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The Project Physics Course 



The Nucleus 

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The Project Physics Course 



Q The Nucleus 

A Component of the 
Project Physics Course 

Published by 


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 

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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. 


(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 

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 

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 

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, 

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


PSCTiPfW''* ....wrW\ 




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. 
















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 


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 


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- 

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 


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 

"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. 


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. 


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 



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- 


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

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 



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 

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 


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 



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 

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 


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 

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 



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 


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. 


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). 


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 


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 

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. 


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 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 


The Nature of the Alpha Particle 

been detected a few hours after the introduction o£ the 

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. 


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. 


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- 

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. 

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Some Personal Notes on the Search for the Neutron 

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Some Personal Notes on the Search for the Neutron 

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Some Personal Notes on the Search for the Neutron 


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 

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 

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 

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 



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 

• 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). 


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. 


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 

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 


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 

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- 


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. 


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. 















1840 ± 90 

— 5x10-8 











e + 







272.8 ± 0.3 



71 + 

273.3 i 0.2 





263.7 ± 0.7 

5x10 -'5 



207 ± 0.5 



206.9 1 0.4 



T + 

965.5 1 0.7 




965 ± 10 





963 ± 9 



960 - 7 




955 ±9 



~ 960 



2182 ± 2 




Z + 

2327 ± 4 





— lO-'O 



2582 ± 10 

- lO-'C 


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 


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- 





"■%/- e 

e + 


,....-^ n- 




S- c- - 



— - 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 

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 



• • 

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 


The Tracks of Nuclear Particles 




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 

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 


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 











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






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. 



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 


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 

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 

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- 










\ - p > 








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. 






ARGON and/alcohol VAPOR 






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. 







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 


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- 












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. 


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 


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 


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. 


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 

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 


t <. 



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". 


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 


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 


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. 


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- 


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 


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 


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. 


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. 


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 


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. 


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. 


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. 


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 

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. 









4\ A A A A A A 











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- 


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 


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. 


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. 


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. 


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. 


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. 


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. 


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. 


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. 


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 

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 

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. 


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). 


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 




The Cyclotron as seen by the Electrical Engineer 

The Cyclotron as seen by the operator 


The Cyclotron As Seen By. 


The Cyclotron as seen by the Theoretical Physicist 

The Cyclotron as seen by the Visitor 


The Cyclotron as seen by the Health Physicist 

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

The Cyclotron as seen by the Experimental Physicist 


The Cyclotron As Seen By. 

The Cyclotron as seen by the Laboratory Director 

The Cyclotron as seen by the Government Funding 


The Cyclotron as seen by the student 


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 


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 



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 

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- 


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 

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 



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 

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 


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 



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, 



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 



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 

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 


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 

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, 



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 

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 


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 

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. 


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. 


Radioactive piston ring 

»; Radioactive iron, Fe^' 

^ for friction and lubrication studies 


Samples.measured for Fe^^ 

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 

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 


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.) 





Leaves 40% 

trunk and 

branches and 

nfiost roots ~ 47% 


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.) 


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 


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 


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 

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 


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 

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. 


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. 


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 


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. 


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 

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). 


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. 


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." 






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 


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 only part ''.'^^"J^^^^J^^'^.^i ,o^er resolution, 
observed when nucleus is bombarded 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- 

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 

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


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- 

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. 


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 


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 


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 


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 


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- 


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 


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 


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 


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- 


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 


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- 

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." 


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


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 


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. 


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- 

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 


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 " 



"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 


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. 



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 


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 



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

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 


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 

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 

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- 



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) 


F.D. Roosevelt, 

President of the United States, 

White House 

Washington; D.C. 


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 

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) 


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 


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- 



Recent Sales of Water Reactors 






Oyster Creek 

Jersey Central 


General Electric 

San Onofre 

Southern California Edison 



Nine Mile Point 

Niagara Mohawk 


General Electric 

Haddam Neck 

Connecticut Yankee 



Dresden 2 

Commonwealth Edison 


General Electric 


Boston Edison 


General Electric 

Millstone Point 

Northeast Utilities 


General Electric 


Rochester Gas & Electric 



Indian Point 2 

Consolidated Edison 



Turkey Point 3 

Florida Power & Light 



Turkey Point 4 

Florida Power & Light 



Dresden 3 

Commonwealth Edison 


General Electric 


Carolina Power & Light 




Consumers Power Company 


Combustion Engr. 

Point Beach 

Wisconsin Michigan Power 



Quad Cities 1 and 2 

Commonwealth Edison and lowa- 
lUinois G «fe E 

2 X 810 

General Electric 


Northern States Power Co. 


General Electric 

Browns Ferry 


2 X 1100 

General Electric 


Vermont Yankee 


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 




Virginia Electric Power Co. 

2 X 800 



Boston Edison 


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. 


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." 









^ 150 






EN 1 








*-GE 1 


1 N 





1 1 1 




\ > y 1 1 

N^r*" ^■\P0INT2 


' / 


\ 1 






300 500 700 




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/ 

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. 


Power Cost Estimates 

1, 2 

Oyster Creek 




TVA coal 




Expected stretch, 


620 Mw 

1100 Mw 










First 10 

First 12 

First 12 














The Nuclear Energy Revolution 


~ 15 









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 FJrtww i o n, 1962). 


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- 


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 


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: 


, + (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- 


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). 


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 

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 


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 

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 

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+. 



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: 


(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. 


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 


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. 


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 


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, 

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 


Conservation Laws 





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. 



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 




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 


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 


Figure 4.3. 


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 

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?) 









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. 


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) 


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, 


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 



Right hand 


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- 


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. 




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 


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). 


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 

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 


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. 


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 


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 


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. 


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. 


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. 


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 

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 


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 


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- 


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. 


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. 


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. 


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 

1. They could assume that the two K-mesons, even though 
indistinguishable in properties, were really two different par- 


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- 


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 

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 

"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 


The Fail of Parity 

right from the left. Some such possible experiments will be 

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- 

"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 


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. 


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 


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 


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 


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 


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 


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 

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. 


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 


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- 

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 


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- 


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. 


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 


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 

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. 


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 


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 

"Without any mystic appeal to con- 
sciousness," declared Sir Arthur Edding- 

/♦ riMf- Rt </f (Vf» C»*\t€. fTAlf 




" "~"~^ 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. 


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 

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. 


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, 


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- 


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. 


Can Time Go Backward? 




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- 


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- 

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). 


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- 

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. 


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. 


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 


A Report to the Secretary of War 

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. 


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. 


A Report to the Secretary of War 

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 


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. 


A Report to the Secretary of War 

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 

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 

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 


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 

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 


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 

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. 


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 


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 


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. 


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 

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 


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 

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 


The Privilege of Being a Physicist 

more primitive to more sophisticated 

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. 


"There is not a single industry today 

that does not make use of the results of atbnriic 

physics or of rfiodern che-mistry/' 



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- 

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 


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- 


plied; he may work with his govern- 
ment on the manner of application of 
scientific results to military or social 

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 


"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 


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. 


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- 


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 


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. 


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 


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- 

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: 


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 

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 


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 


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. 



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. 

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 


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 

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 


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 

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- 


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 


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 

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 


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? 


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 


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 

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 


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 


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 


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. 


A personal statement, by a noted Polish theoretical 
physicist, shows his excitement with his work and with 

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, 


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 

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 


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 


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" 


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 

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 

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 

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. 


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 

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- 

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 


The Development of the Space-Time View of Quantum 

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 / = 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) 



/..—[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 

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 

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 = 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 t he 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 


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- 

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 


The Development of the Space-Time View of Quantum 

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 

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 


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 

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 «. 


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 


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 

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 

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. 


The Development of the Space-Time View of Quantum 

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 

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 

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 

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 


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 

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 


The Development of the Space-Time View of Quantum 

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 


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. 


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 


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. 


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, 


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- 

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 


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 

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." 


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. 


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- 

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 


We are all aware of the highly fruitful 
relations between advances in atomic 


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. 


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. 


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 

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 

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 


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 

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 

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. 


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- 

In closing, I mention once more the 
consistency, the connectivity, revealed 
by physical studies up to the present. 


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. 


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 



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 

G. P. Thomson, The Foreseeable Fu- 
ture, 2nd ed.. Viking Press, New York 
(1960). D 


Authors and Artists 


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, 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 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 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 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, 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 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 


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 


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, 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, 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 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 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 : 
T he Scientific Elite ond The Uses of Power . His 
interests include cosmic radiation, moss spectro- 
scopy and civil defense. 


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. 


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 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 was born in Berlin in 1907 
and received degrees from several universities, 
including a D.Phil, in Theoretical Physics from 


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. 

(see page 256) 


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 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, 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 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, 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 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. 


Authors and Artists 

(see page 21 2) 


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, 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 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 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. 


'V- I,