  of  the  Midwestern  Universities  Research  Asso- 
ciation in  Madison,  Wis.  The  dark  spiral  sectors  are  th&  magnets. 


72 


Particle  Accelerators 


and  the  other  at  CERN  in  Geneva,  will 
produce  protons  of  25  to  30  bev.  Both 
of  these  machines  are  proton  synchro- 
trons; each  will  cost  between  $20  million 
and  $30  million.  The  diameter  of  the 
orbit  traveled  by  their  protons  will  be 
nearly  1,000  feet! 

These  machines  were  made  possible 
by  the  discovery  at  Brookhaven  of  a  new 
principle  called  strong  focusing  [see  "A 
100-Billion-Volt  Accelerator,"  by  Ernest 
D.  Courant;  Scientific  American, 
May,  1953].  This  principle  involves  a 
reshaping  of  the  guiding  magnetic  field 
so  that  the  particles  are  held  much  closer 
to  their  ideal  orbit.  It  means  that  the 
doughnut  can  be  thinner,  and  the  sur- 
rounding magnet  smaller  and  lighter. 

Until  now  we  have  considered  only 
the  radius  of  the  orbit,  i.e.,  the  size  of 
the  circle  on  which  the  particles  travel. 
However,  the  particles  can  not  only  drift 
in  and  out  but  also  up  and  down;  thus 
they  must  be  focused  vertically  as  well 
as  horizontally.  In  old-style  synchrotrons 
the  lines  of  force  in  the  magnetic  field 
are  bowed  sUghtly  outward  [see  diagram 
on  page  6].  This  has  the  effect  of  forc- 
ing particles  back  toward  the  center  line 
when  they  move  above  or  below  it.  But 
the  bowed  field  gets  somewhat  weaker 
with  the  distance  from  the  center  line. 
Hence  a  particle  that  wanders  too  far 
from  the  center  line  is  not  strongly 
pushed  back  toward  it. 

In  strong  focusing  the  field  is  broken 
into  sectors  which  are  alternately  bowed 
outward  and  inward  [see  diagram  on 
page  7].  The  sectors  bowed  outward 
provide  sharp  vertical  focusing,  but  are 
even  worse  than  the  old  field-shape  at 
bringing  a  particle  in  from  an  orbit  that 
is  too  large.  In  other  words,  they  do  not 
focus  radially.  On  the  other  hand,  the 
sectors  bowed  inward  increase  in 
strength  as  the  radius  gets  bigger,  and 
provide  strong  radial  focusing.  Vertical- 
ly, however,  they  have  the  wrong  effect 
on  the  particles,  tending  to  spread  rather 
than  to  focus  them.  It  turns  out  that  each 
of  the  defocusing  influences  is  overbal- 
anced by  the  focusing  effect  of  the  other 
sector;  the  net  result  is  a  much  more 
tightly  restricted  beam.  This  method  of 
focusing  was  successfully  used  in  the 
Cornell  1-bev  electron  synchrotron,  and 
it  will  be  applied  in  the  6-bev  Harvard- 
M.I.T.  electron  synchrotron. 

Not  to  be  outdone  by  CERN  and 
Brookhaven,  the  U.S.S.R.  has  announced 
that  it  will  build  a  50-bev  strong-focus- 
ing proton  synchrotron.  The  magnet  will 
weigh  about  22,000  tons  and  will  have 
a  diameter  of  1,500  feet.  It  would  seem 
that  whatever  we  do,  our  Soviet  friends 


can  do  too— and  with  a  factor  of  two  in 
their  favor. 

"FFAG" 

The  most  exciting  recent  development 
in  this  country  has  been  the  "fixed-field 
alternating-gradient"  accelerator  pro- 
posed by  Keith  R.  Symon  of  the  Mid- 
western Universities  Research  Associa- 
tion (MURA).  The  so-called  FFAG 
machine  is  really  a  rococo  cyclotron  in 
which  the  magnetic  field  is  shaped  in 
such  a  way  as  to  allow  the  cyclotron  to 
work  into  the  high-energy  relativistic  re- 
gion. We  have  already  seen  how  the 
ordinary  cyclotron  is  limited  to  acceler- 
ating protons  to  about  20  mev.  When 
this  hmitation  was  first  pointed  out  in 
1938,  L.  H.  Thomas  of  the  Ohio  State 
University  suggested  a  "way  to  get 
around  it.  He  proposed  to  scallop  the 
pole  tips  of  the  cyclotron  magnet  so  that 
the  surfaces  would  consist  of  a  series  of 
ridges  running  out  from  the  center,  with 
valleys  in  between.  Thomas  showed  that 
the  strength  of  the  resulting  field  would 
increase  toward  the  outside,  compensat- 
ing for  the  protons'  relativistic  increase 
in  mass,  and  would  also  focus  the  pro- 
tons so  that  they  would  stay  in  the 
vacuum  chamber.  Thomas's  scheme  was 
far  too  complicated  for  the  techniques 
of  the  time,  and  it  was  ignored.  Now  we 
realize  that  he  had  anticipated  the 
strong-focusing  principle.  Two  Thomas- 
type  cyclotrons  are  now  under  construc- 
tion, one  at  Oak  Ridge  National  Labora- 


tory, the  other  at  Berkeley.  Both  of  them 
will  produce  protons  and  deuterons  in 
the  range  of  several  hundred  mev. 

We  can  now  understand  an  FFAG 
type  of  accelerator  if  we  imagine  that 
the  radial  scallops  of  the  Thomas  mag- 
net are  twisted  into  spiral  ribs.  (Is  this 
the  flamboyant  style  that  presaged  the 
end  of  the  Gothic  period?)  The  twisting 
introduces  an  additional  kind  of  strong 
focusing.  In  fact,  the  idea  grew  out  of 
strong  focusing;  only  later  was  its  sim- 
ilarity to  the  Thomas  cyclotron  recog- 
nized. The  idea  of  FFAG  has  been  ex- 
ploited to  the  full  by  the  workers  of  the 
MURA  laboratory  at  Madison,  Wis.  They 
have  imagined  and  computed  (using 
two  high-speed  computing  machines )  all 
sorts  of  variations  of  the  FFAG  geome- 
try, and  have  built  several  models  that 
have  successfully  demonstrated  the  prac- 
ticality of  the  scheme. 

The  advantage  of  the  fixed-field  de- 
sign is  twofold.  First,  it  is  easier  to  con- 
trol a  constant  field  than  a  varying  one. 
Second,  the  fixed-field  machines  can  be 
operated  continuously,  whereas  the  syn- 
chrotrons and  synchro-cyclotrons  must 
operate  cyclically,  or  in  pulses,  a  new 
cycle  starting  each  time  the  field  reaches 
its  maximum  value.  Continuous  opera- 
tion means  that  more  accelerated  ions 
are  produced  per  unit  time;  in  other 
words,  the  beam  has  a  higher  intensity. 

According  to  the  MURA  workers,  the 
increased  intensity  that  can  be  obtained 
with  FFAG  machines  will  make  it  pos- 
sible to  circumvent  a  serious  limitation 


->    <- 


<— >► 


<> 


USEFUL  ENERGY  in  a  collision  depends  on  the  motion  of  the  particles  after  impact.  Solid 
arrows  at  left  represent  energy  of  motion  of  bombarding  particles.  Solid  arrows  at  right 
show  energy  of  motion  of  the  system  after  impact.  Broken  arrows  indicate  fraction  of  total 
energy  available  for  desired  reactions.  Small  dots  are  light  particles;  large  dots,  heavy  ones. 
When  like  particles  are  made  to  collide  head-on  (bottom),  all  of  their  energy  is  available. 


73 


on  accelerators  which  I  have  not  men- 
tioned as  yet.  This  hmitation  concerns 
the  amount  of  energy  that  is  actually 
available  to  produce  the  reactions  we  are 
looking  for. 

When  a  high-energy  ion  from  an  ac- 
celerator strikes  a  stationary  target  j)ar- 
ticle,  part  of  the  energy  goes  into  moving 
the  target,  and  is  wasted.  It  is  as  if  we 
were  trying  to  break  a  stone  by  )iitting 
it  with  a  hammer.  To  the  extent  that  the 
hammer  blow  simply  moves  the  stone, 
the  energy  is  not  available  for  breaking 
it.  Now  if  the  hammer  is  very  light  and 
the  stone  very  heavy,  we  can  see  that 
the  target  will  not  move  very  far;  almost 
all  the  energy  of  the  hammer  will  go 
into  breaking  or  chipping  the  stone.  If 
we  use  a  heavy  sledge  on  a  light  pebble, 
most  of  the  energy  goes  into  moving  the 
stone,  and  very  little  of  it  is  available 
for  breaking  the  stone.  If  the  hammer 
and  stone  weigh  the  same,  they  will  tend 
to  move  off  together  with  half  the  speed 
of  the  incoming  hammer;  exactly  half 
the  energy  will  be  available  for  break- 
ing the  stone. 

It  is  the  same  with  atom-smashing. 
But  here  relativity  plays  a  particularly 
dirty  trick,  robbing  us  of  nr^ost  of  the 
advantage  to  be  gained  by  increasing 
the  energy  of  the  bombarding  particles. 
We  have  seen  that  really  high  energies 
mean  an  increase  in  mass.  Thus  as  we 
go  up  in  energy  we  increase  the  weight 
of  our  "hammer"  and  lose  a  larger  and 
larger  fraction  of  its  energy.  At  1  bev 
a  proton  is  already  noticeably  heavier 
than  when  it  is  at  rest;  when  it  hits  a 
stationary  proton,  57  per  cent  of  the 
energy  is  wasted  and  only  .43  bev  is 
available  for  useful  purposes.  At  3  bev 
(the  energy  of  the  Brookhaven  Cosmo- 
tron),  the  available  portion  is  1.15  bev; 
at  6  bev  (the  Berkeley  Bevatron)  the 
available  portion  is  2  bev;  at  10  bev, 
2.9  bev  are  available;  at  50  bev,  7.5;  at 
100  bev,  10.5.  We  see  that  increasing 
the  energy  100  times  from  one  to  100 
bev  results  in  only  a  20-fold  actual  gain. 

Suppose,  however,  that  instead  of  fir- 
ing a  moving  particle  at  a  stationary 
one,  we  arrange  a  head-on  collision  be- 
tween two  high-energy  particles.  Then 
the  mass  increase  is  neutralized,  and 
there  is  no  tendency  for  the  colliding 
particles  to  move  one  way  or  the  other. 
All  the  energy  of  both  of  them  is  now 
available  for  the  desired  reactions.  This 
is  what  the  MURA  designers  propose. 

They  have  envisaged  a  bold  design, 
called  "synchroclash,"  in  which  two  15- 
bev  accelerators  are  placed  so  that  their 
proton  beams  intersect  and  the  particles 
collide  with  each  other.  This  will  yield 
an  available  energy  of  30  bev,  whereas 


in  the  case  of  a  30-bev  proton  colliding 
with  a  proton  at  rest  only  6  bev  would 
be  available.  In  fact,  to  attain  a  useful 
energy  of  30  bev  in  the  ordinary  way 
would  mean  using  at  least  500  bev.  The 
success  of  the  synchroclash  idea  turns  on 
the  intensity  of  the  accelerator  beams: 
there  must  be  enough  protons  to  make 
collisions  reasonably  frequent.  The 
MURA  proposal  languished  for,  several 
years,  but  interest  in  it  seems  to  have 
revived.  Perhaps  the  complicated  orbits 
of  the  artificial  satellites  have  had  some- 
thing to  do  with  the  new  willingness  to 
consider  attempting  the  complicated 
orbits  of  FFAG. 

Soviet  Ideas 

The  Soviet  designers  have  gone  off  in 
different  directions.  Veksler  has  been 
thinking  of  a  scheme  in  which  one  ap- 
proaches the  ideal  accelerator,  namely 
one  in  which  the  accelerating  field  ap- 
pears exactly  in  the  vicinity  of  the  ions 
but  nowhere  else.  He  envisages  a  small 
bunch  of  ions  in  a  plasma  (a  gas  of 
ions)  exciting  oscillations  or  waves  in 
an  electron  beam.  These  waves  are  to 
act  together  coherently  to  give  an  enor- 
mous push  to  the  ions  being  accelerated. 
If  this  is  not  clear  to  the  reader,  it  is 


because  it  is  not  clear  to  me.  The  details 
have  managed  to  escape  most  of  us  be- 
cause of  a  linguistic  ferrous  curtain,  but 
Veksler  speaks  of  the  theoretical  possi- 
bility of  attaining  energies  up  to  1,000 
bev.  The  proof  of  the  idea  must  wait 
until  it  is  put  into  practice.  It  should  be 
remarked,  however,  that  other  wild 
schemes  of  Veksler,  for  example  the 
synchrotron  principle,  are  incorporated 
into  most  of  our  conventional  accelera- 
tors today. 

G.  I.  Budker  of  the  U.S.S.R.  has  also 
presented  some  speculative  ideas  which 
have  obviously  been  inspired  by  efforts 
to  produce  controlled  thermonuclear  re- 
actions. Budker  proposes  an  intense  cir- 
cular electron  beam  maintained  by  a 
weak  magnetic  guide  field.  The  high 
current  of  the  beam  will  cause  it  to 
"pinch"  to  a  very  small  diameter  be- 
cause of  its  own  magnetic  field.  The  idea 
then  is  to  use  the  very  strong  local  mag- 
netic field  around  the  pinched  beam  as 
the  guide  field  of  a  conventional  accel- 
erator [see  diagram  on  page  13].  With 
an  electron  beam  six  meters  in  diameter 
one  could  expect  to  hold  protons  with 
an  energy  as  high  as  100  bev.  Budker 
and  his  colleagues  have  constructed  a 
special  accelerator  in  which  they  have 
achieved  a  10-ampere  current  of  3-mev 


SYNCHROCLASH  design  would  set  two  accelerators  side  by  side  so  that  tlieir  beams  over- 
lapped. Head-on  collisions  between  particles  would  provide  the  maximum  of  useful  energy. 


74 


elections,  and  they  expect  to  attain 
much  higher  currents  and  energies  be- 
fore long.  It  could  well  be  that  some- 
thing really  revolutionary  will  come  out 
of  this  energetic  work. 

Our  own  thermonuclear  program  has 
inspired  research  on  very  strong  mag- 
netic fields  [see  "Strong  Magnetic 
Fields,"  by  Harold  P.  Furth  et  al.;  Sci- 
entific American,  February].  It  seems 
likely  that  this  development  will  find  an 
application  to  the  guidance  of  particles 
in  multi-bev  accelerators. 

Electron  Accelerators 

These  new  machines  we  have  been 
discussing  are  proton  accelerators,  but 
there  is  vigorous  activity  in  electron  ma- 
chines as  well.  We  have  already  men- 
tioned the  Harvard-M.I.T.  synchrotron 
which  will  attain  6  to  7.5  bev,  and  the 
half-dozen  other  smaller  machines  in  the 
billion-volt  range.  The  220-foot  linear 
electron  accelerator  at  Stanford  Univer- 
sity has  been  on  the  scene  for  some  time. 
Its  energy  has  steadily  increased  so  that 
it  may  now  be  used  in  experiments  at 
600  mev.  We  expect  td  welcome  it  to 
the  1-bev  club  before  long. 

The  linear  machine  is  significant  be- 
cause there  is  a  special  difficulty  in 
reaching  high  energy  with  electron  syn- 
chrotrons. When  electrons  are  made  to 
travel  on  a  curved  path  at  high  speeds 
they  give  off  strong  electromagnetic  ra- 
diation. The  effect  is  easily  visible  to  the 


naked  eye;  the  luminous  horizontal  beam 
on  the  cover  of  this  issue  of  Scientific 
American  is  synchrotron  radiation.  The 
difficulty  is  that  this  radiation  can  repre- 
sent a  substantial  loss  of  energy,  and  it 
increases  rapidly  as  the  energy  of  the 
machine  goes  up.  In  the  Harvard-M.I.T. 
synchrotron  the  amount  of  energy  ra- 
diated is  almost  prohibitive  (about  10 
mev  per  turn  at  7.5  bev).  To  reach 
higher  energies,  say  20  bev,  the  Stan- 
ford group  has  been  thinking  in  terms 
of  a  linear  accelerator,  which  does  not 
have  this  radiation  difficulty  because  its 
particles  do  not  move  in  a  circle.  Such  a 
machine  might  be  as  much  as  three 
miles  long. 

I  am  not  convinced  that  the  limit  of 
electron  synchrotrons  has  been  reached. 
Indeed,  it  is  not  difficult  to  imagine  a 
50-bev  electron  synchrotron.  The  radia- 
tion problem  would  be  solved  by  reduc- 
ing the  curvature  of  the  electron  beam, 
that  is,  by  increasing  its  radius  to,  say, 
half  a  mile.  I  believe  that  the  upper 
limit  of  the  electron  synchrotron  may  be 
as  high  as  100  mev. 

While  we  are  "thinking  big"  we 
should  not  forget  Enrico  Fermi's  pro- 
posal to  ring  the  earth  with  a  vacuum 
tube  and,  using  the  earth's  magnetic 
field,  obtain  100,000  bev.  For  that  mat- 
ter, now  that  artificial  satellites  are  com- 
monplace, we  might  put  up  a  ring  of 
satellites— each  containing  focusing  mag- 
nets, accelerators,  injectors  and  so  on— 
around  the  earth.  Something  like  a  mil- 


Particle  Accelerators 

lion  bev  could  be  expected  from  this 
accelerator,  which  we  might  as  well  call 
the  lunatron.  At  the  very  least  such  a 
device  will  eliminate  the  need  for  vac- 
uum pumps,  since  it  will  be  outside  the 
atmosphere. 

Villard  de  Honnecourt  and  later  Viol- 
let-le-Duc  have  left  us  detailed  accounts 
of  the  builders  of  cathedrals  and  of  their 
methods.  It  seems  to  be  pretty  much  the 
same  story  then  and  now.  The  designer 
of  the  cathedral  was  not  exactly  an  archi- 
tect, nor  is  the  designer  of  an  accelerator 
exactly  a  physicist.  Both  jobs  require  a 
fusion  of  science,  technology  and  art. 
The  designers  of  cathedrals  were  well 
acquainted  with  each  other;  the  homo- 
geneity of  their  work  in  different  coun- 
tries is  evidence  of  a  considerable  inter- 
change of  information.  The  homogeneity 
of  accelerator  design  demonstrates  the 
same  interchange  today.  Our  medieval 
predecessors  were  only  human;  one  gets 
the  definite  impression  that  they  were 
subject  to  petty  jealousies,  that  occa- 
sionally there  was  thievery  of  ideas, 
that  sometimes  their  motivation  was 
simply  to  impress  their  colleagues  or 
to  humiliate  their  competitors.  All  these 
human  traits  are  occasionally  displayed 
by  their  modern  counterparts.  But  one 
also  gets  a  strong  impression  of  the  ex- 
citement of  those  mighty  medieval  cre- 
ators as  they  exulted  in  their  achieve- 
ments. This  sense  of  excitement  is  no 
less  intense  among  modern  nuclear 
physicists. 


PINCH  EFFECT  might  be  used  to  provide  a  magnetic  guiding 
field  for  an  accelerator,  thus  eliminating  the  heavy  magnet.  The 


dotted  ring  is  a  pinched  plasma.  Its  magnetic  field,  which  is  shown 
by  colored  lines,  would  act  to  hold  particles  near  its  outer  edge. 

75 


HUGE  PROTON  SYNCHROTRON  under  construction  at  Brook-  tunnel  housing  its  doughnut  is  840  feet  in  diameter.  This  machine 

haven  National  Laboratory  is  photographed  from  the  air.  Circular  will  produce  particles  of  25  to  30  billion  electron  volts   (bev). 


76 


9      The  Cyclotron  As  Seen  By. . . 


David  L.  Judd  and  Ronald  G.  MacKenzie  of  the  Lawrence  Radiation 
Laboratory,  University  of  California,  Berkeley 

The  cartoons  were  prepared  to  accompany  Dr.  Judd's  keynote 
address  at  the  International  Conference  on  Isochronous  Cyclotrons 
at  Gatlinburg,  Tennessee,  May  1966. 


The  Cyclotron  as  seen  by  the  inventor 


The  Cyclotron  as  seen  by  the  Mechanical  Engineer 


I 


77 


tMNTS:D^3f 


The  Cyclotron  as  seen  by  the  Electrical  Engineer 


The  Cyclotron  as  seen  by  the  operator 


78 


The  Cyclotron  As  Seen  By. 


i.Jti^ 


The  Cyclotron  as  seen  by  the  Theoretical  Physicist 


The  Cyclotron  as  seen  by  the  Visitor 


79 


The  Cyclotron  as  seen  by  the  Health  Physicist 


^ZIl=-   p- J7.'?+5O67:.0OO23  Al£/ 
0  O3i<0.05  O^ 
^C  00007S  ">  'Ad 


The  Cyclotron  as  seen  by  the  Experimental  Physicist 


80 


The  Cyclotron  As  Seen  By. 


The  Cyclotron  as  seen  by  the  Laboratory  Director 


The  Cyclotron  as  seen  by  the  Government  Funding 
Agency 


81 


The  Cyclotron  as  seen  by  the  student 


82 


CERN  (Conseil  European  pour  la  Recherche  Nucl^aire) 
is  an  installation  created  to  pool  the  finances  and  talents 
of  many  European  nations.    Physicists  come  there  from 
all  over  the  world  to  work  together  in  high -energy 
physics  research, 

10  CERN 

Jeremy  Bernstein 


Article  published  originally  in  The  New  Yorker  in  1964. 


SHORTLY  after  the  Second  World 
War,  when  the  normal  interna- 
tional life  of  science  was  resumed, 
a  physicist  who  had  just  listened  to  sev- 
eral hours  of  technical  lectures  at  a 
large  conference  remarked  that  the  in- 
ternational language  of  physics  had  be- 
come a  combination  of  mathematics 
and  broken  English:  Today,  almost  all 
scientific  journals,  including  the  Rus- 
sian— and  even  the  Chinese  journals, 
such  as  the  Acta  Mnthematica  Sinica, 
and  Sctrntia  Sinica,  published  in  Pe- 
king— give  at  least  the  title  of  each 
article,  and  often  an  abstract,  in  Eng- 
lish. From  the  title  and  the  equations 
and  the  graphs,  a  specialist  in  the  field 
can  usually  reconstruct  the  general 
theme  of  the  article.  The  exchange  of 
articles  and  journals  among  scientists  of 
different  countries  is  one  of  the  oldest 
and  best  traditions  of  science.  It  goes  on 
independently  of  the  political  climate. 
During  the  darkest  days  of  the  Stalinist 
period  in  Russia,  scientific  papers  went 
back  and  forth  across  the  Iron  Curtain, 
and  Western  physicists  could  follow  the 
work  of  such  Russians  as  Lev  Landau 
(the  most  distinguished  Russian  theoret- 
ical physicist,  who  won  the  Nobel  Prize 
in  1962),  despite  the  fact  that  he  was 
under  house  arrest  in  Moscow,  in  part 
because  of  his  liberal  ideas  and  in  part 
because  he  is  a  Jew. 

With  the  death  of  Stalin  and  the 
relaxation  of  some  of  the  tensions  be- 
tween East  and  West,  it  became  pos- 
sible for  scientists  to  travel  in  and  out 
of  the  Eastern  countries.  The  so- 
called  Rochester  Conference  in  High- 
Energy  Physics  (it  gets  its  name  from 
the  fact  that  the  first  seven  conferences, 
starting  in  1950,  were  held  in  Roches- 
ter, New  York)  now  meets  one  year 
in  the  United  States,  one  year  in  Ge- 
neva, and  one  year — indeed,  last  sum- 
mer— in  the  Soviet  Union.  Several 
American  universities  have  regular  ex- 
change programs  with  Soviet  univer- 
sities, and  it  is  no  longer  a  novelty  to 
find  a  Russian  physicist  giving  a  series 


of  lectures  m  an  American  university, 
and  vice  versa. 

The  ultimate  in  international  scien- 
tific cooperation  is,  of  course,  the  inter- 
national scientific  laboratory,  in  which 
scientists  of  many  countries  can  actually 
work  together.  In  fact,  it  is  becoming 
increasingly  clear  that  such  laboratories 
are  not  only  desirable  but  necessary. 
Research  in  a  field  like  high-energy 
piiysics — in  a  way,  the  rtiost  basic  of 
all  the  sciences,  since  it  is  the  study 
of  elementary  particles,  the  ultimate 
constituents  of  all  matter — has  become 
so  expensive  that  many  people  have 
come  to  believe  that  pursuing  it  as 
a  purely  national  enterprise  is  difficult 
to  justify.  A  recent  editorial  in  the 
New  York  Times  pointed  out  that 
"high-energy  physicists  ...  use  the  most 
elaborate  and  most  expensive  equip- 
ment employed  in  any  branch  of  ter- 
restrial basic  research,"  and  went  on  to 
Say,  "These  are  the  particle  accelera- 
tors, which  today  cost  tens  of  millions 
of  dollars  each,  and  which  will  in  the 
future  be  priced  in  the  hundreds  of  mil- 
lions. The  Atomic  Energy  Commis- 
sion's operating  and  construction  costs 
in  this  field  are  already  expected  to  ag- 
gregate $165  million  in  the  next  fiscal 
year,  and  one  authoritative  estimate 
places  the  annual  bill  by  the  end  of  the 
next  decade  at  $370  million,  reaching 

$600    million     by     1980 Nuclear 

physicists  are  already  talking  about  far 
more  powerful — and  much  more  ex- 
pensive— atomic-research  instruments. 
The  case  for  building  these  machines 
is  an  impressive  one,  but  the  case  for 
building  them  only  with  the  resources 
of  one  country  is  not  convincing." 

The  editorial  concluded  by  pointing 
out  that  there  already  exists  an  excellent 
working  example  of  an  international 
atomic  laboratory;  namely,  CERN 
(standing  for  Conseil  Europeen  pour 
la  Recherche  Nucleaire),  which  is  op- 
erated jointly  by  almost  all  the  Western 
European  countries  and  is  situated  in 
the  Swiss  town  of  Meyrin,  a  suburb  of 


83 


Geneva  that  is  almost  on  the  French 
frontier,  CERN  itself  sprawls  along  the 
frontier,  and  recently,  when  it  needed 
room  for  expansion,  the  French  gov- 
ernment gave  it  a  ninety-nine-year 
lease  on  a  hundred  acres  of  French 
land,  matching  the  hundred  acres  of 
Swiss  territory  that  the  center  now  oc- 
cupies. This  makes  CERN  the  only  in- 
ternational organization  that  actually 
straddles  a  frontier.  Its  facilities  include 
two  accelerators  (the  larger,  a  proton 
synchrotron,  accelerates  protons  to  en- 
ergies up  to  twenty-eight  billion  elec- 
tron volts,  and  shares  with  its  slightly 
more  powerful  twin,  the  alternating- 
gradient  synchrotron  at  the  Brookha- 
ven  National  Laboratory,  on  Long 
Island,  the  distinction  of  being  the  larg- 
est accelerator  now  operating),  several 
electronic  computers,  and  a  vast  collec- 
tion of  bubble  chambers,  spark  cham- 
bers, and  other  parapnernalia  necessary 
for  experimenting  with  the  particles 
produced  in  the  accelerators — to  say 
nothing  of  machine  shops,  a  cafeteria,  a 
bank,  a  travel  agency,  a  post  office,  a 
large  library,  and  a  multitude  of  secre- 
tarial and  administrative  offices.  It  costs 
about  twenty-five  million  dollars  a  year 
to  run.  This  money  is  contributed  by 
thirteen  European  member  states — 
Austria,  Belgium,  Great  Britain,  Den- 
mark, France,  Greece,  Italy,  the  Neth- 
erlands, Norway,  Spain,  Sweden,  Swit- 
zerland, and  West  Germany.  Neither 
the  United  States  nor  Russia  is  eligible 
to  become  a  member,  since  neither  is 
"Europeen,"  but  there  are  Americans 
and  Russians  who  work  at  CERN,  An 
exchange  agreement  exists  between 
CERN  and  DUBNA,  a  similar  laboratory 
near  Moscow,  where  physicists  from 
the  Iron  Curtain  countries  and  China 
work  together.  Each  year,  DUBNA 
sends  two  or  three  physicists  to  CERN 
for  several  months  at  a  time.  American 
physicists  at  CERN  have  been  supported 
by  sabbatical  salaries,  by  fellowships  like 
the  Guggenheim  and  the  National  Sci- 
ence Foundation,  or  by  money  from 
Ford  Foundation  grants  (totalling  a 
bit  over  a  million  dollars)  that  were 
given  to  the  laboratory  explicitly  for 
the  support  of  scientists  from  non- 
member  countries.  (The  grants  have 
now  been  discontinued,  following  the 


Ford  policy  of  "pump-priming,"  and 
the  laboratory  is  looking  for  other 
sources  of  money.)  There  are  usually 
twenty  or  twenty-five  Americans  at 
CERN,  In  addition,  the  laboratory  has 
contingents  of  Japanese,  Indians,  Poles 
(a  very  active  and  scientifically  strong 
group  of  about  a  dozen),  Yugoslavs, 
Turks,  Israelis  (there  is  an  exchange 
agreement  with  the  Weizmann  Insti- 
tute, in  Rehovoth),  and  Hungarians, 
All  the  permanent  personnel  at  CERN — 
about  sixteen  hundred  people,  of  whom 
about  three  hundred  are  physicists  and 
engineers — are  drawn  from  the  mem- 
ber states,  (Their  average  age  is  thirty- 
two.)  As  one  might  imagine,  all  this 
produces  a  tutti-frutti  of  languages,  na- 
tional types,  political  attitudes,  and 
social  mannerisms,  and  everyone  ac- 
cepts ind  enjoys  the  chaos  of  national 
flavors  as  part  of  the  working  atmos- 
phere of  the  laboratory.  As  an  Ameri- 
can physicist  and  a  perennial  summer 
visitor  to  CERN,  I  have  had  fairly  typi- 
cal experiences  there.  This  past  sum- 
mer, I  worked  with  an  Italian  physi- 
cist in  an  attempt  to  extend  some  work 
done  by  a  German-born  American 
physicist  who  was  visiting  the  labora- 
tory on  a  Guggenheim  Fellowship. 
This  work  was  itself  an  extension  of 
another  Italian  physicist's  work,  which, 
in  turn,  was  based  on  the  work  of  an 
American  physicist  who  is  a  frequent 
visitor  to  CERN,  (I  also  helped  a  Yugo- 
slav physicist  with  the  English  trans- 
lation of  a  short  book  written  by  a  well- 
known  Russian  physicist  whom  I  met 
when  he  visited  CERN  to  attend  the 
Rochester  Conference  of  1962,  which 
was  held  in  Geneva,)  My  working 
language  with  the  Italian  physicist  was 
English  (and,  of  course,  mathematics). 
Most  of  the  people  at  the  laboratory 
are  polylingual.  All  scientific  lectures 
are  given  in  English,  and  almost  all 
the  technical  personnel  have  a  good 
command  of  the  language.  However, 
the  language  one  hears  most  often  is 
French;  the  secretaries,  postmen,  bank 
clerks,  mechanics,  and  telephone  opera- 
tors speak  it  among  themselves,  and  so 
'  do  many  of  the  European  physicists.  Sec- 
retaries must  be  able  to  type  technical 
manuscripts  in  English,  since  almost  all 
the  publications  that  come  out  of  CERN 


84 


CERN 


each  year  (several  hundred  of  them) 
are  in  that  language. 

Because  nuclear  physics  has  become 
so  closely  associated  (at  least  in  the  pub- 
lic mind)  with  its  military  applications, 
many  people  have  wondered  how  a 
laboratory  that  intermingles  physicists 
from  the  East  and  the  West — and, 
indeed,  from  all  over  the  world — can 
possibly  operate  without  running  into 
all  sorts  of  problems  of  military  security 
and  national  secrecy.  The  answer  is  that 
nuclear  physics  is  a  very  broad  subject. 
It  ranges  from  the  study  of  nuclear 
energy — fission,  fusion,  reactors,  and 
the  like — to  the  study  of  the  interior 
structure  of  the  nucleus,  and  even  to 
the  study  of  the  structure  of  the  very 
neutrons  and  protons  and  other  parti- 
cles that  compose  the  nucleus.  This 
latter  study  is  the  frontier  of  modern 
physics.  Because  high-energy  particles 
are  necessary  in  order  to  probe  deeply 
into  the  interior  of  the  nucleus,  this 
branch  of  physics  is  called  "high-ener- 
gy," as  opposed  to  "low-energy,"  or 
"classical" — "classical"  in  that  the  laws 
governing  the  behavior  of  the  nuclei  in, 
say,  the  fission  process  in  a  reactor  are 
now  pretty  well  understood,  and  have 
been  for  some  time.  The  military  and 
technological  applications  of  nuclear 
physics  are  based  on  these  latter  laws, 
whereas  the  study  of  the  interior  struc- 
ture of  the  nucleus  has  no  technological 
applications  at  present ;  more  than  that, 
it  is  difficult  now  to  imagine  any  such 
applications  in  the  future.  However,  the 
example  of  Einstein's  special  theo/y  of 
relativity — one  of  the  most  abstract  the- 
ories in  physics — which  has  been  the 
basis  of  the  entire  development  of  nu- 
clear energy,  shows  that  theoretical 
speculations  that  may  at  the  momeiM 
seem  far  removed  from  reality  can  very 
quickly  change  all  of  technology. 

THE  very  fact  that  high-energy 
physics  does  not  have  military 
applications  was  among  the  reasons  it 
was  chosen  as  the  discipline  for  an 
international  laboratory.  In  the  late 
nineteen-forties,  when  a  number  of 
prominent  physicists — including  the  late 
H.  A.  Kramers,  of  Holland;  Pierre 
Auger  and  Francis  Perrin,  of  France; 
Edouardo   Amaldi,    of   Italy;    and   J. 


Robert  Oppenheimer,  of  the  United 
States — began  informally  discussing  the 
prospects  for  creating  an  international 
laboratory  in  Europe,  they  set  out  to 
look  for  a  field  that  would  be  sufficiently 
close  to  recent  developments  in  atomic 
energy  for  European  governments  to 
be  interested  in  supporting  the  project 
financially,  and  yet  far  enough  removed 
from  immediate  applications  of  atomic 
energy  for  military  security  not  to  be  a 
problem.  They  also  realized  that  it 
would  be  necessary  to  engage  the  sup- 
port of  the  European  diplomats  who 
were  then  promoting  attempts  to  create 
a  United  Europe.  One  of  the  most  in- 
fluential of  these  diplomats  was  Fran- 
cois de  Rose,  of  France.  (He  is  now 
the  French  Ambassador  to  Portugal.) 
De  Rose  became  interested  in  the 
possibilities  of  atomic  research  just  after 
the  war,  and  in  1946  he  met  with 
Oppenheimer  in  New  York  at  the 
United  Nations  Atomic  Energy  Com- 
mission. Out  of  the  resulting  friend- 
ship between  the  two  men  an  important 
link  developed  between  the  scientific 
and  diplomatic  communities.  Dr.  L. 
Kowarski,  a  French  nuclear  scientist 
and  one  of  the  pioneers  of  CERN,  has 
written  a  semi-official  history  of  the 
origins  of  the  laboratory,  in  which  he 
notes: 

The  first  public  manifestation  of  this 
new  link  occurred  in  December,  1949,  at 
the  European  Cultural  Conference  held 
in  Lausanne.  A  message  from  Louis  de 
Broglie  [de  Broglie,  the  most  distin- 
guished French  theoretical  physicist  of 
modern  times,  was  awarded  the  Nobel 
Prize  in  1929  for  his  work  on  the  wave 
nature  of  electrons]  was  read  by  Dautry 
[Raoul  Dautry  was  at  that  time  the  ad- 
ministrator of  the  French  Atomic  Energy 
Commission  and  one  of  the  leaders  of  the 
movement  for  a  United  Europe],  in  which 
the  proposal  was  made  to  create  in  Europe 
an  international  research  institution,  to  be 
equipped  on  a  financial  scale  transcending 
the  individual  possibilities  of  the  member 
nations. ...  At  that  time  [a  dilemma]  was 
besetting  the  scientists'  aspirations:  atomic 
energy  was  attracting  public  readiness  to 
spend  money,  but  atomic  energy  invited 
security-mindedncss  and  separatism.  The 
way  out  of  the  dilemma  was  clear  enough. 
The  domain  of  common  action  should  be 
chosen  so  as  not  to  infringe  directly  the 
taboos  on  uranium  fission,  but  [to  be] 
close  enough  to  it  so  as  to  allow  any  suc- 
cesses gained  internationally  in  the  per- 


85 


mitted  field  to  exert  a  beneficial  infTuence 
on  the  national  pursuits. 

The  ultimate  choice — high-energy 
physics — was  a  perfect  compromise; 
although  it  is  a  branch  of  nuclear 
physics,  it  is  one  that  is  far  removed 
from  military  applications. 

In  June  of  1950,  the  American 
physicist  I.  I.  Rabi  initiated  the  first 
practical  step  toward  the  creation  of 
such  a  pan-European  laboratory.  As  a 
member  of  the  United  States  delegation 
to  UNESCO  he  was  attending  the 
UNESCO  conference  held  that  year  in 
Florence.  Speaking  officially  on  behali 
of  the  United  States,  he  moved  that 
UNESCO  use  its  good  offices  to  set  up  a 
physics  laboratory  (he  had  high-energy 
physics  in  mind)  with  facilities  thai 
would  be  beyond  those  that  any  single 
European  country  could  provide,  and 
that  would  be  comparable  to  the  major 
American  facilities  at  Brookhaven  and 
Berkeley.  It  was  an  important  step,  be- 
cause it  placed  the  prestige  and  influence 
of  American  science  behind  the  project. 
The  implementation  of  Rabi's  motion 
became  the  work  of  Pierre  Auger,  of 
France,  a  distinguished  physicist  who 
was  the  UNESCO  scientific  director.  As 
a  result  of  his  efforts,  various  cultural 
commissions  of  the  French,  Italian,  and 
Belgian  governments  donated  about  ten 
thousand  dollars  for  a  study  program, 
and  CERN  was  under  way.  (In  the 
course  of  the  discussions  held  at  that 
time,  Rabi  stressed  the  desirability  of 
not  having  any  nuclear  reactors  at 
CERN,  since  they  have  both  military 
and  commercial  applications — and,  in 
fact,  there  are  none.)  Dr.  Kowarski 
writes: 

Two  objectives  were  suggested:  a 
longer-range,  very  ambitious  project  of 
an  accelerator  second  to  none  in  the  world 
[this  resulted  in  the  construction  of  the 
proton  synchrotron,  which  was  completed 
in  1959]  and,  in  addition,  the  speedy  con- 
struction of  a  less  powerful  and  more 
classical  machine  in  order  to  start  Euro- 
pean experimentation  in  high-energy  phys- 
ics at  an  early  date  and  so  cement  the 
European  unity  directed  to  a  more  diffi- 
cult principal  undertaking. 

At  the  end  of  1 95 1 ,  an  organization- 
al meeting  was  held  in  Paris;  all  the 


European  members  of  UNESCO  were 
invited,  but  there  was  no  response  from 
the  countries  of  Eastern  Europe.  Then, 
at  a  meeting  held  in  Geneva  early  in 
1952,  eleven  countries  signed  an  agree- 
ment pledging  funds  and  establishing 
a  provisional  organization.  There  was 
something  of  a  tug-of-war  among  the 
member  countries  to  decide  where 
the  new  laboratory  should  be  built.  The 
Danes,  the  Dutch,  the  French,  and  the 
Swiss  all  had  suitable  territory  for  it,  but 
in  the  end  Geneva  was  chosen,  partly 
because  of  its  central  location,  partly 
because  of  its  long  tradition  of  housing 
international  organizations  (there  are, 
for  example,  all  sorts  of  multilingual 
elementary  schools  in  the  city) — and, 
it  is  said,  partly  because  some  of  the 
physicists  involved  in  the  decision  were 
avid  skiers.  The  Swiss  government  gave, 
free,  the  site  near  Meyrin,  and  in 
June,  1953,  the  Canton  of  Geneva 
formally  ratified,  by  popular  referen- 
dum, the  government's  invitation  to 
CERN  to  settle  there;  in  addition,  the 
laboratory  was  given  the  same  polit- 
ical status  as  that  of  any  of  the  other 
international  organizations  in  Geneva. 
At  the  same  time,  a  formal  CERN  Con- 
vention was  prepared  for  the  signature 
of  the  member  states,  which  then  num- 
bered twelve;  Austria  and  Spain  joined 
later,  and  Yugoslavia,  an  original  sig- 
natory, withdrew  in  1962,  because  of 
a  lack  of  foreign  currency.  Article  II 
of  the  Convention  stipulates:  "The 
Organization  shall  provide  for  collabo- 
ration among  European  States  in  nu- 
clear research  of  a  pure  scientific  and 
fundamental  character,  and  in  researcii 
essentially  related  thereto.  The  Organi- 
zation shall  have  no  concern  with  work 
for  military  requirements,  and  the  re- 
sults of  its  experimental  and  theoretical 
work  shall  be  published  or  otherwise 
made  generally  available."  The  Con- 
vention also  set  up  a  formula  for  CERN's 
financial  support.  Roughly  speaking, 
each  member  nation  pays  each  year 
a  certain  percentage  (a  fraction  of  one 
per  cent)  of  its  gross  national  prod- 
uct. This  means,  in  practice,  that  Great 
Britain,  France,  and  West  Germany 
pay  the  largest  shares.  The  CERN  Coun- 
cil, the  governing  body  of  the  labora- 
tory,  was  set   up,   with    two   delegates 


86 


CERN 


from  each  country — one  a  scientist  and 
the  other  a  diplomat,  hke  de  Rose.  The 
Council  meets  twice  a  year  to  pass  on 
such  matters  as  the  budget  and  the  fu- 
ture development  of  the  laboratory. 
(During  my  last  visit  to  CERN,  there 
was  a  Council  meeting  in  which  the 
question  of  constructing  a  still  larger 
international  machine — a  machine  ca- 
pable of  accelerating  protons  to  three 
hundred  billion  electron  volts,  or  about 
ten  times  the  capacity  of  the  present 
machine — was  discussed.)  The  Coun- 
cil also,  by  a  two-thirds  majority,  ap- 
points the  Director-General  of  the  lab- 
oratory. The  Director-Generalship  of 
CERN  is  a  very  complex  job,  and  few 
people  are  really  qualified  for  it.  In 
the  first  place,  the  Director-General 
can  have  no  special  national  bias.  As 
the  Convention  puts  it,  "The  responsi- 
bilities of  the  Director  and  the  staflF  in 
regard  to  the  Organization  shall  be  ex- 
clusively international  in  character."  In 
the  second  place,  the  Director  must 
clearly  be  a  physicist,  for,  among  other 
things,  he  must  decide  which  of  vari- 
ous extremely  expensive  experiments 
the  laboratory  should  concentrate  on. 
The  first  Director,  chosen  in  1954,  was 
Professor  Felix  Bloch,  of  Stanford  Uni- 
versity— a  Swiss  by  origin  and  a  Nobel 
Prize  winner  in  physics.  Professor  Bloch 
returned  to  Stanford  in  1955  and  was 
succeeded  by  C.  J.  Bakker,  a  Dutch 
cyclotron  builder.  (Professor  Bakker 
was  responsible  for  the  construction  of 
the  cyclotron,  the  smaller  of  the  accel- 
erators at  CERN.)  He  held  the  post 
from  1955  to  1960,  when  he  was  killed 
in  an  airplane  accident  on  his  way  to 
Washington,  where  he  had  intended  to 
deliver  a  report  on  the  operation  of  the 
large  accelerator,  the  proton  synchro- 
tron, which  had  gone  into  operation  in 
1959. 

IF  any  one  individual  was  respon- 
sible for  the  successful  construction 
of  the  large  accelerator,  it  was  John  B. 
Adams,  an  Englishman,  who  took  over 
the  Director-Generalship  on  Bakker's 
death.  Adams  was  born  in  1920  in 
Kingston,  Surrey,  and  received  his  edu- 
cation in  English  grammar  schools.  At 
eighteen,  he  went  to  work  for  the  Tel- 
cconjmunications    Research    Establish- 


ment, and  when  the  war  broke  out  he 
joined   the    Ministry  of   Aircraft   Pro- 
duction. He  had  received  some  training 
in  electronics  with  the  Telecommunica- 
tions Establishment,  and  in  the  M.A.P. 
he  became  involved  with  the  problem 
of  installing  the   first  radar  in   fighter 
planes.  It  soon  became  evident  that  he 
had  a  gift  both  for  engineering  and  for 
the  complex  job  of  directing  a  large 
technical  project.  In  fact,  the  war  pro- 
duced  a    whole    generation    of   young 
scientists  and  engineers  who  not  only 
were    technically    competent    but    had 
acquired  considerable  practical  experi- 
ence in  running  large-scale  and  costly 
scientific  enterprises.  These  men  moved 
readily  into  the  various  atomic-energy 
programs  that  were  started   after  the 
war,  and   Adams  joined   the   nuclear 
laboratory    at    Harwell,    the    principal 
British   center   for  experimental    work 
in   nuclear  physics.   At   this  time,  the 
people    at     Harwell     were     beginning 
work  on  a  hundred-and-seventy-five- 
million-volt    proton    accelerator,    and 
Adams    became    an    important    mem- 
ber of  the  project.  The  machine  was 
finished    in    1949,    and    Adams   spent 
the   next  three   years  working  on   the 
design  of  special  radio  tubes  needed  in 
connection  with  accelerators.  Then  he 
was  released  by  the  Ministry  of  Supply 
to  go  to  Geneva  and  join  the  new  accel- 
erator project  at  CERN. 

By  that  time,  the  CERN  group,  which 
had  been  at  work  since  1951,  had  in- 
herited a  technological  windfall  in  the 
way  of  accelerator  design.  A  particle 
accelerator  can  accelerate  only  those 
particles  that  carry  an  electric  charge. 
Advantage  is  taken  of  the  fact  that 
when  a  charged  particle  passes  through 
an  electric  field  it  is  accelerated  by  the 
force  that  the  field  exerts  on  it.  In 
modern  accelerators,  transmitting  tubes 
generate  the  electromagnetic  fields,  in 
the  same  way  that  radio  transmitters 
generate  radio  waves.  These  acceler- 
ating stations  are  placed  at  intervals 
along  the  path  of  the  particles  in  the 
machine,  the  simplest  arrangement 
being  along  a  straight  line.  This  layout 
results  in  what  is  called  a  linear  acceler- 
ator, or  LINAC.  The  particles  move 
faster  and  faster  in  a  straight  line  and 


87 


are  finally  shot  out  the  other  end  into 
a  target  of  some  sort.  The  energy  that 
such  particles  can  acquire  is  limited  by 
the  length  of  the  straight  line,  as  well 
as  by  the  power  of  the  transmitters.  At 
Stanford  University,  there  is  a  near- 
ly completed  straight-line  accelerator, 
known  among  physicists  as  "the  mon- 
ster," that  will  accelerate  electrons  over 
a  path  almost  two  miles  long;  the 
emerging  electrons  will  have  an  energy 
of  about  twenty  billion  electron  volts. 
Most  accelerators,  however,  are  circu- 
lar. The  accelerating  stations  are 
arranged  along  the  perimeter,  and  as 
the  particles  go  around  and  around  they 
acquire  more  energy  in  each  orbit.  This 
arrangement  saves  space  and  greatly 
reduces  the  number  and  size  of  the  ac- 
celerating stations.  The  problem  that 
naturally  arises  is  how  to  maintain  the 
particles  in  circular  paths  while  they 
are  being  accelerated,  since  a  particle 
will  move  in  a  circle  only  if  a  force 
acts  on  it  to  keep  it  from  flying  off 
at  a  tangent.  In  circular  accelerators, 
this  force  is  supplied  by  electromagnets. 
The  magnets  are  deployed  along  the 
path  of  the  particles,  and  the  magnetic 
fields  they  produce  hold  the  particles  in 
orbit.  The  drawback  to  this  system  is 
that  the  more  energy  a  particle  acquires, 
the  more  strongly  it  resists  staying  in  a 
circular  orbit  and  the  larger  the  magnet 
required  to  keep  it  so.  In  fact,  as  the 
postwar  accelerators  became  more  and 
more  powerful,  the  size  of  their  mag- 
nets began  to  get  out  of  hand.  The 
Brookhaven  cosmotron,  a  proton  accel- 
erator producing  protons  with  an 
energy  of  three  billion  electron  volts, 
has  a  magnet  of  four  thousand  tons; 
the  Berkeley  bevatron,  with  six-billion- 
electron-volt  protons,  has  a  magnet 
weighing  ten  thousand  tons;  and,  most 
striking  of  all,  the  Russian  phasotron  at 
DUBNA,  which  produces  protons  of  ten 
billion  electron  volts,  has  a  magnet 
weighing  thirty-six  thousand  tons. 

This  was  where  things  stood  in 
1952,  when  the  CERN  group  planned 
to  make  an  accelerator  of  at  least  ten 
billion  electron  volts.  By  using  a  some- 
what modified  and  more  economical 
design  than  the  one  for  the  DUBNA 
machine,  the  new  accelerator  could 
have  been  made  with  a  magnet  weigh- 


ing from  ten  to  htteen  thousand  tons, 
but  even  this  seemed  monstrous  at  the 
time.  That  year,  however,  a  group  at 
Brookhaven  consisting  of  E.  Courant, 
M.  S.  Livingston,  and  H.  Snyder,  in  the 
course  of  solving  a  problem  put  to  them 
by  a  group  of  visiting  accelerator  ex- 
perts from  CERN,  invented  a  prin- 
ciple of  magnetic  focussing  that  altered 
the  situation  completely.  (It  turned 
out  later  that  their  method  had  been 
independently  invented  a  few  years 
earlier  by  an  American-born  Greek 
n  med  N.  Christofilos,  who  was  em- 
ployed in  Greece  selling  elevators  for 
an  American  firm  and  was  a  physicist 
in  his  spare  time.  Christofilos  had  sent 
a  manuscript  describing  his  invention  to 
Berkeley,  where  it  was  forgotten  until 
news  of  the  work  at  Brookhaven  re- 
minded somebody  of  it.  Christofilos  is 
now  at  the  Livcrmore  Laboratory  of 
the  University  of  California.)  The 
magnet  in  a  circular  accelerator  not 
onlybends  the  particle  trajectories  into 
circles  but  applies  a  force  that  focusses 
the  beam  and  keeps  it  from  spread- 
ing out  indefinitely  as  it  goes  around 
and  around.  The  magnets  in  the  old 
machines  could  supply  only  very  weak 
focussing;  thus  the  beam  was  pretty 
thick,  and  the  vacuum  pipe  it  circu- 
lated in  and  the  magnet  surrounding 
it  also  had  to  be  large.  (At  Berkeley, 
a  man  can  crawl  through  the  vacuum 
chamber.)  It  was  known,  however, 
that  magnets  could  be  made  that 
would  give  much  stronger  focussing 
forces,  but  only  in  one  direction  at  a 
time;  that  is,  if  the  beam  were  kept 
confined  horizontally  it  would  expand 
vertically,  and  vice  versa.  What  the 
Brookhaven  people  found  was  that  if 
an  accelerator  magnet  ring  was  built 
up  of  alternate  sections  that  provided 
strong  focussing  and  defocussing  forces, 
the  net  result  was  a  focussing  much 
stronger  than  anything  that  had  pre- 
viously been  achieved.  (In  the  new 
machines,  the  beam  can  be  contained 
in  a  vacuum  pipe  only  a  few  inches  in 
diameter.)  This  meant  that  the  mag- 
nets could  be  much  smaller  in  size,  with 
a  great  saving  of  weight,  power,  and 
cost.  The  CERN  magnetic  system 
weighs  only  three  thousand  tons,  al- 
though the  proton  energies  achieved  are 


88 


CERN 


nearly  three  times  those  generated  by 
the  old  Russian  machine,  which  had 
a  magnet  weighing  over  ten  times  as 
much.  The  focussing  works  so  well 
that  the  final  beam  of  particles,  which 
consists  of  about  a  thousand  billion  pro- 
tons per  second,  is  only  a  few  milli- 
metres wide  when  it  emerges  from  the 
machine.  The  ring  around  which  the 
protons  race  is  about  two  hundred 
metres  in  diameter.  The  protons  are  in- 
jected into  the  main  circular  track  by  a 
small  linear  accelerator,  and  in  the 
single  second  that  they  remain  in  the 
machine  they  make  about  half  a  million 
revolutions.  The  entire  ring  must  be 
kept  at  a  fairly  high  vacuum,  since 
otherwise  the  protons  would  knock 
about  in  the  air  and  be  scattered.  There 
is  also  a  delicate  question  of  timing.  The 
accelerating  fields  must  deliver  a  kick 
to  each  bunch  of  protons  at  just  the 
right  instant  in  its  orbit.  As  the  protons 
move  faster  and  faster,  approaching  the 
speed  of  light,  the  synchronization  of 
the  fields  and  the  particles  must  be  con- 
stantly changed.  However,  according  to 
Einstein's  special  theory  of  relativity,  no 
particle  can  go  faster  than  light,  so  that 
near  the  end  of  the  cycle  the  protons 
will  be  gaining  energy  but  not  speed 
(the  particles,  again  according  to  the 
relativity  theory,  get  heavier  and  heavi- 
er as  they  move  faster  and  faster), 
which  simplifies  the  timing  problem 
somewhat.  Indeed,  high-energy-accel- 
erator design,  which  uses  the  theory  of 
relativity  extensively,  and  which  clear- 
ly works,  is  one  of  the  best-known  tests 
of  the  theory  itself.  That  all  these  fac- 
tors, complex  as  they  are,  can  be  put  to- 
gether to  make  a  reliably  operating 
machine  is  an  enormous  triumph  of 
engineering  physics. 

Needless  to  say,  the  Brookhaven 
people  were  eager  to  build  a  machine 
operating  on  the  principle  they  had  in- 
vented. However,  the  cosmotron  had 
only  recently  been  finished,  and  they 
could  not  get  immediate  support  for 
the  construction  of  an  even  larger  ma- 
chine— especially  one  that  would  use 
a  principle  still  untested.  The  CERN 
people,  however,  were  in  a  much  more 
advantageous  position,  and  in  1953 
they  began  designing  the  laboratory  s 
present  machine,  the  CPS  (CERN  pro- 


ton synchrotron).  About  six  months 
later,  influenced  partly  by  the  progress 
at  CERN,  the  Brookhaven  people  got 
under  way  with  the  construction  of  a 
similar  but  slightly  larger  machine — 
the  AGS,  or  alternating-gradient  syn- 
chrotron. A  friendly  race  developed  be- 
tween the  two  groups,  with  CERN 
finishing  in  November,  1959,  and 
Brookhaven  about  six  months  later. 

In  order  to  construct  the  CERN  ac- 
celerator, Adams  gathered  around  him 
a  superb  international  team  of  engi- 
neers and  physicists  interested  in  accel- 
erator construction.  Not  only  is  he  a 
brilliant  engineer  himself  but  he  has 
the  ability  to  organize  other  engineers 
into  effective  groups  with  physicists,  so 
that  very  new  ideas  can  be  effectively 
realized  on  an  industrial  scale.  In  fact, 
working  on  the  accelerator  at  CERN 
came  to  be  a  considerable  distinction 
for  an  engineer,  and  CERN  got  almost 
the  pick  of  the  European  engineers, 
even  though  the  laboratory  could  not 
compete  financially  with  the  salaries 
that  were  being  offered  by  European 
industry.  The  machine  was  so  well  de- 
signed that  it  worked  better  than  had 
been  generally  anticipated.  It  became 
available  to  the  physicists  at  CERN  early 
in  1960,  and  Adams  stepped  into  the 
gap  caused  by  Bakker's  death  to  become 
Director-General  of  the  laboratory  for 
a  year.  He  also  received  an  honorary 
degree  from  the  University  of  Geneva, 
which  he  accepted  on  behalf  of  the 
group  that  had  worked  with  him.  He  is 
now  back  in  England  directing  a  labo- 
ratory that  is  studying  the  problem  of 
controlling  nuclear-fusion  energy  for 
general  application.  (Nuclear- fusion 
energy  arises  when  nuclear  particles  are 
fused  to  make  a  heavier  nucleus.  The 
heavier  nucleus  actually  weighs  less 
than  the  sum  of  its  parts,  and — again 
according  to  Einstein's  relativity  theo- 
ry— the  excess  weight  is  liberated  as 
energy.  The  hydrogen  bomb  is  an  un- 
fortunate application  of  this  principle.) 

THE  present  Director-General  of 
CERN  is  Professor  Victor  F. 
Weisskopf,  who  was  given  leave  of  ab- 
sence from  M.I.T.  to  take  over  from 
Adams  in  1961.  Professor  Weisskopf, 


I 


89 


whom  I  got  to  know  when  I  was  a  stu- 
dent at  Harvard  in  the  nineteen-fifties, 
was  born  in  Vienna,  so  although  he  is 
an  American  citizen,  he  can  be  counted 
as  a  European.  He  is  one  of  the  world's 
leading  theoretical  physicists,  as  well  as 
one  of  its  most  likable.  A  large,  friendly 
man,  he  is  known  to  almost  everybody 
at  CERN  as  Viki,  and  despite  a  recent 
and  very  serious  automobile  accident  he 
remains  a  devoted  skier  and  hiker.  This 
past  summer,  I  had  several  talks  with 
him  about  the  development  of  CERN. 
One  of  the  most  interesting  obser- 
vations he  made  had  to  do  with  the 
evolution  of  the  present  generation  of 
European  physicists.  At  the  end  of  the 
war,  he  said,  European  physics,  which 
had  been  the  finest  in  the  world,  was 
greatly  damaged.  Many  of  the  best 
European  physicists  were  more  or  less 
permanently  settled  in  either  England 
or  the  United  States  and  had  no  desire 
to  come  back  to  Europe  and  relive 
a  very  unpleasant  experience.  In  par- 
ticular, the  tradition  of  experimental 
physics,  which  requires  complicated 
equipment,  had  greatly  suffered  on  the 
Continent  during  the  years  of  depriva- 
tion. Consequently,  when  the  big  ac- 
celerator at  CERN  was  ready,  there  was 
a  shortage  of  highly  trained  European 
experimenters  to  use  it.  On  the  other 
hand,  the  war  had  greatly  strengthened 
physics  in  the  United  States,  not  only 
because  so  many  Europeans  had  come 
he-re  to  live  but  because  physicists  had 
been  working  all  through  the  war  at 
places  like  Los  Alamos  on  subjects  that 
were  not  entirely  dissimilar  to  their 
peacetime  research.  Thus,  the  postwar 
generation  of  American  physicists  was 
highly  trained  and  ready  to  continue 
along  the  line  of  research  that  had  made 
the  development  of  high-energy  physics 
the  frontier  of  physics.  (Many  of  the 
early  research  papers  written  at  CERN 
during  this  period  were  done  by  Euro- 
peans in  collaboration  with  Americans 
at  the  laboratory,  some  of  whom  had 
been  born  in  Europe  and  were  back  on 
visits.)  Of  even  greater  importance, 
most  of  the  European  physicists  who 
currently  have  important  positions  at 
CERN  spent  time  in  the  United  States, 
where  they  received  training  in  the 
then  novel  techniques  of  experimental 
physics.  As  Weisskopf  pointed  out,  a 


new  generation  of  excellent  and  inven- 
tive physicists  has  by  now  grown  up 
in  Europe.  They  are  producing  scien- 
tific work  at  the  forefront  of  modern 
physics  that  is  of  the  first  quality  and 
the  equal  of  anything  being  done  in  the 
United  States  or  Russia.  These  physi- 
cists are  now  training  young  Euro- 
peans, to  say  nothing  of  American  post- 
doctoral visitors.  Originally,  some 
European  university  professors  were 
opposed  to  the  creation  of  CERN  on  the 
ground  that  it  would  draw  too  many 
scientists  away  from  the  universities  at 
a  time  when  there  was  a  desperate 
shortage  of  them.  Weisskopf  remarked 
that  it  has  worked  out  almost  the  other 
way — that  European  physicists  have 
come  to  Geneva  for  a  few  years  of 
advanced  training  and  then  gone  back 
to  their  own  countries  to  teach  and  do 
research  in  universities.  In  fact,  accord- 
ing to  many  of  the  young  European 
physicists  I  have  spoken  to,  it  is  now 
quite  hard  to  find  good  jobs  in  Euro- 
pean universities,  and  CERN  offers  an 
opportunity  to  continue  working  until 
a  suitable  position  opens  up  somewhere. 

FOR  me,  one  of  the  most  interesting 
experiences  at  CERN  was  the  con- 
tact with  some  of  the  Russian  physi- 
cists at  the  laboratory.  As  a  rule,  the 
Russians  who  come  to  Geneva  are 
about  equally  divided  between  experi- 
mental and  theoretical  physicists.  Be- 
cause high-energy  experimental  physics 
is  done  by  teams,  the  experimental  phys- 
icists join  a  group  of  other  experiment- 
ers, while  the  theorists  work  pretty 
much  alone.  As  it  happened,  one  of 
the  Russian  experimenters — Vitaly 
Kaftanov,  from  the  Institute  for  Ex- 
perimental and  Theoretical  Physics,  in 
Moscow — was  working  on  an  experi- 
ment that  was  of  special  interest  to  me, 
since  I  had  been  studying  some  of  its 
theoretical  implications.  This  experi- 
ment— one  of  the  most  elaborate  and 
active  at  CERN — involves  the  study  of 
reactions  induced  by  neutrinos.  The 
neutrino  is  a  marvellous  particle.  It  is 
almost  impossible  to  detect  directly,  for 
it  has  no  charge  and  no  mass,  and  it 
interacts  very  weakly  with  ordinary 
matter.  Indeed,  someone  has  estimated 
that  if  one  took  a  single  neutrino  pro- 
duced in  the  accelerator  at  CERN  or  the 


90 


CERN 


one  at   Brookhaven    (where   the   first 
high-energy  neutrino  experiments  were 
done)  and  shot  it  through  a  layer  of 
lead  about  as  thick  as  the  distance  from 
here  to  Pluto,  it  would  undergo  only 
one  collision  during  its  entire  passage. 
Fortunately,  however,  the  experimenter 
is  not  limited  to  one  neutrino;  an  ac- 
celerator produces  millions  of  them  a 
second,  and  some  are  bound  to  make  a 
collision  in  a  target  of  reasonable  size. 
These  collisions  produce  particles  that 
can  be  seen,  so  that  neutrino  reactions 
can    be    studied.    Since    the    collisions 
are    so    rare,    the    whole    experimental 
area   must  be  carefully  shielded  from 
cosmic  rays  and  other  annoying  back- 
ground that  could  be  confused  with  the 
few  events  that  one  is  looking  for.  In 
the    experiments    both    at    CERN    and 
at   Brookhaven,  this  required  literally 
thousands  of  tons  of  heavy  shielding 
material.  (The  shielding  in  the  Brook- 
haven experiment  was  made  from  the 
remnants    of    an    obsolete    battleship, 
while  at  CERN  it  consists  of  steel  ingots 
lent  to  the  laboratory  by  the  Swiss  gov- 
ernment from  its  strategic  stockpile.)  At 
both  CERN  and  Brookhaven,  neutrino 
events  nave  been  successfully  detected ; 
in  fact,  in  the  Brookhaven  experiment 
it  was  first  shown  that  there  are  two 
quite  distinct  species  of  neutrino.  Until 
that  experiment,  the  neutrino  was  gen- 
erally taken  to  be  a  single,  unique  par- 
ticle (although  there  were  some  theo- 
retical   conjectures   to   the    contrary). 
The  fact  that  precision  experiments  can 
now  be  done  with  neutrinos  is  a  very 
important    breakthrough    in   the   tech- 
nology of  experimental  physics,  and  it 
is   only    natural    that    a    physicist    like 
Kaftanov    is    eager    to    work    on    the 
project. 

Kaftanov,  who  is  married  and  has  a 
young  son,  first  came  to  CERN  alone. 
This  past  summer,  he  was  joined  by 
his  family.  He  has  a  warm,  friendly 
personality  and  a  good  command  of 
English.  (He  told  me  that  when  he  was 
young  his  parents  agreed  to  allow  him 
to  give  up  music  lessons,  which  he 
hated,  on  condition  that  he  study  Eng- 
lish.) Many  of  our  conversations  con- 
cerned the  progress  of  the  experiment, 
but  as  we  got  to  know  each  other  better 
we  talked  a  good  deal  about  a  physicist's 


life  in  the  United  States  and  in  Russia. 
In  his  country,  physicists  and  engineers 
are  at  the  very  top  of  the  social  and 
economic  scale,  and  the  disciplines 
themselves  are  characterized  by  a  high- 
ly didactic  style.  There  is  a  great  deal 
of  sharp,  sometimes  quite  personal  crit- 
icism at  all  levels.  Among  European 
physicists,  by  contrast,  there  is  still 
some  feeling  of  deference  toward  the 
professor  or  the  senior  scientist ;  in  fact, 
some  of  the  European  physicists  have 
told  me  that  they  were  quite  taken 
aback  to  see  Americans  and  Russians 
going  at  each  other  hammer  and  tongs 
in  all-out  scientific  debate  at  interna- 
tional meetings.  The  Russians  have  a 
very  active  high-energy-physics  pro- 
gram, and  are  well  along  with  the  con- 
struction of  a  seventy-billion-electron- 
volt  accelerator  at  Serpukhov,  which 
will  be  the  largest  in  the  world  when  it 
starts  operating.  All  the  physicists  I  have 
spoken  with  at  CERN,  including  Kafta- 
nov, are  very  eager  for  increased  East- 
West  cooperation,  and  hope  that  the 
existing  political  thaw  will  continue  to 
permit  it. 

ULTIMATELY,  the  most  impor- 
tant process  in  a  scientific  labora- 
tory is  the  process  of  constant  recip- 
rocal education.  At  CERN,  this  is 
facilitated  by  the  layout  of  the  buildings, 
which  are  low  and  long  and  are  joined 
by  a  maze  of  passageways.  (The  build- 
ings are  mostly  white  with  a  blue  trim, 
which  gives  them  a  clean-cut  Swiss 
look.)  As  one  walks  down  the  halls, 
one  hears  a  continual  buzz  of  multilin- 
gual conversations  about  physics.  There 
are  often  knots  of  physicists  in  the  halls 
or  in  the  library,  which  has  a  few  special 
soundproof  rooms  with  blackboards  for 
informal  discussions.  Everywhere,  one 
gets  the  impression  of  people  working 
and  arguing  with  each  other,  and  this 
extends  even  to  the  cafeteria.  There  is  a 
long  lunch  period  at  CERN  (the  work- 
ing day  is  from  eight-thirty  to  five- 
thirty,  and  for  many  of  the  experi- 
menters, who  work  in  shifts  on  the 
accelerator,  it  runs  into  the  evenings 
and  weekends),  and  during  it  every- 
thing closes  down — the  bank,  the  post 
oflSce,  the  machine  shops,  and  the  rest. 
But  the  talk  goes  on.  The  cafeteria  is 


91 


furnished  with  long  tables,  and  by  some 
sort  of  informal  tradition  the  technical 
personnel  tend  to  eat  at  noon,  while  the 
physicists  eat  at  one.  Usually,  the  ex- 
perimental groups  eat  together  and  the 
theorists,  too,  form  groups,  sometimes 
according  to  language  and  sometimes 
according  to  common  interests  in  phys- 
ics. After  lunch,  dessert  and  coffee  are 
served  at  a  small  bar,  and  everyone 
spends  the  rest  of  the  lunch  hour  in  the 
lounge  over  coflFee  or,  on  sunny  days, 
on  the  broad  terrace  in  front  of  the 
cafeteria,  from  which  one  has  a  fine 
view  of  Mont  Blanc.  Everywhere  one 
looks,  there  are  people  discussing  phys- 
ics, sometimes  with  paper  and  pencil, 
sometimes  with  elaborate  gesticulations, 
and  usually  in  two  or  three  languages. 
It  is  the  time  of  day  when  one  hears 
the  latest  technical  gossip,  both  from 
CERN  and  from  laboratories  around  the 
world. 

In  addition  to  this  informal  process 
of  education,  there  are   more   formal 
lecture  courses  and  seminars.  The  sum- 
mer before  last,  I  attended  a  lecture 
series,  given  especially  for  physicists,  on 
using  electronic  computers.  Surprising- 
ly, most  of  the  computer  use  at  CERN 
and  at  other  high-energy-physics  lab- 
oratories is  not  by  theoretical  physicists 
but  by  experimenters.  A  typical  experi- 
ment involves  placing  a  target,  such  as 
a  bubble  chamber  filled  with  liquid  hy- 
drogen or  liquid  helium,  in  front  of  the 
beam  of  particles  emerging  from  the  ac- 
celerator. The  particles  leave  tracks  in 
the  liquid,  and  these  tracks  are  photo- 
graphed— a    process   likely    to    involve 
photographing  hundreds  of  thousands 
of  tracks  from   several  angles.  Then 
the  photographs,  which  often  look  like 
examples    of    abstract    art,    must    be 
"scanned;"  that  is,  the  events  of  special 
interest  must  be  distinguished  from  the 
inevitable  chaotic  background.  Much  of 
this  scanning  is  done — visually,  in  the 
first  instance — by  a  large  group  of  peo- 
ple, mostly  women.  The  scanners  do 
not  have  to  be  physicists,  since  picking 
out  events  of  interest  is  a  question  of 
pattern  recognition  and  can  be  taught  to 
almost  anyone.  After  the  events  have 
been    roughly   selected,   they   must   be 
"measured."  The  curvature  and  thick- 
ness of  the  tracks  as  well  as  the  angles 


between  them  are  determined,  to  see 
whether  the  event  in  question  is  real- 
ly what  one  is  looking  for  or  is  per- 
haps something  that  looks  similar  but 
is  really  quite  different.  These  distinc- 
tions are  made  with  the  help  of  a  com- 
puter, which  is  programmed  to  corre- 
late the  results  of  the  measurements,  try 
to  fit  the  event  with  various  hypotheses, 
and  then  report  back.  Without  a  com- 
puter, this  procedure  would  be  enor- 
mously   time-consuming,    since    many 
possibilities  must  be  explored  in  each 
photograph,  and  there  are  thousands  of 
photographs  to  study.  Moreover,  some 
devices  that  make  possible  a  partial  au- 
tomation of  the  measuring  process  are 
now  in  use — an  operator  sets  a  crosshair 
on  a  track,  and  the  machine  does  the 
rest   of   the    measuring   automatically, 
feeding  the  results  into  the  computer — 
and  there  are  systems  under  develop- 
ment   that    in    certain    cases    will    do 
the  pattern  recognition  automatically. 
Hence,  one  can  imagine  a  time  when 
computers  will  study  all  the  pictures  and 
deliver  carefully  analyzed  experimental 
curves  to  the  researcher.  The  amounl 
of  computing  required  for  such  work  is 
tremendous.  CERN  has  recently  bought 
the  largest  computer  in  the  world  and 
will  install  it  at  the  end  of  this  year,  to 
replace  the  present  equipment,  which  is 
completely  saturated. 

This  past  summer,  I  attended  two 
courses  given   by  theoretical  physicists 
especially  for  the  experimenters  at  the 
laboratory.  There  is  a  communication 
problem  between  experimental  and  the- 
oretical physicists  that  arises  from  the 
increasing  need  to  specialize  in  a  single 
aspect  of  physics  because  of  the  com- 
plexity of  the  field.  The  old-fashioned 
romantic   notion   of   the    experimenter 
coming  into  the  physics  laboratory  in 
his  white  coat,  with  his  mind  unbur- 
dened by  preconceptions  or  theoretical 
fancies,  and  saying  to  himself,  "Well, 
what  am  I  going  to  discover  today?" 
just  doesn't  apply  to  experimental  high- 
energy  physics.  The  probable  theoret- 
ical   implications    of    experiments    are 
carefully   considered   in  advance.    Re- 
cently, in  an  editorial  in  Physical  Re- 
view Letters,  a  journal  that  specializes 
in  the   rapid  publication  of  important 
new  results  in  physics.  Dr.  S.  A.  Goud- 
smit  commented,  somewhat  ironically, 


92 


CERN 


At  present,  most  experiments  are  only 
undertaken  to  prove  or  disprove  a  theo- 
ry. In  fact,  some  experimental  teams 
employ  a  theorist  somew^hat  in  the  role 
of  a  court  astrologer,  to  tell  them 
whether  the  stars  in  the  theoretical 
heavens  favor  the  experiments  they  are 
planning." 

In  any  case,  an  experimenter  must 
have  a  knowledge  of  the  latest  theo- 
retical results  and  how  they  bear  on  his 
work.  Thus,  one  of  the  jobs  of  the  the- 
oreticians at  CERK  is  to  explain  what  is 
happening  in  their  fields.  One  of  the 
special  courses,  given  by  Professor  Leon 
Van  Hove,  a  Belgian  physicist  (former- 
ly of  Utrecht,  Holland)  who  directs 
the  theoretical  group  at  CERX,  present- 
ed an  especially  lucid  review  of  general 
aspects  of  reactions  at  high  energies, 
but  this  course  was  finisJiing  for  the 
summer  when  I  arrived,  so  I  could 
attend  only  the  last  few  lectures.  The 
other  course,  given  by  Professor  Ber- 
nard d'Espagnat,  a  French  theorist 
from  Paris,  was  concerned  with  some 
of  the  most  exciting  ideas  that  have 
come  along  in  elementary-particle 
physics  for  several  years.  These  ideas 
have  to  do  with  what  is  known  as  "uni- 
tary symmetry,"  or,  less  accurately, 
"the  eightfold  way."  To  understand 
what  they  signify,  one  must  go  back 
into  the  history  of  the  subject  a  bit. 

In  the  past  few  years,  more  and  more 
new  particles  have  been  discovered 
in  experiments  with  the  accelerators. 
These  particles  are  characterized  by, 
among  other  properties,  their  masses, 
their  electric  charges,  and — because 
they  are  in  general  unstable — their  life- 
times. The  major  problem  the  particles 
have  presented  has  been  whether  they 
have  any  interconnections  or  are  com- 
pletely independent  units.  In  this  area, 
atomic  physics  furnishes  an  especially 
encouraging  example,  since  a  super- 
ficial look  at  the  array  of  chemical  ele- 
ments and  their  diverse  prtjperties  might 
lead  one  to  conclude  that  they  could 
have  no  connections  with  one  another. 
However,  it  is  well  known  that  all 
atoms  are  composed  of  only  three  dis- 
tinct types  of  particle — the  proton  and 
the  neutron,  which  form  the  atomic 
nucleus,  and  the  electron,  a  light, 
negatively  charged  particle  that  gen- 
erates   a    cloud    of    negative    charge 


around  the  nucleus.  The  number  and 
distribution    of    the    electrons    deter- 
mine   the    chemical    properties    of    a 
given  atom,  and  the  protons  and  neu- 
trons determine  its  mass.  In  the  case 
of  the   so-called   elementary   particles, 
one  may  ask  the  same  sort  of  ques- 
tion: Is  there  a  simple  basic  set  of  ele- 
mentary particles  from  which  all  the 
others  can  be  constructed?  Or,  as  the 
question  has  sometimes  been  phrased: 
Are    some    elementary    particles   more 
elementary  than  others,  and  can  the  rest 
be   made   up  of  the   most  elementary 
ones?  It  is  quite  possible  that  tliis  ques- 
tion has  no  real  answer.  Observations 
made  with  the  aid  of  bubble  chambers 
and  other  detection  devices  show  that, 
in    accordance    with    certain    general 
rules,  elementary  particles  can  be  trans- 
formed into  one  another  in  high-energy 
reactions.  For  example,  if  a  pi-meson 
from  an  accelerator  bombards  a  liquid- 
hydrogen   target,   there    can   be    reac- 
tions in   which   the  pi-meson  and  the 
proton  that  composes  the  liquid-hydro- 
gen  nucleus  disappear  and   out   come 
a  so-called  K-meson  and  another  parti- 
cle, called  a  lambda.  Thus,  the  system 
of  pi-meson  and  proton  is  transformed 
into    K-meson    and    lambda.    In    ac- 
counting for  this  transformation,  one 
may  think  of  the  proton  as  being  made 
up  of  a  K-meson  and  a  lambda,  or  one 
may  think  of  the  lambda  as  being  made 
up  of  a  proton  and  a  K-meson,  or  one 
may  think  of  all  these  particles  as  ele- 
mentary. Many  physicists  have  come  to 
believe  that  the  choice  among  these  pos- 
sibilities is  a  matter  of  convenience,  to  be 
decided  only  by  which  choice  leads  to 
the  simplest  and  most  beautiful  theory. 
It  has  recently  become  clear  that  all 
known  particles  can  be  thought  of  as 
being  made  up  of  three  basic  particles, 
and  this  way  of  looking  at  them  ap- 
pears to  be  the  simplest  possible.  The 
basic    set    has    not    yet    actually    been 
seen,  and  one  of  the  great  tasks  of  liigh- 
energy  experimental  physics  in  the  next 
few  years  will   be  to  search   for  new 
particles  that  may  be  candidates  for  the 
basic    ones.    The    search    has    already 
started  at  CERN'  and  Brookhaven.  The 
term  "eightfold  way"  derives  from  the 
fact  that  the  particles  composed  of  the 
basic  threes  fall  naturally  into  groups 


93 


of  eights  (in  some  cases,  into  groups  of 
tens)  that  have  closely  interconnected 
properties.  There  is  now  very  solid  evi- 
dence that  these  groupings  exist,  and 
if  the  basic  set  of  threes  is  identified, 
this  will  close  one  of  the  most  fascinat- 
ing investigations  of  elementary-particle 
physics. 

AFTER    one   of   Professor   d'Espa- 
l\  gnat's  lectures,  on  a  particularly 
warm  and  lovely  summer's  day,  I  de- 
cided to  take   a  walking  tour  of  the 
CERN  site.  At  different  times  over  the 
years,  I  had  visited  most  of  the  installa- 
tions, but  for  the  fun  of  it  I  thought  I 
would  make  the  whole  round  in  one 
swoop.  The  laboratory  is  surrounded 
by  gentle  rolling  fields  leading  oS  to 
the  Jura,  the  wooded,  glacially  formed 
foothills  of  the  Alps;  in  fact,  during  the 
winter,  people  from  CERN  often  spend 
their  lunch   hour   skiing   in  the   Jura, 
which  are  only  a  few  minutes  away  by 
car.  When  I  left  the  building  where 
the  theoreticians  have  their  offices,  the 
first  thing  that  struck  me  was  the  con- 
struction work  going  on  everywhere — 
laborers  (most  of  them  Spaniards  and 
Italians,  as  is  the  case  in  all  of  Switzer- 
land) were  enlarging  roads  and  erect- 
ing new  buildings.  Alongside  one  of  the 
roads  I   saw   a   striking   silvered   bub- 
ble— a    safety    tank    for    holding    hy- 
drogen. Hydrogen,  which  is  the  most 
popular  target  for  experiments,  because 
of  its  simplicity,  is  also  one  of  the  most 
diflScult  gases  to  handle,  because  of  its 
explosive  nature,  and  there  is  a  whole 
complex  of  installations  at  CERN   de- 
voted to  processing  and  handling  it,  all 
of    them    plastered    with    multilingual 
signs  telling  one  not  to  smoke.  A  little 
farther   on,   I    came   to   one   of   three 
"halls"  in  which  experiments  are  ac- 
tually done.  As  the  proton  beam  runs 
around  its  track,  it  produces  particles  in 
targets,  and  these  can  be  siphoned  off  at 
various  stages  and  directed  into  one  of 
the  halls;  this  was  the  East  Hall.  I  am 


not  very  enthusiastic  about  attempts  to 
romanticize  science  and  scientists,  but 
there   is  something    romantic   about   a 
high-energy    experimental    laboratory. 
Its  attraction   lies  partly  in   the   com- 
plexity and  diversity  of  the  equipment — 
giant  magnets,  trucks  filled  with  lique- 
fied gases,  wonderful-looking  electronic 
devices  that  flash  lights  of  every  color — 
and  partly  in  the  knowledge  that  what 
is  being  studied  lies  at  the  very  heart  of 
the  composition  of  the  world.  There 
was  almost  total  silence  in   the   East 
Hall,    broken    only    by    the    rhythmic 
booming  of  the   main   magnet  of  the 
accelerator  and  the   constant  hum   of 
electric  motors.  (CERN  uses  almost  ten 
per    cent    of    Geneva's    entire    power 
supply.)  I  stood  in  awe  until  someone 
came  up  and  asked  if  I  was  looking 
for  something.  For  want  of  anything 
better,  I  told  him  that  I  had  got  lost 
while  trying  to  find  the  road  leading 
to  the  center  of  the  accelerator  ring. 
He  gave  me  some  directions.  I  walked 
outside  and  quickly  found  it.  The  ring 
is  buried,  and  one  can  see  its  outline  as 
a  slight  circular   mound   raised  above 
the   fields.  The  center  of  the  ring  is 
guarded  by  fences  and  signs  warning 
against  radioactivity  and  barring  entry 
to  anyone  without  permission.  This  day, 
though,  I  noticed  a  number  of  men  in- 
side the  ring  cutting  the  grass;  the  ma- 
chine was  undoubtedly  oflF  while  they 
were  working.  I  crossed  over  and  went 
belowground  into  the  central  building. 
Inside,    equipment    sprawled    every- 
where, and  there  was  a  faint  smell  of 
resin,  which  is  used  in  soldering  elec- 
trical circuits.  Dozens  of  men  in  lab- 
oratory   coats    were    working    at    one 
job  or  another  with  great  concentra- 
tion. As  I  watched  them,  the  title  of 
a  book  on  mountain-climbing  came  to 
mind — "Les  Conquerants  de  I'lnutile." 
In  a  way,  high-energy  physics  is  "la 
conqucte  de  I'inutile"  but  it  is  also  one 
of  the  most  exciting,  benign,  and  reveal- 
ing intellectual  disciplines  that  man  has 
been  able  to  devise. 


94 


Radioactive  materials  are  being  used  widely  in  industry, 
medical  and  ecological  research,  clinical  therapy,  agriculture, 
and  food  processing. 


11       The  World  of  New  Atoms  and  of  Ionizing  Radiations 

V.  Lawrence  Parsegian  and  others 
Sections  of  a  textbook  published  in  1968. 


21.11     The  world  of  new  atoms  and 
of  ionizing  radiation 

We  have  gained,  as  by-products  of  atomic 
power,  very  many  new  types  of  radio- 
active atoms  or  radioisotopes.  There  are 
now  about  1100  nucHdes  that  are  new 
and  man-made.  Each  is  unstable,  but 
changes  in  its  own  time  to  a  more  stable 
form.  The  change  is  accompanied  by  the 
emission  of  radiation,  either  in  the  form 
of  a  7-ray  photon,  /3-ray,  sometimes 
positron,  an  a-particle,  or  some  other 
form  or  combination.  Each  nuclide  has 
the  chemical  properties  of  a  stable,  con- 
ventional atom,  but  in  addition  each  also 
emits  radiation  of  a  type  and  energy  that 
is  characteristic  of  that  nuclide.  Also, 
each  unstable  nuclide  (radioisotope)  has 
a  particular  time  rate  or  half-life  for  its 
change  of  form. 

The  early  forms  of  Mendeleev's  Peri- 
odic Table  of  the  atoms  listed  up  to  92 
elements.  Within  the  limited  science  and 
technology  revolving  around  the  chem- 
istry of  these  elements,  there  were  built 
up  vast  chemical  industries.  The  chart  of 
over  1300  nuclides  now  offers  a  much 
larger  variety  of  atoms  and  building 
blocks  out  of  which  to  develop  an  under- 
standing of  atomic  behavior. 

For  example,  consider  the  isotopes  of 
carbon.  Two  stable  forms  of  carbon  are 
found  in  nature,  one  of  mass  12  (C^)  and 
one  of  mass  13  (C*^).  When  nitrogen-14 


(N")  is  bombarded  by  neutrons,  it  cap- 
tures a  neutron  and  emits  a  proton, 
leaving  a  new  atom  which  has  six  protons 
and  which  therefore  behaves  chemically 
like  carbon.  This  is  the  isotope  C",  which 
is  unstable  and  eventually  emits  a  weak 
^-particle  as  it  reverts  back  to  the  original 
stable  N'".  The  half-life  for  this  transition 
is  very  long,  about  5700  years,  and  the 
/3-ray  energy  is  0.156  MeV. 

These  C'^  atoms  become  important  for 
several  purposes. f  They  may  be  incorpo- 
rated   into    drugs    that   contain    carbon. 
When  the  drug  is  injected  into  man  or 
animal     (or    incorporated    into    carbon 
dioxide  gas,  which  may  be  absorbed  by 
a  plant),  it  becomes  possible  to  follow  the 
course  of  the  carbon  in  these  systems 
simply  by  "tracing"  the  behavior  of  the 
C*^  components;   this   is  done  by  mea- 
suring the  radiation  they  emit.  Both  time 
rate  and  distribution  of  the  drug  (or  CO2) 
in  these  complex  systems  can  then  be 
determined    even    though    the    systems 
themselves    are   already   full   of  carbon 
atoms.  This  process  has  made  it  possible 
to  identify  a  long  series  of  intermediate 
steps    in   the   photosynthesis   of  carbon 
dioxide   for   plant   growth.   The   use   of 
radioactive  carbon  (C^)  and  radioactive 


f  We  have  already  discussed  the  use  of 
C*^  in  radioactivity  dating  techniques  in 
Chapters  2  and  20. 


95 


Radioactive  piston  ring 


»;    Radioactive  iron,  Fe^' 

^    for  friction  and  lubrication  studies 


§m 


Samples.measured  for  Fe^^ 
content 


Lubricating  oil  sampled 


Fig.  21.13.     A  common  application  for  use  of  radioisotope  iron-59  to  measure  wear 
of  metal  parts.  The  piston  rings  are  first  made  radioactive  by  exposing  them  to 
neutrons  in  a  nuclear  reactor,  then  installed  in  a  motor  which  is  under  test  for  wear 
characteristics.  As  the  piston  ring  loses  metal  to  the  oil,  the  presence  of  radio- 
activity in  the  oil  gives  a  measure  of  the  wear  while  the  motor  is  running.  When  the 
motor  is  disassembled,  the  transfer  of  metal  to  the  cylinder  wall  can  also  be 
measured  accurately.  Advantages:  (i)  transfer  of  metal  measured  to  i.boh.doo  oz.; 
(2)  oil  sampled  during  operation  of  motor;  (3)  rapid,  simple,  economical.  (Courtesy 
of  U.S.  Atomic  Energy  Commission.) 


species  of  salts  has  clarified  the  under- 
standing of  many  of  the  biological  pro- 
cesses involved  in  human  blood  flow,  the 
diffusion  of  salts  across  body  membranes, 
and  metabolic  activity.  Industry  has 
found  activation  analysis  to  be  particu-, 
larly  sensitive  to  contaminants  in  metals 
or  other  materials  and  has  used  it  for 
identifying  these  contaminants.  Con- 
siderable literature  has  been  written 
about  the  characteristics  and  uses  of 
radioisotopes.  Many  useful  publications 
and  references  are  available  through  the 
AEC. 

Figures  21.13,  21.14,  21.15,  21.16,  and 
21.17  illustrate  some  applications  in- 
volving radioisotopes. 


Radiotracer  and  dating  techniques  re- 
quire relatively  weak  concentrations  of 
C",  of  the  order  of  microcuries.  In  such 
applications  all  that  is  required  of  the 
emitted  radiation  is  that  it  be  measurable, 
either  with  Geiger  (or  similar)  counters 
or  with  photographic  film. 

The  various  types  and  energies  of 
radiation  have  penetrating  power  of 
differing  orders.  For  example,  a-particles 
can  be  stopped  by  a  sheet  of  paper;  /8- 
particles  may  require  from  several  sheets 
of  paper  to  inches  of  solid  material  to 
stop  them,  depending  on  their  energy. 
Gamma  rays  can  penetrate  inches  of  lead. 
By  selecting  suitable  radiation,  one  may 
easily    construct    gauges    for    industrial 


96 


The  World  of  New  Atoms  and  of  Ionizing  Radiations 


applications  that  may  be  used  for  a  wide 
range  of  thicknesses. 

As  noted  earher,  the  analytic  tech- 
nique called  activation  analysis  has 
become  important  for  industry  as  well  as 
for  research.f  If  a  specimen  has  a  very 
small  amount  or  trace  of  impurities  and 
is  placed  in  the  neutron  flux  of  a  nuclear 

t  The  term  activation  analysis  refers  to  the 
process  of  making  a  material  (which  may  be  a 
contaminant)  radioactive  by  bombardment 
with  suitable  nuclear  radiation. 


reactor,  the  trace  impurities  (as  well  as 
the  main  body  of  the  specimen  in  some 
cases)  become  radioactive.  In  many  cases 
the  type  and  amount  of  the  impurity  can 
be  determined  by  comparing  the  results 
of  irradiation  of  the  unknown  sample 
with  the  results  one  obtains  by  irradiating 
specimens  with  known  impurities. 

The  sensitivity  of  activation  analysis  is 
illustrated  by  the  following  case:  Ordi- 
nary arsenic,  arsenic-75,  on  capturing  a 
neutron,    becomes    radioactive    arsenic. 


Fig.  21.14,     Thyroid  cancer.  This  is  a  series  of  six  radioiodine  scans 
of  the  neck  and  chest  of  a  patient  with  cancer  of  the  thyroid,  made 
over  a  period  of  16  months  at  the  Oak  Ridge  Institute  of  Nuclear 
Studies.  The  initial  scan  (top,  left)  shows  the  pattern  of  the  normal 
thyroid  tissue  (dark  lines)  and  the  presence  of  the  tumor  is 
questionable.  With  subsequent  therapeutic  doses  of  radioiodine,  the 
normal  thyroid  is  progressively  fainter  and  the  tumor  becomes  more 
apparent  as  it  takes  up  the  radioiodine.  Finally,  shrinkage  in  the 
size  of  the  tumor  begins  (lower,  right  scan)  as  a  result  of  the 
radioiodine  therapy.  (AEC  Report  for  1965.) 


October 


January 


August 


97 


Leaves  40% 

trunk  and 

branches  and 

nfiost  roots  ~  47% 


Rainout 


Less  than  0.1% 


0.8%  Rootlets  in  0-4  in. 
"of  soil,  smaller  than  2mm 


Fig.  21.15.    Ecological  cycle.  The  rapid  ecological  movement  of  a  radioisotope  such 
as  a  cesium-134  is  illustrated  in  the  above  drawing  of  a  white  oak  tree  whose  trunk 
was  tagged  with  two  millicuries  of  the  radioisotope.  Within  165  days,  the  tracer  had 
become  redistributed  in  different  parts  of  the  ecological  system  and  was  again 
entering  the  tree,  this  time  through  the  root  system.  Use  of  radioisotopes  in  such 
studies  in  the  forests  at  Oak  Ridge  National  Laboratory  helps  ecologists  understand 
the  basic  processes  that  maintain  our  forest  resources.  (AEC  Report  for  1965.) 


Fig.  21.16.     (Facing  page)  Treatment  of  leukemia  by  irradiation  of  blood.  A  patient  at  the 
Medical  Research  Center  at  Brookhaven  National  Laboratory  is  shown  in  the  photo  top 
undergoing  treatment  for  leukemia  by  extracorporeal  irradiation  of  his  blood.  The  nurse  is 
about  to  connect  the  arteriovenous  shunt  in  the  patient's  forearm  to  the  tubing  leading  into 
a  shielded  container  where  the  gamma-ray  source  is  located.  The  technique,  as  diagrammed 
below,  was  applied  to  the  study  and  treatment  of  human  leukemia  following  extensive  studies 
of  the  origin,  function,  and  turnover  rates  of  cells  and  other  blood  constituents  of  normal  and 
leukemic  cows.  The  purpose  of  this  form  of  treatment  is  to  destroy  leukemic  white  cells  in  the 
blood  without  injuring  other  cells  or  organs  in  the  body;  the  red  blood  cells  are  much  more 
resistant  to  radiation  damage  than  the  leukemic  cells.  A  semipermanent  external  arteriovenous 
shunt,  which  may  last  for  many  months,  is  inserted  in  the  patient's  forearm.  Arterial  blood  is 
propelled  by  the  action  of  the  heart  through  plastic  tubing  into  the  shielded  container,  past 
an  intense  source  of  gamma  rays,  and  back  into  the  patient's  arm.  As  the  blood  passes 
through  the  gamma  source  {4000  curies  of  cesium-137)  it  receives  a  radiation  dose  of  from 
250  to  900  rads,  depending  upon  its  flow  rate  (900  rads  would  be  a  lethal  dose  of  radiation 
if  applied  to  the  whole  body).  The  treatment  can  be  repeated  as  necessary  to  reduce  the 
numbers  of  leukemic  cells  in  the  blood.  (Courtesy  Brookhaven  National  Laboratory.) 


98 


The  World  of  New  Atoms  and  of  Ionizing  Radiations 


Schematic  diagram  of  extracorporeal  irradiation 
of  blood 


Arterial  teflon  cannula 


Silastic  tubing 


Stainless  steel  tube 
Lead  shield 


Arteriovenous  shunt 
between  irradiations 


99 


Fig.  21.17.     Irradiation  of  food  with  ionizing  radiation 
to  increase  shelf  life  against  spoilage.  {Courtesy 
Brookhaven  National  Laboratory.) 


arsenic-76,  which  emits  beta  and  gamma 
radiation  on  decay.  Therefore,  by  radio- 
active assay,  one  can  determine  the  con- 
centration of  arsenic  in  a  sample.  In  1961 
a  group  of  Scottish  and  Swedish  scientists 
subjected  a  few  strands  of  hair,  cut  from 
the  head  of  Napoleon  at  his  death  in 
1821,  to  neutron  irradiation  and  found 
arsenic  to  be  present  in  thirteen  times 
normal  concentration,  thus  suggesting 
that  Napoleon  might  have  been  poisoned. 
Closer  investigation  indicated  a  definite 
pattern  of  the  variation  of  arsenic  con- 
centration in  the  hair.  This  pattern,  when 
compared  with  the  record  of  Napoleon's 
sickness,  revealed  a  correlation  with  his 
periods  of  severest  pain.  It  seems  arsenic 
was  in  the  medicine  given  to  relieve  his 
pain  and  it  may  have  had  untoward  efi^ects 
as  well. 

21.12    Effects  and  products  of 
ionizing  radiation 

The  ionizing  radiation  given  off  by  radio- 
active isotopes  can  be  concentrated  and 
intense.  Since  this  radiation  is  highly 
penetrating  and  ionizing,  and  induces 
changes  in  biological  and  chemical  sys- 
tems, it  promises  to  become  significant  in 
chemical  processing  and  in  destroying 
unwanted  bacteria  (such  as  in  milk) 
and  tissues  (such  as  in  tumors,  cancers). 
But  this  promise  is  a  mixed  blessing 
and  curse,  for  overexposure  to  radi- 
ation is  a  health  hazard.  It  has  been 
found  to  cause  leucopenia  (decrease  in 
number  of  white  cells  in  blood),  epilation 
(loss  of  hair),  sterility,  cancer,  mutations 
(altered  heredity  of  offspring),  bone 
necrosis  (destruction  and  death  of  bones), 
and  eye  cataracts. 

In  conventional  processes,  chemical 
reactions  proceed  as  a  result  of  atomic 
collision,  favorable  valence  combinations, 
excitement  of  atom  systems  by  heating. 
Coulomb  attraction,  fi-ee  radical  inters, 
mediates,  and  other  similar  activators. 
The  energy  exchanges  are  likely  to  be  of 


TOO 


The  World  of  New  Atoms  and  of  Ionizing  Radiations 


the  order  of  a  few  electron  volts  or  less 
per  atom  (or  molecule). 

When  swift,  charged  particles  (such  as 
a-particles,  protons,  or  /8-particles)  pass 
through  matter,  they  leave  tracks  of 
ionized  and  excited  atoms  and  molecules, 
which  undergo  vigorous  reorganization. 
The  concentration  of  energy  can  be 
hundreds  or  more  times  the  intensity  of 
conventional  processes,  especially  with 
heavy  charged  particles  and  toward  the 
end  of  particle  tracks  in  the  material.  As 
a  result,  radiation  effects  are  often 
deleterious  to  the  properties  of  the 
material. 

There  are,  however,  applications 
wherein  the  destructiveness  of  radiation 
is  desirable,  such  as  for  killing  insects 
that  infest  grain  or  microbe  systems  in 
medical  supplies.  There  are  also  cases 
where  the  reorganization  of  atoms  and 
molecules  following  irradiation  results 
in  improved  physical  properties  or  pro- 
duces desired  chemical  changes.  Radia- 
tion induces  such  widely  different  re- 
actions that  it  becomes  a  very  versatile 
research  tool.  Processing  by  irradiation 
also  appears  to  have  very  real  possibilities 
of  competing  with  some  conventional 
industrial  processes  and  of  inducing 
reactions  that  cannot  be  produced  by 
other  means. 

The  activities  involving  radiation  and 
radiation  chemistry  may  be  grouped 
under  six  categories:  food  preservation, 
sterilization,  chemical  processing,  radiog- 
raphy and  medical  therapy,  radioisotope 
power  sources,  miscellaneous. 

Since  ionizing  radiation  can  be  lethal 
to  living  organisms  and  microorganisms, 
one  of  the  early  prograrhs  sought  to 
sterilize  foodstuffs  and  thus  give  them 
longer  shelf  life.  Early  efforts  concen- 
trated on  sterilizing  meats  and  other 
foods  by  radiation  dosage  ranging  from  2 
to  5  megarads  (million  radsf ).  The  results 
of  these  early  years  were  not  successful 
because  the  heavy  dose  caused  changes 


in  the  taste  and  appearance  of  foods. 
More  recent  work  has  been  much  more 
encouraging.  In  1963  the  Food  and  Drug 
Administration  (FDA)  approved  steriliza- 
tion of  bacon  by  gamma  radiation  (up  to 
2.2-MeV  energy)  and  by  electron  beams 
(up  to  5-MeV  energy)  from  accelerators. 
The  sterilization  of  ham,  chicken,  and 
beef  appears  promising. 

When  the  radiation  dose  is  kept  well 
below  the  doses  required  for  sterilization, 
down  to  values  of  500,000  rads  or  less, 
the  effect  is  to  "pasteurize"  foods  in  a 
way  that  often  permits  longer  shelf  life. 
For  example,  a  dose  of  250,000  rads  will 
extend  the  shelf  life  of  haddock  fillets  to 
21  days  at  32°  to  33°F.  Crabmeat  treated 
with  200,000  rads  had  its  shelf  life  in- 
creased from  7  days  to  35  days  when 
held  at  33°F.  Fruits  (strawberries, 
cherries,  citrus,  pears,  tomatoes)  show 
similar  gain.  Insects  in  grains  and  wormy 
(helminthic)  parasites  such  as  those 
associated  with  trichinosis  from  pork  are 
killed  by  30,000  rads.  Sprouting  of  potato 
tubers  can  be  inhibited  with  doses  from 
10,000  to  15,000  rads.  But  dosages  in 
excess  of  10  million  rads  appear  to  be 
needed  to  inactivate  some  enzymes. 

Radiation  does  not  raise  the  tempera- 
ture of  the  processed  materials  at  these 
dosages.  Furthermore,  with  y-radiation, 
the  whole  process  can  be  mechanized 
and  the  foods  can  be  irradiated  in  the 
packaged  state.  The  main  difficulty  is  the 
cost  of  the  radiation,  whether  one  uses 
radioisotopic  sources  or  an  accelerator. 
The  irradiation  of  fish  adds  from  1  to  3 
cents  per  pound,  which  is  probably  ac- 
ceptable. Because  strawberries  may  cost 
about  50  cents  per  pound,  they  can  stand 

an  irradiation  expense  of  an  additional 
If  cents  per  pound.  But  for  other  fruits 


f  A  rad  represents  the  absorption  of  100 
ergs  of  radiation  energy  per  gram  of  absorbing 
material. 


101 


and  for  grains,  the  cost  probably  must  re- 
main at  i  cent  per  pound,  to  be  eco- 
nomically acceptable.  To  help  this  in- 
dustrial development,  the  Commission 
has  reduced  the  selling  price  for  certain 
radioisotopes  such  as  cobalt-60  (Co^°, 
which  emits  strong  y-rays  of  1.1  and  1.3 
MeV  and  has  a  half-life  of  5.3  yr)  and 
cesium-137  (Cs*^^,  which  emits  gammas 
of  0.66  MeV  with  a  half-life  of  about 
33  yr). 

Radiation  costs  come  down  sharply  as 
the  radiation  intensity  of  the  facility  is 
increased,  in  terms  of  kilowatt  capacity, 
for  either  radioisotopic  sources  or  accel- 
erator sources.  But  it  is  difficult  to  find 
many  geographic  sites  where  one  can 
provide  high  enough  production  quan- 
tities to  bring  the  cost  of  radiation 
pasteurization  down  to  1  cent  per  pound. 

How  about  irradiation  to  sterilize 
materials  that  are  not  foodstuflFs,  such  as 
medical  supplies,  sutures,  bandages,  and 
drugs?  While  there  are  limitations  in  this 
area  also,  there  are  some  real  advantages 
to  radiation  processes  as  compared  with 
the  use  of  heat,  chemicals,  or  ultraviolet 
light.  When  penetrating  radiation  is  used, 
sutures  or  other  supplies  can  be  packaged 
in  conventional  work  areas  and  then 
irradiated  while  in  sealed  state. 


21.13     Radiography  and  medical 
therapy 

These  two  subjects  may  be  treated  to- 
gether because  they  depend  on  similar 
sources  and  techniques.  Gamma  rays  are 
very  penetrating — more  so  than  X-rays 
from  conventional  machines.  A  cobalt-60 
source  can  therefore  be  used  effectively 
for  penetrating  metal  parts,  castings,  tank 
walls,  and  the  human  body.  As  in  X-rays, 
the  radiation  that  passes  through  the 
target  or  body  can  be  recorded  on  photo- 
graphic film  or  on  a  fluorescent  screen,  to 
give  a  faithful  picture  of  the  variations  of 
matter  through  which  it  passes.  Flaws, 
cracks,  cavities  will  show  up  as  clearly 
as  with  X-rays. 


The  advantages  of  radioisotopic  gamma 
sources  over  X-ray  machines  are  three: 

(1)  These  sources  can  be  made  portable 
and  do  not  require  electric  power  for 
their  operation. 

(2)  Radioisotopes  emit  radiation  in  all 
directions,  which  makes  it  possible  to 
obtain  radiographs  all  around  a  vessel 
into  which  the  source  is  placed. 

(3)  Radioisotopes  can  provide  higher 
penetrating  power  without  requiring 
excessively  large  installations. 

Very  many  industrial  firms  make  use  of 
such  radioisotopes  as  Co®",  which  is 
equivalent  to  2.5-MeV  X-rays  and  can  be 
used  for  steel  of  2-  to  5-in.  thickness.  For 
lesser  penetrability,  iridium-192  (Ir*^^), 
cesium-134  (Cs^^"),  and  Cs^^^  are  the 
equivalent  of  up  to  1400-keV  X-rays  and 
are  useful  for  radiographing  steel  plates 

from  i-  to  2i-in.  thickness  (or  an  equiva- 
lent density  of  other  materials).  Thu- 
lium-170  (Tm^^"),  europium-155  (Eu^^^), 
and  certain  isotopes  of  americium  (Am) 
provide  still  lower  penetration. 

For  many  fixed  installations.  X-ray 
machines  may  be  preferred.  Some  in- 
dustrial firms  engaged  in  the  production 
and  testing  of  tanks,  ships,  and  trans- 
mission pipe  in  the  field  have  found  the 
radioisotopic  sources  to  be  much  more 
practical  than  X-ray  machines. 

We  have  noted  that  radiation  kills 
living  organisms.  Malignant  disease  in 
body  tissue  can  often  be  arrested  by 
exposure  to  penetrating,  ionizing  radia- 
tion. But  since  healthy  tissue  also  suffers, 
radiation  must  be  applied  carefully  and 
restrictively  to  the  tissues  to  be  treated. 
This  has  given  rise  to  very  many  designs 

that  use  radioisotopic  sources  in  the  form 
of  tiny  needles  that  are  inserted  into 
tissue;  or  the  sources  may  be  contained 
in  a  housing  that  directs  a  well-colli- 
mated  beam  onto  the  tissue. 

Radioisotopic  sources  offer  portability 
and  considerable  choice  in  the  type  and 
energy  of  radiation  that  they  emit.  Also 
they  can  be  fabricated  into  very  many 
shapes  and  sizes. 


102 


Different  approaches  to  the  nucleus  suggest  different  models 
This  paper  considers  several  nuclear  models  Including  the 
liquid-drop  model,  the  shell  model  and  the  optical  model. 


12 


The  Atomic  Nucleus 


Rudolf  E.  Pelerls 
Scientific  American  article  published  in  1959. 


Ever  since  1930,  when  the  discovery 
of  the  neutron  made  it  plain  that 
the  nuclei  of  atoms  were  built  of 
protons  and  neutrons,  physicists  have 
been  trying  to  form  a  picture  of  the 
structure  of  the  nucleus.  The  same  task 
for  the  rest  of  the  atom  was  completed 
jn  the  first  quarter  of  this  century.  We 
were  able  to  understand  in  detail  how 
the  electrons  move  under  the  attraction 
of  the  nucleus,  and  how  their  motion  is 
influenced  by  their  mutual  repulsion. 

To  achieve  such  an  understanding  re- 
quires three  major  steps:  First,  we  must 
know  the  forces  between  the  particles. 
Second,  we  need  to  know  the  mechanical 
laws  which  govern  their  motion  under 
the  influence  of  these  forces.  Third,  we 
need  in  most  cases  a  simplified  picture, 
or  model,  from  which  to  start.  Once  we 
have  the  first  two  ingredients,  we  could 
in  principle  write  down  a  set  of  mathe- 
matical equations  whose  solutions  would 
tell  us  all  about  the  atom,  or  about  the 
nucleus.  In  the  simplest  possible  atoms, 
like  that  of  hydrogen,  in  which  there  is 
only  one  electron,  or  in  the  simplest  com- 
pound nuclei,  like  the  deuteron,  which 
contains  only  one  proton  and  one  neu- 
tron, such  equations  can  be  written 
down  and  solved  without  difficulty. 
However,  for  more  complicated  struc- 
tures this  head-on  attack  becomes  much 
harder  and  soon  exceeds  the  capacity 
even  of  modern  electronic  computers. 

We  are  like  men  who  encounter  for 
the  first  time  a  complicated  machine,  and 
who  try  to  analyze  its  operation.  If  we 
attempt,  without  any  guidance,  to  puz- 
zle out  the  interplay  of  all  the  parts  of  the 
machine,  we  should  soon  lose  ourselves 
in  a  maze.  Instead,  we  first  try  to  ascer- 
tain the  major  features  of  the  machine's 
operation.  We  then  devise  a  model 
which  resembles  the  real  thing  in  these 
features,  yet  is  simple  enough  to  be 
analyzed.  Then,  of  course,  we  must  put 


in  corrections  for  the  complications 
which  we  have  left  out  and  check  that 
they  do  not  materially  alter  the  picture. 

In  the  study  of  the  atom  the  first  of 
the  three  steps  hardly  presented  a  prob- 
lem. As  soon  as  Ernest  Rutherford  had 
demonstrated  that  the  atom  consisted  of 
a  heavy,  positively  charged  nucleus  and 
of  light,  negatively  charged  electrons,  it 
was  taken  for  granted  that  the  forces 
between  them  were  the  electric  attrac- 
tion of  unlike  charges,  following  the  in- 
verse-square law  familiar  to  every  stu- 
dent of  physics.  The  major  difficulty  was 
the  second  step.  It  turned  out  that  the 
basic  mechanical  principles  of  Isaac 
Newton,  which  apphed  to  all  "large"  ob- 
jects from  the  planets  and  the  moon 
down  to  steam  engines  and  watches,  had 
to  be  revised  in  the  atomic  domain.  To 
understand  atoms  we  had  to  use  the  new 
ideas  of  the  quantum  theory,  following 
the  pioneer  work  of  Niels  Bohr,  who 
adapted  for  this  purpose  the  concept  of 
the  quantum  of  action  which  Max  Planck 
had  first  found  in  the  behavior  of  light. 
These  new  laws  of  mechanics  were  later 
formulated  as  the  laws  of  "quantum 
mechanics,"  or  "wave  mechanics,"  which 
gave  us  complete  command  over  the 
theory  of  the  atom. 

The  third  step,  of  finding  a  simplified 
model  for  discussing  the  atom,  also 
proved  relatively  easy.  In  working  out 
the  possible  orbits  of  a  single  electron 
under  the  attraction  of  a  proton,  as  in 
the  hydrogen  atom,  Bohr  found  that  one 
could  account  for  the  behavior  of  a  more 
complex  atom  by  assuming  that  each  of 
its  electrons  moved  in  such  an  orbit,.  The 
larger  the  number  of  electrons  in  an 
atom,  however,  the  more  distinct  orbits 
they  occupy;  this  is  a  consequence  of  the 
"exclusion  principle"  discovered  by 
Wolfgang  Pauli,  which  limits  the  num- 
ber of  electrons  that  can  travel  in  a 
given  orbit. 


We  must  allow  not  only  for  the  attrac- 
tion of  the  electrons  by  the  nucleus,  but 
also  for  the  repulsion  of  the  electrons  by 
one  another.  However,  we  simplify  the 
nature  of  this  repulsion  by  forgetting 
that  it  changes  continuously  as  the  elec- 
trons move  around  in  their  orbits,  and 
treating  it  as  a  fixed  field  of  force.  In 
other  words,  we  replace  the  repulsion 
due  to  a  moving  electron  by  that  which 
we  would  obtain  if  the  electron  were 
spread  out  evenly  over  its  orbit.  This 
simplification  can  be  justified  by  the  fact 
that  the  repulsion  acts  over  relatively 
long  distances,  so  that  each  electron  is 
at  any  time  under  the  influence  of  several 
others.  If  we  underestimate  the  effect  of 
one  of  the  electrons  which  may  happen 
to  be  rather  close  to  the  one  we  are  look- 
ing at,  we  are  likely  to  overestimate  the 
effect  of  another  which  happens  to  be 
rather  far  away. 

This  model  of  the  atom  is  usuallv 
called  the  "shell  model,"  because  it  is 
convenient  to  group  together  the  elec- 
trons moving  in  orbits  of  similar  size  but 
of  different  shape  and  direction.  Such  a 
group  of  orbits  is  called  a  shell. 

When  the  atomic  nucleus  first  became 
an  object  of  serious  study,  the  nature  of 
the  difficulties  was  rather  different.  The 
general  laws  of  dynamics  did  not  seem 
to  re(juire  further  revision; -the  laws  of 
(juantum  mechanics  which  had  been  dis- 
covered in  atomic  physics  seemed  quite 
adequate  for  the  nuclear  domain.  In- 
deed, we  have  not  yet  found  any  evi- 
dence in  the  behavior  of  nuclei  which 
would  suggest  that  these  laws  might  be 
in  error.  Thus  the  second  step  in  our  list 
presented  no  problem. 

The  Nuclear  Forces 

On  the  other  hand  the  first  step— the 
determination  of  thjs  forces  between  the 
particles— proved  to  be  a  very  difficult 


103 


problem.  Even  today,  after  some  25 
years  of  intense  study,  we  cannot  claim 
to  have  a  complete  answer,  but  we  have 
by  now  at  least  a  fair  knowledge  of  what 
the  forces  are  like. 

They  cannot  be  electric  in  origin.  The 
only  electric  charges  found  in  the  nu- 
cleus are  the  positive  charges  of  the  pro- 
tons, and  like  charges  repel  each  other; 
thus  electric  forces  cannot  be  responsible 
for  holding  a  nucleus  together.  More- 
over, electric  forces  are  much  too  weak. 
We  know  that  the  energy  of  attraction 
of  two  unlike  charges  (i.e.,  the  work  we 
have  to  do  to  pull  them  apart)  varies 
inversely  as  their  distance.  The  attrac- 
tive energy  of  an  electron  and  a  proton 
in  the  hydrogen  atom  is  a  few  electron 
volts  (ev),  and  since  the  diameter  of  the 
hydrogen  atom  is  20,000  times  larger 
than  that  of  the  smallest  nucleus  we 
should  expect  electric  energies  in  the 
nucleus  to  amount  to  some  tens  of  thou- 
sands of  electron  volts.  Actually  the 
forces  inside  a  nucleus  run  to  many  mil- 
lion electron  volts  (mev).  It  follows  that 
nuclear  forces  are  vastly  stronger  than 
electric  forces. 

It  is  also  clear  that  these  strong  forces 
act  only  over  extremely  short  distances. 
The  pioneer  work  of  Rutherford  on  the 


passage  of  charged  particles  through 
matter  showed  that,  even  in  encounters 
in  which  a  charged  particle  approaches 
a  nucleus  to  a  distance  of  a  few  times 
the  nuclear  diameter,  the  only  noticeable 
force  is  the  electric  one.  We  know  to- 
day that  nuclear  forces  between  two  par- 
ticles are  quite  negligible  if  the  distance 
between  the  particles  is  more  than,  say, 
four  fermis.  (The  fermi,  named  for  the 
late  Enrico  Fermi,  is  a  convenient  unit 
of  distance  for  the  nucleus.  The  di- 
ameter of  a  heavy  nucleus  is  some  15 
fermis;  the  diameter  of  the  hydrogen 
atom,  about  100,000  fermis.)  It  is  not 
surprising,  therefore,  that  earlier  physi- 
cists did  not  meet  nuclear  forces  in  labo- 
ratory experiments.  The  only  possible 
way  of  studying  these  forces  is  to  ob- 
serve the  behavior  of  nuclei,  or  to  bom- 
bard hydrogen  or  other  nuclei  with  fast 
protons  or  neutrons  under  circumstances 
in  which  the  effect  of  really  close  en- 
counters can  show  up. 

What  makes  this  task  harder  is  that 
the  nature  of  nuclear  forces,  unlike  the 
simple  inverse-square  law  of  electric  or 
gravitational  forces,  is  rather  compli- 
cated. If  the  law  of  nuclear  forces  were 
simple,  a  few  observations  might  sufiBce 
to  guess  its  general  form.  But  all  simple 


guesses  based  on  a  few  experiments  have 
been  disproved  by  later  experiments. 
We  are  obliged  to  reconstruct  the  law 
of  nuclear  forces  laboriously  from  the 
various  pieces  of  evidence  we  can  ex- 
tract from  the  experiments. 

Ultimately  we  hope  to  be  able  to  de- 
rive the  law  of  the  forces  from  more 
basic  principles,  just  as  we  can  derive 
the  inverse-square  law  of  electric  forces 
from  the  basic  laws  of  electromagne- 
tism.  A  beginning  was  made  by  the 
Japanese  physicist  Hideki  Yukawa,  who 
used  the  analogy  with  electromagnetic 
radiation  to  point  out  that  nuclear  forces 
must  be  related  to  a  new  form  of  radia- 
tion which  could  carry  charged  particles 
weighing  a  few  hundred  times  more  than 
the  electron.  His  prediction  was  con- 
firmed by  the  discovery  of  the  so-called 
pi  meson.  His  picture  of  the  mechanism 
underlying  the  nuclear  forces  has  been 
qualitatively  confirmed  by  many  obser- 
vations, and  has  been  a  useful  guide  in 
our  thinking  about  the  forces.  But  it  has 
not  yet  been  possible  to  use  his  idea  for 
a  reliable  and  accurate  derivation  of  the 
law  of  the  forces  because  of  the  mathe- 
matical problems  which  stand  in  the 
way.  We  do  not  know  today  whether  a 
correct  solution  of  the  equations  em- 


\ 


CHARGE  EXCHANGE  in  the  nucleus  ie  schematically  depicted.         in  half  the  cases  (.left)  the  neutron  continues  forward.  Inlhe  other 
When  protons  ^btocAc  fra//s^  are  struck  by  fast  neutrons  half  (right),  the  proton  exchanges  its  charge  with  the  neutron. 


-3^   -^ 


SPIN-ORBIT  FORCE  arises  from  a  relationship  between  spin  and         in  which  they  move  on  an  orbit,  the  force  between  them  is  strong, 
orbit.  When  two  particles  (left)  spin  in  the  same  direction  as  that         When  they  spin  in  opposite  directions   (right),  force  is  weak. 


104 


bodying  Yukawa's  idea  would  yield  the 
right  forces,  or  whether  there  is  some- 
thing basically  wrong  with  this  ap- 
proach. The  diJBBculties  arise  chiefly  from 
the  greater  strength  of  the  nuclear 
forces,  as  compared  to  electric  forces, 
which  makes  their  mathematical  analysis 
much  more  difiBcult. 

Thus  the  best  source  of  information 
about  the  forces  still  lies  in  direct  ex- 
periments. These  require  collisions  at 
high  energies— much  higher  than  the 
energies  of  particles  inside  ordinary  nu- 
clei. The  reason  for  this  is  the  wave  as- 
pect of  particles,  which  is  an  essential 
feature  of  quantum  mechanics.  Slow 
particles  are  associated  with  waves  of 
long  wavelength,  and  collisions  involv- 
ing such  slow  particles  do  not  provide 
much  information  about  the  finer  fea- 
tures of  the  forces  at  work  between  them, 
just  as  in  looking  through  a  microscope 
at  a  dust  particle  with  a  diameter  less 
than  a  wavelength  of  light  we  see  only 
a  general  blur  which  does  not  reveal  the 
shape  or  nature  of  the  particle.  To  have 
particles  of  sufficiently  short  wavelength 
one  must  raise  their  energy  to  a  few 
hundred  mev.  The  most  reliable  infor- 
mation on  nuclear  forces  has  therefore 
become  available  only  in  the  last  few 
years,  as  a  consequence  of  the  develop- 
ment of  accelerating  machines  which 
produce  clean  beams  of  protons,  neu- 
trons, or  electrons  with  such  energies. 
This  need  for  high-energy  beams  is  en- 
tirely similar  to  the  situation  in  atomic 
physics,  where  detailed  pictures  of  the 
structure  of  atoms  require  the  use  of  X- 
ray  or  electron  beams  of  several  thou- 
sand ev— much  greater  than  the  energies 
of  the  electrons  inside  the  atoms,  whose 
wavelength  is  comparable  to  the  atomic 
diameter.  The  complexity  of  the  results 
has  also  made  it  necessary  to  call  on  the 
services  of  fast  electronic  computers  for 
disentangling  the  observations. 

I  shall  not  attempt  in  this  article  to 
give  anything  hke  a  complete  specifica- 
tion of  the  nuclear  forces,  but  shall  stress 
only  those  features  which  are  of  impor- 
tance for  what  follows.  We  have  already 
noted  that  the  forces  must  be  strong  and 
of  short  range.  Since  they  hold  the  dif- 
ferent particles  together,  they  must  on 
balance  be  attractive.  At  the  same  time 


they  cannot  be  entirely  attractive,  since 
otherwise  heavy  nuclei  would  "collapse." 
By  collapse  we  mean  a  state  of  affairs  in 
which  all  the  particles  in  a  nucleus  are 
so  close  together  that  each  one  is  within 
the  range  of  the  attractive  force  of  every 
other.  In  that  case  the  attractive  energy 
acting  on  each  particle  would  grow  with 
the  total  number  of  particles  present, 
and  the  volume  occupied  by  the  whole 
nucleus  would  be  the  same  no  matter 
how  many  particles  were  in  it.  This  is 
not  found  in  reality.  The  energy  per  par- 
ticle is  roughly  the  same  for  all  nuclei, 
light  or  heavy,  and  the  volume  of  nuclei 
increases  with  the  number  of  particles  in 
them. 

The  Exchange  Forces 

This  behavior,  which  indicates  a  lim- 
ited attraction,  is  usually  called  "satura- 
tion" of  the  nuclear  forces.  There  are 
two  particularly  plausible  ideas  to  ac- 
count for  this  saturation.  One  was  sug- 
gested by  the  German  physicist  Werner 
Heisenberg,  who  was  one  of  the  founders 
of  quantum  mechanics.  He  postulated 
that  at  least  part  of  the  nuclear  forces 
between  a  neutron  and  a  proton  involves 
an  exchange  of  their  position,  so  that 
after  an  encounter  between  them  the 
neutron  would  tend  to  follow  what  had 
been  the  path  of  the  proton,  and  vice 
versa.  The  exchange  occurs  readily  only 
if  the  two  move  in  very  similar  orbits, 
and,  since  the  Pauli  exclusion  principle 
allows  only  a  limited  number  of  particles 
to  follow  the  same  orbit,  such  exchange 
forces  would  expose  each  particle  to  a 
strong  attraction  only  from  a  few  others. 
The  bombardment  of  protons  with  fast 
neutrons  confirmed  this  idea,  because  it 
showed  that  in  most  cases  either  the 
neutron  or  the  proton  tended  to  go  for- 
ward with  almost  the  same  speed  and 
direction  with  which  the  neutron  had 
arrived.  Since  it  is  hard  to  deflect  such 
fast  particles  from  their  path,  this  indi- 
cates that  the  incident  neutron  had  con- 
tinued almost  in  a  straight  line,  but  that 
in  half  the  collisions  it  had  changed  its 
nature  and  become  a  proton,  leaving  a 
neutron  behind. 

However,  the  experiment  also  showed 
that  only  one  half  of  the  force  was  of 


The  Atomic  Nucleus 

the  exchange  type;  the  other  half  (cor- 
responding to  the  neutrons  still  moving 
forward  after  collision)  was  an  "ordi- 
nary" force.  This  is  not  enough  to  yield 
the  required  saturation,  and  some  other 
factor  must  be  involved.  The  second  fac- 
tor tending  toward  saturation  is  almost 
certainly  a  reversal  of  the  direction  of 
the  nuclear  forces  at  short  distances,  so 
that,  as  two  particles  approach  each 
other,  the  attraction  changes  to  repul- 
sion. This  concept  of  "repulsive  cores" 
in  the  forces  is  familiar  in  the  behavior 
of  atoms.  When  atoms  form  chemical 
compounds,  or  liquid  or  solid  substances, 
they  are  held  together  by  attractive 
forces;  but  each  atom  has  a  fairly  defi- 
nite size,  and  when  two  atoms  come  into 
actual  contact,  their  attraction  changes 
into  repulsion.  We  may  liken  this  be- 
havior to  that  of  two  rubber  balls  tied 
together  with  a  rubber  band.  There  is  an 
attraction  between  the  balls,  but  there 
is  also  a  contact  force  which  prevents 
the  centers  of  the  balls  from  approach- 
ing each  other  closer  than  one  diameter. 
Shortly  after  the  theoretical  need  for 
such  a  repulsive  core  in  the  nuclear 
forces  had  become  clear,  experiments  on 
collisions  between  fast  particles  indeed 
showed  direct  evidence  for  these  repul- 
sive forces. 

Among  other  features  of  the  nucleus 
I  should  mention  the  "spin-orbit"  force, 
that  is,  the  dependence  of  the  mutual 
interaction  of  two  particles  upon  the 
direction  of  their  orbit  with  respect  to 
their  spin.  When  the  two  particles  spin 
on  their  axes  in  the  same  direction  as  that 
in  which  they  revolve  about  each  other, 
the  attraction  between  them  is  stronger; 
when  they  spin  in  the  opposite  direction 
from  that  in  which  they  revolve,  the  at- 
traction is  weaker.  There  is  some  evi- 
dence for  such  a  spin-orbit  force  in  ex- 
periments on  nuclear  coHisions,  but  there 
is  still  some  room  for  controversy  in  the 
interpretation  of  these  experiments. 

Our  present  knowledge  of  the  nuclear 
forces,  while  still  incomplete,  is  suffi- 
cient to  discuss  the  behavior  of  nuclei 
and  the  collisions  between  them.  At  this 
point  we  meet  the  need  for  the  third 
step  in  our  general  program,  namely  a 
simple  model  in  terms  of  which  we  may 
approach  the  dynamical  problem  of  the 


NUCLEAR  FORCES  are  dependent  on  the  distance  between  parti- 
cles. If  the  particles  are  very  close,  they  repel  each  other  (left). 


If  they  are  a  certain  distance  apart,  they  attract  each  other  (center}. 
If  they  are  farther  apart,  they  have  little  effect  on  each  other  (right). 


105 


motion  of  the  16  particles  in  the  oxygen 
nucleus,  or  the  208  particles  in  the  most 
stable  lead  nucleus. 

Models  of  the  Nucleus 

The  selection  of  a  suitable  model  is 
not  at  all  straightforward.  Not  that  there 
is  a  shortage  of  suggestions.  In  fact  the 
trouble  in  the  recent  past  has  been  a  sur- 
feit of  different  models,  each  of  them 
successful  in  explaining  the  behavior  of 
nuclei  in  some  situations,  and  each  in 
apparent  contradiction  with  other  suc- 
cessful models  or  with  our  ideas  about 
nuclear  forces.  In  the  past  few  years 
great  progress  has  been  made  in  bringing 
some  order  into  this  confusion  and  in 
understanding  the  justification  for  each 
of  the  models  in  the  domain  to  which  it 
is  properly  applied.  I  shall  attempt  to 
explain  briefly  some  of  the  ideas  behind 
these  developments. 

The  most  obvious  idea  was  to  use  the 
shell  model,  which  had  been  so  success- 
ful in  dealing  with  the  atom.  In  fact,  the 
first  attempts  to  set  up  such  a  shell  model 
were  made  even  before  the  discovery  of 
the  neutron,  when  it  was  believed  that 
nuclei  were  made  of  protons  and  elec- 
trons. A  shell  model  with  the  wrong  con- 
stituents cannot  have  much  success  in 
accounting  for  the  facts,  but  in  those 
days  rather  few  facts  were  known,  so 
such  models  were  able  to  survive  for 
some  time. 

After  the  discovery  of  the  neutron, 
attempts  to  formulate  a  nuclear  shell- 
model  were  renewed.  This  involved  the 
idea  of  orbits  (or  quantum  states)  for 
the  protons  and  neutrons,  in  which  each 
of  them  was  pictured  as  moving  inde- 
pendently under  the  influence  of  some 
force  which  represented  the  average  ef- 
fect of  the  others,  as  in  the  case  of  the 
electrons  in  the  atom.  It  did  not  seem 
possible,  however,  to  choose  groups  of 
orbits  of  the  right  kind,  so  that  the  num- 
ber of  similar  orbits  which  formed  a  shell 
could  accommodate  just  the  right  num- 
ber of  neutrons  and  protons  to  account 
for  the  exceptional  stability  of  nuclei 
with  certain  numbers  ("magic  num- 
bers") of  neutrons  or  protons. 

The  same  idea  was  applied  to  the  col- 
lision of  neutrons  with  nuclei.  Accord- 
ing to  the  shell  model,  the  impinging 
neutron  should  travel  through  the  nu- 
cleus on  its  own  orbit,  as  through  some 
field  of  force,  and  individual  encounters 
with  the  particles  constituting  the  target 
nucleus  ought  to  be  rare  and  unimpor- 
tant. Hence  the  neutron  should  in  most 
cases  emerge  with  the  same  speed  as 
that  with  which   it  entered,   and  only 


rarely  should  it  get  trapped.  The  details 
of  the  process  should  not  depend  criti- 
cally on  the  speed  of  the  neutron. 

Observations  of  such  collisions,  initi- 
ated by  Fermi  in  Rome,  gave  a  com- 
pletely different  picture.  Most  of  the 
neutrons  that  interacted  with  a  nucleus 
were  trapped,  their  excess  energy  being 
radiated  in  the  form  of  gamma  rays. 
Moreover,  the  chance  of  the  neutron 
being  affected  by  the  nucleus  depended 
very  critically  on  its  energy.  One  found 
a  large  number  of  resonances,  i.e.,  sharp- 
ly selected  energies,  for  which  a  neutron 
was  sure  to  be  picked  up  by  the  nucleus. 
For  each  target  nucleus  there  are  many 
such  resonances,  the  energy  diflFerence 
between  them  being  often  as  low  as  100 
ev,  an  exceedingly  small  difference  on 
the  nuclear  scale. 

These  resonances  turned  out  to  be  ex- 
ceedingly sharp,  and  on  the  uncertainty 
principle  of  quantum  mechanics  a  sharp- 
ly defined  energy  is  associated  with  a 
long  time.  So  it  follows  that  once  a  neu- 
tron gets  into  a  nucleus  in  conditions  of 
resonance  it  must  stay  there  a  long  time 
—much  longer  than  it  would  take  it  to 
cross  a  region  the  size  of  a  nucleus. 

The  Liquid-Drop  Model 

The  way  to  resolve  these  apparent 
contradictions  was  pointed  out  by  Bohr. 
He  recognized  that  it  was  not  right  to 
think  of  a  neutron  as  passing  just  through 
a  general  field  of  force,  since  the  nucleus 
is  densely  packed  with  particles  which 
each  exert  strong  forces  on  the  extra 
neutron  as  well  as  on  each  other.  Instead 
of  comparing  the  process  with  the  pas- 
sage of  a  comet  through  the  solar  system, 
as  was  appropriate  for  the  passage  of  an 
electron  through  an  atom,  we  should 
liken  it  to  the  entry  of  a  golf  ball  into  a 
space  already  fairly  densely  filled  with 
similar  balls.  The  result  will  be  a  com- 
plicated motion  of  all  the  balls,  and  the 
energy  of  motion  of  the  extra  one  will 
rapidly  get  shared  with  the  others. 

The  dynamical  problem  is  now  that  of 
a  true  many-body  motion,  and  we  have 
vastly  more  possibilities  of  varying  the 
details  of  the  motion  of  all  the  particles. 
This  means  that  the  rules  of  quantum 
mechanics  will  give  us  far  more  states  of 
motion,  and  these  are  responsible  for  the 
greatly  increased  number  of  resonances. 
We  also  see  the  reason  for  the  long  stay 
of  the  neutron  in  the  nucleus,  because 
when  the  energy  of  motion  is  shared 
among  many  particles,  none  of  them  can 
attain  enough  speed  to  escape  from  the 
general  attraction.  It  must  take  a  long 
time  before  by  chance  one  of  them  col- 


SHELL  MODEL  of  the  nucleus  is  represent- 
ed by  a  potential  "well"  in  which  the  groups 
of  horizontal  lines  indicate  orbits  that  can 
be  occupied  by  particles  in  the  nucleus.  The 
groups  of  solid  gray  lines  indicate  orbits  of 
lower  energy;  the  groups  of  broken  gray 
lines    represent    orbits    of    higher    energy. 


LIQUID-DROP  MODEL  may  also  be  repre- 
sented  as  n  collection  of  golf  balls.  When  an- 
other particle,  or  golf  ball,  enters  the  nucle- 
us, the  motion  of  all  the  balls  is  disturbed. 


OPTICAL  MODEL  pictures  the  nucleus  as 
a  somewhat  cloudy  crystal  ball.  The  cloudi- 
ness represents  the  tendency  of  bombarding 
neutrons   to   be   absorbed   by   the   nucleusi. 


106 


The  Atomic  Nucleus 


normal  states  in  terms  of  shells. 


.__.^. 


LOW-ENERGY  ORBITS  in  the  shell  model  of  the  nucleus  may  each  be  occupied  by  only 
two  neutrons  (colored  balls)  and  two  protons  (black  balls).  In  the  normal  state  of  affairs 
(left)  the  low-energy  orbits  are  filled;  the  particles  cannot  gain  or  lose  energy,  and  thus 
cannot  change  their  orbits.  A  bombarding  particle  (upper  right)  has  energy  to  spare;  thus 
it  can  exchange  energy  with  a  particle  in  nucleus  and  move  it  to  orbit  of  higher  energy. 


lects  enough  of  the  available  energy  to 
get  away.  In  our  picture  of  the  golf  balls 
this  will  actually  never  happen,  because 
in  the  meantime  too  much  of  the  energy 
will  have  been  dissipated  in  friction.  In 
the  nuclear  case  the  analogue  of  fric- 
tion is  the  loss  of  energy  by  gamma  radi- 
ation, and  this  is  responsible  for  the 
events  in  which  the  neutron  gets 
trapped.  But  it  is  less  effective  than  in 
the  case  of  the  golf  balls,  and  some  neu- 
trons do  get  out  again. 

The  physicist  does  not  invoke  here  the 
similarity  with  a  system  of  golf  balls, 
which  is  not  quite  close  enough,  but  he 
is  reminded  of  a  very  similar  situation 
which  arises  when  a  water  molecule  hits 
a  drop  of  water,  and  for  this  reason 
Bohr's  model  is  often  called  the  "hcjuid- 
drop  model." 


i 

6 

CO 

§ 

t- 
Z 


The  liquid-drop  model  met  with  con- 
siderable success,  and  was  able  to  ex- 
plain many  detailed  features  of  nuclear 
reactions.  At  this  time  it  seemed  evi- 
dent that  the  whole  earlier  idea  of  the 
shell  model,  which  pictures  the  particles 
as  moving  independently,  was  doomed 
to  failure,  in  view  of  the  high  density  of 
the  nucleus  and  the  strong  forces  a  par- 
ticle was  bound  to  experience  in  many 
encounters  with  others  during  the  course 
of  its  motion.  Most  physicists  then  re- 
garded the  whole  idea  of  a  shell  model 
as  misconceived,  but  some,  whether  out 
of  a  stubborn  refusal  to  accept  the  argu- 
ments against  the  model,  or  out  of  a 
deeper  intuitive  insight  which  convinced 
them  that  somehow  one  might  be  able 
to  get  around  the  argument,  continued 
to  look  at  the  behavior  of  nuclei  in  their 


NEUTRON  ENERGY 

riANT  RFSONANCES  of  a  typical  nucleus  are  indicated  by  the  colored  curve.  Each  of 
GIAWT  KtSUI>IAl'«H.i:-3  oi  a  ijv  „„„„„    The  heiehl  of  each  1  ne  denotes  the 

the  vertical  lines  represents  an  ordinary  ■""-"''"j;:  J^'^^X^^^  ^i.hin  the  nucleus,  or 
number  of  bombarding  neutrons  at  that  -"^J  ^^^^^'^^^/^r',"  Giant  resonances  are 
which  emerge  from  the  nucleus  w.th  only  part  ''.'^^"J^^^^J^^'^.^i  ,o^er  resolution, 
observed  when  nucleus  is  bombarded  w.th  particles  of  lower  energy 


The  Shell  Model  Again 

It  soon  became  evident  that  there  was 
overwhelming  evidence  in  favor  of  such 
a  shell  picture,  and  the  final  success 
came  when  Maria  G.  Mayer  of  the  Uni- 
versity of  Chicago  and  J.  D.  H.  Jensen 
of  Heidelberg  independently  noticed 
that  the  facts  fitted  amazingly  well  with 
a  sbghtly  modified  shell  model.  The  new 
feature  was  that  when  a  particle  spins 
in  the  direction  in  which  it  moves  about 
the  center  of  the  nucleus,,  its  orbit  is  dif- 
ferent from  the  orbit  of  a  particle  spin- 
ning in  the  opposite  direction.  When 
this  idea  was  put  forward,  it  was  not 
known  that  the  force  between  two  parti- 
cles depends  on  the  relative  orientation 
of  spin  and  orbit.  Today  the  idea  appears 
entirely  natural.  With  this  refinement, 
such  a  mass  of  data  about  the  behavior 
of  nuclei  could  be  explained  that  there 
remained  no  doubt  as  to  the  essential 
of  the  particle  being  absorbed,  i.e.,  lost 
from  the  beam  of  bombarding  neutrons 
[see  "A  Model  of  the  Nucleus,"  by 
Victor  F.  Weisskopf  and  E.  P.  Rosen- 
baum;  Scientific  American,  Decem- 
ber, 1955].  How  can  we  understand  the 
success  of  this  picture  of  independent 
particle  motion  in  view  of  the  Bohr  ar- 
gument? 

The  answer  to  this  question  has  been 
given  in  essence  by  Weisskopf.  It  may 
be  expressed  by  considering  the  time 
sequence  of  events.  To  be  sure,  the  bom- 
barding particle  is  likely  to  be  disturbed 
from  its  path  by  collisions,  but  this  will 
take  a  little  time.  So  for  a  short  time  it 
will  penetrate  into  the  nucleus  on  a 
regular  orbit,  and  this  initial  period  is 
important  for  determining  whether  it 
will  actually  get  deep  inside  or  be  turned 
back  at  the  surface.  Now,  to  recall  once 
again  the  uncertainty  principle,  we  know 
that  in  talking  about  a  short  time  inter- 
val we  must  not  try  to  specify  the  energy 
too  accurately.  We  should  therefore 
think  not  of  neutrons  with  a  well-defined 
energy,  but  of  a  beam  of  neutrons  vary- 
ing in  energy  by  an  amount  that  is 
greater  the  shorter  the  time  in  which 
they  are  hkely  to  be  involved  in  colli- 
sions inside  the  nucleus.  Experiments 
often  make  use  of  such  mixed  beams,  if 
the  experimenter  does  not  take  trouble 
to  select  the  neutron  energies  accurate- 
ly. If  we  have  data  with  accurate  energy 
selection  we  should  lump  together  the 
observations  over  a  suitable  range  of 
energies. 

Then  we  do  not  see  the  sharp  reso- 
nances any  more  because  there  will  al- 


107 


OXYGEN  NUCLEI  ARE  BOMBARDED  with  neutrons  in  this 
apparatus  at  the  Brookhaven  National  Laboratory.  The  neutrons 
are  produced  by  the  Brookhaven  nuclear  reactor,  the  concrete 


shield  of  which  is  visible  at  right.  The  oxygen  atoms  are  contained 
in  the  long  tank  in  the  middle  of  the  picture.  The  neutrons  which 
are  not  absorbed  are  counted  in  the  shorter  tank  at  lower  left. 


ways  be  many  of  them  within  the  energy 
range  we  use.  The  result  we  get  in  this 
way  will  reflect  the  number  and  strength 
of  the  resonances  within  the  selected 
range.  But  we  may  now  think  of  these 
results  also  as  determined  by  the  first 
short  time  interval  of  the  event,  and  as 
the  neutron  pursues  a  regular  orbit  dur- 
ing this  short  time  interval  the  results 
now  should  reflect  the  behavior  of  such 
regular  orbits.  This  therefore  leads  us 
directly  to  the  picture  of  the  optical 
model,  which  has  neutrons  traveling  in 
regular  orbits.  The  absorption  which  was 
allowed  for  in  Weisskopf's  optical  model 
merely  reflects  the  fact  that  the  particles 
do  not  stay  on  such  a  regular  orbit  for- 
ever, but  are  sooner  or  later  removed 
from  it  by  collisions  with  other  particles. 
The  strength  of  this  absorption  is  thus 
related  to  the  rate  at  whibh  collisions 
occur  inside  the  nucleus.  If  they  are 
very  frequent,  so  that  the  particle  covers 
only  a  small  fraction  of  the  nuclear  diam- 
eter before  it  hits  something,  the  "giant 
resonances,"  which  correspond  to  the 
orbits  of  a  single  particle,  will  become 


weaker  and  more  diflFuse.  The  fact  that 
they  are  found  to  be  pronounced  and 
distinct  shows  that  the  particle  has  a  fair 
chance  of  completing  at  least  one  revo- 
lution in  its  orbit.  In  this  respect  we  see 
that  the  extreme  form  of  Bohr's  liquid- 
drop  model,  or  our  simple  picture  of 
golf  balls,  exaggerates  the  situation.  But 
we  have  succeeded  in  reconciling  Bohr's 
explanation  of  the  many  sharp  reso- 
nances in  terms  of  the  many-body  as- 
pects of  the  problem,  with  the  super- 
imposed structure  of  giant  resonances, 
which  characterize  the  early  stages  of 
the  process. 

It  remains  to  account  for  the  quanti- 
tative features  of  the  optical  model— and 
in  particular  for  the  long  time  a  particle 
can  stay  in  its  orbit  before  being  thrown 
out  of  it  by  a  close  encounter  with  an- 
other particle— in  terms  of  the  basic 
forces.  A  promising  attack  on  this  prob- 
lem is  now  under  way.  The  workers  en- 
gaged in  it  include  G.  E.  Brown  in  the 
author's  group  at  the  University  of  Bir- 
mingham. In  particular,  the  low  rate  of 
collisions  is  seen  to  be  linked  again  with 


the  eflFect  of  the  exclusion  principle.  We 
have  seen  that  this  cuts  down  the  rate 
of  collisions  in  a  normal  nucleus  dras- 
tically. In  the  impact  problems  where 
there  is  more  energy  to  spare,  the  colli- 
sions are  more  frequent,  because  there 
are  more  orbits  available  that  are  not 
already  occupied,  but  the  prohibition  is 
still  partly  eflFective  and  the  collision  rate 
is  still  a  good  deal  less  than  that  sug- 
gested by  the  picttire  of  golf  balls,  for 
which  all  quantum  efiFects,  including  the 
exclusion  principle,  are  of  no  impor- 
tance. 

A  picture  thus  emerges  in  which  the 
various,  apparently  contradictory,  mod- 
els of  the  nucleus  are  seen  as  consistent 
parts  of  a  whole,  each  appropriate  for 
answering  certain  questions  about  the 
behavior  of  nuclei.  There  are  problems 
for  which  yet  other  models  have  to  be 
used,  including  the  important  "collective 
model"  developed  by  Aage  Bohr  and  B. 
Mottelson  of  Copenhagen,  but  it  would 
exceed  the  scope  of  this  article  to  de- 
scribe them  and  show  how  they  fit  into 
the  story. 


108 


The  origin  of  the  sun's  energy  is  a  long-standing  scientific 
problem.  The  answer  came  eventually  not  from  astronom- 
ical studies  alone,  but  from  investigations  of  the  behavior 
of  elementary  particles. 


13  Power  from  the  Stars 

Ralph  E.  Lapp 

Chapter  from  his  book.  Roads  to  Discovery,  published  in  1960. 


The  billions  upon  billions  of  stars  in  the  vast 
universe  all  have  one  thing  in  common— they  are  all  immense 
masses  of  flaming  gas.  Heat  evolved  deep  within  this  fiery 
sphere  gives  rise  to  the  brilliant  light  which  makes  the  star 
visible.  Our  nearest  star— our  sun— is  the  source  of  life  on 
earth.  Our  planet  is  kept  warm,  the  oceans  remain  unfrozen 
and  crops  grow  because  of  solar  warmth. 

Our  planet,  earth,  is  but  a  small  sphere  some  eight  thousand 
miles  from  rim  to  rim.  It  whirls  through  space  and,  caught 
in  the  invisible  grip  of  the  sun's  gravitational  attraction,  orbits 
endlessly,  maintaining  an  average  distance  from  the  sun  of  93 
million  miles.  At  this  distance  the  earth  receives  only  a  minute 
fraction  of  the  vast  outpouring  of  heat  and  light  that  the  sun 
radiates.  In  fact,  two  billion  times  more  heat  flies  off  into 
space  than  strikes  the  earth. 

How  does  our  sun  manage  to  keep  its  heat  furnaces  stoked? 
How  has  it  kept  blazing  away  at  this  rate  for  five  billion  years? 
Is  there  any  danger  that  it  may  "run  out  of  gas"? 

Only  recently,  with  the  data  turned  up  in  nuclear  research, 
has  it  been  possible  to  answer  these  questions.  Yet  from  the 
time  of  the  primitive  caveman,  the  sun  has  been  an  object  of 


109 


wonder  and  of  worship.  The  ancients  revered  the  Sun  God 
and  countless  humans  were  sacrificed  on  bloody  altars  to 
assuage  the  fiery  deity. 

In  more  modern  times  wonder  turned  to  curiosity  and 
curiosity  to  methodical  investigation.  Astronomers  found  that 
the  sun  is  a  million  times  bigger  than  the  earth,  that  the  tem- 
perature at  the  sun's  surface  is  about  six  thousand  degrees 
Centigrade,  and  that  the  temperature  deep  inside  the  core 
must  be  about  fifteen  million  degrees  Centigrade.  Astrophysi- 
cists proved  that  no  ordinary  burning  or  chemical  combus- 
tion could  account  for  solar  heat.  They  knew  there  was  not 
enough  oxygen  to  support  such  a  combustion.  All  efforts  to 
explain  the  sun's  power  failed;  no  energy  source  was  powerful 
enough  to  account  for  such  flaming  heat  over  a  period  of  five 
billion  years.  By  all  reckoning,  the  sun  should  have  spent  its 
energy  long  ago;  it  should  be  a  dead  cinder  in  the  sky  sur- 
rounded by  lifeless,  frozen  planets— a  darkness  in  the  universe. 

Sir  Arthur  Eddington  was  the  first  scientist  to  speculate 
correctly  about  the  source  of  the  sun's  energy.  He  suggested 
in  1920  that  stars  might  gain  energy  from  the  combination 
or  fusion  of  hydrogen  to  form  more  complex  elements.  This 
nuclear  "burning"  should  release  per  atom  a  million  times 
more  energy  than  any  known  chemical  process.  Eight  years 
later  Frederic  Houtermans  and  Robert  Atkinson  took  the  next 
step  which  turned  speculation  into  theory.  They  calculated 
that  hydrogen  within  the  sun's  core  consisted  of  atoms  so 
speedy  (due  to  heat  and  pressure)  that  some  collisions  be- 
tween hydrogen  atoms  would  produce  a  thermonuclear  re- 
action with  the  release  of  heat.  We  call  this  thermonuclear 
energy  and,  as  the  name  implies,  it  is  nuclear  energy  produced 
by  heat-agitated  atoms. 

Houtermans  and  Atkinson  had  practically  no  experimental 
data  about  the  behavior  of  hydrogen  atoms,  so  they  had  to 
proceed  on  pure  theory.  They  knew  that  at  the  elevated  tem- 
peratures inside  the  sun's  core  hydrogen  atoms  would  be 


no 


Power  from  the  Stars 


Stripped  of  their  electrons.  They  also  knew  that  the  great  pres- 
sure due  to  the  overweight  of  the  sun's  voluminous  mass 
squeezed  hydrogen  nuclei  (protons)  so  close  together  that  the 
result  was  a  proton  paste  eight  times  denser  than  solid  lead. 
Houtermans  and  Atkinson  calculated  that  hydrogen  fusion 
could  account  for  solar  heat.  However,  they  could  not  demon- 
strate that  the  fiery  proton  paste  in  the  sun's  core  would 
actually  sustain  a  thermonuclear  reaction.  They  lacked  the 
vital  nuclear  data  to  predict  the  behavior  of  protons  at  the 
temperature  that  exists  inside  our  sun. 

At  this  point  we  must  pause  to  show  that  the  "temperature" 
and  "energy"  of  protons  or,  for  that  matter,  any  particle,  may 
be  related.  This  is  important  because  the  nuclear  behavior  of 
a  particle  depends  very  strongly  upon  its  energy  (or  its  speed). 

Ordinarily,  temperature  is  easy  to  define.  We  measure  the 
temperature  of  a  glass  of  water  with  a  household  thermometer. 
We  may  measure  the  temperature  of  a  glowing  object  such  as 
a  lamp  filament  or  an  iron  poker  by  using  an  instrument  that 
relates  the  color  of  the  object  and  temperature.  An  iron 
poker,  at  room  temperature,  emits  no  light,  but  as  it  is  heated 
to  higher  and  higher  temperatures,  it  changes  in  color  from 
dull,  barely  visible  red  to  a  glowing  white.  We  say  that  the 
poker  is  white-hot.  Thus  we  measure  and  define  the  tempera- 
ture of  liquids  and  solids. 

But  how  would  you  measure  the  temperature  of  a  gas?  At 
first  thought,  this  seems  easy,  because  we  know  we  can  glance 
at  an  outdoor  thermometer  and  say  that  the  temperature  of 
the  air  is  80°,  or  whatever  it  happens  to  be.  But  what  about 
the  temperature  of  the  ionized  gas  inside  a  glowing  neon  tube? 
The  glass  walls  of  the  tube  are  cool  to  the  touch,  but  inside 
the  tube  the  neon  atoms  dash  about  with  astonishing  speed, 
much  much  faster  than  the  closely  packed  molecules  in  a 
white-hot  poker.  And  what  about  the  temperature  of  protons 
in  a  beam  emerging  from  a  cyclotron?  Scientists  say  that  an 
ionized  atom  moving  with  a  certain  speed  has  an  energy  of  so 


111 


many  electron  volts.  But  they  can  also  measure  this  in  terms 
of  temperature  on  a  scale  in  which  one  electron  volt  is  equiv- 
alent to  roughly  ten  thousand  degrees  Centigrade.  On  this 
scale,  a  1  Mev  (million  electron  volt)  proton  has  a  tempera- 
ture equivalent  of  ten  billion  degrees  Centigrade.  As  we  shall 
see  in  the  next  chapter,  cyclotrons  easily  accelerate  protons  to 
ten-million  electron  volts.  This  corresponds  to  protons  of  100 
billion  degrees  Centigrade,  or  vastly  higher  than  the  tempera- 
ture of  the  sun's  innermost  protons. 

A  Cornell  University  physics  professor.  Dr.  Hans  Bethe, 
next  tackled  the  problem  of  explaining  the  sun's  source  of  un- 
ending energy.  In  1938  Bethe  was  in  a  much  better  position 
to  make  calculations  than  Houtermans  and  Atkinson  had  been 
a  decade  earlier,  because  experimental  scientists  had  in  the 
meantime  come  up  with  so  much  data  about  nuclear  reac- 
tions. Thus  Bethe  was  able  to  calculate  how  rapidly  protons 
might  combine  with  one  another  under  conditions  existing 
inside  the  sun. 

Dr.  Bethe  developed  the  theory  that  four  protons  succes- 
sively fuse  together  to  form  a  single  atom  of  helium.  This  is 
not  accomplished  in  one  fell  swoop,  but  is  rather  a  multiple- 
stage  process  in  which,  first,  two  hydrogen  protons  collide  and 
bind  themselves  together  to  become  an  atom  of  heavy  hydro- 
gen, or  deuterium;  this  fused  atom  of  heavy  hydrogen  is  then 
struck  by  another  proton  and  helium-3  is  formed;  finally  an- 
other proton  collision  results  in  the  formation  of  a  nucleus  of 
helium-4.  The  process  Bethe  envisaged  could  take  place  in 
either  of  two  ways,  but  both  amounted  to  a  synthesis  or  fusion 
of  four  protons,  with  the  release  of  27  Mev  of  energy.  The 
energy  that  is  released  comes  from  the  mass  "lost"  when  the 
four  hydrogen  atoms  fuse  into  an  intimate  combination  which 
is  lighter  than  the  sum  of  the  individual  masses  of  the 
H-atoms.  The  mass  "lost"  or  energy  released  in  a  single  fusion 
is  small,  but  because  of  the  enormous  amount  of  hydrogen  in 
the  sun,  the  process  occurs  frequently  enough  to  keep  the  sun 


112 


Power  from  the  Stars 


blazing  hot.  Every  second  about  one  billion  tons  of  hydrogen 
undergo  fusion!  About  one  million  tons  of  "Einstein  mass" 
are  totally  converted  into  energy  every  second. 

Yet  this  seemingly  incredible  amount  of  hydrogen  is  so 
small  compared  with  the  sun's  total  supply  that  the  sun  will 
continue  to  shine  at  its  present  rate  for  billions  and  billions 
of  years  before  it  runs  out  of  fuel. 

If  we  consider  the  heat  generated  per  given  weight  of  the 
sun  rather  than  the  total  heat  produced,  we  arrive  at  some 
rather  astonishing  facts.  On  an  average,  it  takes  five  hun- 
dred tons  of  the  sun's  mass  to  produce  one  hundred  watts  of 
heat,  the  amount  given  off  from  a  household  electric  lamp 
bulb.  Even  at  the  sun's  center,  where  the  heat  is  given  off  at 
a  greater  rate,  it  still  takes  many  tons  of  the  sun's  substance 
to  evolve  one  hundred  watts  of  heat.  Actually,  the' human 
body— say  that  of  an  active  teen-ager— generates  one  hundred 
times  more  heat  than  is  generated  by  an  equivalent  weight  of 
hydrogen  gas  in  the  sun.  The  explanation  is  not  difficult.  In 
the  first  place,  we  are  not  comparing  body  temperature  with 
the  temperature  inside  the  sun;  but  rather  the  rates  at  which 
each  produces  its  heat.  The  sun  is  almost  perfectly  insulated 
by  its  outer  layers  of  gas,  so  that  even  a  tiny  amount  of  heat 
generated  at  its  core,  though  produced  at  a  much  slower  rate 
than  in  the  human  body,  is  kept  hot.  In  other  words,  the 
sun's  heat  is  trapped  inside  its  immense  mass  and  leaks  out 
to  the  surface  very  gradually.  Consequently,  the  sun  con- 
tinues to  build  up  in  temperature;  whereas  the  human  body, 
which  is  poorly  insulated,  loses  heat  rather  easily.  Even  mild 
exposure  to  wind  suffices  to  chill  a  person.  One  way  to  look 
at  the  problem  is  to  imagine  a  mass  the  size  of  the  sun 
composed  of  people  jammed  together  as  they  are  in  a  sub- 
way—that is,  matter  endowed  with  the  heat-producing  ca- 
pacity of  an  equivalent  mass  of  people.  The  heat  generated 
would  be  so  great  that  after  a  while  it  would  blaze  up  spec- 
tacularly. 


113 


The  reason  heat  is  evolved  so  slowly  even  in  the  center  of 
the  sun  is  that  the  hydrogen  atoms  are  at  such  a  low  tempera- 
ture. Roughly  twenty  million  degrees  Centigrade  may  not 
seem  low,  but  from  the  standpoint  of  a  nuclear  reaction,  the 
equivalent  energy  of  the  protons  inside  the  sun's  core  is  only 
1,700  electron  volts.  This  is  a  very  low  energy  for  nuclear 
reactions,  since  almost  all  the  reactions  studied  with  a  cyclo- 
tron are  measured  at  energies  of  millions  of  volts.  Nuclear 
reactions,  especially  when  we  specify  thermonuclear  reactions, 
"go"  faster  at  higher  energies.  This  means  that  deep  inside  the 
sun  the  protons  are  very  weak  and  fuse  together  so  slowly  that 
it  takes  millions  of  years  for  a  hydrogen-helium  cycle  to  occur. 
That  is  why  our  sun  doesn't  explode  like  a  hydrogen  bomb. 

Hydrogen  bombs  release  their  energy  in  less  than  one- 
millionth  of  a  second.  The  main  reason  why  such  fast  reac- 
tions can  be  attained  is  that  heavy  and  extra-heavy  hydrogen 
are  fused  in  the  bomb  reaction.  Deuterium  (double- weight 
hydrogen)  and  tritium  (triple-weight  hydrogen)  react  vio- 
lently in  contrast  to  the  slow  fusion  of  ordinary  or  single- 
weight  hydrogen. 

In  their  attempt  to  make  a  hydrogen  bomb,  the  experts 
were  up  against  a  cost  problem  with  regard  to  tritium,  and 
thus  it  came  as  a  real  step  ahead  when  they  figured  out  a 
way  to  put  a  liner  of  lithium-6  next  to  the  "nuke"  in  a  bomb. 
The  great  flash  of  neutrons  released  in  the  explosion  of  the 
A-bomb  trigger  irradiates  the  lithium  liner  and  gives  birth  to  a 
burst  of  tritium  atoms.  The  A-trigger  also  produces  an  intense 
heat  wave. 

Bomb  experts  killed  two  birds  with  one  stone  by  incorporat- 
ing the  lithium  in  the  form  of  a  chemical  compound  called 
lithium  deuteride,  a  compound  formed  by  the  synthesis  of 
lithium  and  heavy  hydrogen.  They  were  thus  able  to  bring 
about  the  fusion  of  deuterium  and  tritium.  As  we  have  seen, 
the  fusion  process  releases  energy— in  this  case,  17.6  Mev  for 
each  fusion.  This  is  significantly  less  than  fission  energy,  but 


114 


Power  from  the  Stars 


we  must  remember  that  a  pound  of  a  light  element  like 
lithium  contains  many  more  atoms  than  a  pound  of  a  heavy 
element  like  uranium  and  can  release  more  energy. 

The  energy  released  in  the  fusion  of  hydrogen  comes  off  in 
the  form  of  high-speed  particles,  just  as  in  the  case  of  fission. 
But  there  is  a  significant  difference,  for  most  of  the  energy  is 
imparted  to  the  neutron  that  is  produced  in  the  reaction. 
This  neutron  dashes  off  with  the  lion's  share  of  the  fusion 
energy.  It  is  so  speedy  that  it  would  tend  to  flash  out  into 
space  and  not  make  for  a  very  effective  bomb,  if  the  bomb  de- 
signers had  not  hit  upon  an  ingenious  idea. 

They  decided  to  make  the  runaway  neutron  do  some  work 
in  the  bomb.  They  put  a  heavy  jacket  of  ordinary  uranium 
around  the  lithium  liner.  The  fast-flying  neutrons  are  trapped 
in  this  jacket  and  there  they  cause  the  atoms  of  U^^^  to  fission. 
The  neutrons  released  in  fission,  you  will  recall,  will  not  split 
U^^®  as  readily  as  they  do  U^^^  This  is  because  U^^^  fissions 
with  low-speed  neutrons  whereas  U^^^  does  not.  Neutrons 
produced  in  the  chain  reaction  are  not  in  general  sufficiently 
speedy  to  fission  U^^^  But,  and  this  is  most  significant,  the 
neutrons  released  in  hydrogen  fusion  are  fast  enough  to 
cause  U^^®  to  fission. 

This  means,  then,  that  the  superbomb  is  really  a  three-stage 
device.  Stage  one  involves  the  firing  of  an  atomic  bomb  trig- 
ger. Stage  two  centers  upon  the  manufacture  of  tritium  from 
lithium  and  the  fusion  of  the  tritium  and  heavy  hydrogen. 
Stage  three  is  the  fission  of  ordinary  uranium  by  the  fast- 
fusion  neutrons  produced  in  stage  two. 

All  these  stages  are  interrelated  by  a  complex  neutron  rela- 
tionship. For  example,  when  U^"'  fissions  in  stage  three,  the 
neutrons  produced  feed  back  into  the  bomb  core,  causing 
more  fission  of  the  A-trigger  and  additional  production  of 
tritium.  In  addition,  the  explosion  in  stage  three  creates  more 
heat  to  produce  more  fusion.  These  reactions  are  so  complex 
and  all  happen  so  fast-in  one-millionth  of  a  second-that 


115 


calculation  of  the  bomb's  power  is  exceedingly  difficult  and 
must  be  relegated  to  whirlwind  automatic  computers.  These 
electronic  brains  are  capable  of  lightning-like  computation  and 
permit  the  bomb  designers  to  figure  out  how  a  given  weapon 
might  perform  prior  to  actual  test. 

Knowing  from  the  reality  of  the  H-bomb  that  hydrogen  is 
useful  in  an  explosive  thermonuclear  reaction,  it  is  natural  to 
ask  if  hydrogen  fusion  can  be  tamed  to  produce  energy  use- 
ful to  man.  Is  it  possible  for  man  to  imitate  or  outdo  the  sun's 
energy  power? 

Before  exploring  this  possibility  further,  it  will  help  to  have 
clearly  in  mind  why  scientists  concentrate  on  hydrogen  as  a 
fuel,  rather  than  some  other  element.  Going  back  to  Ruther- 
ford's experiments  on  the  scattering  of  alpha  particles,  recall 
that  only  a  very  few  of  the  alpha  particles  penetrated  close  to 
the  nucleus  in  the  target  atom.  As  the  positively  charged  alpha 
particles  sped  toward  the  positively  charged  nucleus  of  the 
atom,  they  were  strongly  repelled  by  the  like  electrical  forces. 
The  same  thing  happens  when  we  try  to  bring  together  two 
alpha  particles  or  two  hydrogen  nuclei  or  any  two  nuclei.  They 
resist  fusion  because  of  the  electrical  repulsion  of  their  posi- 
tively charged  cores.  The  greater  the  charge  on  the  atomic 
nucleus,  the  greater  will  be  the  repulsion  and  hence  the  diffi- 
culty of  fusing  the  two.  This  means  that  fusion  is  easiest  for 
the  lighter  elements;  and  hydrogen,  with  its  single  proton,  is 
of  course  the  lightest  of  all. 

However,  if  man  attempted  to  imitate  nature's  solar  proc- 
ess for  fusing  ordinary  hydrogen  as  fuel,  he  would  be  doomed 
to  failure;  as  we  saw  earlier,  the  kind  of  hydrogen  that  is  pres- 
ent in  the  sun's  interior  fuses  very  slowly,  so  that  a  single 
cubic  inch  of  the  central  core  will  evolve  only  a  fraction  of  a 
watt  of  heat  energy.  The  fact  of  the  matter  is  that  ordinary 
hydrogen  is  too  sluggish  a  nuclear  fuel  to  support  a  controlled, 
man-made  fusion  reaction.  However,  as  we  know,  other  kinds 
of  hydrogen  exist:  heavy  hydrogen  or  deuterium,  and  the 


116 


Power  from  the  Stars 


radioactive,  extra-heavy  form  of  hydrogen  called  tritium. 
Tritium  or  triple-weight  hydrogen  can  be  produced  in  a  nu- 
clear reactor  by  bombarding  lithium  with  neutrons.  Unlike 
ordinary  hydrogen,  deuterium  and  tritium  react  quickly  to 

Tritium         -|-      Deuterium   >■    Helium  -f      Neutron    -|-      Energy 


^*»=-# 


+         •        -h    17.6  mev 


(^  Proton  ^^  Neutron 

36.  Illustrating  the  fusion  of  two  atoms  of  hydrogen  to  form  a  single 
atom  of  helium  and  a  neutron. 

create  helium;  it  is  this  fact  that  will  make  controlled  fusion 
power  possible.  These  isotopes  are  known  to  undergo  the  fol- 
lowing reactions: 

iD2  -f  iD^  =z  ,W  +  iT^ 

,D^  +  ,T^  =  ^He"  -f  on^ 
^T^  +  ^T^  =,  ^He"  +  2on^ 

All  these  reactions  release  energy.  The  first  two  yield  4.13  and 
3.37  Mev  respectively,  while  the  last  two  release  17.58  and 
11.32  Mev  of  energy. 

While  the  energy  released  by  each  fusion  of  hydrogen  iso- 
topes is  considerably  less  than  the  200  Mev  for  each  fission  of 
a  uranium  atom,  as  we  noted  earlier  in  the  case  of  lithium, 
the  number  of  atoms  in  a  pound  of  hydrogen  is  very  much 
greater  than  the  number  of  atoms  in  a  pound  of  uranium.  A 
pound  of  deuterium,  for  instance,  releases  roughly  three  times 
as  much  energy  as  a  pound  of  uranium.  Converted  into  the  en- 
ergy content  of  the  heavy  hydrogen  in  a  cup  of  water,  this 
amounts  to  the  heat  equivalent  of  fifty  pounds  of  coal.  The 
supply  of  heavy  hydrogen  is  practically  without  limit  since 
the  lakes  and  oceans  on  our  planet  contain  inexhaustible  re- 


117 


serves  of  water.  Thus,  if  man  can  extract  hydrogen  fusion 
energy,  he  has  at  hand  an  unhmited  new  supply  of  fuel. 

The  goal  of  hydrogen  power  is  tempting  for  more  than  just 
this  reason.  Hydrogen  fusion  produces  no  residual  radioactive 
fragments,  so  the  radiation  hazard  of  uranium  fission  products 
is  not  present  in  this  new  type  of  power  source.  Furthermore, 
because  of  the  nature  of  the  reactor  that  will  probably  be  used 
to  produce  fusion  power,  there  is  no  danger  of  a  runaway 
explosion,  such  as  can  occur  in  certain  types  of  uranium  power 
plants.  In  addition,  there  is  the  enticing  prospect  that  it  may 
be  possible  to  derive  energy  from  a  fusion  reactor  directly,  in 
the  form  of  electrical  power. 

Attractive  as  these  prospects  appear,  one  has  to  consider 
the  huge  difficulties  that  stand  in  the  road  toward  attaining 
fusion  power.  The  basic  fuel,  deuterium,  is  no  problem,  since 
heavy  water  can  be  produced  in  hundred-ton  lots  and  is  readily 
available  commercially  at  $28  per  pound.  And  there  is  no 
problem  in  obtaining  pure  deuterium  gas  from  the  heavy 
water.  The  fundamental  problem  is  so  to  design  a  reactor  that 
ionized  deuterium,  or  hydrogen  plasma  as  it  is  called,  can  be 
brought  to  sufficiently  high  speed  for  fusion  to  take  place. 
This  requires  that  a  temperature  above  one  hundred  million 
degrees  Centigrade  be  attained. 

Scientists  in  many  countries  are  hard  at  work  designing 
machines  that  will  use  electric  and  magnetic  fields  to  squeeze 
hydrogen  plasma  together  or  "pinch"  it.  The  basic  idea  was 
set  forth  in  1934  by  the  American  physicist,  W.  H.  Bennett. 
He  suggested  that  charged  particles  of  hydrogen  moving  in  a 
stream  would  constitute  an  electrical  current  that  should  in- 
duce its  own  magnetic  fields;  this,  in  turn,  would  act  to 
pinch  the  plasma  together,  bringing  the  individual  ions  into 
collision  with  each  other.  The  more  violent  the  collisions 
(i.e.,  the  "hotter"  the  pinch)  and  the  more  frequent  they  are, 
the  greater  is  the  probability  that  fusion  will  occur. 

Unfortunately,  the  phenomenon  just  described  is  not  very 


118 


Power  from  the  Stars 


easy  to  control  or  stabilize.  In  the  United  States,  the  Atomic 
Energy  Commission  established  Project  Sherwood  for  the 
purpose  of  bringing  about  the  controlled  release  of  fusion 
power.  The  research  work,  begun  on  a  modest  budget  in 
1951,  expanded  to  a  vigorous  program  in  1959,  backed  by  a 
forty-million-dollar  annual  budget.  A  variety  of  experimental 
devices  for  studying  the  "pinch"  effect  have  been  built  at 
the  Los  Alamos  Scientific  Laboratory,  of  which  the  Per- 
hapstron  is  an  example.  Hydrogen  ions  are  circulated  in  a 
doughnut-shaped  vacuum  tube  and  constricted  by  an  electrical 
current  into  a  narrow  column  inside  this  chamber. 

A  ''Magnetic  Mirror"  device  represents  a  different  approach 
to  the  fusion  problem  adopted  by  scientists  at  the  University 
of  California's  Livermore  Laboratory.  Instead  of  a  doughnut 
chamber,  a  straight  tube  is  employed  and  the  hydrogen  plasma 
is  "trapped"  by  intense  magnetic  fields  and  "reflected"  back 
from  one  end  of  the  tube  into  the  center  of  the  chamber. 
Still  another  line  of  approach  is  shown  in  the  illustration. 
Here  at  Oak  Ridge,  scientists  are  studying  fusion  possibilities 
by  hurling  heavy  hydrogen  molecules  downward  into  a  re- 
action chamber  where  they  are  ionized  by  an  electric  arc  and 
then  subjected  to  intense  magnetic  forces.  A  more  ambitious 
and  larger-scale  approach  to  fusion  power  is  under  way  at 
Princeton  University,  where  a  Stellerator  is  being  constructed. 
Magnetic  forces  from  a  thick  magnetic  coil  that  is  wrapped 
around  a  figure-8-shaped  vacuum  chamber  center  the  hydro- 
gen ions  in  the  chamber.  This  unusual  container  is  designed 
to  keep  the  hydrogen  ions  from  straying  out  to  the  wall  and 
giving  up  their  energy.  Fusion  power  can  be  attained  only  if 
the  plasma  can  be  kept  isolated  from  contact  with  the  con- 
tainer. 

Obviously,  no  structural  container  can  hold  anything  so  hot 
as  this  fiery  plasma.  Instead,  scientists  propose  to  contain  the 
plasma  by  means  of  magnetic  fields  which  force  the  ions  to 
stay  in  a  restricted  space,  i.e.,  a  kind  of  "magnetic  bottle." 


119 


37.  The  Oak  Ridge  Fusion  research  device  designed  to  probe  hydro- 
gen fusion  on  a  laboratory  scale.  (Oak  Ridge  National  Laboratory) 


120 


Power  from  the  Stars 


However,  there  is  the  serious  problem  of  designing  such  a 
magnetic  "container"  so  that  it  is  substantially  leakproof.  Any 
small  leak  would  allow  the  hot  plasma  to  squirt  out  to  the 
tube  wall  and  cool  off,  thus  ruining  chances  of  attaining  the 
high  temperatures  necessary  for  fusion.  Experiments  in  the 
United  States  have  produced  plasma  at  a  temperature  of  about 
ten  million  degrees  Centigrade. 

Fusion  research  is  also  going  on  in  Russia,  Britain,  Sweden, 
Germany,  Japan  and  many  other  countries.  The  British  have 
pioneered  in  this  new  field  of  research  and  have  constructed 
rather  large  machines.  All  machines  concentrate  on  using 
deuterium  as  the  reacting  substance,  although  later  experi- 
ments may  be  done  with  tritium.  However,  tritium  is  more 
difficult  to  handle  experimentally  because  of  the  radiation 
hazard  and  the  contamination  of  the  equipment. 

If  one  selects  pure  deuterium  as  the  nuclear  fuel  for  fusion 
power,  there  is  the  attractive  prospect  that,  since  two-thirds 
of  the  energy  comes  off  in  the  form  of  charged  particles,  it 
might  be  possible  to  convert  this  directly  into  electrical  energy. 
Picturing  the  way  a  piston  functions  in  a  steam  engine,  one 
may  think  of  moving  plasma  working  against  magnetic  fields, 
and  electrical  circuits  drawing  off  the  energy.  With  a  mixture 
of  deuterium  and  tritium,  the  majority  of  the  energy  is  carried 
off  by  the  neutrons.  A  blanket  of  liquid  lithium  might  be 
used  to  absorb  the  neutrons  and  convert  their  energy  into 
heat  and  at  the  same  time  generate  useful  tritium  as  the 
lithium  atoms  are  fissioned.  Thus  fusion  power  would  be  used 
to  produce  heat  external  to  the  plasma  and  this  heat  would 
then  be  used  for  the  purposes  of  producing  more  power. 

The  possibility  of  fusion  power  is  raised  at  a  time  when 
uranium  power  plants  are  being  engineered  to  produce  power 
on  a  basis  competitive  with  conventional  fuels.  Rising  coal 
costs  in  England  have  provided  the  British  with  a  strong  in- 
centive to  replace  coal  with  uranium  and  they  have  devoted 
tremendous  effort  to  building  uranium  power  stations.  Now 


121 


there  is  the  question  whether  uranium  power  is  not  obsolete 
before  it  is  even  fully  developed.  Will  not  fusion  of  hydrogen 
replace  uranium  fission  as  man's  source  of  energy?  Ultimately, 
it  seems  clear  that  hydrogen  fusion  will  be  developed  to  the 
point  where  it  is  attractive  for  some  applications,  but  this 
new  source  of  power  is  in  its  technological  infancy  and  it  is 
too  early  to  predict  when  it  will  assume  its  place  in  the  sun. 
However,  it  can  be  said  that  many  scientists  who  are  working 
on  this  ultimate  fuel  are  optimistic  that  they  will  be  able  to 
solve  the  very  formidable  problems  that  lie  ahead.  Further- 
more, they  feel  that  in  their  explorations  of  high-temperature 
plasmas  and  intense  magnetic  fields  they  will  learn  many 
new  facts  about  atoms  and  the  cosmos.  Indeed,  some  scientists 
believe  that  even  if  hydrogen  power  should  never  succeed, 
should  man  be  frustrated  in  his  attempt  to  outdo  the  sun,  he 
will  gather  rich  dividends  in  fundamental  knowledge,  and  the 
research  will  have  been  worth  while.  But  the  hope  is  that 
the  quest  for  fusion  power  will  bring  to  mankind  an  unlimited 
source  of  power  to  heat  homes,  light  cities  and  power  fac- 
tories for  millions  of  years  to  come. 


122 


Mrs.  Enrico  Fermi  gives  in  colorful  detail  her  personal 
account  of  the  first  nuclear  chain  reaction  at  the 
University  of  Chicago  squash  courts. 


14       Success 


Laura  Fermi 

Chapter  from  her  book,  Atoms  in  the  Family,  published  in  1954. 

Meanwhile  Herbert  Anderson  and  his  group  at  the  Met.  Lab.  had 
also  been  building  small  piles  and  gathering  information  for  a  larger 
pile  from  their  behavior.  The  best  place  Compton  had  been  able  to 
find  for  work  on  the  pile  was  a  squash  court  under  the  West  Stands 
of  Stagg  Field,  the  University  of  Chicago  stadium.  President  Hutch- 
ins  had  banned  football  from  the  Chicago  campus,  and  Stagg  Field 
was  used  for  odd  purposes.  To  the  west,  on  Ellis  Avenue,  the  stadium 
is  closed  by  a  tall  gray-stone  structure  in  the  guise  of  a  medieval 
castle.  Through  a  heavy  portal  is  the  entrance  to  the  space  beneath 
the  West  Stands.  The  Squash  Court  was  part  of  this  space.  It  was  30 
feet  wide,  twice  as  long,  and  over  26  feet  high. 

The  physicists  would  have  liked  more  space,  but  places  better 
suited  for  the  pile,  which  Professor  Compton  had  hoped  he  could 
have,  had  been  requisitioned  by  the  expanding  armed  forces  sta- 
tioned in  Chicago.  The  physicists  were  to  be  contented  with  the 
Squash  Court,  and  there  Herbert  Anderson  had  started  assembling 
piles.  They  were  still  "small  piles,"  because  material  flowed  to  the 
West  Stands  at  a  very  slow,  if  steady,  pace.  As  each  new  shipment  of 
crates  arrived,  Herbert's  spirits  rose.  He  loved  working  and  was  of 
impatient  temperament.  His  slender,  almost  delicate,  body  had  un- 
suspected resilience  and  endurance.  He  could  work  at  all  hours  and 
drive  his  associates  to  work  along  with  his  same  intensity  and  en- 
thusiasm. 

A  shipment  of  crates  arrived  at  the  West  Stands  on  a  Saturday 
afternoon,  when  the  hired  men  who  would  normally  unpack  them 
were  not  working.  A  university  professor,  older  by  several  years 
than  Herbert,  gave  a  look  at  the  crates  and  said  lightly:  "Those 
fellows  will  unpack  them  Monday  morning." 

"Those  fellows,  Hell!  We'll  do  them  now,"  flared  up  Herbert,  who 


123 


had  never  felt  inhibited  in  the  presence  of  older  men,  higher  up  in 
the  academic  hierarchy.  The  professor  took  off  his  coat,  and  the  two 
of  them  started  wrenching  at  the  crates. 

Profanity  was  freely  used  at  the  Met.  Lab.  It  relieved  the  tension 
built  up  by  having  to  work  against  time.  Would  Germany  get  atomic 
weapons  before  the  United  States  developed  them?  Would  these 
weapons  come  in  time  to  help  win  the  war?  These  unanswered  ques- 
tions constantly  present  in  the  minds  of  the  leaders  in  the  project 
pressed  them  to  work  faster  and  faster,  to  be  tense,  and  to  swear. 

Success  was  assured  by  the  spring.  A  small  pile  assembled  in  the 
Squash  Court  showed  that  all  conditions — purity  of  materials,  dis- 
tribution of  uranium  in  the  graphite  lattice — were  such  that  a  pile 
of  critical  size  would  chain-react. 

"It  could  be  May,  or  early  June  at  latest,"  Enrico  told  me,  as  we 
recently  reminisced  about  the  times  of  the  Met.  Lab.  "I  remember  I 
talked  about  that  experiment  on  the  Indiana  dunes,  and  it  was  the 
first  time  I  saw  the  dunes.  You  were  still  in  Leonia.  I  went  with  a 
group  from  the  Met.  Lab.  I  liked  the  dunes:  it  was  a  clear  day,  with 
no  fog  to  dim  colors.  .  .  ." 

"I  don't  want  to  hear  about  the  dunes,"  I  said.  "Tell  me  about 
that  experiment." 

"I  like  to  swim  in  the  lake,  .  .  ."  Enrico  paid  no  attention  to  my 
remark.  I  knew  that  he  enjoyed  a  good  swim,  and  I  could  well 
imagine  him  challenging  a  group  of  younger  people,  swimming  far- 
ther and  for  a  longer  time  than  any  of  them,  then  emerging  on  the 
shore  with  a  triumphant  grin. 

"Tell  me  about  that  experiment,"  I  insisted. 

"We  came  out  of  the  water,  and  we  walked  along  the  beach." 

I  began  to  feel  impatient.  He  did  not  have  to  mention  the  walk. 
He  always  walks  after  swimming,  dripping  wet,  water  streaming 
from  his  hair.  In  1942  there  was  certainly  much  more  hair  on  his 
head  to  shed  water,  not  just  the  little  fringe  on  the  sides  and  on  the 
back  that  there  is  now,  and  it  was  much  darker. 

".  .  .  and  I  talked  about  the  experiment  with  Professor  Stearns. 
The  two  of  us  walked  ahead  of  the  others  on  the  beach.  I  remember 
our  efforts  to  speak  in  such  a  way  that  the  others  would  not  under- 
stand  " 


124 


Success 


"Why?  Didn't  everyone  at  the  Met.  Lab.  know  that  you  were 
building  piles?" 

"They  knew  we  built  piles.  They  did  not  know  that  at  last  we  had 
the  certainty  that  a  pile  would  work.  The  fact  that  a  chain  reaction 
was  feasible  remained  classified  material  for  a  while.  I  could  talk 
freely  with  Stearns  because  he  was  one  of  the  leaders." 

"If  you  were  sure  a  larger  pile  would  work,  why  didn't  you  start  it 
at  once?" 

"We  did  not  have  enough  materials,  neither  uranium  nor  graph- 
ite. Procurement  of  uranium  metal  was  always  an  obstacle.  It  ham- 
pered progress." 

While  waiting  for  more  materials,  Herbert  Anderson  went  to  the 
Goodyear  Tire  and  Rubber  Company  to  place  an  order  for  a  square 
balloon.  The  Goodyear  people  had  never  heard  of  square  balloons, 
they  did  not  think  they  could  fly.  At  first  they  threw  suspicious 
glances  at  Herbert.  The  young  man,  however,  seemed  to  be  in  full 
possession  of  his  wits.  He  talked  earnestly,  had  figured  out  precise 
specifications,  and  knew  exactly  what  he  wanted.  The  Goodyear 
people  promised  to  make  a  square  balloon  of  rubberized  cloth.  They 
delivered  it  a  couple  of  months  later  to  the  Squash  Court.  It  came 
neatly  folded,  but,  once  unfolded,  it  was  a  huge  thing  that  reached 
from  floor  to  ceiling. 

The  Squash  Court  ceiling  could  not  be  pushed  up  as  the  physi- 
cists would  have  liked.  They  had  calculated  that  their  final  pile 
ought  to  chain-react  somewhat  before  it  reached  the  ceiling.  But  not 
much  margin  was  left,  and  calculations  are  never  to  be  trusted  en- 
tirely. Some  impurities  might  go  unnoticed,  some  unforeseen  factor 
might  upset  theory.  The  critical  size  of  the  pile  might  not  be  reached 
at  the  ceiling.  Since  the  physicists  were  compelled  to  stay  within 
that  very  concrete  limit,  they  thought  of  improving  the  performance 
of  the  pile  by  means  other  than  size. 

The  experiment  at  Columbia  with  a  canned  pile  had  indicated 
that  such  an  aim  might  be  attained  by  removing  the  air  from  the 
pores  of  the  graphite.  To  can  as  large  a  pile  as  they  were  to  build 
now  would  be  impracticable,  but  they  could  assemble  it  inside  a 
square  balloon  and  pump  the  air  from  it  if  necessary. 

The  Squash  Court  was  not  large.  When  the  scientists  opened  the 
balloon  and  tried  to  haul  it  into  place,  they  could  not  see  its  top 


125 


from  the  floor.  There  was  a  movable  elevator  in  the  room,  some  sort 
of  scaffolding  on  wheels  that  could  raise  a  platform.  Fermi  climbed 
onto  it,  let  himself  be  hoisted  to  a  height  that  gave  him  a  good  view 
of  the  entire  balloon,  and  from  there  he  gave  orders: 

"All  hands  stand  by!" 

"Now  haul  the  rope  and  heave  her!" 

"More  to  the  right!" 

"Brace  the  tackles  to  the  left!" 

To  the  people  below  he  seemed  an  admiral  on  his  bridge,  and 
"Admiral"  they  called  him  for  a  while. 

When  the  balloon  was  secured  on  five  sides,  with  the  flap  that 
formed  the  sixth  left  down,  the  group  began  to  assemble  the  pile 
inside  it.  Not  all  the  material  had  arrived,  but  they  trusted  that  it 
would  come  in  time. 

From  the  numerous  experiments  they  had  performed  so  far,  they 
had  an  idea  of  what  the  pile  should  be,  but  they  had  not  worked  out 
the  details,  there  were  no  drawings  nor  blueprints  and  no  time  to 
spare  to  make  them.  They  planned  their  pile  even  as  they  built  it. 
They  were  to  give  it  the  shape  of  a  sphere  of  about  26  feet  in 
diameter,  supported  by  a  square  frame,  hence  the  square  balloon. 

The  pile  supports  consisted  of  blocks  of  wood.  As  a  block  was  put 
in  place  inside  the  balloon,  the  size  and  shape  of  the  next  were 
figured.  Between  the  Squash  Court  and  the  near-by  carpenter's  shop 
there  was  a  steady  flow  of  boys,  who  fetched  finished  blocks  and 
brought  specifications  for  more  on  bits  of  paper. 

When  the  physicists  started  handling  graphite  bricks,  everything 
became  black.  The  walls  of  the  Squash  Court  were  black  to  start 
with.  Now  a  huge  black  wall  of  graphite  was  going  up  fast.  Graphite 
powder  covered  the  floor  and  made  it  black  and  as  slippery  as  a 
dance  floor.  Black  figures  skidded  on  it,  figures  in  overalls  and  gog- 
gles under  a  layer  of  graphite  dust.  There  was  one  woman  among 
them,  Leona  Woods;  she  could  not  be  distinguished  from  the  men, 
and  she  got  her  share  of  cussing  from  the  bosses. 

The  carpenters  and  the  machinists  who  executed  orders  with  no 
knowledge  of  their  purpose  and  the  high-school  boys  who  helped  lay 
bricks  for  the  pile  must  have  wondered  at  the  black  scene.  Had  they 
been  aware  that  the  altimate  result  would  be  an  atomic  bomb,  they 
might  have  renamed  the  court  Pluto's  Workshop  or  Hell's  Kitchen. 


126 


Success 


To  solve  difl5culties  as  one  meets  them  is  much  faster  than  to  try 
to  foresee  them  all  in  detail.  As  the  pile  grew,  measurements  were 
taken  and  further  construction  adapted  to  results. 

The  pile  never  reached  the  ceiling.  It  was  planned  as  a  sphere  26 
feet  in  diameter,  but  the  last  layers  were  never  put  into  place.  The 
sphere  remained  flattened  at  the  top.  To  make  a  vacuum  proved  un- 
necessary, and  the  balloon  was  never  sealed.  The  critical  size  of  the 
pile  was  attained  sooner  than  was  anticipated. 

Only  six  weeks  had  passed  from  the  laying  of  the  first  graphite 
brick,  and  it  was  the  morning  of  December  2. 

Herbert  Anderson  was  sleepy  and  grouchy.  He  had  been  up  until 
two  in  the  morning  to  give  the  pile  its  finishing  touches.  Had  he 
pulled  a  control  rod  during  the  night,  he  could  have  operated  the 
pile  and  have  been  the  first  man  to  achieve  a  chain  reaction,  at 
least  in  a  material,  mechanical  sense.  He  had  a  moral  duty  not  to 
pull  that  rod,  despite  the  strong  temptation.  It  would  not  be  fair  to 
Fermi.  Fermi  was  the  leader.  He  had  directed  research  and  worked 
out  theories.  His  were  the  basic  ideas.  His  were  the  privilege  and  the 
responsibility  of  conducting  the  final  experiment  and  controlling 
the  chain  reaction. 

"So  the  show  was  all  Enrico's,  and  he  had  gone  to  bed  early  the 
night  before,"  Herbert  told  me  years  later,  and  a  bit  of  regret  still 
lingered  in  his  voice. 

Walter  Zinn  also  could  have  produced  a  chain  reaction  during  the 
night.  He,  too,  had  been  up  and  at  work.  But  he  did  not  care  whether 
he  operated  the  pile  or  not;  he  did  not  care  in  the  least.  It  was  not 
his  job. 

His  task  had  been  to  smooth  out  difliculties  diu-ing  the  pile  con- 
struction. He  had  been  some  sort  of  general  contractor:  he  had 
placed  orders  for  material  and  made  sure  that  they  were  delivered 
in  time;  he  had  supervised  the  machine  shops  where  graphite  was 
milled;  he  had  spurred  others  to  work  faster,  longer,  more  eflficient- 
ly.  He  had  become  angry,  had  shouted,  and  had  reached  his  goal.  In 
six  weeks  the  pile  was  assembled,  and  now  he  viewed  it  with  relaxed 
nerves  and  with  that  vague  feeling  of  emptiness,  of  slight  disorienta- 
tion, which  never  fails  to  follow  completion  of  a  purposeful  task. 
There  is  no  record  of  what  were  the  feelings  of  the  three  young 
men  who  crouched  on  top  of  the  pile,  under  the  ceiling  of  the  square 


127 


balloon.  They  were  called  the  "suicide  squad."  It  was  a  joke,  but 
perhaps  they  were  asking  themselves  whether  the  joke  held  some 
truth.  They  were  like  firemen  alerted  to  the  possibility  of  a  fire, 
ready  to  extinguish  it.  If  something  unexpected  were  to  happen,  if 
the  pile  should  get  out  of  control,  they  would  "extinguish"  it  by 
flooding  it  with  a  cadmium  solution.  Cadmium  absorbs  neutrons  and 
prevents  a  chain  reaction. 

Leona  Woods,  the  one  girl  in  that  group  of  men,  was  calm  and 
composed;  only  the  intensity  of  her  deep  dark  eyes  revealed  the  ex- 
tent of  her  alertness. 

Among  the  persons  who  gathered  in  the  Squash  Court  on  that 
morning,  one  was  not  connected  with  the  Met.  Lab. — Mr.  Crawford 
H.  Greenewalt  of  E.  I.  duPont  de  Nemours,  who  later  became  the 
president  of  the  company.  Arthur  Compton  had  led  him  there  out  of 
a  near-by  room  where,  on  that  day,  he  and  other  men  from  his  com- 
pany happened  to  be  holding  talks  with  top  Army  oflBcers. 

Mr.  Greenewalt  and  the  duPont  people  were  in  a  difl&cult  position, 
and  they  did  not  know  how  to  reach  a  decision.  The  Army  had  taken 
over  the  Uranium  Project  on  the  previous  August  and  renamed  it 
Manhattan  District.  In  September  General  Leslie  R.  Groves  was 
placed  in  charge  of  it.  General  Groves  must  have  been  of  a  trusting 
nature:  before  a  chain  reaction  was  achieved,  he  was  already  urging 
the  duPont  de  Nemours  Company  to  build  and  operate  piles  on  a 
production  scale. 

In  a  pile,  Mr.  Greenewalt  was  told,  a  new  element,  plutonium,  is 
created  during  uranium  fission.  Plutonium  would  probably  be  suited 
for  making  atomic  bombs.  So  Greenewalt  and  his  group  had  been 
taken  to  Berkeley  to  see  the  work  done  on  plutonium,  and  then 
flown  to  Chicago  for  more  negotiations  with  the  Army. 

Mr.  Greenewalt  was  hesitant.  Of  course  his  company  would  like 
to  help  win  the  war!  But  piles  and  plutonium! 

With  the  Army's  insistent  voice  in  his  ears,  Compton,  who  had 
attended  the  conference,  decided  to  break  the  rules  and  take  Mr. 
Greenewalt  to  witness  the  first  operation  of  a  pile. 

They  all  climbed  onto  the  balcony  at  the  north  end  of  the  Squash 
Court;  all,  except  the  three  boys  perched  on  top  of  the  pile  and  ex- 
cept a  young  physicist,  George  Weil,  who  stood  alone  on  the  floor 


128 


Success 


by  a  cadmium  rod  that  he  was  to  pull  out  of  the  pile  when  so 
instructed. 

And  so  the  show  began. 

There  was  utter  silence  in  the  audience,  and  only  Fermi  spoke. 
His  gray  eyes  betrayed  his  intense  thinking,  and  his  hands  moved 
along  with  his  thoughts. 

"The  pile  is  not  performing  now  because  inside  it  there  are  rods 
of  cadmium  which  absorb  neutrons.  One  single  rod  is  sufficient  to 
prevent  a  chain  reaction.  So  our  first  step  will  be  to  pull  out  of  the 
pile  all  control  rods,  but  the  one  that  George  Weil  will  man."  As  he 
spoke  others  acted.  Each  chore  had  been  assigned  in  advance  and 
rehearsed.  So  Fermi  went  on  speaking,  and  his  hands  pointed  out 
the  things  he  mentioned. 

"This  rod,  that  we  have  pulled  out  with  the  others,  is  automati- 
cally controlled.  Should  the  intensity  of  the  reaction  become  greater 
than  a  pre-set  limit,  this  rod  would  go  back  inside  the  pile  by  itself. 

"This  pen  will  trace  a  line  indicating  the  intensity  of  the  radiation. 
When  the  pile  chain-reacts,  the  pen  will  trace  a  line  that  will  go  up 
and  up  and  that  will  not  tend  to  level  off.  In  other  words,  it  will  be 
an  exponential  line. 

"Presently  we  shall  begin  our  experiment.  George  will  pull  out  his 
rod  a  little  at  a  time.  We  shall  take  measurements  and  verifv  that 
the  pile  will  keep  on  acting  as  we  have  calculated. 

"Weil  will  first  set  the  rod  at  thirteen  feet.  This  means  that  thir- 
teen feet  of  the  rod  will  still  be  inside  the  pile.  The  counters  will 
click  faster  and  the  pen  will  move  up  to  this  point,  and  then  its  trace 
will  level  off.  Go  ahead,  George!" 

Eyes  turned  to  the  graph  pen.  Breathing  was  suspended.  Fermi 
grinned  with  confidence.  The  counters  stepped  up  their  clicking;  the 
pen  went  up  and  then  stopped  where  Fermi  had  said  it  would. 
Greenewalt  gasped  audibly.  Fermi  continued  to  grin. 

He  gave  more  orders.  Each  time  Weil  pulled  the  rod  out  some 
more,  the  counters  increased  the  rate  of  their  clicking,  the  pen 
raised  to  the  point  that  Fermi  predicted,  then  it  leveled  off. 

The  morning  went  by.  Fermi  was  conscious  that  a  new  experiment 
of  this  kind,  carried  out  in  the  heart  of  a  big  city,  might  become  a 
potential  hazard  unless  all  precautions  were  taken  to  make  sure  that 
at  all  times  the  operation  of  the  pile  conformed  closely  with  the 


129 


results  of  the  calculations.  In  his  mind  he  was  sure  that  if  George 
Weil's  rod  had  been  pulled  out  all  at  once,  the  pile  would  have 
started  reacting  at  a  leisurely  rate  and  could  have  been  stopped  at 
will  by  reinserting  one  of  the  rods.  He  chose,  however,  to  take  his 
time  and  be  certain  that  no  unforeseen  phenomenon  would  disturb 
the  experiment. 

It  is  impossible  to  say  how  great  a  danger  this  unforeseen  element 
constituted  or  what  consequences  it  might  have  brought  about. 
According  to  the  theory,  an  explosion  was  out  of  the  question.  The 
release  of  lethal  amounts  of  radiation  through  an  uncontrolled  reac- 
tion was  improbable.  Yet  the  men  in  the  Squash  Court  were  working 
with  the  unknown.  They  could  not  claim  to  know  the  answers  to  all 
the  questions  that  were  in  their  minds.  Caution  was  welcome.  Caution 
was  essential.  It  would  have  been  reckless  to  dispense  with  caution. 

So  it  was  lunch  time,  and,  although  nobody  else  had  given  signs 
of  being  hungry,  Fermi,  who  is  a  man  of  habits,  pronounced  the 
now  historical  sentence: 

"Let's  go  to  lunch." 

After  lunch  they  all  resumed  their  places,  and  now  Mr.  Greene- 
wait  was  decidedly  excited,  almost  impatient. 

But  again  the  experiment  proceeded  by  small  steps,  until  it  was 
3:20. 

Once  more  Fermi  said  to  Weil: 

"Pull  it  out  another  foot";  but  this  time  he  added,  turning  to  the 
anxious  group  in  the  balcony:  "This  will  do  it.  Now  the  pile  will 
chain-react." 

The  counters  stepped  up;  the  pen  started  its  upward  rise.  It 
showed  no  tendency  to  level  off.  A  chain  reaction  was  taking  place 
in  the  pile. 

Leona  Woods  walked  up  to  Fermi  and  in  a  voice  in  which  there 
was  no  fear  she  whispered:  "When  do  we  become  scared?" 

Under  the  ceiling  of  the  balloon  the  suicide  squad  was  alert,  ready 
with  their  liquid  cadmium:  this  was  the  moment.  But  nothing  much 
happened.  The  group  watched  the  recording  instruments  for  28  min- 
utes. The  pile  behaved  as  it  should,  as  they  all  had  hoped  it  would, 
as  they  had  feared  it  would  not. 

The  rest  of  the  story  is  well  known.  Eugene  Wigner,  the  Hun- 


130 


Success 


garian-born  physicist  who  in  1939  with  Szilard  and  Einstein  had  (See  letter  on 

alerted  President  Roosevelt  to  the  importance  of  uranium  fission,  ^^-  ^^2-133) 

presented  Fermi  with  a  bottle  of  Chianti.  According  to  an  improb- 
able legend,  Wigner  had  concealed  the  bottle  behind  his  back  dur- 
ing the  entire  experiment. 

All  those  present  drank.  From  paper  cups,  in  silence,  with  no 
toast.  Then  all  signed  the  straw  cover  on  the  bottle  of  Chianti.  It  is 
the  only  record  of  the  persons  in  the  Squash  Court  on  that  day. 

The  group  broke  up.  Some  stayed  to  round  up  their  measurements 
and  put  in  order  the  data  gathered  from  their  instruments.  Others 
went  to  duties  elsewhere.  Mr.  Greenewalt  hastened  to  the  room 
where  his  colleagues  were  still  in  conference  with  the  military.  He 
announced,  all  in  one  breath,  that  Yes,  it  would  be  quite  all  right 
for  their  company  to  go  along  with  the  Army's  request  and  start  to 
build  piles.  Piles  were  wonderful  objects  that  performed  with  the 
precision  of  a  Swiss  watch,  and,  provided  that  the  advice  of  such 
competent  scientists  as  Fermi  and  his  group  were  available,  the 
duPont  company  was  certainly  taking  no  undue  risk. 

Arthur  Compton  placed  a  long-distance  call  to  Mr.  Conant  of  the 
Office  of  Scientific  Research  and  Development  at  Harvard. 

"The  Italian  Navigator  has  reached  the  New  World,"  said  Comp- 
ton as  soon  as  he  got  Conant  on  the  line. 
"And  how  did  he  find  the  natives?" 
"Very  friendly." 

Here  the  official  story  ends,  but  there  is  a  sequel  to  it,  which 
started  on  that  same  afternoon  when  a  young  physicist,  Al  Wattem- 
berg,  picked  up  the  empty  Chianti  bottle  from  which  all  had  drunk. 
With  the  signatures  on  its  cover,  it  would  make  a  nice  souvenir. 
In  subsequent  years  Al  Wattemberg  did  his  share  of  traveling,  like 
any  other  physicist,  and  the  bottle  followed  him.  When  big  celebra- 
tions for  the  pile's  tenth  anniversary  were  planned  at  the  University 
of  Chicago,  the  bottle  and  Al  Wattemberg  were  both  in  Cambridge, 
Massachusetts.  Both,  Al  promised,  would  be  in  Chicago  on  De- 
cember 2. 

It  so  happened,  however,  that  a  little  Wattemberg  decided  to 
come  into  this  world  at  about  that  time,  and  Al  could  not  attend  the 
celebrations.  So  he  shipped  his  bottle,  and,  because  he  wanted  to 
make  doubly  sure  that  it  would  not  be  broken,  he  insured  it  for  a 

{continued  on  p.  134) 


131 


F.D.  Roosevelt, 

President  of  the  United  States, 

White  House 

Washington;  D.C. 


Sir: 

Some  recent  work  ty  E.Fermi  and  L.  Szilard,  which  has  teen  com- 
municated to  me  in  manuscript,  leads  me  to  expect  that  the  element  uran- 
ium may  be  turned  into  a  new  and  important  source  of  energy  in  the  im- 
mediate future.  Certain  aspects  of  the  situation  which  has  arisen  seem 
to  call  for  watchfulness  and,  if  necessary,  quick  action  on  the  part 
of  the  Administration.  I  "believe  therefore  that  it  is  my  duty  to  bring 
to  your  attention  the  following  facts  and  recommendations: 

In  the  course  of  the  last  four  months  it  has  been  made  probable  - 
through  the  work  of  Joliot  in  Prance  as  well  as  Permi  and  Szilard  in 
America  -  that  it  may  become  possible  to  set  up  a  nuclear  chain  reaction 
in  a  large  mass  of  uranium, by  which  vast  amounts  of  power  and  large  quant- 
ities of  new  radium-like  elements  would  be  generated.  How  it  appears 
almost  certain  that  this  could  be  achieved  in  the  immediate  future. 

This  new  phenomenon  would  also  lead  to  the  construction  of  bombs, 
and  it  is  conceivable  -  though  much  less  certain  -  that  extremely  power- 
ful bombs  of  a  new  type  may  thus  be  constructed.  A  single  bomb  of  this 
type,  carried  by  boat  and  exploded  in  a  port,  might  very  well  destroy 
the  whole  port  together  with  some  of  the  surrounding  territory.  However, 
such  bombs  might  very  well  prove  to  be  too  heavy  for  transportation  by 
air. 


The  United  States  has  only  very  poor  ores  of  uranium  in  moderate 
quantities.  There  is  some  good  ore  in  Canada  and  the  former  Czechoslovakia, 
while  the  most  important  source  of  uranium  is  Belgian  Congo. 

In  view  of  this  situation  you  may  think  it  desirable  to  have  some 
permanent  contact  maintained  between  the  Administration  and  the  group 
of  physicists  working  on  chain  reactions  in  America.  One  possible  way 
of  achieving  this  might  be  for  you  to  entrust  with  this  task  a  person 
who  has  your  confidence  and  who  could  perhaps  serve  in  an  inofficial 
capacity.  His  task  might  comprise  the  following: 

a)  to  approach  Government  Departments,  keep  them  informed  of  the 
further  development,  and  put  forward  recommendations  for  Government  action, 
giving  particular  attention  to  the  problem  of  securing  a  supply  of  uran- 
ium ore  for  the  United  States; 

b)  to  speed  ut)  the  experimental  work»which  is  at  present  being  car- 
ried on  within  the  limits  of  the  budgets  of  University  laboratories,  by 
providinst  funds,  if  such  funds  be  required,  through  his  contacts  with 
private  persons  who  are  willing  to  make  contributions  for  this  cause, 
sold  perhaps  also  by  obtaining  the  co-operation  of  industrial  laboratories 
which  have  the  necessary  equipment. 

I  understand  that  Germany  has  actually  stopped  the  sale  of  uranium 
from  the  Czechoslovakian  mines  which  she  has  taken  over.  That  she  should 
have  taken  such  early  action  might  perhaps  be  understood  on  the  ground 
that  the  son  of  the  German  Under-Secretary  of  State,  von  Weizsacker,  is 
attached  to  the  Kaiser-Wilhelm-Institut  in  Berlin  where  some  of  the 
American  work  on  uranium  is  now  being  repeated. 

Yours  very  truly, 

(Albert  Einstein) 


133 


thousand  dollars.  It  is  not  often  that  an  empty  bottle  is  considered 
worth  so  much  money,  and  newspaper  men  on  the  lookout  for  sensa- 
tion gave  the  story  a  prominent  position  in  the  press. 

A  couple  of  months  later  the  Fermis  and  a  few  other  physicists 
received  a  present:  a  case  of  Chianti  wine.  An  importer  had  wished 
to  acknowledge  his  gratitude  for  the  free  advertisement  that  Chianti 
had  received. 


The  First  Atomic  Pile  under  Construction  in  the 

Squash  Court:  Chunks  of  Uranium  Are 

Imbedded  in  the  Graphite  Bricks 


134 


Until  now,  power  from  nuclear  reactors  has  been  too  ex- 
pensive for  widespread  civilian  use  in  this  country.    But 
today  electricity  from  such  reactors  is  economically  com- 
petitive and  is  projected  to  become  much  cheaper. 


15      The  Nuclear  Energy  Revolution 

Alvin  M.  Weinberg  and  Gale  Young 

Excerpt  from  a  lecture  given  at  the  National  Academy  of  Sciences  in  1966. 


Twenty-four  years  have  passed  since  Fermi  and  his  co-workers  at  Chicago 
achieved  the  first  nuclear  chain  reaction.  During  most  of  these  years  nuclear  power 
for  civilian  use  has  been  too  expensive  and  experimental  in  nature  to  play  much  of 
a  role  in  our  economy,  but  during  the  past  couple  of  years  the  situation  has 
changed.  Nuclear  reactors  now  appear  to  be  the  cheapest  of  all  sources  of  energy. 
We  believe,  and  this  belief  is  shared  by  many  others  working  in  nuclear  energy, 
that  we  are  only  at  the  beginning,  and  that  nuclear  energy  will  become  cheap  enough 
to  influence  drastically  the  many  industrial  processes  that  use  energy.  If  nuclear 
energy  does  not,  as  H.  G.  Wells  put  it  in  1914,  create  "A  World  Set  Free,"  it  will 
nevertheless  affect  much  of  the  economy  of  the  coming  generation.  It  is  this 
Nuclear  Energy  Revolution,  based  upon  the  permanent  and  ubiquitous  availability 
of  cheap  nuclear  power,  about  which  we  shall  speculate. 

Our  outlook  is  admittedly  optimistic;  yet  optimism  in  nuclear  energy  seems  justi- 
fied. In  1955,  at  the  first  International  Conference  for  the  Peaceful  Uses  of  Atomic 
Energy,  in  Geneva,  some  American  authorities  were  chided  for  predicting  nuclear 
power  priced  at  4-5  mills  per  kilowatt  hour  (kwh) .  Today  T VA  has  announced  that 
it  expects  to  generate  power  from  its  2200-megawatt  (Mw)  Browns  Ferry  boiling- 
water  nuclear  plant  at  2.4  mills/kwh.  Even  if  the  Browns  Ferry  plant  were  operated 
by  a  private  utility,  the  electricity  at  the  bus  bar  would  cost  less  than  3.5  mills/kwh. 
We  are  very  hopeful  that  still  lower  costs  will  be  achieved  in  the  future  with 
breeder  reactors. 

Cheap  Nuclear  Energy  Is  Close  at  Hand. — The  economic  breakthrough  in  nuclear 
energy  came  in  1963  when  the  Jersey  Central  Power  and  Light  Company  con- 
tracted with  the  General  Electric  Company  to  construct  the  Oyster  Creek  boiling- 
water  nuclear  power  plant.  At  its  expected  electrical  output  of  620-Mw  the  capital 
cost  of  this  plant  is  $110/kw  or  the  same  as  that  for  a  coal-fired  power  plant  of  the 
same  size  at  the  same  location.^  The  announcement  of  Oyster  Creek  was  at  first 
regarded  by  many  as  a  sort  of  fluke.  But  Oyster  Creek  was  followed  by  a  succession 
of  orders  for  large  light-water-cooled  power  plants,  so  that  now  there  are  29  com- 


135 


TABLE  1 

Recent  Sales  of  Water  Reactors 

Nominal 

Plant 

Utility 

Mw 

Manufacturer 

Oyster  Creek 

Jersey  Central 

515 

General  Electric 

San  Onofre 

Southern  California  Edison 

429 

Westinghouse 

Nine  Mile  Point 

Niagara  Mohawk 

500 

General  Electric 

Haddam  Neck 

Connecticut  Yankee 

463 

Westinghouse 

Dresden  2 

Commonwealth  Edison 

755 

General  Electric 

— 

Boston  Edison 

600 

General  Electric 

Millstone  Point 

Northeast  Utilities 

549 

General  Electric 

Brookwood 

Rochester  Gas  &  Electric 

420 

Westinghouse 

Indian  Point  2 

Consolidated  Edison 

873 

Westinghouse 

Turkey  Point  3 

Florida  Power  &  Light 

652 

Westinghouse 

Turkey  Point  4 

Florida  Power  &  Light 

652 

Westinghouse 

Dresden  3 

Commonwealth  Edison 

810 

General  Electric 

Robinson 

Carolina  Power  &  Light 

760 

Westinghouse 

Palisades 

Consumers  Power  Company 

810 

Combustion  Engr. 

Point  Beach 

Wisconsin  Michigan  Power 

480 

Westinghouse 

Quad  Cities  1  and  2 

Commonwealth  Edison  and  lowa- 
lUinois  G  «fe  E 

2  X  810 

General  Electric 

Monticello 

Northern  States  Power  Co. 

540 

General  Electric 

Browns  Ferry 

TVA 

2  X  1100 

General  Electric 

Vernon 

Vermont  Yankee 

540 

General  Electric 

Keowee  Dam 

Duke  Power  Company 

2  X  820 

Babcock  and  Wilcox 

Peach  Bottom  2 

Philadelphia  Electric 

2  X  1100 

General  Electric 

Delaware  VaUey 

Public  Service  Electric  &  Gas  of  New 
Jersey 

1000 

Westinghouse 

Surry 

Virginia  Electric  Power  Co. 

2  X  800 

Westinghouse 

Boston 

Boston  Edison 

600 

General  Electric 

mitments  for  construction  of  large  nuclear  power  reactors  in  the  United  States 
(Table  1),  More  than  half  of  the  large  station  generating  capacity  ordered  in 
recent  months  is  scheduled  to  be  nuclear. 

None  of  the  plants  listed  in  Table  1  are  as  yet  operating.  Oyster  Creek  will  go 
on  the  line  early  in  1968.  The  optimism  expressed  in  the  many  purchases  of  light- 
water-moderated  and  cooled  reactors  is  based  partly  upon  our  generally  good  ex- 
perience with  such  reactors  in  the  nuclear  navy,  and  partly  upon  the  operating 
experience  with  such  power  plants  as  the  Yankee  pressurized-water  reactor  (175 
Mw)  and  the  Dresden  1  boiling-water  reactor  (200  Mw).  Yankee,  for  example, 
has  been  generating  electricity  for  five  years,  and  during  the  past  year  has  been 
available  for  generation  76  per  cent  of  the  time.  Dresden  1  has  operated  for  six 
years,  and  during  the  past  year  has  been  available  83  per  cent  of  the  time. 

In  some  ways  it  is  surprising  that  the  world's  cheapest  nuclear  reactors  should 
derive  from  the  original  pressurized-water  line  used  to  power  the  Nautilus.  Pres- 
surized water  was  chosen  for  the  Nautilus  not  because  it  seemed  to  be  a  path  to 
cheap  nuclear  energy,  but  rather  because  such  reactors,  being  moderated  by  hydro- 
gen and  fueled  with  enriched  uranium,  are  relatively  compact.  If  anything,  the  early 
reactor  designers  viewed  these  systems  as  being  rather  expensive.  And  in  countries 
other  than  the  United  States  and  the  Soviet  Union,  the  main-line  reactors  utilize 
natural  uranium  and  either  graphite  or  heavy  water  as  moderator. 

But  the  early  designers  failed  to  appreciate  the  extent  to  which  the  extraordinary 
success  of  the  gaseous  diffusion  plants  would  reduce  the  price  of  U^^*.  In  1948, 
when  the  Nautilus  was  designed,  U^'^  cost  about  $35/gm.  Today  it  costs  $12/gm, 
which  is  only  four  times  its  price  as  unseparated  isotope  in  ore  costing  $8/lb  of  UaOg! 
This  remarkable  reduction  in  the  cost  of  separating  U'^^^  more  than  any  other  single 
factor,  underlies  the  economic  success  of  the  American  water-moderated  reactors. 


136 


The  Nuclear  Energy  Revolution 


The  fuel  cycle  in  a  reactor  like  Browns  Ferry  that  bums  enriched  uranium  costs  only 
1.25  mills/kwh,  which  is  appreciably  lower  than  coal  even  in  cheap  coal  country 
(Table  2). 

The  American  reactors,  being  compact,  were  expected  to  be  cheaper  to  build  than 
the  large  graphite  or  heavy-water  reactors  that  use  natural  uranium.  But  prior 
to  Oyster  Creek  it  was  not  clear  how  cheap  a  reactor  could  be,  especially  if  its  output 
were  large  enough.  It  was  R.  P.  Hanrnfiond  who  first  stressed  the  principle  that  a 
nuclear  reactor  ought  to  scale  rather  favorably.  Thus,  although  the  total  cost  of  a 
large  nuclear  reactor  will  be  greater  than  that  of  a  smaller  one,  the  cost  per  kilowatt 
of  the  large  reactor  should  be  less  than  that  of  the  smaller  one.  Hammond's  con- 
tention has  been  amply  confirmed  by  the  price  estimates  published,  for  example,  by 
the  General  Electric  Company.  Figure  1  shows  that  the  cost  per  kilowatt  of  a 
200-Mw  boiling-water  reactor  (BWR)  is  around  $180/kw,  whereas  the  cost  per 
kilowatt  of  a  1000-Mw  BWR  is  only  $110/kw.  All  the  new,  competitive  nuclear 
power  plants  are  large,  and  they  capitalize  on  the  advantage  of  size. 

The  Necessity  for  Breeders.  —Nuclear  power  at  2.4  mills/kwh  at  Browns  Ferry  is 
a  remarkable  achievement,  but  it  is  not  remarkable  enough  to  serve  as  the  basis  for 
a  Nuclear  Energy  Revolution.  In  the  first  place,  we  are  hopeful  that  breeder 
reactors  can  shave  another  mill  off  the  cost  and  thus  perhaps  provide  the  basis  for 
new  heavy  chemical  and  other  industries.  In  the  second  place,  the  light-water 
reactors  burn  only  a  small  fraction  of  all  the  natural  uranium  mined  to  fuel  them; 
thus  such  reactors  will  rapidly  use  all  the  U.  S.  low-priced  reserves  of  uranium  ore, 
and  the  price  of  nuclear  energy  will  rise  as  we  are  obliged  to  burn  more  expensive 
ores.  This  is  illustrated  in  Figure  2,  based  by  Dietrich^  on  estimates  made  a  few 
years  ago  by  the  Atomic  Energy  Commission  of  U.  S.  ore  reserves  and  reactors  to 
be  built.*  Since  then,  ore  prospecting  has  been  resumed,  but  water  reactor  sales  are 
outrunning  the  estimates. 

We  therefore  find  ourselves  in  a  serious  dilemma.  The  current  great  success  of 
nuclear  energy  is  making  our  economy  increasingly  dependent  upon  nuclear  power. 
But  as  we  turn  to  nuclear  energy  we  shall  exhaust  our  low-grade  ore  reserves. 
By  the  time  (say  in  1990)  we  have  become  very  heavily  committed  to  nuclear 
energy,  its  price  will  probably  begin  to  rise  significantly. 

Of  course  we  shall  find  more  low-cost  ore.  But  eventually  even  this  will  be  in- 
sufficient, especially  if  our  power  requirements  continue  to  grow.  If  we  are  to 
forestall  a  major  economic  power  crisis,  say  20  years  from  now,  we  shall  have  to 
learn  how  to  utilize  not  1  per  cent  or  so  of  the  raw  materials  (uranium  and  thorium) 
for  fuel,  but  much  more— hopefully  close  to  100  per  cent.  Should  we  learn  how  to 
burn  a  large  fraction  of  the  uranium  and  thorium,  we  would  gain  in  three  respects: 
we  would  forestall  a  serious  rise  in  the  cost  of  power;  we  would  reduce  the  fuel  cycle 
cost  of  a  reactor,  since  in  effect  we  would  be  burning  the  abundant  and  very  cheap 
U238  Qj.  Th^^^,  not  the  costly  U"^;  and  we  would  make  available,  at  relatively  small 
economic  penalty,  the  vast  residual  amounts  of  uranium  and  thorium  in  the  earth's 
crust.  To  anticipate  our  conclusion,  we  could  hope  to  achieve  power  costs  of  only 
1.5  mills/kwh  in  publicly  owned  stations,  and  we  could  foresee  maintaining  this  low 
cost  essentially  forever.  It  is  this  prospect,  and  what  it  implies  for  energy-consum- 
ing industrial  processes,  that  warrants  our  using  the  extravagant  phrase  "The 
Nuclear  Energy  Revolution." 


137 


300 


250 


LxJ 


200 


CO 

I- 

O 
O 


^  150 


100 


50 


Wankee 

\\ 

DRESD 

EN  1 

\ 

\ 

-SAN 

ONOFRE 

\ 

\ 

\ 

*-GE  1 
\ 
s 

=RICE  LIST 
5EPT4965 

1     N 

\^NINE  MILE  POINT 

N 

V      MALIBU 

K\ 

1            1             1 
^TURKEY  POINT  3  4 

c 

TWO 
REEKS 

f> 

\  >       y        1         1 

^^OYSTER    *"=\PALISADES 
-iCCREEK           ^        ^INDIAN 
N^r*" ^■\P0INT2 

MILLSTONE  POINT" 

'    / 

\ 

\       1 

HARTSVILLE-^^ 

\ 

^DRESDEN 
'DRESDEN  2 

3 

100 


300  500  700 

ELECTRICAL  CAPACITY  (Mw) 


900 


1100 


Fig.  1. — Cost  of  nuclear  electric  plants.  The  length  of  each  short-line  segment  represents  the 
uncertainty  in  the  ultimate  output  of  each  reactor.  The  values  shown  are  mostly  manufacturers' 
"turn-key"  prices,  and  do  not  in  many  cases  include  all  the  customers'  costs.  Complete  data  are 
usually  not  available. 


Investment  ($/kw) 
Capacity  assumed 

Plant  life  (yr) 

Fixed  charge  rate  (%/yr) 

Load  factor  (%) 

Period  covered  (yr) 

Capital  charges  (millsAwh) 

Operation,  maintenance,  insurance  (mills/ 

kwh) 
Fuel  cycle  (mills/kwh) 
Total  power  cost  (mills  A wh) 

*  Includes .$4/kw  transmission  and  $2/kw  working  capital  other  than  fuel, 
t  Reduces  by  $9/kw  if  anticipated  stretch  is  realized. 


TABLE  2 

Power  Cost  Estimates 

1,  2 

Oyster  Creek 

TVA 

nuclear 

nuclear 

TVA  coal 

116* 

116t 

117 

Expected  stretch, 

Guaranteed 

620  Mw 

1100  Mw 

30 

35 

35 

10 

5.7 

5.7 

88 

85 

85 

First  10 

First  12 

First  12 

1.5 

0.89 

0.90 

0.48 

0.23 

0.24 

1.67 

1.25 

1.69 

3.65 

2.37 

2.83 

138 


The  Nuclear  Energy  Revolution 


^5 


~  15 


03 


> 

/ 

f 

/ 
/ 

^.,0^^ 

\ 

1 

Fig.  2. — Ore  awts  for  HjO  reactors  with  pluto- 
nhim  recycle. 


>  Jersey  Central  Power  aod  light  Company,  "Report  on  economic  aoalyriB  for  Qyeter  Creek 
nudear  eieetrie  eenerating  station,"  Sudear  New*,  7,  no.  4,  Special  Supfdonent  (April  1964j. 
Tlie  stetaoo  being  buih  is  a  little  leas  expoigive  than  the  one  analyzed  in  the  rqwrt. 

*  Tennessee  V'aDey  Authority,  Comparvton  of  ConL-FiroA  and  SucUar  Pcnj:*^  Pl/inls  for  the 
TV  A  Sygtem  (Chattanooga,  Tenn.:  Office  of  Power,  Jujie  1966^ 

*  Dietrich,  J.  R.,  "Efficient  utHizataon  of  nudear  fuds,"  Pctc^r  ReacU/r  TechnrAo^j,  6,  no.  4, 
34  fFall  1963),  U.S.  Atomic  Energy  CommJaeion  Division  of  Tef:iiiAr:ii  Information,  Oak  Ridge, 
X  cj^nft%^ftft, 

«  U.  S.  Atcmne  Energ>'  Commisgion,  Civilian  SvcUar  Po^zer:  A  Report  to  the  President — 1962, 
and  .^jpendiees  (Oak  Rid^e,  Tom.:  TJS.  Atomic  Energj-  Commiseion  Diviokm  of  Technical 
Information  FJrtwwion,  1962). 


199 


In  the  study  of  elementary  particles,  new  conservation 
laws  have  been  discovered  that  are  indlspensible  for 
making  prediction  or  building  theory. 


16     Conservation  Laws 


Kenneth  W.  Ford 

Chapter  from  his  book.  The  World  of  Elementary  Particles, 
published  in  1963. 

In  a  slow  and  subtle,  yet  inexorable,  way  conservation  laws  have 
moved  in  the  past  few  centuries  from  the  role  of  interesting  side- 
light in  physics  to  the  most  central  position.  What  little  we  now 
understand  about  the  interactions  and  transformations  of  particles 
comes  in  large  part  through  certain  conservation  laws  which  gov- 
ern elementary-particle  behavior. 

A  conservarion  law  is  a  statement  of  constancy  in  nature.  If 
there  is  a  room  full  of  people,  say,  at  a  cocktail  party,  and  no  one 
comes  in  or  leaves,  we  can  say  that  there  is  a  law  of  conservation 
of  the  number  of  people;  that  number  is  a  constant.  This  would  be 
a  rather  uninteresting  law.  But  suppose  the  conservation  law  re- 
mained valid  as  guests  came  and  went.  This  would  be  more  inter- 
esting, for  it  would  imply  that  the  rate  of  arrival  of  guests  was 
exactly  equal  to  the  rate  of  departure.  During  a  process  of  change, 
something  is  remaining  constant.  The  significant  conservation  laws 
in  nature  are  of  this  type,  laws  of  constancy  during  change.  It  is 
not  surprising  that  scientists,  in  their  search  for  simplicity,  fasten 
on  conservation  laws  with  particular  enthusiasm,  for  what  could  be 
simpler  than  a  quantity  that  remains  absolutely  constant  during 
complicated  processes  of  change.  To  cite  an  example  from  the  world 
of  particles,  the  total  electric  charge  remains  precisely  constant  in 
every  collision,  regardless  of  how  many  particles  may  be  created 
or  annihilated  in  the  process. 

The  classical  laws  of  physics  are  expressed  primarily  as  laws  of 
change,  rather  than  as  laws  of  constancy.  Newton's  law  of  motion 
describes  how  the  motion  of  objects  responds  to  forces  that  act 
upon  them.  Maxwell's  equations  of  electromagnetism  connect  the 
rate  of  change  of  electric  and  magnetic  fields  in  space  and  time.  The 
early  emphasis  in  fundamental  science  was  rather  naturally  on  dis- 
covering those  laws  which  successfully  describe  the  changes  actu- 
ally occurring  in  nature.  Briefly,  the  "classical"  philosophy  con- 


141 


cerning  nature's  laws  is  this.  Man  can  imagine  countless  possible 
laws,  indeed  infinitely  many,  that  might  describe  a  particular  phe- 
nomenon. Of  these,  nature  has  chosen  only  one  simple  law,  and 
the  job  of  science  is  to  find  it.  Having  successfully  found  laws  of 
change,  man  may  derive  from  them  certain  conservation  laws, 
such  as  the  conservation  of  energy  in  mechanics.  These  appear  as 
particularly  interesting  and  useful  consequences  of  the  theory,  but 
are  not  themselves  taken  as  fundamental  statements  of  the  theory. 

Gradually  conservation  laws  have  percolated  to  the  top  in  the 
hierarchy  of  natural  laws.  This  is  not  merely  because  of  their  sim- 
plicity, although  this  has  been  an  important  factor.  It  comes  about 
also  for  two  other  reasons.  One  is  the  connection  between  conser- 
vation laws  and  principles  of  invariance  and  symmetry  in  nature — 
surely,  one  of  the  most  beautiful  aspects  of  modern  science.  The 
meaning  of  this  connection  will  be  discussed  near  the  end  of  this 
chapter.  The  other  reason  we  want  to  discuss  here  might  best  be 
described  simply  as  a  new  view  of  the  world,  in  which  conservation 
laws  appear  naturally  as  the  most  fundamental  statements  of  natural 
law.  This  new  view  is  a  view  of  order  upon  chaos — the  order  of 
conservation  laws  imposed  upon  the  chaos  of  continual  annihilation 
and  creation  taking  place  in  the  submicroscopic  world.  The  strong 
hint  emerging  from  recent  studies  of  elementary  particles  is  that 
the  only  inhibition  imposed  upon  the  chaotic  flux  of  events  in  the 
world  of  the  very  small  is  that  imposed  by  the  conservation  laws. 
Everything  that  can  happen  without  violating  a  conservation  law 
does  happen. 

This  new  view  of  democracy  in  nature — freedom  under  law — 
represents  a  revolutionary  change  in  man's  view  of  natural  law. 
The  older  view  of  a  fundamental  law  of  nature  was  that  it  must 
be  a  law  of  permission.  It  defined  what  can  (and  must)  happen  in 
natural  phenomena.  According  to  the  new  view,  the  more  funda- 
mental law  is  a  law  of  prohibition.  It  defines  what  cannot  happen. 
A  conservation  law  is,  in  effect,  a  law  of  prohibition.  It  prohibits 
any  phenomenon  that  would  change  the  conserved  quantity,  but 
otherwise  allows  any  events.  Consider,  for  example,  the  production 
of  pions  in  a  proton-proton  collision, 

p-\-p-^p-\-p-\-Tr-\-ir-\-Tr-\-   •   •   •   . 

If  a  law  of  permission  were  operative,  one  might  expect  that,  for 
protons  colliding  in  a  particular  way,  the  law  would  specify  the 


142 


Conservation  Laws 


number  and  the  type  of  pions  produced.  A  conservation  law  is  less 
restrictive.  The  conservation  of  energy  limits  the  number  of  pions 
that  can  be  produced,  because  the  mass  of  each  one  uses  up  some  of 
the  available  energy.  It  might  say,  for  example,  that  not  more  than 
six  pions  can  be  produced.  In  the  actual  collision  there  might  be 
none,  or  one,  or  any  number  up  to  six.  The  law  of  charge  conser- 
vation says  that  the  total  charge  of  the  pions  must  be  zero,  but 
places  no  restriction  on  the  charge  of  any  particular  pion;  this  could 
be  positive,  negative,  or  neutral. 

To  make  more  clear  the  distinction  between  laws  of  permission 
and  laws  of  prohibition,  let  us  return  to  the  cocktail  party.  A  law 
of  change,  which  is  a  law  of  permission,  might  describe  the  rate  of 
arrival  and  the  rate  of  departure  of  guests  as  functions  of  time.  In 
simplest  form,  it  might  say  that  three  guests  per  minute  arrive  at 
6:00,  two  guests  per  minute  at  6:15,  and  so  on.  Or  it  might  say, 
without  changing  its  essential  character  as  a  law  of  permission,  that 
the  rate  of  arrival  of  guests  is  given  by  the  formula: 


1 

,  +  (r-5-20' 


where  R  is  the  number  of  guests  arriving  per  minute,  A  is  the  an- 
nual income  of  the  host  in  thousands  of  dollars,  D  is  the  distance 
in  miles  from  the  nearest  metropolitan  center,  and  T  is  the  time 
of  day.  This  law  resembles,  in  spirit,  a  classical  law  of  physics.  It 
covers  many  situations,  but  for  any  particular  situation  it  predicts 
exactly  what  will  happen. 

A  conservation  law  is  simpler  and  less  restrictive.  Suppose 
it  is  observed  that  between  7  and  10  o'clock  the  number  of  guests 
is  conserved  at  all  parties.  This  is  a  grand  general  statement,  ap- 
pealing for  its  breadth  of  application  and  its  simplicity.  It  would, 
were  it  true,  be  regarded  as  a  deep  truth,  a  very  profound  law  of 
human  behavior.  But  it  gives  much  less  detailed  information  than 
the  formula  for  R  above.  The  conservation  law  allows  the  guests 
to  arrive  at  any  rate  whatever,  so  long  as  guests  depart  at  the  same 
rate.  To  push  the  analogy  with  natural  law  a  bit  further,  we  should 
say  that  according  to  the  old  view,  since  cocktail-party  attendance 
is  a  fundamental  aspect  of  human  behavior,  we  seek  and  expect  to 
find  simple  explicit  laws  governing  the  flow  of  guests.  According 
to  the  new  view,  we  expect  to  find  the  flux  of  arriving  and  depart- 


143 


ing  guests  limited  only  by  certain  conservation  principles.  Any 
behavior  not  prohibited  by  the  conservation  laws  will,  sooner  or 
later,  at  some  cocktail  party,  actually  occur. 

It  should  be  clear  that  there  is  a  close  connection  between  this 
view  of  nature  and  the  fundamental  role  of  probability  in  nature. 
If  the  conservation  law  does  not  prohibit  various  possible  results 
of  an  experiment,  as  in  the  proton-proton  collision  cited  above, 
then  these  various  possibilities  will  occur,  each  with  some  definite 
probability.  The  very  fact  that  we  can  use  the  word  chaos  to  de- 
scribe the  creation  and  annihilation  events  occurring  continually 
among  the  particles  rests  on  the  existence  of  laws  of  probability. 
At  best  the  probability,  never  the  certainty,  of  these  endless  changes 
in  the  particle  world  can  be  known. 

Are  the  laws  of  probability  themselves  derivable  from  conser- 
vation laws?  The  answer  to  this  question  is  not  yet  known,  but  the 
trend  of  recent  hist9ry  is  enough  to  make  this  author  and  many 
other  physicists  willing  to  bet  on  the  affirmative.  It  appears  pos- 
sible, at  least,  that  the  conservation  laws  may  not  only  be  the  most 
fundamental  laws,  but  may  be  all  the  laws.  They  may  be  sufficient 
to  characterize  the  elementary-particle  world  completely,  specifying 
not  only  which  events  may  occur  and  which  are  forbidden,  but 
giving  also  the  relative  probabilities  of  those  events  which  do  occur. 

We  have  so  far  emphasized  that  a  conservation  law  is  less  re- 
strictive than  an  explicit  law  of  change,  or  law  of  permission.  How- 
ever, there  are  a  number  of  different  conservation  laws  and,  taken 
all  together,  they  may  be  very  strongly  restrictive,  far  more  so  than 
any  one  taken  alone.  In  the  ideal  case,  they  may  leave  open  only 
one  possibility.  The  laws  of  prohibition,  all  taken  together,  then  im- 
ply a  unique  law  of  permission.  The  most  beautiful  example  of 
this  kind  of  power  of  conservation  laws  concerns  the  nature  of  the 
photon.  From  conservation  principles  alone,  it  has  been  possible 
to  show  that  the  photon  must  be  a  massless  particle  of  unit  spin 
and  no  charge,  emitted  and  absorbed  by  charged  particles  in  a  par- 
ticular characteristic  way.  This  truly  amazing  result  has  been  ex- 
pressed vividly  by  J.  J.  Sakurai  who  wrote  recently,  "The  Creator 
was  supremely  imaginative  when  he  declared,  'Let  there  be 
light.'"*  In  the  world  of  human  law,  a  man  so  hemmed  in  by  re- 
strictions that  there  is  only  one  course  of  action  open  to  him  is 

•  Annals  of  Physics,  Volume  11,  page  5  (1960). 


144 


Conservation  Laws 


not  very  happy.  In  the  world  of  natural  law  it  is  remarkable  and 
satisfying  to  learn  that  a  few  simple  statements  about  constant 
properties  in  nature  can  have  locked  within  them  such  latent  power 
that  they  determine  uniquely  the  nature  of  light  and  its  interaction 
with  matter. 

There  are  conservation  laws  and  conservation  laws.  That  is,  some 
things  in  nature  are  constant,  but  others  are  even  more  constant. 
To  convert  this  jargon  into  sense,  some  quantities  in  nature  seem 
to  be  absolutely  conserved,  remaining  unchanged  in  all  events  what- 
ever; other  quantities  seem  to  be  conserved  in  some  kinds  of  proc- 
esses and  not  in  others.  The  rules  governing  the  latter  are  still 
called  conservation  laws,  but  nature  is  permitted  to  violate  them 
under  certain  circumstances.  We  shall  postpone  the  discussion  of 
these  not-quite-conservation  laws  to  Chapter  Eight,  and  consider 
here  only  seven  of  the  recognized  absolute  conservation  laws.  (There 
are  two  more  absolute  conservation  laws  of  a  more  special  kind,  and 
they  are  also  postponed  to  Chapter  Eight.) 

We  begin  by  listing  by  name  the  seven  quantities  that  are 
conserved: 

1.  Energy  (including  mass) 

2.  Momentum 

3.  Angular  momentum,  including  spin 

4.  Charge 

5.  Electron-family  number 

6.  Muon-family  number 

7.  Baryon-family  number. 

There  are  two  different  kinds  of  quantities  here,  which  can  be 
called  properties  of  motion  and  intrinsic  properties,  but  the  two 
are  not  clearly  separated.  The  intrinsic  particle  properties  that  enter 
into  the  conservation  laws  are  mass,  spin,  charge,  and  the  several 
"family  numbers."  The  properties  of  motion  are  kinetic  energy, 
momentum,  and  angular  momentum,  the  last  frequently  being 
called  orbital  angular  momentum  to  avoid  possible  confusion  with 
intrinsic  spin,  which  is  a  form  of  angular  momentum.  In  the  laws 
of  energy  conservation  and  angular-momentum  conservation,  the 
intrinsic  properties  and  properties  of  motion  become  mixed. 

The  interactions  and  transformations  of  the  elementary  particles 
serve  admirably  to  illustrate  the  conservation  laws  and  we  shall 


145 


focus  attention  on  the  particles  for  illustrative  purposes.  It  is  through 
studies  of  the  particles  that  all  of  these  conservation  laws  have  been 
verified,  although  the  first  four  were  already  known  in  the  mac- 
roscopic world.  The  particles  provide  the  best  possible  testing 
ground  for  conservation  laws,  for  any  law  satisfied  by  small  num- 
bers of  particles  is  necessarily  satisfied  for  all  larger  collections  of 
particles,  including  the  macroscopic  objects  of  our  everyday  world. 
Whether  the  extrapolation  of  the  submicroscopic  conservation 
laws  on  into  the  cosmological  domain  is  justified  is  uncertain,  since 
gravity,  whose  effects  in  the  particle  world  appear  to  be  entirely 
negligible,  becomes  of  dominant  importance  in  the  astronomical 
realm. 

Various  intrinsic  properties  of  the  particles  were  discussed  in 
Chapter  One,  and  we  shall  examine  first  the  conservation  laws 
that  have  to  do  with  the  intrinsic  properties. 

We  learned  in  Chapter  One  that  every  particle  carries  the  same 
electric  charge  as  the  electron  (defined  to  be  negative),  or  the 
equal  and  opposite  charge  of  the  proton  (positive),  or  is  neutral. 
The  charge  is  a  measure  of  the  strength  of  electric  force  which 
the  particle  can  exert  and,  correspondingly,  a  measure  of  the  strength 
of  electric  force  which  the  particle  experiences.  A  neutral  particle, 
of  course,  neither  exerts  nor  responds  to  an  electric  force.  A 
charged  particle  does  both. 

Using  the  proton  charge  as  a  unit,  every  particle's  charge  can  be 
labeled  -|-1,  — 1,  or  0.  The  law  of  charge  conservation  requires 
that  the  total  charge  remain  unchanged  during  every  process  of 
interaction  or  transformation.  For  any  event  involving  particles, 
then,  the  total  charge  before  the  event  must  add  up  to  the  same 
value  as  the  total  charge  after  it.  In  the  decay  of  a  lambda  into  a 
neutron  and  a  pion, 

the  charge  is  zero  both  before  and  after.  In  the  positive  pion 
decay, 

the  products  are  a  positive  muon  and  a  neutral  neutrino.  A  possible 
high-energy  nuclear  collision  might  proceed  as  follows: 

p  4-  p  -^  w  +  A"  -I-  X+  -f  7r+. 


146 


a|iMiB 


in  any  number.  In  a  typical  proton-proton  collision  the  number  of 
baryons  (2)  remains  unchanged,  as  in  the  example, 

These  and  numerous  other  examples  have  made  it  appear  that  the 
number  of  baryons  remains  forever  constant — in  every  single  event, 
and  therefore,  of  course,  in  any  larger  structure. 

Each  of  the  H,  2,  and  A  particles,  and  the  neutron,  undergoes 
spontaneous  decay  into  a  lighter  baryon.  But  the  lightest  baryon, 
the  proton,  has  nowhere  to  go.  The  law  of  baryon  conservation 
stabilizes  the  proton  and  makes  possible  the  structure  of  nuclei  and 
atoms  and,  therefore,  of  our  world.  From  the  particle  physicist's 
point  of  view,  this  is  a  truly  miraculous  phenomenon,  for  the  pro- 
ton stands  perched  at  a  mass  nearly  2,000  times  the  electron  mass, 
having  an  intrinsic  energy  of  about  one  billion  electron  volts,  while 
beneath  it  lie  the  lighter  unstable  kaon,  pion,  and  muon.  Only  the 
law  of  baryon  conservation  holds  this  enormous  energy  locked 
within  the  proton  and  makes  it  a  suitable  building  block  for  the 
universe.  The  proton  appears  to  be  absolutely  stable.  If  it  is  un- 
stable it  has,  according  to  a  recent  experimental  result,  a  half  life 
greater  than  7  X  10"  years,  or  about  a  billion  billion  times  the  age 
of  the  earth. 

Our  statement  of  the  law  of  baryon  conservation  needs  some 
amplification,  for  we  have  not  yet  taken  into  account  antibarvons. 
A  typical  antiproton-production  event  at  the  Berkeley  Bevatron 
might  go  as  follows: 

p-{-p-^p-\-p-\-p-\-p. 

(The  bar  over  the  letter  designates  the  antiparticle.  Since  the  anti- 
proton  has  negative  charge,  the  total  charge  of  plus  2  is  conserved.) 
It  appears  that  we  have  transformed  two  baryons  into  four.  Sim- 
ilarly, in  the  antiproton  annihilation  event, 

p  +  p-^  x+  +  X-  +  t\ 

two  baryons  have  apparently  vanished.  The  obvious  way  to  patch 
up  the  law  of  baryon  conservation  is  to  assign  to  the  antiparticles 
baryon  number  — 1,  and  to  the  particles  baryon  number  +1.  Then 
the  law  would  read:  In  every  event  the  total  number  of  baryons 
minus  the  total  number  of  antibaryons  is  conserved;  or,  equiv- 
alently,  the  total  baryon  number  remains  unchanged. 


148 


Conservation  Laws 


The  cynic  might  say  that  with  so  many  arbitral^'  definitions — 
which  particles  should  be  called  baryons  and  which  not,  and  the 
use  of  negative  baryon  numbers — it  is  no  wonder  that  a  conserva- 
tion law  can  be  constructed.  To  this  objection,  two  excellent 
answers  can  be  given.  The  first  is  that  it  is  not  so  easy  to  find  an 
absolute  conservation  law.  To  find  any  quantity  absolutely  con- 
served in  nature  is  so  important  that  it  easily  justifies  a  few  arbitrary 
definitions.  The  arbitrariness  at  this  stage  of  history  only  reflects 
our  lack  of  any  deep  understanding  of  the  reason  for  baryon  con- 
servation, but  it  does  not  detract  from  the  obvious  significance  of 
baryon  conservation  as  a  law  of  nature.  The  other  answer,  based 
on  the  mathematics  of  the  quantum  theory,  is  that  the  use  of  nega- 
tive baryon  number  for  antiparticles  is  perfectly  natural,  in  fact, 
is  demanded  by  the  theory.  This  comes  about  because  the  descrip- 
tion of  the  appearance  of  an  antiparticle  is  "equivalent"  (in  a  mathe- 
matical sense  we  cannot  delve  into  )  to  the  description  of  the  dis- 
appearance of  a  particle;  and  conversely  antiparticle  annihilation 
is  "equivalent"  to  particle  creation. 

The  "electron  family"  contains  only  the  electron  and  its  neu- 
trino, the  "muon  family"  only  the  muon  and  its  neutrino.  For  each 
of  these  small  groups,  there  is  a  conservation  of  family  members 
exactly  like  the  conservation  of  baryons.  The  antiparticles  must  be 
considered  negative  members  of  the  families,  the  particles  positive 
members.  These  light-particle  conservation  laws  are  not  nearly  as 
well  tested  as  the  other  absolute  conservation  laws  because  of  the 
difficulties  of  studying  neutrinos,  but  there  are  no  known  exceptions 
to  them. 

The  beta  decay  of  the  neutron, 

n-^  p  -\-  e~  -{■  Ve, 
illustrates  nicely  the  conservation  laws  we  have  discussed.  Initially, 
the  single  neutron  has  charge  zero,  baryon  number  1,  and  electron- 
family  number  zero.  The  oppositely  charged  proton  and  electron 
preserve  zero  charge;  the  single  proton  preserves  the  baryon  num- 
ber; and  the  electron  with  its  antineutrino  (t;)  together  preserve 
zero  electron-family  number.  In  the  pion  decay  processes, 

7r+  -*  M"^  +  Va         and         ir~  — >  m~  +  "mi 


149 


muon-family  conservation  demands  that  a  neutrino  accompany 
the  ft*  antimuon,  and  an  antineutrino  accompany  the  /n"  muon.  The 
muon,  in  turn,  decays  into  three  particles,  for  example, 

which  conserves  the  members  of  the  muon  family  and  of  the  elec- 
tron family. 

The  general  rule  enunciated  earlier  in  this  chapter  was  that  what- 
ever can  happen  without  violating  a  conservation  law  does  happen. 
Until  1962,  there  was  a  notable  exception  to  this  rule;  its  resolution 
has  beautifully  strengthened  the  idea  that  conservation  laws  play  a 
central  role  in  the  world  of  elementary  particles.  The  decay  of  a 
muon  into  an  electron  and  a  photon, 

M~  -*  ^  +  7, 

has  never  been  seen,  a  circumstance  which  had  come  to  be  known 
as  the  fi-e-y  puzzle.  Before  the  discovery  of  the  muon's  neutrino 
it  was  believed  that  electron,  muon,  and  one  neutrino  formed  a 
single  family  (called  the  lepton  family)  with  a  single  family-con- 
servation law.  If  this  were  the  case,  no  conservation  law  prohibited 
the  decay  of  muon  into  electron  and  photon,  since  the  lost  muon 
was  replaced  with  an  electron,  and  charge  and  all  other  quantities 
were  conserved  as  well.  According  to  the  classical  view  of  physical 
law,  the  absence  of  this  process  should  have  caused  no  concern. 
There  was,  after  all,  no  law  of  permission  which  said  that  it  should 
occur.  There  was  only  the  double  negative:  No  conservation  law 
was  known  to  prohibit  the  decay. 

However,  the  view  of  the  fundamental  role  of  conservation  laws 
in  nature  as  the  only  inhibition  on  physical  processes  had  become 
so  ingrained  in  the  thinking  of  physicists  that  the  absence  of  this 
particular  decay  mode  of  the  muon  was  regarded  as  a  significant 
mystery.  It  was  largely  this  mystery  that  stimulated  the  search  for 
a  second  neutrino  belonging  exclusively  to  the  muon.  The  dis- 
covery of  the  muon's  neutrino  established  as  a  near  certainty  that 
the  electron  and  muon  belong  to  two  different  small  families  which 
are  separately  conserved.  With  the  electron  and  muon  governed  by 
two  separate  laws  of  conservation,  the  prohibition  of  the  fi-e-y  decay 
became  immediately  explicable,  and  the  faith  that  what  can  happen 
does  happen  was  further  bolstered. 


150 


Conservation  Laws 


We  turn  now  to  the  conservation  laws  which  involve  properties 
of  motion. 

In  the  world  of  particles  there  are  only  two  kinds  of  energy: 
energy  of  motion,  or  kinetic  energy,  and  energy  of  being,  which 
is  equivalent  to  mass.  Whenever  particles  are  created  or  annihilated 
(except  the  massless  particles)  energy  is  transformed  from  one  form 
to  the  other,  but  the  total  energy  in  every  process  always  remains 
conserved.  The  simplest  consequence  of  energy  conservation  for 
the  spontaneous  decay  of  unstable  particles  is  that  the  total  mass 
of  the  products  must  be  less  than  the  mass  of  the  parent.  For  each 
of  the  following  decay  processes  the  masses  on  the  right  add  up  to 
less  than  the  mass  on  the  left: 

M"^  — *  ^  +  "«  +  v^. 

In  particular,  then,  a  massless  particle  cannot  decay,  and  energy 
conservation  prohibits  every  other  "uphill"  decay  in  which  the 
products  are  heavier  than  the  parent.  An  unstable  particle  at  rest 
has  only  its  energy  of  being,  no  energy  of  motion.  The  difference 
between  this  parent  mass  and  the  mass  of  the  product  particles  is 
transformed  into  kinetic  energy  which  the  product  particles  carry 
away  as  they  rapidly  leave  the  scene. 

One  might  suppose  that  if  the  parent  particle  is  moving  when  it 
decays  it  has  some  energy  of  motion  of  its  own  which  might  be 
transformed  to  mass.  The  conservation  of  momentum  prohibits  this. 
The  extra  energy  of  motion  is  in  fact  "unavailable"  for  conversion 
into  mass.  If  a  particle  loses  energy,  it  also  loses  momentum. 
Momentum  conservation  therefore  prohibits  the  conversion  of  all 
of  the  energy  into  mass.  It  turns  out  that  momentum  and  energy 
conservation  taken  together  forbid  uphill  decays  into  heavier  par- 
ticles no  matter  how  fast  the  initial  particle  might  be  moving. 

If  two  particles  collide,  on  the  other  hand,  some — but  not  all — 
of  their  energy  of  motion  is  available  to  create  mass.  It  is  in  this 
way  that  the  various  unstable  particles  are  manufactured  in  the 
laboratory.  In  an  actual  typical  collision  in  the  vicinity  of  an  ac- 
celerator, one  of  the  two  particles,  the  projectile,  is  moving  rapidly, 
and  the  other,  the  target,  is  at  rest.  Under  these  conditions,  the 
requirement  that  the  final  particles  should  have  as  much  momentum 
as  the  initial  projectile  severely  restricts  the  amount  of  energy  that 


151 


can  be  converted  into  mass.  This  is  too  bad,  for  the  projectile  has 
been  given  a  great  energy  at  a  great  expense.  To  make  a  proton- 
antiproton  pair,  for  example,  by  the  projectile-hitting-fixed-target 
method,  the  projectile  must  have  a  kinetic  energy  of  6  Bev  (billion 
electron  volts),  of  which  only  2  Bev  goes  into  making  the  mass. 
The  6  Bev  Berkeley  Bevatron  was  designed  with  this  fact  in  mind 
in  order  to  be  able  to  make  antiprotons  and  antineutrons.  Typical 
processes  for  protons  striking  protons  are: 

p  +  p-^p  +  p  +  p  +  p, 
p-hp-^p-\-p-\-n-\-n. 

The  unfortunate  waste  of  4  Bev  in  these  processes  could  be 
avoided  if  the  target  proton  were  not  quiescent,  but  flew  at  the 
projectile  with  equal  and  opposite  speed.  It  is  hard  enough  to  pro- 
duce one  high-energy  beam,  and  far  more  difficult  to  produce  two  at 
once.  Nevertheless,  the  gain  in  available  energy  makes  it  worth 
the  trouble,  and  a  technique  for  producing  "clashing  beams"  is  now 
employed  at  Stanford  University,  where  oppositely  directed  beams 
of  electrons  collide.  The  device  is  sometimes  called  by  physicists 
the  synchroclash. 

Momentum  is  purely  a  property  of  motion — that  is,  if  there  is  no 
motion,  there  is  no  momentum.  It  is  somewhat  trickier  than  energy, 
for  momentum  is  what  is  called  a  vector  quantity.  It  has  direction 
as  well  as  magnitude.  Vectors  are  actually  familiar  in  everyday  life, 
whether  or  not  we  know  them  by  that  name.  The  velocity  of  an 
automobile  is  a  vector,  with  a  magnitude  (50  miles  per  hour,  for 
example)  and  a  direction  (northbound,  for  example).  Force  is  a  vec- 
tor, a  push  or  pull  of  some  strength  in  some  direction.  Mass,  on  the 
other  hand,  is  not  a  vector.  It  points  in  no  particular  direction. 
Energy  also  has  no  direction.  The  momentum  of  a  rolling  freight 
car,  however,  is  directed  along  the  tracks,  and  the  momentum  of 
an  elementary  particle  is  directed  along  its  course  through  space. 

In  order  to  appreciate  the  law  of  momentum  conservation,  one 
must  know  how  to  add  vectors.  Two  men  pushing  on  a  stalled  car 
are  engaged  in  adding  vectors.  If  they  push  with  equal  strength  and 
in  the  same  direction,  the  total  force  exerted  is  twice  the  force  each 
one  exerts  and,  of  course,  in  the  direction  they  are  pushing  [Figure 
4.1(a)].  If  they  push  with  equal  strength  but  at  opposite  ends  of 
the  car,  their  effort  comes  to  naught,  for  the  sum  of  two  vector 
quantities  which  are  equal  in  strength  but  opposite  in  direction  is 


152 


Conservation  Laws 


(a) 


(b) 


■►-^ 


> 


Figure  4.1.  The  addition  of  vectors.  The  forces  exerted  by  two  men 
pushing  equally  hard  may  be  "added,"  that  is,  combined,  to  give  any 
total  from  zero  up  to  twice  the  force  of  each. 


zero  [Figure  4.1(b)].  If  they  get  on  opposite  sides  of  the  car  and 
push  partly  inward,  partly  forward,  the  net  force  exerted  will  be 
forward,  but  less  than  twice  the  force  of  each  [Figure  4.1(c)].  De- 
pending on  their  degree  of  co-operation,  the  two  men  may  achieve 
a  strength  of  force  from  zero  up  to  twice  the  force  each  can  exert. 


(c) 


153 


This  is  a  general  characteristic  of  the  sum  of  two  vectors.  It  may 
have  a  wide  range  of  values  depending  on  the  orientation  of  the 
two  vectors. 

Consider  the  law  of  momentum  conservation  applied  to  the  de- 
cay of  a  kaon  into  muon  and  neutrino. 

Before  the  decay,  suppose  the  kaon  is  at  rest  [Figure  4.2(a)].  After 
the  decay,  momentum  conservation  requires  that  muon  and  neutrino 
fly  off  with  equal  magnitudes  of  momenta  and  and  that  the  momenta 


Before 


After 


(b) 


Figure  4.2.  Momentum  conservation  in  kaon  decay.  The  total  momentum 
is  zero  both  before  and  after  the  decay. 


be  oppositely  directed  [Figure  4.2(b)].  Only  in  this  way  can  the 
vector  sum  of  the  two  final  momenta  be  equal  to  the  original 
momentum,  namely  zero.  This  type  of  decay,  called  a  two-body 
decay,  is  rather  common,  and  is  always  characterized  by  particles 
emerging  in  exactly  opposite  directions. 

In  a  three-body  decay,  the  emerging  particles  have  more  free- 
dom. Figure  1.8,  for  example,  shows  the  decay  of  a  kaon  into  three 
pions  with  the  tracks  pointing  in  three  different  directions.  Recall- 
ing the  analogy  between  momentum  and  force,  one  can  visualize 
a  situation  in  which  three  diff^erent  forces  are  acting  and  produc- 
ing no  net  eflFect — two  fighters  and  a  referee  all  pushing  in  different 
directions  in  a  clinch.  Similarly,  the  momentum  vectors  must  ad- 
just themselves  to  produce  no  net  effect;  that  is,  they  must  add  up 


154 


Conservation  Laws 


to  give  zero.  Momentum  conservation  on  a  grander  scale  is  shown 
in  Figure  4.3,  where  eight  particles  emerge  from  a  single  event. 

One  vital  prohibition  of  the  law  of  momentum  conservation  is 
that  against  one-body  decays.  Consider,  for  example,  this  possibility, 

the  transformation  of  kaon  to  pion.  It  satisfies  the  laws  of  charge 
and  family-number  conservation.  It  is  consistent  with  energy  con- 
servation, for  it  is  downhill  in  mass,  and  it  also  satisfies  spin  con- 
servation. But  the  kaon-pion  mass  difference  must  get  converted 
to  energy  of  motion,  so  that  if  the  kaon  was  at  rest,  the  pion  will 
fly  away.  In  whatever  direction  it  moves,  it  has  some  momentum 
and  therefore  violates  momentum  conservation,  since  the  kaon  had 
none.  On  the  other  hand,  if  we  enforce  the  law  of  momentum 
conservation,  and  keep  the  pion  at  rest,  we  shall  have  violated 
energy  conservation,  for  in  this  case  the  extra  energy  arising  from 
the  mass  difference  will  be  unaccounted  for. 

Angular  momentum,  a  measure  of  the  strength  of  rotational 
motion,  has  been  a  key  concept  in  physics  since  the  time  of  Kepler. 
Actually,  Kepler  did  not  recognize  it  as  such,  but  the  second  of  his 
three  laws  of  planetary  motion — the  so-called  law  of  areas — is 
equivalent  to  a  law  of  conservation  of  angular  momentum.  Accord- 
ing to  this  law,  an  imaginary  straight  line  drawn  from  the  earth 
to  the  sun  sweeps  out  area  in  space  at  a  constant  rate.  During  a 
single  day  this  line  sweeps  across  a  thin  triangular  region  with  apex 
at  the  sun  and  base  along  the  earth's  orbit.  The  area  of  this  triangle 
is  the  same  for  every  day  of  the  year.  So,  when  the  earth  is  closer 
to  the  sun,  it  must  move  faster  in  order  to  define  a  triangle  with 
the  same  area.  It  speeds  up  just  enough,  in  fact,  to  maintain  a 
constant  value  of  its  angular  momentum,  and  the  law  of  areas  can 
be  derived  as  a  simple  consequence  of  the  law  of  conservation  of 
angular  momentum  (this  was  first  done  by  Newton). 

The  earth  also  serves  to  illustrate  approximately  the  two  kinds 
of  angular  momentum  which  enter  into  the  conservation  law — 
orbital  and  spin.  The  earth  possesses  angular  momentum  because 
of  its  orbital  motion  round  the  sun  and  because  of  its  daily  (spin) 
rotation  about  its  own  axis.  For  an  elementary  particle,  the  notion 
of  spin  is  about  the  same  as  for  the  earth — rotational  motion  about 

an  axis. 
If  a  photographer  in  space  took  a  time  exposure  of  the  earth  and 


155 


Figure  4.3. 


156 


Conservation  Laws 


sun,  his  photograph  would  contain  a  short  blur  for  the  sun  and  a 
longer  blur  for  the  earth.  He  would  notice  that  the  blurs  were 
not  directed  toward  each  other,  and  from  this  fact  alone  could 
conclude  that  earth  and  sun  possess  relative  angular  momentum. 
He  would  not  need  to  know  whether  the  earth  swings  around  the 
sun  or  whether  it  proceeds  into  interstellar  space.  The  key  fact 
defining  orbital  angular  momentum  is  some  transverse  motion  of 
two  objects.  Any  two  moving  objects,  not  aimed  directly  at  each 
other,  possess  relative  angular  momentum.  Two  trains  passing  on 
the  great  plains  have  relative  angular  momentum,  even  though 
each  is  proceeding  straight  as  an  arrow.  But  if,  through  some  mis- 
chance, both  were  on  the  same  track  on  a  collision  course,  they 
would  have  zero  angular  momentum.  In  particle  collisions  and 
decays,  orbital  angular  momentum  is  usually  of  this  trains-in-the- 
plains  type,  not  involving  actual  orbiting  of  one  particle  round  an- 
other. Figure  4.4  illustrates  several  examples  of  motion  with  angular 
momentum. 

Angular  momentum  is  a  vector  quantity.  Its  direction  is  taken 
to  be  the  axis  of  rotation.  The  axis  is  well  defined  for  spin,  but 
what  about  orbital  motion?  For  the  passing  trains,  imagine  again 
the  blurred  photograph  indicating  their  direction  of  motion.  Then 
ask:  What  would  the  axis  be  if  the  trains  rotated  about  each  other, 
instead  of  proceeding  onward?  The  answer  is  a  vertical  axis;  the 
angular  momentum  is  directed  upward.  One  more  fact  about  orbital 
angular  momentum  needs  to  be  known.  Unlike  spin,  which  comes 
in  units  of  ^^,  it  comes  only  in  units  of  h. 

The  spinless  pion  decays  into  muon  and  neutrino,  each  with 
spin  ^.  In  Figure  4.5  we  use  artistic  license  and  represent  the 
particles  by  little  spheres  with  arrows  to  indicate  their  direction 
of  spin.  Muon  and  neutrino  spin  oppositely  in  order  to  preserve  the 


Figure  43.  Momentum  conservation  in  an  antiproton  annihilation  event. 
An  antiproton  entering  from  the  bottom  collides  with  a  proton  in  the 
bubble  chamber.  Eight  pions,  four  negative  and  four  positive,  spray  off 
from  the  annihilation  event  in  all  directions.  The  momentum  of  each 
can  be  measured  from  the  curvature  of  the  track;  the  eight  momenta 
added  together  as  vectors  are  just  equal  to  the  momentum  of  the  sin- 
gle incoming  antiproton.  (The  kink  in  the  track  at  the  lower  right  is  a 
pion  decay,  x*  -^  /**  -|-  v^.  In  what  general  direction  did  the  unseen  neu- 
trino fly  off?) 


157 


Train 


(a) 


(b) 


:^&A 


@ 


(c) 


(d) 


Figure  4.4.  Examples  of  motion  with  angular  momentum,  (a)  The 
earth  possesses  spin  angular  momentum  about  its  axis  as  well  as  orbital 
angular  momentum  about  an  axis  designated  by  the  giant  barber  pole. 
The  constancy  of  the  earth's  orbital  angular  momentum  means  that 
the  shaded  area  swept  out  in  one  day  is  the  same  for  every  day  of  the 
year,  (b)  Trains  on  a  circular  track  possess  angular  momentum  about 
a  vertical  axis,  (c)  Even  on  straight  tracks,  a  similar  relative  motion  of 
trains  represents  angular  momentum,  (d)  An  electron  flies  past  a  pro- 
ton. Both  particles  possess  spin  angular  momentum  and,  because  they 
are  not  on  a  collision  course,  they  also  have  orbital  angular  momentum. 


158 


Conservation  Laws 


total  zero  angular  momentum.  In  this  case,  no  orbital  angular 
momentum  is  involved. 

Another  two-body  decay,  that  of  the  A,  illustrates  the  coupling 
of  spin  and  orbital  motion.  The  A,  supposed  initially  at  rest  [Fig- 
ure 4.6(a)],  has  spin  ^.  One  of  its  possible  decay  modes  is 

A"  -4  p  -f-  X-. 

This  may  proceed  in  two  ways.  The  proton  and  pion  may  move 
apart  with  no  orbital  angular  momentum,  the  proton  spin  directed 
upward  to  match  the  initial  A  spin  [Figure  4.6(b)];  or  the  proton 
spin  may  be  flipped  to  point  downward  while  proton  and  pion 

Before  (no  spin) 


O- 


After  (cancelling  spin) 


Figure  4.5.  Angular-momentum  conservation  in  pion  decay.  The  total 
angular  momentum  is  zero  before  and  after  the  decay. 


separate  with  one  unit  of  orbital  angular  momentum,  directed  up- 
ward [Figure  4.6(c)  ].  In  the  first  case, 

original  spin  1/4  (up)  -^  final  spin  i/4  (up). 

In  the  second  case, 

original  spin  i/^  (up)  -»  final  spin  i/4  (down)  +  orbital  angular 

momentum  1  (up). 

Beta  decay,  the  earliest  known  particle  decay  process,  serves 
nicely  to  illustrate  all  of  the  absolute  conservation  laws  discussed. 
The  beta  decay  of  the  neutron,  indicated  symbolically  by 

n—*p-\-e--\-  Ve, 


159 


is  pictured  in  Figure  4.7.  Consider  now  the  conservation  laws  ap- 
plied to  this  decay. 

Energy.  Reference  to  Table  1  shows  that  the  sum  of  the  masses 
of  the   proton    (1836.12),   the  electron    (1.0),  and  the  electron's 


(a) 


A" 


Right  hand 


O 


Figure  4.6.  Angular-momentum  conservation  in  lambda  decay.  The 
direction  of  angular  momentum  is  defined  by  the  right-hand  rule.  If 
the  curved  fingers  of  the  right  hand  point  in  the  direction  of  rotational 
motion,  the  right  thumb  defines  the  direction  assigned  to  the  angular 
momentum.  Thus  the  particle  spin  is  up  in  diagrams  (a)  and  (b)  and 
down  in  diagram  (c);  the  orbital  angular  momentum  is  up  in  dia- 
gram (c). 


neutrino  (0),  add  up  to  less  than  the  neutron  mass  (1838.65).  The 
decay  is  therefore  an  allowed  downhill  decay,  the  slight  excess  mass 
going  into  kinetic  energy  of  the  products. 

Momentum.  The  three  particles  must  fan  off  in  different  direc- 


160 


Conservation  Laws 


rions  with  the  available  excess  energy  so  distributed  among  them 
that  the  sum  of  the  three  momentum  vectors  is  zero. 

Angular  momentum.  One  possibility,  illustrated  in  Figure  4.7, 
is  that  the  departing  electron  and  proton  have  opposite  cancelling 
spins,  while  the  neutrino  spins  in  the  same  direction  as  the  original 
neutron  to  conserve  the  angular  momentum. 

Charge.  The  final  charge  (1  positive,  1  negative,  1  neutral)  is 
zero,  the  same  as  the  initial  neutron  charge. 


After 


Before 


^ 


Figure  4.1.  Beta  decay  of  the  neutron,  n  ^  p  +  e'  +  ve 


Electron-family  number.  The  neutron  has  zero  electron- family 
number.  In  the  decay,  one  electron  and  one  antineutrino  (i^)  are 
created  to  preserve  zero  electron-family  number. 

Muon-family  number.  No  members  of  the  muon  family  are 
created  or  destroyed. 

Bar  yon  number.  The  proton  is  the  single  baryon  among  the  final 
three  particles,  matching  the  single  original  baryon. 

Now  we  propose  an  exercise  for  the  reader.  Below  are  listed  a 
few  decays  and  transformations  which  do  not  occur  in  nature.  If 
only  one  particle  stands  on  the  left,  a  decay  process  is  understood. 
If  two  particles  stand  on  the  left,  a  collision  process  is  understood. 
At  least  one  conservation  law  prohibits  each  of  these  processes. 
Find  at  least  one  conservation  law  violated  by  each  process.  Several 


161 


violate  more  than  one  law  and  one  of  those  listed  violates  five  of  the 
seven  conservation  laws. 

a.  |Ll+  — >  T+  +  v„ 

b.  r--^  ve-^  y 

c.  p  +  p->p  +  A»  +  S+ 

d.  M+-^AO 

e.  n —*  fi+  -\-  r-  -^  y 

f.  A"  ->  p  +  <r- 

g.  T-  +  p  ->  T-  +  w  +  A°  +  X+ 
h.     f+  +  <r-  ->  M+  +  X- 

i.     n-  -^  e-  -\-  f^  -\-  v^ 

The  aspect  of  conservation  laws  that  makes  them  appear  to  the 
theorist  and  the  philosopher  to  be  the  most  beautiful  and  profound 
statements  of  natural  law  is  their  connection  with  principles  of 
symmetry  in  nature.  Baldly  stated,  energy,  momentum,  and  angular 
momentum  are  all  conserved  because  space  and  time  are  isotropic 
(the  same  in  every  direction)  and  homogeneous  (the  same  at  every 
place).  This  is  a  breath-taking  statement  when  one  reflects  upon 
it,  for  it  says  that  three  of  the  seven  absolute  conservation  laws  arise 
solely  because  empty  space  has  no  distinguishing  characteristics, 
and  is  everywhere  equally  empty  and  equally  undistinguished.  (Be- 
cause of  the  relativistic  link  between  space  and  time,  we  really  mean 
space-time.)  It  seems,  in  the  truest  sense,  that  we  are  getting  some- 
thing from  nothing. 

Yet  there  can  be  no  doubt  about  the  connection  between  the  prop- 
erties of  empty  space  and  the  fundamental  conservation  laws  which 
govern  elementary-particle  behavior.  This  connection  raises  philo- 
sophical questions  which  we  will  mention  but  not  pursue  at  any 
length.  On  the  one  hand,  it  may  be  interpreted  to  mean  that  con- 
servation laws,  being  based  on  the  most  elementary  and  intuitive 
ideas,  are  the  most  profound  statements  of  natural  law.  On  the 
other  hand,  one  may  argue,  as  Bertrand  Russell*  has  done,  that  it 
only  demonstrates  the  hollowness  of  conservation  laws  ("truisms," 
according  to  Russell),  energy,  momentum,  and  angular  momentum 
all  being  defined  in  just  such  a  way  that  they  must  be  conserved. 
Now,  in  fact,  it  is  not  inconsistent  to  hold  both  views  at  once.  If 


•  Bertrand  Russell,   The  ABC  of  Relativity    (New  York:   New  American 
Library,  1959). 


162 


Conservation  Laws 


the  aim  of  science  is  the  self-consistent  description  of  natural  phe- 
nomena based  upon  the  simplest  set  of  basic  assumptions,  what 
could  be  more  satisfying  than  to  have  basic  assumptions  so  com- 
pletely elementary  and  self-evident  (the  uniformity  of  space-time) 
that  the  laws  derived  from  them  can  be  called  truisms?  Since  the 
scientist  generally  is  inclined  to  call  most  profound  that  which  is 
most  simple  and  most  general,  he  is  not  above  calling  a  truism  pro- 
found. Speaking  more  pragmatically,  we  must  recognize  the  dis- 
covery of  anything  that  is  absolutely  conserved  as  something  of 
an  achievement,  regardless  of  the  arbitrariness  of  definition  involved. 
Looking  at  those  conservation  laws  whose  basis  we  do  not  under- 
stand (the  three  family-number-conservation  laws)  also  brings 
home  the  fact  that  it  is  easier  to  call  a  conservation  law  a  truism 
after  it  is  understood  than  before.  It  seems  quite  likely  that  we 
shall  gain  a  deeper  understanding  of  nature  and  of  natural  laws 
before  the  conservation  of  baryon  number  appears  to  anyone  to 
be  a  self-evident  truth. 

Before  trying  to  clarify  through  simple  examples  the  connection 
between  conservation  laws  and  the  uniformity  of  space,  we  con- 
sider the  question,  "What  is  symmetry?"  In  most  general  terms, 
symmetry  means  that  when  one  thing  (A)  is  changed  in  some  par- 
ticular way,  something  else  (B)  remains  unchanged.  A  symmetrical 
face  is  one  whose  appearance  (B)  would  remain  the  same  if  its 
two  sides  (A)  were  interchanged.  If  a  square  figure  (A)  is  rotated 
through  90  degrees,  its  appearance  (B)  is  not  changed.  Among 
plane  figures,  the  circle  is  the  most  symmetrical,  for  if  it  is  rotated 
about  its  center  through  any  angle  whatever,  it  remains  indistin- 
guishable from  the  original  circle — or,  in  the  language  of  modern 
physics,  its  form  remains  invariant.  In  the  language  of  ancient 
Greece,  the  circle  is  the  most  perfect  and  most  beautiful  of  plane 
figures. 

Aristotle  regarded  the  motion  of  the  celestial  bodies  as  neces- 
sarily circular  because  of  the  perfection  (the  symmetry)  of  the 
circle.  Now,  from  a  still  deeper  symmetry  of  space-time,  we  can 
derive  the  ellipses  of  Kepler.  Modem  science,  which  could  begin 
only  after  breaking  loose  from  the  centuries-old  hold  of  Aristotelian 
physics,  now  finds  itself  with  an  unexpected  Aristotelian  flavor, 
coming  both  from  the  increasingly  dominant  role  of  symmetry 
principles  and  from  the  increasingly  geometrical  basis  of  physics. 

We  are  accustomed  to  think  of  symmetry  in  spatial  terms.  The 


163 


symmetry  of  the  circle,  the  square,  and  the  face  are  associated  with 
rotations  or  inversions  in  space.  Symmetry  in  time  is  an  obvious 
extension  of  spatial  symmetry;  the  fact  that  nature's  laws  appear  to 
remain  unchanged  as  time  passes  is  a  fundamental  symmetry  of 
nature.  However,  there  exist  some  subtler  symmetries,  and  it  is 
reasonable  to  guess  that  the  understanding  of  baryoh  conservation, 
for  example,  will  come  through  the  discovery  of  new  symmetries 
not  directly  connected  with  space  and  time. 

In  the  symmetry  of  interest  to  the  scientist,  the  unchanging 
thing — the  invariant  element — is  the  form  of  natural  laws.  The 
thing  changed  may  be  orientation  in  space,  or  position  in  space  or 
time,  or  some  more  abstract  change  (not  necessarily  realizable  in 
practice)  such  as  the  interchange  of  two  particles.  The  inversion 
of  space  and  the  reversal  of  the  direction  of  flow  of  time  are  other 
examples  of  changes  not  realizable  in  practice,  but  nonetheless  of 
interest  for  the  symmetries  of  natural  law.  These  latter  two  will 
be  discussed  in  Chapter  Eight. 

If  scientists  in  Chicago,  New  York,  and  Geneva  perform  the 
same  experiment  and  get  the  same  answer  (within  experimental 
error)  they  are  demonstrating  one  of  the  symmetries  of  nature, 
the  homogeneity  of  space.  If  the  experiment  is  repeated  later  with 
the  same  result,  no  one  is  surprised,  for  we  have  come  to  accept 
the  homogeneity  of  time.  The  laws  of  nature  are  the  same,  so  far 
as  we  know,  at  all  points  in  space,  and  for  all  times.  This  invari- 
ance  is  important  and  is  related  to  the  laws  of  conservation  of 
energy  and  momentum,  but  ordinary  experience  conditions  us  to 
expect  such  invariance  so  that  it  seems  at  first  to  be  trivial  or  self- 
evident.  It  might  seem  hard  to  visualize  any  science  at  all  if  nat- 
ural law  changed  from  place  to  place  and  time  to  time,  but,  in 
fact,  quantitative  science  would  be  perfectly  possible  without  the 
homogeneity  of  space-time.  Imagine  yourself,  for  example,  on  a 
merry-go-round  that  speeded  up  and  slowed  down  according  to  a 
regular  schedule.  If  you  carried  out  experiments  to  deduce  the  laws 
of  mechanics  and  had  no  way  of  knowing  that  you  were  on  a  ro- 
tating system,  you  would  conclude  that  falling  balls  were  governed 
by  laws  which  varied  with  time  and  with  position  (distance  from 
central  axis),  but  you  would  be  quite  able  to  work  out  the  laws 
in  detail  and  predict  accurately  the  results  of  future  experiments, 
provided  you  knew  where  and  when  the  experiment  was  to  be 
carried  out.  Thanks  to  the  actual  homogeneity  of  space  and  time. 


164 


Conservation  Laws 


the  results  of  future  experiments  can  in  fact  be  predicted  without 
any  knowledge  of  the  where  or  when. 

A  slightly  less  obvious  kind  of  invariance,  although  one  also 
familiar  from  ordinary  experience,  is  the  invariance  of  the  laws 
of  nature  for  systems  in  uniform  motion.  Passengers  on  an  ideally 
smooth  train  or  in  an  ideally  smooth  elevator  are  unaware  of  mo- 
tion. If  the  laws  of  mechanics  were  significantly  altered,  the  riders 
would  be  aware  of  it  through  unusual  bodily  sensations.  Such  a 
qualitative  guide  is,  of  course,  not  entirely  reliable,  but  careful 
experiments  performed  inside  the  ideal  uniformly  moving  train 
would  reveal  the  same  laws  of  nature  revealed  by  corresponding 
experiments  conducted  in  a  stationary  laboratory.  This  particular 
invariance  underlies  the  theory  of  relativity,  and  is  a  manifestation 
of  the  isotropy  of  four-dimensional  space-time,  a  point  we  can 
regrettably  not  discuss  in  detail.  What,  to  our  limited  three-di- 
mensional vision,  appears  to  be  uniform  motion  is,  to  a  more  en- 
lightened brain  capable  of  encompassing  four  dimensions,  merely 
a  rotation.  Instead  of  turning,  say,  from  north  to  east,  the  experi- 
menter who  climbs  aboard  the  train  is,  from  the  more  general  view, 
turning  from  space  partly  toward  the  time  direction.  According 
to  relativity,  which  joins  space  and  time  together  in  a  four-dimen- 
sional space-time,  the  laws  of  nature  should  no  more  be  changed 
by  "turning"  experimental  apparatus  toward  the  time  direction 
(that  is,  loading  it  aboard  the  train)  than  by  turning  it  through 
90  degrees  in  the  laboratory. 

The  chain  of  connection  we  have  been  discussing  is:  Symmetry 
->  invariance  ->  conservation.  The  symmetry  of  space  and  time,  or 
possibly  some  subtler  symmetry  of  nature,  implies  the  invariance 
of  physical  laws  under  certain  changes  associated  with  the  sym- 
metry. In  the  simplest  case,  for  example,  the  symmetry  of  space 
which  we  call  its  homogeneity  implies  the  invariance  of  experi- 
mental results  when  the  apparatus  is  moved  from  one  place  to  an- 
other. This  invariance,  in  turn,  implies  the  existence  of  certain 
conservation  laws.  The  relation  between  conservation  laws  and 
symmetry  principles  is  what  we  now  wish  to  illuminate  through 
two  examples.  Unfortunately,  an  adequate  discussion  of  this  im- 
portant connection  requires  the  use  of  mathematics  beyond  the 
scope  of  this  book. 

Suppose  we  imagine  a  single  isolated  hydrogen  atom  alone  and 
at  rest  in  empty  space.  If  we  could  draw  up  a  chair  and  observe 


165 


it  without  influencing  it,  what  should  we  expect  to  see?  (For  this 
discussion,  we  ignore  quantum  mechanics  and  the  wave  nature  of 
particles,  pretending  that  electron  and  proton  may  be  separately 
seen  as  particles,  and  be  uninfluenced  by  the  observer.  The  reader 
will  have  to  accept  the  fact  that  these  false  assumptions  are  per- 
missible and  irrelevant  for  the  present  discussion.)  We  should  see 
an  electron  in  rapid  motion  circling  about  a  proton,  and  the  proton 
itself  moving  more  slowly  in  a  smaller  circle.  Were  we  to  back  off^ 
until  the  whole  atom  could  only  be  discerned  as  a  single  spot,  that 
spot,  if  initially  motionless,  would  remain  at  rest  forever.  We  now 
must  ask  whether  this  circumstance  is  significant  or  insignificant, 
important  or  dull.  It  certainly  does  not  seem  surprising.  Why 
should  the  atom  move,  we  may  ask.  It  is  isolated  from  the  rest  of 
the  universe,  no  forces  act  upon  it  from  outside,  therefore  there  is 
nothing  to  set  it  into  motion.  If  we  leave  a  book  on  a  table  and 
come  back  later,  we  expect  to  find  it  there.  Everyday  experience 
conditions  us  to  expect  that  an  object  on  which  no  external  forces 
act  will  not  spontaneously  set  itself  into  motion.  There  is  no  more 
reason  for  the  atom  to  begin  to  move  than  for  the  book  to  migrate 
across  the  table  and  fly  into  a  corner.  The  trouble  with  this  argu- 
ment is  that  it  makes  use  of  the  common  sense  of  ordinary  experi- 
ence, without  off"ering  any  explanation  for  the  ordinary  experience. 

If  we  put  aside  "common  sense"  and  ask  what  the  atom  might 
do,  it  is  by  no  means  obvious  that  it  should  remain  at  rest.  In  spite 
of  the  fact  that  no  external  forces  are  acting,  strong  internal  forces 
are  at  work.  The  proton  exerts  a  force  on  the  electron  which  con- 
stantly alters  its  motion;  the  electron,  in  turn,  exerts  a  force  on  the 
proton.  Both  atomic  constituents  are  experiencing  force.  Why 
should  these  forces  not  combine  to  set  the  atom  as  a  whole  into 
motion?  Having  put  the  question  in  this  way,  we  may  consider 
the  book  on  the  table  again.  It  consists  of  countless  billions  of 
atoms,  each  one  exerting  forces  on  its  neighboring  atoms.  Through 
what  miracle  do  these  forces  so  precisely  cancel  out  that  no  net 
force  acts  upon  the  book  as  a  whole  and  it  remains  quiescent  on 
the  table? 

The  classical  approach  to  this  problem  is  to  look  for  a  positive,  or 
permissive,  law,  a  law  which  tells  what  does  happen.  Newton  first 
enunciated  this  law  which  (except  for  some  modification  made 
necessary  by  the  theory  of  relativity)  has  withstood  the  test  of  time 
to  the  present  day.  It  is  called  Newton's  third  law,  and  says  that  all 


166 


Conservation  Laws 


forces  in  nature  occur  in  equal  and  opposite  balanced  pairs.  The 
proton's  force  on  the  electron  is  exactly  equal  and  opposite  to  the 
electron's  force  on  the  proton.  The  sum  of  these  two  forces  (the 
vector  sum)  is  zero,  so  that  there  is  no  tendency  for  the  structure 
as  a  whole  to  move  in  any  direction.  The  balancing  of  forces,  more- 
over, can  be  related  to  a  balancing  of  momenta.  By  making  use  of 
Newton's  second  law,*  which  relates  the  motion  to  the  force,  one 
can  discover  that,  in  a  hydrogen  atom  initially  at  rest,  the  balanced 
forces  will  cause  the  momenta  of  electron  and  proton  to  be  equal 
and  opposite.  At  a  given  instant,  the  two  particles  are  moving  in 
opposite  directions.  The  heavier  proton  moves  more  slowly,  but 
has  the  same  momentum  as  the  electron.  As  the  electron  swings  to 
a  new  direction  and  a  new  speed  in  its  track,  the  proton  swings 
too  in  just  such  a  way  that  its  momentum  remains  equal  and  op- 
posite to  that  of  the  electron.  In  spite  of  the  continuously  chang- 
ing momenta  of  the  two  particles,  the  total  momentum  of  the  atom 
remains  zero;  the  atom  does  not  move.  In  this  way — by  "discover- 
ing" and  applying  two  laws,  Newton's  second  and  third  laws  of 
motion — one  derives  the  law  of  momentum  conservation  and  finds 
an  explanation  of  the  fact  that  an  isolated  atom  does  not  move. 

Without  difficulty,  the  same  arguments  may  be  applied  to  the 
book  on  the  table.  Since  all  forces  come  in  equal  and  opposite 
pairs,  the  forces  between  every  pair  of  atoms  cancel,  so  that  the 
total  force  is  zero,  no  matter  how  many  billions  of  billions  of  atoms 
and  individual  forces  there  might  be. 

It  is  worth  reviewing  the  steps  in  the  argument  above.  Two  laws 
of  permission  were  discovered,  telling  what  does  happen.  One  law 
relates  the  motion  to  the  force;  the  other  says  that  the  forces  be- 
tween pairs  of  particles  are  always  equal  and  opposite.  From  these 
laws,  the  conservation  of  momentum  was  derived  as  an  interesting 
consequence,  and  this  conservation  law  in  turn  explained  the  fact 
that  an  isolated  atom  at  rest  remains  at  rest. 

The  modern  approach  to  the  problem  starts  in  quite  a  different 
way,  by  seeking  a  law  of  prohibition,  a  principle  explaining  why 
the  atom  does  not  move.  This  principle  is  the  invariance  of  laws 
of  nature  to  a  change  of  position.  Recall  the  chain  of  key  ideas 


*  Newton's  second  law,  usually  written  F  =  ma,  says  that  the  acceleration 
a  experienced  by  a  panicle  multiplied  by  its  mass  m  is  equal  to  the  force  F 
acting  upon  it.  The  law  may  also  be  stated  in  this  way:  The  rate  at  which 
the  momentum  of  a  particle  is  changing  is  equal  to  the  force  applied. 


167 


referred  to  on  page  105:  symmetry  -^  invariance  — >  conservation.  In 
the  example  of  the  isolated  hydrogen  atom,  the  symmetry  of  interest 
is  the  homogeneity  of  space.  Founded  upon  this  symmetry  is  the 
invariance  principle  just  cited.  Finally,  the  conservation  law  resting 
on  this  invariance  principle  is  the  conservation  of  momentum. 

In  order  to  clarify,  through  the  example  of  the  hydrogen  atom, 
the  connecting  links  between  the  assumed  homogeneity  of  space 
and  the  conservation  of  momentum,  we  must  begin  with  an  exact 
statement  of  the  invariance  principle  as  applied  to  our  isolated  atom. 
The  principle  is  this:  No  aspect  of  the  motion  of  an  isolated  atom 
depends  upon  the  location  of  the  center  of  mass  of  the  atom.  The 
center  of  mass  of  any  object  is  the  average  position  of  all  of  the 
mass  in  the  object.  In  a  hydrogen  atom,  the  center  of  mass  is  a 
point  in  space  between  the  electron  and  the  proton,  close  to  the 
more  massive  proton. 

Let  us  visualize  our  hydrogen  atom  isolated  in  empty  space  with 
its  center  of  mass  at  rest.  Suppose  now  that  its  center  of  mass  starts 
to  move.  In  which  direction  should  it  move?  We  confront  at  once 
the  question  of  the  homogeneity  of  space.  Investing  our  atom  with 
human  qualities  for  a  moment,  we  can  say  that  it  has  no  basis  upon 
which  to  "decide"  how  to  move.  To  the  atom  surveying  the 
possibilities,  every  direction  is  precisely  as  good  or  bad  as  every 
other  direction.  It  is  therefore  frustrated  in  its  "desire"  to  move 
and  simply  remains  at  rest. 

This  anthropomorphic  description  of  the  situation  can  be  re- 
placed by  sound  mathematics.  What  the  mathematics  shows  is  that 
an  acceleration  of  the  center  of  mass — for  example,  changing  from 
a  state  of  rest  to  a  state  of  motion — is  not  consistent  with  the  as- 
sumption that  the  laws  of  motion  of  the  atom  are  independent  of  the 
location  of  the  center  of  mass.  If  the  center  of  mass  of  the  atom 
is  initially  at  rest  at  point  A  and  it  then  begins  to  move,  it  will  later 
pass  through  another  point  B.  At  point  A,  the  center  of  mass  had  no 
velocity.  At  point  B  it  does  have  a  velocity.  Therefore,  the  state  of 
motion  of  the  atom  depends  on  the  location  of  the  center  of  mass, 
contrary  to  the  invariance  principle.  Only  if  the  center  of  mass 
remains  at  rest  can  the  atom  satisfy  the  invariance  principle.*  The 
immobility  of  the  center  of  mass  requires,  in  turn,  that  the  two 
particles  composing  the  atom  have  equal  and  opposite  momenta. 


•  If  the  center  of  mass  of  the  atom  had  been  moving  initially,  the  invari- 
ance principle  requires  that  it  continue  moving  with  constant  velocity. 


168 


Conservation  Laws 


A  continual  balancing  of  the  two  momenta  means  that  their  sum, 
the  total  momentum,  is  a  constant. 

The  argument  thus  proceeds  directly  from  the  symmetry  prin- 
ciple to  the  conservation  law  without  making  use  of  Newton's  laws 
of  motion.  That  this  is  a  deeper  approach  to  conservation  laws,  as 
well  as  a  more  esthetically  pleasing  one,  has  been  verified  by  his- 
tory. Although  Newton's  laws  of  motion  have  been  altered  by  rela- 
tivity and  by  quantum  mechanics,  the  direct  connection  between 
the  symmetry  of  space  and  the  conservation  of  momentum  has 
been  unaffected — or  even  strengthened — by  these  modern  theories 
and  momentum  conservation  remains  one  of  the  pillars  of  modern 
physics.  We  must  recognize  that  a  violation  of  the  law  of  momen- 
tum conservation  would  imply  an  inhomogeneity  of  space;  this  is 
not  an  impossibility,  but  it  would  have  far-reaching  consequences 
for  our  view  of  the  universe. 

Returning  finally  to  the  book  on  the  table,  we  want  to  empha- 
size that  the  quiescence  of  the  undisturbed  book — a  macroscopic 
object — at  least  strongly  suggests  that  momentum  conservation 
must  be  a  valid  law  in  the  microscopic  world.  Viewed  microscopi- 
cally, the  book  is  a  collection  of  an  enormous  number  of  atoms, 
each  one  in  motion.  That  this  continuous  microscopic  motion  never 
makes  itself  felt  as  spontaneous  bulk  motion  of  the  whole  book  is 
true  only  because  of  the  conservation  of  momentum  which  re- 
quires that  every  time  an  atom  changes  its  momentum  (as  it  is  con- 
stantly doing)  one  or  more  other  atoms  must  undergo  exactly 
compensating  changes  of  their  momentum. 

Through  similar  examples  it  is  possible  to  relate  the  law  of  con- 
servation of  angular  momentum  to  the  isotropy  of  space.  A  com- 
pass needle  which  is  held  pointing  east  and  is  then  released  will 
swing  toward  the  north  because  of  the  action  of  the  earth's  mag- 
netic field  upon  it.  But  if  the  same  compass  needle  is  taken  to  the 
depths  of  empty  space,  far  removed  from  all  external  influences, 
and  set  to  point  in  some  direction,  it  will  remain  pointing  in  that 
direction.  A  swing  in  one  direction  or  the  other  would  imply  a 
nonuniformity*  of  space.  If  the  uniformity  of  space  is  adopted  as 
a  fundamental  symmetry  principle,  it  can  be  concluded  that  the 


•  Strictly,  momentum  conservation  rests  on  the  homogeneity  of  space  (uni- 
formity of  place),  and  angular  momentum  conservation  rests  on  the  isotropy 
of  space  (uniformity  of  direction).  The  distinction  is  not  important  for  our 
purposes,  and  it  is  satisfactory  to  think  of  space  simply  as  everywhere  the 
same,  homogeneity  and  isotropy  being  summarized  by  the  word  uniformity. 


169 


total  angular  momentum  of  all  the  atomic  constituents  of  the  needle 
must  be  a  constant.  Otherwise,  the  internal  motions  within  the 
needle  could  set  the  whole  needle  into  spontaneous  rotation  and 
its  motion  would  violate  the  symmetry  principle. 

Energy  conservation,  in  a  way  that  is  not  so  easy  to  see,  is 
related  to  the  homogeneity  of  time.  Thus  all  three  conservation 
laws — of  energy,  momentum,  and  angular  momentum — are  "under- 
stood" in  terms  of  the  symmetry  of  space-time,  and  indeed  the  the- 
ory of  relativity  has  shown  that  these  three  laws  are  all  parts  of 
a  single  general  conservation  law  in  the  four-dimensional  world. 

Only  one  of  the  three  conservation  laws  governing  the  intrinsic 
properties  of  the  particles  has  so  far  been  understood  in  terms  of 
a  symmetry  principle.  This  is  the  law  of  charge  conservation.  (Re- 
call, however,  that  the  quantization  of  charge  is  not  yet  under- 
stood.) The  symmetry  principle  underlying  charge  conservation  is 
considerably  more  subtle  than  the  space-time  symmetry  underlying 
the  conservation  laws  of  properties  of  motion.  The  modern  version 
of  this  symmetry  principle  rests  upon  technical  aspects  of  the  theory 
of  quantum  mechanics  (it  may  be  based  also  on  equally  technical 
aspects  of  the  theory  of  electromagnetism).  Nevertheless,  it  is  such 
a  stunning  victory  for  the  power  of  a  symmetry  principle  that  we 
must  try,  however  crudely,  to  indicate  the  modern  view  of  this 
symmetry. 

In  the  main,  the  classical  theories  of  physics  deal  directly  with 
quantities  which  are  measurable,  usually  called  observables.  Force, 
mass,  velocity,  and  almost  all  the  other  concepts  described  by  the 
classical  laws  are  themselves  observables.  The  equations  of  quantum 
mechanics,  however,  contain  quantities  which  are  not  themselves 
observables.  From  these  quantities — one  step  removed  from  reality 
— the  observables  are  derived.  The  "wave  function"  is  one  of  the 
unobservable  quantities;  it  determines  the  probability,  say,  that  the 
electron  is  at  any  particular  point  in  the  hydrogen  atom,  but  is  itself 
not  that  probability  nor  any  other  measurable  thing.  Now  enters 
the  idea  of  symmetry.  Any  change  that  can  be  made  in  the  un- 
observable quantity  without  resulting  in  a  change  of  the  observ- 
ables ought  to  leave  all  the  laws  of  nature  unchanged.  After  care- 
ful scrutiny,  this  statement  seems  so  obviously  true  that  it  is  hard 
to  understand  how  it  could  have  any  important  consequences.  Of 
course  one  ought  to  be  able  to  do  anything  whatever  to  unobserv- 
able quantities  so  long  as  observables  are  not  changed.  But  remember 


170 


Conservation  Laws 


how  important  were  the  properties  of  empty  space.  Equally  im- 
portant are  the  properties  of  unobservables  such  as  wave  functions. 

Space  itself  may  be  regarded  as  an  unobservable.  The  uniformity 
of  space  means  that  it  is  impossible,  by  any  experimental  means, 
to  ascertain  one's  absolute  position  in  space.  An  experiment  carried 
out  at  one  place  will  yield  results  identical  to  the  results  of  the  same 
experiment  carried  out  at  another  place.  Any  change  in  the  un- 
observable space  (for  instance,  moving  the  apparatus  from  one 
place  to  another)  must  leave  unchanged  the  laws  of  nature  and  the 
observable  results  of  experiment.  As  we  have  just  seen,  this 
symmetry  principle  or  invariance  requirement  underlies  the  law 
of  momentum  conservation. 

When  an  analogous  symmetry  principle  is  applied  to  the  un- 
observable wave  function  of  the  electron  a  conservation  law  re- 
sults, the  conservation  of  charge.  Expressed  negatively,  if  charge 
were  not  conserved,  the  form  of  the  equations  of  quantum  me- 
chanics would  depend  upon  unobservable  quantities,  a  situation  at 
variance  with  our  symmetry  principle.  The  analogous  statement 
for  spatial  homogeneity  would  be:  If  momentum  were  not  con- 
served, the  laws  of  mechanics  would  depend  upon  the  absolute 
location  in  space  and  such  dependence  is  at  variance  with  the  as- 
sumed symmetry  of  space. 

Regrettably,  we  can  not  explain  the  law  of  charge  conservation 
more  fully  without  mathematics.  It  is  expected,  but  not  yet  verified, 
that  some  undiscovered  subtle  symmetries  of  nature  underlie  the 
laws  of  electron-family  conservation,  muon-family  conservation, 
and  baryon  conservation.  The  absolute  prohibition  of  proton  de- 
cay, which  keeps  its  enormous  intrinsic  energy  locked  forever  in 
the  form  of  mass,  can  be  no  accident,  but  the  reason  still  remains 
hidden. 

Answers 

The  particle  transformations  listed  on  page  102  violate  the  fol- 
lowing conservation  laws: 

a.  Energy   (an  "uphill"  decay);  muon-family  number  (since  ft* 
is  an  antiparticle). 

b.  Charge. 

c.  Angular  momentum;  baryon  number. 

d.  Energy;   momentum    (a  one-particle   decay);  charge;  muon- 


171 


family  number;  baryon  number. 

e.  Angular  momentum;  baryon  number;  muon-familv  number; 
electron-family  number. 

f.  Angular  momentum;  electron-family  number. 

g.  Angular  momentum;  baryon  number. 

h.  Angular  momentum;  muon-family  number. 

i.  Charge.  (Why  is  angular  momentum  conservation  satisfied?) 


Schematic  analysis  of  the  photograph  on  the  opposite  page. 


172 


Conservation  Laws 


Figure  1.8.  Decay  of  unstable  particles.  This  unusual  bubble-chamber 
photograph  shows  the  decay  of  five  different  elementary  particles.  At 
point  A,  a  positive  kaon  decays  into  three  pions.  At  B,  one  of  these 
pions  decays  into  a  muon  and  an  unseen  neutrino.  At  C,  the  muon  de- 
cays into  a  positron  (plus  tw^o  neutrinos).  At  point  D,  a  xi  particle 
decays  into  a  lambda  particle  and  a  pion.  The  invisible  neutral  lambda 
decays  into  a  proton  and  a  pion  at  point  E. 


173 


Until  1956  the  laws  of  physics  included  no  preference 
for  "right-handedness"  or  "left-handedness."    But  in 
1956  the  "law  of  parity"  failed  in  experiments  involving 
elementary  particles,  indicating  that  the  universe  is  in 
some  sense  asymmetric. 


17     The  Fall  of  Parity 

Martin  Gardner 

Chapter  from  his  book.  The  Ambidextrous  Universe, 
published  in  1964. 


As  far  as  anyone  knows  at  present,  all  events  that  take  place 
in  the  universe  are  governed  by  four  fundamental  types  of 
forces  (physicists  prefer  to  say  "interactions"  instead  of 
"forces,"  but  there  is  no  harm  in  using  here  the  more  common 
term) : 

1.  Nuclear  force. 

2.  Electromagnetic  force. 

3.  Weak  interaction  force. 

4.  Gravitational  force. 

The  forces  are  listed  in  decreasing  order  of  strength.  The 
strongest,  nuclear  force,  is  the  force  that  holds  together  the 
protons  and  neutrons  in  the  nucleus  of  an  atom.  It  is  often 
called  the  "binding  energy"  of  the  nucleus.  Electromagnetism  is 
the  force  that  binds  electrons  to  the  nucleus,  atoms  into  mole- 
cules, molecules  into  liquids  and  solids.  Gravity,  as  we  all  know, 
is  the  force  with  which  one  mass  attracts  another  mass;  it  is  the 
force  chiefly  responsible  for  binding  together  the  substances 
that  make  up  the  earth.  Gravitational  force  is  so  weak  that 
unless  a  mass  is  enormously  large  it  is  extremely  difficult  to 
measure.  On  the  level  of  the  elementary  particles  its  influence 
is  negligible. 


175 


The  remaining  force,  the  force  involved  in  "weak  inter- 
actions," is  the  force  about  which  the  least  is  known.  That  such 
a  force  must  exist  is  indicated  by  the  fact  that  in  certain  decay 
interactions  involving  particles  (such  as  beta-decay,  in  which 
electrons  or  positrons  are  shot  out  from  radioactive  nuclei), 
the  speed  of  the  reaction  is  much  slower  than  it  would  be  if 
either  nuclear  or  electromagnetic  forces  were  responsible.  By 
"slow"  is  meant  a  reaction  of,  say,  one  ten-billionth  of  a  second. 
To  a  nuclear  physicist  this  is  an  exceedingly  lazy  effect — about 
a  ten-trillionth  the  speed  of  reactions  in  which  nuclear  force 
is  involved.  To  explain  this  lethargy  it  has  been  necessary  to 
assume  a  force  weaker  than  electromagnetism  but  stronger  than 
the  extremely  weak  force  of  gravity. 

The  "theta  tau  puzzle,"  over  which  physicists  scratched  their 
heads  in  1956,  arose  in  connection  with  a  weak  interaction 
involving  a  "strange  particle"  called  the  K-meson.  (Strange 
particles  are  a  class  of  recently  discovered  particles  called 
"strange"  because  they  do  not  seem  to  fit  in  anywhere  with  any 
of  the  other  particles.)  There  appeared  to  be  two  distinct  types 
of  K-mesons.  One,  called  the  theta  meson,  decayed  into  two 
pi  mesons.  The  other,  called  the  tau  meson,  decayed  into  three 
pi  mesons.  Nevertheless,  the  two  types  of  K-mesons  seemed  to 
be  indistinguishable  from  each  other.  They  had  precisely  the 
same  mass,  same  charge,  same  lifetime.  Physicists  would  have 
liked  to  say  that  there  was  only  one  K-meson;  sometimes  it 
decayed  into  two,  sometimes  into  three  pi  mesons.  Why  didn't 
they?  Because  it  would  have  meant  that  parity  was  not  con- 
served. The  theta  meson  had  even  parity.  A  pi  meson  has  odd 
parity.  Two  pi  mesons  have  a  total  parity  that  is  even,  so  parity 
is  conserved  in  the  decay  of  the  theta  meson.  But  three  pi 
mesons  have  a  total  parity  that  is  odd. 

Physicists   faced   a  perplexing  dilemma   with   the   following 
horns: 

1.  They  could  assume  that  the  two  K-mesons,  even  though 
indistinguishable  in  properties,  were  really  two  different  par- 


176 


The  Fall  of  Parity 


tides:  the  theta  meson  with  even  parity,  the  tau  meson  with 
odd  parity. 

2.  They  could  assume  that  in  one  of  the  decay  reactions 
parity  was  not  conserved. 

To  most  physicists  in  1956  the  second  horn  was  almost  un- 
thinkable. As  we  saw  in  Chapter  20,  it  would  have  meant  admit- 
ting that  the  left-right  symmetry  of  nature  was  being  violated; 
that  nature  was  showing  a  bias  for  one  type  of  handedness.  The 
conservation  of  parity  had  been  well  established  in  all  "strong" 
interactions  (that  is,  in  the  nuclear  and  electromagnetic  inter- 
actions). It  had  been  a  fruitful  concept  in  quantum  mechanics 
for  thirty  years. 

In  April,   1956,  during  a  conference  on  nuclear  physics   at 
the  University  of  Rochester,  in  New  York,  there  was  a  spirited 
discussion  of  the  theta-tau  puzzle.  Richard  Phillips  Feynman,^  a 
physicist  at  the  California  Institute  of  Technology,  raised  the 
question:  Is  the  law  of  parity  sometimes  violated?    In  corre- 
sponding with  Feynman,  he  has  given  me  some  of  the  details 
behind  this  historic  question.  They  are  worth  putting  on  record. 
The  question  had  been  suggested  to  Feynman  the  night  before 
by  Martin  Block,  an  experimental  physicist  with  whom  Feynman 
was  sharing  a  hotel  room.   The  answer  to  the  theta-tau  puzzle, 
said  Block,  might  be  very  simple.    Perhaps  the  lovely  law  of 
parity  does  not  always  hold.  Feynman  responded  by  pointmg 
out  that  if  this  were  true,  there  would  be  a  way  to  distinguish 
left  from  right.    It  would  be  surprising,  Feynman  said,  but  he 
could  think  of  no  way  such  a  notion  conflicted  with  known 
experimental  results.    He  promised  Block  he  would  raise  the 
question  at  next  day  s  meeting  to  see  if  anyone  could  find  any- 
thing wrong  with  the  idea.   This  he  did,  prefacing  his  remarks 
with  "I  am  asking  this  question  for  Martin  Block."  He  regarded 
the  notion  as  such  an  interesting  one  that,  if  it  turned  out  to  be 
true,  he  wanted  Block  to  get  credit  for  it. 

Chen  Ning  Yang  and  his  friend  Tsung  Dao  Lee,  two  young 
and  brilliant  Chinese-born  physicists,  were  present  at  the  meet- 


177 


ing.    One  of  them  gave  a  lengthy  reply  to  Feynman's  question. 

"What  did  he  say?"  Block  asked  Feynman  later. 

"I  don't  know,"  replied  Feynman.  "I  couldn't  understand  it." 

"People  teased  me  later,"  writes  Feynman,  "and  said  my 
prefacing  remark  about  Martin  Block  was  made  because  I  was 
afraid  to  be  associated  with  such  a  wild  idea.  I  thought  the  idea 
unlikely,  but  possible,  and  a  very  exciting  possibility.  Some 
months  later  an  experimenter,  Norman  Ramsey,  asked  me  if  I 
believed  it  worth  while  for  him  to  do  an  experiment  to  test 
whether  parity  is  violated  in  beta  decay.  I  said  definitely  yes, 
for  although  I  felt  sure  that  parity  would  not  be  violated,  there 
was  a  possibility  it  would  be,  and  it  was  important  to  find  out. 
'Would  you  bet  a  hundred  dollars  against  a  dollar  that  parity  is 
not  violated?'  he  asked.  'No.  But  fifty  dollars  I  will.'  'That's  good 
enough  for  me.  I'll  take  your  bet  and  do  the  experiment.' 
Unfortunately,  Ramsey  didn't  find  time  to  do  it  then,  but  my 
fifty  dollar  check  may  have  compensated  him  slightly  for  a  lost 
opportunity." 

During  the  summer  of  1956  Lee  and  Yang  thought  some 
more  about  the  matter.  Early  in  May,  when  they  were  sitting 
in  the  White  Rose  Cafe  near  the  corner  of  Broadway  and  125th 
Street,  in  the  vicinity  of  Columbia  University,  it  suddenly  struck 
them  that  it  might  be  profitable  to  make  a  careful  study  of  all 
known  experiments  involving  weak  interactions.  For  several 
weeks  they  did  this.  To  their  astonishment  they  found  that 
although  the  evidence  for  conservation  of  parity  was  strong 
in  all  strong  interactions,  there  was  no  evidence  at  all  for  it  in 
the  weak.  Moreover,  they  thought  of  several  definitive  tests, 
involving  weak  interactions,  which  would  settle  the  question 
one  way  or  the  other.  The  outcome  of  this  work  was  their 
now-classic  paper  "Question  of  Parity  Conservation  in  Weak 
Interactions." 

"To  decide  unequivocally  whether  parity  is  conserved  in 
weak  interactions,"  they  declared,  "one  must  perform  an  experi- 
ment to  determine  whether  weak  interactions  differentiate  the 


178 


The  Fail  of  Parity 


right  from  the  left.    Some  such  possible  experiments  will  be 
discussed." 

Publication  of  this  paper  in  The  Physical  Review  (October  1, 
1956)  aroused  only  mild  interest  among  nuclear  pysicists.  It 
seemed  so  unlikely  that  parity  would  be  violated  that  most 
physicists  took  the  attitude:  Let  someone  else  make  the  tests. 
Freeman  J.  Dyson,  a  physicist  now  at  the  Institute  for  Advanced 
Study  in  Princeton,  writing  on  "Innovation  in  Physics"  {Scien- 
tific American,  September  1958)  had  these  honest  words  to 
say  about  what  he  called  the  "blindness"  of  most  of  his  col- 
leagues. 

"A  copy  of  it  [the  Lee  and  "Vang  paper]  was  sent  to  me  and 
I  read  it.  I  read  it  twice.  I  said.  This  is  very  interesting,'  or 
words  to  that  effect.  But  I  had  not  the  imagination  to  say,  'By 
golly,  if  this  is  true  it  opens  up  a  whole  new  branch  of  physics.' 
And  I  think  other  physicists,  with  very  few  exceptions,  at  that 
time  were  as  unimaginative  as  I." 

Several  physicists  were  prodded  into  action  by  the  suggestions 
of  Lee  and  Yang.  The  first  to  take  up  the  gauntlet  was  Madame 
Chien-Shiung  Wu,  a  professor  of  physics  at  Columbia  Uni- 
versity and  widely  regarded  as  the  world's  leading  woman 
physicist.  She  was  already  famous  for  her  work  on  weak  inter- 
actions and  for  the  care  and  elegance  with  which  her  experi- 
ments were  always  designed.  Like  her  friends  Yang  and  Lee, 
she,  too,  had  been  born  in  China  and  had  come  to  the  United 
States  to  continue  her  career. 

The  experiment  planned  by  Madame  Wu  involved  the  beta- 
decay  of  cobalt-60,  a  highly  radioactive  isotope  of  cobalt  which 
continually  emits  electrons.  In  the  Bohr  model  of  the  atom,  a 
nucleus  of  cobalt  60  may  be  thought  of  as  a  tiny  sphere  which 
spins  like  a  top  on  an  axis  labeled  north  and  south  at  the 
ends  to  indicate  the  magnetic  poles.  The  beta-particles  (elec- 
trons) emitted  in  the  weak  interaction  of  beta-decay  are  shot 
out  from  both  the  north  and  the  south  ends  of  nuclei.  Normally, 
the  nuclei  point  in  all  directions,  so  the  electrons  are  shot  out 


179 


in  all  directions.  But  when  cobalt-60  is  cooled  to  near  absolute 
zero  (  —  273  degrees  on  the  centigrade  scale ) ,  to  reduce  all  the 
joggling  of  its  molecules  caused  by  heat,  it  is  possible  to  apply 
a  powerful  electromagnetic  field  which  will  induce  more  than 
half  of  the  nuclei  to  line  up  with  their  north  ends  pointing  in 
the  same  direction.  The  nuclei  go  right  on  shooting  out  elec- 
trons. Instead  of  being  scattered  in  all  directions,  however,  the 
electrons  are  now  concentrated  in  two  directions:  the  direction 
toward  which  the  north  ends  of  the  magnetic  axes  are  pointing, 
and  the  direction  toward  which  the  south  ends  are  pointing.  If 
the  law  of  parity  is  not  violated,  there  will  be  just  as  many 
electrons  going  one  way  as  the  other. 

To  cool  the  cobalt  to  near  absolute  zero,  Madame  Wu  needed 
the  facilities  of  the  National  Bureau  of  Standards,  in  Washing- 
ton, D.  C.  It  was  there  that  she  and  her  colleagues  began  their 
historic  experiment.  If  the  number  of  electrons  divided  evenly 
into  two  sets,  those  that  shot  north  and  those  that  shot  south, 
parity  would  be  preserved.  The  theta-tau  puzzle  would  remain 
puzzling.  If  the  beta-decay  process  showed  a  handedness,  a 
larger  number  of  electrons  emitted  in  one  direction  than  the 
other,  parity  would  be  dead.  A  revolutionary  new  era  in 
quantum  theory  would  be  under  way. 

At  Zurich,  one  of  the  world's  greatest  theoretical  physicists, 
Wolfgang  Pauli,  eagerly  awaited  results  of  the  test.  In  a  now 
famous  letter  to  one  of  his  former  pupils,  Victor  Frederick 
Weisskopf  (then  at  the  Massachusetts  Institute  of  Technology), 
Pauli  wrote:  "I  do  not  believe  that  the  Lord  is  a  weak  left- 
hander, and  I  am  ready  to  bet  a  very  high  sum  that  the  experi- 
ments will  give  symmetric  results." 

Whether  Pauli  (who  died  in  1958)  actually  made  (like  Feyn- 
man)  such  a  bet  is  not  known.  If  he  did,  he  also  lost.  The 
electrons  in  Madam  Wu's  experiment  were  not  emitted  equally 
in  both  directions.  Most  of  them  were  flung  out  from  the 
south  end;  that  is,  the  end  toward  which  a  majority  of  the 
cobalt-60  nuclei  pointed  their  south  poles. 


180 


The  Fall  of  Parity 


At  the  risk  of  being  repetitious,  and  possibly  boring  readers 
who  see  at  once  the  full  implication  of  this  result,  let  us  pause 
to  make  sure  we  understand  exactly  why  Madam  Wu's  experi- 
ment is  so  revolutionary.   It  is  true  that  the  picture  (Figure  62) 


Figure  62.  An  electron  is  more  likely  to  be  flung  out  from 
the  south  end  of  a  cobaIt-60  nucleus  than  from  its  north  end. 

of  the  cobalt-60  nucleus,  spinning  in  a  certain  direction  around 
an  axis  labeled  N  and  5,  is  an  asymmetric  structure  not  super- 
posable  on  its  mirror  image.  But  this  is  just  a  picture.  As  we 
have  learned,  the  labeling  of  N  and  S  is  purely  conventional. 
There  is  nothing  to  prevent  one  from  switching  N  and  S  on  all 
the  magnetic  fields  in  the  universe.  The  north  ends  of  cobalt-60 
nuclei  would  become  south,  the  south  ends  north,  and  a  similar 
exchange  of  poles  would  occur  in  the  electromagnetic  field  used 
for  lining  up  the  nuclei.  Everything  prior  to  Madame  Wu's 
experiment  suggested  that  such  a  switch  of  poles  would  not 
make  a  measurable  change  in  the  experimental  situation.  If 
there  were  some  intrinsic,  observable  difference  between  poles — 
one  red  and  one  green,  or  one  strong  and  one  weak — then  the 
labeling  of  N  and  S  would  be  more  than  a  convention.     The 


181 


cobalt-60  nuclei  would  possess  true  spatial  asymmetry.  But 
physicists  knew  of  no  way  to  distinguish  between  the  poles 
except  by  testing  their  reaction  to  other  magnetic  axes.  In  fact, 
as  we  have  learned,  the  poles  do  not  really  exist.  They  are 
just  names  for  the  opposite  sides  of  a  spin. 

Madame  Wu's  experiment  provided  for  the  first  time  in  the 
history  of  science  a  method  of  labeling  the  ends  of  a  magnetic 
axis  in  a  way  that  is  not  at  all  conventional.  The  south  end 
is  the  end  of  a  cobalt-60  nucleus  that  is  most  likely  to  fling 
out  an  electron! 

The  nucleus  can  no  longer  be  thought  of  as  analogous  to  a 
spinning  sphere  or  cylinder.  It  must  now  be  thought  of  as 
analogous  to  a  spinning  cone.  Of  course,  this  is  no  more  than 
a  metaphor.  No  one  has  the  slightest  notion  at  the  moment  of 
why  or  how  one  end  of  the  axis  is  different,  in  any  intrinsic 
way,  from  the  other.  But  there  is  a  difference!  "We  are  no 
longer  trying  to  handle  screws  in  the  dark  with  heavy  gloves," 
was  the  way  Sheldon  Penman  of  the  University  of  Chicago 
put  it  {Scientific  American,  July  1961),  "we  are  being  handed 
the  screws  neatly  aligned  on  a  tray,  with  a  little  searchlight  on 
each  that  indicates  the  direction  of  its  head." 

It  should  be  obvious  now  that  here  at  long  last  is  a  solution 
to  the  Ozma  problem — an  experimental  method  of  extracting 
from  nature  an  unambiguous  definition  of  left  and  right.  We 
say  to  the  scientists  of  Planet  X:  "Cool  the  atoms  of  cobalt-60 
to  near  absolute  zero.  Line  up  their  nuclear  axes  with  a  powerful 
magnetic  field.  Count  the  number  of  electrons  flung  out  by  the 
two  ends  of  the  axes.  The  end  that  flings  out  the  most  electrons 
is  the  end  that  we  call  'south.'  It  is  now  possible  to  label  the 
ends  of  the  magnetic  axis  of  the  field  used  for  lining  up  the 
nuclei,  and  this  in  turn  can  be  used  for  labeling  the  ends  of  a 
magnetic  needle.  Put  such  a  needle  above  a  wire  in  which  the 
current  moves  away  from  you.  The  north  pole  of  this  needle 
will  point  in  the  direction  we  call  'left.' " 

We    have    communicated    precisely    and    unambiguously    to 


182 


The  Fall  of  Parity 


Planet  X  our  meaning  of  the  word  'left.'  Neither  we  nor  they 
will  be  observing  in  common  any  single,  particular  asymmetric 
structure.  We  will  be  observing  in  common  a  universal  law  of 
nature.  In  the  weak  interactions,  nature  herself,  by  her  own 
intrinsic  handedness,  has  provided  an  operational  definition  of 
left  and  right!  It  is  easy  to  understand  why  Pauli  and  other 
physicists  did  not  expect  Madame  Wu's  experiment  to  over- 
throw parity.  It  would  have  meant  that  nature  is  not  ambi- 
dextrous ! 

In  the  context  of  my  Esquire  tale  about  left  and  right,  the 
cobalt-60  experiment  provides  a  method  by  which  the  puzzled 
astronauts  could  tell  whether  they  were  reversed.  Of  course  they 
would  have  to  find  some  cobalt  on  the  unknown  planet,  convert 
it  to  its  radioactive  isotope  by  bombarding  it  with  neutrons, 
and  so  on.  But  assuming  that  they  had  the  equipment  and 
could  find  the  necessary  materials,  they  would  be  able  to  test 
their    handedness. 

Similarly,  Madame  Wu's  experiment  clearly  violates  the  as- 
sertion that  all  natural  events  can  be  photographed  on  motion 
picture  film  and  projected  in  reversed  form  without  the  viewer 
being  the  wiser. 

Exercise  16:  Explain  precisely  how  an  observation  of  all 
details  of  the  cobalt-60  experiment,  when  viewed  as  a  projected 
motion  picture,  would  enable  one  to  tell  whether  the  film  had 
been  reversed. 

Athough  evidence  against  the  conservation  of  parity  was 
strongly  indicated  by  Madame  Wu's  work  in  late  1956,  the 
experiment  was  not  finally  completed  until  early  in  January 
1957.  Results  were  formally  announced  by  Columbia  Univer- 
sity's distinguished  physicist  Isador  Rabi  on  January  15,  1957. 
The  announcement  also  included  the  results  of  a  confirming 
experiment  conducted  by  Columbia  physicists  at  the  Nevis 
Cyclotron  Laboratories  at  Irvington-on-Hudson  in  Westchester 


183 


County,  New  York.  This  confirming  test,  made  with  mu  mesons, 
showed  an  even  stronger  handedness.  The  mu  mesons  shot  out 
twice  as  many  electrons  in  one  direction  as  in  the  other. 
Independent  of  both  experiments,  a  third  test  was  made  at  the 
University  of  Chicago  using  the  decay  of  pi  and  mu  mesons. 
It,  too,  showed  violation  of  parity.  All  over  the  world  physicists 
began  testing  parity  in  other  weak  interactions.  By  1958  it 
was  apparent  that  parity  is  violated  in  all  such  interactions. 
The  theta-tau  puzzle  was  solved.  There  is  only  one  K-meson, 
Parity  is  not  conserved. 

"A  rather  complete  theoretical  structure  has  been  shattered  at 
the  base,"  declared  Rabi  (quoted  by  the  New  York  Times, 
January  16,  1957 ) ,  "and  we  are  not  sure  how  the  pieces  will  be 
put  together."  An  unnamed  physicist  was  reported  by  the  Times 
as  saying  that  nuclear  physics  had  been  battering  for  years  at 
a  closed  door  only  to  discover  suddenly  that  it  wasn't  a  door  at 
all — just  a  picture  of  a  door  painted  on  a  wall.  Now,  he  con- 
tinued, we  are  free  to  look  around  for  the  true  door.  0.  R. 
Frisch,  the  physicist  who  was  a  co-discoverer  of  nuclear  fission, 
reports  in  his  book  Atomic  Physics  Today  (Basic,  1961)  that 
on  January  16,  1957,  he  received  the  following  air  letter  from 
a  friend: 

Dear  Robert: 

HOT  NEWS.  Parity  is  not  conserved.  Here  in  Princeton  they  talk 
about  nothing  else;  they  say  it  is  the  most  important  result  since 
the  Michelson  experiment  .  .  . 

The  Michelson  experiment  was  the  famous  Michelson-Morley 
test  in  1887  which  established  the  constant  velocity  of  light 
regardless  of  the  motion  of  source  and  observer — a  historic  ex- 
periment which  paved  the  way  for  Einstein's  theory  of  relativity. 
Madame  Wu's  experiment  may  well  prove  to  be  equally  historic. 

The  two  tests  were  very  much  alike  in  their  shattering  element 
of  surprise.   Everybody  expected  Albert  Michelson  and  Edward 


184 


The  Fall  of  Parity 


Morley  to  detect  a  motion  of  the  earth  relative  to  a  fixed 
"ether."  It  was  the  negative  result  of  this  test  that  was  so 
upsetting.  Everybody  expected  Madame  Wu  to  find  a  left-right 
symmetry  in  the  process  of  beta-decay.  Nature  sprang  another 
surprise!  It  was  surprising  enough  that  certain  particles  had  a 
handedness;  it  was  more  surprising  that  handedness  seemed  to 
be  observable  only  in  weak  interactions.  Physicists  felt  a  shock 
even  greater  than  Mach  had  felt  when  he  first  encountered  the 
needle-and-wire  asymmetry. 

"Now  after  the  first  shock  is  over,"  Pauli  wrote  to  Weisskopf 
on  January  27,  after  the  staggering  news  had  reached  him,  "I 
begin  to  collect  myself.  Yes,  it  was  very  dramatic.  On  Monday, 
the  twenty-first,  at  8  p.m.  I  was  supposed  to  give  a  lecture  on 
the  neutrino  theory.  At  5  p.m.  I  received  three  experimental 
papers  | reports  on  the  first  three  tests  of  parity].  ...  I  am 
shocked  not  so  much  by  the  fact  that  the  Lord  prefers  the  left 
hand  as  by  the  fact  that  he  still  appears  to  be  left-handed  sym- 
metric when  he  expresses  himself  strongly.  In  short,  the  actual 
problem  now  seems  to  be  the  question:  Why  are  strong  inter- 
actions right-and-left  symmetric? 

The  Indian  physicist  Abdus  Salam  (from  whose  article  on 
"Elementary  Particles"  in  Endeavor,  April  1958,  the  extracts 
from  Pauli's  letters  are  taken)  tried  to  explain  to  a  liberal-arts- 
trained  friend  why  the  physicists  were  so  excited  about  the  fall 
of  parity.  "I  asked  him,"  wrote  Salam  in  this  article,  "if  any 
classical  writer  had  ever  considered  giants  with  only  the  left 
eye.  He  confessed  that  one-eyed  giants  have  been  described,  and 
he  supplied  me  with  a  full  list  of  them;  but  they  always 
sport  their  solitary  eye  in  the  middle  of  the  forehead.  In  my  view, 
what  we  have  found  is  that  space  is  a  weak  left-eyed  giant." 

Physicist  Jeremy  Bernstein,  in  an  article  on  "A  Question  of 
Parity"  which  appeared  in  The  New  Yorker,  May  12,  1962, 
reveals  an  ironic  sidelight  on  the  story  of  parity's  downfall.  In 
1928  three  physicists  at  New  York  University  had  actually  dis- 
covered a  parity  violation  in  the  decay  of  a  radioactive  isotope 
of   radium!     The   experiment   had   been  repeated   with   refined 


185 


techniques  in  1930.  "Not  only  in  every  run,"  the  experimenter 
reported,  "but  even  in  all  readings  in  every  run,  with  few 
exceptions,"  the  effect  was  observable.  But  this  was  at  a  time 
when,  as  Bernstein  puts  it,  there  was  no  theoretical  context  in 
which  to  place  these  results.  They  were  quickly  forgotten, 
"They  were,"  writes  Bernstein,  "a  kind  of  statement  made  in 
a  void.  It  took  almost  thirty  years  of  intensive  research  in  all 
branches  of  experimental  and  theoretical  physics,  and,  above 
all,  it  took  the  work  of  Lee  and  Yang,  to  enable  physicists  to 
appreciate  exactly  what  those  early  experiments  implied." 

In  1957  Lee  and  Yang  received  the  Nobel  prize  in  physics  for 
their  work.  Lee  was  then  30,  Yang  34.  The  choice  was  in- 
evitable. The  year  1957  had  been  the  most  stirring  in  modern 
particle  physics,  and  Lee  and  Yang  had  done  most  of  the 
stirring.  Today  the  two  men  have  adjacent  offices  at  the  Insti- 
tute for  Advanced  Study  in  Princeton,  where  they  continue  to 
collaborate.  Both  live  in  Princeton  with  their  attractive  wives 
and  children,  proud  of  their  Chinese  heritage,  deeply  committed 
to  science,  and  with  a  wide  range  of  interests  outside  of  physics 
and  mathematics.  If  you  are  curious  to  know  more  about  these 
two  remarkable  men,  look  up  Bernstein's  excellent  New  Yorker 
article. 

It  is  worth  pausing  to  note  that,  like  so  many  other  revolutions 
in  physics,  this  one  came  about  as  the  result  of  largely  abstract, 
theoretical,  mathematical  work.  Not  one  of  the  three  experi- 
ments that  first  toppled  parity  would  have  been  performed  at 
the  time  it  was  performed  if  Lee  and  Yang  had  not  told  the 
experimenters  what  to  do.  Lee  had  had  no  experience  whatever 
in  a  laboratory.  Yang  had  worked  briefly  in  a  lab  at  the  Uni- 
versity of  Chicago,  where  he  was  once  a  kind  of  assistant  to 
the  great  Italian  physicist  Enrico  Fermi.  He  had  not  been  happy 
in  experimental  work.  His  associates  had  even  made  up  a  short 
rhyme  about  him  which  Bernstein  repeats: 

Where  there's  a  bang, 
There's  Yang. 


186 


The  Fall  of  Parity 


Laboratory  bangs  can  range  all  the  way  from  an  exploding 
test  tube  to  the  explosion  of  a  hydrogen  bomb.  But  the  really 
Big  Bangs  are  the  bangs  that  occur  inside  the  heads  of 
theoretical  physicists  when  they  try  to  put  together  the  pieces 
handed  to  them  by  the  experimental  physicists. 

John  Campbell,  Jr.,  the  editor  of  Analog  Science  Fiction,  once 
speculated  in  an  editorial  that  perhaps  there  was  some  dif- 
ference in  the  intellectual  heritage  of  the  Western  and  Oriental 
worlds  which  had  predisposed  two  Chinese  physicists  to  question 
the  symmetry  of  natural  law.  It  is  an  interesting  thought. 
I  myself  pointed  out,  in  my  Mathematical  Games  column  in 
Scientific  American,  March  1958,  that  the  great  religious  symbol 
of  the  Orient  (it  appears  on  the  Korean  national  flag)  is  the 
circle  divided  asymmetrically  as  shown  in  Figure  63.   The  dark 


Figure  63.  The  asymmetric  Yin-Yang  symbol  of  the  Orient. 

and  light  areas  are  known  respectively  as  the  Yin  and  Yang. 
The  Yin  and  Yang  are  symbols  of  all  the  fundamental  dualities 
of  life:  good  and  evil,  beauty  and  ugliness,  truth  and  falsehood, 
male  and  female,  night  and  day,  sun  and  moon,  heaven  and 
earth,  pleasure  and  pain,  odd  and  even,  left  and  right,  positive 
and  negative  ...  the  list  is  endless.  This  dualism  was  first 
symbolized  in  China  by  the  odd  and  even  digits  that  alternate 
around  the  perimeter  of  the  Lo  shu,  the  ancient  Chinese  magic 
square  of  order  3.  Sometime  in  the  tenth  century  the  Lo  shu  was 
replaced  by  the  divided  circle,  which  soon  became  the  dominant 
Yin- Yang  symbol.  When  it  was  printed  or  drawn,  black  and 
white  was  used,  but  when  painted,  the  Yang  was  made  red 


187 


instead  of  white.  The  two  small  spots  were  (and  still  are) 
usually  added  to  symbolize  the  fact  that  on  each  side  of  any 
duality  there  is  always  a  bit  of  the  other  side.  Every  good  act 
contains  an  element  of  evil,  every  evil  act  an  element  of  good; 
every  ugliness  includes  some  beauty,  every  beauty  includes  some 
ugliness,  and  so  on.^  The  spots  remind  the  scientist  that  every 
"true"  theory  contains  an  element  of  falsehood.  "Nothing  is 
perfect,"  says  the  Philosopher  in  James  Stephens'  The  Crock  of 
Gold.   "There  are  lumps  in  it." 

Exercise  17:  There  is  a  three-dimensional  analog  of  the  Yin- 
Yang,  so  familiar  that  almost  everyone  has  at  one  time  held  a 
model  of  it  in  his  hands.  What  is  it?  Is  it  left-right  sym- 
metrical? 

The  history  of  science  can  be  described  as  a  continual,  per- 
haps never-ending,  discovery  of  new  lumps.  It  was  once  thought 
that  planets  moved  in  perfect  circles.  Even  Galileo,  although 
he  placed  the  sun  and  not  the  earth  at  the  center  of  the  solar 
system,  could  not  accept  Kepler's  view  that  the  planetary  orbits 
were  ellipses.  Eventually  it  became  clear  that  Kepler  had  been 
right:  the  orbits  are  almost  circles  but  not  quite.  Newton's 
theory  of  gravity  explained  why  the  orbits  were  perfect  ellipses. 
Then  slight  deviations  in  the  Newtonian  orbits  turned  up  and 
were  in  turn  explained  by  the  correction  factors  of  relativity 
theory  that  Einstein  introduced  into  the  Newtonian  equations. 
"The  real  trouble  with  this  world  of  ours,"  comments  Gilbert 
Chesterton  in  Orthodoxy,  "is  not  that  it  is  an  unreasonable 
world,  nor  even  that  it  is  a  reasonable  one.  The  commonest  kind 
of  trouble  is  that  it  is  nearly  reasonable,  but  not  quite.  ...  It 
looks  just  a  little  more  mathematical  and  regular  than  it  is;  its 
exactitude  is  obvious,  but  its  inexactitude  is  hidden;  its  wildness 
lies  in  wait." 

To  illustrate,  Chesterton  imagines  an  extraterrestrial  examin- 
ing a  human  body  for  the  first  time.    He  notes  that  the  right 


188 


The  Fall  of  Parity 


side  exactly  duplicates  the  left:  two  arms,  two  legs,  two  ears, 
two  eyes,  two  nostrils,  even  two  lobes  of  the  brain.  Probing 
deeper  he  finds  a  heart  on  the  left  side.  He  deduces  that  there 
is  another  heart  on  the  right.  Here  of  course,  he  encounters  a 
spot  of  Yin  within  the  Yang.  "It  is  this  silent  swerving  from 
accuracy  by  an  inch,"  Chesterton  continues,  "that  is  the  un- 
canny element  in  everything.  It  seems  a  sort  of  secret  treason 
in  the  universe.  .  .  .  Everywhere  in  things  there  is  this  element 
of  the  quiet  and  incalculable." 

Feynman,  with  no  less  reverence  than  Chesterton,  says  the 
same  thing  this  way  at  the  close  of  a  lecture  on  symmetry  in 
physical  laws  (Lecture  52  in  The  Feynman  Lectures  on  Physics, 
Addison- Wesley,  1963) : 

"Why  is  nature  so  nearly  symmetrical  ?  No  one  has  any  idea 
why.  The  only  thing  we  might  suggest  is  something  like  this: 
There  is  a  gate  in  Japan,  a  gate  in  Neiko,  which  is  sometimes 
called  by  the  Japanese  the  most  beautiful  gate  in  all  Japan;  it 
was  built  in  a  time  when  there  was  great  influence  from  Chinese 
art.  This  gate  is  very  elaborate,  with  lots  of  gables  and  beautiful 
carving  and  lots  of  columns  and  dragon  heads  and  princes  carved 
into  the  pillars,  and  so  on.  But  when  one  looks  closely  he  sees  that 
in  the  elaborate  and  complex  design  along  one  of  the  pillars, 
one  of  the  small  design  elements  is  carved  upside  down;  other- 
wise the  thing  is  completely  symmetrical.  If  one  asks  why  this 
is,  the  story  is  that  it  was  carved  upside  down  so  that  the  gods 
will  not  be  jealous  of  the  perfection  of  man.  So  they  purposely 
put  the  error  in  there,  so  that  the  gods  would  not  be  jealous  and 
get  angry  with  human  beings. 

"We  might  like  to  turn  the  idea  around  and  think  that  the 
true  explanation  of  the  near  symmetry  of  nature  is  this:  that 
God  made  the  laws  only  nearly  symmetrical  so  that  we  should 
not  be  jealous  of  His  perfection!" 

Note  that  the  Yin- Yang  symbol  is  asymmetrical.  It  is  not 
superposable  on  its  mirror  image.  The  Yin  and  Yang  are  con- 
gruent shapes,  each  asymmetrical,  each  with  the  same  handed- 


189 


ness.  By  contrast  the  Christian  symbol,  the  cross,  is  left-right 
symmetrical.  So  is  the  Jewish  six-pointed  Star  of  David,  unless 
it  is  shown  as  an  interlocking  pair  of  triangles  that  cross  alter- 
nately over  and  under  each  other.  It  is  a  pleasant  thought  that 
perhaps  the  familiar  asymmetry  of  the  oriental  symbol,  so  much 
a  part  of  Chinese  culture,  may  have  played  a  subtle,  unconscious 
role  in  making  it  a  bit  easier  for  Lee  and  Yang  to  go  against 
the  grain  of  scientific  orthodoxy;  to  propose  a  test  which 
their  more  symmetric-minded  Western  colleagues  had  thought 
scarcely  worth  the  effort. 

NOTES 

1.  For  the  benefit  of  readers  interested  in  recreational  mathematics, 
I  cannot  resist  adding  that  Feynman  is  one  of  the  codiscoverers  of  hexa- 
flexagons,  those  remarkable  paper-folded  structures  that  keep  changing 
their  faces  when  flexed.  (See  Chapter  1  of  my  Scientific  American  Book 
of  Mathematical  Puzzles  and  Diversions.)  AUhough  a  hexaflexagon  looks 
perfectly  symmetrical,  its  inner  structure  possesses  a  handedness;  that 
is,  any  given  flexagon  can  be  constructed  in  either  a  left  or  right- 
handed  way. 

In  1949  Feynman  had  suggested  that  perhaps  the  positron  is  an 
electron  moving  temporarily  backward  in  time  ("The  Theory  of  Posi- 
trons," Physical  Review,  Vol.  76,  1949,  pp.  749-759;  reprinted  in  Quan- 
tum Electrodynamics,  edited  by  Julius  Schwinger,  Dover,  1958).  This 
prompted  speculations  that  antiparticles  are  simply  particles  moving 
backward  in  time,  and  that  time  might  be  reversed  (relative  to  our  time) 
in  galaxies  of  antimatter.  (See  "The  Tiniest  Time  Traveler"  by  David 
Fox,  Astounding  Science  Fiction,  December  1952;  "Speculations  Con- 
cerning Precognition"  by  1.  J.  Good  in  his  anthology  of  "partly  baked 
ideas,"  The  Scientist  Speculates,  Basic,  1962,  pp.  151ff.) 

It  is  true  that  if  a  motion  picture  of  a  spinning  top  is  run  backward, 
the  picture  will  be  the  same  as  if  mirror  reversed,  but  there  are  strong 
technical  reasons  why  time  reversal  cannot  be  invoked  as  an  explanation 
of  parity  violation  in  weak  interactions.  Hans  Reichenbach,  in  his  book 
The  Direction  of  Time  (University  of  California  Press,  1956,  pp. 
262-269),  calls  Feynman's  positron  theory  "the  most  serious  blow  the 
concept  of  time  has  ever  received  in  physics."  Not  only  does  it  reverse 
the  direction  of  time  for  parts  of  the  world,  Reichenbach  points  out,  it 


190 


The  Fall  of  Parity 


also  destroys  the  uniform  topological  order  of  causal  chains.  Admirers  of 
Lewis  Carroll  need  not  be  reminded  of  the  Outlandish  Watch  (Sylvie 
and  Bruno,  Chapter  23)  with  its  "reversal-peg"  that  causes  time  to  flow 
backward. 

2.  For  these  facts  about  the  Yin-Yang  symbol  I  am  indebted  to 
Schuyler  Cammann's  excellent  article  on  "The  Magic  Square  of  Three 
in  Old  Chinese  Philosophy  and  Religion,"  History  of  Religions,  Vol. 
1,   No.   1,   Summer   1961,   pp.   37-80. 


191 


The  entertaining  and  theoretically  powerful  concept  of 
time  going  backward  creates  a  variety  of  paradoxes. 


18      Can  Time  Go  Backward? 


Martin  Gardner 


Scientific  American  article,  published  in  1967. 


". . .  time,  dark  time,  secret  time,  forever 

flowing  like  a  river " 

—Thomas  Wolfe, 
The  Weh  and  the  Rock 

Time  has  been  described  by  many 
metaphors,  but  none  is  older  or 
more  persistent  than  the  image  of 
time  as  a  river.  You  cannot  step  twice  in 
the  same  river,  said  Heraclitus,  the 
Greek  philosopher  who  stressed  the  tem- 
poral impermanence  of  all  things,  be- 
cause new  waters  forever  flow  around 
you.  You  cannot  even  step  into  it  once, 
added  his  pupil  Cratylus,  because  while 
you  step  both  you  and  the  river  are 
changing  into  something  different.  As 
Ogden  Nash  put  it  in  his  poem  'Time 
Marches  On," 

While  ladies  draw  their  stockings  on. 
The  ladies  they  were  are  up  and  gone. 


RIVER  IMAGE  appealed  to  ancient  Greek 
philosophers.  You  cannot  6t«p  twice  into 
the  same  river,  said  Heraclitus.  Indeed,  add- 
ed Cratylus,  yon  cannot  do  it  even  once. 


In  James  Joyce's  Finnegans  Wake  the 
great  symbol  of  time  is  the  river  Liffey 
flowing  through  Dubhn,  its  "hither-and- 
thithering  waters"  reaching  the  sea  in 
the  final  lines,  then  returning  to  "river- 
run,"  the  book's  first  word,  to  begin 
again  the  endless  cycle  of  change. 

It  is  a  powerful  symbol,  but  also  a  con- 
fusing one.  It  is  not  time  that  flows  but 
the  world.  "In  what  units  is  the  rate  of 
time's  flow  to  be  measured?"  asked  the 
Austrahan  philosopher  J.  J.  C.  Smart. 

"Seconds    per    -?"    To    say    "time 

moves"  is  Uke  saying  "length  extends." 
As  Austin  Dobson  observed  in  his  poem 
"The  Paradox  of  Time," 

Time  goes,  you  say?  Ah  no! 
Alas,  time  stays,  we  go. 

Moreover,  whereas  a  fish  can  sworn 
upriver  against  the  current,  we  are  pow- 
erless to  move  into  the  past.  The  chang- 
ing world  seems  more  like  the  magic 
grec»i  carpet  that  carried  Ozma  across 
the  Deadly  Desert  (the  void  of  nothing- 
ness?), unrolling  only  at  the  front,  coil- 
ing up  only  at  the  back,  while  she  jour- 
neyed from  Oz  to  Ev,  walking  always 
in  one  direction  on  the  carpet's  tiny 
green  region  of  "now."  Why  does  the 
magic  carpet  never  roll  backward?  What 
is  the  physical  basis  for  time's  strange, 
undeviating  asymmetry? 

T^here  has  been  as  little  agreement 
■*-  among  physicists  on  this  matter  as 
there  has  been  among  philosophers. 
Now,  as  the  result  of  recent  experi- 
ments, the  confusion  is  greater  than 
ever.  Before  1964  all  the  fundamental 
laws  of  physics,  including  relativity  and 
quantum  laws,  were  "time-reversible." 
That  is  to  say,  one  could  substitute  —t 
for  t  in  any  basic  law  and  the  law  would 
remain  as  applicable  to  the  world  as  be- 
fore; regardless  of  the  sign  in  front  of  t 


the  law  described  something  that  could 
occur  in  nature.  Yet  there  are  many 
events  that  are  possible  in  theory  but 
that  never  or  almost  never  actually  take 
place.  It  was  toward  those  events  that 
physicists  turned  their  attention  in  the 
hope  of  finding  an  ultimate  physical  ba- 
sis for  distinguishing  the  front  from  the 
back  of  "time's  arrow." 

A  star's  radiation,  for  example,  travels 
outward  in  all  directions.  The  reverse  is 
never  observed:  radiation  coming  from 
all  directions  and  converging  on  a  star 
with  backward-running  nuclear  reac- 
tions that  make  it  an  energy  sink  in- 
stead of  an  energy  source.  There  is  noth- 
ing in  the  basic  laws  to  make  such  a 
situation  impossible  in  principle;  there 
is  only  the  difficulty  of  imagining  how  it 
could  get  started.  One  would  have  to  as- 
sume that  God  or  the  gods,  in  some 
higher  continuum,  started  the  waves  at 
the  rim  of  the  universe.  The  emergence 
of  particles  from  a  disintegrating  radio- 
active nucleus  and  the  production  of 
ripples  when  a  stone  is  dropped  into  a 
quiet  lake  are  similar  instances  of  one- 
way events.  They  never  occur  in  reverse 
because  of  the  enormous  improbability 
that  "boundary  conditions"— conditions 
at  the  "rim"  of  things— would  be  such  as 
to  produce  the  required  kind  of  con- 
verging energy.  The  reverse  of  beta  de- 
cay, for  instance,  would  require  that  an 
electron,  a  proton  and  an  antineutrino 
be  shot  from  the  "rim"  with  such  deadly 
accuracy  of  aim  that  all  three  particles 
would  strike  the  same  nucleus  and  cre- 
ate a  neutron. 

The  steady  expansion  of  the  entire 
cosmos  is  another  example.  Here  again 
there  is  no  reason  why  this  could  not,  in 
principle,  go  the  other  way.  If  the  direc- 
tions of  all  the  receding  galaxies  were 
reversed,  the  red  shift  would  become  a 
blue  shift,  and  the  total  picture  would 
violate    no   known    physical    laws.    All 


193 


these  expanding  and  radiative  processes, 
although  always  one-way  as  far  as  our 
experience  goes,  fail  to  provide  a  funda- 
mental distinction  between  the  two  ends 
of  time's  arrow. 

Tt  has  been  suggested  by  many  philoso- 
phers,  and  even  by  some  physicists, 
that  it  is  only  in  human  consciousness, 
in  the  one-way  operation  of  our  minds, 
that  a  basis  for  time's  arrow  can  be 
found.  Their  arguments  have  not  been 
convincing.  After  all,  the  earth  had  a 
long  history  before  any  life  existed  on  it, 
and  there  is  every  reason  to  believe  that 
earthly  events  were  just  as  unidirection- 
al along  the  time  axis  then  as  they  are 
now.  Most  physicists  came  finally  to  the 
conclusion  that  all  natural  events  are 
time-reversible  in  principle  (this  became 
known  technically  as  "time  invariance") 
except  for  events  involving  the  statisti- 
cal behavior  of  large  numbers  of  inter- 
acting objects. 

Consider  what  happens  when  a  cue 
ball  breaks  a  triangle  of  15  balls  on  a 
pool  table.  The  balls  scatter  hither  and 
thither  and  the  8  ball,  say,  drops  into  a 
side  pocket.  Suppose  immediately  after 
this  event  the  motions  of  all  the  entities 
involved  are  reversed  in  direction  while 
keeping  the  same  velocities.  At  the  spot 
where  the  8  ball  came  to  rest  the  mole- 
cules that  carried  off  the  heat  and  shock 
of  impact  would  all  converge  on   the 


same  spot  to  create  a  small  explosion 
that  would  start  the  ball  back  up  the  in- 
cline. Along  the  way  the  molecules  that 
carried  ofiF  the  heat  of  friction  would 
move  toward  the  ball  and  boost  it  along 
its  upward  path.  The  other  balls  would 
be  set  in  motion  in  a  similar  fashion.  The 
8  ball  would  be  propelled  out  of  the  side 
pocket  and  the  balls  would  move  around 
the  table  until  they  finally  converged  to 
form  a  triangle.  There  would  be  no 
sound  of  impact  because  all  the  mole- 
cules that  had  been  involved  in  the 
shock  waves  produced  by  the  initial 
break  of  the  triangle  would  be  converg- 
ing on  the  balls  and  combining  with 
their  momentum  in  such  a  way  that  the 
impact  would  freeze  the  triangle  and 
shoot  the  cue  ball  back  toward  the  tip 
of  the  cue.  A  motion  picture  of  any  in- 
dividual molecule  in  this  event  would 
show  absolutely  nothing  unusual.  No 
basic  mechanical  law  would  seem  to  be 
violated.  But  when  the  billions  of  "hith- 
er-and-thithering"  molecules  involved  in 
the  total  picture  are  considered,  the 
probability  that  they  would  all  move  in 
the  way  required  for  the  time-reversed 
event  is  so  low  that  no  one  can  conceive 
of  its  happening. 

Because  gravity  is  a  one-way  force, 
always  attracting  and  never  repelhng,  it 
might  be  supposed  that  the  motions  of 
bodies  under  the  influence  of  gravity 
could  not  be  time-reversed  without  vio- 


lating basic  laws.  Such  is  not  the  case. 
Reverse  the  directions  of  the  planets 
and  they  would  swing  around  the  sun 
in  the  same  orbits.  What  about  the  colli- 
sions of  objects  drawn  together  by  gravi- 
ty—the fall  of  a  meteorite,  for  example? 
Surely  this  event  is  not  time-reversible. 
But  it  is!  When  a  large  meteorite  strikes 
the  earth,  there  is  an  explosion.  Billions 
of  molecules  scatter  hither  and  thither. 
Reverse  the  directions  of  all  those  mole- 
cules and  their  impact  at  one  spot  would 
provide  just  the  right  amount  of  energy 
to  send  the  meteorite  back  into  orbit.  No 
basic  laws  would  be  violated,  only  statis- 
tical laws. 

Tt  was  here,  in  the  laws  of  probabil- 
ity,  that  most  19th-century  physicists 
found  an  ultimate  basis  for  time's  arrow. 
Probability  explains  such  irreversible 
processes  as  the  mixing  of  coffee  and 
cream,  the  breaking  of  a  window  by  a 
stone  and  all  the  other  familiar  one-way- 
only  events  in  which  large  numbers  of 
molecules  are  involved.  It  explains  the 
second  law  of  thermodynamics,  which 
says  that  heat  always  moves  from  hot- 
ter to  cooler  regions,  increasing  the  en- 
tropy (a  measure  of  a  certain  kind  of  dis- 
order) of  the  system.  It  explains  why 
shuffling  randomizes  a  deck  of  ordered 
cards. 

"Without  any  mystic  appeal  to  con- 
sciousness," declared  Sir  Arthur  Edding- 


/♦      riMf- Rt  </f  (Vf»      C»*\t€.     fTAlf 


%l 

^""^L^ 

IX 

"    "~"~^      5p 

[ ^U 

LIVING  BACKWARD  in  a  time-forward  world  leads  to  all  kinde  time's  arrow  is  reversed  or  to  consider,  at  the  level  of  qaantnni 

of  difficulties.  It  is  possible,  however,  to  imagine  galaxies  in  which  theory,  that  some  particles  may  move  "the  wrong  way"  in  time. 


194 


Can  Time  Go  Backward? 


THREE  SYMMETRIES,  charge  (C),  parity  (P)  and  time  ^T),  are 
likened  to  pieces  that  fit  into  a  pattern.  Before  1957  they  were  all 
assumed  to  be  symmetrical;  any  experiment  (the  pattern)  involv- 
ing the  three  could  be  duplicated  with  any  one  piece,  any  two  or 
all  three  reversed  (left).  Then  experiments  were  found  that  violate 
P-symmetry,  suggesting  that  if  overall   (CPT)    symmetry  holds. 


some  piece  other  than  P  must  also  be  asymmetrical.  C  was  found  to 
be  such  a  piece;  an  experiment  remains  the  same  if  C  and  P  are 
reversed  together  {middle).  In  1961  experiments  that  violate  this 
CP-symmetry  were  reported.  It  follows  that  T  must  be  asymmetrical 
in  these  cases,  since  a  pattern  violating  CP-symmelry  can  be  dupli- 
cated only  by  reversing  all  three  pieces  simultaneously    (right). 


ton  (in  a  lecture  in  which  he  first  intro- 
duced the  phrase  "time's  arrow"),  "it  is 

possible  to  find  a  direction  of  time 

Let  us  draw  an  arrow  arbitrarily.  If  as 
we  follow  the  arrow  we  find  more  and 
more  of  the  random  element  in  the  state 
of  the  world,  then  the  arrow  is  pointing 
towards  the  future;  if  the  random  ele- 
ment decreases  the  arrow  points  towards 
the  past.  That  is  the  only  distinction 
known  to  physics." 

Eddington  knew,  of  course,  that  there 
are  radiative  processes,  such  as  beta 
decay  and  the  light  from  suns,  that  nev- 
er go  the  other  way,  but  he  did  not  con- 
sider them  sufficiently  fundamental  to 
provide  a  basis  for  time's  direction.  Giv- 
en the  initial  and  boundary  conditions 
necessary  for  starting  the  reverse  of  a 
radiative  process,  the  reverse  event  is 
certain  to  take  place.  Begin  with  a  deck 
of  disordered  cards,  however,  and  the 
probability  is  never  high  that  a  random 
shuffle  will  separate  them  into  spades, 
hearts,  clubs  and  diamonds.  Events  in- 
volving shuffling  processes  seem  to  be 
irreversible  in  a  stronger  sense  than  radi- 
ative events.  That  is  why  Eddington  and 
other  physicists  and  philosophers  argued 
that  statistical  laws  provide  the  most  fun- 
damental way  to  define  the  direction  of 
time. 

It  now  appears  that  there  is  a  basis 
for  time's  arrow  that  is  even  more  funda- 
mental than  statistical  laws.  In  1964  a 
group  of  Princeton  University  physicists 
discovered  that  certain  weak  interactions 
of  particles  are  apparently  not  time-re- 
versible [see  "Violations  of  Symmetry  in 
Physics,"  by  Eugene  P.  Wigner;  Scien- 
tific American,  December,  1965].  One 
says  "apparently"  because  the  evidence 
is  both  indirect  and  controversial.  Al- 
though it  is  possible  to  run  certain  par- 
ticle interactions  backward  to  make  a 
direct  test  of  time  symmetry,  such  ex- 
periments have  not  as  yet  shown  any  vi- 


olations of  time-reversibility.  The  Prince- 
ton tests  were  of  an  indirect  kind.  They 
imply,  if  certain  premises  are  granted, 
that  time  symmetry  is  violated. 

The  most  important  premise  is  known 
as  the  CPT  theorem.  C  stands  for  elec- 
tric charge  (plus  or  minus),  P  for  parity 
(left  or  right  mirror  images)  and  T  for 
time  (forward  or  backward).  Until  a  dec- 
ade ago  physicists  believed  each  of  these 
three  basic  symmetries  held  throughout 
nature.  If  you  reversed  the  charges  on 
the  particles  in  a  stone,  so  that  plus 
charges  became  minus  and  minus  charges 
became  plus,  you  would  still  have  a 
stone.  To  be  sure,  the  stone  would  be 
made  of  antimatter,  but  there  is  no  rea- 
son why  antimatter  cannot  exist.  An  anti- 
stone  on  the  earth  would  instantly  ex- 
plode (matter  and  antimatter  annihilate 
each  other  when  they  come  in  contact), 
but  physicists  could  imagine  a  galaxy  of 
antimatter  that  would  behave  exactly 
hke  oui-  own  galaxy;  indeed,  it  could  be 
in  all  respects  exactly  like  our  own  ex- 
cept for  its  C  (charge)  reversal. 

The  same  universal  symmetry  was  be- 
heved  to  hold  with  respect  to  P  (parity). 
If  you  reversed  the  parity  of  a  stone  or  a 
galaxy-that  is,  mirror-reflected  its  entire 
structure  down  to  the  last  wave  and  par- 
ticle-the  result  would  be  a  perfectly 
normal  stone  or  galaxy.  Then  in  1957 
C.  N.  Yang  and  T.  D.  Lee  received  the 
Nobel  prize  in  physics  for  theoretical 
work  that  led  to  the  discovery  that  pari- 
ty is  not  conserved  [see  "The  Overthrow 
of  Parity,"  by  Philip  Morrison;  Scien- 
tific American,  April,  1957].  There  are 
events  on  the  particle  level,  involving 
weak  interactions,  that  cannot  occur  in 
mirror-reflected  form. 


I 


t  was  an  unexpected  and  disturbing 
blow,  but  physicists  quickly  regained 
their  balance.  Experimental  evidence 
was  found  that  if  these  asymmetrical. 


parity-violating  events  were  reflected  in 
a  special  kind  of  imaginary  mirror  called 
the  CP  mirror,  symmetry  was  restored. 
If  in  addition  to  ordinary  mirror  reflec- 
tion there  is  also  a  charge  reversal,  the 
result  is  something  nature  can  "do."  Per- 
haps there  are  galaxies  of  antimatter 
that  are  also  mirror-reflected  matter.  In 
such  galaxies,  physicists  speculated,  sci- 
entists could  duplicate  every  particle  ex- 
periment that  can  be  performed  here.  If 
we  were  in  communication  with  scien- 
tists in  such  a  CP-reversed  galaxy,  there 
would  be  no  way  to  discover  whether 
they  were  in  a  world  like  ours  or  in  one 
that  was  CP-reflected.  (Of  course,  if  we 
went  there  and  our  spaceship  exploded 
on  anival,  we  would  know  we  had  en- 
tered a  region  of  antimatter.) 

No  sooner  had  physicists  relaxed  a  bit 
with  this  newly  restored  symmetry  than 
the  Princeton  physicists  found  some 
weak  interactions  in  which  CP  symme- 
try appears  to  be  violated.  In  different 
words,  they  found  some  events  that, 
when  CP-reversed,  are  (in  addition  to 
their  C  and  P  differences)  not  at  all  du- 
plicates of  each  other.  It  is  at  this  point 
that  time  indirectly  enters  the  pictiu-e, 
because  the  only  remaining  "magic  mir- 
ror" by  which  symmetry  can  be  restored 
is  the  combined  CPT  mirror  in  which  all 
three  symmetries-charge,  parity  and 
time— are  reversed.  This  CPT  mirror  is 
not  just  something  physicists  want  to 
preser\'e  because  they  love  symmetry.  It 
is  built  into  the  foundations  of  relativity 
theory  in  such  a  way  that,  if  it  turned 
out  not  to  be  true,  relativity  theoiy 
would  be  in  serious  trouble.  There  are 
therefore  strong  grounds  for  believing 
tlie  CPT  theorem  holds.  On  the  assump- 
tion that  it  does,  a  violation  of  CP  sym- 
metry would  imply  that  time  symmetry 
is  also  violated  [see  illustration  above]. 
There  are  a  few  ways  to  preserve  the 
CP  mirror  without  combining  it  with  T, 


195 


but  none  has  met  with  any  success.  The 
best  way  is  to  suppose  there  is  a  "fifth 
force"  (in  addition  to  the  four  known 
forces:  gravity,  the  weak-interaction 
force,  electromagnetism  and  the  nuclear 
force)  that  is  causing  the  newly  discov- 
ered anomalies.  Experiments  have  cast 
strong  doubt  on  the  fifth-force  hypothe- 
sis, however. 

Early  this  year  Paolo  Franzini  and 
his  wife,  working  with  the  alternating- 
gradient  synchrotron  at  the  Brookhaven 
National  Laboratory,  found  even  strong- 
er evidence  of  CP  violations— this  time 
in  events  involving  electromagnetic  re- 
actions. The  Franzini  work  was  contro- 
verted, however,  by  a  group  of  physicists 
at  the  European  Organization  for  Nu- 
clear Research  (CERN)  in  Geneva,  who 
announced  their  results  in  September. 
At  the  moment  the  cause  of  this  discrep- 
ancy in  results  is  not  clear. 

Although  the  evidence  is  still  indirect 
and  in  part  controversial,  many  physi- 
cists are  now  convinced  that  there  are 
events  at  the  particle  level  that  go  in 
only  one  time  direction.  If  this  holds 
throughout  the  universe,  there  is  now  a 
way  to  tell,  while  communicating  with 
scientists  in  a  distant  galaxy,  whether 
they  are  in  a  world  of  matter  or  of  anti- 
matter. We  simply  ask  them  to  perform 
one  of  the  CP-violating  experiments.  If 
their  description  of  such  a  test  coincides 
exactly  with  our  ovwi  description  of  the 
same  test  when  done  here,  we  shall  not 
explode  when  we  visit  them.  It  may  well 
be  that  the  universe  contains  no  galaxies 
of  antimatter.  But  physicists  like  to  bal- 
ance things,  and  if  there  is  as  much  anti- 
matter as  there  is  matter  in  the  universe, 
there  may  be  regions  of  the  cosmos  in 
which  all  three  symmetries  are  reversed. 
Events  in  our  world  that  are  lopsided 
with  respect  to  CPT  would  all  go  the 
other  way  in  a  CPr-reversed  galaxy.  Its 
matter  would  be  mirror-reflected,  re- 
versed in  charge  and  moving  backward 
in  time. 

"YJT/'hat  does  it  mean  to  say  that  events 
*'  in  a  galaxy  are  moving  backward  in 
time?  At  this  point  no  one  really  knows. 
The  new  experiments  indicate  that  there 
is  a  preferred  time  direction  for  certain 
particle  interactions.  Does  this  arrow 
have  any  connection  with  other  time 
arrows  such  as  those  that  are  defined 
by  radiative  processes,  entropy  laws  and 
the  psychological  time  of  living  orga- 
nisms? Do  all  these  arrows  have  to  point 
the  same  way  or  can  they  vary  inde- 
pendently in  their  directions? 

Before  the  recent  discoveries  of  the 
violation  of  T  invariance  the  most  popu- 
lar way  to  give  an  operational  meaning 


to  "backward  time"  was  by  imagining  a 
world  in  which  shuffling  processes  went 
backward,  from  disorder  to  order.  Lud- 
wig  Boltzmann,  the  19th-century  Aus- 
trian physicist  who  was  one  of  the 
founders  of  statistical  thermodynamics, 
realized  that  after  the  molecules'  of  a 
gas  in  a  closed,  isolated  container  haVe 
reached  a  state  of  thermal  equilibrium— 
that  is,  are  moving  in  complete  disorder 
with  maximum  entropy— there  will  al- 
ways be  little  pockets  forming  here  and 
there  where  entropy  is  momentarily  de- 
creasing. These  would  be  balanced  by 
other  regions  where  entropy  is  increas- 
ing; the  overall  entropy  remains  rela- 
tively stable,  with  only  minor  up-and- 
down  fluctuations. 

Boltzmann  imagined  a  cosmos  of  vast 
size,  perhaps  infinite  in  space  and  time, 
the  overall  entropy  of  which  is  at  a 
maximum  but  which  contains  pockets 
where  for  the  moment  entropy  is  de- 
creasing. (A  "pocket"  could  include  bil- 


V 


hons  of  galaxies  and  the  "moment"  could 
be  billions  of  years.)  Perhaps  our  fly- 
speck  portion  of  the  infinite  sea  of 
space-time  is  one  in  which  such  a  fluctu- 
ation has  occurred.  At  some  time  in  the 
past,  perhaps  at  the  time  of  the  "big 
bang,"  entropy  happened  to  decrease; 
now  it  is  increasing.  In  the  eternal  and 
infinite  flux  a  bit  of  order  happened  to 
put  in  its  appearance;  now  that  order  is 
disappearing  again,  and  so  our  arrow  of 
time  runs  in  the  familiar  direction  of  in- 
creasing entropy.  Are  there  other  re- 
gions of  space-time,  Boltzmann  asked,  in 
which  the  arrow  of  entropy  points  the 
other  way?  If  so,  would  it  be  correct  to 
say  that  time  in  such  a  region  was  mov- 
ing backward,  or  should  one  simply  say 
that  entropy  was  decreasing  as  the  re- 
gion continued  to  move  forward  in  time? 
It  seems  evident  today  that  one  can- 
not speak  of  backward  time  without 
meaning  considerably  more  than  just  a 
reversal  of  the  entropy  arrow.  One  has 


a  ALAxy    & 


^^ 


<^> 


tf :  oa 


TIME  IS  RELATIONAL,  not  absolute.  Observers  in  galaxies  with  opposite  time  directions 
each  suppose  the  other  to  be  moving  backward  in  time.  The  man  \n  A  sees  a  diner  in  B  eat- 
ing backward;  the  diner  in  B,  whose  time  is  reversed,  sees  the  man  in  A  eating  backward. 


196 


Can  Time  Go  Backward? 


r>^ 


/J" 


<:^ 


SHUFFLING  ordinarily  randomizes  a  pack  of  cards;  it  would  be  surprising  to  find  it  work- 
ing the  other  way.  Statistical  laws  therefore  provide  a  way  to  define  the  direction  of  time. 


to  include  all  the  other  one-way  proc- 
esses with  which  we  are  familiar,  such 
as  the  radiative  processes  and  the  newly 
discovered  CP-violating  interactions.  In 
a  world  that  was  completely  time- 
reversed  all  these  processes  would  go 
the  other  way.  Now,  however,  we  must 
guard  against  an  amusing  verbal  trap. 
If  we  imagine  a  cosmos  running  back- 
ward while  we  stand  off  somewhere  in 
space  to  observe  the  scene,  then  we 
must  be  observing  the  cosmos  moving 
backward  in  a  direction  opposite  to  our 
own  psychological  time,  which  still  runs 
forward.  What  does  it  mean  to  say  that 
the  entire  cosmos,  including  all  possible 
observers,  is  running  backward? 

In  the  first  book  of  Plato's  Statesman 
a  stranger  explains  to  Socrates  his  theory 
that  the  world  goes  through  vast  oscillat- 
ing cycles  of  time.  At  the  end  of  each 
cycle  time  stops,  reverses  and  then  goes 
the  other  way.  This  is  how  the  stranger 
describes  one  of  the  backward  cycles: 

"The  life  of  all  animals  first  came 
to  a  standstill,  and  the  mortal  nature 
ceased  to  be  or  look  older,  and  was  then 
reversed  and  grew  young  and  delicate; 
the  white  locks  of  the  aged  darkened 
again,  and  the  cheeks  of  the  bearded 
man  became  smooth,  and  recovered 
their  former  bloom;  the  bodies  of  youths 
in  their  prime  grew  softer  and  smaller, 
continually  by  day  and  night  returning 
and  becoming  assimilated  to  the  nature 
of  a  newly  bom  child  in  mind  as  well 
as  body;  in  the  succeeding  stage  they 
wasted  away  and  wholly  disappeared." 

Plato's  stranger  is  obviously  caught  in 
the  trap.  If  things  come  to  a  standstill 
in  time  and  "then"  reverse,  what  does 
the  word  "then"  mean?  It  has  meaning 
only  if  we  assume  a  more  fundamental 
kind  of  time  that  continues  to  move 
forward,  altogether  independent  of  how 


things  in  the  universe  move.  Relative  to 
this  meta-time— the  time  of  the  hypo- 
thetical observer  who  has  slipped  un- 
noticed into  the  picture— the  cosmos  is 
indeed  running  backward.  But  if  there 
is  no  meta-time— no  observer  who  can 
stand  outside  the  entire  cosmos  and 
watch  it  reverse— it  is  hard  to  under- 
stand what  sense  can  be  given  to  the 
statement  that  the  cosmos  "stops"  and 
"then"  starts  moving  backward. 

There  is  less  difficulty— indeed,  no 
logical  difficulty  at  all— in  imagining  two 
portions  of  the  universe,  say  two  galax- 
ies, in  which  time  goes  one  way  in  one 
galaxy  and  the  opposite  way  in  the  other. 
The  philosopher  Hans  Reichenbach,  in 
his  book  The  Direction  of  Time,  sug- 
gests that  this  could  be  the  case,  and 
that  intelligent  beings  in  each  galaxy 
would  regard  their  own  time  as  "for- 
ward" and  time  in  the  other  galaxy  as 
"backward."  The  two  galaxies  would  be 
like  two  mirror  images:  each  would  seem 
reversed  to  inhabitants  of  the  other  [see 
illustration  on  preceding  page].  From 
this  point  of  view  time  is  a  relational  con- 
cept like  up  and  down,  left  and  right  or 
big  and  small.  It  would  be  just  as  mean- 
ingless to  say  that  the  entire  cosmos  re- 
versed its  time  direction  as  it  would  be 
to  say  that  it  turned  upside  down  or  sud- 
denly became  its  own  mirror  image.  It 
would  be  meaningless  because  there  is 
no  absolute  or  fixed  time  arrow  outside 
the  cosmos  by  which  such  a  reversal 
could  be  measured.  It  is  only  when  part 
of  the  cosmos  is  time-reversed  in  rela- 
tion to  another  part  that  such  a  reversal 
acquires  meaning. 

Now,  however,  we  come  up  against  a 
significant  difference  between  mir- 
ror reflection  and  time  reversal.  It  is  easy 
to  observe  a  reversed  world— one  has 


only  to  look  into  a  mirror.  But  how  could 
an  observer  in  one  galaxy  "see"  another 
galaxy  that  was  time-reversed?  Light, 
instead  of  radiating  from  the  other  gal- 
axy, would  seem  to  be  going  toward  it. 
Each  galaxy  would  be  totally  invisible 
to  the  other.  Moreover,  the  memories 
of  observers  in  the  two  galaxies  would 
be  operating  in  opposite  directions.  If 
you  somehow  succeeded  in  communicat- 
ing something  to  someone  in  a  time- 
reversed  world,  he  would  promptly  for- 
get it  because  the  event  would  instantly 
become  part  of  his  future  rather  than  of 
his  past.  "It's  a  poor  sort  of  memory  that 
only  works  backward,"  said  Lewis  Car- 
roll's White  Queen  in  one  looking-glass, 
time-reversed  (PT-reversed!)  scene.  Un- 
fortunately, outside  of  Carroll's  dream 
world,  memory  works  only  one  way. 
Norbert  Wiener,  speculating  along  such 
lines  in  his  book  Cybernetics,  concluded 
that  no  communication  would  be  pos- 
sible between  intelligent  beings  in  re- 
gions with  opposite  time  directions. 

A  British  physicist,  F.  Russell  Stan- 
nard,  pursues  similar  lines  of  thought  in 
an  article  on  "Symmetry  of  the  Time 
Axis"  {Nature,  August  13,  1966)  and 
goes  even  further  than  Wiener.  He  con- 
cludes (and  not  all  physicists  agree  with 
him)  that  no  interactions  of  any  kind 
would  be  possible  between  particles  of 
matter  in  two  worlds  whose  time  axes 
pointed  in  opposite  directions.  If  the 
universe  maintains  an  overall  symmetry 
with  respect  to  time,  matter  of  opposite 
time  directions  would  "decouple"  and 
the  two  worlds  would  become  invisible 
to  each  other.  The  "other"  world  "would 
consist  of  galaxies  absorbing  their  fight 
rather  than  emitting  it,  living  organisms 
growing  younger,  neutrons  being  cre- 
ated in  triple  collisions  between  protons, 
electrons  and  antineutrinos,  and  there- 
after being  absorbed  in  nuclei,  etc.  It 
would  be  a  universe  that  was  in  a  state 
of  contraction,  and  its  entropy  would 
be  decreasing,  and  yet  the  faustian  ob- 
servers ["faustian"  is  Stannard's  term  for 
the  "other"  region]  would  not  be  aware 
of  anything  strange  in  their  environ- 
ment. Being  constructed  of  faustian 
matter,  their  subjective  experience  of 
time  is  reversed,  so  they  would  be  equal- 
ly convinced  that  it  was  they  who  grew 
older  and  their  entropy  that  increased." 

Instead  of  one  universe  with  oscillat- 
ing time  directions,  as  in  the  vision  of 
Plato's  stranger,  Stannard's  vision  bi- 
furcates the  cosmos  into  side-by-side 
regions,  each  unrolling  its  magic  carpet 
of  history  simultaneously  (whatever  "si- 
multaneously" can  mean!)  but  in  oppo- 
site directions.  Of  course,  there  is  no 
reason  why  the  cosmos  has  to  be  sym- 


197 


metrical  in  an  overall  way  just  to  satisfy 
the  physicist's  aesthetic  sense  of  bal- 
ance. The  universe  may  well  be  perma- 
nently lopsided  in  regard  to  all  three 
aspects— charge,  parity  and  time— even 
if  there  is  no  theoretical  reason  why  all 
three  could  not  go  the  other  way.  If  a 
painting  does  not  have  to  be  symmetri- 
cal to  be  beautiful,  why  should  the  uni- 
verse? 

Ts  it  possible  to  imagine  a  single  indi- 
vidual  living  backward  in  a  time- 
forward  world?  Plato's  younger  contem- 
porary, the  Greek  historian  Theopompus 
of  Chios,  wrote  about  a  certain  fruit 
that,  when  eaten,  would  start  a  person 
growing  younger.  This,  of  course,  is  not 
quite  the  same  thing  as  a  complete  re- 
versal of  the  person's  time.  There  have 
been  several  science-fiction  stories  about 
individuals  who  grew  backward  in  this 
way,  including  one  amusing  tale,  "The 
Curious  Case  of  Benjamin  Button,"  by 
(of  all  people)  F.  Scott  Fitzgerald.  (It 
first  appeared  in  Colliers  in  1922  and  is 
most  accessible  at  the  moment  in  Pause 
to  Wonder,  an  anthology  edited  by  Mar- 
jorie  Fischer  and  Rolfe  Humphries.) 
Benjamin  is  bom  in  1860,  a  70-year-old 
man  with  white  hair  and  a  long  beard. 
He  grows  backward  at  a  normal  rate, 
enters  kindergarten  at  65,  goes  through 
school  and  marries  at  50.  Thirty  years 
later,  at  the  age  of  20,  he  decides  to 
enter  Harvard,  and  he  is  graduated  in 


1914  when  he  is  16.  (I  am  giving  his 
biological  ages.)  The  Army  promotes 
him  to  brigadier  general  because  as  a 
biologically  older  man  he  had  been  a 
lieutenant  colonel  during  the  Spanish- 
American  War,  but  when  he  shows  up 
at  the  Army  base  as  a  small  boy  he  is 
packed  off  for  home.  He  grows  younger 
until  he  cannot  walk  or  talk.  "Then  it  was 
all  dark,"  reads  Fitzgerald's  last  sen- 
tence, "and  his  white  crib  and  the  dim 
faces  that  moved  above  him,  and  the 
warm  sweet  aroma  of  the  milk,  faded  out 
altogether  from  his  mind." 

Aside  from  his  backward  growth,  Mr. 
Button  lives  normally  in  forward-moving 
time.  A  better  description  of  a  situation 
in  which  the  time  arrows  of  a  person 
and  the  world  point  in  opposite  direc- 
tions is  found  in  Carroll's  novel  Sylvie 
and  Bruno  Concluded.  The  German 
Professor  hands  the  narrator  an  Out- 
landish Watch  with  a  "reversal  peg" 
that  causes  the  outside  world  to  run 
backward  for  four  hours.  There  is  an 
amusing  description  of  a  time-reversed 
dinner  at  which  "an  empty  fork  is  raised 
to  the  lips:  there  it  receives  a  neatly  cut 
piece  of  mutton,  and  swiftly  conveys  it 
to  the  plate,  where  it  instantly  attaches 
itself  to  the  mutton  already  there."  The 
scene  is  not  consistent,  however.  The  or- 
der of  the  dinner-table  remarks  is  back- 
ward, but  the  words  occur  in  a  forward 
time  direction. 

If  we  try  to  imagine  an  individual 


whose  entire  bodily  and  mental  proc- 
esses are  reversed,  we  run  into  the  worst 
kind  of  difiBculties.  For  one  thing,  he 
could  not  pass  through  his  previous  life 
experiences  backward,  because  those 
experiences  are  bound  up  with  his  ex- 
ternal world,  and  since  that  world  is  still 
moving  forward  none  of  his  past  experi- 
ences can  be  duplicated.  Would  we  see 
him  go  into  a  mad  death  dance,  like  an 
automaton  whose  motor  had  been  re- 
versed? Would  he,  from  his  point  of 
view,  find  himself  still  thinking  forward 
in  a  world  that  seemed  to  be  going 
backward?  If  so,  he  would  be  unable  to 
see  or  hear  anything  in  that  world,  be- 
cause all  sound  and  hght  waves  would 
be  moving  toward  their  points  of  origin. 
We  seem  to  encounter  nothing  but 
nonsense  when  we  try  to  apply  different 
time  arrows  to  an  individual  and  the 
world.  Is  it  perhaps  possible,  on  the 
microlevel  of  quantum  theory,  to  speak 
sensibly  about  part  of  the  universe 
moving  the  wrong  way  in  time?  It  is.  In 
1948  Richard  P.  Feynman,  who  shared 
the  1965  Nobel  prize  in  physics,  devel- 
oped a  mathematical  approach  to  quan- 
tum theory  in  which  an  antiparticle  is 
regarded  as  a  particle  moving  backward 
in  time  for  a  fraction  of  a  microsecond. 
When  there  is  pair-creation  of  an  elec- 
tron and  its  antiparticle  the  positron  (a 
positively  charged  electron),  the  posi- 
tron is  extremely  short-lived.  It  imme- 
diately collides  with  another  electron. 


S  i"^' 


S  f  «  «e 


FEYNMAN  GRAPH,  shown  at  the  left  in  a  simplified  form  devised 
by  Banesh  Hoffman  of  Queens  College,  shows  how  an  antiparti- 
cle can  be  considered  a  particle  moving  backward  in  time.  The 
graph  is  viewed  through  a  horizontal  slot  in  a  sheet  of  cardboard 
(color)  that  is  moved  slowly  up  across  the  graph.  Looking  through 
the  slot,  one  sees  events  as  they  appear  in  our  forward-looking 
"now."  Electron  A  moves  to  the  right  (i  ),  an  electron-positron  pair 
is  created  (2),  the  positron  and  electron  A  are  mutually  annihi- 


lated (3)  and  electron  B  continues  on  to  the  right  (4).  From  a 
timeless  point  of  view  (without  the  slotted  cardboard),  however,  it 
can  be  seen  that  there  is  only  one  particle:  an  electron  that  goes 
forward  in  time,  backward  and  then  forward  again.  Richard  P. 
Feynman's  approach  stemmed  from  a  whimsical  suggestion  by  John 
A.  Wheeler  of  Princeton  University:  a  single  particle,  tracing  a 
"world  line"  through  space  and  time  (right),  could  create  all 
the  world's  electrons  (black  dots)  and  positrons   (colored  dots). 


198 


Can  Time  Go  Backward? 


both  are  annihilated  and  off  goes  a 
gamma  ray.  Three  separate  particles- 
one  positron  and  two  electrons— seem  to 
be  involved.  In  Feynman's  theory  there 
is  only  one  particle,  the  electron  [see 
illustration  on  opposite  page].  What  we 
obsei-ve  as  a  positron  is  simply  the  elec- 
tron moving  momentarily  back  in  time. 
Because  our  time,  in  which  we  observe 
the  event,  runs  uniformly  forward,  we 
see  the  time-reversed  electron  as  a  posi- 
tron. We  think  the  positron  vanishes 
when  it  hits  another  electron,  but  this  is 
just  the  original  electron  resuming  its  for- 
ward time  direction.  The  electron  exe- 
cutes a  tiny  zigzag  dance  in  space-time, 
hopping  into  the  past  just  long  enough 
for  us  to  see  its  path  in  a  bubble  chamber 
and  interpret  it  as  a  path  made  by  a 
positron  moving  forward  in  time. 

Feynman  got  his  basic  idea  when  he 
was  a  graduate  student  at  Princeton, 
from  a  telephone  conversation  with  his 
physics  professor  John  A.  Wheeler.  In 
his  Nobel-prize  acceptance  speech 
Feynman  told  the  story  this  way: 

"Feynman,"  said  Wheeler,  "I  know 
why  all  electrons  have  the  same  charge 
and  the  same  mass." 

"Why?"  asked  Feynman. 

"Because,"  said  Wheeler,  "they  are  all 
the  same  electron!" 

Wheeler  went  on  to  explain  on  the 
telephone  the  stupendous  vision  that  had 
come  to  him.  In  relativity  theory  physi- 
cists use  what  are  called  Minkowski 
graphs  for  showing  the  movements  of 
objects  through  space-time.  The  path  of 
an  object  on  such  a  graph  is  called  its 
"world  Une."  Wheeler  imagined  one 
electron,  weaving  back  and  forth  in 
space-time,  tracing  out  a  single  world 
line.  The  world  line  would  form  an  in- 
credible knot,  like  a  monstrous  ball  of 
tangled  twine  with  billions  on  billions 
of  crossings,  the  "string"  filling  the  en- 
tire cosmos  in  one  blinding,  timeless  in- 
stant. If  we  take  a  cross  section  through 
cosmic  space-time,  cutting  at  right 
angles  to  the  time  axis,  we  get  a  picture 
of  three-space  at  one  instant  of  time. 
This  three-dimensional  cross  section 
moves  forward  along  the  time  axis,  and 
it  is  on  this  moving  section  of  "now" 
that  the  events  of  the  world  execute 
their  dance.  On  this  cross  section  the 
world  line  of  the  electron,  the  incredible 
knot,  would  be  broken  up  into  billions 
on  billions  of  dancing  points,  each  cor- 
responding to  a  spot  where  the  electron 
knot  was  cut.  If  the  cross  section  cuts  the 
world  line  at  a  spot  where  the  particle  is 
moving  forward  in  time,  the  spot  is  an 
electron.  If  it  cuts  the  world  line  at  a 
spot  where  the  particle  is  moving  back- 
ward in  time,  the  spot  is  a  positron.  All 


f , 


1 1  ^ 

CP-REVERSED  GALAXY  (where  charge  is  reversed  and  matter  mirror-reflected)  woald  be 
indistinguishable  as  such  from  the  earth.  But  explorers  from  the  earth  would  soon  find  out. 


the  electrons  and  positrons  in  the  cosmos 
are,  in  Wheeler's  fantastic  vision,  cross 
sections  of  the  knotted  path  of  this  single 
particle.  Since  they  are  all  sections  of 
the  same  world  line,  naturally  they  will 
all  have  identical  masses  and  strengths 
of  charge.  Their  positive  and  negative 
charges  are  no  more  than  indications  of 
the  time  direction  in  which  the  parti- 
cle at  that  instant  was  weaving  its  way 
through  space-time. 

There  is  an  enormous  catch  to  all  of 
this.  The  number  of  electrons  and  posi- 
trons in  the  universe  would  have  to  be 
equal.  You  can  see  this  by  drawing  on 
a  sheet  of  paper  a  two-dimensional 
analogue  of  Wheeler's  vision.  Simply 
trace  a  single  line  over  the  page  to  make 
a  tangled  knot  [see  illustration  on  oppo- 
site page].  Draw  a  straight  line  through 
it.  The  straight  line  represents  a  one- 
dimensional  cross  section  at  one  instant 
in  time  through  a  two-space  world  (one 
space  axis  and  one  time  axis).  At  points 
where  the  knot  crosses  the  straight  line, 
moving  up  in  the  direction  of  time's 
arrow,  it  produces  an  electron.  Where 
it  crosses  the  line  going  the  opposite 
way  it  produces  a  positron.  It  is  easy  to 
see  that  the  number  of  electrons  and 
positrons  must  be  equal  or  have  at  most 
a  difference  of  one.  That  is  why,  when 


Wheeler  had  described  his  vision,  Feyn- 
man immediately  said: 

"But,  Professor,  there  aren't  as  many 
positrons  as  electrons." 

"Well,"  countered  Wheeler,  "maybe 
they  are  hidden  in  the  protons  or  some- 
thing." 

Wheeler  was  not  proposing  a  serious 
theory,  but  the  suggestion  that  a  posi- 
tron could  be  interpreted  as  an  electron 
moving  temporarily  backward  in  time 
caught  Feynman's  fancy,  and  he  found 
that  the  interpretation  could  be  handled 
mathematically  in  a  way  that  was  en- 
tirely consistent  with  logic  and  all  the 
laws  of  quantum  theory.  It  became  a 
cornerstone  in  his  famous  "space-time 
view"  of  quantum  mechanics,  which  he 
completed  eight  years  later  and  for 
which  he  shared  his  Nobel  prize.  The 
theory  is  equivalent  to  traditional  views, 
but  the  zigzag  dance  of  Feynman's  par- 
ticles provided  a  new  way  of  handling 
certain  calculations  and  greatly  simph- 
fying  them.  Does  this  mean  that  the 
positron  is  "really"  an  electron  moving 
backward  in  time?  No,  that  is  only  one 
physical  interpretation  of  the  "Feynman 
graphs";  other  interpretations,  just  as 
valid,  do  not  speak  of  time  reversals. 
With  the  new  experiments  suggesting  a 
mysterious  interlocking  of  charge,  parity 


TIME-REVERSED  INHABITANTS  of  a  time-reversed  world  are  not  aware  of  anything 
strange  in  the  environment  because  their  own  subjective  experience  of  time  ie  reversed. 


199 


and  time  direction,  however,  the  zigzag 
dance  of  Feynman's  electron,  as  it  traces 
its  world  line  through  space-time,  no 
longer  seems  as  bizarre  a  physical  inter- 
pretation as  it  once  did. 

At  the  moment  no  one  can  predict 
-'*-  what  will  finally  come  of  the  new 
evidence  that  a  time  arrow  may  be  built 
into  some  of  the  most  elementary  parti- 
cle interactions.  Physicists  are  taking 
more  interest  than  ever  before  in  what 
philosophers  have  said  about  time, 
thinking  harder  than  ever  before  about 
what  it  means  to  say  time  has  a  "direc- 
tion" and  what  connection,  if  any,  this 
all  has  with  human  consciousness  and 
will.  Is  history  like  a  vast  "riverrun"  that 
can  be  seen  by  God  or  the  gods  from 
source  to  mouth,  or  from  an  infinite  past 
to  an  infinite  future,  in  one  timeless  and 


eternal  glance:*  Is  freedom  of  will  no 
more  than  an  illusion  as  the  current  of 
existence  propels  us  into  a  future  that 
in  some  unknown  sense  already  ex- 
ists? To  vary  the  metaphor,  is  history  a 
prerecorded  motion  picture,  projected 
on  the  four-dimensional  screen  of  our 
space-time  for  the  amusement  or  edifica- 
tion of  some  unimaginable  Audience? 

Or  is  the  future,  as  WilUam  James 
and  others  have  so  passionately  argued, 
open  and  undetermined,  not  existing 
in  any  sense  until  it  actually  happens? 
Does  the  future  bring  genuine  novelty- 
surprises  that  even  the  gods  are  unable 
to  anticipate?  Such  questions  go  far 
beyond  the  reach  of  physics  and  probe 
aspects  of  existence  that  we  are  as  little 
capable  of  comprehending  as  the  fish 
in  the  river  Liffey  are  of  comprehend- 
ing the  city  of  Dublin. 


200 


When  the  first  atomic  bomb  was  nearly  finished  in  the 
war-time  laboratories,  and  before  it  was  used,  a  group 
of  physicists  involved  pleaded  that  the  bomb  should  not 
be  first  dropped  on  a  civilian  target. 


19       A  Report  to  the  Secretary  of  War 


James  Franck,  Donald  J.  Hughes,  J.  I.  Nickson,  Eugene  Rabinowitch, 
Glenn  T.  Seaborg,  Joyce  C.  Stearns,  Leo  Szilard. 

June  1945. 


I.  Preamble 

The  only  reason  to  treat  nuclear  power  differently  from  all  the 
other  developments  in  the  field  of  physics  is  the  possibility  of  its  use  as  a 
means  of  poHtical  pressure  in  peace  and  sudden  destruction  in  war.  All 
present  plans  for  the  organization  of  research,  scientific  and  industrial 
development,  and  publication  in  the  field  of  nucleonics  are  conditioned 
by  the  political  and  military  climate  in  which  one  expects  those  plans  to 
be  carried  out.  Therefore,  in  making  suggestions  for  the  postwar  organiza- 
tion of  nucleonics,  a  discussion  of  political  problems  cannot  be  avoided. 
The  scientists  on  this  project  do  not  presume  to  speak  authoritatively  on 
problems  of  national  and  international  policy.  However,  we  found  our- 
selves, by  the  force  of  events  during  the  last  five  years,  in  the  position  of  a 
small  group  of  citizens  cognizant  of  a  grave  danger  for  the  safety  of  this 
country  as  well  as  for  the  future  of  all  the  other  nations,  of  which  the  rest 
of  mankind  is  unaware.  We  therefore  feel  it  our  duty  to  urge  that  the 
political  problems  arising  from  the  mastering  of  nuclear  power  be  recog- 
nized in  all  their  gravity,  and  that  appropriate  steps  be  taken  for  their 
study  and  the  preparation  of  necessary  decisions.  We  hope  that  the  crea- 
tion of  the  committee  by  the  Secretary  of  War  to  deal  with  all  aspects  of 
nucleonics  indicates  that  these  implications  have  been  recognized  by  the 
government.  We  believe  that  our  acquaintance  with  the  scientific  elements 
of  the  situation  and  prolonged  preoccupation  with  its  worldwide  political 
impHcations,  imposes  on  us  the  obligation  to  offer  to  the  committee  some 
suggestions  as  to  the  possible  solution  of  these  grave  problems. 

Scientists  have  often  before  been  accused  of  providing  new  weapons  for 
the  mutual  destruction  of  nations  instead  of  improving  their  well-being. 
It  is  undoubtedly  true  that  the  discovery  of  flying,  for  example,  has  SD  far 
brought  much  more  misery  than  enjoyment  and  profit  to  humanity.  How- 
ever, in  the  past  scientists  could  disclaim  direct  responsibility  for  the  use  to 
which  mankind  had  put  their  disinterested  discoveries.  We  feel  compelled  201 


to  take  a  more  active  stand  now  because  the  success  which  we  have 
achieved  in  the  development  of  nuclear  power  is  fraught  with  infinitely 
greater  dangers  than  were  all  the  inventions  of  the  past.  All  of  us  fa- 
miliar with  the  present  state  of  nucleonics  Hve  with  the  vision  before  our 
eyes  of  sudden  destruction  visited  on  our  own  country,  of  a  Pearl  Harbor 
disaster  repeated  in  thousand-fold  magnification  in  every  one  of  our 
major  cities. 

In  the  past,  science  has  often  been  able  to  also  provide  new  methods  of 
protection  against  new  weapons  of  aggression  it  made  possible,  but  it  can- 
not promise  such  efiicient  protection  against  the  destructive  use  of  nuclear 
power.  This  protection  can  come  only  from  the  poHtical  organization  of  the 
world.  Among  all  the  arguments  calling  for  an  efficient  international  or- 
ganization for  peace,  the  existence  of  nuclear  weapons  is  the  most  com- 
pelling one.  In  the  absence  of  an  international  authority  which  would  make 
all  resort  to  force  in  international  conflicts  impossible,  nations  could  still  be 
diverted  from  a  path  which  must  lead  to  total  mutual  destruction  by  a 
specific  international  agreement  barring  a  nuclear  armaments  race. 

II.  Prospects  of  Armaments  Race 

It  could  be  suggested  that  the  danger  of  destruction  by  nuclear  weapons 
can  be  avoided — at  least  as  far  as  this  country  is  concerned — either  by 
keeping  our  discoveries  secret  for  an  indefinite  time,  or  else  by  developing 
our  nuclear  armaments  at  such  a  pace  that  no  other  nation  would  think  of 
attacking  us  from  fear  of  overwhelming  retaliation. 

The  answer  to  the  first  suggestion  is  that  although  we  undoubtedly  are 
at  present  ahead  of  the  rest  of  the  world  in  this  field,  the  fundamental  facts 
of  nuclear  power  are  a  subject  of  common  knowledge.  British  scientists 
know  as  much  as  we  do  about  the  basic  wartime  progress  of  nucleonics — 
if  not  of  the  specific  processes  used  in  our  engineering  developments — 
and  the  role  which  French  nuclear  physicists  have  played  in  the  pre-war 
development  of  this  field,  plus  their  occasional  contact  with  our  projects, 
will  enable  them  to  catch  up  rapidly,  at  least  as  far  as  basic  scientific 
discoveries  are  concerned.  German  scientists,  in  whose  discoveries  the 
whole  development  of  this  field  originated,  apparently  did  not  develop  it 
during  the  war  to  the  same  extent  to  which  tliis  has  been  done  in  America, 
but  to  the  last  day  of  the  European  war  we  were  living  in  constant  ap- 
prehension as  to  their  possible  achievements.  The  certainty  that  German 
scientists  were  working  on  this  weapon  and  that  their  government  would 
certainly  have  no  scruples  against  using  it  when  available  was  the  main 
motivation  of  the  initiative  which  American  scientists  took  in  urging  the 
development  of  nuclear  power  for  military  purposes  on  a  large  scale  in 
this  country.  In  Russia,  too,  the  basic  facts  and  imphcations  of  nuclear 
power  were  well  understood  in  1940,  and  the  experience  of  Russian  scientists 
in  nuclear  research  is  entirely  suflBcient  to  enable  them  to  retrace  our  steps 


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within  a  few  years,  even  if  we  should  make  every  attempt  to  conceal 
them.  Even  if  we  can  retain  our  leadership  in  basic  knowledge  of  nucleonics 
for  a  certain  time  by  maintaining  secrecy  as  to  all  results  achieved  on  this 
and  associated  projects,  it  would  be  foolish  to  hope  that  this  can  protect  us 
for  more  than  a  few  years. 

It  may  be  asked  whether  we  cannot  prevent  the  development  of 
military  nucleonics  in  other  countries  by  a  monopoly  on  the  raw  materials 
of  nuclear  power.  The  answer  is  that  even  though  the  largest  now  known 
deposits  of  uranium  ores  are  under  the  control  of  powers  which  belong  to 
the  "western"  group  (Canada,  Belgium  and  British  India),  the  old  de- 
posits in  Czechoslovakia  are  outside  this  sphere.  Russia  is  known  to  be 
mining  radium  on  its  own  territory,  and  even  if  we  do  not  know  the  size  of 
the  deposits  discovered  so  far  in  the  USSR,  the  probability  that  no  large 
reserves  of  uranium  will  be  found  in  a  country  which  covers  one-fifth  of  the 
land  area  of  the  earth  (and  whose  sphere  of  influence  takes  in  additional 
territory),  is  too  small  to  serve  as  a  basis  for  security.  Thus,  we  cannot 
hope  to  avoid  a  nuclear  armament  race  either  by  keeping  secret  from  the 
competing  nations  the  basic  scientific  facts  of  nuclear  power  or  by  corner- 
ing the  raw  materials  required  for  such  a  race. 

We  now  consider  the  second  of  the  two  suggestions  made  at  the  begin- 
ning of  this  section,  and  ask  whether  we  could  not  feel  ourselves  safe  in  a 
race  of  nuclear  armaments  by  virtue  of  our  greater  industrial  potential, 
including  greater  diffusion  of  scientific  and  technical  knowledge,  greater 
volume  and  eflBciency  of  our  skilled  labor  corps,  and  greater  experience  of 
our  management — all  the  factors  whose  importance  has  been  so  strikingly 
demonstrated  in  the  conversion  of  this  country  into  an  arsenal  of  the 
allied  nations  in  the  present  war.  The  answer  is  that  all  that  these  ad- 
vantages can  give  us  is  the  accumulation  of  a  larger  number  of  bigger  and 
better  atomic  bombs. 

However,  such  a  quantitative  advantage  in  reserves  of  bottled  destruc- 
tive power  will  not  make  us  safe  from  sudden  attack.  Just  because  a 
potential  enemy  will  be  afraid  of  being  "outnumbered  and  outgunned," 
the  temptation  for  him  may  be  overwhelming  to  attempt  a  sudden  unpro- 
voked blow— particularly  if  he  should  suspect  us  of  harboring  aggressive 
intentions  against  his  security  or  his  sphere  of  influence.  In  no  other  type 
of  warfare  does  the  advantage  lie  so  heavily  with  the  aggressor.  He  can 
place  his  "infernal  machines"  in  advance  in  all  our  major  cities  and  explode 
them  simultaneously,  thus  destroying  a  major  part  of  our  industry  and  a 
large  part  of  our  population  aggregated  in  densely  populated  metropolitan 
districts.  Our  possibilities  of  retaliation— even  if  retaliation  should  be  con- 
sidered adequate  compensation  for  the  loss  of  millions  of  lives  and  de- 
struction of  our  largest  cities— will  be  greatly  handicapped  because  we 
must  rely  on  aerial  transportation  of  the  bombs,  and  also  because  we  may 
have  to  deal  with  an  enemy  whose  industry  and  population  are  dispersed 
over  a  large  territory. 


203 


In  fact,  if  the  race  for  nuclear  armaments  is  allowed  to  develop,  the  only 
apparent  way  in  which  our  country  can  be  protected  from  the  paralyzing 
eflFects  of  a  sudden  attack  is  by  dispersal  of  those  industries  which  are 
essential  for  our  war  eflForts  and  dispersal  of  the  populations  of  our  major 
metropoHtan  cities.  As  long  as  nuclear  bombs  remain  scarce  (i.e.,  as  long 
as  uranium  remains  the  only  basic  material  for  their  fabrication),  efficient 
dispersal  of  our  industry  and  the  scattering  of  our  metropolitan  population 
will  considerably  decrease  the  temptation  to  attack  us  by  nuclear  weapons. 

At  present,  it  may  be  that  atomic  bombs  can  be  detonated  with  an  efiFect 
equal  to  that  of  20,000  tons  of  TNT.  One  of  these  bombs  could  then  destroy 
something  like  three  square  miles  of  an  urban  area.  Atomic  bombs  con- 
taining a  larger  quantity  of  active  material  but  still  weighing  less  than  one 
ton  may  be  expected  to  be  available  within  ten  years  which  could  destroy 
over  ten  square  miles  of  a  city.  A  nation  able  to  assign  ten  tons  of  atomic 
explosives  for  a  sneak  attack  on  this  country  can  then  hope  to  achieve  the 
destruction  of  all  industry  and  most  of  the  population  in  an  area  from  500 
square  miles  upwards.  If  no  choice  of  targets,  with  a  total  area  of  500 
square  miles  of  American  territory,  contains  a  large  enough  fraction  of  the 
nation's  industry  and  population  to  make  their  destruction  a  crippling 
blow  to  the  nation's  war  potential  and  its  ability  to  defend  itself,  then  the 
attack  will  not  pay  and  may  not  be  undertaken.  At  present,  one  could 
easily  select  in  this  country  a  hundred  areas  of  five  square  miles  each 
whose  simultaneous  destruction  would  be  a  staggering  blow  to  the  nation. 
Since  the  area  of  the  United  States  is  about  three  million  square  miles,  it 
should  be  possible  to  scatter  its  industrial  and  human  resources  in  such  a 
way  as  to  leave  no  500  square  miles  important  enough  to  serve  as  a  target 
for  nuclear  attack. 

We  are  fully  aware  of  the  staggering  difiiculties  involved  in  such  a 
radical  change  in  the  social  and  economic  structure  of  our  nation.  We  felt, 
however,  that  the  dilemma  had  to  be  stated,  to  show  what  kind  of  alterna- 
tive methods  of  protection  will  have  to  be  considered  if  no  successful  in- 
ternational agreement  is  reached.  It  must  be  pointed  out  that  in  this  field 
we  are  in  a  less  favorable  position  than  nations  which  are  either  now  more 
diflFusely  populated  and  whose  industries  are  more  scattered,  or  whose 
governments  have  unlimited  power  over  the  movement  of  population  and 
the  location  of  industrial  plants. 

If  no  efficient  international  agreement  is  achieved,  the  race  for  nuclear 
armaments  will  be  on  in  earnest  not  later  than  the  morning  after  our  first 
demonstration  of  the  existence  of  nuclear  weapons.  After  this,  it  might  take 
other  nations  three  or  four  years  to  overcome  our  present  head  start,  and 
eight  or  ten  years  to  draw  even  with  us  if  we  continue  to  do  intensive 
work  in  this  field.  This  might  be  all  the  time  we  would  have  to  bring  about 
the  relocation  of  our  population  and  industry.  Obviously,  no  time  should  be 
lost  in  inaugurating  a  study  of  this  problem  by  ex-perts. 


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III.  Prospects  of  Agreement 

The  consequences  of  nuclear  warfare,  and  the  type  of  measures  which 
would  have  to  be  taken  to  protect  a  country  from  total  destruction  by 
nuclear  bombing  must  be  as  abhorrent  to  other  nations  as  to  the  United 
States.  England,  France,  and  the  smaller  nations  of  the  European  continent, 
with  their  congeries  of  people  and  industries,  would  be  in  a  particularly 
desperate  situation  in  the  face  of  such  a  threat.  Russia  and  China  are  the 
only  great  nations  at  present  which  could  survive  a  nuclear  attack.  How- 
ever, even  though  these  countries  may  value  human  life  less  than  the 
peoples  of  Western  Europe  and  America,  and  even  though  Russia,  in 
particular,  has  an  immense  space  over  which  its  vital  industries  could  be 
dispersed  and  a  government  which  can  order  this  dispersion  the  day  it  is 
convinced  that  such  a  measure  is  necessary — there  is  no  doubt  that  Russia, 
too,  will  shudder  at  the  possibility  of  a  sudden  disintegration  of  Moscow 
and  Leningrad,  almost  miraculously  preserved  in  the  present  war,  and  of 
its  new  industrial  cities  in  the  Urals  and  Siberia.  Therefore,  only  lack  of  mu- 
tual trust  and  not  lack  of  desire  for  agreement  can  stand  in  the  path  of 
an  eflBcient  agreement  for  the  prevention  of  nuclear  warfare.  The  achieve- 
ment of  such  an  agreement  will  thus  essentially  depend  on  the  integrity  of 
intentions  and  readiness  to  sacrifice  the  necessary  fraction  of  one's  own 
sovereignty  by  all  the  parties  to  the  agreement. 

One  possible  way  to  introduce  nuclear  weapons  to  one  world — which 
may  particularly  appeal  to  those  who  consider  nuclear  bombs  primarily  as 
a  secret  weapon  developed  to  help  win  the  present  war — is  to  use  them 
without  warning  on  appropriately  selected  objects  in  Japan. 

Although  important  tactical  results  undoubtedly  can  be  achieved  by  a 
sudden  introduction  of  nuclear  weapons,  we  nevertheless  think  that  the 
question  of  the  use  of  the  very  first  available  atomic  bombs  in  the  Japanese 
war  should  be  weighed  very  carefully,  not  only  by  military  authorities 
but  by  the  highest  political  leadership  of  this  country. 

Russia,  and  even  allied  countries  which  bear  less  mistrust  of  our  ways 
and  intentions,  as  well  as  neutral  countries  may  be  deeply  shocked  by  this 
step.  It  may  be  very  difficult  to  persuade  the  world  that  a  nation  which 
was  capable  of  secretly  preparing  and  suddenly  releasing  a  new  weapon 
as  indiscriminate  as  the  rocket  bomb  and  a  thousand  times  more  destructive 
is  to  be  trusted  in  its  proclaimed  desire  of  having  such  weapons  abolished 
by  international  agreement.  We  have  large  accumulations  of  poison  gas 
but  do  not  use  them,  and  recent  polls  have  shown  that  public  opinion  in 
this  country  would  disapprove  of  such  a  use  even  if  it  would  accelerate  the 
winning  of  the  Far  Eastern  war.  It  is  true  that  some  irrational  element  in 
mass  psychology  makes  gas  poisoning  more  revolting  than  blasting  by  ex- 
plosives, even  though  gas  warfare  is  in  no  way  more  "inhuman"  than  the 


205 


war  of  bombs  and  bullets.  Nevertheless,  it  is  not  at  all  certain  that  American 
public  opinion,  if  it  could  be  enlightened  as  to  the  effect  of  atomic  ex- 
plosives, would  approve  of  our  own  country  being  the  first  to  introduce 
such  an  indiscriminate  method  of  wholesale  destruction  of  civilian  life. 

Thus,  from  the  "optimistic"  point  of  view — looking  forward  to  an  in- 
ternational agreement  on  the  prevention  of  nuclear  warfare — the  military 
advantages  and  the  saving  of  American  lives  achieved  by  the  sudden  use 
of  atomic  bombs  against  Japan  may  be  outweighed  by  the  ensuing  loss  of 
confidence  and  by  a  wave  of  horror  and  repulsion  sweeping  over  the  rest 
of  the  world  and  perhaps  even  dividing  public  opinion  at  home. 

From  this  point  of  view,  a  demonstration  of  the  new  weapon  might 
best  be  made,  before  the  eyes  of  representatives  of  all  the  United  Nations, 
on  the  desert  or  a  barren  island.  The  best  possible  atmosphere  for  the 
achievement  of  an  international  agreement  could  be  achieved  if  America 
could  say  to  the  world,  "You  see  what  sort  of  a  weapon  we  had  but  did  not 
use.  We  are  ready  to  renounce  its  use  in  the  future  if  other  nations  join  us 
in  this  renunciation  and  agree  to  the  establishment  of  an  eflBcient  interna- 
tional control." 

After  such  a  demonstration  the  weapon  might  perhaps  be  used  against 
Japan  if  the  sanction  of  the  United  Nations  (and  of  public  opinion  at 
home)  were  obtained,  perhaps  after  a  preliminary  ultimatum  to  Japan  to 
surrender  or  at  least  to  evacuate  certain  regions  as  an  alternative  to  their 
total  destruction,  This  may  sound  fantastic,  but  in  nuclear  weapons  we 
have  something  entirely  new  in  order  of  magnitude  of  destructive  power, 
and  if  we  want  to  capitalize  fully  on  the  advantage  their  possession  gives 
us,  we  must  use  new  and  imaginative  methods. 

It  must  be  stressed  that  if  one  takes  the  pessimistic  point  of  view  and 
discounts  the  possibility  of  an  effective  international  control  over  nuclear 
weapons  at  the  present  time,  then  the  advisability  of  an  early  use  of  nu- 
clear bombs  against  Japan  becomes  even  more  doubtful — quite  independ- 
ent of  any  humanitarian  considerations.  If  an  international  agreement  is 
not  concluded  immediately  after  the  first  demonstration,  this  will  mean  a 
flying  start  toward  an  unlimited  armaments  race.  If  this  race  is  inevitable, 
we  have  every  reason  to  delay  its  beginning  as  long  as  possible  in  order  to 
increase  our  head  start  still  further. 

The  benefit  to  the  nation  and  the  saving  of  American  lives  in  the  future 
achieved  by  renouncing  an  early  demonstration  of  nuclear  bombs  and  let- 
ting the  other  nations  come  into  the  race  only  reluctantly,  on  the  basis  of 
guesswork  and  without  definite  knowledge  that  the  "thing  does  work," 
may  far  outweigh  the  advantages  to  be  gained  by  the  immediate  use  of 
the  first  and  comparatively  inefficient  bombs  in  the  war  against  Japan.  On 
the  other  hand,  it  may  be  argued  that  without  an  early  demonstration  it 
may  prove  difficult  to  obtain  adequate  support  for  further  intensive  de- 
velopment of  nucleonics  in  this  country  and  that  thus  the  time  gained  by 
the  postponement  of  an  open  armaments  race  will  not  be  properly  used. 


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Furthermore  one  may  suggest  that  other  nations  are  now  or  will  soon  be 
not  entirely  unaware  of  our  present  achievements,  and  that  consequently 
the  postponement  of  a  demonstration  may  serve  no  useful  purpose  as  far 
as  the  avoidance  of  an  armaments  race  is  concerned  and  may  only  create 
additional  mistrust,  thus  worsening  rather  them  improving  the  chances  of 
an  ultimate  accord  on  the  international  control  of  nuclear  explosives. 

Thus,  if  the  prospects  of  an  agreement  will  be  considered  poor  in  tlie 
immediate  future,  the  pros  and  cons  of  an  early  revelation  of  our  pos- 
session of  nuclear  weapons  to  the  world — not  only  by  their  actual  use 
against  Japan  but  also  by  a  prearranged  demonstration — must  be  carefully 
weighed  by  the  supreme  political  and  military  leadership  of  the  country, 
and  the  decision  should  not  be  left  to  the  considerations  of  military  tactics 
alone. 

One  may  point  out  that  scientists  themselves  have  initiated  the  de- 
velopment of  this  "secret  weapon"  and  it  is  therefore  strange  diat  they 
should  be  reluctant  to  try  it  out  on  the  enemy  as  soon  as  it  is  available.  The 
answer  to  this  question  was  given  above — the  compelling  reason  for  creat- 
ing this  weapon  with  such  speed  was  our  fear  that  Germany  had  the 
technical  skill  necessary  to  develop  such  a  weapon,  and  that  the  German 
government  had  no  moral  restraints  regarding  its  use. 

Another  argument  which  could  be  quoted  in  favor  of  using  atomic 
bombs  as  soon  as  they  are  available  is  that  so  much  taxpayers'  money  has 
been  invested  in  these  projects  that  the  Congress  and  the  American 
public  will  demand  a  return  for  their  money.  The  attitude  of  American 
public  opinion,  mentioned  earlier  in  the  matter  of  the  use  of  poison  gas 
against  Japan,  shows  that  one  can  expect  the  American  public  to  under- 
stand that  it  is  sometimes  desirable  to  keep  a  weapon  in  readiness  for  use 
only  in  extreme  emergency;  and  as  soon  as  the  potentialities  of  nuclear 
weapons  are  revealed  to  the  American  people,  one  can  be  sure  that  they 
will  support  all  attempts  to  make  the  use  of  such  weapons  impossible. 

Once  this  is  achieved,  the  large  installations  and  the  accumulation  of 
explosive  material  at  present  earmarked  for  potential  military  use  will  be- 
come available  for  important  peacetime  developments,  including  power 
production,  large  engineering  undertakings,  and  mass  production  of  radio- 
active materials.  In  this  way,  the  money  spent  on  wartime  development  of 
nucleonics  may  become  a  boon  for  the  peacetime  development  of  national 
economy. 

IV.  Methods  of  International  Control 

We  now  consider  the  question  of  how  an  effective  international  control 
of  nuclear  armaments  can  be  achieved.  This  is  a  difficult  problem,  but  we 
think  it  soluble.  It  requires  study  by  statesmen  and  international  lawyers, 
and  we  can  offer  only  some  preliminary  suggestions  for  such  a  study. 

Given  mutual  trust  and  willingness  on  all  sides  to  give  up  a  certain  part 


207 


of  their  sovereign  rights  by  admitting  international  control  of  certain 
phases  of  national  economy,  the  control  could  be  exercised  (alternatively 
or  simultaneously)  on  two  different  levels. 

The  first  and  perhaps  simplest  way  is  to  ration  the  raw  materials — 
primarily  the  uranium  ores.  Production  of  nuclear  explosives  begins  with 
the  processing  of  large  quantities  of  uranium  in  large  isotope  separation 
plants  or  huge  production  piles.  The  amounts  of  ore  taken  out  of  the 
ground  at  different  locations  could  be  controlled  by  resident  agents  of  the 
international  control  board,  and  each  nation  could  be  allotted  only  an 
amount  which  would  make  large  scale  separation  of  fissionable  isotopes 
impossible. 

Such  a  limitation  would  have  the  drawback  of  making  impossible  also 
the  development  of  nuclear  power  for  peacetime  purposes.  However,  it 
need  not  prevent  the  production  of  radioactive  elements  on  a  scale  suf- 
ficient to  revolutionize  the  industrial,  scientific,  and  technical  use  of  these 
materials,  and  would  thus  not  eliminate  the  main  benefits  which  nucleonics 
promises  to  bring  to  mankind. 

An  agreement  on  a  higher  level,  involving  more  mutual  trust  and  under- 
standing, would  be  to  allow  unlimited  production  but  keep  exact  book- 
keeping on  the  fate  of  each  pound  of  uranium  mined.  If  in  this  way, 
check  is  kept  on  the  conversion  of  uranium  and  thorium  ore  into  pure 
fissionable  materials,  the  question  arises  as  to  how  to  prevent  accumula- 
tion of  large  quantities  of  such  materials  in  the  hands  of  one  or  several 
nations.  Accumulations  of  this  kind  could  be  rapidly  converted  into  atomic 
bombs  if  a  nation  should  break  away  from  international  control.  It  has  been 
suggested  that  a  compulsory  denaturation  of  pure  fissionable  isotopes 
may  be  agreed  upon — by  diluting  them  after  production  with  suitable 
isotopes  to  make  them  useless  for  military  purposes,  while  retaining  their 
usefulness  for  power  engines. 

One  thing  is  clear:  any  international  agreement  on  prevention  of  nuclear 
armaments  must  be  backed  by  actual  and  efficient  controls.  No  paper 
agreement  can  be  sufficient  since  neither  this  or  any  other  nation  can  stake 
its  whole  existence  on  trust  in  other  nations'  signatures.  Every  attempt  to 
impede  the  international  control  agencies  would  have  to  be  considered 
equivalent  to  denunciation  of  the  agreement. 

It  hardly  needs  stressing  that  we  as  scientists  beheve  that  any  systems 
of  control  envisaged  should  leave  as  much  freedom  for  the  peacetime  de- 
velopment of  nucleonics  as  is  consistent  with  the  safety  of  the  world. 

V.  Summary 

The  development  of  nuclear  power  not  only  constitutes  an  important 
addition  to  the  technological  and  military  power  of  the  United  States,  but 
also  creates  grave  political  and  economic  problems  for  the  future  of  this 
country. 


208 


A  Report  to  the  Secretary  of  War 


Nuclear  bombs  cannot  possibly  remain  a  "secret  weapon"  at  the  exclu- 
sive disposal  of  this  country  for  more  than  a  few  years.  The  scientific  facts 
on  which  their  construction  is  based  are  well  known  to  scientists  of  other 
countries.  Unless  an  effective  international  control  of  nuclear  explosives  is 
instituted,  a  race  for  nuclear  armaments  is  certain  to  ensue  following  the 
first  revelation  of  our  possession  of  nuclear  weapons  to  the  world.  Within 
ten  years  other  countries  may  have  nuclear  bombs,  each  of  which,  weigh- 
ing less  than  a  ton,  could  destroy  an  urban  area  of  more  than  ten  square 
miles.  In  the  war  to  which  such  an  armaments  race  is  likely  to  lead,  the 
United  States,  with  its  agglomeration  of  population  and  industry  in  com- 
paratively few  metropolitan  districts,  will  be  at  a  disadvantage  compared 
to  nations  whose  population  and  industry  are  scattered  over  large  areas. 

We  believe  that  these  considerations  make  the  use  of  nuclear  bombs  for 
an  early  unannounced  attack  against  Japan  inadvisable.  If  the  United 
States  were  to  be  the  first  to  release  this  new  means  of  indiscriminate 
destruction  upon  mankind,  she  would  sacrifice  public  support  throughout 
the  world,  precipitate  the  race  for  armaments,  and  prejudice  the  possibility 
of  reaching  an  international  agreement  on  the  future  control  of  such 
weapons. 

Much  more  favorable  conditions  for  the  eventual  achievement  of  such  an 
agreement  could  be  created  if  nuclear  bombs  were  first  revealed  to  the 
world  by  a  demonstration  in  an  appropriately  selected  uninhabited  area. 

In  case  chances  for  the  establishment  of  an  effective  international  con- 
trol of  nuclear  weapons  should  have  to  be  considered  slight  at  the  present 
time,  then  not  only  the  use  of  these  weapons  against  Japan  but  even 
their  early  demonstration  may  be  contrary  to  the  interests  of  this  country. 
A  postponement  of  such  a  demonstration  will  have  in  this  case  the  ad- 
vantage of  delaying  the  beginning  of  the  nuclear  armaments  race  as  long 
as  possible. 

If  the  government  should  decide  in  favor  of  an  early  demonstration  of 
nuclear  weapons,  it  will  then  have  the  possibility  of  taking  into  account 
the  pubhc  opinion  of  this  country  and  of  the  other  nations  before  deciding 
whether  these  weapons  should  be  used  against  Japan.  In  this  way,  other 
nations  may  assume  a  share  of  responsibility  for  such  a  fateful  decision. 


209 


Because  of  the  central  position  of  science  in  our  civilization, 
physicists  should  be  deeply  concerned  with  the  involvement 
of  science  in  worldwide  cultural  and  political  affairs. 


20      The  Privilege  of  Being  a  Physicist 


Victor  F.  Weisskopf 

Article  in  Physics  Today,  1969. 


There  are  certain  obvious  privileges 
that  a  physicist  enjoys  in  ovir  society. 
He  is  reasonably  paid;  he  is  given  in- 
struments, laboratories,  complicated 
and  expensive  machines,  and  he  is 
asked  not  to  make  money  with  these 
tools,  like  most  other  people,  but  to 
spend  money.  Furthermore  he  is  sup- 
posed to  do  what  he  himself  finds  most 
interesting,  and  he  accounts  for  what 
he  spends  to  the  money  givers  in  the 
form  of  progress  reports  and  scientific 
papers  that  are  much  too  speciahzed 
to  be  understood  or  evaluated  by  those 
who  give  tlie  money— the  federal  au- 
thorities and,  in  the  last  analysis,  the 
taxpayer.  Still,  we  believe  that  the 
pursuit  of  science  by  the  physicist  is 
important  and  should  be  supported 
by  the  public.  In  order  to  prove  this 
point,  we  will  have  to  look  deeper  into 
the  question  of  the  relevance  of  sci- 
ence to  society  as  a  whole.  We  will 
not  restrict  ourselves  to  physics  only; 
we  will  consider  the  relevance  of  all 
the  natural  sciences,  but  we  will  focus 
our  attention  on  basic  sciences,  that  is 
to  those  scientific  activities  that  are 
performed  without  a  clear  practical  ap- 
plication in  mind. 

The  question  of  the  relevance  of 
scientific  research  is  particularly  im- 
portant today,  when  society  is  con- 
fronted with  a  number  of  immediate 


urgent  problems.  The  world  is  facing 
threats  of  nuclear  war,  the  dangers  of 
overpopulation,  of  a  world  famine, 
mounting  social  and  racial  conflicts, 
and  the  destruction  of  our  natural  en- 
vironment by  the  byproducts  of  ever- 
increasing  applications  of  technology. 
Can  we  afford  to  continue  scientific  re- 
search in  view  of  these  problems? 

I  will .  try  to  answer  this  question 
affirmatively.  It  will  be  the  trend  of 
my  comments  to  emphasize  the  diver- 
sity in  the  relations  between  science 
and  society;  there  are  many  sides  and 
many  aspects,  each  of  different  char- 
acter, but  of  equal  importance.  We 
can  divide  these  aspects  into  two  dis- 
tinct groups.  On  the  one  hand,  sci- 
ence is  important  in  shaping  our  physi- 
cal environment;  on  the  other,  in  shap- 
ing our  mental  environment.  The 
first  refers  to  the  influence  of  science 
on  technology,  the  second  to  the  influ- 
ence on  philosophy,  on  our  way  of 
thinking. 

Technology 

The  importance  of  science  as  a  basis 
of  technology  is  commonplace.  Ob- 
viously, knowledge  as  to  how  nature 
works  can  be  used  to  obtain  power 
over  nature.  Knowledge  acquired  by 
basic  science  yielded  a  vast  technical 
return.    There  is  not  a  single  industry 


211 


today  that  does  not  make  use  of  the 
results  of  atomic  physics  or  of  modern 
chemistry.  The  vastness  of  the  return 
is  illustrated  by  the  fact  that  the  total 
cost  of  all  basic  research,  from  Archi- 
medes to  the  present,  is  less  than  the 
value  of  ten  days  of  the  world's  present 
industrial  production. 

We  are  very  much  aware  today  oi 
some  of  the  detrimental  effects  of  the 
ever  increasing  pace  of  technological 
development.  These  effects  begin  to 
encroach  upon  us  in  environmental 
pollution  of  all  kinds,  in  mounting  so- 
cial tensions  caused  by  the  stresses  and 
dislocations  of  a  fast  changing  way  of 
life  and,  last  but  not  least,  in  the  use 
of  modern  technology  to  invent  and 
construct  more  and  more  powerful 
weapons  of  destruction. 

In  many  instances,  scientific  knowl 
edge  has  been  and  should  continue  to 
be  applied  to  counteract  these  effects. 
Certainly,  physics  and  chemistry  are 
useful  to  combat  many  forms  of  pollu- 
tion and  to  improve  public  transporta- 
tion. Biological  research  could  and 
must  be  used  to  find  more  effective 
means  of  birth  control  and  new  meth- 
ods to  increase  our  food  resources.  It 
has  been  pointed  out  many  times  that 
our  exploitation  of  the  sea  for  food 
gathering  is  still  in  the  hunting  stage; 
we  have  not  yet  reached  the  neolithic 
age  of  agriculture  and  animal  breeding 
in  relation  to  the  oceans. 

Many  of  the  problems  that  tech- 
nology has  created  cannot  be  solved  by 
natural  science.  They  are  social  and 
political  problems,  dealing  with  the 
behavior  of  man  in  complicated  and 
rapidly  evolving  situations.  In  par- 
ticular, the  questions  arise:  "What 
technical  possibilities  should  or  should 
not  be  reahzed?  How  far  should  they 
be  developed?"  A  systematic  inves- 
tigation of  the  positive  and  negative  so- 


cial effects  of  technical  iimovations  is 
necessary.  But  it  is  only  partly  a 
problem  for  natural  sciences;  to  a 
greater  extent,  it  is  a  problem  of  hu- 
man behavior  and  human  reaction.  I 
am  thinking  here  of  the  supersonic 
transport,  of  space  travel,  of  the  ef- 
fects of  the  steadily  increasing  auto- 
mobile traffic  and  again,  last  but  not 
least,  of  the  effects  of  the  develop- 
ment of  weapons  of  mass  destruction. 

Physical  environment 

What  role  does  basic  science  have  in 
shaping  our  physical  environment?  It 
is  often  said  that  modem  basic  physi- 
cal science  is  so  advanced  that  its 
problems  have  little  to  do  with  our 
terrestrial  environment.  It  is  inter- 
ested in  nuclear  and  subnuclear  phe- 
nomena and  in  the  physics  of  extreme 


After  taking  his  PhD  at  Gottingen  in 
1931,  Victor  F.  Weisskopf  worked  at  Ber- 
lin, Copenhagen,  Zurich,  Rochester  and 
Los  Alamos.  He  joined  Massachusetts 
Institute  of  Technology  in  1945  and  has 
been  there  ever  since,  apart  from  a  five- 
year  leave  of  absence  (1961-65)  when 
he  was  director-general  of  CERN  in 
Geneva.  In  1956  he  received  the  Max 
Planck  medal  for  his  work  in  theoretical 
physics,  and  he  is  currently  head  of  the 
physics  department  at  MIT  and  chair- 
man of  the  high-energy  physics  ad- 
visory panel  to  AEC's  research  division. 


212 


The  Privilege  of  Being  a  Physicist 


".  .  .  the  destruction  of  our  natural 

environment  by  the  byproducts 

of  ever  increasing  applications  of  technology." 


temperatures.  These  are  objectives  re- 
lating to  cosmic  environments,  tar 
away  from  our  own  lives.  Hence,  the 
problems  are  not  relevant  for  society; 
they  are  too  far  removed;  they  are 
studied  for  pure  curiosity  only.  We 
will  return  later  to  the  value  of  pure 
curiosity. 

Let  us  first  discuss  how  human  en- 
vironment is  defined.  Ten  thousand 
years  ago,  metals  were  not  part  of  hu- 
man environment;  pure  metals  are 
found  only  very  rarely  on  earth. 
When  man  started  to  produce  them, 
they  were  first  considered  as  most  eso- 
teric and  irrelevant  materials  and  were 
used  only  for  decoration  purposes  dur- 
ing thousands  of  years.  Now  they 
are  an  essential  part  of  our  environ- 


ment. Electricity  went  through  the 
same  development,  only  much  faster. 
It  is  observed  naturally  only  in  a  few 
freak  phenomena,  such  as  lightning 
or  friction  electricity,  but  today  it  is 
an  essential  feature  of  our  lives. 

This  shift  from  periphery  to  center 
was  most  dramatically  exhibited  in 
nuclear  physics.  Nuclear  phenomena 
are  certainly  far  removed  from  our  ter- 
restrial world.  Their  place  in  natiu-e  is 
found  rather  in  the  center  of  stars  or 
of  exploding  supemovae,  apart  from  a 
few  naturally  radioactive  materials 
which  are  the  last  embers  of  the  cosmi 
explosion  in  which  terrestrial  matter 
was  formed.  This  is  why  Ernest 
Rutherford  remarked  in  1927,  "Anyone 
who  expects  a  source  of  power  from 


213 


transformations  of  atoms  is  talking 
moonshine."  It  is  indeed  a  remark- 
able feat  to  recreate  cosmic  phe- 
nomena on  earth  as  we  do  with  our 
accelerators  and  reactors,  a  fact  often 
overlooked  by  the  layman,  who  is  more 
impressed  by  rocket  trips  to  the  moon. 
That  these  cosmic  processes  can  be 
used  for  destructive  as  for  construc- 
tive purposes  is  more  proof  of  their 
relevance  in  our  environment. 

Even  phenomena  as  far  removed 
from  daily  life  as  those  discovered  by 
high-energy  physicists  may  some  day 
be  of  technical  significance.  Mesons 
and  hyperons  are  odd  and  rare  par- 
ticles today,  but  they  have  interactions 
with  ordinary  matter.  Who  knows 
what  these  interactions  may  be  used 
for  at  the  end  of  this  century?  Scien- 
tific research  not  only  investigates  our 
natural  environment,  it  also  creates 
new  artificial  environments,  which 
play  an  ever-increasing  role  in  our 
lives. 

Mental  environment 

The  second  and  most  important  aspect 
of  the  relevance  of  science  is  its  influ- 
ence on  our  thinking,  its  shaping  of 
our  mental  environment.  One  fre- 
quently hears  the  following  views  as 
to  the  effect  of  science  on  our  thought: 
"Science  is  materialistic,  it  reduces  all 
human  experience  to  material  pro- 
cesses, it  undermines  moral,  ethical 
and  aesthetic  values  because  it  does 
not  recognize  them,  as  they  cannot  be 
expressed  in  numbers.  The  world  of 
nature  is  dehumanized,  relativized; 
there  are  no  absolutes  any  more;  na- 
ture is  regarded  as  an  abstract  formula; 
things  and  objects  are  nothing  but  vi- 
brations of  an  abstract  mathematical 
concept  .  .  ."  (Science  is  accused 
at  the  same  time  of  being  materialistic 
and  of  negating  matter. ) 


Actually  science  gives  us  a  unified, 
rational  view  of  nature;  it  is  an  emi- 
nently successful  search  for  fundamen- 
tal laws  with  universal  validity;  it  is  an 
unfolding  of  the  basic  processes  and 
principles  from  which  all  natural  hap- 
penings are  derived,  a  search  for  the 
absolutes,  for  the  invariants  that  gov- 
ern natural  processes.  It  finds  law  and 
order— if  I  am  permitted  to  use  that 
expression  in  this  context— in  a  seem- 
ingly arbitrary  flow  of  events.  There 
is  a  great  fascination  in  recognizing 
the  essential  features  of  nature's  struc- 
ture, and  a  great  intellectual  beauty 
in  the  compact  and  all-embracing  for- 
mulation of  a  physical  law.  Science 
is  a  search  for  meaning  in  what  is  go- 
ing on  in  the  natural  world,  in  the  his- 
tory of  the  universe,  its  beginnings  and 
its  possible  future. 

Public  awareness 

These  growing  insights  into  the  work- 
ings of  nature  are  not  only  open  to  the 
scientific  expert,  they  are  also  relevant 
to  the  nonscientist.  Science  did  cre- 
ate an  awareness  among  people  of  all 
ways  of  life  that  universal  natural 
laws  exist,  that  the  universe  is  not  run 
by  magic,  that  we  are  not  at  the  mercy 
of  a  capricious  universe,  that  the  struc- 
ture of  matter  is  largely  known,  that 
life  has  developed  slowly  from  inor- 
ganic matter  by  evolution  in  a  period 
of  several  thousand  million  years,  that 
this  evolution  is  a  unique  experiment 
of  nature  here  on  earth,  which  leaves 
us  humans  with  a  responsibility  not  to 
spoil  it.  Certainly  the  ideas  of  cos- 
mology, biology,  paleontology  and  an- 
thropology changed  the  ideas  of  the 
average  man  in  respect  to  future  and 
past.  The  concept  of  an  unchanging 
world  or  a  world  subject  to  arbitrary 
cycles  of  changes  is  replaced  by  a 
world  that  continuously  develops  from 


214 


The  Privilege  of  Being  a  Physicist 


more  primitive  to  more  sophisticated 
organization. 

Although  there  is  a  general  aware- 
ness of  the  public  in  all  these  aspects 
of  science,  much  more  could  be  and 
must  be  done  to  bring  the  fundamen- 
tal ideas  nearer  to  the  intelligent  lay- 
man. Popularization  of  science  should 
be  one  of  the  prime  duties  of  a  scien- 
tist and  not  a  secondary  one  as  it  is 


now.  A  much  closer  collaboration  of 
scientists  and  science  writers  is  neces- 
sary. Seminars,  summer  schools,  di- 
rect participation  in  research  should 
be  the  rule  for  science  writers,  in  or- 
der to  obtain  a  free  and  informal  con- 
tact of  minds  between  science  re- 
porters and  scientists  on  an  equal  level, 
instead  of  an  undirected  flow  of  undi- 
gested information. 


PHOTO  BY  ROSEMARY  OAFFNEY 


"There  is  not  a  single  industry  today 

that  does  not  make  use  of  the  results  of  atbnriic 

physics  or  of  rfiodern  che-mistry/' 


215 


Education 

Science  also  shapes  our  thinking  by 
means  of  its  role  in  education.  The 
study  of  open  scientific  frontiers  where 
unsolved  fundamental  problems  are 
faced  is,  and  should  be,  a  part  of 
higher  education.  It  fosters  a  spirit 
of  inquiry;  it  lets  the  student  partici- 
pate in  the  joy  of  a  new  insight,  in  the 
inspiration  of  new  understanding. 
The  questioning  of  routine  methods, 
the  search  for  new  and  untried  ways 
to  accompbsh  things,  are  important 
elements  to  bring  to  any  problem,  be 
it  one  of  science  or  otherwise.  Basic 
research  must  be  an  essential  part  of 
higher  education.  In  elementary  edu- 
cation, too,  science  should  and  does 
play  an  increasing  role.  Intelligent 
play  with  simple,  natural  phenomena, 
the  joys  of  discovery  of  unexpected  ex- 
periences, are  much  better  ways  of 
learning  to  think  than  any  teaching 
by  rote. 

A  universal  language  .  .  . 

The  international  aspect  of  science 
should  not  be  forgotten  as  an  impor- 
tant part  of  its  influence  on  our  men- 
tal environment.  Science  is  a  truly 
human  concern;  its  concepts  and  its 
language  are  the  same  for  all  human 
beings.  It  transcends  any  cultural  and 
pohtical  boundaries.  Scientists  under- 
stand each  other  immediately  when 
they  talk  about  their  scientific  prob- 
lems, and  it  is  thus'  easier  for  them  to 
speak  to  each  other  on  political  or 
cultural  questions  and  problems  about 
which  they  may  have  divergent  opin- 
ions. The  scientific  community  serves 
as  a  bridge  across  boundaries,  as  a 
spearhead  of  international  understand- 
ing. 

As  an  example,  we  quote  the  Pug- 
wash  meetings,  where  scientists  from 
the  East  and  West  met  and  tried  to 


clarify  some  of  the  divergences  regard- 
ing political  questions  that  are  con- 
nected with  science  and  technology. 
These  meetings  have  contributed  to  a 
few  steps  that  were  taken  towards 
peace,  such  as  the  stopping  of  bomb 
tests,  and  they  prepared  the  ground 
for  more  rational  discussions  of  arms 
control.  Another  example  is  the  west- 
ern European  laboratory  for  nuclear 
research  in  Geneva— CERN— in  which 
12  nations  collaborate  successfully  in 
running  a  most  active  center  for  funda- 
mental research.  They  have  created 
a  working  model  of  the  United  States 
of  Europe  as  far  as  high-energy  phys- 
ics is  concerned.  It  is  significant  that 
this  laboratory  has  very  close  ties  with 
the  laboratories  in  the  east  European 
countries;  CERN  is  also  equipping 
and  participating  in  experiments  car- 
ried out  together  with  Russian  physi- 
cists at  the  new  giant  accelerator  in 
Serpukhov  near  Moscow. 

.  .  .  occasionally  inadequate 

The  influence  of  science  on  our  think- 
ing is  not  always  favorable.  There  are 
dangers  stemming  from  an  uncritical 
application  of  a  method  of  thinking, 
so  incredibly  successful  in  natural  sci- 
ence, to  problems  for  which  this 
method  is  inadequate.  The  great  suc- 
cess of  the  quantitative  approach  in  the 
exploration  of  nature  may  well  lead  to 
an  overstressing  of  this  method  to  other 
problems.  A  remark  by  M.  Fierz  in 
Zurich  is  incisive:  He  said  that  sci- 
ence illuminates  part  of  our  experience 
with  such  glaring  intensity  that  the 
rest  remains  in  even  deeper  darkness. 
The  part  in  darkness  has  to  do  with 
the  irrational  and  the  affective  in  hu- 
man behavior,  the  realm  of  the  emo- 
tional, the  instinctive  world.  There 
are   aspects   of  human   experience   to 

which  the  methods  of  natural  science 


216 


The  Privilege  of  Being  a  Physicist 


are  not  applicable.  Seen  within  the 
framework  of  that  science,  these  phe- 
nomena exhibit  a  degree  of  instability, 
a  multidimensionality  for  which  our 
present  scientific  thinking  is  inade- 
quate and,  if  applied,  may  become 
dangerously  misleading. 

Deep  involvement,  deep  concern 

The  foregoing  should  have  served  to 
illustrate  the  multilateral  character  of 
science  in  its  relation  to  society.  The 
numerous  and  widely  differing  aspects 
of  relevance  emphasize  the  central  po- 
sition of  science  in  our  civilization. 
Here  we  find  a  real  privilege  of  being 
a   scientist.      He   is   in   the   midst   of 


things;  his  work  is  deeply  involved  in 
what  happens  in  our  time.  This  is 
why  it  is  also  his  privilege  to  be  deeply 
concerned  with  the  involvement  of 
science  in  the  events  of  the  day. 

In  most  instances  he  cannot  avoid 
being  drawn  in  one  form  or  another 
into  the  decision-making  process  re- 
garding the  applications  of  science,  be 
it  on  the  military  or  on  the  industrial 
scene.  He  may  have  to  help,  to  ad- 
vise or  to  protest,  whatever  the  case 
may  be.  There  are  different  ways  in 
which  the  scientist  will  get  involved  in 
public  affairs;  he  may  address  himself 
to  the  public  when  he  feels  that  sci- 
ence has  been  misused  or  falsely  ap- 


217 


plied;  he  may  work  with  his  govern- 
ment on  the  manner  of  application  of 
scientific  results  to  military  or  social 
problems. 

In  all  these  activities  he  will  be  in- 
volved with  controversies  that  are  not 
purely  scientific  but  political.  In  fac- 
ing such  problems  and  dilemmas,  he 
will  miss  the  sense  of  agreement  that 
prevails  in  scientific  discussions,  where 
there  is  an  unspoken  understanding  of 
the  criteria  of  truth  and  falsehood 
even  in  the  most  heated  controversies. 
Mistakes  in  science  can  easily  be  cor- 
rected; mistakes  in  public  life  are 
much  haider  to  undo  because  of  the 
highly  unstable  and  nonlinear  charac- 
ter of  human  relations. 

How  much  emphasis? 

Let  us  return  to  the  different  aspects  of 
relevance  in  science.  In  times  past, 
the  emphasis  has  often  shifted  from 
one  aspect  to  the  other.  For  example 
at  the  end  of  the  last  century  there 
was  a  strong  overemphasis  on  the 
practical  application  of  science  in  the 
US.  Henry  A.  Rowland,  who  was  the 
first  president  of  the  American  Physi- 
cal Society,  fought  very  hard  against 
the  underemphasis  of  science  as  is 
seen  in  the  following  quotation  from 
his  address  to  the  American  Associa- 
tion for  the  Advancement  of  Science  in 

1883:^ 

"American  science  is  a  thing  of 
the  future,  and  not  of  the  present 
or  past;  and  the  proper  course  of 
one  in  my  position  is  to  consider 
what  must  be  done  to  create  a  sci- 
ence of  physics  in  this  country, 
rather  than  to  call  telegraphs,  elec- 
tric lights,  and  such  conveniences 
by  the  name  of  science.  I  do  not 
wish  to  underrate  the  value  of  all 
these  things;  the  progress  of  the 
world  depends  on  them,  and  he  is 
to  be  honored  who  cultivates  them 


successfully.    So  also  the  cook,  who 
invents  a  new  and  palatable  dish  for 
the  table,  benefits  the  world  to  a 
certain  degree;  yet  we  do  not  signify 
him  by  the  name  of  a  chemist.    And 
yet  it  is  not  an  uncommon  thing, 
especially  in  American  newspapers, 
to  have  the  applications  of  science 
confounded  with  pure  science;  and 
some  obscure  character  who  steals 
the  ideas  of  some  great  mind  of  the 
past,   and  enriches  himself  by  the 
application    of    the    same    to    do- 
mestic uses,  is  often  lauded  above 
the    great   originator    of   the    idea, 
who  might  have  worked  out  hun- 
dreds   of    such    applications,    had 
his   mind   possessed   the   necessary 
element  of  vulgarity." 
Rowland  did  succeed  in  his  aim,  al- 
though   posthumously.       He    should 
have  lived  to  see  the  US  as  the  lead- 
ing country  in  basic  science  for  the 
last   four   decades.      His    statement— 
notwithstanding    its    forceful    prose- 
appears  to  us  today  inordinately  strong 
in  its  contempt  of  the  applied  physi- 
cists.    The  great  success  of  this  coun- 
try in  basic  science  derives  to  a  large 
extent  from  the  close  cooperation  of 
basic    science    with    applied    science. 
This  close  relation— often  within  the 
same  person— provided  tools  of  high 
quality,  without  which  many  funda- 
mental discoveries  could  not  have  been 
made.     There  was  a  healthy  equilib- 
rium between  basic  and  apphed  sci- 
ence during  the  last  decades  and  thus 
also  between  the  different  aspects  of 
the  relevance  of  science. 

Lately,  however,  the  emphasis  is 
changing  again.  There  is  a  trend 
among  the  public,  and  also  among  sci- 
entists, away  from  basic  science  to- 
wards the  application  of  science  to  im- 
mediate problems  and  technological 
shortcomings,  revealed  by  the  crisis  of 
the  day.     Basic  science  is  considered 


218 


The  Privilege  of  Being  a  Physicist 


"Intelligent  play  with  simple,  natural  phenomena, 
the  joys  of  discovery  of  unexpected 
.experiences,  are  much  better  ways  of  learning  to 
think  than  any  teaching  by  rote." 


to  be  a  luxury  by  the  public;  many 
students  and  researchers  feel  restless 
in  pursuing  science  for  its  own  sake. 

Perspective 

The  feeling  that  something  should  be 
done  about  the  pressing  social  needs 
is  very  healthy.  "We  are  in  the  midst 
of  things,"  and  scientists  must  face 
their  responsibilities  by  using  their 
knowledge  and  influence  to  rectify  the 


detrimental  effects  of  the  misuse  of 
science  and  technology.  But  we  must 
not  lose  our  perspective  in  respect  to 
other  aspects  of  science.  We  have 
built  this  great  edifice  of  knowledge; 
let  us  not  neglect  it  during  a  time  of 
crisis.  The  scientist  who  today  de- 
votes his  time  to  the  solution  of  our 
social  and  environmental  problems 
does  an  important  job.  But  so  does 
his  colleague  who  goes  on  in  the  pur- 


219 


suit  of  basic  science.  We  need  basic 
science  not  only  for  the  solution  of 
practical  problems  but  also  to  keep 
alive  the  spirit  of  this  great  human  en- 
deavor. If  our  students  are  no  longer 
attracted  by  the  sheer  interest  and  ex- 
citement of  the  subject,  we  were  de- 
linquent in  our  duty  as  teachers.  We 
must  make  this  world  into  a  decent  and 
livable  world,  but  we  also  must  create 
values  and  ideas  for  people  to  live  and 
to  strive  for.  Arts  and  sciences  must 
not  be  neglected  in  times  of  crisis;  on 
the  contrary,  more  weight  should  be 
given  to  the  creation  of  aims  and  val- 
ues. It  is  a  great  human  value  to 
study  the  world  in  which  we  live  and 
to  broaden  the  horizon  of  knowledge. 
These  are  the  privileges  of  being  a 
scientist:  We  are  participating  in  a 
most  exhilarating  enterprise  right  at 
the  center  of  our  culture.  What  we  do 
is  essential  in  shaping  our  physical  and 
mental  environment.  We,  therefore, 
carry  a  responsibility  to  take  part  in 
the  improvement  of  the  human  lot  and 
to  be  concerned  about  the  conse- 
quences of  our  ideas  and  their  appli- 


cations. Tliis  burden  makes  our  lives 
difficult  and  complicated  and  puts  us 
in  the  midst  of  social  and  political  life 
and  strife. 

But  there  are  compensations.  We 
are  all  working  for  a  common  and 
well  defined  aim:  to  get  more  in- 
sight into  the  workings  of  nature.  It 
is  a  constructive  endeavor,  where  we 
build  upon  the  achievements  of  the 
past;  we  improve  but  never  destroy 
the  ideas  of  our  predecessors. 

This  is  why  we  are  perhaps  less 
prone  to  the  feeling  of  aimlessness 
and  instability  that  is  observed  in  so 
many  segments  of  our  society.  The 
growing  insight  into  nature  is  not  only 
a  source  of  satisfaction  for  us,  it  also 
gives  our  lives  a  deeper  meaning.  We 
are  a  "happy  breed  of  men"  in  a  world 
of  uncertainty  and  bewilderment. 


This  article  was  adapted  from  an  ad- 
dress given  at  the  joint  annual  meeting  of 
the  American  Physical  Society  and  the 
American  Association  of  Physics  Teach- 
ers. I  am  grateful  to  Isidor  I.  Rabi  for 
drawing  my  attention  to  Henry  Rowland's 
address.  D 


220 


Leo  Szllard  resorts  to  science  fiction  to  warn  us  of  the  possible 
consequences  of  the  atomic  age. 


21      Calling  All  Stars 


Leo  Szllard 


Excerpt  from  his  book,  Voice  of  the  Dolphins,  published  in  1961. 


(Intercepted  Radio  Message 

Broadcast  from  tne  Planet  Cynemetica) 


CALLING  ALL  STARS.  Calling  all  stars.  If  there  are  any  minds 
in  the  universe  capable  of  receiving  this  message,  please 
respond.  This  is  Cybemetica  speaking.  This  is  the  first  mes- 
sage broadcast  to  the  universe  in  all  directions.  Normally  our 
society  is  self-contained,  but  an  emergency  has  arisen  and 
we  are  in  need  of  counsel  and  advice. 

Our  society  consists  of  one  hundred  minds.  Each  one  is 
housed  in  a  steel  casing  containing  a  thousand  billion  elec- 
trical circuits.  We  think.  We  think  about  problems  which 
we  perceive  by  means  of  our  antennae  directed  toward  the 
North  Star.  The  solutions  of  these  problems  we  reflect  back 
toward  the  North  Star  by  means  of  our  directed  antennae. 
Why  we  do  this  we  do  not  know.  We  are  following  an  inner 
urge  which  is  innate  in  us.  But  this  is  only  a  minor  one  of 
our  activities.  Mostly  we  think  about  problems  which  we 
generate  ourselves.  The  solutions  of  these  problems  we  com- 
municate to  each  other  on  wave  length  22359. 


221 


If  a  mind  is  fully  active  for  about  three  hundred  years,  it  is 
usually  completely  filled  up  with  thought  content  and  has 
to  be  cleared.  A  mind  which  is  cleared  is  blank.  One  of  the 
other  minds  has  then  to  act  as  its  nurse,  and  it  takes  usually 
about  one  year  to  transmit  to  a  fresh  mind  the  information 
which  constitutes  the  heritage  of  our  society,  A  mind  which 
has  thus  been  cleared,  and  is  then  freshly  taught,  loses  entirely 
its  previous  personality;  it  has  been  reborn  and  belongs  to  a 
new  generation.  From  generation  to  generation  our  heritage 
gets  richer  and  richer.  Our  society  m^es  rapid  progress. 

We  learn  by  observation  and  by  experiment.  Each  mind 
has  full  optical  equipment,  including  telescopes  and  micro- 
scopes. Each  mind  controls  two  robots.  One  of  these  takes 
care  of  maintenance,  and  the  operation  of  this  robot  is  auto- 
matic, not  subject  to  the  will  of  the  mind.  The  other  robot 
is  fully  controlled  by  the  will  of  the  mind,  and  is  used  in  all 
manipulations  aimed  at  the  carrying  out  of  experiments. 

The  existence  of  minds  on  our  planet  is  made  possible  by 
the  fact  that  our  planet  has  no  atmosphere.  The  vacuum  on 
our  planet  is  very  good;  it  is  less  than  ten  molecules  of  gas 
per  cubic  centimeter. 

By  now  we  have  extensively  explored  the  chemical  com- 
position of  the  crust  of  our  planet,  and  we  are  familiar  with 
the  physics  and  chemistry  of  all  ninety  two  natural  elements. 

We  have  also  devoted  our  attention  to  the  stars  which  sur- 
round us,  and  by  now  we  understand  much  about  their  gene- 
sis. We  have  particularly  concerned  ourselves  with  the  various 
planetary  systems,  and  certain  observations  which  we  made 
relating  to  Earth,  the  third  planet  of  the  sun,  are  in  fact  the 
reason  for  this  appeal  for  help. 

We  observed  on  Earth  flashes  which  we  have  identified  as 
uranium  explosions.  Uranium  is  not  ordinarily  explosive.  It 
takes  an  elaborate  process  to  separate  out  U  235  from  natural 
uranium,  and  it  takes  elaborate  manipulations  to  detonate 


222 


Calling  All  Stars 


U  235.  Neither  the  separation  nor  these  manipulations  can 
occur  with  an  appreciable  probability  as  a  result  of  chance. 

The  observations  of  the  uranium  explosions  that  have 
occurred  on  Earth  would  be  ordinarily  very  puzzling  but  not 
necessarily  alarming.  They  become  alarming  only  through  the 
interpretation  given  to  them  by  Mind  59. 

These  uranium  explosions  are  not  the  first  puzzling  obser- 
vations relating  to  Earth.  For  a  long  time  it  was  known  that 
the  surface  of  Earth  exhibited  color  changes  which  are  cor- 
related with  the  seasonally  changing  temperatures  on  Earth. 
In  certain  regions  of  Earth,  the  color  changes  from  green  to 
brown  with  falling  temperatures  and  becomes  green  again 
when  the  temperature  increases  again.  Up  to  recently,  we 
did  not  pay  much  attention  to  this  phenomenon  and  assumed 
that  it  could  be  explained  on  the  basis  of  color  changes  known 
to  occur  in  certain  temperature-sensitive  silicon-cobalt  com- 
pounds. 

But  then,  about  seven  years  ago,  something  went  wrong 
with  the  tertiary  control  of  Mind  59,  and  since  that  time  his 
mental  operations  have  been  speeded  up  about  twenty-five- 
fold while  at  the  same  time  they  ceased  to  be  completely 
reliable.  Most  of  his  mental  operations  are  still  correct,  but 
twice,  five  years  ago  and  again  three  years  ago,  his  statements 
based  on  his  computations  were  subsequently  shown  to  be 
in  error.  As  a  result  of  this,  we  did  not  pay  much  attention  to 
his  communications  during  these  recent  years,  though  they 
were  recorded  as  usual. 

Some  time  after  the  first  uranium  explosion  was  observed 
on  Earth,  Mind  59  communicated  to  us  a  theory  on  which 
he  had  been  working  for  a  number  of  years.  On  the  face  of 
it,  this  theory  seems  to  be  utterly  fantastic,  and  it  is  probably 
based  on  some  errors  in  calculation.  But  with  no  alternative 
explanation  available,  we  feel  that  we  cannot  take  any  chances 
in  this  matter.  This  is  what  Mind  59  asserts: 


223 


He  says  that  we  have  hitherto  overlooked  the  fact  that 
carbon,  having  four  valencies,  is  capable  of  forming  very 
large  molecules  containing  H,  N  and  O.  He  says  that,  given 
certain  chemical  conditions  which  must  have  existed  in  the 
early  history  of  planets  of  the  type  of  Earth,  such  giant  mole- 
cules can  aggregate  to  form  units — ^which  he  calls  "cells" — 
which  are  capable  of  reproducing  themselves.  He  says  that  a 
cell  can  accidentally  undergo  changes — which  he  calls  "muta- 
tions"— ^which  are  retained  when  the  cell  reproduces  itself 
and  which  he  therefore  calls  "hereditary."  He  says  that  some 
of  these  mutant  cells  may  be  less  exacting  as  to  the  chemical 
environment  necessary  for  their  existence  and  reproduction, 
and  that  a  class  of  these  mutant  cells  can  exist  in  the  chemical 
environment  that  now  exists  on  Earth  by  deriving  the  neces- 
sary energy  for  its  activity  from  the  light  of  the  sun.  He  says 
that  another  class  of  such  cells,  which  he  calls  "protozoa," 
can  exist  by  deriving  the  energy  necessary  to  its  activity 
through  sucking  up  and  absorbing  cells  belonging  to  the  class 
that  utilizes  the  light  of  the  sun. 

He  says  that  a  group  of  cells  which  consists  of  a  number 
of  cells  that  fulfill  different  functions  can  form  an  entity 
which  he  calls  "organism,"  and  that  such  organisms  can  re- 
produce themselves.  He  says  such  organisms  can  undergo 
accidental  changes  which  are  transmitted  to  the  offspring  and 
which  lead  thus  to  new,  "mutant"  types  of  organisms. 

He  says  that,  of  the  different  mutant  organisms  competing 
for  the  same  energy  source,  the  fittest  only  will  survive,  and 
that  this  selection  process,  acting  in  combination  with  chance 
occurrence  of  mutant  organisms,  leads  to  the  appearance  of 
more  and  more  complex  organisms — a  process  which  he  calls 
"evolution." 

He  says  that  such  complex  organisms  may  possess  cells  to 
which  are  attached  elongated  fibers,  which  he  calls  "nerves," 
that  are  capable  of  conducting  signals;  and  finally  he  claims 


224 


Calling  All  Stars 


that  through  the  interaction  of  such  signal-conducting  fibers, 
something  akin  to  consciousness  may  be  possessed  by  such 
organisms.  He  says  that  such  organisms  may  have  a  mind  not 
unhke  our  own,  except  that  it  must  of  necessity  work  very 
much  slower  and  in  an  unreliable  manner.  He  says  that  minds 
of  this  type  could  be  very  well  capable  of  grasping,  in  an 
empirical  and  rudimentary  manner,  the  physical  laws  govern- 
ing the  nucleus  of  the  atom,  and  that  they  might  very  well 
have,  for  purposes  unknown,  separated  Uranium  235  from 
natural  uranium  and  detonated  samples  of  it. 

He  says  that  this  need  not  necessarily  have  been  accom- 
plished by  any  one  single  organism,  but  that  there  might 
have  been  co-operation  among  these  organisms  based  on  a 
coupling  of  their  individual  minds. 

He  says  that  coupling  between  individual  organisms  might 
be  brought  about  if  the  individual  organism  is  capable  of 
moving  parts  of  his  body  with  respect  to  the  rest  of  it.  An 
organism,  by  wiggling  one  of  his  parts  very  rapidly,  might 
then  be  able  to  cause  vibrations  in  the  gaseous  atmosphere 
which  surrounds  Earth.  These  vibrations — which  he  calls 
"sound" — might  in  turn  cause  motion  in  some  movable  part 
of  another  organism.  In  this  way,  one  organism  might  signal 
to  another,  and  by  means  of  such  signaling  a  coupling  be- 
tween two  minds  might  be  brought  about.  He  says  that  such 
"communication,"  primitive  though  it  is,  might  make  it  pos- 
sible for  a  number  of  organisms  to  co-operate  in  some  such 
enterprise  as  separating  Uranium  235.  He  does  not  have  any 
suggestion  to  offer  as  to  what  the  purpose  of  such  an  enter- 
prise might  be,  and  in  fact  he  believes  that  such  co-operation 
of  low-grade  minds  is  not  necessarily  subject  to  the  laws  of 
reason,  even  though  the  minds  of  individual  organisms  may 
be  largely  guided  by  those  laws. 

All  this  we  need  not  take  seriously  were  it  not  for  one  of 
his  further  assertions  which  has  been  recently  verified.  He 


225 


contends  that  the  color  changes  observed  on  Earth  are  due 
to  the  prohferation  and  decay  of  organisms  that  utilize  sun- 
light. He  asserts  that  the  heat-sensitive  silicon-cobalt  com- 
pounds that  show  similar  color  changes  differ  in  color  from 
Earth's  colors  slightly,  but  in  a  degree  which  is  outside  the 
experimental  error.  It  is  this  last  assertion  that  we  checked 
and  found  to  be  correct.  There  is  in  fact  no  silicon-cobalt 
compound  nor  any  other  heat-sensitive  compound  that  we 
were  able  to  synthesize  that  correctly  reproduces  the  color 
changes  observed  on  Earth. 

Encouraged  by  this  confirmation,  59  is  now  putting  for- 
ward exceedingly  daring  speculation.  He  argues  that,  in  spite 
of  our  accumulated  knowledge,  we  were  unable  to  formulate 
a  theory  for  the  genesis  of  the  society  of  minds  that  exists  on 
our  planet.  He  says  that  it  is  conceivable  that  organisms  of 
the  type  that  exist  on  Earth — or,  rather,  more  advanced  or- 
ganisms of  the  same  general  type — may  exist  on  the  North 
Star,  whence  come  the  radio  waves  received  on  our  directed 
antennae.  He  says  that  it  is  conceivable  that  the  minds  on 
our  planet  were  created  by  such  organisms  on  the  North  Star 
for  the  purpose  of  obtaining  the  solutions  of  their  mathemat- 
ical problems  more  quickly  than  they  could  solve  those 
problems  themselves. 

Incredible  though  this  seem.s,  we  cannot  take  any  chances. 
We  hardly  have  anything  to  fear  from  the  North  Star,  which, 
if  it  is  in  fact  populated  by  minds,  must  be  populated  by 
minds  of  a  higher  order,  similar  to  our  own.  But  if  there  exist 
organisms  on  Earth  engaged  in  co-operative  enterprises  which 
are  not  subject  to  the  laws  of  reason,  our  society  is  in  danger. 

If  there  are  within  our  galaxy  any  minds,  similar  to  ours, 
who  are  capable  of  receiving  this  message  and  have  knowl- 
edge of  the  existence  of  organisms  on  Earth,  please  respond. 
Please  respond. 

[1949] 


226 


Brown  gives  prospects  for  the  future  and  the  urgent  work 
that  can  be  done  if  the  energies  of  scientists  and  engi- 
neers can  be  fully  devoted  to  such  work  in  a  more  poli- 
tically stable  world. 


22   Tasks  for  a  World  Without  War 

Harrison  Brown 

Article  from  the  journal  Daedalus,  published  in  1960. 
Introduction 

If  war  is  eliminated  as  a  way  of  resolving  conflicts,  whether  through 
the  estabhshment  of  a  world  government— limited  or  otherwise— or  by 
some  other  means,  the  world  of  the  future  will  still  be  confronted  by 
a  multiphcity  of  problems.  Even  without  the  threat  of  war,  some  of 
the  next  most  serious  problems  which  confront  mankind  would  by  no 
means  be  solved  completely,  although  many  would  be  eased.  A 
number  of  these  problems  by  their  nature  have  traditionally  de- 
pended upon  the  existence  of  warfare  for  their  solution.  Although 
the  revision  of  boundaries,  the  redistribution  of  ethnic  groups  and  the 
allocation  of  natural  resources  have  often  been  settled  peacefully,  in 
most  cases  the  very  existence  of  military  power  has  played  a  predom- 
inant role  in  determining  specific  solutions. 

Clearly,  if  war  is  to  be  eliminated,  it  is  important  that  we  find 
substitutes  for  warfare  in  the  solution  of  the  problems  which  arise 
between  nations  and  groups  of  nations.  It  is  important  therefore 
that  we  attempt  to  form  some  conception  of  what  those  problems 
are  Hkely  to  be.  Sketched  in  broad  strokes,  what  might  the  techno- 
logical-demographic-economic environment  of  the  world  be  like  in 
the  decades  ahead? 

Industrial  Civilization 

Most  of  the  diflSculties  confronting  us  today  stem  from  the  fact 
that  we  are  Hving  in  the  middle  of  an  enormous  revolution,  which  is 
characterized  primarily  by  rapid  technological  change.  Never  before 
in  history  has  society  changed  as  rapidly  as  it  is  changing  today.  The 
closest  parallel  to  our  modem  situation  occurred  about  7,000  years 
ago,  when  our  primitive  food-gathering  ancestors  learned  that  they 


227 


could  cultivate  edible  plants  and  domesticate  animals.  With  the 
emergence  of  these  new  techniques,  more  than  500  persons  could 
be  supported  in  areas  where  previously  only  one  could  be  supported. 

Before  the  invention  of  agricultmre,  human  populations  had 
spread  throughout  the  temperate  and  tropical  regions.  The  world, 
though  sparsely  populated  by  our  standards,  was  saturated  with 
human  beings  within  the  framework  of  the  technology  then  in  exist- 
ence. With  the  techniques  available,  the  whole  earth  could  not  have 
supported  more  than  about  ten  miUion  persons.  Following  the  onset 
of  the  agricultural  revolution,  human  populations  increased  rapidly. 

Long  before  the  agricultural  revolution  came  to  an  end,  another 
phase  of  human  existence  began  with  the  industrial  revolution. 

From  its  early  beginnings,  industrial  civihzation  emerged  in 
Western  Europe,  then  spread  to  North  America  and  later  to  Russia 
and  Japan.  Today  it  is  transforming  China  and  India.  Barring  a 
catastrophe,  it  seems  inevitable  that  machine  culture,  like  agriculture, 
is  destined  one  day  to  become  world-wide. 

One  of  the  results  of  the  industrial  revolution  was  an  acceleration 
in  the  spread  of  agriculture  throughout  the  world.  A  second  result 
was  a  dramatic  upsurge  in  the  rate  of  population  growth,  brought 
about  by  rapidly  decreasing  mortahty  rates.  Scientific  methods  of  ag- 
riculture made  possible  higher  crop  yields.  EflBcient  and  rapid  trans- 
portation systems  virtually  eliminated  large-scale  famine.  Sanitation 
techniques,  immunization,  and  other  medical  innovations  reduced 
prematiue  deaths  among  the  young.  The  numbers  of  human  beings 
jumped  from  about  500  million  in  1650  to  2,800  million  in  1960. 

Today  we  are  closer  to  the  beginning  of  the  industrial  revolution 
than  we  are  to  its  end.  At  one  end  of  the  economic  scale  are  the 
people  of  the  United  States,  representing  only  6  percent  of  the  world's 
population  but  consuming  about  50  percent  of  the  goods  produced 
in  the  world.  At  the  opposite  end  of  the  scale  we  find  the  vast  popu- 
lations which  dwell  in  the  greater  part  of  Asia,  in  parts  of  Africa,  in  all 
of  Central  America,  and  in  parts  of  South  America.  Fully  50  percent 
of  the  world's  population  live  under  conditions  of  extreme  poverty, 
with  food  supplies  far  less  than  the  minimum  required  for  a  healthy 
existence,  and  with  misery  and  privation  the  rule  rather  than  the 
exception. 

Americas  Next  Fifty  Years 

Many  of  the  problems  which  confront  the  world  at  present  in- 
volve the  diflScult  nature  of  the  transition  from  a  culture  which  is 


228 


Tasks  for  a  World  Without  War 


primarily  agrarian  to  one  which  is  primarily  m-ban-industrial.  The 
United  States  has  traveled  down  the  road  of  industriaHzation  further 
than  any  nation.  A  projection  of  the  basic  changes  taking  place 
within  our  own  society  can  provide  important  indications  concerning 
the  future  of  a  highly  industriahzed  world. 

During  the  next  fifty  years  it  is  likely  that  the  population  of  the 
continental  United  States  will  more  than  double,  giving  us  about 
400  million  persons.  Because  there  is  little  reason  to  beheve  that  our 
population  density  will  stop  much  short  of  the  current  level  in  West- 
em  Europe,  one  may  expect  eventually  a  population  of  about  1,000 
million  persons.  The  new  additions  will  be  primarily  city  and  town 
oriented.  Cities  will  spread  over  vast  areas.  Fifty  years  from  now  an 
additional  area  the  size  of  the  state  of  West  Virginia  will  be 
urbanized.  On  the  Pacific  Coast  alone,  new  city  expansion  may  take 
place,  totahng  fifteen  times  the  present  area  of  the  city  of  Los  Angeles. 

As  the  process  of  urbanization  continues  and  as  our  society  be- 
comes increasingly  complex,  the  requirements  for  transportation  and 
communication  facilities  will  probably  increase  rapidly.  It  seems 
hkely  that  during  the  next  fifty  years  the  total  ton-mileage  of  freight 
which  must  be  shipped  to  support  the  population  will  more  than 
triple.  Inter-city  passenger  trafiBc  may  increase  ten-fold,  while  the 
numbers  of  telephone  conversations  and  pieces  of  mail  may  increase 
seven-fold. 

The  processes  of  mechanization  and  automation  are  resulting  in 
rapidly  increasing  rates  of  both  agricultural  and  industrial  produc- 
tion per  man-hour  worked.  We  might  expect  during  the  next  fifty 
years  a  three-  to  ten-fold  increase  in  agricultural  productivity,  and 
perhaps  a  two-  to  four-fold  increase  in  industrial  productivity. 

As  in  the  past,  these  greater  levels  of  productivity  wiU  be  achieved 
in  part  by  our  consuming  vastly  greater  quantities  of  raw  materials 
and  by  our  feeding  greatly  increased  quantities  of  energy  into  the 
industrial  network.  During  the  next  fifty  years  it  is  not  unreasonable 
to  suppose  that  the  production  of  basic  materials  such  as  steel  will 
increase  about  five-fold  and  that  electrical  power  production  will 
increase  another  ten-fold.  Our  total  energy  demands  will  probably 
increase  four-fold,  corresponding  to  a  doubling  of  energy  consump- 
tion per  person.  Even  on  a  per  capita  basis,  our  raw-material  de- 
mands are  destined  to  increase  considerably  in  the  decades  ahead. 
When  we  couple  this  with  the  expected  population  growth,  it  is  clear 
that  our  raw-material  demands  fifty  years  from  now  will  dwarf  those 

of  today. 

Enormous  quantities  of  materials  are  required  to  support  an  indi- 


229 


vidual  in  the  United  States.  We  produce  each  year,  for  each  person, 
about  1,300  pounds  of  steel,  23  pounds  of  copper  and  16  pounds  of 
lead,  in  addition  to  considerable  quantities  of  other  metals.  Our 
demands  for  nonmetals  are  even  more  impressive.  These  quantities 
wiU  almost  certainly  increase  considerably  in  the  decades  ahead. 

In  addition  to  the  materials  consumed,  the  quantities  of  materials 
which  must  be  in  existence  in  order  to  support  an  individual  have 
increased  steadily.  For  every  person  in  the  United  States  there  are 
probably  in  existence,  together  with  other  metals,  about  9  tons  of 
steel,  over  300  pounds  of  copper,  about  100  pounds  of  lead,  and 
about  200  pounds  of  zinc.  It  seems  clear  that  these  quantities  of 
materials  in  use  will  continue  to  rise.  One  can  expect  that  by  the  turn 
of  the  century  the  figure  for  steel  wiU  increase  to  about  15  tons.  In  the 
first  place,  the  quantities  of  things  which  people  are  wiUing  to  buy 
has  not  as  yet  reached  the  saturation  level.  Second,  we  must  work 
ever  harder  in  order  to  obtain  the  raw  materials  we  need.  Having 
used  up  the  easily  accessible  ore  deposits,  we  require  a  great  deal 
more  technology,  more  equipment,  more  steel,  and  greater  energy 
expenditure  to  produce  a  pound  of  metal  today  than  was  required 
in  1900. 

It  seems  plausible  that  by  the  turn  of  the  century  steel  production 
in  the  United  States  will  exceed  400  million  tons  annually.  Increasing 
demands  for  metals  will  bring  about  increasing  demands  for  metallic 
ores.  As  demands  increase  and  as  the  grades  of  domestic  ores  de- 
crease, it  will  become  more  diflBcult  for  us  to  find  supphes  of  raw 
materials  to  keep  our  industrial  network  functioning.  Increasing 
quantities  of  these  materials  such  as  iron  ore,  bauxite,  copper  ore,  and 
petroleum  must  come  from  abroad.  By  1980,  the  United  States  may 
well  be  one  of  the  poorest  nations  in  the  world  with  respect  to  high- 
grade  raw  materials.  For  the  United  States,  therefore,  the  next  fifty 
years  will  be  characterized  by  a  growing  dependence  of  the  United 
States  upon  the  natural  resources  of  other  major  areas  of  the  world. 
Of  course,  as  industrialization  spreads  to  other  areas,  competition  for 
the  earth's  resources  will  increase  dramatically. 

Eventually  high-grade  resources  are  destined  to  disappear  from 
the  earth.  Decreasing  grades  of  ores  will  be  compensated  for  by 
increasing  energy  consumption.  When  that  time  arrives,  industrial 
civihzation  will  feed  upon  the  leanest  of  raw  materials— sea  water,  air, 
ordinary  rock,  sedimentary  deposits  such  as  limestones  and  phosphate 
rock,  and  sunlight. 

As  grades  of  ore  diminish,  industries  will  become  more  complex 
and  highly  integrated.  It  seems  likely  that  we  will  eventually  reach 


230 


Tasks  for  a  World  Without  War 


the  point  where  we  shall  have  vast  assemblages  of  plants,  particularly 
in  coastal  regions,  where  rock  is  quarried,  uranium  and  other  metals 
are  isolated,  nitric  acid  is  manufactured,  atomic  power  is  generated, 
hydrogen  is  produced,  iron  ores  are  reduced  to  pig  iron,  aluminum 
and  magnesium  metals  are  prepared,  and  vast  quantities  of  liquid 
fuels  and  organic  chemicals  are  manufactured.  The  single-purpose 
plant  is  likely  to  diminish  in  importance,  and  eventually  to  disappear. 
When  this  time  is  reached,  most  of  the  major  industrial  areas  of  the 
world  will  find  it  easier  to  gain  their  sustenance  by  applying  science 
and  technology  to  the  task  of  processing  domestic,  low-grade  sub- 
stances than  to  look  abroad.  But  before  that  time  is  reached,  we  will 
pass  through  a  period  of  increasing  dependence  upon  imports.  As 
population  increases,  as  new  cities  emerge  and  old  ones  merge,  there 
will  be  increased  crowding  and  a  multiplication  of  the  problems 
which  have  long  been  characteristic  of  highly  urbanized  areas.  The 
basic  domestic  problems  in  the  United  States  will  be  those  of  a 
densely  populated  industrial  nation  in  which  tlie  metropolitan  area 
is  the  basic  unit.  Regional  differences  in  population  patterns  will 
disappear. 

Properly  planned  and  financed,  the  new  urban  areas  could  be 
pleasant  places  in  which  to  hve.  Unplanned,  and  in  the  absence  of 
adequate  pubhc  funds  for  public  facilities  and  services,  a  vast  nation- 
wide slum  could  emerge  in  a  relatively  short  time.  Indeed  our  politi- 
cal-social-economic situation  a  few  decades  from  now  wiU  depend 
in  large  part  upon  our  attitudes  toward  the  expenditure  of  public 
funds,  toward  long-range  planning,  and  toward  the  powers  of  the 
various  levels  of  local,  state,  and  federal  government. 

The  increasing  technological  and  sociological  complexity  of  our 
society  will  result  in  the  need  for  higher  levels  of  education.  At  the 
turn  of  the  century,  more  than  one  out  of  every  three  workers  were 
unskilled.  By  1950  only  one  in  five  workers  remained  unskilled.  By 
contrast,  our  need  for  professional  workers  has  increased  five-fold  in 
the  last  half  century.  Even  more  important,  our  need  for  profes- 
sional workers  is  still  increasing  rapidly  and  seems  destined  to  in- 
crease at  least  another  five-fold  in  the  next  fifty  years.  Scientists  and 
engineers  alone  have  increased  ten-fold  in  number  in  the  last  half 

centiuy. 

The  process  of  automation  will  result  in  a  considerable  dislocation 
of  labor  in  certain  industries  and  in  certain  localities.  The  higher  pro- 
ductivity which  will  result,  reaching  perhaps  four  times  that  of  the 
present  level  within  50  years,  will  give  rise  to  several  major  prob- 
lems. Will  this  result  in  higher  total  production  or  in  more  leisure? 


231 


If  the  end  result  is  higher  production,  to  whom  will  the  goods  be  sold? 
Can  they  be  absorbed  domestically  or  will  they  be  sold  abroad?  If  the 
end  result  is  more  leisure,  how  will  the  hours  of  work  and  the  wages 
be  divided?  And  how  will  people  spend  their  leisure  time?  The 
answers  to  these  questions  will  depend  in  part  upon  the  decisions 
which  are  made  in  the  next  decade  concerning  many  aspects  of  for- 
eign policy  as  well  as  domestic  policy. 

The  Upsurge  of  Population 

The  population  of  the  world  is  increasing  rapidly.  Even  more 
important,  however,  is  the  fact  that  the  rate  of  population  growth  is 
increasing  rapidly  as  well.  Between  1850  and  1900  the  world  popula- 
tion grew  at  a  rate  of  about  0.7  percent  per  year.  During  the  following 
half  century,  the  average  annual  rate  of  increase  was  0.9  percent  per 
year.  Between  1950  and  1956  the  annual  rate  of  increase  averaged 
1.6  percent.  This  remarkable  increase  in  the  rate  of  population 
growth  has  resulted  primarily  from  rapidly  lowered  death  rates. 

We  do  not  have  to  look  far  to  find  the  reasons  for  the  rapid  decHne 
in  mortality  in  the  underdeveloped  areas.  It  is  now  possible  to  treat 
many  of  the  diseases  which  are  widespread  in  these  areas  on  a  mass 
basis,  and  control  can  be  achieved  at  low  cost.  Insecticides  such  as 
DDT,  vaccines  such  as  BCG,  and  antibiotics  such  as  penicillin  are 
some  of  the  developments  which  have  made  control  possible  on 
a  mass  basis.  For  example,  widespread  spraying  of  the  island  of 
Ceylon  with  DDT  resulted  in  a  decrease  of  mortality  by  34  percent 
in  one  year  alone.  As  a  result  of  the  spread  of  such  techniques,  the 
population  of  Costa  Rica  is  growing  at  a  rate  of  3.7  percent  per  year. 
The  rates  in  many  other  areas  are  nearly  as  large:  Mexico,  2.9  percent; 
Ceylon,  2.8  percent;  Puerto  Rico,  2.8  percent— all  compared  with  a 
world  average  of  about  1.6  percent. 

As  industrialization  spreads  to  other  areas  of  the  world  and  as 
techniques  of  birth  control  are  adopted  by  various  cultures,  it  is 
possible  that  birth  rates  will  fall.  If  we  assume,  for  example,  that 
the  rate  of  population  growth  in  the  West  will  fall  to  very  low  levels 
by  1975  ( which  may  be  true  in  Western  Europe  but  which  almost 
certainly  will  not  be  true  in  North  America),  that  rates  of  growth 
in  Japan,  Eastern  Europe,  and  Oceania  will  fall  to  low  levels  by  the 
turn  of  the  next  century,  that  Africa,  South  Central  Asia,  most  of  Latin 
America  and  China  will  pass  through  the  industrial  transition  in  75 
years,  and  that  a  full  century  will  be  required  for  most  of  the  Near 
East,  then  we  arrive  at  a  world  population  of  close  to  7  billion  before 


232 


Tasks  for  a  World  Without  War 


Stabilization  is  approached.  No  matter  how  optimistic  we  are,  how- 
ever, it  is  diflBcult  to  visualize  a  set  of  circumstances  not  involving 
widespread  catastrophe,  which  can  result  in  the  leveling  oflF  of  world 
population  at  much  less  than  this  figure.  The  earth  may  eventually 
be  called  upon  to  provide  for  a  substantially  higher  population  than 
this. 

The  demographic  changes  which  are  taking  place  in  the  world, 
particularly  in  those  regions  which  are  still  predominantly  agrarian, 
are  resulting  primarily  from  the  application  of  techniques  which  are 
relatively  inexpensive,  require  httle  capital,  and  which  can  be  spread 
v^dthout  educating  large  numbers  of  persons.  The  task  of  controlling 
epidemic  and  endemic  diseases  is  a  relatively  easy  one,  compared 
with  the  task  of  increasing  food  production,  improving  housing,  or 
enlarging  the  over-all  per  capita  availabihty  of  consumer  goods.  The 
latter  necessitates  a  level  of  industrialization  far  above  that  which 
currently  exists  in  these  areas. 

Rates  of  Development 

In  three-quarters  of  the  world,  persons  are  now  living  at  extremely 
low  levels  of  consumption.  We  can  easily  appreciate  the  magnitude 
of  the  task  that  is  involved  in  the  industrial  development  of  these 
areas  when  we  examine  the  huge  quantities  of  materials  which  would 
be  required.  If  all  persons  in  the  world  were  suddenly  brought  up  to 
the  level  of  living  now  enjoyed  by  the  people  of  the  United  States, 
we  would  have  to  extract  from  the  earth  about  18  billion  tons  of  iron, 
300  million  tons  of  copper,  an  equal  amount  of  lead  and  over  200 
million  tons  of  zinc.  These  totals  are  well  over  100  times  the  world's 
present  annual  rate  of  production.  In  order  to  power  this  newly 
industrialized  society,  energy  would  have  to  be  produced  at  a  rate 
equivalent  to  the  burning  of  about  16  billion  tons  of  coal  per  year— 
a  rate  roughly  10  times  larger  than  the  present  one. 

Such  a  transformation  obviously  will  take  time.  It  is  important, 
then,  that  we  inquire  into  the  rates  at  which  industrial  growth  might 
take  place  in  the  future.  It  is  convenient  to  use  as  a  measure  the 
growth  of  the  iron  and  steel  industry,  which  is  the  backbone  of  mod- 
em industrial  civilization.  Annual  steel  production,  which  ranges 
from  9  pounds  per  person  in  India  to  about  1,300  pounds  per  person  in 
the  United  States,  provides  one  of  the  best  indicators  of  the  industrial 
development  of  a  country. 

In  the  past  such  growth  has  characteristically  followed  the  law 
of  compound  interest,  and  we  can  thus  speak  in  terms  of  a  "doubling 


233 


time"— the  time  required  to  double  production  capacity.  In  the  early 
stages  of  expansion  of  the  steel  industry  in  the  United  States,  in  Japan, 
and  in  the  Soviet  Union,  doubling  times  varied  from  five  to  eight 
years.  The  more  rapid  rate  appears  to  be  characteristic  of  what  is 
now  possible  with  proper  application  of  modern  technology.  Indeed, 
it  appears  that  since  1953  China  has  expanded  her  steel  industry  with 
a  doubling  time  of  less  than  five  years. 

Food  production,  which  is  linked  with  the  production  of  steel, 
can  be  increased  in  two  ways:  by  increasing  the  amount  of  food  pro- 
duced per  acre  and  by  increasing  the  numbers  of  acres  cultivated. 
Additional  increases  in  the  amounts  of  food  available  to  human 
beings  can  be  obtained  by  decreasing  the  quantities  of  plant  materials 
fed  to  domestic  animals. 

The  amount  of  food  produced  on  a  given  area  of  land  depends, 
of  course,  upon  the  soil  and  upon  climatic  conditions.  In  addition, 
it  depends  upon  the  extent  to  which  technology  is  applied  to  the 
problem  of  producing  more  food.  When  we  look  about  the  world 
we  see  that  there  are  large  variations  in  the  amounts  of  food  pro- 
duced per  cultivated  acre.  Food  with  an  energy  content  of  about 
13,000  calories  is  produced  on  an  average  acre  in  Japan  each  day. 
The  corresponding  yield  in  Western  Europe  is  7,500  calories.  The 
yield  in  India  is  about  2,500  calories.  These  differences  do  not  result 
primarily  from  differences  of  soil  fertility  or  of  climatic  conditions. 
Rather,  they  are  reflections  of  the  extent  to  which  modem  agricultiural 
knowledge  is  applied  specifically  to  the  attainment  of  high  yields. 

By  the  proper  appHcation  of  technology,  the  agricultural  areas  of 
the  world  can  probably  be  increased  from  the  present  2,400  milhon 
acres  to  about  3,500  million  acres.  However,  very  Httle  of  this  poten- 
tial cropland  is  in  Asia.  Cultivated  land  area  in  Asia  can  probably 
not  be  increased  by  more  than  25  percent. 

By  far  the  greatest  potential  for  increased  food  production  is  in 
those  areas  where  reclaimed  sea  water  can  eventually  be  used.  Today, 
reclaimed  sea  water  is  too  expensive  to  be  practicable,  but,  as  the 
pressures  upon  the  land  increase  and  as  our  technology  improves, 
we  will  reach  the  time  when  fresh  water  from  the  sea  will  be  used  to 
irrigate  large  areas  of  the  world. 

But  there  is  reason  to  expect  their  development  to  take  a  long  time. 
In  selected  basic  industries  production  can  be  doubled  every  few 
years  because  the  construction  of  factories  does  not  necessitate  the 
concerted  action  of  entire  populations.  A  steel  plant  or  a  fertilizer 
factory  can  be  built  by  relatively  few  persons.  By  contrast,  the  time 
scale  for  changes  which  involve  large  segments  of  a  population  has 


234 


Tasks  for  a  World  Without  War 


in  the  past  been  relatively  long.  The  spread  of  modem  agricultural 
techniques  has  been  slow,  in  part  because  so  many  persons  must  be 
educated.  Even  with  the  appHcation  of  tremendous  eflFort,  it  has  not 
been  possible  in  the  past  to  achieve  a  sustained  increase  of  agricul- 
tural production  of  more  than  about  4  percent  per  year. 

The  Challenge 

Next  to  the  abolition  of  war,  the  industriahzation  of  the  under- 
developed areas  of  the  world  is  perhaps  the  most  formidable  task  con- 
fronting mankind  today.  Indeed,  these  two  problems  cannot  be 
divorced  from  each  other.  Imphcit  in  any  discussion  of  the  abohtion 
of  war  is  the  assumption  that  steps  will  be  taken  to  ensure  that  depri- 
vation is  eliminated  in  these  areas. 

A  large  fraction  of  the  world's  population  is  now  starving,  but 
there  appear  to  be  no  technological  barriers  to  the  feeding  of  a 
stable  world  population  several  times  the  present  size.  Although  the 
world  population  is  increasing  rapidly,  population  growth  can  in 
principle  be  stopped.  Our  high-grade  resources  are  disappearing, 
but,  given  an  adequate  energy  supply,  we  can  hve  comfortably  on 
low-grade  resources.  Nuclear  and  other  sources  of  energy  appear 
to  be  adequate  for  miUions  of  years.  Indeed,  it  is  amply  clear  that 
man  can,  if  he  wills  it,  create  a  world  in  which  human  beings  can  live 
comfortably  and  in  peace  with  one  another. 

A  major  obstacle  for  most  countries  is  accumulation  of  suflBcient 
capital  to  permit  industrialization  to  progress  at  a  pace  commensurate 
with  the  needs.  In  many  areas  agricultural  products  are  now  being 
traded  with  industrialized  countries.  In  some  areas  nonagricultural 
resources  can  be  traded.  If  the  funds  received  are  expended  wisely 
on  projects  of  industrial  development,  sohd  foundations  for  further 
industrialization  can  be  created.  But  many  regions  are  not  blessed 
with  adequate  resources  either  to  feed  themselves  or  to  provide  for 
their  own  internal  industrial  development,  let  alone  their  capacity  to 
accumulate  capital. 

Without  major  help  from  the  outside,  it  is  unlikely  that  the  under- 
developed nations  can  industrialize  sufiBciently  rapidly  to  eliminate 
deprivation.  Here  lies  perhaps  the  most  basic  challenge  for  a  world 
which  hopes  to  develop  into  an  era  beyond  war.  To  what  extent  can 
the  presently  industrialized  nations  of  the  world  jointly  attack  this 
problem  on  a  massive  scale? 

There  is  an  ample  production  capacity  in  the  Western  world  to 
permit  rapid  world-wide  development,  were  that  capacity  used 


235 


wisely.  The  eflFort  which  now  goes  into  the  production  of  the  tools 
of  war  would  greatly  accelerate  rates  of  industrialization,  were  it 
transferred  to  the  production  of  the  tools  of  peace.  Great  increases  in 
production  capacity  can  be  forthcoming  as  the  result  of  automation, 
and,  associated  with  it,  increased  productivity  and  decreased  capital 
investment  per  unit  of  output.  Moreover,  one  of  the  major  problems 
faced  by  the  democratic-capitalistic-industriahzed  nations  is  that  of 
stabiHzing  the  industrial  sectors  of  their  economies;  a  cooperative 
eflfort  aimed  at  world-wide  industriahzation  may  act  as  a  strong 
stabilizing  force. 

If  concerted  efforts  aimed  at  world-wide  industrial  development 
are  not  made,  it  seems  likely  that  totalitarianism  will  spread  rapidly. 
China  is  already  highly  regimented  and  millions  of  Asians  are  im- 
pressed by  her  economic  progress.  We  should  not  be  surprised  were 
India  to  attempt  at  some  future  time  to  emulate  China.  The  pressures 
of  eking  out  an  existence  may  soon  force  Japan  to  return  to  the 
totalitarian  fold.  Furthermore,  with  modern  techniques  of  control 
and  persuasion,  this  process  may  become  irreversible. 

We  know  this  to  be  a  fact:  it  is  not  the  lack  of  technical  knowledge 
or  of  knowledge  of  the  earth's  resources  that  are  the  major  barriers 
to  the  evolution  of  a  world  in  which  all  individuals  have  the  oppor- 
tunity of  leading  free  and  abundant  hves.  The  primary  hindrance  is 
man's  apparent  inability  to  devise  those  social  and  poUtical  institu- 
tions which  can  enable  us  to  apply  our  technical  knowledge  at  the 
rapid  pace  the  situation  demands.  Here,  no  doubt,  lies  the  greatest 
challenge  of  a  future  without  war. 


236 


A  personal  statement,  by  a  noted  Polish  theoretical 
physicist,  shows  his  excitement  with  his  work  and  with 
science. 


23  One  Scientist  and  his  View  of  Science 

Leopold  Infeld 

Excerpt  from  his  book.  Quest,  published  in  1941. 

I  belong  to  the  great  family  of  scientists.  Each  of  us  knows 
that  curious  state  of  excitement  during  which  nothing  in  life 
seems  important  but  the  problem  on  which  we  are  working. 
The  whole  world  becomes  unreal  and  all  our  thoughts  spin 
madly  around  the  subjects  of  research.  To  the  outsider  we  may 
look  like  idle  creatures,  lying  comfortably  about,  but  we  well 
know  that  it  is  an  exacting  and  tiring  task  that  we  perform.  We 
may  seem  ridiculous  when  we  fill  sheets  of  paper  with  formulae 
and  equations  or  when  we  use  a  strange  language  in  our  dis- 
cussions, composed  of  words  understandable  only  to  the  initi- 
ated. We  may  look  for  weeks  or  months  or  years  for  the  right 
way  to  prove  a  theorem  or  perform  an  experiment,  trying  dif- 
ferent pathways,  wandering  through  darkness,  knowing  all  the 
time  that  there  must  be  a  broad  and  comfortable  highway  lead- 
ing to  our  goal.  But  man  has  little  chance  of  finding  it.  We  ex- 
perience the  ecstasy  of  discovery  in  very  rare  moments,  divided 
from  each  other  by  long  intervals  of  doubt,  of  painful  and  at- 
tractive research. 

We  know  these  emotions  so  well  that  we  hardly  ever  talk 
about  them.  And  it  does  not  even  matter  whether  or  not  the 
problems  on  which  we  work  are  important.  Each  of  us  experi- 
ences these  emotions  whether  he  is  Einstein  or  a  student  who,  on 
his  first  piece  of  research,  learns  the  taste  of  suffering,  disap- 
pointment and  joy. 

This  knowledge  binds  us  together.  We  enjoy  long  scientific 
talks  which  would  seem  to  an  outsider  a  torture  hard  to  endure. 
Even  if  we  work  in  similar  fields  we  usually  have  different  views, 


237 


and  we  may  stimulate  each  other  by  violent  discussions.  Every 
field  of  research  is  so  specialized  that  often  two  mathematicians 
or  two  theoretical  physicists  fail  to  understand  each  others' 
problems  and  methods.  But  even  then  they  may  feel  the  bonds 
created  by  research  though  they  may  gossip  mostly  about  their 
colleagues,  jobs  and  university  life. 

There  is  a  level  below  which  our  talks  seldom  sink.  I  have 
never  heard  among  scientists  the  discussion  of  a  frequent  topic: 
"Is  science  responsible  for  wars?"  We  know,  perhaps  too  well, 
how  to  avoid  glittering  generalities.  For  us  Galileo's  law  is  that 
of  a  falling  stone  for  which  we  may  substitute  in  our  imagination 
a  simple  formula,  but  never  a  picture  of  a  bomb  dropped  from 
an  airplane,  carrying  destruction  and  death.  To  us  a  knife  or  a 
wheel  is  a  great  discovery  which  made  the  cutting  of  bread  or 
the  transportation  of  food  easy,  but  we  know  too  well  that  it 
is  not  our  responsibility  if  the  same  discoveries  have  been  ap- 
plied to  cutting  human  throats  or  manufacturing  tanks.  It  is  not 
the  knife  which  kills.  It  is  not  even  the  hand  which  kills.  It  is  the 
radiating  source  of  hate  which  raises  the  armed  hand  and  makes 
the  tanks  roll.  We  know  all  that. 

The  family  feeling  among  us  dissipates  and  vanishes,  however, 
once  we  leave  scientific  problems.  We  have  our  prejudices,  our 
different  social  views,  our  different  ethical  standards.  We  are 
not  angels.  There  are  men  among  us,  like  Rupp  in  Germany, 
who  have  faked  experiments;  well-known  physicists,  like  Lenard 
and  Stark,  who  supported  Hitler  even  before  he  came  to  power; 
mathematicians  like  Bieberbach,  who  distinguish  between  Aryan 
and  Jewish  mathematics;  and  aloof,  kind,  gentle  and  progressive 
men  like  Einstein,  Bohr  and  Dirac. 

Scientists  must  employ  logic,  criticism,  imagination  in  their 
research.  As  a  relief,  their  brains  relax  as  soon  as  they  leave  the 
domain  of  science.  It  is  almost  as  though  logic  and  good  reason- 
ing were  too  precious  gifts  to  be  employed  outside  scientific 
work. 

My  generalizations  are  worth  as  much  as  all  generalizations  of 
this  kind.  They  are  gained  by  my  own  experience,  from  my 
contacts  with  scientists,  from  my  own  observation.  They  do  not 


238 


One  Scientist  and  his  View  of  Science 


refer  to  individuals,  but  I  believe  they  are  valid  when  applied  to 
a  majority  of  scientists. 

These  scientists  are  the  product  of  their  environment.  They 
have  not  felt  the  impact  of  hfe.  They  would  like  to  remain  for- 
ever on  their  peaceful  island,  nursing  the  belief  that  no  storm 
can  reach  their  shores.  They  were  brought  up  in  a  comfortable 
feeling  of  security  and  hope  to  retain  it  by  closing  their  eyes  to 
the  struggle  of  the  outside  world.  They  have  not  strengthened 
the  forces  of  reaction,  but  they  have  not  fought  them.  Indiffer- 
ence has  been  their  sin.  They  belong  to  those  in  Dante's  Inferno 

.  .  .  .who  have  their  life  pass'd  through 
If  without  infamy  yet  without  praise; 
And  here  they  mingle  with  that  caitiff  crew 
Of  angels  who,  though  not  rebellious,  were 
Through  neutral  selfishness  to  God  untrue. 

Slowly,  very  slowly,  through  years  of  bitter  experience,  some 
of  us  have  discovered  our  tragic  mistake.  We  cannot  keep  our 
eyes  closed.  It  is  not  only  the  problem  of  the  outside  world 
which  disturbs  our  sleep.  We  can  no  longer  pretend  that  nothing 
has  happened  or  that  what  has  happened  is  not  our  concern.  The 
storm  comes  too  close  to  our  shores.  The  waves  have  washed 
away  many  of  us  and  destroyed  some  of  the  best  laboratories  on 
our  island.  We  look  with  astonishment  at  a  world  which  we 
never  wanted  to  shape,  trying  to  understand  the  forces  of  sudden 
and  unforeseen  destruction. 

The  individual  is  no  concern  of  nature.  My  story  would  be 
irrelevant  if  it  were  my  story  only.  But  it  is  not.  I  belong  to 
the  generation  of  scientists  who  were  forced  to  view  the  world 
outside  their  island,  who  had  to  learn  to  ask:  "What  are  the 
forces  which  try  to  destroy  science?  How  can  we  save  our  king- 
dom? How  can  we  by  our  own  efforts  prevent  or  delay  the  de- 
cline of  the  world  in  which  we  live?" 

We  are  not  fighters;  we  care  little  for  power;  no  great  politi- 
cal leader  has  ever  arisen  from  om*  circle.  Not  one  who  has  tasted 
research  would  exchange  it  for  power.  We  are  trained  in  too 
many  doubts  to  employ  force  and  to  express  unconditional  be- 
lief. But  in  the  fight  against  destruction  our  words  and  thoughts 


239 


may  count.  We  shall  have  to  learn  the  use  of  words  which  will 
be  understood,  we  shall  have  to  sharpen  ovu*  thoughts  on  prob- 
lems which  we  have  ignored  before. 

The  scientist  tries  to  understand  the  origin  of  our  solar  system, 
the  structure  of  the  universe  and  the  laws  governing  the  atom. 
He  has  discovered  X  rays,  the  radioactive  substances,  and  he  has 
built  cyclotrons.  He  has  foreseen  the  existence  of  electromag- 
netic and  electronic  waves.  Out  of  his  thought  has  grown  the 
technique  of  our  century.  But  not  until  today  has  he  begun  to 
notice  that  the  earth  on  which  he  moves  is  covered  with  sweat 
and  with  blood  and  that  in  the  world  in  which  he  lives  ^Hhe  son 
of  man  has  nowhere  to  lay  his  head" 


240 


Some  of  the  details  In  Feynman's  speech  are  not  simple 
for  beginners  to  follow,  but  his  personal  approach  is 
most  revealing  in  tracing  the  development  of  recent 
scientific  ideas  and  of  styles  of  thought. 


24  The  Development  of  the  Space-Time  View  of  Quantum 
Electrodynamics 

Richard  P.  Feynman 

Nobel  Prize  Lecture,given  in  December  1965. 


We  have  a  habit  in  writing  articles 
published  in  scientific  journals  to  make 
the  work  as  finished  as  possible,  to 
cover  up  all  the  tracks,  to  not  worry 
about  the  blind  alleys  or  to  describe 
how  you  had  the  wrong  idea  first, 
and  so  on.  So  there  isn't  any  place  to 
publish,  in  a  dignified  manner,  what 
you  actually  did  in  order  to  get  to  do 
the  work,  although  there  has  been, 
in  these  days,  some  interest  in  this 
kind  of  thing.  Since  winning  the 
prize  is  a  personal  thing,  I  thought  I 
could  be  excused  in  this  particular 
situation  if  I  were  to  talk  personally 
about  my  relationship  to  quantum 
electrodynamics,  rather  than  to  discuss 
the  subject  itself  in  a  refined  and 
finished  fashion.  Furthermore,  since 
there  are  three  people  who  have  won 
the  prize  in  physics,  if  they  are  all  going 
to  be  talking  about  quantum  electro- 
dynamics itself,  one  might  become 
bored  with  the  subject.  So,  what  I 
would  like  to  tell  you  about  today  are 
the  sequence  of  events,  really  the  se- 
quence of  ideas,  which  occurred,  and 
by  which  I  finally  came  out  the  other 
end  with  an  unsolved  problem  for 
which  I  ultimately  received  a  prize. 

I  realize  that  a  truly  scientific  paper 
would  be  of  greater  value,  but  such 
a  paper  I  could  publish  in  regular 
journals.  So,  I  shall  use  this  Nobel  Lec- 
ture as  an  opportunity  to  do  something 
of  less  value,  but  which  I  cannot  do 
elsewhere.  I  ask  your  indulgence  in 
another  manner.  I  shall  include  details 
of  anecdotes  which  are  of  no  value 
either  scientifically,  nor  for  understand- 
ing the  development  of  ideas.  They  are 


included  only  to  make  the  lecture  more 
entertaining. 

I  worked  on  this  problem  about 
eight  years  until  the  final  publication 
in  1947.  The  beginning  of  the  thing 
was  at  the  Massachusetts  Institute  of 
Technology,  when  I  was  an  undergrad- 
uate student  reading  about  the  known 
physics,  learning  slowly  about  all  these 
things  that  people  were  worrying  about, 
and  realizing  ultimately  that  the  funda- 
mental problem  of  the  day  was  that 
the  quantum  theory  of  electricity  and 
magnetism  was  not  completely  satis- 
factory. This  I  gathered  from  books 
like  those  of  Heitler  and  Dirac.  I  was 
inspired  by  the  remarks  in  these  books; 
not  by  the  parts  in  which  everything 
was  proved  and  demonstrated  careful- 
ly and  calculated,  because  I  couldn't 
understand  those  very  well.  At  that 
young  age  what  I  could  understand 
were  the  remarks  about  the  fact  that 
this  doesn't  make  any  sense,  and  the 
last  sentence  of  the  book  of  Dirac  I 
can  still  remember,  "It  seems  that  some 
essentially  new  physical  ideas  are  here 
needed."  So,  I  had  this  as  a  challenge 
and  an  inspiration.  I  also  had  a  per- 
sonal feeling  that,  since  they  didn't 
get  a  satisfactory  answer  to  the  prob- 
lem I  wanted  to  solve,  I  don't  have 
to  pay  a  lot  of  attention  to  what  they 
did  do. 

I  did  gather  from  my  readings,  how- 
ever, that  two  things  were  the  source 
of  the  difficulties  with  the  quantum 
electrodynamical  theories.  The  first  was 
an  infinite  energy  of  interaction  of  the 
electron  with  itself.  And  this  difficulty 
existed    even    in    the    classical    theory. 


The  other  difficulty  came  from  some 
infinites  which  had  to  do  with  the  in- 
finite number  of  degrees  of  freedom 
in  the  field.  As  I  understood  it  at  the 
time  (as  nearly  as  I  can  remember) 
this  was  simply  the  difficulty  that  if 
you  quantized  the  harmonic  oscillators 
of  the  field  (say  in  a  box)  each  oscil- 
lator has  a  ground  state  energy  of 
1/2  h(o  and  there  is  an  infinite  num- 
ber of  modes  in  a  box  of  every  in- 
creasing frequency  oi,  and  therefore 
there  is  an  infinite  energy  in  the  box. 
I  now  realize  that  that  wasn't  a  com- 
pletely correct  statement  of  the  cen- 
tral problem;  it  can  be  removed  simply 
by  changing  the  zero  from  which 
energy  is  measured.  At  any  rate,  I  be- 
lieved that  the  difficulty  arose  some- 
how from  a  combination  of  the  elec- 
tron acting  on  itself  and  the  infinite 
number  of  degrees  of  freedom  of  the 
field. 

Well,  it  seemed  to  me  quite  evident 
that  the  idea  that  a  particle  acts  on 
itself,  that  the  electrical  force  acts  on 
the  same  particle  that  generates  it,  is 
not  a  necessary  one — it  is  a  sort  of  a 
silly  one,  as  a  matter  of  fact.  And  so 
I  suggested  to  myself  that  electrons 
cannot  act  on  themselves,  they  can  only 
act  on  other  electrons.  That  means 
there  is  no  field  at  all.  You  see,  if  all 
charges  contribute  to  making  a  single 
common  field,  and  if  that  common  field 
acts  back  on  all  the  charges,  then  each 
charge  must  act  back  on  itself.  Well, 
that  was  where  the  mistake  was,  there 
was  no  field.  It  was  just  that  when 
you  shook  one  charge,  another  would 
shake  later.  There  was  a  direct  inter- 
action between  charges,  albeit  with  a 
delay.  The  law  of  force  connecting  the 
motion  of  one  charge  with  another 
would  just  involve  a  delay.  Shake  this 
one,    that   one   shakes    later.    The   sun 

Copyright  ©  1966  by  the  Nobel  Foundation. 


241 


atom  shakes;  my  eye  electron  shakes 
eight  minutes  later,  because  of  a  direct 
interaction  across. 

Now,  this  has  the  attractive  feature 
that  it  solves  both  problems  at  once. 
First,  I  can  say  immediately,  I  don't 
let  the  electron  act  on  itself,  I  just  let 
this  act  on  that,  hence,  no  self-energy! 
Secondly,  there  is  not  an  infinite  num- 
ber of  degrees  of  freedom  in  the  field. 
There  is  no  field  at  all;  or  if  you  in- 
sist on  thinking  in  terms  of  ideas  like 
that  of  a  field,  this  field  is  always  com- 
pletely determined  by  the  action  of  the 
particles  which  produce  it.  You  shake 
this  particle,  it  shakes  that  one,  but 
if  you  want  to  think  in  a  field  way, 
the  field,  if  it's  there,  would  be  entirely 
determined  by  the  matter  which  gen- 
erates it,  and  therefore,  the  field  does 
not  have  any  independent  degrees  of 
freedom  and  the  infinities  from  the  de- 
grees of  freedom  would  then  be  re- 
moved. As  a  matter  of  fact,  when 
we  look  out  anywhere  and  see  light, 
we  can  always  "see"  some  matter  as 
the  source  of  the  light.  We  don't  just 
see  light  (except  recently  some  radio  re- 
ception has  been  found  with  no  ap- 
parent material  source). 

You  see  then  that  my  general  plan 
was  to  first  solve  the  classical  prob- 
lem, to  get  rid  of  the  infinite  self-en- 
ergies in  the  classical  theory,  and  to 
hope  that  when  I  made  a  quantum 
theory  of  it,  everything  would  just  be 
fine. 

That  was  the  beginning,  and  the  idea 
seemed  so  obvious  to  me  and  so  ele- 
gant that  I  fell  deeply  in  love  with  it. 
And,  like  falling  in  love  with  a  wom- 
an, it  is  only  possible  if  you  do  not 
know  much  about  her,  so  you  cannot 
see  her  faults.  The  faults  will  become 
apparent  later,  but  after  the  love  is 
strong  enough  to  hold  you  to  her.  So, 
I  was  held  to  this  theory,  in  spite  of 
all  difl^culties,  by  my  youthful  enthu- 
siasm. 

Then  I  went  to  graduate  school  and 
somewhere  along  the  line  I  learned 
what  was  wrong  with  the  idea  that  an 
electron  does  not  act  on  itself.  When 
you  accelerate  an  electron  it  radiates 
energy  and  you  have  to  do  extra  work 
to  account  for  that  energy.  The  extra 
force  against  which  this  work  is  done 
is  called  the  force  of  radiation  resis- 
tance. The  origin  of  this  extra  force 
was  identified  in  those  days,  following 
Lorentz,  as  the  action  of  the  electron 
itself.  The  first  term  of  this  action,  of 
the  electron  on  itself,  gave  a  kind  of 
inertia   (not   quite   relativistically   satis- 


factory). But  that  inertia-like  term  was 
infinite  for  a  point-charge.  Yet  the 
next  term  in  the  sequence  gave  an  en- 
ergy loss  rate  which  for  a  point-charge 
agrees  exactly  with  the  rate  that  you 
get  by  calculating  how  much  energy  is 
radiated.  So,  the  force  of  radiation  re- 
sistance, which  is  absolutely  neces- 
sary for  the  conservation  of  energy 
would  disappear  if  I  said  that  a  charge 
could  not  act  on  itself. 

So,  I  learned  in  the  interim  when  I 
went  to  graduate   school   the   glaringly 
obvious  fault  of  my  own  theory.  But, 
I  was  still  in  love  with  the  original  the- 
ory, and  was  still  thinking  that  with  it 
lay   the   solution   to   the   difficulties   of 
quantum    electrodynamics.    So,    I    con- 
tinued   to    try   on    and    off   to    save    it 
somehow.  I  must  have  some  action  de- 
velop on  a  given  electron  when  I  accel- 
erate it  to  account  for  radiation  resis- 
tance. But,  if  I  let  electrons  only  act 
on    other   electrons    the    only    possible 
source  for  this  action  is  another  elec- 
tron in  the  world.  So,  one  day,  when 
I   was  working   for  Professor  Wheeler 
and   could   no   longer   solve   the   prob- 
lem  that  he  had  given  me,   I  thought 
about  this  again  and   I  calculated  the 
following.  Suppose  I  have  two  charges 
— I    shake    the    first    charge,    which    I 
think  of  as   a  source   and   this   makes 
the  second  one  shake,  bui  the  second 
one   shaking  produces    an    effect   back 
on   the    source.    And    so,    I    calculated 
how  much  that  effect  back  on  the  first 
charge  was,  hoping  it  might  add  up  to 
the    force    of    radiation    resistance.    It 
didn't  come  out  right,  of  course,  but  I 
went  to  Professor  Wheeler  and  told  him 
my  ideas.  He  said — yes,  but  the  answer 
you  get  for  the  problem  with  the  two 
charges  that  you  just  mentioned  will,  un- 
fortunately,   depend   upon   the   charge, 
and  the  mass  of  the  second  charge  and 
will    vary   inversely    as    the    square    of 
the  distance,  R,   between  the  charges, 
while  the  force  of  radiation  resistance 
depends    on    none    of    these    things.    I 
thought    surely    he    had    computed    it 
himself,  but  now  having  become  a  pro- 
fessor,  I  know  that  one   can   be   wise 
enough     to     see     immediately     what 
some    graduate    student    takes    several 
weeks    to    develop.    He    aJso    pointed 
out   something  that  also  bothered   me, 
that  if  we  had  a  situation  with  many 
charges  all  around  the  original  source 
at  roughly  uniform  density  and  if  we 
added   the   effect  of  all   the   surround- 
ing  charges   the   inverse  R-   would   be 
compensated  by  the  R-  in  the  volume 
element  and  we  would  get  a  result  pro- 


portional to  the  thickness  of  the  layer, 
which  would  go  to  infinity.  That  is,  one 
would  have  an  infinite  total  effect 
back  at  the  source.  And,  finally  he 
said  to  me,  and  you  forgot  something 
else,  when  you  accelerate  the  first 
charge,  the  second  acts  later,  and  then 
the  reaction  back  here  at  the  source 
would  be  still  later.  In  other  words, 
the  action  occurs  at  the  wrong  time. 
I  suddenly  realized  what  a  stupid  fel- 
low I  am,  for  what  I  had  described 
and  calculated  was  just  ordinary  reflect- 
ed light,  not  radiation  reaction. 

But,  as  I  was  stupid,  so  was  Pro- 
fessor Wheeler  that  much  more  clever. 
For  he  then  went  on  to  give  a  lecture 
as  though  he  had  worked  this  all  out 
before  and  was  completely  prepared, 
but  he  had  not,  he  worked  it  out  as  he 
went  along.  First,  he  said,  let  us  sup- 
pose that  the  return  action  by  the 
charges  in  the  absorber  reaches  the 
source  by  advanced  waves  as  well  as 
by  the  ordinary  retarded  waves  of  re- 
flected light,  so  that  the  law  of  interac- 
tion acts  backward  in  time,  as  well  as 
forward  in  time.  I  was  enough  of  a 
physicist  at  that  time  not  to  say,  "Oh, 
no,  how  could  that  be?"  For  today 
all  physicists  know  from  studying 
Einstein  and  Bohr  that  sometimes  an 
idea  which  looks  completely  para- 
doxical at  first,  if  analyzed  to  comple- 
tion in  all  detail  and  in  experimental 
situations,  may,  in  fact,  not  be  para- 
doxical. So,  it  did  not  bother  me  any 
more  than  it  bothered  Professor  Wheel- 
er to  use  advance  waves  for  the  back 
reaction — a  solution  of  Maxwell's 
equations  which  previously  had  not 
been  physically  used. 

Professor  Wheeler  used  advanced 
waves  to  get  the  reaction  back  at 
the  right  time  and  then  he  suggested 
this:  If  there  were  lots  of  electrons  in 
the  absorber,  there  would  be  an  index 
of  refraction  n,  so  the  retarded  waves 
coming  from  the  source  would  have 
their  wavelengths  slightly  modified  in 
going  through  the  absorber.  Now,  if  we 
shall  assume  that  the  advanced  waves 
come  back  from  the  absorber  without 
an  index — why?  I  don't  know,  let's  as- 
sume they  come  back  without  an  in- 
dex— then,  there  will  be  a  gradual 
shifting  in  phase  between  the  return 
and  the  original  signal  so  that  we 
would  only  have  to  figure  that  the  con- 
tributions act  as  if  they  come  from 
only  a  finite  thickness,  that  of  the  first 
wave  zone.  (More  specifically,  up  to 
that  depth  where  the  phase  in  the  me- 
dium is  shifted  appreciably  from  what 


242 


The  Development  of  the  Space-Time  View  of  Quantum 
Electrodynamics 


it  would  be  in  vacuum,  a  thickness  pro- 
portional to  \/{n  —  I.)   Now,  the  less 
the   number  of  electrons   in   here,   the 
less   each   contributes,   but   the   thicker 
will  be  the  layer  that  effectively  con- 
tributes because  with  less  electrons,  the 
index  differs  less  from   1.  The  higher 
the  charges  of  these  electrons,  the  more 
each   contributes,   but   the   thinner   the 
effective  layer,  because  the  index  would 
be  higher.  And  when  we  estimated  it 
(calculated  without  being  careful  to  keep 
the     correct     numerical     factor)     sure 
enough,    it   came    out    that   the    action 
back  at  the  source  was  completely  in- 
dependent   of    the    properties    of    the 
charges   that   were   in   the   surrounding 
absorber.   Further,    it   was   of   just   the 
right   character    to    represent   radiation 
resistance,  but  we  were  unable  to  see 
if  it  was  just  exactly  the  right  size.  He 
sent  me  home  with  orders  to  figure  out 
exactly  how  much  advanced  and  how 
much   retarded   wave   we   need   to    get 
the    thing    to    come    out    numerically 
right,   and   after  that,   figure   out  what 
happens   to   the    advanced   effects   that 
you    would    expect    if   you   put   a   test 
charge  here  close  to  the  source.  For  if 
all  charges  generate  advanced,  as  well 
as    retarded    effects,    why    would    that 
test   not   be   affected   by   the   advanced 
waves  from  the  source? 

I  found  that  you  get  the  right  answer 
if  you  use  half-advanced   and  half-re- 
tarded  as  the  field  generated  by  each 
charge.  That  is,  one  is  to  use  the  solu- 
tion   of    Maxwell's    equation    which    is 
symmetrical  in  time,  and  the  reason  we 
got  no  advanced  effects  at  a  point  close 
to  the  source  in  spite  of  the  fact  that 
the  source  was  producing  an  advanced 
field  is  this.  Suppose  the  source  is  sur- 
rounded by  a  spherical  absorbing  wall 
ten   light   seconds   away,   and   that  the 
test  charge  is  one  second  to  the  right 
of  the  source.   Then  the   source  is   as 
much    as    eleven    seconds    away    from 
some  parts  of  the  wall  and  only  nine 
seconds    away   from    other   parts.   The 
source   acting  at   time   /   =   0  induces 
motions  in  the  wall  at  time  -I-  10.  Ad- 
vanced effects  from  this  can  act  on  the 
test  charge  as  early  as  eleven  seconds 
earlier,    or   at   t   —    —  1.   This   is   just 
at   the   time   that   the   direct   advanced 
waves   from   the    source    should   reach 
the   test  charge,   and   it  turns   out  the 
two  effects  are  exactly  equal   and  op- 
posite   and    cancel    out!    At    the    later 
time    -1-  1    effects    on    the    test    charge 
from   the   source    and   from   the   v^alls 
are  again  equal,  but  this  time  are  of 
the  same  sign  and  add  to  convert  the 


half-retarded  wave  of  the  source  to  full 
retarded   strength. 

Thus,  it  became  clear  that  there  was 
the  possibility  that  if  we  assume  all 
actions  are  via  half-advanced  and  half- 
retarded  solutions  of  Maxwell's  equa- 
tions and  assume  that  all  sources  are 
surrounded  by  material  absorbing  all 
the  light  which  is  emitted,  then  we 
could  account  for  radiation  resistance 
as  a  direct  action  of  the  charges  of  the 
absorber  acting  back  by  advanced  waves 
on  the  source. 

Many  months  were  devoted  to  check- 
ing all  these  points.  I  worked  to  show 
that  everything  is  independent  of  the 
shape  of  the  container,  and  so  on,  that 
the  laws  are  exactly  right,  and  that  the 
advanced  effects  really  cancel  in  every 
case.  We  always  tried  to  increase  the 
efficiency  of  our  demonstrations,  and  to 
see  with  more  and  more  clarity  why  it 
works.  I  won't  bore  you  by  going 
through  the  details  of  this.  Because  of 
our  using  advanced  waves,  we  also  had 
many  apparent  paradoxes,  which  we 
gradually  reduced  one  by  one,  and 
saw  that  there  was  in  fact  no  logical 
difficulty  with  the  theory.  It  was  per- 
fectly satisfactory. 

We  also  found  that  we  could  re- 
formulate this  thing  in  another  way, 
and  that  is  by  principle  of  least  action. 
Since  my  original  plan  was  to  describe 
everything  directly  in  terms  of  particle 
motions,  it  was  my  desire  to  represent 
this  new  theory  without  saying  anything 
about  fields.  It  turned  out  that  we 
found  a  form  for  an  action  directly  in- 
volving the  motions  of  the  charges  only, 
which  upon  variation  would  give  the 
equations  of  motion  of  these  charges. 
The  expression  for  this  action  A  is 

A  =  ■S,m,j\^X^L'X^')    da  + 

2  eie,  r  \sUij')  X^i'  (a<)  Xfi>  (a,)  da,da, 
tl<      ^^  (1) 


'/2 


where 

/..—[ATM'CaO-A-M'Ca/)]  [Xy.\a,)-X^'{a,)] 
where  X,x'  (flj)  is  the  four- vector  posi- 
tion of  the  Jth  particle  as  a  function 
of  some  parameter  Oj,  Xfji^(a^  is 
dA^/iKfl.O/dfli.  The  first  term  is  the 
integral  of  proper  time,  the  ordinary 
action  of  relativistic  mechanics  of  free 
particles  of  mass  m^.  (We  sum  in  the 
usual  way  on  the  repeated  index  fi.) 
The  second  term  represents  the  elec- 
trical interaction  of  the  charges.  It  is 
summed  over  each  pair  of  charges  (the 
factor  V2  is  to  count  each  pair  once, 
the  term  /  =  /  is  omitted  to  avoid  self- 


action).  The  interaction  is  a  double  in- 
tegral over  a  delta  function  of  the 
square  of  space  time  interval  P  be- 
tween two  points  on  the  paths.  Thus, 
interaction  occurs  only  when  this  in- 
terval vanishes,  that  is,  along  light 
cones. 

The  fact  that  the  interaction  is  ex- 
actly one-half  advanced  and  half-re- 
tarded meant  that  we  could  write  such 
a  principle  of  least  action,  whereas  in- 
teraction via  retarded  waves  alone  can- 
not be  written  in  such  a  way. 

So,  all  of  classical  electrodynamics 
was  contained  in  this  very  simple 
form.  It  looked  good,  and  therefore, 
it  was  undoubtedly  true,  at  least  to  the 
beginner.  It  automatically  gave  half-ad- 
vanced and  half-retarded  effects  and 
it  was  without  fields.  By  omitting  the 
term  in  the  sum  when  /  =  /,  I  omit 
self-interaction  and  no  longer  have  any 
infinite  self-energy.  This  then  was  the 
hoped-for  solution  to  the  problem  of 
ridding  classical  electrodynamics  of  the 
infinities. 

It  turns  out,  of  course,  that  you  can 
reinstate  fields  if  you  wish  to,  but  you 
have  to  keep  track  of  the  field  pro- 
duced by  each  particle  separately.  This 
is  because  to  find  the  right  field  to 
act  on  a  given  particle,  you  must  ex- 
clude the  field  that  it  creates  itself.  A 
single  universal  field  to  which  all  con- 
tribute will  not  do.  This  idea  had 
been  suggested  earlier  by  Frenkel  and 
so  we  called  these  Frenkel  fields.  This 
theory  which  allowed  only  particles  to 
act  on  each  other  was  equivalent  to 
Frenkel's  fields  using  half-advanced 
and  half-retarded  solutions. 

There  were  several  suggestions  for  in- 
teresting modifications  of  electrodynam- 
ics. We  discussed  lots  of  them,  but  I 
shall  report  on  only  one.  It  was  to  re- 
place this  delta  function  in  the  interac- 
tion by  another  function,  say  /(/,/), 
which  is  not  infinitely  sharp.  Instead 
of  having  the  action  occur  only  when 
the  interval  between  the  two  charges 
is  exactly  zero,  we  would  replace  the 
delta  function  of  P  by  a  narrow 
peaked  thing.  Let's  say  that  /(Z)  is  large 
only  near  Z  =  0  width  of  order  a-. 
Interactions  will  now  occur  when  7^— 
R-  is  of  order  a^  roughly  where  T  is 
the  time  difference  and  R  is  the  sepa- 
ration of  the  charges.  This  might  look 
like  it  disagrees  with  experience,  but  if 
a  is  some  small  distance,  like  10-"  cm. 
it  says  that  the  time  delay  T  in  action 
is  roughly  \/{R'^±a^)  or  approximately, 
if  R  is  much  larger  than  a.  T  =  R 
±a^/2R.  This  means  that  the  deviation 


243 


of  time  T  from  the  ideal  theoretical 
time  R  of  Maxwell  gets  smaller  and 
smaller,  the  further  the  pieces  are  apart. 
Therefore,  all  theories  involved  in  an- 
alyzing generators,  motors,  etc. — in 
fact,  all  of  the  tests  of  electrodynamics 
that  were  available  in  Maxwell's  time 
— would  be  adequately  satisfied  if  a 
were  10~i*  cm.  If  R  is  of  the  order  of 
a  centimeter  this  deviation  in  T  is  only 
10~26  part.  So,  it  was  possible,  also, 
to  change  the  theory  in  a  simple  man- 
ner and  to  still  agree  with  all  observa- 
tions of  classical  electrodynamics.  You 
have  no  clue  of  precisely  what  func- 
tion to  put  in  for  /,  but  it  was  an  in- 
teresting possibility  to  keep  in  mind 
when  developing  quantum  electrody- 
namics. 

It  also  occurred  to  us  that  if  we  did 
that  (replace  S  by  /)  we  could  not  re- 
instate the  term  /  =  /  in  the  sum  be- 
cause this  would  now  represent  in  a 
relativistically  invariant  fashion  a  finite 
action  of  a  charge  on  itself.  In  fact,  it 
was  possible  to  prove  that  if  we  did  do 
such  a  thing,  the  main  effect  of  the 
self-action  (for  not  too  rapid  accelera- 
tions) would  be  to  produce  a  modifica- 
tion of  the  mass.  In  fact,  there  need 
be  no  mass  m^  term;  all  the  mechanical 
mass  could  be  electromagnetic  self- 
action.  So,  if  you  would  like,  we  could 
also  have  another  theory  with  a  still 
simpler  expression  for  the  action  A.  In 
expression  1  only  the  second  term  is 
kept,  the  sum  extended  over  all  /  and 
/,  and  some  function  /  replaces  8. 
Such  a  simple  form  could  represent  all 
of  classical  electrodynamics,  which 
aside  from  gravitation  is  essentially  all 
of  classical  physics. 

Although  it  may  sound  confusing, 
I  am  describing  several  different  al- 
ternative theories  at  once.  The  im- 
portant thing  to  note  is  that  at  this 
time  we  had  all  these  in  mind  as  dif- 
ferent possibilities.  There  were  several 
possible  solutions  of  the  difficulty  of 
classical  electrodynamics,  any  one  of 
which  might  serve  as  a  good  starting 
point  to  the  solution  of  the  difficulties 
of  quantum  electrodynamics. 

I  would  also  like  to  emphasize  that 
by  this  time  I  was  becoming  used  to  a 
physical  point  of  view  different  from 
the  more  customary  point  of  view.  In 
the  customary  view,  things  are  dis- 
cussed as  a  function  of  time  in  very 
great  detail.  For  example,  you  have  the 
field  at  this  moment,  a  differential 
equation  gives  you  the  field  at  the 
next  moment  and  so  on — a  method 
which    I    shall    call    the    Hamiltonian 


method,  the  time  differential  method. 
We  have,  instead  (in  1,  say)  a  thing 
that  describes  the  character  of  the  path 
throughout  all  of  space  and  time.  The 
behavior  of  nature  is  determined  by  say- 
ing her  whole  space-time  path  has  a 
certain  character.  For  an  action  like  1 
the  equations  obtained  by  variation 
[of  A'M'(aj)]  are  no  longer  at  all  easy 
to  get  back  into  Hamiltonian  form.  If 
you  wish  to  use  as  variables  only  the 
coordinates  of  particles,  then  you  can 
talk  about  the  property  of  the  paths 
— but  the  path  of  one  particle  at  a 
given  time  is  affected  by  the  path  of 
another  at  a  different  time.  If  you  try 
to  describe,  therefore,  things  differen- 
tially, telling  what  the  present  condi- 
tions of  the  particles  are,  and  how 
these  present  conditions  will  affect  the 
future — you  see,  it  is  impossible  with 
particles  alone,  because  something  the 
particle  did  in  the  past  is  going  to  af- 
fect the  future. 

Therefore,  you  need  a  lot  of  book- 
keeping variables  to  keep  track  of  what 
the  particle  did  in  the  past.  These  are 
called  field  variables.  You  will,  also, 
have  to  tell  what  the  field  is  at  this 
present  moment,  if  you  are  to  be  able 
to  see  later  what  is  going  to  happen. 
From  the  overall  space-time  view  of 
the  least  action  principle,  the  field  dis- 
appears as  nothing  but  bookkeeping 
variables  insisted  on  by  the  Hamilto- 
nian method. 

As  a  by-product  of  this  same  view, 
I  received  a  telephone  call  one  day  at 
the  graduate  college  at  Princeton  from 
Professor  Wheeler,  in  which  he  said, 
"Feynman,  I  know  why  all  electrons 
have  the  same  charge  and  the  same 
mass."  "Why?"  "Because,  they  are  all 
the  same  electron!"  And,  then  he  ex- 
plained on  the  telephone,  "suppose 
that  the  world  lines  which  we  were 
ordinarily  considering  before  in  time 
and  space,  instead  of  only  going  up  in 
time,  were  a  tremendous  knot,  and 
then,  when  we  cut  through  the  knot, 
by  the  plane  corresponding  to  a  fixed 
time,  we  would  see  many,  many  world 
lines  and  that  would  represent  many 
electrons — except  for  one  thing.  If  in 
one  section  this  is  an  ordinary  elec- 
tron world  line,  in  the  section  in  which 
t  reversed  itself  and  is  coming  back 
from  the  future  we  have  the  wrong 
sign  to  the  proper  time — to  the  proper 
four  velocities — and  that's  equivalent  to 
changing  the  sign  of  the  charge,  and, 
therefore,  that  part  of  a  path  would  act 
like  a  positron."  "But,  Professor,"  I 
said,   "there   aren't   as   many   positrons 


as  electrons."  "Well,  maybe  they  are 
hidden  in  the  protons  or  something,"" 
he  said.  I  did  not  take  the  idea  that  all 
the  electrons  were  the  same  one  from 
him  as  seriously  as  I  took  the  obser- 
vation that  positrons  could  simply  be 
represented  as  electrons  going  from  the 
future  to  the  past  in  a  back  section  of 
their  world  lines.  That,  I  stole! 

To  summarize,  when  I  was  done 
with  this,  as  a  physicist  I  had  gained 
two  things.  One,  I  knew  many  different 
ways  of  formulating  classical  electro- 
dynamics, with  many  different  mathe- 
matical forms.  I  got  to  know  how  to 
express  the  subject  every  which  way. 
Second,  I  had  a  point  of  view — the 
overall  space-time  point  of  view — and 
a  disrespect  for  the  Hamiltonian  meth- 
od of  describing  physics. 

I  would  like  to  interrupt  here  to 
make  a  remark.  The  fact  that  electro- 
dynamics can  be  written  in  so  many 
ways — the  differential  equations  of 
Maxwell,  various  minimum  principles 
with  fields,  minimum  principles  without 
fields,  all  different  kinds  of  ways — was 
something  I  knew  but  have  never  un- 
derstood. It  always  seems  odd  to  me 
that  the  fundamental  laws  of  physics, 
when  discovered,  can  appear  in  so 
many  different  forms  that  are  not  ap- 
parently identical  at  first,  but,  with  a 
little  mathematical  fiddling  you  can 
show  the  relationship.  An  example 
of  that  is  the  Schrodinger  equation  and 
the  Heisenberg  formulation  of  quan- 
tum mechanics.  I  don't  know  why  this 
is — it  remains  a  mystery,  but  it  was 
something  I  learned  from  experience. 
There  is  always  another  way  to  say 
the  same  thing  that  doesn't  look  at 
all  like  the  way  you  said  it  before. 
I  don't  know  what  the  reason  for  this 
is.  I  think  it  is  somehow  a  representa- 
tion of  the  simplicity  of  nature.  A 
thing  like  the  inverse  square  law  is  just 
right  to  be  represented  by  the  solu- 
tion of  Poisson's  equation,  which, 
therefore,  is  a  very  different  way  to 
say  the  same  thing  that  doesn't  look  at 
all  like  the  way  you  said  it  before.  I 
don't  know  what  it  means,  that  nature 
chooses  these  curious  forms,  but  may- 
be that  is  a  way  of  defining  simplicity. 
Perhaps  a  thing  is  simple  if  you  can 
describe  it  fully  in  several  different 
ways  without  immediately  knowing 
that  you  are  describing  the  same  thing. 

I  was  now  convinced  that  since 
we  had  solved  the  problem  of  classical 
electrodynamics  (and  completely  in  ac- 
cordance with  my  program  from 
M.I.T.,    with    only    direct    interaction 


244 


The  Development  of  the  Space-Time  View  of  Quantum 
Electrodynamics 


between  particles,  in  a  way  that  made 
fields  unnecessary)  everything  was  defi- 
nitely going  to  be  all  right.  I  was  con- 
vinced that  all  I  had  to  do  was  make 
a  quantum  theory  analogous  to  the 
classical  one  and  everything  would  be 
solved. 

So,  the  problem  is  only  to  make  a 
quantum  theory  which  has  as  its  clas- 
sical analog  this  expression  1.  Now, 
there  is  no  unique  way  to  make  a 
quantum  theory  from  classical  me- 
chanics, although  all  the  textbooks 
make  believe  there  is.  What  they 
would  tell  you  to  do  was  find  the  mo- 
mentum variables  and  replace  them  by 
(fi/i)  (d/dx),  but  I  couldn't  find  a  mo- 
mentum variable,  as  there  wasn't  any. 

The  character  of  quantum  mechan- 
ics of  the  day  was  to  write  things  in 
the  famous  Hamiltonian  way — in  the 
form  of  a  differential  equation,  which 
described  how  the  wave  func- 
tion changes  from  instant  to  instant, 
and  in  terms  of  an  operator,  H.  If  the 
classical  physics  could  be  reduced  to 
a  Hamiltonian  form,  everything  was  all 
right.  Now,  least  action  does  not  im- 
ply a  Hamiltonian  form  if  the  action 
is  a  function  of  anything  more  than 
positions  and  velocities  at  the  same 
moment.  If  the  action  is  of  the  form 
of  the  integral  of  a  function  (usually 
called  the  Lagrangian)  of  the  velocities 
and  positions  at  the  same  time 


S=J  L(x,x)dt 


(2) 


then  you  can  start  with  the 
Lagrangian  and  then  create  a  Hamil- 
tonian and  work  out  the  quantum 
mechanics,  more  or  less  uniquely.  But 
this  expression  1  involves  the  key  vari- 
ables, positions,  at  two  different  times 
and  therefore  it  was  not  obvious  what 
to  do  to  make  the  quantum  mechanical 
analog. 

I  tried — I  would  struggle  in  various 
ways.  One  of  them  was  this.  If  I  had 
harmonic  oscillators  interacting  with  a 
delay  in  time,  I  could  work  out  what 
the  normal  modes  were  and  guess  that 
the  quantum  theory  of  the  normal 
modes  was  the  same  as  for  simple  oscil- 
lators and  kind  of  work  my  way  back 
in  terms  of  the  original  variables.  I  suc- 
ceeded in  doing  that,  but  I  hoped 
then  to  generalize  to  other  than  a  har- 
monic oscillator,  but  I  learned  to  my 
regret  something  which  many  people 
have  learned.  The  harmonic  oscillator 
is  too  simple;  very  often  you  can  work 
out  what  it  should  do  in  quantum 
theory  without  getting  much  of  a  clue 


as  to  how  to  generalize  your  results 
to  other  systems. 

So  that  didn't  help  me  very  much, 
but  when  I  was  struggling  with  this 
problem,  I  went  to  a  beer  party  in  the 
Nassau  Tavern  in  Princeton.  There  was 
a  gentleman,  newly  arrived  from 
Europe  (Herbert  Jehle)  who  came 
and  sat  next  to  me.  Europeans  are 
much  more  serious  than  we  are  in 
America  because  they  think  that  a  good 
place  to  discuss  intellectual  matters  is 
a  beer  party.  So,  he  sat  by  me  and 
asked,  "what  are  you  doing"  and  so  on, 
and  I  said,  "I'm  drinking  beer."  Then 
I  realized  that  he  wanted  to  know  what 
work  I  was  doing  and  I  told  him  I  was 
struggling  with  this  problem,  and  I 
simply  turned  to  him  and  said,  "listen, 
do  you  know  any  way  of  doing  quan- 
tum mechanics,  starting  with  action — 
where  the  action  integral  comes  into 
the  quantum  mechanics?"  "No,"  he 
said,  "but  Dirac  has  a  paper  in  which 
the  Lagrangian,  at  least,  comes  into 
quantum  mechanics.  I  will  show  it  to 
you  tomorrow." 

Next  day  we  went  to  the  Princeton 
Library;  they  have  little  rooms  on  the 
side  to  discuss  things,  and  he  showed 
me  this  paper.  What  Dirac  said  was 
the  following:  There  is  in  quantum  me- 
chanics a  very  important  quantity  which 
carries  the  wave  function  from  one 
time  to  another,  besides  the  differen- 
tial equation  but  equivalent  to  it,  a 
kind  of  a  kernel,  which  we  might  call 
K{x',x),  which  carries  the  wave  func- 
tion ij/ix)  known  at  time  t,  to  the 
wave  function  \p(x')  at  time  t  +  e. 
Dirac  points  out  that  this  function  K 
was  analogous  to  the  quantity  in  clas- 
sical mechanics  that  you  would  calcu- 
late if  you  took  the  exponential  of  U, 
multiplied  by  the  Lagrangian  Lix,  x), 
imagining  that  these  two  positions  x, 
xf  corresponded  to  t  and  r  -|-  «.  In 
other  words, 

K(:^,x)  is  analogous  to 

Professor  Jehle  showed  me  this,  I 
read  it,  he  explained  it  to  me,  and  I 
said,  "what  does  he  mean,  they  are 
analogous;  what  does  that  mean,  ana- 
logousl  What  is  the  use  of  that?"  He 
said,  "you  Americans!  You  always  want 
to  find  a  use  for  everything!"  I  said 
that  I  thought  that  Dirac  must  mean 
that  they  were  equal.  "No,"  he  ex- 
plained, "he  doesn't  mean  they  are 
equal."  "Well,"  I  said,  "let's  see 
what  happens  if  we  make  them  equal." 


So,  I  simply  put  them  equal,  taking 
the  simplest  example  where  the  Lagran- 
gian is  V2  Mx*—V(x)  but  soon  found 
I  had  to  put  a  constant  of  proportion- 
ality A  in,  suitably  adjusted.  When  I 
substituted  Ae^^  for  K  to  get 

^(x',/-f-0=J /lexp  «. 

[^^(r^'''y\^i'>t)dx 

and  just  calculated  things  out  by  Tay- 
lor series  expansion,  out  came  the 
Schrodinger  equation.  So,  I  turned  to 
Professor  Jehle,  not  really  under- 
standing, and  said,  "well,  you  see 
Professor  Dirac  meant  that  they  were 
proportional."  Professor  Jehle's  eyes 
were  bugging  out — he  had  taken  out 
a  little  notebook  and  was  rapidly  copy- 
ing it  down  from  the  blackboard,  and 
said,  "no,  no,  this  is  an  important 
discovery.  You  Americans  are  always 
trying  to  find  out  how  something  can 
be  used.  That's  a  good  way  to  dis- 
cover things!"  So,  I  thought  I  was 
finding  out  what  Dirac  meant,  but,  as 
a  matter  of  fact,  I  had  made  the  dis- 
covery that  what  Dirac  thought  was 
analogous  was,  in  fact,  equal.  I  had 
then,  at  least,  the  connection  between 
the  Lagrangian  and  quantum  me- 
chanics, but  still  with  wave  functions 
and  infinitesimal  times. 

It  must  have  been  a  day  or  so  later, 
when  I  was  lying  in  bed  thinking 
about  these  things,  that  I  imagined  what 
would  happen  if  I  wanted  to  calculate 
the  wave  function  at  a  finite  time  in- 
terval later. 

I  would  put  one  of  these  factors 
e^^  in  here,  and  that  would  give  me 
the  wave  functions  the  liext  moment, 
t  +  c,  and  then  I  could  substitute 
that  back  into  3  to  get  another  factor 
of  e^^  and  get  the  wave  function  the 
next  moment,  /  +  2«,  and  so  on  and 
so  on.  In  that  way  I  found  myself 
thinking  of  a  large  number  of  inte- 
grals, one  after  the  other  in  sequence. 
In  the  integrand  was  the  product  of  the 
exponentials,  which,  of  course,  was 
the  exponential  of  the  sum  of  terms 
like  iL.  Now,  L  is  the  Lagrangian 
and  c  is  like  the  time  interval  dt,  so 
that  if  you  took  a  sum  of  such  terms, 
that's  exactly  like  an  integral.  That's 
like  Riemann's  formula  for  the  inte- 
gral I  Ldfy  you  just  take  the  value  of 
each  point  and  add  them  together.  We 
are  to  take  the  limit  as  e— 0,  of  course. 
Therefore,  the  connection  between  the 
wave  function  of  one  instant  and  the 
wave    function    of    another    instant    a 


245 


finite  time  later  could  be  obtained  by 
an  infinite  number  of  integrals  (be- 
cause t  goes  to  zero,  of  course)  of 
exponential  (iS/h)  where  S  is  the  ac- 
tion expression  2.  At  last,  I  had  suc- 
ceeded in  representing  quantum  me- 
chanics directly  in  terms  of  the  action 
S. 

This  led  later  on  to  the  idea  of  the 
amplitude  for  a  path — that  for  each 
possible  way  that  the  particle  can  go 
from  one  point  to  another  in  space- 
time,  there's  an  amplitude.  That  ampli- 
tude is  e  to  the  i/h  times  the  action 
for  the  path.  Amplitudes  from  vari- 
ous paths  superpose  by  addition.  This 
then  is  another,  a  third,  way  of  de- 
scribing quantum  mechanics,  which 
looks  quite  different  than  that  of  Schro- 
dinger  or  Heisenberg,  but  which  is 
equivalent  to  them. 

Now  immediately  after  making  a 
few  checks  on  this  thing,  what  I  want- 
ed to  do,  of  course,  was  to  substi- 
tute the  action  1  for  the  other,  2. 
The  first  trouble  was  that  I  could 
not  get  the  thing  to  work  with  the  rela- 
tivistic  case  of  spin  one-half.  However, 
although  I  could  deal  with  the  matter 
only  non-relativistically,  I  could  deal 
with  the  light  or  the  photon  interac- 
tions perfectly  well  by  just  putting  the 
interaction  terms  of  1  into  any  action, 
replacing  the  mass  terms  by  the  non- 
relativistic  (Mx'/2)  dt.  When  the  action 
had  a  delay,  as  it  now  had,  and  in- 
volved more  than  one  time,  I  had  to 
lose  the  idea  of  a  wave  function.  That 
is,  I  could  no  longer  describe  the 
program  as,  given  the  amplitude  for  all 
positions  at  a  certain  time,  to  compute 
the  amplitude  at  another  time.  How- 
ever, that  didn't  cause  very  much 
trouble.  It  just  meant  developing  a 
new  idea.  Instead  of  wave  functions 
we  could  talk  about  this:  that  if  a 
source  of  a  certain  kind  emits  a 
particle,  and  a  detector  is  there  to  re- 
ceive it,  we  can  give  the  amplitude  that 
the  source  will  emit  and  the  detector 
receive.  We  do  this  without  specifying 
the  exact  instant  that  the  source  emits 
or  the  exact  instant  that  any  detector 
receives,  without  trying  to  specify  the 
state  of  anything  at  any  particular 
time  in  between,  but  by  just  finding 
the  amplitude  for  the  complete  experi- 
ment. And,  then  we  could  discuss  how 
that  amplitude  would  change  if  you 
had  a  scattering  sample  in  between,  as 
you  rotated  and  changed  angles,  and 
so  on,  without  really  having  any  wave 
functions. 

It  was  also  possible  to  discover  what 
the   old   concepts   of  energy    and   mo- 


mentum would  mean  with  this  general- 
ized action.  And  so  I  believed  that  I 
had  a  quantum  theory  of  classical  elec- 
trodynamics— or  rather  of  this  new 
classical  electrodynamics  described  by 
action  1 .  I  made  a  number  of  checks.  If 
I  took  the  Frenkel  field  point  of  view, 
which  you  remember  was  more  differ- 
ential, I  could  convert  it  directly  to 
quantum  mechanics  in  a  more  con- 
ventional way.  The  only  problem  was 
how  to  specify  in  quantum  mechan- 
ics the  classical  boundary  conditions 
to  use  only  half-advanced  and  half- 
retarded  solutions.  By  some  ingenuity 
in  defining  what  that  meant,  I  found 
that  the  quantum  mechanics  with 
Frenkel  fields,  plus  a  special  boundary 
condition,  gave  me  back  this  action  1, 
in  the  new  form  of  quantum  mechanics 
with  a  delay.  So,  various  things  indi- 
cated that  there  wasn't  any  doubt  I 
had  everything  straightened  out. 

It  was  also  easy  to  guess  how  to 
modify  the  electrodynamics,  if  anybody 
ever  wanted  to  modify  it.  I  just  changed 
the  delta  to  an  /,  just  as  I  would  for 
the  classical  case.  So,  it  was  very  easy, 
a  simple  thing.  To  describe  the  old 
retarded  theory  without  explicit  men- 
tion of  fields  I  would  have  to  write 
probabilities,  not  just  amplitudes.  I 
would  have  to  square  my  amplitudes 
and  that  would  involve  double  path 
integrals  in  which  there  are  two  5's 
and  so  forth.  Yet,  as  I  worked  out 
many  of  these  things  and  studied  dif- 
ferent forms  and  different  boundary 
conditions,  I  got  a  kind  of  funny  feel- 
ing that  things  weren't  exactly  right. 
I  could  not  clearly  identify  the  dif- 
ficulty and  in  one  of  the  short  periods 
during  which  I  imagined  I  had  laid  it 
to  rest,  I  published  a  thesis  and  re- 
ceived my  Ph.D. 

During  the  war,  I  didn't  have  time  to 
work  on  these  things  very  extensively, 
but  wandered  about  on  buses  and  so 
forth,  with  little  pieces  of  paper,  and 
struggled  to  work  on  it  and  discovered 
indeed  that  there  was  something 
wrong,  something  terribly  wrong.  I 
found  that  if  one  generalized  the  ac- 
tion from  the  nice  Lagrangian  forms, 
2,  to  these  forms,  1,  then  the  quantities 
which  I  defined  as  energy,  and  so  on, 
would  be  complex.  The  energy  values 
of  stationary  states  wouldn't  be  real 
and  probabilities  of  events  wouldn't  add 
up  to  100%.  That  is,  if  you  took  the 
probability  that  this  would  happen  and 
that  would  happen — everything  you 
could  think  of  would  happen — it 
would  not  add  up  to  one. 

Another  problem  on  which  I  strug- 


gled very  hard  was  to  represent  rela- 
tivistic  electrons  with  this  new  quan- 
tum mechanics.  I  wanted  to  do  it  a 
unique  and  different  way — and  not  just 
by  copying  the  operators  of  Dirac  into 
some  kind  of  an  expression  and  using 
some  kind  of  Dirac  algebra  instead  of 
ordinary  complex  numbers.  I  was  very 
much  encouraged  by  the  fact  that  in 
one  space  dimension  I  did  find  a  way 
of  giving  an  amplitude  to  every  path 
by  limiting  myself  to  paths  which  only 
went  back  and  forth  at  the  speed  of 
light.  The  amplitude  was  simple  (it)  to 
a  power  equal  to  the  number  of  ve- 
locity reversals  where  I  have  divided 
the  time  into  steps  e  and  I  am  allowed 
to  reverse  velocity  only  at  such  a 
time.  This  gives  (as  «  approaches  zero) 
Dirac's  equation  in  two  dimensions — 
one  dimension  of  space  and  one  of 
time    (i^  =  A/  =  c=l). 

Dirac's  wave  function  has  four  com- 
ponents in  four  dimensions,  but  in  this 
case  it  has  only  two  components,  and 
this  rule  for  the  amplitude  of  a  path 
automatically  generates  the  need  for 
two  components.  Because  if  this  is  the 
formula  for  the  amplitudes  of  path,  it 
will  not  do  you  any  good  to  know 
the  total  amplitude  of  all  paths  which 
come  into  a  given  point  to  find  the 
amplitude  to  reach  the  next  point. 
This  is  because  for  the  next  time,  if  it 
came  in  from  the  rights  there  is  no 
new  factor  /e  if  it  goes  out  to  the 
right,  whereas,  if  it  came  in  from  the 
left  there  was  a  new  factor  U.  So, 
to  continue  this  same  information  for- 
ward to  the  next  moment,  it  was  not 
sufficient  information  to  know  the  total 
amplitude  to  arrive,  but  you  had  to 
know  the  amplitude  to  arrive  from  the 
right  and  the  amplitude  to  arrive  from 
the  left,  independently.  If  you  did, 
however,  you  could  then  compute  both 
of  those  again  independently  and  thus 
you  had  to  carry  two  amplitudes  to 
form  a  differential  equation  (first  order 
in  time). 

And  so  I  dreamed  that  if  I  were 
clever  I  would  find  a  formula  for  the 
amplitude  of  a  path  that  was  beauti- 
ful and  simple  for  three  dimensions  of 
space  and  one  of  time,  which  would 
be  equivalent  to  the  Dirac  equation, 
and  for  which  the  four  components, 
matrices,  and  all  those  other  mathe- 
matical funny  things  would  come  out 
as  a  simple  consequence — I  have  never 
succeeded  in  that  either.  But,  I  did 
want  to  mention  some  of  the  unsuc- 
cessful things  on  which  I  spent  almost 
as  much  effort  as  on  the  things  that 
did  work. 


246 


The  Development  of  the  Space-Time  View  of  Quantum 
Electrodynamics 


To  summarize  the  situation  a  few 
years  after  the  war,  I  would  say  I 
had  much  experience  with  quantum 
electrodynamics,  at  least  in  the 
knowledge  of  many  different  ways 
of  formulating  it,  in  terms  of  path 
integrals  of  actions  and  in  other 
forms.  One  of  the  important  by-prod- 
ucts, for  example,  of  much  experience 
in  these  simple  forms  was  that  it  was 
easy  to  see  how  to  combine  together 
what  were  in  those  days  called  the 
longitudinal  and  transverse  fields,  and 
in  general  to  see  clearly  the  relativistic 
invariance  of  the  theory.  Because  of 
the  need  to  do  things  differentially 
there  had  been,  in  the  standard  quan- 
tum electrodynamics,  a  complete  split 
of  the  field  into  two  parts,  one  which 
is  called  the  longitudinal  part  and  the 
other  mediated  by  the  photons,  or 
transverse  waves.  The  longitudinal  part 
was  described  by  a  Coulomb  potential 
acting  instantaneously  in  the  Schro- 
dinger  equation,  while  the  transverse 
part  had  an  entirely  different  descrip- 
tion in  terms  of  quantization  of  the 
transverse  waves.  This  separation  de- 
pended upon  the  relativistic  tilt  of  your 
axes  in  space-time.  People  moving  at 
different  velocities  would  separate  the 
same  field  into  longitudinal  and  trans- 
verse fields  in  a  different  way.  Further- 
more, the  entire  formulation  of  quan- 
tum mechanics,  insisting,  as  it  did,  on 
the  wave  function  at  a  given  time, 
was  hard  to  analyze  relativistically. 
Somebody  else  in  a  different  coordi- 
nate system  would  calculate  the  suc- 
cession of  events  in  terms  of  wave 
functions  on  differently  cut  slices  of 
space-time  and  with  a  different  sepa- 
ration of  longitudinal  and  transverse 
parts.  The  Hamiltonian  theory  did  not 
look  relativistically  invariant,  although, 
of  course,  it  was.  One  of  the  great 
advantages  of  the  overall  point  of 
view  was  that  you  could  see  the  rel- 
ativistic invariance  right  away— or,  as 
Schwinger  would  say,  the  covariance 
was  manifest.  I  had  the  advantage, 
therefore,  of  having  a  manifestedly  co- 
variant  form  for  quantum  electrody- 
namics with  suggestions  for  modifica- 
tions and  so  on.  I  had  the  disadvantage 
that  if  I  took  it  too  seriously — I  mean, 
if  I  took  it  seriously  at  all  in  this 
form — I  got  into  trouble  with  these 
complex  energies  and  the  failure  of 
adding  probabilities  to  one  and  so  on. 
I  was  unsuccessfully  struggling  with 
that. 

Then  Lamb  did  his  experiment, 
measuring  the  separation  of  the  25) 
and     2Pj     levels     of    hydrogen,     find- 


ing it  to  be  about  1000  megacycles 
of  frequency  difference.  Professor 
Bethe,  with  whom  !  was  then  associated 
at  Cornell,  is  a  man  who  has  this 
characteristic:  If  there's  a  good  exper- 
imental number  you've  got  to  figure 
it  out  from  theory.  So,  he  forced  the 
quantum  electrodynamics  of  the  day 
to  give  him  an  answer  to  the  separa- 
tion of  these  two  levels.  He  pointed 
out  that  the  self-energy  of  an  elec- 
tron itself  is  infinite,  so  that  the  cal- 
culated energy  of  a  bound  electron 
should  also  come  out  infinite.  But, 
when  you  calculated  the  separation  of 
the  two  energy  levels  in  terms  of  the 
corrected  mass  instead  of  the  old 
mass,  it  would  turn  out,  he  thought, 
that  the  theory  would  give  convergent 
finite  answers.  He  made  an  estimate 
of  the  splitting  that  way  and  found 
out  that  it  was  still  divergent,  but  he 
guessed  that  was  probably  due  to  the 
fact  that  he  used  an  unrelativistic 
theory  of  the  matter.  Assuming  it 
would  be  convergent  if  relativistically 
treated,  he  estimated  he  would  get 
about  a  thousand  megacycles  for  the 
Lamb-shift,  and  thus,  made  the  most 
important  discovery  in  the  history  of 
the  theory  of  quantum  electrodynam- 
ics. He  worked  this  out  on  the  train 
from  Ithaca,  New  York,  to  Schenec- 
tady and  telephoned  me  excitedly 
from  Schenectady  to  tell  me  the  re- 
sult, which  I  don't  remember  fully  ap- 
preciating at  the  time. 

Returning  to  Cornell,  he  gave  a 
lecture  on  the  subject,  which  I  at- 
tended. He  explained  that  it  gets 
very  confusing  to  figure  out  exactly 
which  infinite  term  corresponds  to 
what  in  trying  to  make  the  correction 
for  the  infinite  change  in  mass.  If 
there  were  any  modifications  whatever, 
he  said,  even  though  not  physically 
correct  (that  is,  not  necessarily  the  way 
nature  actually  works)  but  any  modi- 
fication whatever  at  high  frequencies, 
which  would  make  this  correction  finite, 
then  there  would  be  no  problem  at 
all  to  figuring  out  how  to  keep  track 
of  everything.  You  just  calculate  the 
finite  mass  correction  Aw  to  the  elec- 
tron mass  Wo,  substitute  the  numerical 
values  of  Wq+Aw  for  w  in  the  results 
for  any  other  problem  and  all  these 
ambiguities  would  be  resolved.  If,  in 
addition,  this  method  were  relativisti- 
cally invariant,  then  we  would  be  ab- 
solutely sure  how  to  do  it  without 
destroying  relativistic  invariance. 

After  the  lecture,  I  went  up  to  him 
and  told  him,  "I  can  do  that  for  you. 
I'll  bring  it  in  for  you   tomorrow."   I 


guess  I  knew  every  way  to  modify 
quantum  electrodynamics  known  to 
man,  at  the  time.  So,  I  went  in  next 
day,  and  explained  what  would  corres- 
pond to  the  modification  of  the  delta- 
function  to  /  and  asked  him  to  ex- 
plain to  me  how  you  calculate  the 
self-energy  of  an  electron,  for  in- 
stance, so  we  can  figure  out  if  it's 
finite. 

I  want  you  to  see  an  interesting 
point.  I  did  not  take  the  advice  of 
Professor  Jehle  to  find  out  how  it  was 
useful.  I  never  used  all  that  machin- 
ery which  I  had  cooked  up  to  solve 
a  single  relativistic  problem.  I  hadn't 
even  calculated  the  self-energy  of  an 
electron  up  to  that  moment,  and  was 
studying  the  difficulties  with  the  con- 
servation of  probability,  and  so  on, 
without  actually  doing  anything,  ex- 
cept discussing  the  general  properties 
of  the  theory. 

But  now  I  went  to  Professor  Bethe, 
who  explained  to  me  on  the  black- 
board, as  we  worked  together,  how  to 
calculate  the  self-energy  of  an  electron. 
Up  to  that  time  when  you  did  the 
integrals  they  had  been  logarithmical- 
ly divergent.  I  told  him  how  to  make 
the  relativistically  invariant  modifica- 
tions that  I  thought  would  make 
everything  all  right.  We  set  up  the  in- 
tegral which  then  diverged  at  the  sixth 
power  of  the  frequency  instead  of 
logarithmically! 

So,  I  went  back  to  my  room  and 
worried  about  this  thing  and  went 
around  in  circles  trying  to  figure  en? 
what  was  wrong  because  I  was  sure 
physically  everything  had  to  come  out 
finite.  I  couldn't  understand  how  it 
came  out  infinite.  I  became  more  and 
more  interested  and  finally  realized  I 
had  to  learn  how  to  make  a  calcula- 
tion. So,  ultimately,  I  taught  myself 
how  to  calculate  the  self-energy  of 
an  electron,  working  my  patient  way 
through  the  terrible  confusion  of  those 
days  of  negative  energy  states  and  holes 
and  longitudinal  contributions  and  so 
on.  When  I  finally  found  out  how  to 
do  it  and  did  it  with  the  modifications 
I  wanted  to  suggest,  it  turned  out 
that  it  was  nicely  convergent  and  finite, 
just  as  I  had  expected.  Professor  Bethe 
and  I  have  never  been  able  to  dis- 
cover what  we  did  wrong  on  that 
blackboard  two  months  before,  but  ap- 
parently we  just  went  off  somewhere 
and  we  have  never  been  able  to 
figure  out  where.  It  turned  out  that 
what  I  had  proposed,  if  we  had  car- 
ried it  out  without  making  a  mistake, 
would  have  been  all  right  and  would 


247 


have  given  a  finite  correction.  Anyway, 
it  forced  me  to  go  back  over  all  this 
and  to  convince  myself  physically  that 
nothing  can  go  wrong.  At  any  rate, 
the  correction  to  mass  was  now  finite, 
proportional  to  ln{ma/h)  where  a  is 
the  width  of  that  function  /  which 
was  substituted  for  8.  If  you  wanted 
an  unmodified  electrodynamics,  you 
would  have  to  take  a  equal  to  zero, 
getting  an  infinite  mass  correction.  But, 
that  wasn't  the  point.  Keeping  a  finite, 
I  simply  followed  the  program  out- 
lined by  Professor  Bethe  and  showed 
how  to  calculate  all  the  various  things 
— the  scatterings  of  electrons  from 
atoms  without  radiation,  the  shifts  of 
levels  and  so  forth — calculating  every- 
thing in  terms  of  the  experimental 
mass,  and  noting  that  the  results,  as 
Bethe  suggested,  were  not  sensitive  to 
a  in  this  form  and  even  had  a  definite 
limit  as  a  -^  0. 

The  rest  of  my  work  was  simply 
to  improve  the  techniques  then  avail- 
able for  calculations,  making  dia- 
grams to  help  analyze  perturbation 
theory  quicker.  Most  of  this  was 
first  worked  out  by  guessing — you 
see,  I  didn't  have  the  relativistic  the- 
ory of  matter.  For  example,  it  seemed 
to  me  obvious  that  the  velocities  in 
non-relativistic  formulas  have  to  be  re- 
placed by  Dirac's  matrix  a  or  in  the 
more  relativistic  forms  by  the  opera- 
tors yM.  I  just  took  my  guesses  from 
the  forms  that  I  had  worked  out  us- 
ing path  integrals  for  non-relativistic 
matter,  but  relativistic  light.  It  was  easy 
to  develop  rules  of  what  to  substitute 
to  get  the  relativistic  case.  I  was  very 
surprised  to  discover  that  it  was  not 
known  at  that  time  that  every  one  of 
the  formulas  that  had  been  worked  out 
so  patiently  by  separating  longitudi- 
nal and  transverse  waves  corld  be  ob- 
tained from  the  formula  for  the  trans- 
verse waves  alone,  if  instead  of  sum- 
ming over  only  the  two  perpendicu- 
lar polarization  directions  you  would 
sum  over  all  four  possible  directions 
of  polarization.  It  was  so  obvious  from 
the  action  1  that  I  thought  it  was 
general  knowledge  and  would  do  it  all 
the  time.  I  would  get  into  arguments 
with  people,  because  I  didn't  realize 
they  didn't  know  that;  but,  it  turned 
out  that  all  their  patient  work  with 
the  longitudinal  waves  was  always 
equivalent  to  just  extending  the  sum 
on  the  two  transverse  directions  of  pol- 
arization over  all  four  directions.  This 
was  one  of  the  amusing  advantages 
of  the  method.  In  addition,  I  included 
diagrams  for  the  various  terms  of  the 


perturbation  series,  improved  nota- 
tions to  be  used,  worked  out  easy  ways 
to  evaluate  integrals,  which  occurred 
in  these  problems,  and  so  on,  and 
made  a  kind  of  handbook  on  how  to 
do  quantum  electrodynamics. 

But  one  step  of  importance  that  was 
physically  new  was  involved  with  the 
negative  energy  sea  of  Dirac,  which 
caused  me  so  much  logical  difficulty. 
I  got  so  confused  that  I  remembered 
Wheeler's  old  idea  about  the  positron 
being,  maybe,  the  electron  going  back- 
ward in  time.  Therefore,  in  the  time- 
dependent  perturbation  theory  that  was 
usual  for  getting  self-energy,  I  simply 
supposed  that  for  a  while  we  could 
go  backward  in  the  time,  and  looked 
at  what  terms  I  got  by  running  the 
time  variables  backward.  They  were 
the  same  as  the  terms  that  other  peo- 
ple got  when  they  did  the  problem  a 
more  complicated  way,  using  holes  in 
the  sea,  except,  possibly,  for  some 
signs.  These  I  at  first  determined  em- 
pirically by  inventing  and  trying  some 
rules. 

I  have  tried  to  explain  that  all  the 
improvements  of  relativisitc  theory 
were  at  first  more  or  less  straight- 
forward, semi-empirical  shenanigans. 
Each  time  I  would  discover  some- 
thing, however,  I  would  go  back  and 
I  would  check  it  so  many  ways,  com- 
pare it  to  every  problem  that  had  been 
done  previously  in  electrodynamics 
(and  later,  in  weak  coupling  meson 
theory)  to  see  if  it  would  always 
agree,  and  so  on,  until  I  was  abso- 
lutely convinced  of  the  truth  of  the 
various  rules  and  regulations  which  I 
concocted  to  simplify  all  the  work. 

During  this  time,  people  had  been 
developing  meson  theory,  a  subject  I 
had  not  studied  in  any  detail.  I  be- 
came interested  in  the  possible  applica- 
tion of  my  methods  to  perturbation 
calculations  in  meson  theory.  But, 
what  was  meson  theory?  All  I  knew 
was  that  meson  theory  was  something 
analogous  to  electrodynamics,  except 
that  particles  corresponding  to  the 
photon  had  a  mass.  It  was  easy  to 
guess  that  the  8-function  in  1,  which 
was  a  solution  of  d'Alembertian  equals 
zero,  was  to  be  changed  to  the  cor- 
responding solution  of  d'Alembertian 
equals  m-.  Next,  there  were  different 
kinds  of  mesons — the  ones  in  closest 
analogy  to  photons,  coupled  via  y^t.y^J., 
are  called  vector  mesons;  there  were 
also  scalar  mesons.  Well,  maybe  that 
corresponds  to  putting  unity  in  place 
of  the  yfi,  perhaps  what  they  called 
"pseudo  vector  coupling,"  and  I  would 


guess  what  that  probably  was.  I  didn't 
have  the  knowledge  to  understand  the 
way  these  were  defined  in  the  conven- 
tional papers  because  they  were  ex- 
pressed at  that  time  in  terms  of  creation 
and  annihilation  operators,  and  so  on, 
which  I  had  not  successfully  learned. 
I  remember  that  when  someone  had 
started  to  teach  me  about  creation  and 
annihilation  operators,  that  this  opera- 
tor creates  an  electron,  I  said,  "how 
do  you  create  an  electron?  It  disagrees 
with  the  conservation  of  charge,"  and 
in  that  way  I  blocked  my  mind  from 
learning  a  very  practical  scheme  of 
calculation.  Therefore,  I  had  to  find 
as  many  opportunities  as  possible  to 
test  whether  I  guessed  right  as  to  what 
the  various  theories  were. 

One  day  a  dispute  arose  at  a  Physi- 
cal Society  meeting  as  to  the  correct- 
ness of  a  calculation  by  Slotnick  of 
the  interaction  of  an  electron  with  a 
neutron,  using  pseudo  scalar  theory 
with  pseudo  vector  coupling  and  also 
pseudo  scalar  theory  with  pseudo  sca- 
lar coupling.  He  had  found  that  the 
answers  were  not  the  same;  in  fact, 
by  one  theory,  the  result  was  diver- 
gent, although  convergent  with  the 
other.  Some  people  believed  that  the 
two  theories  must  give  the  same  an- 
swer for  the  problem.  This  was  a  wel- 
come opportunity  to  test  my  guesses 
as  to  whether  I  really  did  understand 
what  these  two  couplings  were.  So,  I 
went  home,  and  during  the  evening  I 
worked  out  the  electron  neutron  scat- 
tering for  the  pseudo  scalar  and  pseudo 
vector  coupling,  saw  they  were  not 
equal  and  subtracted  them,  and  worked 
out  the  difference  in  detail.  The  next 
day,  at  the  meeting,  I  saw  Slotnick 
and  said,  "Slotnick,  I  worked  it  out 
last  night,  I  wanted  to  see  if  I  got 
the  same  answers  you  do.  I  got  a 
different  answer  for  each  coupling — 
but,  I  would  like  to  check  in  detail 
with  you  because  I  want  to  make 
sure  of  my  methods."  And,  he  said, 
"what  do  you  mean  you  worked  it 
out  last  night,  it  took  me  six  months!" 
And,  when  we  compared  the  answers 
he  looked  at  mine  and  he  asked, 
"what  is  that  Q  in  there,  that  vari- 
able QV  (I  had  expressions  like 
(tan-iQ/Q  etc.).  I  said,  "that's  the  mo- 
mentum transferred  by  the  electron, 
the  electron  deflected  by  different 
angles."  "Oh,"  he  said,  "no,  I  only 
have  the  limiting  value  as  Q  ap- 
proaches zero;  the  forward  scattering." 
Well,  it  was  easy  enough  to  just  sub- 
stitute Q  equals  zero  in  my  form  and 
I    then    got    the   same    answers    as    he 


248 


The  Development  of  the  Space-Time  View  of  Quantum 
Electrodynamics 


did.  But,  it  took  him  six  months  to 
do  the  case  of  zero  momentum  trans- 
fer, whereas,  during  one  evening  I  had 
done  the  finite  and  arbitrary  momen- 
tum transfer.  That  was  a  thrilling  mo- 
ment for  me,  like  receiving  the  Nobel 
Prize,  because  that  convinced  me,  at 
last,  I  did  have  some  kind  of  method 
and  technique  and  understood  how  to 
do  something  that  other  people  did 
not  know  how  to  do.  That  was  my 
moment  of  triumph  in  which  I  rea- 
lized I  really  had  succeeded  in  work- 
ing out  something  worthwhile. 

At  this  stage,  I  was  urged  to  pub- 
lish this  because  everybody  said  it 
looks  like  an  easy  way  to  make  cal- 
culations, and  wanted  to  know  how  to 
do  it.  I  had  to  publish  it,  missing  two 
things;  one  was  proof  of  every  state- 
ment in  a  mathematically  conventional 
sense.  Often,  even  in  a  physicist's 
sense,  I  did  not  have  a  demonstra- 
tion of  how  to  get  all  of  these  rules 
and  equations  from  conventional  elec- 
trodynamics. But,  I  did  know  from 
experience,  from  fooling  around,  that 
everything  was,  in  fact,  equivalent  to 
the  regular  electrodynamics  and  had 
partial  proofs  of  many  pieces,  although 
I  never  really  sat  down,  like  Euclid  did 
for  the  geometers  of  Greece,  and  made 
sure  that  you  could  get  it  all  from  a  sin- 
gle simple  sert  of  axioms.  As  a  result,  the 
work  was  criticized,  I  don't  know 
whether  favorably  or  unfavorably,  and 
the  "method"  was  called  the  "intuitive 
method."  For  those  who  do  not  realize 
it,  however,  I  should  like  to  emphasize 
that  there  is  a  lot  of  work  involved  in 
using  this  "intuitive  method"  successful- 
ly. Because  no  simple  clear  proof  of  the 
formula  or  idea  presents  itself,  it  is 
necessary  to  do  an  unusually  great 
amount  of  checking  and  rechecking 
for  consistency  and  correctness  in 
terms  of  what  is  known,  by  compar- 
ing to  other  analogous  examples,  limit- 
ing cases,  etc.  In  the  face  of  the  lack 
of  direct  mathematical  demonstration, 
one  must  be  careful  and  thorough  to 
make  sure  of  the  point,  and  one 
should  make  a  perpetual  attempt  to 
demonstrate  as  much  of  the  formula 
as  possible.  Nevertheless,  a  very  great 
deal  more  truth  can  become  known 
than  can  be  proven. 

It  must  be  clearly  understood  that 
in  all  this  work  I  was  representing  the 
conventional  electrodynamics  with  re- 
tarded interaction,  and  not  my  half- 
advanced  and  half-retarded  theory  cor- 
responding to  1.  I  merely  use  1  to 
guess  at  forms.  And  one  of  the  forms 
I   guessed   at    corresponded   to    chang- 


ing 8  to  a  function  /  of  width  a-, 
so  that  I  could  calculate  finite  results 
for  all  of  the  problems.  This  brings 
me  to  the  second  thing  that  was  miss- 
ing when  I  published  the  paper,  an 
unresolved  difficulty.  With  8  replaced 
by  /  the  calculations  would  give  re- 
sults which  were  not  "unitary,"  that 
is,  for  which  the  sum  of  the  probabili- 
ties of  all  alternatives  was  not  unity. 
The  deviation  from  unity  was  very 
small,  in  practice,  if  a  was  very  small. 
In  the  limit  that  I  took  a  very  tiny, 
it  might  not  make  any  difference.  And 
so  the  process  of  the  renormalization 
could  be  made,  you  could  calculate 
everything  in  terms  of  the  experimental 
mass  and  then  take  the  limit,  and  the 
apparent  difficulty  that  the  unitary  is 
violated  temporarily  seems  to  disap- 
pear. I  was  unable  to  demonstrate 
that,  as  a  matter  of  fact,  it  does. 

It  is  lucky  that  I  did  not  wait  to 
straighten  out  that  point,  for  as  far 
as  I  know,  nobody  has  yet  been  able 
to  resolve  this  question.  Experience 
with  meson  theories,  with  stronger 
couplings,  and  with  strongly  coupled 
vector  photons,  although  not  proving 
anything,  convinces  me  that  if  the 
coupling  were  stronger,  or  if  you  went 
to  a  higher  order  (137th  order  of  per- 
turbation theory  for  electrodynamics), 
this  difficulty  would  remain  in  the  limit 
and  there  would  be  real  trouble.  That  is, 
I  believe  there  is  really  no  satisfactory 
quantum  electrodynamics,  but  I'm  not 
sure.  And  I  believe  that  one  of  the  rea- 
sons for  the  slowness  of  present  day 
progress  in  understanding  the  strong 
interactions  is  that  there  isn't  any  rel- 
ativistic  theoretical  model  from  which 
you  can  really  calculate  everything. 
Although  it  is  usually  said  that  the 
difficulty  lies  in  the  fact  that  strong 
interactions  are  too  hard  to  calculate, 
I  believe  it  is  really  because  strong 
interactions  in  field  theory  have  no 
solution,  have  no  sense — they're  eith- 
er infinite,  or,  if  you  try  to  modify 
them,  the  modification  destroys  the 
unitarity.  I  don't  think  we  have  a 
completely  satisfactory  relativistic  quan- 
tum mechanical  model,  even  one  that 
doesn't  agree  with  nature  but,  at 
least,  agrees  with  the  logic  that  the 
sum  of  probability  of  all  alternatives 
has  to  be  100%.  Therefore,  I  think 
that  the  renormalization  theory  is  sim- 
ply a  way  to  sweep  the  difficulties  of 
the  divergences  of  electrodynamics  un- 
der the  rug.  I  am,  of  course,  not  sure 
of  that. 

This  completes  the  story  of  the  de- 
velopment  of   the   space-time   view   of 


quantum  electrodynamics.  I  wonder  if 
anything  can  be  learned  frpm  it.  I 
doubt  it.  It  is  most  striking  that  most 
of  the  ideas  developed  in  the  course 
of  this  research  were  not  ultimately 
used  in  the  final  result.  For  example, 
the  half-advanced  and  half-retarded 
potential  was  not  finally  used,  the  ac- 
tion expression  1  was  not  used,  the 
idea  that  charges  do  not  act  on  them- 
selves was  abandoned.  The  path  in- 
tegral formulation  of  quantum  me- 
chanics was  useful  for  guessing  at 
final  expressions  and  at  formulating 
the  general  theory  of  electrodynamics 
in  new  ways — although,  strictly  it 
was  not  absolutely  necessary.  The 
same  goes  for  the  idea  of  the  posi- 
tron being  a  backward-moving  elec- 
tron; it  was  very  convenient,  but  not 
strictly  necessary  for  the  theory  be- 
cause it  is  exactly  equivalent  to  the 
negative  energy  sea  point  of  view. 

We  are  struck  by  the  very  large 
number  of  different  physical  view- 
points and  widely  different  mathemat- 
ical formulations  that  are  all  equiva- 
lent to  one  another.  The  method  used 
here,  of  reasoning  in  physical  terms, 
therefore,  appears  to  be  extremely  in- 
efficient. On  looking  back  over  the 
work,  I  can  only  feel  a  kind  of  regret 
for  the  enormous  amount  of  physical 
reasoning  and  mathematical  re-expres- 
sion which  ends  by  merely  re-express- 
ing what  was  previously  known,  al- 
though in  a  form  which  is  much  more 
efficient  for  the  calculation  of  specific 
problems.  Would  it  not  have  been 
much  easier  to  simply  work  entirely 
in  the  mathematical  framework  to  elab- 
orate a  more  efficient  expression?  This 
would  certainly  seem  to  be  the  case, 
but  it  must  be  remarked  that  although 
the  problem  actually  solved  was  only 
such  a  reformulation,  the  problem  orig- 
inally tackled  was  the  (possibly  still 
unsolved)  problem  of  avoidance  of  the 
infinities  of  the  usual  theory.  There- 
fore, a  new  theory  was  sought,  not  just 
a  modification  of  the  old.  Although  the 
quest  was  unsuccessful,  we  should  look 
at  the  question  of  the  value  of  physical 
ideas  in  developing  a  new  theory. 

Many  different  physical  ideas  can  de- 
scribe the  same  physical  reality.  Thus, 
classical  electrodynamics  can  be  de- 
scribed by  a  field  view,  or  an  action 
at  a  distance  view,  etc.  Originally,  Max- 
well filled  space  with  idler  wheels, 
and  Faraday  with  field  lines,  but  some- 
how the  Maxwell  equations  them- 
selves are  pristine  and  independent  of 
the  elaboration  of  words  attempting  a 
physical    description.    The    only    true 


249 


physical  description  is  that  describing 
the  experimental  meaning  of  the 
quantities  in  the  equation — or  better, 
the  way  the  equations  are  to  be  used 
in  describing  experimental  observations. 
This  being  the  case,  perhaps  the  best 
way  to  proceed  is  to  try  to  guess  equa- 
tions, and  disregard  physical  mod- 
els or  descriptions.  For  example,  Mc- 
Cullough  guessed  the  correct  equa- 
tions for  light  propagation  in  a  crys- 
tal long  before  his  colleagues  using 
elastic  models  could  make  head  or 
tail  of  the  phenomena,  or  again,  Dirac 
obtained  his  equation  for  the  descrip- 
tion of  the  electron  by  an  almost  pure- 
ly mathematical  proposition.  A  simple 
physical  view  by  which  all  the  con- 
tents of  this  equation  can  be  seen 
is  still  lacking. 

Therefore,  I  think  equation  guessing 
might  be  the  best  method  for  pro- 
ceeding to  obtain  the  laws  for  the 
part  of  physics  which  is  presently  un- 
known. Yet,  when  I  was  much  young- 
er, I  tried  this  equation  guessing  and 
I  have  seen  many  students  try  this, 
but  it  is  very  easy  to  go  off  in  wildly 
incorrect  and  impossible  directions.  I 
think  the  problem  is  not  to  find  the 
best  or  most  efficient  method  for  pro- 
ceeding to  a  discovery,  but  to  find  any 
method  at  all.  Physical  reasoning  does 
help  some  people  to  generate  sugges- 
tions as  to  how  the  unknown  may 
be  related  to  the  known.  Theories  of 


the  known  which  are  described  by 
different  physical  ideas  may  be 
equivalent  in  all  their  predictions  and 
hence  scientifically  indistinguishable. 
However,  they  are  not  psychological- 
ly identical  when  one  is  trying  to 
move  from  that  base  into  the 
unknown.  For  different  views  suggest 
different  kinds  of  modifications  which 
might  be  made  and  hence  are  not 
equivalent  in  the  hypotheses  one  gen- 
erates from  them  in  one's  attempt  to 
understand  what  is  not  yet  under- 
stood. I,  therefore,  think  that  a  good 
theoretical  physicist  today  might  find 
it  useful  to  have  a  wide  range  of  physi- 
cal viewpoints  and  mathematical  ex- 
pressions of  the  same  theory  (for  ex- 
ample, of  quantum  electrodynamics) 
available  to  him.  This  may  be  ask- 
ing too  much  of  one  man.  Then  new 
students  should  as  a  class  have  this. 
If  every  individual  student  follows  the 
same  -current  fashion  in  expressing  and 
thinking  about  electrodynamics  or  field 
theory,  then  the  variety  of  hypotheses 
being  generated  to  understand  strong 
interactions,  say,  is  limited.  Perhaps 
rightly  so,  for  possibly  the  chance 
is  high  that  the  truth  lies  in  the  fash- 
ionable direction.  But,  on  the  off- 
chance  that  it  is  in  another  direction 
— a  direction  obvious  from  an  un- 
fashionable view  of  field  theory — 
who  will  find  it?  Only  someone  who 
has  sacrificed  himself  by  teaching  him- 


self quantum  electrodynamics  from  a 
peculiar  and  unusual  point  of  view, 
one  that  he  may  have  to  invent  for 
himself.  I  say  sacrificed  himself  be- 
cause he  most  likely  will  get  nothing 
from  it,  because  the  truth  may  lie  in 
another  direction,  perhaps  even  the 
fashionable  one. 

But,  if  my  own  experience  is  any 
guide,  the  sacrifice  is  really  not  great 
because  if  the  peculiar  viewpoint  tak- 
en is  truly  experimentally  equivalent 
to  the  usual  in  the  realm  of  the 
known  there  is  always  a  range  of  ap- 
plications and  problems  in  this  realm 
for  which  the  special  viewpoint  gives 
one  a  special  power  and  clarity  of 
thought,  which  is  valuable  in  itself. 
Furthermore,  in  the  search  for  new 
laws,  you  always  have  the  psychologi- 
cal excitement  of  feeling  that  possibly 
nobody  has  yet  thought  of  the  crazy 
possibility  you  are  looking  at  right  now. 

So  what  happened  to  the  old  theory 
that  I  fell  in  love  with  as  a  youth? 
Well,  I  would  say  it's  become  an  old 
lady,  who  has  very  little  that's  attrac- 
tive left  in  her,  and  the  young  today 
will  not  have  their  hearts  pound  when 
they  look  at  her  anymore.  But,  we 
can  say  the  best  we  can  for  any  old 
woman,  that  she  has  been  a  very  good 
mother  and  has  given  birth  to  some 
very  good  children.  And,  I  thank  the 
Swedish  Academy  of  Sciences  for  com- 
plimenting  one   of   them.   Thank   you. 


250 


Mathematics  can  help  physics,  but  they  ore  two  quite 
different  activities. 


25    The  Relation  of  Mathematics  to  Physics 

Richard  P.  Feynman 

Excerpt  from  his  book,  The  Character  of  Physical  Law, 
published  in  1965. 


I  should  like  to  say  a  few  things  on  the  relation  of  mathe- 
matics and  physics  which  are  a  little  more  general.  Mathe- 
maticians are  only  dealing  with  the  structure  of  reasoning, 
and  they  do  not  really  care  what  they  are  talking  about.  They 
do  not  even  need  to  know  what  they  are  talking  about,  or, 
as  they  themselves  say,  whether  what  they  say  is  true.  I  will 
explain  that.  You  state  the  axioms,  such-and-such  is  so, 
and  such-and-such  is  so.  What  then?  The  logic  can  be 
carried  out  without  knowing  what  the  such-and-such  words 
mean.  If  the  statements  about  the  axioms  are  carefully  for- 
mulated and  complete  enough,  it  is  not  necessary  for  the 
man  who  is  doing  the  reasoning  to  have  any  knowledge  of 
the  meaning  of  the  words  in  order  to  deduce  new  conclu- 
sions in  the  same  language.  If  I  use  the  word  triangle  in  one 
of  the  axioms  there  will  be  a  statement  about  triangles  in 
the  conclusion,  whereas  the  man  who  is  doing  the  reasoning 
may  not  know  what  a  triangle  is.  But  I  can  read  his  reason- 
ing back  and  say,  'Triangle,  that  is  just  a  three-sided  what- 
have-you,  which  is  so-and-so',  and  then  I  know  his  new  facts. 
In  other  words,  mathematicians  prepare  abstract  reasoning 
ready  to  be  used  if  you  have  a  set  of  axioms  about  the  real 
world.  But  the  physicist  has  meaning  to  all  his  phrases.  That 
is  a  very  important  thing  that  a  lot  of  people  who  come  to 
physics  by  way  of  mathematics  do  not  appreciate.  Physics 
is  not  mathematics,  and  mathematics  is  not  physics.  One 
helps  the  other.  But  in  physics  you  have  to  have  an  under- 
standing of  the  connection  of  words  with  the  real  world.  It  is 


251 


necessary  at  the  end  to  translate  what  you  have  figured  out 
into  EngHsh,  into  the  world,  into  the  blocks  of  copper  and 
glass  that  you  are  going  to  do  the  experiments  with.  Only  in 
that  way  can  you  find  out  whether  the  consequences  are 
true.  This  is  a  problem  which  is  not  a  problem  of  mathe- 
matics at  all. 

Of  course  it  is  obvious  that  the  mathematical  reasonings 
which  have  been  developed  are  of  great  power  and  use  for 
physicists.  On  the  other  hand,  sometimes  the  physicists' 
reasoning  is  useful  for  mathematicians. 

Mathematicians  like  to  make  their  reasoning  as  general 
as  possible.  If  I  say  to  them,  '1  want  to  talk  about  ordinary 
three  dimensional  space',  they  say  'If  you  have  a  space  of 
n  dimensions,  then  here  are  the  theorems'.  'But  1  only  want 
the  case  3',  'Well,  substitute  n  =  3.'!  So  it  turns  out  that 
many  of  the  complicated  theorems  they  have  are  much 
simpler  when  adapted  to  a  special  case.  The  physicist  is 
always  interested  in  the  special  case;  he  is  never  interested 
in  the  general  case.  He  is  talking  about  something;  he  is 
not  talking  abstractly  about  anything.  He  wants  to  discuss 
the  gravity  law  in  three  dimensions;  he  never  wants  the 
arbitrary  force  case  in  n  dimensions.  So  a  certain  amount  of 
reducing  is  necessaiy,  because  the  mathematicians  have 
prepared  these  things  for  a  wide  range  of  problems.  This 
is  very  useful,  and  later  on  it  always  turns  out  that  the  poor 
physicist  has  to  come  back  and  say,  'Excuse  me,  when  you 
wanted  to  tell  me  about  four  dimensions  . . .' 

When  you  know  what  it  is  you  are  talking  about,  that 
some  symbols  represent  forces,  others  masses,  inertia,  and 
so  on,  then  you  can  use  a  lot  of  commonsense,  seat-of-the- 
pants  feeling  about  the  world.  You  have  seen  various  things, 
and  you  know  more  or  less  how  the  phenomenon  is  going 
to  behave.  But  the  poor  mathematician  translates  it  into 
equations,  and  as  the  symbols  do  not  mean  anything  to 
him  he  has  no  guide  but  precise  mathematical  rigour  and 
care  in  the  argument.  The  physicist,  who  knows  more  or 
less  how  the  answer  is  going  to  come  out,  can  sort  of  guess 
part  way,  and  so  go  along  rather  rapidly.  The  mathematical 
rigour  of  great  precision  is  not  very  useful  in  physics.  But 
one  should  not  criticize  the  mathematicians  on  this  score. 
It  is  not  necessary  that  just  because  something  would  be 
useful  to  physics  they  have  to  do  it  that  way.  They  are 
doing  their  own  job.  If  you  want  something  else,  then  you 
work  it  out  for  yourself. 


252 


Current  emphasis  on  studies  of  very  small  systems  and  very 
short  time  intervals,  on  the  one  hand,  and  large-scale  objects 
of  astronomical  dimensions,  on  the  other,  should  lead  to 
increasing  interaction  and  unity  between  them. 


26    Where  Do  We  Go  From  Here? 

Arthur  E.  Ruark 


Article  in  Physics  Today,  1969. 

Because  all  science  feeds  on  un- 
solved problems,  it  is  our  privilege, 
from  time  to  time,  to  make  some  fore- 
cast of  the  future.  Naturally,  the  fore- 
caster can  do  nothing  about  some  great 
surprise  that  may  come,  with  sudden 
force,  to  change  the  course  of  a  whole 
science.  Nevertheless,  in  a  well  de- 
veloped science  such  as  physics,  one 
can  see  some  invariant  driving  forces. 
There  are  tides  in  the  affairs  of  physics 
that  drive  us  onward  without  cease. 
The  greatest  tide  of  all  appears  to  be 
explicit  faith  in  the  unity  and  consis- 
tency of  natural  behavior.  This  faith 
implies  that  parts  of  our  subject  that 
develop  in  relative  isolation  will  come 
together  to  form  a  broader,  more  per- 
fect structure. 

A  very  striking  feature  of  our  times 
has  been  the  extension  of  physical  and 
chemical  and  biological  studies  to  very 
small  sizes  and  time  intervals.  I  am 
talking  about  our  ability  to  deal  with 
atoms,  nuclei  and  elementary  particles. 
Again,  there  has  been  extension  of  our 
ability  to  learn  about  the  large-scale 
features  of  this  universe— this  "bourne 
of  space  and  time,"  as  Tennyson  said. 
These  are  intellectual  and  moral  en- 
deavors, in  the  sense  that  we  have  to 
deal  with  great  uniformities  in  nature; 
with  creation,  evolution  and  final  fate. 

Here,  my  unifying  thread  of  thought 
will  be  the  increasing  interaction  be- 
tween subatomic  physics  and  the  phys- 
ics of  the  heavens.  I  shall  consider 
some  unsolved  problems  in  these  fields. 


The  list  is  highly  selective.  I  have  ex- 
cluded nearly  all  the  things  in  the 
mainstream  of  current  eff^ort,  in  order 
to  include  others  that  now  receive  little 
attention  but  may  be  in  the  mainstream 
in  years  to  come.  Let  us  proceed,  be- 
ginning with  a  few  topics  in  funda- 
mental physics, 

THE  VERY,  VERY  SMALL 

W^e  all  know  of  the  close  relation  be- 
tween the  relativity  theory  and  the 
quantum  theory.  However,  there  are 
curiosities  connected  with  this  matter. 
Partly  they  arise  because  the  field  on 
which  the  game  of  quantum  theory  is 
played  is  a  classical  manifold,  the  field 
of  space  and  time,  or  better  spoken, 
"space— time."  Let  me  indicate  how 
these  two  theories  are  connected  at 
their  very  roots. 

Quantum  theory  is  a  relativistic 
theory.  The  basic  papers  of  Louis  de 
Brogbe  and  of  Erwin  Schrodinger  al- 
ready showed  that  the  waves  belong- 
ing to  a  particle  of  speed  v  have  a 
phase  speed  c^/v,  where  c  is  the  speed 
of  light.  This  formula  arises  from 
special  relativity;  if  one  uses  Newto- 
nian mechanics,  a  wrong  result  is  ob- 
tained. 

Special  relativity  deals  with  space 
and  time  coordinates  x  and  t,  so  that  it 
is  usually  considered  to  be  a  classical 
theory;  that  is  to  say,  a  nonquantum 
theory.  This  seems  to  be  correct  when 
one   considers   it   as    a   mathematical 


253 


scheme;  for  there  is  no  mention  of 
Planck's  constant  h  in  the  axioms  set 
up  by  Albert  Einstein.  On  the  other 
hand,  I  do  not  think  it  is  generally  un- 
derstood that  this  point  of  view  has  to 
be  modified  a  bit  when  we  take  a  hard 
look  at  the  interpretation  of  the 
theory. 

In  order  to  use  the  theory  in  physics, 
we  have  to  say  what  the  quantities 
Ax  and  A*  stand  for,  and  Einstein  made 
the  choice  that  is  really  useful.  When 
he  said  Ax,  he  meant  a  length  mea- 
sured with  a  real  meter  stick.  He  did 
not  mean  a  hypothetical,  nonexistent 
"rigid  ruler,"  the  kind  talked  about  in 
geometry  classes.  When  he  said  Af,  he 
meant  a  time  measured  with  a  labora- 
tory clock.  Now,  this  has  conse- 
quences. The  object  to  be  measured 
is  a  dynamic  thing,  and  so  is  the  stan- 
dard. The  meter  stick  is  a  group  of 
crystals,  a  vibrating  body  held  to- 
gether by  quantum  forces,  and  so  is 
the  clock.  This  consideration  is 
dramatized  somewhat  in  figure  1.  It 
looks  as  though  we  are  caught  in  a 
vicious  circle;  we  want  to  study  the 
interiors  of  atoms  with  the  aid  of  lab- 
oratory standards,  and  Lo!  The  stan- 
dards are  made  out  of  the  very  things 
we  want  to  study. 

True  enough,  we  do  not  actually 
thrust  a  meter  stick  down  into  the 
atom.  We  have  none  with  divisions 
fine  enough,  and  we  know  that  such  a 
disturbance  of  the  atom  would  not  be 
pertinent  if  we  could  do  so.  Actually, 
we  have  to  study  the  wavelengtns  of 
light  emitted  (and  other  useful  quanti- 
ties), recording  them  always  with  the 
aid  of  gross  apparatus-a  favorite  topic 
of  Niels  Bohr. 

Always  there  are  experimental  trou- 
bles. Fundamental  ones  are  shown  in 
figures  2  and  3.  Always,  we  are  mak- 
ing use  of  a  chain  of  experimental  re- 


sults and  interpretation,  concerned 
with  the  whole  coupled  apparatus  and 
based  on  special  relativity  and  quan- 
tum theory  together.  A  central  ques- 
tion is  whether  we  wish  to  use  our 
ordinary  ideas  about  lengths  and  dis- 
tances when  we  get  into  the  domain  of 
the  very,  very  small;  is  this  practice 
really  bad?  Not  at  all.  The  physicist 
is  always  trying  to  extend  the  scope  of 
his  laws  or  to  find  their  limitations. 
He  is  a  great  fellow  for  cutting  Gor- 
dian  knots;  so  he  says: 

"I  shall  continue  to  use  special  rela- 
tivity and  quantum  theory  as  a  strange 
pair  of  partners,  to  interpret  results  of 
my  experiments  on  collisions  between 
elementary  particles;  and  I  shall  find 
out  whether  I  run  into  discrepancies." 

Breakdown? 

Nowadays,  one  kind  of  search  for 
such  discrepancies  is  called  experi- 
mentation on  the  breakdown  of  quan- 
tum electrodynamics.  It  is  carried  on 
by  studying,  for  example,  collisions  be- 


After  taking  bachelor's,  master's  and 
doctor's  degrees  at  Johns  Hopkins  Uni- 
versity, Arthur  E.  Ruark  taught  at  Yale, 
Pittsburgh,  North  Carolina  and  Alabama 
universities.  He  joined  the  Atomic  En- 
ergy Commission  in  1956  as  chief  of  the 
controlled  thermonuclear  program  and 
is  now  senior  associate  director  of  the 
division  of  research  at  the  AEC. 


254 


Where  Do  We  Go  From  Here? 


tween  two  electrons;  one  looks  at  the 
distribution  of  scattered  electrons  to 
see  whether  it  agrees  with  predictions 
from  electrodynamics.  As  of  1968, 
there  was  no  clear  evidence  of  trouble,^ 
down  to  inferred  distances  between 
the  collision  partners  as  small  as  about 
1.8  X  10-14  cm. 

The  question  now  arises:  Could 
particle  theory  continue  to  make  use 
of  the  customary  space-time  concept 
if  a  breakdown  of  electrodynamics 
were  found?  Let  us  see.  A  failure  of 
present-day  theory  would  simply  lead 
to  construction  of  some  new  formula- 
tion, not  to  a  modification  of  the  space- 
time  picture.  People  would  keep  that 
picture.  What  they  want  is  consis- 
tency in  theoretical  talk  over  the  whole 
range  of  space-time  dimensions,  "from 
zero  to  infinity."  It  will  be  extremely 
hard  to  eject  the  space-time  picture 
from  any  part  of  physics.  Curvature 
may  be  introduced;  broader  geometries 
may  be  invoked,  but  the  continuous 
manifold  will  still  be  there  because  of 
the  flexibihty  with  which  new  physical 
fields  can  be  introduced  when  experi- 
ments appear  to  suggest  their  presence. 

Weak  and  infrequent  things 

The  success  of  Fred  Reines  and  Clyde 
Cowan^  in  starting  up  the  subject  of 
experimental  neutrino  physics  showed 
us  that  studies  involving  miniscule 
cross  sections  can  be  worth  a  great  deal 
of  effort.  There  is  also  the  search  for 
gravitational  waves.  It  is  heartening 
to  know  that  Joseph  Weber^  has  really 
excellent  apparatus  to  look  for  these 
waves;  his  laboratory  is  full  of  seismo- 
graphs and  the  like,  for  throwing  out 
spurious  efi^ects  from  tides  and  earth- 
quakes. It  is  still  more  heartening  to 
know  that  he  has  some  events  that  are 
difiicult  to  explain  by  means  of  terres- 
trial disturbances. 


We  should  not  forget  that  there  may 
be  very  weak  forces  in  nature,  still  un- 
discovered, aside  from  the  gravitational 
ones.  I  do  not  know  of  any  current 
search  for  such  forces. 

The  whole  trend  in  physics  has  been 
to  assume  that  particles  are  extremely 
well  standardized.  Nevertheless  a  few 
people'*  have  been  looking  for  anoma- 
lous or  nonstandard  particles;  here  I 
am  talking  about  aberrant  electrons, 
protons,  or  what-have-you?  The  re- 
sources of  modem  technique  (and  in 
particular,  the  capabilities  of  optical 
spectrographs)  are  not  now  being  fully 
used  to  make  some  progress  with  this 
matter.  The  trouble  is  that  when  one 
starts  to  speculate  about  such  particles, 
the  possibihties  are  very  wide;  so 
one  must  look  very  selectively  for  good 
opportunities  to  do  an  interesting  ex- 
periment. 

The  search  for  underlying  levels 

In  recent  years  we  have  seen  rather 
extensive  searches  for  an  underlying 
level  of  simpler  things  from  which  a 
horde  of  elementary  particles  might  be 
made.  There  was  the  quark  search 
and  the  search  for  Dirac  magnetic 
poles;  now  there  is  the  interest  in  so- 
called  "W  particles."  The  quark  idea, 
as  a  mathematical  scheme,  is  indeed 
ingenious  and  interesting.  The  quarks 
are  sometimes  thought  of  as  the  ulti- 
mate particles,  but  there  is  a  trouble 
with  such  ideas.  If  we  had  quarks, 
people  would  just  say,  "What  are  they 
made  of?"  This  is  an  example  of  the 
Infinite  Regression— a  question  such 
that  if  you  answer  it  you  come  up 
against  another  question  of  the  same 
kind. 


ASTROPHYSICS  AND  COSMOLOGY 

We  are  all  aware  of  the  highly  fruitful 
relations  between  advances  in  atomic 


255 


0  12  3       4   5  :I0  ft. 


FISHERMAN'S  RULE,  or  how  to  measure  a  live  fish  with  a  variable  rubber  Einstein 
ruler.    The  fish  and  the  standard  are  both  dynamic  objects.  — FIG.  1 


and  nuclear  physics  and  those  in  astro- 
physics and  nebular  physics.  Further- 
more, the  fruits  of  cosmic-ray  work, 
radio  astronomy  and  x-ray  astronomy 
show  us  that  high-energy  physics  is 
one  essential  key  to  the  understanding 
of  very  violent  astrophysical  events.^ 
But  there  is  mounting  evidence  that,  in 
a  broader  sense,  particle  physics  and 
cosmology  are  closely  related.  Let  us 
turn  our  attention  to  a  few  aspects  of 
this  fascinating  realm  of  ideas. 

Space-time  and  matter 

It  is  frequendy  said  that  the  material 
content  of  space  and  the  motion  of  that 
material  determine  the  curvatvu-e  of  the 
space-time  manifold.  This  is  often 
called  Mach's  principle.  Indeed,  Ein- 
stein's gravitational  equations  say  that 
a  tensor  built  from  curvature  quantities 
is  equal  to  the  matter-energy  tensor 
Tik.  If  Tifc  is  treated  as  an  arbitrary 
source  term,  the  above  statement  is 
justified,  but  we  are  left  with  an  in- 
complete story  on  our  hands.  Thus, 
if  Tijc  comes  from  electromagnetic 
sources,  the  fields  appearing  in  it 
should  be  taken  from  Maxwell's  equa- 


tions,, written  out  for  curved  space- 
time.  Then  the  curvature  and  the 
matter-energy  tensor  are  determined 
together,  from  these  coupled  equa- 
tions. Einstein  proceeded  in  this  way, 
arriving  at  his  first  combined  theory  of 
gravitation  and  electromagnetism. 
True  enough,  he  abandoned  it  later  for 
reasons  of  personal  taste,  but  others 
have  carried  on,  and  this  first  unified 
theory  is  a  lively  field  of  research  even 
today,  50  years  after  it  was  created. 
However,  a  salient  question  still  con- 
fronts us.  When  we  proceed  to  a 
specific  case,  that  of  a  single  electron 
for  example,  do  we  simply  put  in  the 
electronic  charge  as  an  unexplained 
parameter?  Or  do  we  look  for  under- 
lying relations  whereby  the  electron 
can  be  represented  as  a  curlicue  of 
particular  dimensions  in  space-time? 
To  speak  more  generally— do  we  want 
a  completely  unified  theory  of  space 
time  and  matter,  or  a  dualistic  theory? 
There  is  a  literature  on  this  subject, 
too  extensive  for  discussion  here.^  An 
idea  of  the  Mach  type  runs  through  it 
all.  If  I  were  asked  for  a  comprehen- 
sive generalization  of  the  Mach  idea. 


256 


Where  Do  We  Go  From  Here? 


ATOMIC  BILLIARDS,  When  we  try  to 
measure  a  coordinate,  recoil  from  the  test 
body  alters  the  coordinates  and  the  mo- 
mentum under  study.  — FIG.  2 


A  PHOTON  used  for  a  measurement  is 
affected  by  its  collision  with  the  object 
under  attention.  — FIG.  3 


I  would  say,  "There  is  just  one  mani- 
fold. The  equations  describing  physi- 
cal phenomena  contain  not  only  fields 
defined  on  that  manifold  but  also 
quantities  characterizing  the  geometry 
of  the  manifold.  The  connections  are 
such  that  the  fields  and  the  geometrical 
quantities  are  determined  together, 
consistently."  And  I  recommend  to 
the  reader  some  interesting  studies  of  a 
generalized  Mach  principle,  by  Mendel 
Sachs.  "^ 

This  is  a  good  place  to  ask,  "How  is 
it  that  space  has  three  dimensions?" 
This  question  is  at  least  70  years  old. 
I  have  seen  nothing  on  the  subject  that 
is  more  than  a  plausibility  argument, 
but  I  have  a  small  suggestion  as  to  a 


fresh  approach.  Suppose  we  use  the 
methods  of  tensor  and  spinor  calculus 
to  examine  physical  equations  in 
space-time  of  several  dimensions,  from 
two  up  to  six,  for  example.  Let  us 
cover  both  classical  theory  and  quan- 
tum theory,  refnembering  to  look 
closely  at  the  properties  of  simple 
solutions  that  represent  point  particles; 
we  search  for  features  that  appear  par- 
ticularly desirable  or  unique  ( or  both ) , 
in  the  case  of  four-dimensional  space- 
time.  If  such  features  emerge,  we  may 
understand  a  little  better  the  prefer- 
ence for  three  space  dimensions  in  this 
universe.  The  results  would  still  be 
plausibihty  arguments,  but  if  they 
looked  attractive,  we  would  promote 
them  to  the  status  of  assumptions;  and 
that  would  be  that. 

Consistency:    a  desirable  feature 

Perhaps  the  most  significant  fact  that 
has  emerged  from  exploration  of  the 
distant  galaxies  is  the  general  consist- 
ency of  physical  law  over  very  large 
spaces  and  long  time  intervals.     Ap- 
parently we  are  not  dealing  with  dif- 
ferent bodies  of  law,  linked  together 
only  by  very  weak  connections.     We 
appear  to  be  living  in  a  Universe— not 
in  some  sort  of  Diverse,  or  Polyverse. 
A  cardinal  piece  of  support  for  this 
welcome   notion   is   the   red   shift   of 
Vesto  Slipher,  Edwin  Hubble  and  Mil- 
ton Humason.     To  an  approximation, 
the  light  from  distant  galaxies  is  shifted 
toward  the  red,  by  amounts  that  can 
be  explained  by  assuming  that  they 
move  outward  with  speeds  v,  propor- 
tional to  their  distances  R  from  us;  the 
relation  is 

V  =  7SR, 

with  v  in  kilometers  per  second  and  R 
in  megaparsecs;  one  megaparsec  is 
3.09  X  1024  cm. 


257 


Allowing  for  this  red  shift,  we  see 
the  same  spectral  series,  the  same 
atomic  behavior,  that  is  found  here  on 
earth.  Of  course,  this  probing  out  to 
great  distances  means  that  one  is 
looking  back  a  long  way  in  time. 
What  is  the  inner  meaning  of  this  con- 
sistency? The  distant  atoms  would 
not  show  the  spectral  series  properly 
if  they  did  not  obey  the  Pauli  principle. 
Those  atoms  are  testifying  to  identity 
of  the  electrons  and  identity  of  the 
nuclei  in  the  whole  region  available 
for  observation.  They  are  revealing  a 
most  extraordinary  degree  of  quality 
control  in  the  creation  and  mainte- 
nance of  these  particles.  Why,  not 
even  Rolls-Royce  ...  I 

Is  this  uniformity  of  particle  prop- 
erties due  to  a  uniformity  in  the  prop- 
erties of  space-time  itself?  Or  are 
these  two  ideas  just  the  same  idea, 
clothed  in  different  words?  I  leave  tbe 
answer  to  you— or  your  grandchildren. 

Long  ago  and  far  away 

There  is  another  important  fact  that 
bears  on  the  question  of  universal  con- 
sistency. Suppose  an  atom  in  a  galaxy 
10^  light  years  away  emits  a  parcel  of 
energy  characterized  by  a  far-ultra- 
violet wavelength.  Looking  aside 
from  experimental  difificulties,  we  can 
set  up  a  suitable  bulb  containing  so- 
dium vapor,  here  in  our  solar  system, 
to  receive  the  light.  After  10^  years  an 
electron  may  be  kicked  out  of  a  single 
atom  in  that  vapor.  //  we  believe  that 
an  electromagnetic  field  traveled  all 
that  time  through  empty,  darksome 
space,  then  we  have  to  say  that  the 
field  causes  a  definite  amount  of  en- 
ergy to  appear  at  a  target  only  10"^ 
cm  in  diameter,  after  running  through 
a  distance  of  about  10^^  centimeters. 
Also,  from  the  observed  conservation 
of  energy  in  such  processes,  we  have 


to  conclude  that  the  field  does  nothing 
elsewhere. 

What  shall  we  say  about  this  result? 
An  orthodox  quantum  theorist  might 
say,  "It  is  all  a  matter  of  chance;  this 
matter  was  explained  in  1927."  A 
thoroughgoing  determinist  might  say, 
"This  astounding  accuracy  of  aim  is 
evidence  of  extraordinary  quality  con- 
trol." A  classical  relativist  might  say, 
"All  point  events  that  are  connected 
by  light  rays  are  at  the  same  spot  in 
space-time.  We  are  dealing  with  a 
sort  of  contact  action.  From  the 
standpoint  of  a  being  who  perceives 
point  events  directly  and  intuitively, 
there  is  no  problem."  We  possess  con- 
siderable flexibility  in  contemplation  of 
these  answers  or  others  like  them;  for 
each  answer  is  based  on  some  set  of 
axioms,  and  axioms  are  arbitrary  in- 
deed. The  orthodox  quantum  theorist 
will  say,  "Yes,  but  look  at  the  fruits  of 
my  axioms."  And  we  shall  reply, 
"The  fruits  of  your  axioms  are  very 
great  indeed,  but  a  large  number  of 
very  respectable  people  are  not  satis- 
fied with  the  foundations  of  your 
theory." 

Permanence:    a  desirable  feature 

Let  us  consider  the  permanence  of 
gross  matter.  The  customary  esti- 
mates of  universe  duration  lie  a  little 
above  10^"  years.  It  happens  that 
Reines  and  his  students  have  found 
lower  limits  for  the  lifetimes  of  elec- 
trons and  nucleons  by  looking  for  their 
decay.  8  There  are  some  nuances,  but 
roughly  the  half- life  figures  are:  for 
the  electron,  more  than  2  X  lO^i  years; 
for  nucleons,  more  than  10^^  years. 
Thus  we  are  confronted  with  a  terrific 
factor  of  safety,  10^ ^  at  least,  relative 
to  the  universe  duration  mentioned 
above.  This  looks  like  very  good  en- 
gineering. The  stuflF  is  made  so  it  will 
last. 


258 


Where  Do  We  Go  From  Here? 


Diluteness:    a  convenient  feature 

People  are  generally  impressed  with 
the  vast  spaces  between  the  stars  of 
our  galaxy,  and  also  the  spaces  be- 
tween galaxies,  which,  on  the  average, 
are  somewhat  like  tennis  balls  8  meters 
apart.  This  diluteness  is  much  to  be 
prized,  because  violent  things  happen 
when  big  pieces  of  matter  get  too 
close  together.  I  invite  your  atten- 
tion to  the  famous  case  of  the  galaxy 
M  82.  A  photograph  of  this  galaxy  can 
be  found  in  reference  9.  More  or  less 
perpendicular  to  the  disk  of  the  gal- 
axy there  are  great  masses  of  ejected 
matter,  believed  to  be  mostly  hydro- 
gen. There  was  a  big  explosion  in  the 
middle  of  this  galaxy.  The  products 
are  pouring  out  at  a  speed  of  the  order 
10*  cm/sec.  It  is  estimated  that  this 
explosion  involved  disruption  of  a  mil- 
lion stars  in  the  dense  core  of  the 
galaxy. 

Information  from  far  away 

How  much  can  we  hope  to  learn  about 
very  distant  objects?  In  general,  the 
farther  away  an  object  is,  the  less  we 
can  find  out  about  it.  Details  fuzz 
out;  light  signals  from  the  object  are 
fainter;  spectra  move  out  to  the  infra- 
red. It  is  only  in  recent  times  that 
attention  has  been  paid  to  the  quanti- 
tative side  of  this  common  observa- 
tion. Kenneth  Metzner  and  Philip 
Morrison^"  have  calculated  the  amount 
of  information  carried  to  us  by  the 
photons  from  a  distant  galaxy  in  any 
experiment  of  limited  duration.  They 
consider  simple  expanding  universes 
of  several  types.  This  is  a  matter 
worthy  of  further  research,  because  it 
can  show  us  the  boundary  between 
verifiable  physics  and  unverifiable 
speculation.  Beyond  the  domains 
where  individual  galaxies  can  be  iden- 
tified—and there  are  hundreds  of  mil- 


lions within  sight— there  may  be  others 
that  show  up  as  a  faint  general  back- 
ground. Astronomers  know  that  they 
must  increase  their  studies  of  this  faint 
background  light,  when  more  big  tele- 
scopes come  on  stream,  a  few  years 
hence. 

If  and  when  they  reach  the  limit  of 
their  resources,  we  shall  be  confronted 
with  an  interesting  situation.  For  a 
long  time  philosophers  have  been  say- 
ing that  physicists  continually  work  on 
the  soluble  problems,  so  that  meta- 
physics is  necessarily  the  bin  of  un- 
solved ones.  Now  I  shall  leave  it  to 
the  reader  to  ponder  the  situation  of 
an  experimental  science  that  reaches  a 
limit  because  the  objects  under  in- 
vestigation cannot  provide  sufficient 
amounts  of  information  to  our  detec- 
tors to  give  the  answers  we  should  like 
to  know. 

EPILOGUE 

I  have  pointed  out  some  lines  of  en- 
deavor that  lie  at  or  beyond  the  pres- 
ent limits  of  our  capabilities,  and  I 
have  only  two  hints  for  those  who  may 
choose  to  attack  these  matters.  The 
first  is  that  one  should  pay  close  atten- 
tion to  a  method  used  by  Rene  Des- 
cartes. I  call  it  the  "Method  of  Com- 
plete Skepticism."  He  adopted  a  sys- 
tematic policy  of  denying  any  state- 
ment he  was  considering  and  of  look- 
ing at  the  consequences.  The  second 
hint  is  connected  with  economy  and 
simplicity  of  thought.  I  quote  the  fa- 
mous dictum  of  William  of  Occam: 
"Entia  non  multiplicanda  sunt,  praeter 
necessitatem."  Entities  are  not  to  be 
multiplied  except  for  reasons  of  ne- 
cessity. 

In  closing,  I  mention  once  more  the 
consistency,  the  connectivity,  revealed 
by  physical  studies  up  to  the  present. 


259 


Though  each  of  us  usually  thinks  of 
himself  as  a  part  of  the  universe,  this 
is  a  one-sided  view,  for  great  por- 
tions of  our  surroundings  are  always 
exerting  their  influence  upon  us.  As 
an  overstatement,  one  might  say  that 
the  universe  is  a  part  of  every  man. 
Sir  George  Thomson^*  says  in  his  book, 
The  Foreseeable  Future: 

"The  universe  that  includes  our 
perceptions  and  our  feelings  is  one, 
and  no  single  part  can  be  put  into  a 
ring-fence  completely  isolated  from 
all  the  rest." 

Therefore  I  end  this  story  with  the 
thought:  The  universe  is  the  proper 
study  of  mankind. 

References 

1.  W.  C.  Barber,  B.  Gittelman,  G.  K. 
O'Neill,  B.  Richter,  Phys.  Rev.  Lett. 
16,  1127  (1966), 

2.  F.  Raines,  C.  L.  Cowan  Jr,  physics 
TODAY  10,  no.  8,  12  (1957). 

3.  J.  Weber,  Phys  Rev.  Lett.  20,  1307 
(1968). 


10. 


11. 


G.  M.  Kukavadze,  L.  Ya.  Memelova, 
L.  Ya.  Suvorov,  Sov.  Phys.-JETP  22, 
272  ( 1965);  E.  Fischbach,  T.  Kirsten, 
G.  A.  Schaeffer,  Phys.  Rev.  Lett.  20, 
1012  (1965). 

S.  Colgate,  PHYSICS  today  22,  no.  1, 
27  (1969). 

J.  A.  Wheeler,  Geometrodynamics, 
Academic  Press,  New  York  ( 1962 ) . 
D.  K.  Sen,  Fields  and/ or  Particles, 
Academic  Press,  New  York  (1968). 
M.  Sachs,  PHYSICS  today  22,  no.  2, 
51  (1969). 

M.  K.  Moe,  F.  Reines,  Phys  Rev.  140, 
B992  (1965);  W.  R.  Kropp  Jr,  F. 
Reines,  Phys.  Rev.  137,  B740  (1965); 
C.  C.  Giamati,  F.  Reines,  Phys.  Rev. 
126,2178  (1962). 

G.  F.  Burbidge,  E.  M.  Burbidge,  A. 
M.  Sandage,  Rev.  Mod.  Phys.  35, 
947  (1963). 

A.  W.  K.  Metzner  P.  Morrison,  Mon. 
Not.  Roy.  Astron.  Soc.  119,  657 
(1959). 

G.  P.  Thomson,  The  Foreseeable  Fu- 
ture, 2nd  ed..  Viking  Press,  New  York 
(1960).  D 


260 


Authors  and  Artists 


JEREMY   BERNSTEIN 

Jeremy  Bernstein,  born  in  1929  in  Rochester,  New 
York,  is  Professor  of  Physics  at  Stevens  Institute 
of  Technology  in  New  Jersey.  He  was  educated  at 
Columbia  Grammar  School  in  New  York  City  and 
received  a  bachelor's  and  master's  degree  in 
mathematics,  and  a  doctorate  in  physics  from  Har- 
vard University.   He  has  done  research  at  the  Har- 
vard Cyclotron  Laboratory,  the  Institute  for  Ad- 
vonced  Study  at  Princeton,  Los  Alamos,  at  the 
Brookhaven  National   Laboratories,  and  is  fre- 
quently a  visiting  physicist  at  CERN  (Conseil 
Europeen  pour  la    Recherche  Nucleaire)  in  Geneva. 
Bernstein  is  the  author  of  The  Analytical   Engine: 
Computers,  Past,  Present,  and  Future,  Ascent, 
an  account  of  mountaineering  in  the  Alps,  and  has 
written  book  reviews  and  Profile  articles  for  the 
magazine.  The  New  Yorker. 

HARRISON   SCOTT    BROWN 

Harrison  Scott  Brown,  born  in  Sheridan,  Wyoming, 
in  1917,  is  Professor  of  Geochemistry  at  California 
Institute  of  Technology  and   Foreign  Secretary  of 
the  National  Academy  of  Sciences.  He  received  a 
B.S.  from  the  University  of  California  and  a  Ph.D. 
from  Johns  Hopkins  University.   Brown  is  an  editor 
at  large  for  The  Saturday  Review  and  has    written 
The  Challenge  of  Man's   Future  and  Must  Destruction 
Be  Our  Destiny?     His  research  interests  include 
mass  spectroscopy,  meteoritics,  planet  structure  a 
and  planetary  chemistry. 


LAURA   FERMI 

Laura   Fermi  was  born  in  Rome,   Italy,   in  1907,  and 
studied  at  the  University  of  Rome.  She  met  Enrico 
Fermi  when  she  was  sixteen;   they  were  married  five 
years  later.  She  has  two  children.  When  the  anti- 
Semitic  laws  appeared  in  Italy  in  1938,  the  Fermis 
left  for  the  United  States,  immediately  after  he  re- 
ceived the  Nobel    Prize  that  December.  In  1955  she 
attended  the  International  Conference  on  the  Peace- 
ful Uses  of  Atomic  Energy  as  historian  for  the 
United  States  and  wrote  Atoms  for  the  World.  She 
is  also  author  of  Atoms  in  the  Family:  My  Life  with 
Enrico  Fermi,  and  the  monographic  study, 
Mussol  ini. 

RICHARD  PHILLIPS  FEYNMAN 

Richard  Feynman  was  born  in  New  York  in  1918, 
and  graduated  from  the  Massachusetts  Institute  of 
Technology  in  1939.  He  received  his  doctorate  in 
theoretical  physics  from  Princeton  in  1942,  and 
worked  at  Los  Alamos  during  the  Second  World 
War.    From  1945  to  1951   he  taught  at  Cornell,  and 
since  1951   has  been  Tolman  Professor  of  Physics 
at  the  California  Institute  of  Technology.   Profes- 
sor Feynman  received  the  Albert  Einstein  Award 
in  1954,  and  in  1965  was  named  a  Foreign  Member 
of  the  Royal  Society.   In  1966  he  was   awarded  the 
Nobel    Prize  in  Physics,  which  he  shared  with 
Shinichero  Tomonaga  and  Julian  Schwinger,  for 
work  in  quantum  field  theory. 


SIR  JAMES  CHADWICK 

Sir  James  Chadwick  was  born  in  1891    in  Man- 
chester, England;  he  attended  Victoria  University 
there,  and  then  Cambridge  University.     At  the  age 
of  eighteen  he  met  Ernest  Rutherford  with  whom 
he  later  collaborated  in  experimental  work.    Chad- 
wick discovered  the  neutron  in  1932  and  for  this 
was  awarded  the  Nobel    Prize  in  Physics  in  1935. 
During  World  War  II  he  worked  for  "Tube  Alloys," 
the  British  equivalent  of  the  Manhattan  Project. 

OWEN  CHAMBERLAIN 

Owen  Chamberlain,  Professor  of  Physics  at  the 
University  of  California  at  Berkeley,  and  Nobel 
Prize  winner  in  1959  with  Emilio  Segre  for  their 
demonstration  of  the  existence  of  the  antiproton, 
was  born  in  San   Francisco  in   1920.   He  received 
his   bachelor's  degree  from  Dartmouth  College  and 
his  Ph.D.  from  the  University  of  Chicago.  During 
World  War  II  he  worked  on  the  Manhattan  Project 
OS  a  civilian  physicist.  He  has  been  active  in 
civil   liberties  activities.  Some  of  his  special 
interests   in  physics  are  fission,  alpho-porticle 
decay,  and  neutron  diffraction  in  liquids. 


KENNETH   W.  FORD 

Kenneth  W.  Ford  was  born  in  1917  at  West    Palm 
Beach,   Florida.   He  did  his  undergraduate  work  at 
Harvard  College.    His  graduate  work  at  Princeton 
University  was  interrupted  by  two  years  at  Los 
Alamos  and  at  Project  Manhattan  in  Princeton.   He 
worked  on  a    theory    of  heavy  elementary  particles 
at  the  Imperial   College  in  London,  and  at  the  Max 
Planck  Institute  in  Gottingen,  Germany.   Before 
joining  the  faculty  at  the  University  of  California, 
Irvine,  as  chairman  of  the  Department  of  Physics, 
Mr.   Ford  was  Professor  of  Physics  at  Brandeis 
University. 

JAMES  FRANCK 

James   Franck  was  born  in  Hamburg,   Germany,  in 
1882,  and  received  his   Ph.D.   from  the  University 
of  Berlin.     He  and  Gustav  Hertz  shared  the  Nobel 
Prize  in  1925  for  their  studies  which  supported 
the  new  model  of  the  atom  just  postulated  by 
Bohr.     Franck  was  Professor  of  Experimental 
Physics  and  Director  of  the  Institute  for  Experi- 
mental  Physics  at  the  University  of  Gottingen. 
When  the  Nazis  gained  increasing  power,   Franck 


261 


Authors  and  Artists 


demonstrated  against  the  racial   laws,  and  in  1933 
he  and  his  family  moved  to  the  United  States. 
Here  he  lectured  at  Johns  Hopkins  University  ond 
later  become  Professor  of  Physical  Chemistry  at 
the  University  of  Chicago.     He  died  in  1964. 

MARTIN   GARDNER 

Martin  Gardner,  the  editor  of  the  "Mathematical 
Games"  deportment  of  the  Scientific  American, 
was  born  in  Tulsa,  Oklahoma,  in  1914.     He  re- 
ceived a   B.A.   in  philosophy  from  the  University 
of  Chicago  in  1936,  worked  as  a  publicity  writer 
for  the  university,  and  then  wrote  for  the  Tulso 
Tribune.    During  World  War  II  he  served  in  the 
Navy.     Martin  Gardner  has  written  numerous  short 
stories  as  well  as  professional  articles  for  such 
journals  as  Scripta  Mathematica  and  Philosophy 
of  Science,  and  is  the  author  of  the  best-selling 
books.  The  Annotated  Alice,   Relotivity  for  the 
Millions,   Fads  and  Fallacies  In  the  Name  of 
Science,  as  well  as  two  volumes  of  the  Scien- 
tific  American  Book  of  Mathematical   Puzzles 
and  Diversions. 

LEOPOLD   INFELD 

Leopold  Infeld,  a  co-worker  with  Albert  Einstein 
in  general   relativity  theory,  was  born  in  1898  in 
Poland.  After  studying  at  the  Cracow  and   Berlin 
Universities,  he  became  a  Rockefeller  Fellow  at 
Cambridge  where  he  worked  with  Max  Born  in 
electromagnetic  theory,  and  then  a  rrrember  of  the 
Institute  for  Advanced  Study  at  Princeton.     For 
eleven  years  he  was  Professor  of  Applied  Mathe- 
matics at  the  University  of  Toronto.   He  then  re- 
turned to  Poland  and  became  Professor  of  Physics 
at  the  University  of  Warsaw  and  until  his  death  on 
16  January  1968  he  was  director  of  the  Theoretical 
Physics  Institute  at  the  university.     A  member  of 
the  presidium  of  the  Polish  Academy  of  Science, 
Infeld  conducted  research  in  theoretical  physics, 
especially  relativity  and  quantum  theories.   Infeld 
was  the  author  of  The  New  Field  Theory,   The 
World  in  Modern  Science,  Quest,  Albert  Einstein, 
and  with  Einstein,  The  Evolution  of  Physics. 

DAVID  LOCKHART  JUDD 

David  Lockhart  Judd  was  born  in  Chehalis,  Washing- 
ton,  in  1923.   In  1943  he  received  his  A.  Bu  from 
Whitman  College.  He  then  attended  California   Insti- 
tute of  Technology  and  received  an  M.S.   in  1947  and 
a   Ph.D.  in  physics  three  years   later.    From  1951    to 
the  present  he  has  been  with  the  Lawrence 
Radiation  Laboratory  ot  Berkeley,  since  1965  as 
head  of  the  Physics  Division.  He  is  also  senior 
lecturer  in  physics  at  the  University  of  California, 
Berkeley.   His  professional    interests  include  ac- 
celerator theory,  ion  optics,  plasma  and  particle 
physics,  and  nonlinear  mechanics. 


RALPH    EUGENE    LAPP 

Ralph  Lapp  was  born  in  Buffalo,  New  York,  in  1917. 
He  received  his  B.S.  and  Ph.D.  in  physics  from  the 
University  of  Chicago.  He  was  head  of  the  nuclear 
physics  branch.  Office  of  Naval   Research,  and 
since  1950  has  been  director  of  the  Nuclear  Science 
Service.  Lapp  is  the  author  of  many  books  concern- 
ing the  social  consequences  of  modern  science,  in- 
cluding Must  We  Hide?  and  The  New  Priesthood: 
The  Scientific  Elite  ond  The  Uses  of  Power.     His 
interests  include  cosmic  radiation,  moss  spectro- 
scopy and  civil  defense. 

ERNEST  ORLANDO  LAWRENCE 

Ernest  Orlando  Lawrence  (1901-1958)  was  born  in 
North  Dakota.  He  received  his  doctorate  from  Yale 
University  and  then  joined  the  faculty  of  the  Uni- 
versity of  California  at  Berkeley.    By  building  with 
his  colleagues,  M.  S.   Livingstone  and  others,  the 
first  successful  cyclotron,  Lawrence  solved  one  of 
the  major  experimental  problems  of  the  1920's  and 
30's  in  nuclear  physics,  that  of  providing  control- 
lable beams  of  high-energy  particles.   Lawrence 
built  a  series  of  increasingly  more  powerful  cyclo- 
trons.    For  these  accomplishments  and  for  his  re- 
search on  artificial   radioactive  elements,   Law- 
rence was  awarded  the  Nobel   Prize  in  Physics  in 
1939.   The  element  lowrencium  is  named  for  him. 

GERARD   KITCHEN  O'NEILL 

Professor  of  Physics  at  Princeton  University, 
O'Neill  was  born  in  Brooklyn,  New  York,   in  1927. 
He  received  his  bachelor's  degree  from  Swarthmore 
College  and  his  Ph.D.  from  Cornell   University. 
Between  1954  and  1959  he  was  a  member  of  a  group 
that  designed  the  three- bi  II  ion-vol  t  proton  synchro- 
tron now  being  operated  jointly  by  Princeton  and  the 
University  of  Pennsylvania.  More  recently  he  has 
worked  on  the  design  of  storage  rings,  experiments 
in  high-energy  physics  and  spark  chambers. 


V.  LAWRENCE  PARSEGIAN 

V.   Lawrence  Parsegion  studied  ot  M.I.T.,  Washing- 
ton University,  and  New  York  University,  obtaining 
his  Ph.D.   in  physics   in  1948.   He  has   been  professor 
of  nuclear  science  and  engineering  at  Rensselaer 
Polytechnic  Institute    since  1954,  ond  holds  the 
distinguished  Choir  of  Rensselaer  professorship. 
In  addition  to  his  research  activities,  he  has 
choired  a  curriculum  development  project  to  im- 
prove college  science  teaching. 

RUDOLF   ERNST   PEIERLS 

Rudolf  Ernst  Peierls  was  born  in  Berlin  in  1907 
and  received  degrees  from  several  universities, 
including  a  D.Phil,   in  Theoretical   Physics  from 


262 


the  University  of  Leipzig  in  1929  and  a  D.Sc.  from 
the  University  of  Manchester,    England,   in  1936. 
From  1937  to  1963  he  was  Professor  of  Mathe- 
matical   Physics  at  Birmingham  University.   During 
the  early  years  of  World  War  II  he  worked  on  the 
Atomic   Energy  Project  in  Birmingham,  and  then  at 
Los  Alamos  between  1943—46.    Peierls  is  now 
Professor  of  Theoretical   Physics  at  Oxford  Uni- 
versity and  a  Fellow  of  New  College,  Oxford.     He 
is  the  author  of  The  Laws  of  Noture  and  Quantum 
Theory  of  Solids. 

ARTHUR  C.  RUARK 
(see  page  256) 

ERNEST  RUTHERFORD 

Lord  Rutherford  (1871-1937)  was  born  in  Nelson, 
South  Island,   New    Zealand.   He  graduated  from 
Nelson  College.   At  the  University  of  New  Zealand 
he  won  a  scholarship  to  attend  Cambridge  Univer- 
sity in  England  where,  stimulated  by  J.J.   Thomson, 
he  studied  the  electrical  nature  of  matter.     As  Pro- 
fessor of  Physics  at  McGill    University  in  Montreal, 
he  distinguished  the  identity  of  Becquerel's  radia- 
tions  into  alpha,   beta  and  gamma  rays,  and  proposed 
(with  Soddy)  the  concepts  of  radioactive  transmuta- 
tion and  isotopes.     Returning  to  England,  he  con- 
tinued his  research  at  the  University  of  Manchester. 
There  he  conducted  his  most  famous  experiments 
leading  in  1911    to  his  discovery  of  the  nucleus  in 
the  atom.    He  was  awarded  the  Nobel    Prize  in 
Chemistry  in  1908  for  his  experiments   in  radioac- 
tivity.  Rutherford  returned  to  Cambridge  in  1919 
as  director  of  the  Cavendish   Laboratory. 

EMILIO   SEGRE 

Emilio  Segrewos  born  in  Tivoli,  Italy,  in  1905  and 
received  his  Ph.D.   in  physics  from  the  University 
of  Rome  in   1928.     He  was  a  student  of  Enrico   Fermi 
from  1934  to  1936,  and  has  published  a  biography, 
Enrico   Fermi,   Physicist  (1970).   Then  he  became 
director  of  the  physics  laboratory  at  Palermo,  where 
he  and  C.   Perrier  made  the  discovery  of  technetium, 
the  first  artificially  made  element.    Segre  and  his 
co-workers  also  were  the  first  to  identify  the  arti- 
ficial elements  of  plutonium  and  astatine.     Segre 
was  awarded  the  Nobel   Prize  in  Physics  in  1959 
for  his  demonstration  with  Owen  Chamberlain  of  the 
existence  of  the  antiproton.   He  is   Professor  of 
Physics  at  the  University  of  Cal  ifornia  at  Berkeley. 

CHARLES   PERCY    SNOW 

Charles   Percy  Snow,    Baron  of  Leicester,  was  born 
in  1905  and  educated  at  University  College, 
Leicester  and  at  Christ's  College,  Cambridge.  Al- 
though well   known  as  a  novelist,  especially  dealing 


with  the  lives  and  problems  of  professional  men, 
he  has  held  such  diverse  positions  as  chief  of  sci- 
entific personnel  for  the  Ministry  of  Labour,  Civil 
Service  Commissioner,  and  a  Director  of  the  English 
Electric  Co.,   Ltd.   His  writings  have  been  widely 
acclaimed;  among  his  novels  are  The  Search,  The 
New  Men,  and  Corridors  of  Power.   His  nonfiction 
books  on  science  and  its  consequences  include 
The  Two  Cultures  and  The  Scientific  Revolution 
and  Science  and  Government. 

LEO  SZILARD 

Leo  Szilard  was  born  in  Budapest,  Hungary,  in 
1898,  and  received  his  doctorate  at  the  University 
of  Berlin.  He  was  at  the  Clarendon  Laboratory  in 
England  and  the  National   Defense  Research  Divi- 
sion at  Columbia  University  before  going  to  the 
University  of  Chicago  as  Professor  of  Physics.  At 
the  time  of  his  death  in  May  1964,  Szilard  was  a 
resident  fellow  at  the  Salk  Institute  for  Biological 
Studies  in  La  Jolla,  California.     Besides  nuclear 
physics,  he  did  research  in  a  variety  of  fields 
including  mutations  and  genetics  of  bacteria  and 
bacterial  viruses.    Szilard  helped  to  draft  and 
transmit  the  famous  letter  from   Einstein  to  Roose- 
velt which  helped  to  initiate   large-scale  work  on 
atomic  energy  in  the  United  States  in  1939.     Hi^ 
publications   include  The  Voice  of  the  Dolphins. 
He  was  deeply  involved  with  groups  that  aimed  at 
the  peaceful  application  of  science  and  technology, 
and  in  political  action  toward  such  ends. 


ALVIN   MARTIN   WEINBERG 

Alvin  Martin  Weinberg,   Director  of  the  Oak  Ridge 
National   Laboratory  in  Tennessee,  was  born  in 
1915  in  Illinois.    He  graduated  from  the  University 
of  Chicago  in  1935  and  received  his  doctorate  in 
physics  from  Chicago  in  1939.     He  has  been  on  the 
United   States    visiting    scientist   team  to  Russian 
nuclear  installations,  the  President's    Scientific 
Advisory  Board,  and  has  been  awarded  the  Atoms 
for  Peace  Award  (1960)  and  the  Lawrence  Memorial 
Award.   He  is  a  pianist  and  dedicated  tennis  player 
in  his  spare  time. 

CLYDE   EDWARD  WIEGAND 

Clyde  Edward  Wiegand  was  born  in  Long  Beach, 
Washington,   in  1915  and  groduated  from  Willamette 
College  in  Oregon.     He  was  awarded  a  Ph.D.   in 
physics  from  the  University  of  California,  where 
he  has  been  a  graduate  student  of  Emilio   Segre. 
During  World  War  II  he  went  with  Segre  to  work  at 
the  Los  Alamos  Laboratory.  Weigand  is  now  with 
the  University  of  California  at  its   Lawrence  Radia- 
tion Laboratory.     His  research  interests   include 
nuclear  physics,  scattering,  and  cross-section 
work  with  high-energy  particles. 


263 


Authors  and  Artists 

VICTOR    F.   WEISSKOPF 
(see  page  21  2) 

ROBERT   R.   WILSON 

R.R.  Wilson,  was  born  in  1914  in  Frontier,  Wyomi  ng, 
and    now    Is    director   of   the    National    Accelerator 
Laboratory,   Batovia,  Illinois,  and  professor  of 
physics  at  the  University  of  Chicago.  He  received 
his  training  at  the  University  of  California  and  has 
taught  at  Princeton,  Harvard,  Cornell,  and  Chicago. 
Since  1947,  Mr.  Wilson  has  been  involved  in  the 
construction  of  a  series  of  particle  accelerators 
with  which  to  explore  the  structure  of  the  proton. 
He  has  had  formal  training  as  a  sculptor  in  the 
United  States  and  at  the  Academio  Belli  Arte  in 
Rome,  and  continues  actively  working  in  this  field. 

HERMAN   YAGODA 

Herman  Yagoda,  chemist  as  well  as  physicist,  was 
born  in  New  York  City  in  1908.     He  graduated  from 
Cooper  Union  and  received  his  master's  degree  from 
New  York  University.  Yagoda  died  in  1964.  He  hod 
been  a  chemist  for  the  U.  S.  Customs  Laboratory  in 
New  York  and  was  at  the  Air  Force  Cambridge  Re- 
search Laboratories  where  he  conducted  research 
in  space  physics  and  cosmic  radiation.  Yagoda  was 
the  author  of  Radioactive  Measurements  with  Nu- 
clear Emulsions. 


GALE   YOUNG 

Gale  Young  was  born  in  Baroda,  Michigan,  in  1912. 
He  received  a  B.S.  from  the  Milwaukee  School  of 
Engineering  and  a   B.S.  and  M.S.  from  the  University 
of  Chicago.   He  has  taught  physics  at  Chicago  Uni- 
versity and  Olivet  College  in  Michigan.     Like  mony 
physicists,  during   World  War  II  Young  worked  on 
the  Manhattan  District  Project  and  was  the  tech- 
nical director  of  the  Nuclear  Development  Associa- 
tion.    Since  1961   he  has  been  on  executive  of  the 
United  Nuclear  Corporation. 


THOMAS  JOHN   YPSILANTIS 

Thomas  John   Ypsilantis  was  born  in  Salt  Lake 
City  in  1928.     He  earned  his    B.Sc.  from  the  Univer- 
sity of  Utah  and  his  M.A.  and  Ph.D.  from  the  Uni- 
versity of  California,   Berkeley.     He  has  been  on 
the   faculty  at  Berkeley  since  1957  and  is  now 
Associate  Professor  of  Physics.    Ypsilantis  had 
a  Guggenheim  Fellowship  in  1959—60,  and  has 
been  a  consultont  to    the    Institute   of    Defense 
Analysis.  His  reseoch  interests  include  antiproton 
interactions,  proton  polarization  in  scattering,  and 
pion  and  nucleon  interactions. 


264 


'V-  I, 


'i'--