Inside the Centre: The Life of J. Robert Oppenheimer (27 page)

What Born and Oppenheimer presented in their joint paper was a mathematical technique – which has since become a cornerstone of the entire discipline of quantum chemistry – for calculating the energy of a molecule through a series of approximations. First, the energies of the electrons are calculated on the assumption that the nuclei are stationary. This is an approximation, but not a wild one, since the mass of the nucleus is so much greater than that of the electrons that, from the point of view of the electron, so to speak, the nucleus
is
stationary. Then, the vibrations of the nucleus are calculated, and finally the rotational energy of the molecule. Though each of these calculations is an approximation, the result is to turn what had previously been a completely impossible calculation into one that, though difficult, is at least possible, thereby enabling one to bring the insights of quantum mechanics to bear on the questions that had attracted Oppenheimer to science in the first place – questions about the fundamental nature of chemical substances.

The paper had a difficult gestation. Its first draft, produced by Oppenheimer during the Easter vacation of 1927, was only five pages long. ‘I thought this was about right,’ Oppenheimer later said. ‘It was very light of touch and it seemed to me all that was necessary.’ Born thought otherwise. He was, he later recalled, ‘horrified’ by Oppenheimer’s first draft and used his position as the senior partner to insist upon a more expansive rewrite. ‘I didn’t like it,’ Oppenheimer later said, ‘but it was obviously not possible for me to protest to a senior author.’

Because of the wrangling over presentation, and the rewriting it necessitated, the paper was not sent off for publication until the end of August 1927. In the meantime, in June, Edwin Kemble visited Göttingen and reported to a Harvard colleague:

About two weeks later, much to Born’s relief, Oppenheimer left Göttingen.

He left with a doctorate, a growing international reputation as one of the most brilliant young physicists of his generation, and a small but important circle of friends united by their brilliant intelligence, their eminence and their shared passion for understanding the strange world
of quantum mechanics. It was this last aspect that dominated his own memories of Göttingen. ‘In the sense which had not been true in Cambridge and certainly not at Harvard,’ Oppenheimer remembered, ‘I was part of a little community of people who had some common interests and tastes and many common interests in physics.’

One should not, however, be misled by these memories to think of Oppenheimer being part of a community of people at Göttingen itself. As Born told Ehrenfest, the paralysing effect that Oppenheimer had exerted on Born himself was felt also by his students (as Born rather melodramatically put it, Oppenheimer ‘ruined my young people’). No, the ‘community’ that Oppenheimer had in mind consisted of people who came to Göttingen as visiting scholars from other institutions. Of the people he mentioned by name as members of that community, not one of them was a physicist based at Göttingen. Indeed, with regard to two of them – Gregor Wentzel, who was at the University of Leipzig, and Wolfgang Pauli, from the University of Hamburg – he was not even sure whether he met them in Göttingen or in Hamburg (the latter seems more likely). The one person he mentioned by name who was actually based at Göttingen was Richard Courant, who was a mathematician rather than a physicist and had very little to do with the development of quantum mechanics. The final person named by Oppenheimer in connection with the ‘little community’ was Werner Heisenberg, who continued to be based at Copenhagen until the autumn of 1927, when he was made a professor at Leipzig.

If there is a single person at Göttingen with whom Oppenheimer might conceivably have formed some sort of community during his time there, it is Paul Dirac, who in June 1927 left Göttingen for Leiden, where he stayed as a guest of Paul Ehrenfest for a month before returning to Cambridge. When it was time for Oppenheimer to leave Göttingen, he followed Dirac to Leiden, joining him as Ehrenfest’s guest. This is what prompted Born, in the letter quoted previously, to write to Ehrenfest about Oppenheimer. Most of the letter, dated 16 July 1927, is typewritten and concerned with matters of a professional interest. Then, in a handwritten postscript, Born wrote:

Ehrenfest evidently replied in a way that indicated that he did not share Born’s view of Oppenheimer. ‘Your information about Oppenheimer was very valuable to me,’ Born told him in a letter of 7 August 1927. ‘I know that he is a very fine and decent man but one can’t help it if someone gets on your nerves.’ By the time Born wrote this, Oppenheimer himself was back in the US, having set sail to New York from Liverpool in mid-July. His plan was to spend the rest of the summer with his family before taking up his postdoctoral fellowship at Harvard in October.

It was, in some ways, an unfortunate time for a quantum physicist to be leaving Europe, since two of the most significant events in the history of quantum mechanics were about to happen, the first in Italy and the second in Belgium. The first was the announcement by Niels Bohr of the principle of complementarity, the importance of which Oppenheimer himself in later life was to emphasise at every opportunity and which, together with Heisenberg’s uncertainty principle, forms the so-called Copenhagen Interpretation of quantum mechanics. The principle of complementarity says that waves and particles are inconsistent, but complementary, features of the reality of photons and electrons. Light really is made up of particle-like quanta (photons)
and
it really does consist of waves. Depending on how we measure it, we see it as waves or as particles, but never both. Nevertheless, for a complete understanding of photons and electrons, both are necessary. We must not attempt to reduce waves to particles or particles to waves, Bohr thought; we must rather accept each as complementing the other.

Bohr announced the principle of complementarity in a paper called ‘The Quantum Postulate and the Recent Development of Atomic Theory’, which he delivered at the International Physics Congress, held in Como, Italy, in September 1927. In the paper, he argued that complementarity was the bedrock upon which quantum theory was based. The uncertainty principle, for example, Bohr claimed, was simply a consequence of complementarity; that we cannot measure
both
position and momentum precisely at the same time is a special case of the more general truth that we cannot see an electron or a photon both as a particle and a wave at the same time. To complementarity and the uncertainty principle, Bohr added Born’s statistical interpretation of Schrödinger’s wave function, to form what he now regarded as a complete and finished theory – that is, quantum mechanics – but which others regard as the three essential elements merely of the Copenhagen Interpretation of quantum mechanics. Either way, it is the most
influential and most important set of ideas in twentieth-century physics, with consequences that go way beyond physical science to the most basic and general philosophical ideas. If Born’s statistical interpretation of the wave function requires one to abandon determinism, the uncertainty principle forces one to abandon the age-old conception of causality, which held that, given a complete description of the position and momentum of an object, one could causally predict its future. Meanwhile, the principle of complementarity seems to force one to rethink the very idea of an ‘outside world’, the idea that we can observe the things and events around us without interfering with them. On Bohr’s understanding, to observe is to measure, and to measure is to influence which side of the wave-particle duality we are dealing with (since it is the method of measurement that determines whether we see waves or particles). Virginia Woolf, in emphasising the importance of the art exhibition ‘Manet and the Post-Impressionists’, once famously remarked: ‘On or about December 1910, human character changed.’ In the same spirit, one might say: ‘On or about September 1927, the physical world changed.’

Attending the Como conference were more than seventy physicists from all over the world. Born was there to give a paper on the statistical interpretation of the wave function. Heisenberg was there and, though he did not give a paper, he spoke in support of Bohr’s paper, giving, in the process, his own outline of uncertainty. Also there were Rutherford, de Broglie, Wolfgang Pauli, Arnold Sommerfeld and Arthur Compton. If Oppenheimer had been in Europe at the time, he would surely have attended.

The second momentous event in the autumn of 1927 was the fifth Solvay Congress, held in Brussels during the last week in October. The Solvay Congresses (named after their sponsor, the Belgian industrialist Ernest Solvay) had begun in 1911, the first in the series having the theme ‘Radiation and the Quanta’. The idea was to gather together the twenty or so most distinguished physicists in the world to hammer out an ongoing, open question. The star of the first conference had been the young Albert Einstein. After the second conference in 1913, the series was interrupted by the First World War and then deeply affected by the post-war exclusion of German physicists, which condemned the third and fourth conferences, held respectively in 1921 and 1924, to a discussion of the most fundamental questions in the absence of many of the leading physicists.

No such problems beset the fifth Solvay Congress, which was anticipated with great excitement within the international community of theoretical physicists for a number of reasons. First, its theme of ‘Electrons and Photons’ was the hot topic of the day, and the wording of the invitation (the Solvay Congresses were strictly invitation-only)
made it clear that the ‘conference will be devoted to the new quantum mechanics and to questions connected with it’. Second, since the admission of Germany into the League of Nations in 1926, German scientists could no longer be treated as members of an enemy country, which meant that the conference could invite not only quantum pioneers like Max Planck, but also the leading members of the younger generation of German physicists, such as Heisenberg and Born, who had founded, developed and shaped the new quantum theory. Finally, the readmittance of German physicists into the international community meant that Albert Einstein, the leading opponent of the new theory, could engage publicly with its chief proponents.

And so the stage was set for what has gone down in history as
the
great debate about the science and philosophy of quantum mechanics, in which almost all the most notable defenders and opponents of the new theory – the radical consequences of which had been spelled out and emphasised by Born, Heisenberg and Bohr – were gathered in one place. The defenders included, as well as Bohr, Born and Heisenberg, Paul Dirac and Wolfgang Pauli. Representing the opposition were Einstein, Planck, Schrödinger and de Broglie. Also present were Marie Curie, Arthur Compton and Ralph Fowler. It was an extraordinarily prestigious group; of the twenty-nine people who attended, seventeen were or would become Nobel Prize-winners. At stake in their discussions was not only a new physical theory, but a proposed fundamental change in the way we think about determinism, causality and the nature of scientific theory. One way of crystallising the issue that lay at the centre of the debates, using a phrase that recurred again and again during the conference, is to ask the question that Einstein had raised in his letter to Born: does God play dice or not?

The congress ran from Monday 24 October to Friday 28 October. The format chosen was for reports to be delivered on various aspects of quantum mechanics, with each of them followed by a lengthy discussion. Only five reports were delivered during the entire conference, such was the determination of the organisers to give plenty of time for discussion. On the first day, reports were given by William L. Bragg from Manchester on X-ray reflection and Arthur Compton on the photoelectric effect. The following day, Louis de Broglie reported on ‘The New Dynamics of Quanta’, outlining and defending his own view – which received almost no support from the delegates – that both wave and particle existed, although not as envisaged by Bohr and Born, but rather in a way that visualised particles being guided or ‘piloted’ by waves.

Throughout these early papers, Einstein remained silent. He even stayed silent when, on Wednesday 26 October, Born and Heisenberg presented a joint report on quantum mechanics that seemed calculated to provoke him into discussion. After outlining matrix mechanics,
transformation theory, the probability interpretation, uncertainty and complementarity, Born and Heisenberg ended with the uncompromising statement: ‘We consider quantum mechanics to be a closed theory, whose fundamental physical and mathematical assumptions are no longer susceptible of any modification.’

Einstein finally broke his silence on the last day of the conference, when, in place of reports, the organisers had arranged for the entire day to be taken up with a wide-ranging general discussion that was to be the climax of the whole event. As it turned out, the discussion was dominated by a series of exchanges between Bohr and Einstein. First Einstein would propose what he took to be a fatal flaw in quantum mechanics, then Bohr would respond, invariably identifying a flaw in Einstein’s own arguments. In a letter to his students at Leiden, Ehrenfest described Bohr as ‘towering over everybody . . . step by step defeating everybody’. This reflected the general view. As the conference closed, Heisenberg wrote: ‘I am satisfied in every respect with the scientific results. Bohr’s and my views have been generally accepted; at least serious objections are no longer being made, not even by Einstein and Schrödinger.’