Inside the Centre: The Life of J. Robert Oppenheimer

by Ray Monk

  Having insisted that his contract permit him to devote some of his time to teaching graduate students, Oppenheimer abandoned the plan of depending on the trustees of the institute to identify suitable students at Princeton, and instead took the precaution of bringing his own. In a move that recalls the annual migration from Berkeley to Pasadena that students like Serber were prepared to take in the 1920s and ’30s in order to maximise their time with Oppenheimer, in the summer of 1947 no fewer than five students – Hal Lewis, Robert Finkelstein, Saul Epstein, Leslie Foldy and Sig Wouthuysen – came with Oppenheimer when he left California for the east.

  In December 1947, soon after Oppenheimer moved to Princeton, Life magazine ran an article about the institute under the heading ‘The Thinkers: The Institute for Advanced Study is their Haven’. The atomic bomb, the piece began by saying, was a ‘devastating projection of this century’s most abstruse thinking’. In the light of this demonstration of the power of thought, ‘the thinker has come into his own’, and therefore the institute, being ‘one of the most imposing collections of minds gathered in one place’, had become recognised as ‘one of the most important places on earth’.

  The photographs accompanying the article, however, picture for the most part a distinctly unimposing collection of elderly men: the economist Walter W. Stewart reclining on his couch and looking as if he is about to fall asleep; the classics scholar Benjamin Merritt peering through a magnifying glass at an ancient Greek inscription; the mathematician Oswald Veblen leaning back in his chair and staring with apparent bewilderment at his desk; and, of course, Einstein, who is pictured twice, once in front of an audience and again sitting with Oppenheimer, telling him, according to the caption, ‘about his newest attempts to explain matter in terms of space’, and looking in both pictures like an ancient Old Testament prophet.

  In the starkest contrast to these pictures are two of Oppenheimer. In the first – captioned ‘talking shop’ – he is shown engaged in obviously earnest and intense discussion with Dirac and Pais, all three of them looking quite sure that what they are discussing is of great importance. In the second – captioned ‘Oppenheimer’s students’ – Oppenheimer is shown perched on a desk, with five young men evidently hanging on to his every word. By accident or design, the contrast between the two sets of photographs sends a very clear message: under Oppenheimer, the institute would no longer be the resting place for eminent old men whose best work was behind them; it was to be a place where up-and-coming young men who meant business would make new and fundamental contributions to scientific knowledge.

  These were exciting years for physics, as Oppenheimer, after the Shelter Island Conference, knew they would be, and he was determined to be, as far as possible, at the centre of developments. Indeed, though Oppenheimer was at this time chairman of the Atomic Energy Commission’s General Advisory Committee, and as such perhaps the most influential person in the country in the development of America’s atomic policies, and though he had moved east partly so that his regular trips to Washington would not be so difficult or time-consuming, it was actually physics, rather than politics, that dominated his first two years at Princeton. As he had done in the 1930s, he published jointly with his students. In October 1947 he submitted the paper he had given at Shelter Island, ‘The Multiple Production of Mesons’, to the Physical Review as a joint publication, co-written with Hal Lewis and Sig Wouthuysen. A few months later he submitted another paper, ‘Note on the Stimulated Decay of Negative Mesons’, this time co-written with Saul Epstein and Robert Finkelstein. But, more importantly, he directed his students to the area where, in the wake of Shelter Island, the fundamentally important new steps would be taken: that is, to quantum electrodynamics, in which, as Oppenheimer knew, the solution to the puzzles posed by the recent experiments conducted at Columbia would be found.

  Oppenheimer encouraged the young physicists at the institute to attend the many important seminars and conferences being given at that time, not just in America, but also in Europe. For example, he encouraged Pais to travel to a small conference in Copenhagen in September 1947, where Cecil Powell reported on his recent experiments at Bristol, which demonstrated the truth of Marshak’s ‘two-meson’ hypothesis. It was there that Pais first heard the names that would soon become accepted for the two particles: the pi-meson and the mu-meson. When he returned, Oppenheimer asked Pais to give a seminar reporting on what he had learned at Copenhagen. To Pais’s surprise, Einstein turned up to hear his account of Powell’s work. ‘It was,’ says Pais, ‘the only occasion in all my institute years that I saw Einstein present at a physics seminar given by someone other than himself.’

  Oppenheimer was so excited by the developments in physics during this time that he could not resist mentioning them, or at least alluding to them, even in his public, non-technical lectures. One example of this – his mention of the experiments conducted by the three Italian scientists, Conversi, Pancini and Piccioni, in his lecture ‘Atomic Energy as a Contemporary Problem’, given in September 1947 – has already been mentioned. Another example occurred a couple of months later. On 13–15 November 1947, Oppenheimer was in Washington to attend the tenth Washington Conference on Theoretical Physics. Also there was Schwinger, who gave a report on a series of calculations that he had made relating to the quantum-mechanical interactions between electrons and photons, particles and radiation, in a relativistic field. These calculations were so subtle, so intricate and so complicated that Schwinger was possibly the only man then alive who could have performed them, but they also pointed to the fundamental change in QED that was needed to account for the Lamb shift and the anomalous magnetic moment of the electron. Feynman, who was at the conference, reports that he himself ‘did not have time to understand what exactly Schwinger had done’, but he knew that, whatever it was, it had to be interesting, because ‘it got Oppy so excited’. What excited Oppenheimer was the possibility that both the energy shift of electrons observed by Lamb and the anomalous increase in the magnetic charge of electrons reported by Rabi, Nafe and Nelson could be accounted for by the same set of calculations. This strongly suggested that something new and important had been discovered about the way electrons react to their own magnetic fields.

  ‘The importance of Schwinger’s calculation cannot be underestimated,’ writes the physicist and historian of physics Silvan Schweber:

  In the course of theoretical developments there sometimes occur important calculations that alter the way the community thinks about particular approaches. Schwinger’s calculation is one such instance. By indicating, as Feynman had noted, that ‘the discrepancy in the hyperfine structure of the hydrogen atom . . . could be explained on the same basis as that of the electromagnetic self-energy, as can the line shift of Lamb’, Schwinger had transformed the perception of quantum electrodynamics. He had made it into an effective, coherent, and consistent computational scheme.

  Just ten days after the Washington conference, on 25 November, Oppenheimer gave a public lecture at MIT entitled ‘Physics in the Contemporary World’. His theme was the ‘temporarily disastrous effect on the prosecution of pure science’ that the Second World War had had, because of the ‘demands of military technology’, and the speed with which the science of physics, especially, had recovered from that disastrous effect. ‘It has,’ he told his audience, ‘been an exciting and an inspiring sight to watch the recovery – a recovery testifying to extraordinary vitality and vigor in this human activity. Today, barely two years after the end of hostilities, physics is booming.’

  As examples of the booming progress that physics was then making, Oppenheimer mentioned three things: 1. the new discoveries about mesons and the consequent progress in understanding elementary particles (‘Almost every month has surprises for us in the findings about these particles. We are meeting new ones for which we are not prepared. We are learning how poorly we had identified the properties even of our old friends among them’); 2. Schwinger’s dramatic improvements to Dirac’s QED
(‘A newly vigorous criterion for the adequacy of our knowledge of the interactions of radiation and matter. Thus we are beginning to see in this field at least a partial resolution, and I am myself inclined to think rather more than that, of the paradoxes that have plagued the professional theorists for two decades’); and 3. the identification of the pi-meson as the Yukawa particle (‘the increasing understanding of those forces which give to atomic nuclei their great stability, and to their transmutations their great violence’). Finally he mentioned the importance of recognising the connections between these three:

  It is the prevailing view that a true understanding of these forces may well not be separable from the ordering of our experience with regard to elementary particles, and that it may also turn on an extension to new fields of recent advances in electrodynamics.

  Unfortunately, Oppenheimer’s central message in this lecture – that physics was emerging from its wartime shackles into a new golden era of exciting fundamental progress – has been largely lost to posterity because of a momentary lapse into hyperbole. Referring to the role that scientists played not only in developing the atomic bomb, but also in recommending their development and advising on their use, he remarked: ‘In some sort of crude sense which no vulgarity, no humor, no overstatement can quite extinguish, the physicists have known sin; and this is a knowledge which they cannot lose.’ So arresting was this remark, and so widely reported, that it came to overshadow everything else Oppenheimer said in this lecture. The lecture has thus acquired a reputation for being a gloomy and introspective confession of guilt rather than for being what it is – a cheery celebration of the dawn of a golden age of physics.

  As is shown by Oppenheimer’s mention in his MIT lecture of the possibility of solving ‘the paradoxes that have plagued the professional theorists for two decades’, the reason he was so excited about Schwinger’s calculations was not just that they promised to explain both sets of the experiments conducted at Columbia, but also because, in doing so, they promised to solve the problems in quantum electrodynamics that Oppenheimer himself had worked on before the war. In particular, Schwinger’s work offered a way of overcoming the problems that Oppenheimer had long believed pointed to a fundamental flaw in Dirac’s theory. These problems centred on the fact that, though the theory seemed in general to work well, at various points when one tried to use it to make very precise or detailed calculations, the answers it gave had to be wrong because they involved infinities, where the answer (for example, to questions about the energy of an electron at a certain state) had to be finite. This was the problem that Sidney Dancoff came so close to solving back in 1939. Now, it seemed, Schwinger was on the brink of providing the definitive solution.

  Oppenheimer was not the only physicist excited at the progress promised by Schwinger’s calculations. On his way back to Harvard from the Washington conference, Schwinger paid a visit to Columbia, where he gave a progress report on his groundbreaking work. After he had gone, Rabi wrote to Bethe, telling him that, in his view, Schwinger’s theory was undoubtedly correct. He concluded: ‘God is great!’ Bethe replied in equally excited terms: ‘I have heard about Schwinger’s theory and find it very wonderful . . . It is certainly wonderful how those experiments of yours have given a completely new slant to a theory and how the theory has blossomed in a relatively short time. It is as exciting as in the early days of quantum mechanics.’

  In late December 1947, Schwinger sent a report of his treatment of the anomalous magnetic moment to the Physical Review, in the course of which he mentioned the work that Dancoff had done with Oppenheimer in 1939 and the ‘confusion’ it had generated. Before this paper appeared in print, physicists had a chance to hear Schwinger report on his new theory at the annual meeting of the American Physical Society, which was held at Columbia from 29 to 31 January 1948. Oppenheimer and Pais attended the meeting, taking the train together from Princeton to New York on 29 January. Their main interest, of course, was to hear Schwinger. He, however, was not scheduled to speak until the last day. In the meantime, Pais remembers, he and Oppenheimer were impressed by one of the other speakers, a young British physicist called Freeman Dyson. ‘As he proceeded to give his talk,’ Pais recalls, ‘Robert and I nodded at each other: this kid is smart.’ Educated at Winchester public school and at Trinity College, Cambridge, Dyson was a member of a very distinguished British family, his father being the well-known composer George Dyson. At the time Oppenheimer and Pais met him, Dyson was six months into a visiting fellowship at Cornell, where he had been working with Bethe and Feynman. After his talk, Oppenheimer approached him and invited him to spend the following academic year at the institute, an invitation Dyson promptly accepted.

  When the time came for Schwinger’s lecture on the final day of the conference, it was discovered that 1,600 people had registered to hear him speak. The afternoon’s session was hurriedly rearranged so that he could give his lecture twice. In a letter that Dyson wrote his parents about the meeting, the excitement generated by Schwinger’s paper is vividly captured:

  The great event came on Saturday morning, and was an hour’s talk by Schwinger, in which he gave a masterly survey of the new theory which he has the greatest share in constructing and at the end made a dramatic announcement of a still newer and more powerful theory, which is still in embryo. This talk was so brilliant that he was asked to repeat it in the afternoon session, various unfortunate lesser lights being displaced in his favour. There were tremendous cheers when he announced that the crucial experiment had supported his theory: the magnetic splitting of two of the spectral lines of gallium (an obscure element hitherto remarkable only for being a liquid metal like mercury) were found to be in the ratio of 2.00114 to 1: the old theory gave for this ratio exactly 2 to 1, while the Schwinger theory gave 2.0016 to 1.

  Feynman was at this historic talk and, in the discussion period, said that he had a different method of calculating the magnetic moment of the electron and that his calculations supported Schwinger. ‘I was not showing off,’ Feynman later said. ‘I was just trying to say that there’s no problem, for I had done the same thing that he had done and it had come out all right.’ The problem was, as Feynman later conceded: ‘People knew Schwinger, but most of them did not know me.’

  I heard later from several people who were at the APS meeting that I sounded funny to them. ‘The great Julian Schwinger was talking when this little squirt got up and said, “I have already done this, Daddy, you’re in no trouble at all! Everything will be OK!”’

  Feynman’s time would come, but, for the moment, all eyes were on Schwinger. Schwinger’s opportunity to present in detail the ‘still newer and more powerful theory’ mentioned by Dyson came at the end of March 1948, when the second conference in the series that had begun the previous year at Shelter Island was held. As, by common consent, Oppenheimer was the dominant figure at Shelter Island, it was only natural that he should take a lead in organising and securing funding for this second conference. Just a few days after the Shelter Island Conference had finished, Oppenheimer was writing to the National Academy of Sciences, urging them to support a second conference.

  On 10 December 1947, Oppenheimer had circulated all the participants of the Shelter Island Conference, suggesting that the next one should be held from 30 March to 2 April 1948 – days when he knew his distinguished visitors to the institute, Bohr and Dirac, would be free to attend. As the Ram’s Head Inn was not available during those days, Oppenheimer and Pais went looking for an alternative and found what they considered to be an ideal place, a hotel in the Pocono Mountains, Pennsylvania, called Pocono Manor.

  At Pocono, Schwinger was given as much time as he wanted and delivered a talk that took up almost an entire day. At the end of it, Oppenheimer was heard to remark: ‘Now it does not matter any more whether things are infinite.’ John Wheeler took notes of the talk, which covered no fewer than forty pages. Pais has described Schwinger’s talk as ‘a major tour de force in which he unveiled a detailed
new calculus’. Dyson was not there (‘I was not invited because I was not yet an expert’), but he had good first-hand accounts of the talk from Bethe and Feynman. Schwinger, Dyson writes, ‘had a new theory of quantum electrodynamics which explained all the Columbia experiments. His theory was built on orthodox principles and was a masterpiece of mathematical technique. His calculations were extremely complicated, and few in the audience stayed with him all the way through the eight-hour exposition. But Oppy understood and approved everything.’

  One thing Oppenheimer did not yet understand, however, was Feynman’s own version of quantum electrodynamics, which he presented after Schwinger in a paper called ‘Alternative Formulation of Quantum Electrodynamics’. Dyson, who had got to know Feynman well by this time and liked him a great deal, writes: ‘Dick tried to tell the exhausted listeners how he could explain the same experiments much more simply using his own unorthodox methods. Nobody understood a word that Dick said. At the end Oppy made some scathing comments and that was that. Dick came home from the meeting very depressed.’ Pais, who was, of course, actually there, remembers it slightly differently. No one could follow Feynman’s methods, he recalled, but ‘the speed with which Feynman could reproduce results also found by Schwinger convinced us that he was on to something’. Certainly Schwinger thought Feynman was on to something. ‘The Pocono conference,’ he later said, ‘was my first opportunity to learn what Feynman was doing’, and ‘as his talk proceeded, I could see points of similarity’. Feynman himself remembers that at Pocono he and Schwinger ‘got together in the hallway and although we’d come from the end of the earth with different ideas, we had climbed the same mountain from different sides and we could check each other’s equations’.

  Actually, Feynman and Schwinger were not the only two climbers of this particular mountain, as Oppenheimer discovered when he returned to Princeton. Waiting for him there was a letter from the Japanese physicist Sin-Itiro Tomonaga, telling him about recent work done in Japan that seemed in some important respects to anticipate Schwinger’s work, or, anyway, to have arrived independently at very similar results. Tomonaga and his colleagues had been stimulated by reading Sidney Dancoff’s 1939 paper to attempt exactly what Schwinger had achieved: a way of avoiding infinities in QED. Moreover, their method of accomplishing this, though not as fully worked out as Schwinger’s, was, from a mathematical point of view, practically identical. With the letter Tomonaga sent Oppenheimer a collection of papers by the Japanese scientists that would appear in the Japanese English-language journal Progress of Theoretical Physics.