by Ray Monk
On 12 May 1941, just seven months into her marriage to Oppenheimer, Kitty gave birth to a baby boy, whom they named Peter. By this time of the year they were in Pasadena, from where they wrote to the Chevaliers inviting them to meet the new arrival. Thus it was that Chevalier was with Oppenheimer in Pasadena on 22 June 1941, when they heard the news that Nazi Germany had broken its pact with the Soviet Union and had begun an invasion of Russia. ‘It was on our way to the beach that we heard over the radio of the Nazi invasion of the Soviet Union,’ Chevalier remembered, ‘and as we drove, both shocked and terribly excited by the news, we heard the whole of Churchill’s speech denouncing Hitler and welcoming the Soviet Union as an ally, pledging Great Britain’s full cooperation in a united war effort.’
That night, Chevalier recalls, ‘we sat up with Opje and Kitty till the small hours, listening to the news broadcasts and trying to analyse the significance of this latest Nazi move’. Apparently blind to the irony of what he is describing, Chevalier remarks: ‘Hitler had destroyed at one stroke the dangerous fiction, so prevalent in liberal and political circles, that fascism and communism were but two different versions of the same totalitarian philosophy. The communist and democratic forces were now allies committed to fighting their common fascist enemy.’
The entry of the Soviet Union into the war came as something of a relief to many American communists, since it brought them back on to the same side as their countrymen. The US had not yet actually entered the conflict, but preparations for war were going on everywhere. In a letter to the Uehlings written just a few days after Peter was born and about a month before the Nazi attack on Russia, Oppenheimer, again warmly thanking the Uehlings for their hospitality the previous summer (‘even now a year later I want to thank you for it again’), wrote disconsolately: ‘I think we’ll go to war.’ At this time the imminence of war was something from which he felt detached and which he saw as inimical to the development of his subject. ‘I expect,’ he told the Uehlings, ‘unless there is a drastic change in policy on the part of the research boards, that physics in our sense will just about stop by next year.’
As he conceded, though, the war preparations had been good for physics in at least one sense: for the first time since he started teaching, students graduating with higher degrees in physics were almost guaranteed employment. With regard to a position that the Uehlings’ own university had advertised for a spectroscopist, Oppenheimer wrote: ‘You are going to find it pretty hard to get any decent physicist these days, with the demand suddenly exceeding the supply.’ If they were wanting to employ any theoretical physicists at Seattle, Oppenheimer added, ‘there will be some first rate ones coming up next year.’
This year’s crop is pretty well spoken for already. The situation in Berkeley & here in Pasadena is in some ways very gloomy: here specially almost all the men active in physics have been taken away for war work. Those left are swamped in administrative & teaching duties & their own defense problems. The number of graduates too is way down: the losses heaviest among the men about to finish in nuclear physics, but noticeable all along the line. In Berkeley we’ve lost Alvarez, McMillan, 2/3 Lawrence, [Bernice] Brode, Loeb. Only the last is not missed.
Oppenheimer always liked to be ‘at the centre’, and it is clear from this letter that he felt that he and his diminishing band of graduates working on, as he put it, ‘theories of mesotron field & the light they throw on “Heisenberg” showers, proton isobars, scattering & other such recondite matters’ were being confined to the periphery – the centre being the secret work on nuclear physics from which, for the moment at least, he was excluded.
Actually, just as Oppenheimer’s work on neutron stars and black holes, which had looked so exotic in 1938–9, now looks like his greatest achievement in physics, so the ‘recondite’ work that he and his students pursued in the period 1939–41, which looked peripheral at a time when nuclear physics held sway with physicists and research funding bodies, now looks more central. This is particularly true in the area of quantum electrodynamics, in which the work done under Oppenheimer at Berkeley is now being recognised as a precursor to the work done by the people – Julian Schwinger, Richard Feynman, Freeman Dyson and Sin-Itiro Tomonaga – who finalised that theory in the late 1940s. In fact, one of Oppenheimer’s students, Sidney Dancoff, came extremely close to arriving at something similar to that later version of quantum electrodynamics, and, according to the historian of science, Silvan Schweber, would have got there before Schwinger et al., had he not made a mistake in his calculations.
Oppenheimer thought very highly of Dancoff, as can be seen by a letter he wrote to F. Wheeler Loomis, the chairman of the physics department at the University of Illinois, in May 1940, urging him to offer Dancoff the one-year instructorship that, through Serber, Oppenheimer knew Loomis had going vacant. Lawrence, Oppenheimer told Loomis, had wanted to employ Dancoff in the Radiation Laboratory, ‘but we are all agreed that that is not the ideal place for him’. Dancoff was, Oppenheimer wrote, ‘a good physicist, well trained and with good ideas and great technical facility in calculation’. The only reason he did not already have an academic position was that ‘jobs for theorists are not too common, and he has had the competition of older men of greater reputation: Schiff, Schwinger, Snyder for instance in Berkeley’. In response, Loomis offered the one-year post to Dancoff, who then remained at Illinois for the rest of his short life (he died in 1951, shortly before his thirty-eighth birthday).
Of the ‘older men of greater reputation’ that Oppenheimer mentions, the odd one out would seem to be Hartland Snyder, who was not, in fact, any older than Dancoff (they were the same age) and who was, academically, slightly behind, finishing his PhD six months after Dancoff. Snyder, though, had the enormous advantage of being Oppenheimer’s co-author on their classic paper on black holes, while, for some reason, Oppenheimer never published anything with Dancoff.
The other two were indeed more senior. Leonard Schiff, a New York Jew of Lithuanian background, had come to Berkeley as an NRC fellow, and in the summer of 1938 was given the challenging task of replacing Robert Serber as Oppenheimer’s research associate. Schiff would later become known as a brilliant teacher and the author of a commonly used textbook on quantum mechanics that was based on Oppenheimer’s lectures at Berkeley. However, possibly because he was dissatisfied with him as a replacement for Serber, Oppenheimer is recorded as being rather cruel to Schiff. Edward Gerjuoy, who started work as a graduate student of Oppenheimer’s at Berkeley in 1938, the same time that Schiff began his two-year stint as Oppenheimer’s research associate, reports that when Schiff gave a seminar discussion of a book with some rather difficult mathematics in it, Oppenheimer ‘asked Schiff searching questions about each and every equation Schiff wrote down’. ‘On more than a few occasions,’ Gerjuoy remembers, ‘Oppie had Schiff, who was a gentle soul, visibly on the verge of tears.’
Julian Schwinger, who replaced Schiff as Oppenheimer’s research associate in the summer of 1940, though no less gentle than Schiff, was too brilliant a physicist and too confident in his abilities to be browbeaten in the same way. Schwinger, who would go on to win the Nobel Prize in 1965 for his part in the development of the modern formulation of quantum electrodynamics, could hold his own with any physicist. At Schwinger’s first seminar as Oppenheimer’s research assistant, Gerjuoy recalls, Oppenheimer’s students ‘were wondering how long it would take Julian to shrivel under Oppie’s questioning’. They were in for a shock:
Julian started talking and very soon Oppie, in accordance with his usual practice, asked Julian a question, which Julian answered. More questions came; more questions were answered. After about a dozen questions, answered by Julian with no visible sign of distress whatsoever, Oppie stopped firing questions and let him finish his seminar essentially without further interruption. Nor did he ever again unduly interrupt during any succeeding seminar of Julian’s.
By the time he gave this seminar Schwinger had already been at Berkeley f
or more than a year, having arrived as an NRC postdoctoral student in the summer of 1939. He was from a remarkably similar background to Oppenheimer. His father, a German-speaking Jew from Central Europe, arrived in the US as a teenager in 1880, made a fortune in the clothing trade and bought a large apartment in the Upper West Side of Manhattan. In fact, the apartment in which Schwinger grew up was on the very same street, Riverside Drive, that Oppenheimer had grown up in. As a child, Schwinger was precocious even by Oppenheimer’s standards. At the age of thirteen he read, and understood, Dirac’s The Principles of Quantum Mechanics, which he later described as ‘my bible’. At the age of sixteen Schwinger wrote his first paper on quantum-field theory, which remained unpublished, but the following year he published two letters to the editor of the Physical Review. He was then a student at City College, New York, where, bored by lectures that just told him what he already knew, he developed what would become a lifelong habit of sleeping during the day and beginning work in the evening. After a year, he transferred to Columbia to work with Isidor Rabi, who held him in such high regard that he asked Schwinger – while still an undergraduate – to lecture to his graduate students when he was out of town. In the summer of 1936, still only eighteen, Schwinger received his undergraduate degree.
Less than a year later, Schwinger was ready to submit his PhD thesis, but Columbia’s rules would not allow him to graduate with a PhD without spending at least two years in residence, so he published the work as a collection of articles, which, when the time came, he bound together and presented as his PhD work. To stop him getting bored, Rabi arranged for Schwinger to spend the first part of the academic year 1937–8 at the University of Wisconsin, working with Gregory Breit and Eugene Wigner. By the time he returned to Columbia in the spring of 1938, Schwinger was a young physicist with an international reputation and was being offered academic positions at several universities, both in Europe and in the US. Despite the many alternatives offered to him, Schwinger chose to apply for an NRC fellowship in order to spend some time at Berkeley working with Oppenheimer. Rabi recalls that he tried to persuade Schwinger to go to Zurich to work with Wolfgang Pauli, but Schwinger ‘thought Oppenheimer was a more interesting physicist’. When he was asked later in life about this decision, Schwinger replied: ‘Oppenheimer was the name in American theoretical physics. Where else could I have gone?’ That the most intellectually precocious, sought-after young theoretical physicist in the country should choose to stay in America rather than sit at the feet of one of the founders of quantum theory in Europe, and that, in particular, he would rather work with Oppenheimer than with Pauli, was the fulfilment of Oppenheimer’s ambitions for himself and for the department he had built up. One might, without too much hyperbole, regard Schwinger’s decision to go to Berkeley as a symbolic moment, marking the point at which the centre of gravity in theoretical physics shifted from Europe to the United States.
Of course it is possible that, in the summer of 1939, Schwinger had other reasons for not wanting to travel to Europe. In fact, as it happened, the day he arrived in Berkeley was 1 September, the very day the German army invaded Poland. Nevertheless, it is clear from the things he read, the things he cites in his published papers and the problems that he chose to work on, that Schwinger regarded the work being done by Oppenheimer and those influenced by him as more interesting and more important than anything going on in Europe at that time.
Though he was still only twenty-one when he came to Berkeley as an NRC fellow, Schwinger’s reputation among theoretical physicists as a rare talent preceded him and Oppenheimer had heard much about him before they met. In turn, Schwinger knew about Oppenheimer’s reputation as a teacher and he arrived determined to resist any attempt at domination. It would, he knew, be a struggle. Oppenheimer, Schwinger recalls, ‘was overwhelming. [He] was not only impressive, he liked to impress. He was a showman. I was impressed, no question about it. But I also resisted him.’ At first, this resistance led to some friction between the two. Rabi remembered:
I spoke to Oppenheimer later and he was terribly disappointed. He came to the point of writing a letter to the National Research Council suggesting that Julian go somewhere else, because it took a man like Oppenheimer quite a bit to get used to Julian. Pauli once referred to Oppenheimer’s students as being Zunicker. Somebody who knows enough German knows what this means – people who nod heads – and Julian was not that way – that, and his hours.
One source of tension was Schwinger’s refusal to cooperate with Oppenheimer’s way of working with his graduate students. As Schwinger recalls:
At the early stage perhaps I didn’t measure up in the sense of ritual, in which everybody would come into Oppenheimer’s office at some early hour of the morning and they would sit around and talk. I presume I was still a late riser and so never came to these get-togethers. Maybe he didn’t like my dissident ways at first. I never heard a direct statement, but it’s very plausible that I was a strange fish to begin with until he appreciated that I could produce nevertheless. So perhaps in the first month he didn’t quite like the ‘cut of my jib’.
Oppenheimer and Schwinger soon overcame their initial difficulties with each other – Oppenheimer learned to accept Schwinger’s refusal to come to his office in the morning, and Schwinger reminded himself that ‘After all, I was there to learn from him’ – and, within two months of Schwinger’s arrival, they were collaborating on a joint letter to the editor of the Physical Review. The subject was a problem arising out of some results of experiments performed by Lauritsen and Fowler at Caltech, the explanation of which Oppenheimer thought might require the postulation of new physical forces. Oppenheimer had given the problem to Schiff, who had made no headway with it. In an interview much later in life, Schwinger remembered: ‘Schiff was then Oppenheimer’s assistant in Berkeley, and the problem got handed down from one to the next. Oppenheimer was interested in this, so Schiff said, “Hey, Schwinger, why don’t you look into this?” So I did. And obviously it got done in a day or so.’ The key to solving the puzzle, as it had been several times in the past when Oppenheimer had thought that experimental results showed the breakdown of quantum electrodynamics, was the realisation that the existing theory was perfectly adequate to explain the laboratory observations.
The solution to the problem was Schwinger’s, but the wording of the letter was Oppenheimer’s, and Schwinger was not entirely happy with the result. In particular, he was irritated that, having presented Schwinger’s solution, Oppenheimer then, characteristically, went on to speculate about cases where the theory might not hold. Schwinger later said:
He wrote that letter to the Physical Review incorporating whatever calculations and ideas I had but at the same time mentioning other possibilities. To me it was a purely electrodynamic process and exactly what was to be expected. On the other hand he, in the spirit of the time, was convinced that electrodynamics had broken down and so in the letter there is still a reference to the possibility of some new short-range force between electrons and protons, which I had no great stock in, but there it was.
One of the most important things demonstrated by Oppenheimer and Schwinger in their jointly written letter was the physical reality of what is called ‘vacuum polarisation’, which, in Schwinger’s words ‘means no more than the fact that an electron-positron combination is coupled to the electromagnetic field and it may show itself as real or virtual’. Ironically, it was Sidney Dancoff’s neglect of this very phenomenon that lay at the heart of his error mentioned earlier, the error that prevented him from anticipating Schwinger’s greatest contribution to physics: the ‘renormalisation’ of quantum electrodynamics. As Schwinger’s biographers remark, though he and Dancoff got to know each other well at Berkeley, ‘history might have developed differently if the two of them had had more time to discuss their respective research interests in greater detail’.
After Schwinger’s year as an NRC fellow expired in the summer of 1940, Oppenheimer immediately appointed him as Schiff
’s replacement as research associate. In that role Schwinger stayed for just a year, during which the focus of his and Oppenheimer’s research interests was the attempt to understand the particle that Oppenheimer in those days still called the ‘mesotron’ (it was not until after the war that he began calling it the ‘meson’). As we now know, and as mentioned in the previous chapter, what Oppenheimer called the ‘mesotron’ was, in fact, two very different particles: the mu-meson (or muon), which is a component part of cosmic rays, and the pi-meson (or pion), which is the carrier of the strong nuclear force. In the period from 1939 to 1942 almost all of Oppenheimer’s published work, and a good deal of the work undertaken by his graduate and postdoctoral students, was devoted to solving the puzzles of the ‘mesotron’, most of which arose from the mistaken assumption that the mu-meson and pi-meson were the same thing.
In trying to explain how Oppenheimer exerted such an inspirational influence on his students, Edward Gerjuoy writes: ‘I feel Oppie did his physics, talked about his physics, lived his physics, with an unusual passion, which had to inspire students; in any event it sure inspired me.’ As an example of this passion, Gerjuoy describes Oppenheimer’s absorption in the problem of the ‘mesotron’:
To give you just one of many possible illustrations, it bothered him, it tore at him, that he didn’t understand how the pi mesons, which in nuclei were so strongly interacting, penetrated the earth’s atmosphere so readily. Maybe he should have hit upon the idea that the mesons reaching the earth’s surface really weren’t pi mesons, but instead were other weakly interacting mesons – those we now term mu mesons; but since he hadn’t conceived of mu mesons he couldn’t stop talking about the anomaly that atmospheric penetration by pi mesons represented, in seminar after seminar and in less formal conversations with groups of his students.