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Full text of "Personal Recollections about P.A.M. Dirac, by Dr. Maurice Pryce"

JOURNAL 

of the 

U.B.C. Physics Society 



APRIL 1985 
VOLUME 24 NUMBER 1 



1 

PERSONAL RECOLLECTIONS OF P. A.M. DIRAC. 

Dr.M.H.L.Pryce 
Honorary Professor of Physics 
University of British Columbia 

To call P. A.M. Dirac a remarkable thinker is an extreme understatement. The 
originality of his formulation of physical phenomena can hardly be matched 
in the history of scientific thinking. His first paper on quantum mechanics, 
published while he was still a graduate student, creates a conceptual framework 
quite different from the classical description of physical happenings, which 
still endures to this day. It tacitly implied that this is the way things 
are, whereas the classical language is just a rather clumsy attempt at talking 
about the "real 11 world. 

Dirac did not actually originate the new concepts. He had the good fortune 
to hear the visiting Heisenberg give a seminar at the Kapitza Club in Cambridge, 
in July 1925, about his new ideas on quantum mechanics, and a month later, 
to see the proofs of Heisenberg 1 s first paper before publication. He appears 
to have recognized immediately that here was a vital clue, and he clearly 
believed that no excuse was needed for using abstract ideas . It was this 
independence from accepted preconceptions which was the mark of Dirac* s style. 

To look at, Dirac was very different from the popular image of a scientist, 
and more like a poet or an artist. He was of slight build, to the point of 
appearing frail, and sparing of speech to the point of taciturnity. He husbanded 
his inner resources in order not to waste them on idle social intercourse. If 
asked a simple question to which he knew the answer, he would give it in simple, 
succinct language, in a very quiet voice. If he did not know the answer, quite 
often he would not reply at all. He rarely volunteered a comment, and when 
he did it could be very naive if it dealt with things that did not greatly 
concern him, or could be very penetrating. He avoided tea, coffee and other 
stimulants. 

I was nineteen years old when I first met him personally, a second year 
undergraduate at Cambridge and secretary of the Trinity Mathematical Society, 
with the task of organizing speakers for the coming year ! s programme. I called 
on Dirac in his rooms in St. Johns College and shyly explained what I wanted. He 
replied, equally shyly, that yes, he had been working on a mathematical structure 
called spinor analysis, which the society might find interesting. We fixed 
a date in the somewhat distant future, and I left without further conversation. In 
due course, by which time I was president of the TMS, Dirac arrived at the 



PHYSICS UNDERGRADUATE SOCIETY JOURNAL 
DEPARTMENT OF PHYSICS 
UNIVERSITY OF BRITISH COLUMBIA 
Vol.24 Number 1, April 1985 



2 

meeting room and proceeded to give a beautifully crafted lecture devoid of 
any unnecessary words. I believe it was my first experience of chairing a 
meeting, and I can still remember the tongue-tied feeling of trying to find 
appropriate words with which to thank the speaker for a fascinating lecture. 
such an occasion as this, Dirac, though noticeably aloof, was completely at 
ease and ready to give a direct answer to any relevant question. It was also 
my first introduction to Dirac *s lecturing style - though soon thereafter 
I started to attend his lectures on quantum mechanics. 

As I look back on the many occasions when I have listened to Dirac lecture, 
in university courses, seminars or conferences, I am struck by the completely 
unique quality of his style. In his university course, which was usually attended 
by a mix of fresh and mature postgraduate students, he went directly into 
the subject, essentially in the words of the first chapter of his book. In 
a bound notebook I still have my notes from those lectures. The title page 
reads "^lantum Mechanics. Dirac. Jan. 1934 ." The next page reads, "Fundamental 
of science is the fact that we have to deal only with observables. Every 
observation has an inevitable disturbing effect on the observed system and 
there is a minimum disturbance beyond which the experiment cannot be idealized. 
From standpoint of philosophy this gives a limit to the reduction of things 
to smaller things. The law of causality of classical mechanics is meaningless 
when this is taken into account. For if we wish to know all about a system 
we must observe it at every instant and this causes an infinite disturbance. 
"Superposition of States. 

Given a system of known masses and forces, a state in classical theory 
is defined by values of coordinates and momenta at given time. In Q.M. this 
knowledge is impossible. In fact we will see that a state is defined by coordinate 
alone , or momenta alone ..." 

No doubt this is a somewhat distorted rendering of what Dirac actually 
said, but it rings true. In this first lecture he started straight into the 
subject with a highly condensed introduction, as quoted above. In subsequent 
lectures he continued where he had left off, with no repetition of the last 
sentence, or a reminder of the previous lecture. 

I have alluded to Dirac' s readiness to answer direct questions. It is 
part of the legend that he would totally ignore an indirectly framed question, 
such as "It seems to me that...", and that once when the chairman asked Dirac 
if he was not going to answer a question, Dirac replied "That was not a 
question -it was a statement." Though I was not present on that occasion, 






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I have observed many similar occurences, and I can illustrate with a personal 
anecdote. In 1938 when Dirac was married and living a mile or so out from 
the centre of Cambridge, and I was living in Trinity College, Gritli Born 
came from Edinburgh to visit me and was staying with the Diracs. She was due 
to meet me one evening after dinner for a theatre performance. She had no 
watch, and the buses were infrequent, so she was anxious about the time and 
asked Mrs, Dirac - whose watch had stopped. The Hungarian cook was called 
from the kitchen, but the kitchen clock had stopped too. During all this Dirac 
sat eating his meal, paying no attention to the hubbub. Then Gritli had a 
bright idea and said "Professor Dirac, what is the time?", whereupon he took 
his watch out of his pocket, looked at it, and told her ! 

Dirac' s apparent physical frailness was deceptive, as I discovered in 
the summer of 1940 when I shared a cottage in the Lake District with the Diracs 
for a week, and went on hikes and rock climbs with Dirac and his stepson Gabor. He 
was a very competent rock climber, his lightness combined with long reach 
being an asset. He was adept at striding rapidly down scree slopes, a technique 
he told me he had learned from the Lakeland slate quarriers. His physical 
stamina was in fact quite remarkable. 

I will close these rambling reminiscences with an example of the fallibility 
of memory. In late 1932 or early 1933 I was attending the weekly colloquium 
in the Cavendish Laboratory and heard Blackett reporting on the experiments 
in which he first detected in the cosmic radiation charged particles of apparently 
the mass of the electron, but which were either coming from outer space and 
with positive charge, or were negatively charged ordinary electrons coming 
with great energy out of the earth. Dirac and Kapitza, who were close friends, 
were sitting together in the front row. It was Kapitza' s habit to tease Dirac, 
and on this occasion he turned to him and said "Positive electrons, Dirac, 
positive electrons ! Put that in your theory ", and in his usual quiet voice 
Dirac said "Positive electrons have been in the theory for quite a long time 
now." Some years ago Kapitza was visiting Vancouver and I reminded him of 
the episode. He was amazed, and had totally forgotten that it had happened. 

M.H.L.Pryce 

Vancouver, March 1985