Inside the Centre: The Life of J. Robert Oppenheimer (23 page)

Why did this passage mean so much to Oppenheimer that he learned it by heart? And why was reading it one of the great experiences of his life?

A clue to this might be contained in some remarks Oppenheimer made towards the end of his life, when he took part in a series of conferences partially sponsored by the Congress for Cultural Freedom – a group of politcally liberal intellectuals united by their opposition to communism – on the relationship between science and culture. ‘We most of all should try to be experts on the worst among ourselves,’ he said at one such meeting, and, as if to confirm that he had always been acutely aware of the worst in himself, he made the following confession:

In other words, Oppenheimer was able to live with other people only when he came to see that they did not necessarily see him as he saw himself, and therefore that his words and deeds did not arouse in others the sense of revulsion they aroused in him. This realisation, brought about perhaps by a combination of Proust, conversations with good friends and the pleasures of the Corsican countryside, had an enormous effect on him. When he returned from Corsica, he said, he ‘felt much kinder and more tolerant’ and ‘could now relate to others’.

Oddly, Oppenheimer chose to leave Corsica in a way that persuaded Wyman and Edsall that he was still, after all, a little mad – or at least, as Wyman later put it, ‘passing through a great emotional crisis’. Having reached Bonifacio, the plan was to proceed to Sardinia, but, while the three of them were having dinner at their inn, a waiter approached Oppenheimer to tell him when the next boat left for France. Naturally, Edsall and Wyman immediately asked him why he was leaving so abruptly and unexpectedly. ‘I can’t bear to speak of it,’ said Oppenheimer, ‘but I’ve got to go.’ As Wyman later remembered it, the three of them drank a little more wine and then Oppenheimer said: ‘Well, perhaps I can tell you why I have to go. I’ve done a terrible thing. I’ve put a poisoned apple on Blackett’s desk and I’ve got to go back and see what happened.’ Why Oppenheimer should, in an effort to explain his decision to return to Cambridge in the spring of 1926, confess to something he had done seven months earlier, suggesting that he had only just done it, takes some understanding. Indeed, it might well be inexplicable, though it seems reasonable to suppose that, having spent ten days and nights living so closely with Edsall and Wyman, Oppenheimer felt obliged to confess to them the secret of his bizarre behaviour the previous autumn.

However, the reason he wanted to get back to Cambridge, it seems safe to suppose, is that he wanted to finish writing his paper on quantum physics, and, with this in mind, it is possible to arrive at the following speculative explanation of his ‘confession’ to Wyman and Edsall: before he set off for Corsica, Oppenheimer had left on Blackett’s desk an early draft of his first paper on quantum mechanics, which he now, ten days into his holiday in Corsica, realised contained serious errors (that is, it was ‘poisoned’). Desperate to get back to Cambridge to continue work on his paper and to correct the mistakes it contained, and feeling obliged to confess his previous sins to Edsall and Wyman, he exploited the analogy
to tell them a story that contained, at one and the same time, a literal confession about what he had done the previous autumn and a metaphorical explanation of why he had to break short his holiday and return to Cambridge. Whether there is any truth at all in this speculative suggestion, what is not in doubt is that, after his holiday in Corsica, Oppenheimer was a different person. Whereas just a few months earlier he had felt paralysed, depressed and unequal to the demands made upon him, now he was confident, productive and energetic.

When he returned to England in the spring of 1926 the country was in political turmoil, heading towards the general strike, which lasted from 3 May to 12 May. The country was engulfed in class war, with the middle classes doing everything they could to negate the effects of the strike. At Cambridge, this involved undergraduates temporarily abandoning their studies in order to drive buses or trains or lorries – anything to keep deliveries going and prevent the economy and society from grinding to a halt. So many Cambridge students took part in these strike-breaking activities that the university decided to postpone the summer exams. A minority of students and academics, including Patrick Blackett, were on the side of the workers and voiced their support for the strike. Paul Dirac, meanwhile, ignored it and spent this time finishing his PhD thesis. Entitled simply ‘Quantum Mechanics’, it was the first doctoral thesis ever to be submitted on the subject and contained work that had already been recognised as being of fundamental importance.

Oppenheimer, who had not yet even begun work on his PhD thesis, managed, within a few weeks of returning from Corsica, to complete the paper that was to become his first publication. Its title was ‘On the Quantum Theory of Vibration-Rotation Bands’. On 24 May 1926, it was received by the Cambridge Philosophical Society, the venerable scientific society (established in 1819 ‘for the purpose of promoting scientific inquiry’) to which Oppenheimer had been elected as an ‘associate’ in January 1926, and was published in their
Proceedings
in July. Though he was later disparaging about it (‘That was a mess, that first paper’), the very fact that he was able, so soon after his severe problems during the winter, to write a publishable paper on a subject right at the cutting edge of advanced physical theory was a notable achievement.

The paper might be seen as one of the earliest contributions to the subject of ‘quantum chemistry’, in that it attempts to apply the new quantum mechanics of Heisenberg, Born, Jordan, Dirac and Schrödinger (all of whose papers are cited in it) to the understanding not of atoms, but of molecules. In particular, Oppenheimer seeks to show the applicability of Dirac’s version of the mathematics of quantum mechanics to the understanding of diatomic molecules; that is, molecules, like those of oxygen (O
₂
) and hydrogen (H
₂
), that consist of two atoms. The vibration
and rotation of these molecules produce characteristic spectra of electromagnetic radiation, the frequencies of which Oppenheimer attempts in this paper to derive from within Dirac’s theory.

Compared with what Dirac was producing at this time, Oppenheimer’s first paper was a minor piece of work. It addresses a problem that is of secondary, rather than fundamental, importance, and, moreover, it has a weakness that would have been unthinkable in anything by Dirac: it contains mathematical errors. Nevertheless, its publication was enough to transform Oppenheimer from a failing experimental physicist to an up-and-coming theorist. When distinguished visitors came to Cambridge, Oppenheimer was now introduced to them as one of the
Knaben
leading the revolution in theoretical physics. When Paul Ehrenfest, the professor of physics at Leiden, came to Cambridge, for example, Oppenheimer remembers that ‘we went out on the river and talked about collision problems, Coulomb’s law . . . and so on’. A short while after Ehrenfest’s visit to Cambridge, Oppenheimer met him again, when he and other American physicists at Cambridge spent a week at the University of Leiden. There he met Ehrenfest’s young, but already famous, assistants, Samuel Goudsmit and George Uhlenbeck, who together had been the first to put forward the idea that electrons possess the property of spin. Oppenheimer’s reception among the theoretical physicists at Leiden recalls his acceptance by the literary ‘troika’ in New Mexico in 1922. Uhlenbeck remembers Oppenheimer as being a ‘very warm person’ who was ‘so involved in physics’ that it ‘was as if we were old friends because [we] had so many things in common’. Oppenheimer, for his part, recalls that it was ‘wonderful’ at Leiden and that he ‘realised then that some of the troubles of the winter had been exacerbated by the English customs’.

Back in Cambridge, Oppenheimer resumed his theoretical studies and began work on a second paper on quantum mechanics, this time on what is known as the ‘two-body problem’. This is, in general, the problem of providing a mathematical model of two bodies orbiting one another. Newton had provided a solution of this problem for classical physics, and Dirac and Schrödinger had investigated it from the point of view of quantum mechanics. Oppenheimer’s aim was to provide a more complete quantum-mechanical solution to this problem than had so far been achieved.

At the beginning of June 1926, while hard at work on this problem, Oppenheimer had one of the most memorable moments of his time at Cambridge – indeed, of his entire life – when he was introduced to Niels Bohr. Bohr, who was in England to receive the honour of being made a foreign member of the Royal Society, happened to be in Rutherford’s room at the Cavendish when Oppenheimer walked in.
Rutherford, who by then looked upon Oppenheimer as a promising theorist rather than a distinctly unpromising experimentalist, immediately introduced him to Bohr. As custom and politeness demanded in such a situation, Bohr asked Oppenheimer what he was working on and, on being told that it was the two-body problem, asked him how it was going. ‘I’m in difficulties,’ Oppenheimer replied. ‘Are the difficulties mathematical or physical?’ Bohr asked. ‘I don’t know,’ Oppenheimer answered, prompting Bohr to remark: ‘That’s bad.’ The encounter made a deep and lasting impression on Oppenheimer. After meeting Bohr, he once said, ‘I forgot about beryllium and films and decided to try to learn the trade of becoming a theoretical physicist.’ Bohr’s question to him, he thought, was a very good one, a question that went right to the heart of his difficulties. ‘I thought it put a rather useful glare on the extent to which I became embroiled in formal questions without stepping back to see what they really had to do with the physics of the problem.’

Perhaps because of the arithmetical mistakes in his first paper, Oppenheimer took immense care to ensure that the mathematics in this second paper was free from error. Edsall remembers how, at Oppenheimer’s request, he spent hours one Sunday checking the figures in this paper, even though he himself had little idea what they meant. His reward was a footnote acknowledging his help while misspelling his name (‘I am indebted to Mr J.T. Edsahl for checking these calculations’). By the middle of July the paper was finished and it appeared that month in the
Proceedings of the Cambridge Philosophical Society
under the title ‘On the Quantum Theory of the Problem of the Two Bodies’.

By a fortuitous coincidence, this second paper brought Oppenheimer to the attention of one of the leading figures in quantum mechanics at the very point when he was making his greatest contribution to the subject. That figure was Max Born, who had already played a key role in the development of the matrix version of quantum mechanics and was on the brink of providing the definitive interpretation of the theory. A summary of that interpretation had been given in a short paper that Born published on 10 July 1926 called ‘Zur Quantenmechanik der Stossvorgänge’ (‘On the Quantum Mechanics of Collision Processes’). Ten days later, Born sent off a longer, more polished and refined paper with the same title to the journal
Zeitschrift für Physik
, and on 29 July – three days after the publication of Oppenheimer’s second paper – Born came to Cambridge to deliver this paper as a talk to the Kapitza Club with the English title ‘On the Quantum Mechanics of Collisions of Atoms and Electrons’. This paper was to have a profound impact on the way quantum mechanics was understood, addressing head-on exactly the question raised by Bohr’s brief discussion with Oppenheimer, the
question about how one was to understand the physical reality that lay behind the mathematics of quantum mechanics.

The immediate aim of Born’s paper was to bring quantum mechanics to bear on the subject of how particles behave when they collide with each other; his more general intention was to provide an interpretation of the mathematical formulae of quantum mechanics. In both respects, his conclusions were startling, from both a physical and a philosophical point of view; so startling that many people, including Einstein, refused to accept them. Still more remarkable, especially in the light of Einstein’s resistance, is the fact that those conclusions became widely accepted and remain today the generally held view among scientists.

Regarding collisions, Born showed that quantum mechanics, unlike classical Newtonian mechanics, is non-deterministic. In Newtonian mechanics, what happens to one body after it collides with another (for example, a billiard ball hitting another billiard ball) is entirely determined by the laws of motion. So, if you repeat a collision (hit a billiard ball into another in exactly the same way), exactly the same thing will happen. If the ball deflected to the left the first time, it would deflect to the left every time you repeated the shot. In quantum mechanics, however, the situation is very different. According to Born, quantum mechanics allows identical experiments to have different outcomes: one time, the particle might be deflected to the left; another time, to the right. Any outcome is
possible
; some outcomes, however, are more
probable
than others. It is this feature of quantum mechanics that persuaded Einstein that the theory could not possibly be right and prompted him to make his famous remark (in a letter to Born): ‘God does not play dice.’