IN THE LATE 1950s and the 1960s, most theoretical physicists were working on particle physics and quantum field theory, to understand the workings of nature at the subatomic level. This endeavour was boosted by the large amount of data coming out of the new accelerators at CERN, Geneva and Dubna near Moscow. In 1956–57, when I had partially completed my Ph. D. thesis, and felt more confident about where I was headed, I, too, turned from gravity to these two popular topics and joined the informal particle theory group at Cambridge. I wanted to be involved in the exciting activities of my colleagues.
In his position as Lucasian Professor of Mathematics, the chair filled originally by Isaac Newton, Paul Dirac was the senior professor of theoretical physics at Cambridge. His office was in the Mathematics Department at the Arts School on Benet Street, where the Theoretical Physics Department was also housed. The Cavendish Laboratory, where much of the experimental physics took place, could be reached from the Arts School by an alley. Dirac was titular head of the theory group in physics, which consisted of some senior lecturers and the research students. He appeared every Thursday for our theoretical physics seminars. Otherwise, he did not show up very often.
Paul Adrian Maurice Dirac was born in 1902 in Bristol, England, where my parents and I had suffered through such intense Nazi bombings during the war. A fellow pupil at the school Dirac attended there was Archibald Leach, who later became the Hollywood film star Cary Grant. An abstract sculpture in Bristol today celebrates Dirac as a famous native son of the city, and not far away is a lifelike bronze statue of Cary Grant.
Dirac took an engineering degree, but later switched to theoretical physics and became a student and later a fellow of St. John’s College, Cambridge. He was one of the co-founders of quantum mechanics, and in 1928, when he was a twenty-five-year-old research student at St. John’s, he published his famous Dirac equation describing the spinning electron as both a particle and a wave. The equation also predicted the existence of anti-matter, in the form of the positively charged electron. There was opposition to Dirac’s paper from such physicists as Bohr and Oppenheimer. Yet experiment vindicated Dirac in 1932,when Carl Anderson at Cal-tech discovered experimentally an electron with a positive electric charge. He christened this anti-matter particle the “positron.” Dirac and Schrödinger shared the Nobel Prize for physics in 1933, the year after I was born, for their contributions to developing quantum mechanics.
The Dirac equation is one of the most significant discoveries in the history of physics because it predicted a new particle and the existence of anti-matter. At Cambridge, Dirac was a famous but reclusive figure. Because of his extreme reticence and private nature, he was not well known to the public as Einstein was, but behind the scenes he was revered by physicists.
Dirac was not seen much at the Arts School when I was at Cambridge, and much of the supervision of the students was performed by James Hamilton. As a student, I pondered how Hamilton reacted psychologically to doing research in particle physics in the shadow of Dirac’s genius and fame. Hamilton seemed to regard me as an odd student due to my anomalous academic background, but was pleasant enough to me most of the time.
Hamilton conducted the weekly Thursday seminar on quantum field theory techniques and particle physics. Two of my fellow students at this seminar were John Polkinghorne and Geoffrey Gold-stone. Much later, in 1979, Polkinghorne left theoretical physics and trained for the Anglican priesthood. He has since become famous for his writings and lectures on the role of religion in scientific research, and how his personal beliefs in God and Christianity do not, in his opinion, conflict with the search for truth in scientific research. He was awarded the Templeton Prize in 2002 “for progress towards research or discoveries about spiritual realities.” This prize, established by the British-American entrepreneur Sir John Templeton in 1972, is awarded annually with the stipulation that the cash award be slightly larger than the Nobel Prize, which is currently valued at almost $1.5 million. The Templeton Prize is awarded to people in many different fields and of different religious backgrounds, but the awards to scientists have been criticized as encouraging the undermining of rational scientific thought.
My other fellow research student at Trinity, Geoffrey Gold-stone, showed early brilliance as a theoretical physicist. At the time of the annual Trinity fellowship competition, I discussed with him how he was faring with the preparation of his required research essay. He told me in a casual manner that he would write one soon.
I asked him when the deadline for the essay competition was, and he calmly responded that it was in a few days. I felt deflated by this news, knowing that I could never have met such a deadline, or treated it with such aplomb. Goldstone later collaborated with the celebrated Hans Bethe on problems in nuclear theory. In 1961, he published a fundamental paper introducing the idea of spontaneous symmetry breaking in particle physics.*
Another student at Hamilton’s seminars with whom I became friends was Walter Gilbert, who would later switch to molecular biology, become a professor at Harvard and win the Nobel Prize in 1980 for his work on recombinant DNA. While at Cambridge, he was also an accomplished particle physicist and contributed important papers to the subject in the 1960s.
In this talented company, I entered the active field of particle physics.
Invariably, at each Thursday seminar, Dirac would sit in the first row, a slim, nondescript figure in his tweed suit. Early on in the talk, he would look at the speaker in his rather distant way and ask, “Is this a three-vector or a four-vector?” This referred to a vector field in quantum field theory either being in three-dimensional space or in the four-dimensional spacetime of special relativity theory. The question really had no relevance to the talk, but Dirac always asked it because, being the senior professor, he probably felt obliged to ask a question. We students had decided that one week the answer would be “three-vector,” and the next week whoever was speaking would reply “four-vector.” Dirac seemed completely unaware of this conspiracy, and after the question was answered, he would promptly fall asleep, with his chin sagging onto his chest and his mouth open.
Uncannily, however, very near the end of the talk, he would rouse himself, stare intently at the speaker, and, whatever the subject of the lecture, would ask his second question:“Can you fit the electron into your scheme?” Again, most often we considered this to be a totally irrelevant question because the talk would be about some technical subject such as dispersion relations in the scattering of elementary particles, in which the electron did not play a role at all. Again we had a conspiracy going, with three possible answers. One answer would be: “I’m not sure, Professor Dirac.” The second one would be, “No, I don’t think it can be fitted into this scheme.” And the third one would be, of course: “Yes, you can fit it into this scheme.” Dirac seemed unmoved by whichever answer we gave, and after some other desultory questions were asked, the seminar would end.
Dirac was particularly concerned about the electron because he did not keep up with the parade of new particles being discovered in accelerators at that time, and so could not ask a question about any of them. And of course the electron figured very prominently in his celebrated work on quantum mechanics, in his famous Dirac equation and in his early development of quantum field theory.
At the beginning of my second year of research, at one of the theory group seminars, Hamilton announced that the half-dozen or so students who had completed their first year of graduate work had to speak at each successive seminar on an original research topic of their choice. This was a daunting assignment, for finding an original research topic was a serious challenge so early in our careers.
When my turn came, I talked about quantum field theory and Hilbert space. This was a technically difficult corner of quantum field theory that had caught my interest. At my talk, everybody was surprised to see that Dirac did not fall asleep, and instead of asking his standard question about whether the field was a three- or four-vector, he asked me a specific question about the notation I was using. In fact, I was using a notation that was popular among American quantum physicists, in which a round parenthesis was used to open and close a symbol in the equations of a quantum field theory. I explained this to Dirac. At this time, it was also common practice to use Dirac’s notation, which he had invented, called the “bra” and “ket” notation, which of course meant “bracket.” The “bra” was in the form of a triangular bracket to the left, and the “ket” was a closing triangular bracket to the right. Amusingly, Dirac was very curious about my choice of notation, and why I had not used the famous notation he had invented. Unlike Schrödinger, who took me to task for using Einstein’s method of deriving the unified field equations rather than his own, Dirac did not seem upset about my choice of notation; he merely wanted to know why I was using it. I replied that I was using the American notation because it was used in several of the papers I had studied that were pertinent to my talk.
Later in my talk, Dirac was very interested in a particular result I had discovered in relation to Haag’s theorem in quantum field theory. This theorem concerns the technical details of Hilbert space in quantum field theory. Most surprisingly, Dirac focused on that topic, and did not ask his famous question about whether I could fit the electron into my scheme.
As it turned out, three or four of the students who had given talks at the Thursday seminars in Dirac’s presence received letters from the Board of Research Studies informing them that they were being sent down from Cambridge—they were no longer research students at Cambridge! They had received no prior warning about this. Thus, although the circumstances of our graduate research and supervision at Cambridge seemed very lax—allowing me, for example, to neither attend courses nor take exams—it turned out that this was an illusion. Giving a talk as a second-year student on a piece of original research was very difficult because most students had not yet acquired the knowledge or the originality in physics to have their own ideas and to meet such a challenge. To be “guillotined” after such a talk was an ultimate and abrupt sentence.
Many years later, I attended a talk by Dirac at the annual meeting of the American Physical Society in New York, when he was a visiting professor at Yeshiva University there. He brought up the very issue that I had raised in my talk years earlier at the Cambridge Thursday seminar—Haag’s theorem and non-separable Hilbert space—which he had shown such an interest in then. He now called this problem the “deadwood” of quantum electrodynamics (QED),* meaning that it was a part of quantum field theory that should not be retained in the theory, but was not easily removed from it.
As Dirac was leaving the auditorium after his talk, I stopped him, shook his hand and said, “I’m John Moffat. You may recall that I was a student in your theory group at Cambridge. At a Thursday seminar, I once talked about what you now call the deadwood of quantum electrodynamics.” Dirac smiled and said in his cryptic way, “Yes, I do recall that. Very interesting.”
I had some infrequent but memorable interactions with the great Dirac in my student days at Trinity. One afternoon I was walking up the stairs to the students’ study room on the second floor of the Arts School and met Dirac on his way down. He stopped me and said, “Oh, Moffat. I want to ask you a question. Can you explain to me what this particle is that they call the K-meson?”
I explained to him in some detail what the recently discovered unstable, electrically neutral K-meson was and its place in the lineup of elementary particles. He smiled graciously, thanked me for the information and continued down the stairs. I stood still a moment before continuing up the stairs, pondering how a physicist ofDirac’s stature could not have been following the growing and exciting literature on the new elementary particles being discovered in accelerators almost every month.
Another time, Dirac came out of his office as I was leaving the Arts School study room, and again he stopped me and said, “Oh, Moffat! You know, I have been studying Einstein’s gravity theory, and I came upon this fascinating mathematical result.” We went into the study room and he wrote on the blackboard a set of mathematical identities—equations that are always satisfied in an identical way—involving the Riemann curvature tensor and the conservation of energy in Einstein’s field equations.
“Ah, Professor Dirac!” I exclaimed. “These are known as the Bianchi Identities, discovered by the Italian mathematician Luigi Bianchi in 1880.”
Dirac smiled and said, “Oh, now, isn’t that interesting!”
This incident revealed to me two very intriguing things about Dirac: first, it was not only the particle physics literature that he was not cognizant of, but the standard literature on relativity theory as well; and second, he was so ingenious that he had actually discovered the fundamental Bianchi identities by himself.
Towards the end of the class, Dirac wrote another equation on the blackboard, paused, and walked to the window, looking out at the grey spring morning with his hands behind his back. We students sat and waited. Finally he turned and said, “We will continue next week.” Thus ended another memorable lecture by Dirac based on his famous book published in 1930, Principles of Quantum Mechanics.
We all stormed out of the Arts School lecture room. I nodded at a fellow student, James, as we entered Benet Street and walked together down King’s Parade towards Trinity College and lunch. It had started to drizzle and I turned up the collar of my jacket, rearranged my black gown over it and said, “I heard a rumour that Heisenberg might be coming to Cambridge this week.”
James was a second-year research student like myself at Trinity College, who had chosen quantum field theory as his Ph. D. subject. He had a pallid, scholarly look, and a slightly stooped posture even at the age of twenty-one. He sniffed, looked up at the sky and said, “Well, it looks like we’re in for another wet spring. As for Heisenberg, yes, he is coming this week. He’s giving a public lecture at the Cavendish Maxwell Lecture Theatre Thursday evening at seven.”
“Can’t miss that,” I said. “I heard about his visit to New York a month ago when he presented his new ideas on a unified field theory of particle forces. Apparently that didn’t go down well with Bohr and Pauli.”
We passed the market and the Senate House and entered Trinity Street, making our way single file down the narrow pavement, as students jostled one another to get to Trinity and St. John’s for lunch.
“I wonder if Heisenberg’s activities with the German bomb research during the war caused any rift between him and Pauli and Bohr,” I ruminated as we walked. “Those two weren’t happy about his activities with the Nazis during the war. But I hope it wouldn’t influence their behaviour towards him and the physics he’s doing.
Heisenberg is always working on some important new developments. I’m looking forward to his talk.”
We reached the entrance of Trinity College, walked in past the porter’s lodge and entered Great Court. “Did you see the British Times last week?” James asked. “They actually published Heisenberg’s unified theory equation below the obituaries.”
“Hmm,” I said. “Is that where it belongs?”
Thursday evening, after my usual dinner of baked beans, chips and tea at the Lion’s Restaurant, I made my way to the Cavendish Laboratory early, to get a seat for Heisenberg’s public lecture. Already some of my black-gowned compatriots were seated in the front rows of the gloomy, austere Maxwell Theatre. I sat in the second row instead of the first in order not to make myself too conspicuous, but close enough to see the great man in action. About twenty-five minutes remained until the lecture, and already the hall was filling up. It was an audience consisting of professors and students from different faculties, the spouses of professors and their families. This was obviously going to be a popular lecture, I thought.
A side door opened and in walked Professors Neville Mott and Paul Dirac, and a slightly built man in a grey suit with greying blond hair, a smile and twinkling eyes. The trio came over and stood near us, talking physics. Our group leader, Jim Hamilton, joined them. Soon he led the visitor over to where we were sitting, and in his usual breezy autocratic manner, said, “These are the students in our group studying quantum mechanics and quantum field theory.” We stood up and were introduced in turn to Professor Heisenberg, who leaned over and shook our hands one by one in a friendly way. “I just shook the hand of the famous Werner Heisenberg!” I thought, thrilled.
When the amphitheatre was filled, the doors were closed. Mott introduced Heisenberg, enumerating his successes and telling how he had helped to develop quantum mechanics and had won the Nobel Prize. Then Heisenberg walked to the podium and began lecturing. He spoke in an animated way, with a slight German accent, about his new unified theory, waving his hands enthusiastically for emphasis and striding back and forth across the stage in front of the podium. I thought that he had probably always been enthusiastic about physics and that his love of the subject must be the driving force that led him to make such great discoveries. Now he was in his fifties and had undergone difficult times during the war, being part of the Nazi war machine’s effort to make an atomic bomb.
He had also suffered serious privations at the end of the war when Germany was occupied by the Allies. It showed on his face. In spite of all this, he managed to inject a sense of humour into his talk, and smiled at the audience as he spoke. I noticed that he wore a gold tie pin in the shape of an h-bar, which stands for Planck’s famous constant divided by two pi. This must have been a private joke, or perhaps a personal totem, for the h-bar was a significant element in Heisenberg’s famous equation describing the uncertainty principle in quantum mechanics.
When the lecture ended, Mott invited questions from the audience. There were a couple of questions from lay people about developments in quantum mechanics, and how Heisenberg came to the idea of his unified field theory.
Then, suddenly, Dirac stood up and asked, “Werner, can you fit the electron into your scheme?” We students in the first and second rows looked at one another and couldn’t help smiling at the familiar question, but the moment was disturbing too, because in Cambridge academic circles it was unheard of that a professor would seriously question another professor at a public lecture, particularly a speaker as renowned as Heisenberg. This was not cricket, not the English way of doing things. Dirac, widely referred to as “the silent physicist,” normally never asked any questions at public lectures, and, in fact, was known for not expressing his views about anything at all. He preferred to hide out at his house on Cavendish Road, repairing his old Rolls-Royce and thinking about physics in his inimitable way.
Heisenberg stopped his pacing and suddenly looked quite pale in the face, staring uncomfortably at Dirac. We had heard from a professor who had visited Columbia University recently, where the meeting between Heisenberg, Pauli and Bohr had taken place, that this very point was the downfall of Heisenberg’s unified theory as far as Bohr and Pauli were concerned. It was at this meeting that Niels Bohr had uttered his much-quoted comment: “Werner, your theory is crazy, but it is not crazy enough!” Pauli, too, had berated Heisenberg in his normal aggressive way, and the meeting had ended with Heisenberg depressed and upset. With the exception of the war years, when there was little contact between the two men, Heisenberg had often been subjected to Pauli’s criticisms throughout his career, particularly when he was developing quantum mechanics in his early twenties. But Pauli had treated him particularly badly at the recent New York meeting because, in fact, he could not fit the electron into his scheme. And of what use is a unified theory without any electrons?
We all turned around to look at Dirac some rows back as we waited for an answer. Even Dirac, in his unworldly way, was beginning to look uncomfortable. He had simply asked Heisenberg whether the electron could fit into his scheme because that was his stock question. Knowing Dirac, he had probably not discussed Heisenberg’s meeting with Bohr and Pauli with the professor who had been at Columbia and indeed was most likely unaware of the event.
The silence continued, and the audience waited. Finally, Heisenberg shook himself out of his reverie and said, “Well, Paul, this is the one serious problem I have with this theory. I cannot yet fit the electron into the scheme.” The hushed silence of the audience was broken by murmurings. It was clear to everyone that Dirac had unwittingly hit the bull’s eye with his question. It was also clear to Mott that Dirac had precipitated a socially disastrous situation.
There were no further questions, and the public lecture broke up.
As we students left, we observed Dirac, Mott and Hamilton gathered around Heisenberg, soothing the wounds of our honoured guest.
As James and I walked into King’s Parade past King’s College, he laughed loudly and said, “Well, I wonder if Dirac is going to ask this question next Thursday at my talk.”
I said, “Yes, indeed, that would be an interesting development, to see whether Dirac finally realizes that his question is irrelevant. But then again, it turned out that it was very relevant for Heisenberg!”
In contrast to Einstein’s attempts to discover a unified field theory based on classical field equations for gravity and electromagnetism, Heisenberg’s scheme only attempted to unify the subatomic forces, that is, the electromagnetic force, the strong nuclear force and the weak interactions, excluding gravity, and he used the techniques of relativistic quantum field theory. In my work on unified field theory, like Einstein, I initially left out the subatomic forces, although in Copenhagen I did attempt to make Einstein’s unified field theory into a relativistic quantum field theory with the nuclear forces.
Heisenberg had put his group in Munich to work on his unified theory, and since he was the director of the Max Planck Institute there, everyone in the group was forced to work on this subject, whether they believed in the validity of the project or not. Unfortunately, the electron seemed to sit by itself in an antisocial way in this scheme because of its small mass, and it did not interact strongly with the other particles, such as the nucleons, the pi-meson, the K-meson and the rest of the strongly interacting particles discovered in the 1950s. This antisocial behaviour of the electron showed itself in Heisenberg’s scheme. He had a method of calculating the masses of elementary particles from his equations, and the electron mass simply did not come out of the calculations. Heisenberg’s scheme, which he claimed was a unified theory of particle physics, failed because clearly the electron, an essential stable elementary particle that played an important role in the theory of weak interactions, was simply not present in Heisenberg’s model.
As it turned out in the following years, after much effort by those working with Heisenberg at the institute in Munich, the theory did not live up to his expectations. After his death in 1976, only a few devotees continued to try to solve the problems posed by Heisenberg’s unified field theory. With younger physicists coming along and wanting to engage in more promising research, the project was eventually abandoned. Heisenberg’s immense success with the discovery of the uncertainty principle, which is at the foundation of quantum mechanics, and his development of particle-based matrix mechanics, which was an alternative to Schrödinger’s wave mechanics, had made him one of the most influential figures in postwar physics in Germany and internationally. But he was criticized by his peers both abroad and in Germany for demanding that his group, and anyone who was hired in Munich, or indeed at other physics institutes in Germany, should work on his unified theory project, which was not considered a worthwhile effort by physicists such as Bohr and Pauli and others in the United States.
In contrast, Dirac worked in isolation, rarely collaborating with other physicists, and had little influence on the progress of physics at Cambridge, despite the fact that he was just as famous as Heisenberg for his part in the development of quantum mechanics. Heisenberg won the Nobel Prize in physics in 1932, and the following year Dirac and Schrödinger shared the prize.
By 1957, near the end of my studies at Cambridge, I had to concern myself with my prospects of finding a job. I was then married to Bridget, who worked as a secretary at a firm outside Cambridge. We had hopes of starting a family someday, and I needed to find a secure position. There wasn’t a great demand around the world for physicists specializing in gravitational theory. But I hoped that my recent work in particle physics would bolster my resumé. I applied for a government fellowship at the U. K. Department of Scientific and Industrial Research, and also for a position at the British Atomic Energy Laboratory at Aldermaston. Alarmingly, I had received notification that I could be conscripted into the army. The prospect of spending two years in the army, in the wake of the Korean War, did not please me. In my mind, this would be a serious impediment to a future academic career. I decided to ask Professor Dirac to write a letter of reference for the government fellowship I’d applied for and the position at Aldermaston. I would also ask him for a letter recommending that any army conscription be deferred.(As it turned out, military conscription continued in Britain until 1960.)
I made an appointment with Professor Dirac’s secretary to see him at his house, a villa in one of the wealthier neighbourhoods of Cambridge. When I arrived, I spied a pair of legs in grey flannel trousers sticking out from underneath an old Rolls-Royce in the garage. I decided not to interfere with Dirac’s mechanical repair work. I went to the door, rang the bell and was ushered in by a maid in a black-and-white uniform. She took me to the back of the house and invited me to wait in a sunlit drawing room overlooking a colourful garden. I saw no sign of Mrs. Dirac. I knew that she was the sister of Professor Eugene Wigner, a Nobel Prize–winning physicist at Princeton University. The story had gone around in the student circles at Cambridge that on occasion, when introducing his wife, Dirac would announce, “This is Wigner’s sister.”
I sat in an easy chair looking out at the garden, and listened to the birds singing. It was early spring, and sunlight streamed into the room through the French doors. Eventually Dirac appeared, with clean clothes and with washed hands. We shook hands cordially and he sat down opposite me. A silence ensued, during which I expected him to ask me why I had come to see him. But he said nothing. Eventually I said hesitantly, “Professor Dirac, I’ve come to ask whether you would write a letter of recommendation on my behalf. I have applied for a fellowship in the Department of Scientific and Industrial Research and a possible position at Aldermas-ton Laboratory doing nuclear physics research.” I didn’t add that the British government was busy trying to make an atomic bomb at Aldermaston.
Again, a silence ensued. We both stared out the French doors, and only the chirping of the birds broke the silence. Dirac simply smiled and said nothing. I moved uneasily in my chair and said, “Perhaps I should tell you about my current research activities. As you know, I gave a seminar when you were present on non-separable Hilbert space in quantum field theory.” Dirac said nothing. “I’m planning to continue this research, and also further some work I have been doing on gravitation theory,” I added.
I expected he would ask me some questions about my research, but he remained silent. “I also have to ask you for a letter so that I can get a deferment from military conscription. I’ve received notification from the military that I’m facing conscription after I finish my Ph. D.”
The maid knocked on the door and arrived with a tray of tea and cakes. This was a welcome relief from the silent tension, broken only by my voice. We drank the tea in silence and I munched a piece of cake noisily. I began to feel that this interview was becoming ridiculous and surpassed any of my expectations regarding Dirac’s reputation as being a kind of deaf-mute. He had talked to me in the past when he wanted information about some physics problem. Even then, the conversation on his part had been terse. Later in life, I would realize that Dirac showed symptoms of autism or Asperger’s syndrome in his seeming inability to relate normally to people. Once when I was a visiting professor at the University of Texas at Austin, I attended a lecture given by Dirac when he was also a visiting professor there. After he finished his talk, Bryce Dewitt, one of the senior professors in theoretical physics, known for his pioneering work on quantum gravity, stood up at the back of the room and asked a complex question that took at least five minutes to complete. “Do you agree with my impression that what I am asking is true, Paul?” he concluded.
Dirac stood in his tweed suit looking gaunt and much older than when I had known him at Cambridge. He contemplated what Dewitt had said, and then he merely said, “No.”
Eventually the strain of the silence in the spring drawing room began to overwhelm me, and I stood up nervously and said, “Thank you, Professor Dirac, for seeing me. I hope that you can provide the needed letters of recommendation.”
He accompanied me to the door, opened it and smiled at me. The maid came and nodded sympathetically. Without a word, I left. This was the strangest interview that I would ever experience in my life.
As it turned out, Dirac did provide a letter of recommendation for my government fellowship, and my application for a job at Aldermaston. And since I never heard another word from the army, I could only assume that he had been equally persuasive in a letter to them.
I would meet up with Dirac again in later years. In addition to the meeting in New York where he discussed the deadwood of quantum electrodynamics, he was often present at the Coral Gables conferences on particle physics held at Miami University, as he had moved to Florida State University at Tallahassee, after retiring from Cambridge. At one of these meetings, supersymmetry was the latest physics fad, and there was a special session on this new topic. Supersymmetry is a symmetry occurring in particle physics in which it is postulated that for every boson particle with integer spin, there is a corresponding fermion particle with half-integer spin. For example, the photon, which is a boson, would have a super-symmetric partner called the photino, a fermion. The theory was called supersymmetry because it would constitute the biggest symmetry you can have in particle physics and spacetime. Supersym-metry was invented to get rid of certain technical problems in particle physics.
After hearing a talk on supersymmetry by an excited young physicist, Dirac stood up at question time and asked, “Why are we interested in this so-called supersymmetry, for no experiment has ever detected supersymmetric particles?” This was typical of Dirac’s literal and logical approach to physics. In spite of his preaching that physics theories should be elegant and beautiful, he still maintained that such “beautiful” theories had to be verified by experiment.
One afternoon during that Miami conference, all the attendees congregated in a courtyard outside the conference auditorium for a group photograph. While the photographer was preparing his equipment, a journalist from a local newspaper interviewed Dirac. Behram 
, a Turkish-born physicist who was director of a theoretical physics research institute at Miami University, organized the annual Coral Gables conferences and idolized Dirac, was hovering over Dirac and the journalist in his usual protective manner. I was standing nearby and overheard the conversation.
said to the journalist, “Professor Dirac is one of the greatest physicists of the twentieth century.”
The journalist turned to Dirac and asked him, “Do you agree with this assessment, Professor Dirac?”
There was a long silence, and then Dirac said, “No.”
Several seconds passed while and the journalist digested Dirac’s response. Then the journalist asked, “Professor Dirac, why would you not agree with this assessment of your career?”
Dirac brooded on this question and then answered, “Because I have only won one Nobel Prize.” Dirac was referring to the fact that the American physicist John Bardeen had won two Nobel Prizes in physics—one in 1956 and the other in 1972—doing one better than Marie Curie who had won one prize in physics, but her second prize only in chemistry. In his literal way, Dirac found his single prize lacking.
Many years later, Dirac and his wife, Manci (Wigner’s sister), attended a conference at Abdus Salam’s International Center for Theoretical Physics in Trieste, Italy, where I was also an attendee. We all stayed at the Adriatico Palace Hotel on the waterfront, with the marina and its splendid yachts nearby. One evening before dinner, I was sitting in the hotel lobby with and Manci Dirac. This was the first time I had met Dirac’s wife. She was a strong-looking, assertive woman who was not particularly attractive.
Eventually Dirac appeared out of the elevator in his perennial brown, crumpled, three-piece tweed suit. Manci frowned at him, and said sharply, “Paul, you are so stupid! You can’t even put on your own trousers.” It came out in the conversation that she had had to help him get dressed prior to her leaving the hotel room. Dirac looked sheepishly at us and smiled, and did not respond to this comment. I was surprised but not shocked by Manci’s behaviour, because I had heard that she could be quite sharp and critical towards her famous autistic-savant husband. looked upset and said, “Mrs. Dirac, how can you call one of the greatest physicists of the twentieth century stupid?”
Manci turned on Behram and barked, “He’s my husband. If I want to call him stupid, that’s my business.” Behram had nothing further to say to this.
On the other hand, Manci could be very supportive of Dirac. After his retirement from the Lucasian chair at Cambridge, the university took away his parking space at the Cavendish Laboratory, which he had had for decades, and even worse, took away his office at the Arts School, forcing him to work at home. Manci wrote outraged letters on Dirac’s behalf to the Cambridge administration. Although the letters certainly proved her support for her husband, they had no effect on the administrators’ treatment of him. Neither did the fact that Paul Dirac was, arguably, the greatest British physicist since James Clerk Maxwell and Isaac Newton.
*In particle physics the vacuum is the state of lowest energy. This state can have a symmetry, described by certain mathematical groups. The spontaneous breaking of the vacuum state symmetry, Goldstone discovered, creates quantum spin zero massless particles, which were later called the Goldstone bosons. The spontaneous aspect of the symmetry breaking can be pictured as a ball balanced on top of a hill. The balancing is precarious, and any slight perturbation triggers the ball to roll down the hill. Once the ball chooses a direction in rolling down the hill, then it is said that the symmetry of the balancing ball has been spontaneously broken.
*Quantum electrodynamics is the theory that quantizes James Clerk Maxwell’s classical electromagnetic field equations. It is a quantum theory of charged particles,
*such as the electron, and particles that convey the electromagnetic interaction between them, namely, photons.