by Cherrill M. Spencer

The author, an experimental physicist, is retired from the SLAC National Accelerator Laboratory, Stanford University.

When I was a mere postdoc at the Florida State University back in 1977-79 I had the privilege of spending a couple of days out with Dirac. Yes, that Dirac who formulated the Dirac equation in 1928 and won the Nobel Prize with Schrödinger in 1933 for “the discovery of new productive forms of atomic theory.” Who would have thought he was alive and well and living in Tallahassee, Florida in March 1977? But there he was, sitting in the front row of the audience for my colloquium on the particle physics experiment I had worked on during my previous post-doctoral job (the “Iron Ball” at SPEAR). And when Professor Dirac, the founder of quantum electrodynamics, told me in question time he didn’t believe in the Feynman diagram I had used to explain how we had collided an electron and a positron and made a muon and anti-muon, I was completely flustered and had to appeal to the other theoretical physicists in the auditorium for help¹. Nevertheless Dirac sought me out at the reception that followed and praised me for my youthful enthusiasm for physics. Then he told me he’d be joining me the next Saturday in a tour by car of northern Florida with the head of the Physics Department at Florida State University (FSU), Professor Joe Lannutti, and Assistant Professor of Physics, Ron Diamond.

So that’s how I came to be sitting in the back of a car for a day out with arguably the most famous physicist of the twentieth century. But no-one had told me that Dirac was also famous for being very precise, quite shy and bordering on autistic in his social interactions. As we drove to Wakulla Springs to take a boat ride to see some alligators I was ready to pepper him with questions about his life at Cambridge University, how he came up with his equation that describes the behaviour of electrons and predicted the existence of anti-matter, and why he’d come to work at Florida State University when he’d had the choice of any university in the world for where to go after his mandated retirement from Cambridge. When he didn’t answer my first question right away I asked him a different one and after three un-answered questions I took a break to wonder what was wrong with them. Then after what felt like an age Dirac answered my first question in a carefully-thought out way; to start with I was a bit confused because I supposed he was answering my most recent question, but then I realised what he had answered was my first question, asked over 5 minutes before! Having learnt how to converse with Professor Dirac I spread out my questions and waited patiently for his short but precise answers as we sped in the car towards the Gulf of Mexico to go for a swim after our boat ride.

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The tide was out at the deserted beach and one had to wade from shore over 50 meters to reach water deep enough to swim in. But first Professor Dirac had to check the water temperature, which he did by throwing a thermometer on a string into the water and dragging it back after a minute or two to read it. It was 73⁰F and Dirac declared that the water was warm enough to swim in, so we four took off our outer clothes, having put on our swimming costumes before we set out, and waded into the calm Gulf water with no-one else in sight. Dirac was further out than me and swimming slowly parallel to the shore when I saw what I thought was a shark’s fin gliding above the water a few meters beyond him. Oh dear, I thought, I’m going to have to save the most famous physicist in the world, who established the general theory of quantum mechanics about 45 years earlier, from a shark attack, so I shouted out “Professor Dirac, there’s a shark over there!” He stood up (the water was not deep) and looked at where I was pointing, “Oh no,” he said, “that’s not a shark’s fin, that’s a dolphin’s fin,” and proceeded to explain the difference in their fin shapes, a subject he had apparently studied in preparation for swimming in Florida. You can imagine my great relief.

After a sea-food dinner at a restaurant Professor Lannutti drove us back to Professor Dirac’s house in Tallahassee where his wife Margit (known as Manci) invited us in for a cup of coffee and to find out about our sight-seeing trip. While we chatted with her, Professor Dirac disappeared into the back of their house and eventually returned with a pile of presentation boxes which he opened up one by one to show me what was inside. They each contained one of the many prizes and honors that Professor Dirac had received over his lifetime, for example, the Royal Medal, the Copley Medal, the Max Planck Medal, his Order of Merit and his Nobel Prize medal. What a treat it was to see all those medals and to read their accompanying citations; as we left Mrs. Dirac pulled me aside and told me Dirac had never before brought out his prizes to show anyone and I must have been a good influence on him. That was a marvelous remark to hear at the end of my first amazing day out with Dirac.

A year passed before I saw Dirac again because I was working on an experiment for FSU at the Stanford Linear Accelerator Center (BC67) and living in Palo Alto, California. I returned to Tallahassee in April 1978 to attend a symposium at FSU: “Current Trends in the Theory of Fields, A Symposium in Honor of P.A.M. Dirac.” There was Dirac standing in the queue for coffee at the first break, also in the rather long and slow queue were other famous physicists (and Nobel Prize winners, past and future) who had come to pay homage to Dirac: Eugene Wigner (his brother-in-law), Murray Gell-Mann, Gerard ‘t Hooft, J.J. Sakurai, Kenneth Wilson, Frank Wilczek, Freeman Dyson, John Ellis (whom I knew from my graduate student days at CERN) and so on. But were any of them talking with Dirac? No! So I greeted Dirac and engaged him in some conversation; maybe the others knew it was difficult to have small talk with Dirac. He gave the last scientific talk of the symposium—on variations of Big G—and and I took a slide of him speaking.

One of Dirac’s two daughters, Mary, lived in Tallahassee, and we became friends during my several stays there. Florida State University threw a dinner for Dirac and his family the day after the symposium finished, which I attended, and my diary (upon which I have depended for this article) reports that I spoke with Dirac about films and Shakespeare.

Dirac’s theoretical physicist colleague at FSU, and close friend, Leopold Halpern, was also at the dinner and invited me to join him and Dirac on a canoe trip on the Wakulla River the next day, which I happily accepted. Mary Dirac drove her father and me (I never saw Dirac drive) to Wakulla Springs where we met Leopold and helped him launch his large canoe a little downstream of the Springs. We helped to paddle the canoe to a small island where we beached it and all went swimming in the refreshing but full of weeds river. I was relieved to see a water snake only after I got out of the water and a long alligator only later when we had paddled further downstream. We also saw osprey, a turtle, a blue heron and an enormous spider which turned up when we got stuck in a side channel and Leopold had to get out and push us off the mud. Many years later I read in Graham Farmelo’s biography of Dirac² that he and Halpern took canoe trips on the Wakulla River most weekends and “occasionally, they would invite a visitor to join them—but it had to be someone who could be relied upon to stay silent most of the time.” I don’t think I fit that criteria but I recall we all had a lovely time on my second day out with Dirac. I gave a seminar a few days after the canoe trip on the experiment I was working on at SLAC with the FSU High Energy Physics (HEP) group, I was happy to see Dirac in the audience and he had no startling questions for me that time.

In September 1978 I spent three weeks in Tallahassee working on that experiment’s data analysis, my office was in the same building as Dirac’s and so sometimes I would see him in the corridor. One day he gave me some preprints of his more recent work and asked if I would sit in on an interview he was giving the next day. I agreed to, it was a young newspaper reporter who asked Dirac lots of questions about his latest theory (probably to do with the variation of the “Big G” gravitational constant, I don’t recall) and I tried to help the interview process, having learnt how long it could take Dirac to answer a question. The next day Dirac flew off to Germany for a month, he often took trips away from Tallahassee to attend symposia. While I was back in Tallahassee working on our HEP experiment’s data in the spring of 1979 I had dinner with Mary Dirac and her parents a couple of times. At the first dinner I heard about Dirac’s March trip to Israel where he had addressed the Jerusalem Einstein Centennial Symposium and at the second dinner I heard about his May trip to Paris, where he had been guest of honour at UNESCO’s Einstein celebration. On the 24th May 1979 I attended a colloquium at the FSU Physics Department given by Dirac- the talk he had given at the Einstein Centennial: “The Early Years of Relativity”, quite a thrill to hear about that period from someone who was there. The next day I visited Dirac in his office to say goodbye, after much soul-searching and discussions with particle physics colleagues who had already left the field to work in industry, I had decided to leave particle physics and was flying back to Palo Alto to take up a staff scientist position at a large science consulting firm. When I explained this to Dirac he asked “Will you have to punch in and out?” I think this was a genuine question and indicates how little he knew about life outside academia, but I didn’t hold it against him and have treasured the times I was privileged to spend with one of the greatest physicists of all times.

Endnotes

1. Dirac had invented quantum electrodynamics in the 1930s and using his mathematical equations one arrived at predictions that were—more often than not—infinite and therefore unacceptable. A workaround known as renormalization was developed, but Dirac never accepted this. “I must say that I am very dissatisfied with the situation,” he said in 1975, “because this so-called ‘good theory’ does involve neglecting infinities which appear in its equations, neglecting them in an arbitrary way. This is just not sensible mathematics. Sensible mathematics involves neglecting a quantity when it is small—not neglecting it just because it is infinitely great and you do not want it!” This 1975 quote comes from Helge S.Kragh, Dirac: A Scientific Biography, Cambridge University Press, p.184 (30 March 1990). I have since understood that any Feynman diagram has underlying mathematics for calculating cross-sections that may need renormalizations to be carried out to match with the experimental data; so when Dirac saw my Feynman diagram, (which represented a first order reaction, not needing any renormalization) it brought to mind his many-decades long dissatisfaction with the renormalization process used in particle physics.

2. G. Farmelo, The Strangest Man: The Hidden Life of Paul Dirac, Mystic of the Atom, Basic Books, 2009.

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