Inside the Centre: The Life of J. Robert Oppenheimer

by Ray Monk

  Oppenheimer was by this time a graduate research student, though not, as he had earlier hoped, supervised by Rutherford, but rather by Rutherford’s predecessor, the aged and semi-retired J.J. Thomson. When Rutherford took over as director of the Cavendish in 1919, he insisted on having complete control and got Thomson to put in writing that he would not in any way interfere with Rutherford’s running of the place. In return for this assurance, Thomson was granted space in the laboratory for his own research and was allowed to supervise some research students. These tended to be the ones, like Oppenheimer, that Rutherford did not want to supervise.

  Thomson was nearly seventy years old and had been for many years somewhat off the pace in the rapidly changing world of theoretical physics. The monumental developments in the subject that had occurred in the early twentieth century were things that he either ignored or resisted. He never accepted either Einstein’s theory of relativity or Rutherford’s planetary model of the atom, and quantum theory had passed him by altogether. In his old age, he remained deeply devoted to Trinity College and developed an absorbing interest in gardening. He is remembered fondly by those who knew him as genial and kind, but he was not the man to guide an emotionally turbulent, brilliant young man – suffering agonies of sexual frustration, social isolation and a crippling ineptitude for practical laboratory work – through the intricacies of modern physics.

  The details of Oppenheimer’s research under Thomson’s supervision are now lost. In a letter to Fergusson of November 1925, he says that Thomson ‘thought my experiments quite good, but didn’t help much otherwise’, but he does not say what those experiments were. Later in life he described his research as a study of ‘what happened with beams of electrons and thin films of metal’, a description that could perfectly well apply to a good deal of the research conducted at the Cavendish during this period, but which also ties in with Oppenheimer’s description to Priestley that September of his intended research topic: the theory of electronic conduction, especially those aspects of it ‘which can give an indication of the laws of force to which the motion of electrons is subject’. To make the ‘thin films of metal’ he needed for this research, Oppenheimer had to undergo what he later recalled as ‘the miseries of evaporating beryllium on to collodion, and then getting rid of the collodion, and so on’. The resulting beryllium films were used not only by Oppenheimer, but also by James Chadwick, Rutherford’s second-in-command at the Cavendish, famous for the discovery of neutrons in 1932.

  ‘The business in the laboratory was really quite a sham,’ Oppenheimer later remarked, ‘but it got me into the laboratory, where I heard talk and found out a good deal of what people were interested in.’ In other words, the only value he later saw in his experience of experimental physics at Cambridge was that it stimulated his interest in contemporary developments in theoretical physics. As it turns out, that stimulus was enough to help him eventually overcome the acute psychological problems he had suffered in the autumn of 1925. John Edsall remembers that in the New Year of 1926, though it was obvious that Oppenheimer was undergoing some sort of crisis (‘there was a tremendous inner turmoil’), he nevertheless ‘kept on doing a tremendous amount of work, thinking, reading, discussing things’.

  What brought about this burst of activity was Oppenheimer’s discovery that theoretical physics was undergoing what the Nobel laureate Steven Weinberg has described as ‘the most profound revolution in physical theory since the birth of modern physics in the seventeenth century’. Most of the important contributions to this ‘profound revolution’ were made by young physicists just a few years older than Oppenheimer himself. It was, it was commonly remarked, the period of Knabenphysik (boy physics).

  The ‘boys’ in question fully realised that they were living in exciting times. Oppenheimer, soon after arriving at Cambridge, found himself caught up in that excitement. In November 1925, he had written to Fergusson saying that there were ‘certainly some good physicists’ at Cambridge, emphasising ‘the young ones I mean’. He had, he told Fergusson, ‘been taken to all sorts of meetings’, including ‘several rather pallid science clubs’. Pallid or not, it was at these science clubs that Oppenheimer was introduced to the epoch-making work in theoretical physics that was then going on, and where he got to meet and get to know some of the Knaben who were ushering in the new epoch.

  The best known of these clubs is the Kapitza Club, which had been formed by the Russian physicist Peter Kapitza upon his arrival at the Cavendish in 1921, to provide an informal atmosphere within which ideas in physics could be discussed and debated. Kapitza, the son of a tsarist general, but a fervent supporter of the Bolshevik revolution, was one of the most colourful characters at the Cavendish and a favourite of Rutherford’s. He and Blackett vied with each other to be regarded as Rutherford’s chief assistant. The club Kapitza established in his own name became an important forum for the exploration of new ideas in physics, providing both a means by which experimentalists and theorists at Cambridge could learn from each other and an opportunity for Cambridge physicists to hear papers from distinguished physicists from other countries. Blackett was a member of this club and it was no doubt he who introduced Oppenheimer to it. The club met at the young experimentalist John Cockcroft’s room in the Cavendish, where, in addition to Kapitza himself, Oppenheimer would have encountered not only all the leading experimental physicists at Cambridge, but also the man who would very soon become recognised as one of the world’s leading theorists: Paul Dirac.

  Just two years older than Oppenheimer, Dirac had been a research graduate student in physics at St John’s College since 1923, having previously completed degrees in both electrical engineering and applied mathematics at the University of Bristol. He was tall and thin and had a reputation for saying as little as possible. He would now almost certainly be diagnosed as ‘autistic’; the many stories that circulated about him describe the kind of behaviour characteristic of Asperger’s syndrome. He combined an extraordinarily intense, obsessive interest in mathematics and physics with an almost complete lack of interest in anything else, including politics, literature and everyday conversation. Oppenheimer, remembering Dirac later in life, remarked that he was ‘not easily understood [and] not concerned to be understood. I thought he was absolutely grand.’

  Coming from a relatively impoverished, lower-middle-class family in Bristol, Dirac was certainly not ‘grand’ in the social sense, but there was, undeniably, a certain grandeur in his exceptional intellectual ability. That he was socially awkward may have been, for Oppenheimer, an advantage; there is no sign that Dirac, for all his extraordinary brilliance, induced in Oppenheimer the murderous envy that Fergusson and Blackett had provoked. Dirac may have been the cleverest graduate physicist at Cambridge, and possibly even the greatest scientist the university had produced since Newton, but he did not, like Fergusson, mix with Europe’s literary, artistic and philosophical elite; nor was he, like Blackett, widely regarded as the most handsome, best-dressed and most charismatic figure on the Cambridge social scene. Oppenheimer was thus able to admire him without feeling awed or envious.

  Though still a graduate student, Dirac was invited to give a course of lectures on quantum theory in the academic year 1925–6. Entitled ‘Quantum Theory (Recent Developments)’, it was the first course on quantum mechanics ever given at a British university. Among the few students who attended it was Oppenheimer, who, like the other attendees, was no doubt conscious of the privilege of being given access to Dirac’s latest thoughts on the subject before they were announced and published to the outside world. ‘Dirac gave us what he himself had recently done,’ remembers one member of this privileged group, adding: ‘We did not, it is true, form a very sociable group, but for anyone there it was impossible to forget the sense of excitement at the new work.’

  Possibly through Dirac, or possibly through Blackett, Oppenheimer was introduced to the 2V Club, usually referred to as the ‘Del Squared V Club’, being a mathematical symbol and 2 b
eing an operator (the ‘Laplacian operator’) frequently used in theoretical physics. Where the Kapitza Club consisted mainly of experimental physicists, the 2V Club was for theorists. There Oppenheimer would have met all the leading theoretical physicists at Cambridge, including most notably Ralph Fowler, Dirac’s supervisor and Rutherford’s son-in-law. Fowler, who has been described as ‘a generous-spirited man with the build of Henry VIII and the voice of a drill sergeant’, was, until Dirac’s fame overshadowed him, the foremost theoretical physicist at Cambridge and, crucially for both Dirac and Oppenheimer, the one most fully abreast of developments on the continent.fn21

  It was Fowler, for instance, who was first aware of the importance of the work of the French physicist Louis de Broglie, the man who took the initial steps towards the quantum-mechanics revolution. De Broglie, a member of one of the most ancient and distinguished French aristocratic families, had studied medieval history at the University of Paris before, under the influence of his elder brother, switching to physics. In the autumn of 1923, two years before Oppenheimer’s arrival at Cambridge, de Broglie had published a series of three short papers in the French journal Comptes rendus, putting forward the outlandish suggestion that electrons should be regarded as being both particles and waves.

  The inspiration for this was Einstein’s Nobel Prize-winning suggestion in 1905 that light, previously thought of as consisting of waves, should be thought of as being made up of discrete ‘quanta’, or ‘photons’ as they are now called. Einstein had used this idea to account for the ‘photoelectric effect’ – that is, the fact that, when light is shone onto a metal surface, electrons are emitted, the energy of the electrons depending not on the intensity of the light, but on its frequency. This quantum theory of light (or, more generally, of electromagnetic radiation) was confirmed in 1922 in a series of experiments conducted by the American physicist Arthur Compton. De Broglie, in a flash of inspiration, saw that, if Einstein’s suggestion regarding light were extended to electrons, some of the difficulties faced by the Rutherford–Bohr–Sommerfeld model of the atom might be overcome. In particular, it would be possible to answer the question that Rutherford, with his unerring instinct for the heart of a problem, had raised about Bohr’s model of the atom: how do electrons ‘know’ which orbits to travel on? Or, to put it another way, why are electrons only ‘allowed’ certain orbits? De Broglie’s hypothesis of the wave-particle duality of electrons provided a brilliant answer to this: as electrons are waves, they can only circle the nucleus in certain orbits, namely those that correspond to multiple whole units of their wavelengths.

  To begin with, de Broglie’s brilliant idea aroused remarkably little interest among physicists. Fowler was one of the first to see any value in it, and it was he who in October 1923 submitted to the Philosophical Magazine an English version of de Broglie’s articles. Entitled ‘A Tentative Theory of Light Quanta’, this appeared in print in February 1924, and, though it made de Broglie’s revolutionary idea accessible to English-speaking physicists, it failed to attract very much attention. In fact, it required the advocacy of Einstein himself to make theorists take de Broglie seriously. In the spring of 1924, de Broglie wrote up his ideas and presented them as a PhD thesis, which was examined the following November. One of the examiners was Paul Langevin, who sent de Broglie’s thesis to Einstein, asking him what he thought. The reply was unequivocal: ‘He has lifted a corner of the great veil,’ wrote Einstein. De Broglie was duly awarded his doctorate and, five years later, after his hypothesis had been confirmed experimentally, was awarded the Nobel Prize.

  Once it had been applauded by Einstein, de Broglie’s audacious idea of wave-particle duality caught the imagination of physicists everywhere. Patrick Blackett was reported to have returned from his year in Göttingen ‘brimful of talk and enthusiasm about de Broglie and wave mechanics’. In August 1925, a month before Oppenheimer arrived at Cambridge, Paul Dirac gave a paper to the Kapitza Club on de Broglie’s ideas.

  By then, however, the attention of the few physicists keeping abreast of these developments had shifted to the work of the young German physicist Werner Heisenberg. Having received his doctorate (supervised by Arnold Sommerfeld) from the University of Munich in 1923, when he was still only twenty-one, Heisenberg moved to Göttingen to take up a position as Max Born’s assistant. During the first half of the academic year 1924–5, as Born was due to be in the United States on a lecture tour (which, in the event, he postponed until the following year), Heisenberg arranged to spend some months at Bohr’s institute in Copenhagen. There at the same time, taking sabbatical leave from Cambridge, was Ralph Fowler, who was thus able to add Heisenberg to his already impressive list of personal contacts among the leading and up-and-coming physicists in Europe. Meanwhile, Patrick Blackett was at Göttingen, discussing with Franck and Born (and then, when he returned to Göttingen in April 1925, with Heisenberg) the wave-particle duality of the electron posited by de Broglie.

  Though de Broglie’s theory gave a convincing explanation of why electrons were confined to the orbits, or energy states, specified in Bohr’s model of the atom, it introduced an enormous problem of its own: how could an electron possibly be both a particle and a wave? We can picture electrons as waves vibrating around the nucleus, or we can picture them as material objects orbiting the nucleus, but we cannot, surely, picture them as both at the same time. De Broglie’s initial attempt to solve this conundrum was to imagine electrons as particles moving along a wave-like path, but this stripped the theory of its power to explain Bohr’s orbits, since no good explanation could be given as to why electrons were tied to those wave-like paths. The beauty of de Broglie’s theory lay precisely in the thought that an electron was a wave, the wavelength of which explained the ‘static orbits’ of Bohr’s theory. And yet there were very good reasons for believing, and abundant experimental evidence to suggest, that electrons were particles.

  Heisenberg’s novel response to this problem was to jettison all talk of orbits, particles and waves and refrain from picturing the electron at all. We must, he declared, confine ourselves to what can be observed. We cannot observe the orbiting of the nucleus by the electron; all we can observe is the energy given off by an electron when it ‘jumps’ from one state to another. The reason we can observe this is that the energy in question takes the form of visible light, thus enabling the technique of investigation known as spectroscopy: the study of the spectra of light emitted by electrons of various elements, which allows physicists to associate each element with its characteristic and unique spectrum of coloured light. It is upon the data provided by spectroscopy that Bohr’s theory of atomic structure was built (hence the title of Sommerfeld’s classic book on the subject: Atombau und Spektrallinien [Atomic Structure and Spectral Lines]), and when Heisenberg announced his intention of confining himself to what can be observed, he meant primarily: observed using the techniques of spectroscopy.

  In June 1925, shortly after he returned to Göttingen from Copenhagen, Heisenberg, ill with hay fever, decided to recuperate on the North Sea island of Helgoland. There, thinking alone about the strictly observable properties of electrons, inspiration struck him and he formulated the basic ideas of the branch of physics that was to claim the attention of Oppenheimer and most of his contemporaries: quantum mechanics. The fundamental aim of this branch of physics is to provide quantum theory with a mechanics – that is, a mathematical model that would explain the apparently bizarre movements of electrons and of subatomic particles generally. What occurred to Heisenberg in Helgoland was (to him) a brand-new kind of mathematics, which one could use to model the behaviour of electrons.

  At the heart of this mathematics was a numbering system that assigned to electrons a pair of numbers, p (representing the electron’s momentum – that is, its mass multiplied by its velocity) and q (representing the electron’s position), and a technique of multiplying these pairs of numbers. The troubling aspect of this new mathematical model was that the multiplication rules for
it were not commutative – that is, p x q was not, in general, equal to q x p. Heisenberg had no explanation for this departure from the basic rules of arithmetic, nor could he offer a picture of the physical processes that obeyed such odd rules. What he did have was a mathematical modelling of the behaviour of electrons, and this itself was exciting enough to ensure that he did not sleep very much in Helgoland; and enough, too, to ensure that, six years later, he won the Nobel Prize.

  Returning to Göttingen in a state of excitement and optimism about his new work, Heisenberg hurriedly wrote up his new theory as a paper entitled ‘Quantum Theoretical Reinterpretation of Kinematic and Mechanical Relations’, which he gave to Born to submit for publication, while he himself left for Cambridge to fulfil a prior arrangement to deliver a talk to the Kapitza Club. The talk, delivered on 28 July 1925, was not on his revolutionary new ideas, but Heisenberg did mention his recently written paper to his host, Fowler, who asked to see it when Heisenberg had proof copies available. When, at the beginning of September, Fowler duly received a proof copy, he sent it to Dirac with a scribbled message on the front page: ‘What do you think of this? I shall be glad to hear.’

  Dirac was at this time in Bristol for the summer vacation. After an initial glance at Heisenberg’s paper, he put it to one side, seeing little interest in it. When he returned to Cambridge in October, he took up the paper again and this time became fascinated with it and quickly convinced of its fundamental importance. He realised that the key to it was the non-commutative multiplications that had puzzled Heisenberg, and, unlike Heisenberg, he recognised these as being akin to a mathematical construction called a ‘Poisson bracket’, which had been introduced into mathematics in the nineteenth century. Using the method of Poisson brackets, Dirac provided Heisenberg’s theory with a new mathematical foundation, the centre of which was the equation (p x q) – (q x p) = ih/2π, which not only says that the multiplication of p and q is non-commutative (if it were commutative, of course, (p x q) – (q x p) would be equal to zero), but also provides an exact quantity by which p x q differs from q x p, a quantity that uses the magical ingredient h, Planck’s constant, together with that equally mysterious ‘imaginary’ number, i, which is the square root of –1.