by Ray Monk
In his original submission to the NRC for a postdoctoral fellowship, Oppenheimer had stated his intention of starting on 16 September, working first with Fowler in Cambridge and then with either Ehrenfest in Leiden or Bohr in Copenhagen. In the event, with the fellowship starting in November, he went straight to Leiden. Of all the great physicists he had met during his previous two years in Europe, it was Ehrenfest with whom he formed the closest attachment. They all admired Oppenheimer’s manifest intellect, but Ehrenfest really liked him.
And for Ehrenfest, more than perhaps any other great scientist, liking people and being liked by them was important. A working-class Jew from Vienna who became the successor to the great H.A. Lorentz at Leiden, Ehrenfest was a man of passionate intensity, who inspired admiration as a physicist and devotion as a teacher and friend. His biographer Martin Klein has written of him:
His way of being alive involved thinking about physics, talking and arguing about physics, working to his utmost to understand physics, and teaching it to anyone who showed an interest in it – students, colleagues, laymen, casual acquaintances, children. Others have been as intensely committed to science, but Ehrenfest was unique in his need to have close human contacts as an essential part of doing physics, in the breadth of human experience and the range of emotions that went into his scientific activity.
His close friend Einstein said:
He was not merely the best teacher in our profession whom I have ever known; he was also passionately preoccupied with the development and destiny of men, especially his students. To understand others, to gain their friendship and trust, to aid anyone embroiled in outer or inner struggles, to encourage youthful talent – all this was his real element, almost more than his immersion in scientific problems.
Though perfectly capable of following highly abstract mathematics, Ehrenfest was famous among physicists for distrusting overly complicated formalistic treatments of physical problems. In this, he was often contrasted with Max Born. The great physicist Victor Weisskopf, who studied at Göttingen, remarked that Ehrenfest taught him ‘to distrust the complicated mathematics and formalisms that were then very popular at Göttingen’ and thereby ‘showed me how to get at the real physics’.
When, in the early summer of 1928, Oppenheimer expressed a desire to spend some of his time as a postdoctoral student working with Ehrenfest at Leiden, he naturally wrote to Ehrenfest asking for his support and received in reply the following characteristically forthright and warm response:
If you intend to mount heavy mathematical artillery again during your coming year in Europe, I would ask you not only not to come to Leiden, but if possible not even to Holland, and just because I am really so fond of you and want to keep it that way. But if, on the contrary, you want to spend at least your first few months patiently, comfortably and joyfully in discussions that keep coming back to the same few points, chatting about a few basic questions with me and our young people – and without thinking much about publishing (!!!) – why then I welcome you with open arms!!
Though it had been Ehrenfest who had spotted the mathematical mistakes in Oppenheimer’s Ramsauer paper, it is typical of him that his concern was not that Oppenheimer was incompetent in mathematics, but that he would attach too much importance to it. Ehrenfest’s greatest concern in physics was always with attaining clarity, genuine understanding.
Oppenheimer in later life emphasised how much he admired Ehrenfest. ‘I thought of him,’ he once said, ‘in semi-Socratic terms, and I thought I would learn something from him and indeed certainly did.’ The intention of both Oppenheimer and Ehrenfest was that Oppenheimer would, during his time at Leiden, not only pursue his own research, but also act as Ehrenfest’s assistant. To everybody’s astonishment, Oppenheimer, in this latter capacity, gave a few seminars at Leiden in Dutch, a language he seemed to have learned in a matter of months. ‘I don’t think it was very good Dutch,’ he later recalled, but it was, nevertheless, greatly appreciated.
However, Oppenheimer’s principal interest was his own research, and, despite his great admiration of Ehrenfest, he could not be persuaded to abandon altogether his tendency to look for mathematical techniques to solve the questions of physics. ‘I think that his [Ehrenfest’s] interest in simplicity and clarity was really a great thing,’ Oppenheimer once said, ‘but I probably still had a fascination with formalism and complication, so that the large part of what had me stuck or engaged was not his dish.’ Very quickly after arriving at Leiden, therefore, Oppenheimer came to think that – his affection for, and admiration of, Ehrenfest notwithstanding – he would be better off somewhere else. ‘There was not a great deal of life in the physics in Leiden at the time,’ he recalled. ‘I think Ehrenfest was depressed: I don’t think that I was of great interest to him then. I don’t think he told me what was on his mind and I have a recollection of quiet and gloom.’
Indeed, Ehrenfest was depressed, far more so than anybody realised at the time. Two things drove him to depression. The first was the state of physics, which seemed to move further and further away from the kind of clarity he himself sought to achieve, in favour of mathematical techniques, the physical interpretation of which remained clouded in mystery and controversy. The second was his youngest son, Vassily (‘Wassik’), who was born with Down’s syndrome. Within a few years these two pressures would weigh more and more heavily on Ehrenfest. Finally, he could stand no more, as he tried to explain in a letter that he wrote (but never sent) to a number of his closest friends, including Bohr and Einstein. ‘I absolutely do not know any more how to carry further during the next few months the burden of my life, which has become unbearable,’ he began, adding:
In recent years it has become ever more difficult for me to follow the developments in physics with understanding. After trying, ever more enervated and torn, I have finally given up in desperation. This made me completely weary of life . . . I did feel condemned to live on mainly because of the economic cares for the children. I tried other things, but that helps only briefly. Therefore I concentrate more and more on the precise details of suicide. I have no other practical possibility than suicide, and that after having first killed Wassik. Forgive me . . .
On 25 September 1933, having made arrangements for his other children, Ehrenfest accompanied Wassik to the Professor Watering Institute in Amsterdam, where he was being treated. While they sat in the waiting room, Ehrenfest shot Wassik and then himself. ‘None of us,’ Oppenheimer wrote to Ehrenfest’s former assistant Uhlenbeck, ‘who were his students, shall be quite free of guilt in this his desperation.’
Of course, in November 1928, Oppenheimer had no way of knowing just how deep Ehrenfest’s depression was and to what it would drive him. He knew only that he wanted to get away from Leiden, which, he said later, ‘spoiled this period from the point of view of physics’. After his disappointing time with Ehrenfest, Oppenheimer intended to go to Copenhagen to work with Bohr, but, as an interim measure, he spent a few weeks at the University of Utrecht with Bohr’s old student and disciple Hendrik Kramers. ‘Bohr is Allah,’ Wolfgang Pauli once said, ‘and Kramers is his Prophet.’ Another frequently made comparison was with Michael Faraday and James Clerk Maxwell, with Bohr being the intuitive Faraday and Kramers the mathematically-minded Maxwell. Kramers was in many ways ideally suited to being Oppenheimer’s mentor, not only because of his inclination towards formal ways of approaching physics, but because of the many other things they had in common, including a veneration of Bohr and a wide range of intellectual and cultural interests. Kramers, for example, combined being a professor of physics with playing the cello to a very high standard, writing poetry and editing a literary magazine. Indeed, though they did not become especially close, Oppenheimer and Kramers got on very well, and Oppenheimer enjoyed his time at Utrecht, the most lasting legacy of which was the nickname he acquired there: Opje. Though the anglicised version, Oppie, became more widely known and used, among Oppenheimer’s closest friends Opje was the preferred form.
From Utrecht, on 30 December 1928, Oppenheimer wrote a long letter to Frank, who had written an essay on aesthetics and had sent it to his brother. Consequently, most of the letter is taken up with an interesting extended discussion of the subject, which shows not only how close the two brothers had become, but also how much deep thought Oppenheimer had given to the subject. Frank’s central point in the essay had evidently been that an expression of personal, individual taste (‘I like it’, and so on) is not an artistic judgement. With this, Oppenheimer agreed. In a rather schoolmasterly way, however, he ticked Frank off for showing ‘a lamentable ignorance of history in the matter’, but added that that was ‘almost irrelevant’. More problematic, Oppenheimer maintained, was a difficulty that faced anybody who wanted to insist on the universality and objectivity of artistic standards, namely that ‘appreciation of art is in fact neither universal nor objective, that it depends on education, experience, taste; that, in its critical aspects, it is defined only by the “I like its” of the sensitive and the initiated’. The solution to this difficulty, Oppenheimer suggested, was to accept that ‘the value of a picture is best defined as relative, not to the person, but to what one may vaguely call the civilization: the public, traditional culture and experience of the civilization for which it was painted’. He recommended that Frank read Roger Fry’s Transformations: Critical and Speculative Essays on Art, which had been published the previous year. The letter ended with some entirely general advice: ‘discipline, work, honesty, and, toward other people, a solicitude for their welfare and as complete an indifference as possible to their good opinion’.
A few days later, on 3 January 1929, Oppenheimer was back in Leiden, from where he wrote to the IEB’s Paris office, saying that, ‘at the suggestion of Ehrenfest and of Kramers’, he had changed his plans: instead of going from Leiden to Copenhagen to work with Bohr, he now intended to go to Zurich to work with Wolfgang Pauli. He had, he said, written to Pauli asking for his consent and still intended to go to Copenhagen after he had worked with Pauli. ‘I hope,’ he concluded, ‘that it will be possible for you to grant this permission without waiting for the discretion of the American Board; for I should like to leave Leiden in the next weeks.’ The following week, Ehrenfest wrote to the IEB, saying that Oppenheimer (‘a very ingenious physicist’) would be better off going to Zurich, not only for educational reasons, but also because of his health, in particular ‘that obstinate cough which had not been in order since his arrival in Holland’. ‘Please,’ pleaded Ehrenfest, ‘put this charming, fine – but whose health is questionable – young man under medical control, but without letting him know that I wrote you about it.’
Ehrenfest, it seems, had very strong views about where Oppenheimer should go in order to develop to its fullest his potential as a physicist. Part of Ehrenfest being, as Einstein put it, ‘passionately preoccupied with the development and destiny of men’ was that he took an intense interest in where and with whom his students should study. In the case of Oppenheimer, he felt strongly, as Oppenheimer later told an interviewer, ‘that Bohr with his largeness and vagueness was not the medicine I needed’. Rather, Ehrenfest felt that Oppenheimer needed ‘someone who was a professional calculating physicist’, who could give him ‘more discipline and more schooling’, and the man he chose for this task was Wolfgang Pauli.
As Abraham Pais has said, Ehrenfest’s view that Pauli rather than Bohr could offer Oppenheimer what he most needed was ‘a wise judgement with far-reaching consequences for Robert’s career’. Ehrenfest had evidently come to this conclusion soon after Oppenheimer’s arrival in Leiden. In a letter to Pauli of 26 November 1928, he urged him to accept Oppenheimer, and showed, in the process, how perceptive he was about Oppenheimer’s strengths and weaknesses, how much he liked him and how much he cared about making sure that Oppenheimer worked with the right person. He was writing, he told Pauli, ‘about a physicist (a good one though), namely Oppenheimer’.
The poor devil is with us in Leiden . . . under pressure of my schoolmasterly character. He has always very witty ideas . . . But then the great misery starts that I cannot grasp anything that cannot be ‘visualised’. And, although he then with imperturbable calm and kindness tries to meet my wishes, the result is that I bother more than help him. He does not think of complaining . . . I am really convinced that, for the full development of his (great) scientific talent, Oppenheimer still needs ‘RECHTZEITIG a bisserl (!) LIEBEVOLL zurechtgeprügelt warden sollte’ [timely and a bit lovingly to be beaten into shape]. He thoroughly deserves this kindness since he is a rare and decent fellow . . . Therefore I would like it very much if he can come to you after Leiden. This idea appeals very much to him.
The man at the IEB’s Paris office charged to deal with Oppenheimer was Dr W.J. Robbins, to whom Oppenheimer wrote on 23 January 1929, enclosing a note from Pauli approving his plan of working with him, and saying that he was now already in Zurich, having made the trip at his own expense, which came to $15 for his fare and $29 for his luggage. In a subsequent letter of 4 February, he explained: ‘The luggage was frightfully expensive, because of the weight of the books and offprints. I can see no reason, a priori, why the Board should pay for this.’ He also told Robbins, perhaps confusingly: ‘I did not, of course, leave Holland until I had assurance from Professor Pauli that I might work with him; but I had no letter which I could submit to the Board to indicate his consent.’
A possible explanation for this last statement is that Pauli gave his consent not in a letter, but face-to-face. In the middle of January 1929, both Oppenheimer and Pauli were in Leipzig, attending a regional meeting of the German Physical Society. Both had been drawn by the presence there of Heisenberg: Oppenheimer to hear him lecture on his recent work on ferromagnetism, and Pauli to discuss a piece of work that he and Heisenberg had planned to write jointly.
Heisenberg had been at Leipzig since 1927, when, at the astonishingly young age of twenty-five, he had been appointed to the chair of physics there. At about the same time, his interests and Pauli’s began to converge, both stimulated by the work of Dirac’s that had so excited Oppenheimer at Göttingen. What Dirac had achieved in that work was to take the first step in the direction of a theory that would unite quantum mechanics and electrodynamics into what is now known as QED, or quantum electrodynamics. Oppenheimer had been disappointed that Dirac had not developed that theory at Göttingen; now, Heisenberg and Pauli were about to combine their formidable energies and talents in the pursuit of such a development, and, fortunately for Oppenheimer, they were about to do this just at the moment when he was going to start work with Pauli.
In its classical form, electrodynamics – the understanding of electromagnetic forces – received its definitive formulation in the differential equations of James Clerk Maxwell, who, in the 1860s, was the first to realise that light was a form of electromagnetic radiation. In Clerk Maxwell’s theory, developed and refined by later physicists, such as Heinrich Hertz, electromagnetic radiation was understood to consist of waves in the ‘ether’, which, after the discovery of the electron by Thomson in 1896, was considered to be an electromagnetic ‘field’ that mediated between individual electrons. All this changed with Einstein’s work in 1905: first, the ether was abolished; second, electromagnetic radiation was seen as consisting of discrete ‘quanta’; third, energy and matter were now regarded as equivalent to each other (this is the importance of the famous equation E=Mc2); and finally, in accordance with the theory of relativity, the speed of light (as of all electromagnetic radiation) was held to be the same for all observers, faster than which nothing was allowed to travel, which necessitated fundamental changes in the equations used to calculate the energies of waves of radiation or particles of matter.
The problem was that no consistent theory of electrodynamics had yet emerged that took into account these revolutions in physics brought by Einstein and then later by quantum theory. Einstein had shown how the basic equations of electrodynamics could be made relati
vistic, but they still described the continuous waves of classical electrodynamics, not the discontinuous ‘quantised’ light envisaged by Einstein and in quantum theory. What was needed, and what Dirac had shown might be possible, was a relativistic quantum-field theory.
As soon as he read Dirac’s 1927 paper, Pauli wrote to Heisenberg proposing a project to construct a complete quantum electrodynamics analogous to the Clerk Maxwell formulation of the classical theory. That year, both had too much else to do to make much progress on this, but the following year the project received fresh stimulus, again provided by Dirac, who published a paper with the Royal Society that introduced what is now known as the ‘Dirac equation’. This is an equation for calculating the energy of electrons that, unlike the famous Schrödinger wave function, takes relativity into account, a factor that becomes increasingly important as the speed of the electrons approaches the speed of light.
Furthermore, the Dirac equation could deal more naturally and more easily than the Schrödinger function with the ‘spin’ of electrons and therefore with Pauli’s great contribution to physics: the exclusion principle. Formulated by Pauli in 1924, the exclusion principle was hailed by Einstein as a ‘new law of nature’, and its importance was recognised and appreciated more and more as theoretical physics developed in the 1920s and ’30s. In Heisenberg’s lecture on ferromagnetism that Oppenheimer attended at Leipzig, for example, much use was made of the exclusion principle. Eventually, prompted by Einstein, its importance was recognised by the Nobel Prize committee, who in 1945 awarded Pauli the Nobel Prize in Physics on the basis of the exclusion principle (Heisenberg’s contribution was recognised much earlier – he received his Nobel Prize in 1932).