Dirac will probably have found the celebrations a chore and will have been relieved to spend the next day only with people he knew, a relaxed family Sunday. The many in Cambridge who saw Dirac as a shadow of a man, with no sense of fun, would have been surprised to see him at ease in the Bohrs’ nest, playfully squirting water from an indoor fountain over his mother and Margrethe, both of them laughing and protesting as they tried hopelessly to shield themselves from the dousing. Dirac’s Cambridge acquaintances would not have expected, either, that he would happily spend a day larking around with Bohr, his boys and Heisenberg, playing badminton and sleighing on the hills near Copenhagen. In the evening, Dirac reverted to his usual stand-offishness: he sloped off to bed early, not bothering to wish anyone goodnight. But Bohr wanted Dirac to talk shop and so yanked him back downstairs.
On her return to Bristol late on Monday, Flo was met at the railway station by Betty, who was up until the small hours listening to her mother’s account of her ‘great and wonderful adventure’. Charles was nowhere to be seen.
For the rest of his life, Dirac was curious about how he came to win the prize with Heisenberg and Schrödinger. The Nobel Foundation, always the essence of discretion, releases the papers concerning each year’s prize only after keeping them under lock and key for fifty years. Dirac never did find out about the political machinations that led to the first prizes for quantum mechanics; he eventually learned only that the English crystallographer William Bragg had nominated him and that Einstein had not.20 Only after Dirac died did it come to light that he had been fortunate to win the prize so young.
In the first three decades of the prize, the committee that decided the Nobel Prize for physics was biased against theoretical contributions, probably because of Alfred Nobel’s wish that his prizes should reward practical inventions and discoveries. The committee, not always well informed about theoretical physics, issued a statement in 1929 that the theories of Heisenberg and Schrödinger ‘have not yet given rise to any discovery of a more fundamental nature’.21 Behind the scenes in Stockholm, a long and involved battle was being fought about when to award a prize for the new theory and who should receive it. The Foundation was still arguing about this in 1932, when nominations for Heisenberg and Schrödinger were accumulating by the month. By early 1933, the pressure to award a prize for the theory was overwhelming, but there were still disagreements about how to share it. Dirac’s name had barely registered with the committee.22
By the time the committee met in September 1933, after the discovery of the positron had become widely accepted, his name was much more prominent. The Swedish physicist Carl Oseen, the most influential member of the committee, had heard from his student Ivar Waller of the quality of Dirac’s work. More important, the positron’s discovery was viewed as ‘an actual fact’, an observation that illustrated the utility of Dirac’s theory. At the end of the meeting, the consensus was that Heisenberg, Schrödinger and Dirac were head and shoulders above the other candidates, including Pauli and Born, and that Heisenberg deserved special recognition for being the first to publish the new theory.
Today, the committee’s judgements appear capricious. It would, perhaps, have been fairer to award Heisenberg and Schrödinger individual prizes in 1932 and 1933, leaving Dirac to win his own prize a year later, an outcome that Dirac himself would almost certainly have regarded as just. None of this really matters; today, no one doubts that the three physicists honoured in Stockholm in December 1933 deserved their Nobel status. Dirac, Heisenberg and Schrödinger are now among the select group of winners that give all Nobel Prizes their special lustre.
Notes - Chapter eighteen
1 Dalitz and Peierls (1986: 146).
2 Information from RSAS, 14 September 2004.
3 The main sources of the material in this chapter are in the Dirac Papers (FSU): Letter to Dirac from his mother, 21 November 1933 (2/2/9). Florence Dirac’s account of her trip is in ‘My visit to Stockholm’ (1/2/9) and in a long, descriptive letter to Betty (2/2/9).
4 Reports in Svenska Dagbladet and Dagens Nyheter, both on 9 December 1933.
5 This was one of Dirac’s favourite stories about his absent-minded mother. It is well recounted in Kursunoglu (1987: 18).
6 Reports in the Stockholm newspapers Nya Dagligt Allehanda, 9 December 1933, Stockholms Dagblad, 10 December 1933.
7 Reports in the Stockholm newspapers Nya Dagligt Allehanda, 9 December 1933, Stockholms Dagblad, 10 December 1933.
8 Report in Dagens Nyheter, 11 December 1933.
9 Dagens Nyheter, 11 December 1933; Svenska Dagbladet, 11 December 1933.
10 Women guests were first invited to the banquet in 1909, when the female Swedish wirter Selma Lagerlöf won the Nobel Prize for Literature.
11 Dagens Nyheter, 11 December 1933; Svenska Dagbladet, 11 December 1933; Stockholms Tidningen, 11 December 1933.
12 See http://nobelprize.org/physics/laureates/1933/dirac-speech.html (accessed 14 May 2008).
13 Annemarie Schrödinger notes ‘Stockholm 1933’, AHQP. Letter from Schrödinger to Dirac, 24 December 1933.
14 I thank Professor Sir Partha Dasgupta for identifying this error and clarifying its nature.
15 Flo Dirac, Dirac Papers, 1/2/9 (FSU) and 2/2/9 (FSU).
16 See http://nobelprize.org/nobel_prizes/physics/laureates/1933/dirac-lecture.html (accessed 14 May 2008).
17 Schuster (1898a: 367); see also Schuster’s follow-up article (1898b).
18 Born (1978: 270). See also ‘Eamon de Valera, Erwin Schrödinger and the Dublin Institute’ (McCrea 1987).
19 Flo Dirac, Dirac Papers, 1/2/9 (FSU) and 2/2/9 (FSU).
20 Dirac read Abraham Pais’s book Subtle is the Lord, and remarked ‘Most interesting for its revelation of the working of Nobel Committee’, Dirac Papers, 2/32/12 (FSU). The book mentions that Einstein did not nominate Dirac for a Nobel Prize.
21 Nobel Committee papers, 1929 RSAS.
22 Apart from Bragg, only the comparatively little-known Polish physicist Czeslaw Bialobrzeski nominated Dirac in 1933. No other leading theorist had nominated him.
Nineteen
To fast, to study, and to see no woman –
Flat treason ’gainst the kingly state of youth.
WILLIAM SHAKESPEARE, Love’s Labour’s Lost,
Act IV, Scene III
At the age of thirty-two, Dirac appeared to have everything he could wish for. He was in excellent health, was recognised as one of the best theoretical physicists in the world, had plenty of money and could not have been in a more agreeable job. Apart from worries about his home life, his only problem was that all his friends were men. Most people seemed to take it for granted that Dirac would spend the rest of his life being cosseted in the all-male bastion of St John’s College and would die a bachelor. Over the next three years, he would surprise them all.
As several theoretical physicists guessed, their subject was coming to the end of a golden age. The toolkit of quantum mechanics was now available to solve almost all the practical problems encountered by scientists studying atoms and nuclei. In that domain, the theory worked wonderfully well. But for Dirac and others at the forefront of research, the subject was far from finished: most pressing was the need to find a field theory of electrons, positrons and photons – a theory known as quantum electrodynamics – that is free of infinities.
Based in California, Oppenheimer was an international leader in the field, which he studied when he was not immersed in the Bhagavad Gita and a dozen other books. Early in 1934, Oppenheimer and one of his students had dealt a heavy blow to Dirac’s hole theory when they proved that quantum field theory accommodates the existence of anti-electrons without assuming the existence of a negative-energy sea. Oppenheimer sent Dirac a copy of his paper, but heard nothing in reply. In Europe, Pauli and his young student Vicki Weisskopf proved that particles with no spin also have anti-particles, flatly contradicting Dirac’s theory, which implied that spinless particles should not have anti-particles becaus
e they do not obey the Pauli exclusion principle. Pauli was proud of what he called his ‘anti-Dirac paper’ and pleased that he was ‘able again to stick one on my old enemy – Dirac’s theory of the spinning electron’.1 Pauli and Weisskopf rendered the concept of the negative-energy sea redundant, and it gradually fell into disuse, as physicists became inured to the idea that each positron was just as real as the electron – there was no need to treat the positron as the absence of anything. But Dirac did not accept this – there are no spinless fundamental particles, he noted unconvincingly, so Pauli and Weisskopf’s arguments were academic. For this reason, he continued to use the hole theory, which yielded precisely the same results as theories that dispensed with the sea. His authority ensured that many other physicists followed him, and the hole theory continued to be used, if only as a heuristic device.2
Whichever version of quantum electrodynamics physicists used, it was plain that the theory was in trouble. However hard Dirac and his fellow physicists tried, they could not find a way of removing the infinities in the theory, to make rigorous calculations possible. Theoretical physics was ‘in a hell of a way’, Oppenheimer groaned, though he remained optimistic that either Pauli or Dirac would find a way of rescuing the theory by the following summer. If not, they would have to agree with many others that the theory was beyond salvation.3
Visitors to Cambridge, including Heisenberg and Wigner, found that Dirac was not working on quantum field theory but doing experiments with Kapitza in his new laboratory. Dirac was trying to solve a practical problem for some Cavendish colleagues, who needed pure samples of chemical elements. Each atom of every element contains the same number of electrons and protons, but the nuclei do not all have the same number of neutrons: the different varieties of nuclei, each with a characteristic number of neutrons, are known as the element’s isotopes. There are, for example, three isotopes of hydrogen: most hydrogen nuclei contain no neutrons at all, but there exist others with one and two neutrons. Rutherford’s colleagues needed pure samples of some isotopes for their experiments, but this was difficult, as atoms of naturally occurring samples of elements are a mixture of isotopes, extremely difficult to separate because they behave almost identically in chemical reactions. Dirac thought of a neat way of separating a mixture of two isotopes in a gas, using apparatus with no moving parts. His idea was to force a high-pressure jet of gas to follow a spiral path: the heavier, more sluggish molecules should tend to aggregate on the outside of the rotating mass of gas, while the lighter ones should hog the inside track. Dirac designed his apparatus for this ‘jet stream method of isotope separation’, then rolled up his sleeves and built it, having borrowed one of the compressors in Kapitza’s store. Once again, he was trying his hand at being an engineer.
He was surprised by the results. The apparatus did not separate the isotopes efficiently but produced what he later described as ‘something like a conjuring trick’.4 When he pumped gas at six times ordinary atmospheric pressure into a small copper pipe, he found that, after the gas had undergone its spiral motion, it separated into two streams with very different temperatures – one stream was hotter than the other by about one hundred degrees Celsius. During a visit to Cambridge in May 1934, Wigner saw the apparatus and asked Dirac questions about it, but Dirac’s replies were terse and unhelpful, causing the mannerly Wigner to take umbrage. Wigner understood that Dirac did not want to speak about the apparatus until he knew what he was talking about and that Dirac was unaware of the convention of parrying ignorance with a polite remark. Dirac thought the temperature difference was caused by the differences in the resistance to flow of the two gases, though it is more likely that the rotational motion tends to separate the faster gas molecules from the slower ones. Dirac spent months collaborating with Kapitza under the approving eye of Rutherford, who thought it augured well for theoretical physics that the Lucasian Professor was soiling his hands in the laboratory.5
During his discussions with Dirac, Kapitza will have talked a good deal about his friends at Trinity College High Table and the interdisciplinary wanderings of their conversations. What Kapitza did not know was that, from March 1934, one of his acquaintances, whom he and Anna often welcomed to their home, was an MI5 informant. Codenamed ‘VSO’, the colleague was convinced that ‘it would be impossible for a Soviet citizen to go backwards and forwards to Russia unless his value to the Soviet authorities in this country were greater than his value in Russia’. The reports submitted by VSO, flecked with jealous asides about Kapitza’s scientific reputation, contained no proof that he was a spy but enough circumstantial evidence to worry the security services. Why was Kapitza so sheepish about admitting, even to a friend, that he held a Soviet passport? The rest home for scientists in the Crimea was open only to Communist Party members, so why was Kapitza allowed to stay there if, as he claimed, he was not a member?6 Most suspicious were the clandestine meetings Kapitza had near Cambridge with the new Soviet Ambassador in London, Ivan Maysky.7 So far as MI5 were concerned, Kapitza was now one of their top suspects.
Yet Dirac seems to have aroused no suspicion at all, probably because – to most people – he seemed to be a perfect embodiment of the apolitical, head-in-the-clouds don. If VSO had been as diligent as he was suspicious, he might have wondered why Dirac was able to join Kapitza in the exclusive rest home in the Crimea. But Dirac appears to have entirely escaped the attention of MI5; if they kept a file on him, there remains no public record of it.
The brutality of Hitler’s regime was now clear from press reports, though it seems that Heisenberg made light of them when he visited Cambridge in the spring of 1934 for what turned out to be a fruitless attempt to engage Dirac on the future of quantum electrodynamics. Heisenberg stayed in Born’s home and tried to persuade him to return to his homeland.8 During an afternoon walk in the garden with his host, he mentioned that the Nazi Government had agreed that Born could return to Germany to continue his research but not to teach. His family would not be allowed to go with him. Born, indignant that a close family friend could even contemplate conveying such a message, was furious and broke off the conversation. Only much later could Born bear to listen to Heisenberg describing the privations of trying to be a decent citizen amid the Nazi barbarities.
Conditions were no better in the USSR for scientists unwilling to toe the Stalinist line. George Gamow, worried that his support of orthodox quantum mechanics would result in his deportation to a Siberian concentration camp, used his invitation to the 1933 Solvay Conference as a way to escape. He persuaded the Soviet Prime Minister Vyacheslav Molotov to grant him and his wife Rho exit visas and then fled, leaving the Soviet authorities livid. The Gamows arrived in Cambridge in early 1934 and were soon a popular couple, delighting all comers with their friendly vivacity. Rho was a strikingly attractive brunette, with a Garboesque presence that could light up a roomful of the dourest dons. Stylishly dressed with smart accessories, all colour-coordinated with her lipstick, she sometimes looked as if she had walked off a photo shoot for Vogue.9 She smoked one cigarette after another, but this did not put Dirac off; he adored her. The feeling was mutual, and they soon found ways of doing things together that entailed being alone with each other: she would teach him Russian in exchange for his teaching her to drive. Dirac made steady progress with learning his fourth language, as Rho recorded in the coming months by plotting a graph showing a gradual fall in his ‘error index’, an undefined concept, Dirac could not help noting.10 After spending just a few weeks in Cambridge, the Gamows departed for Copenhagen, leaving Dirac bereft.
According to private comments Dirac made a few years later, he was not in love with Rho.11 Nonetheless, their affectionate notes bounced back and forth across the North Sea for months, in a rally of infatuation. ‘Please read my letters alone,’ she pleaded. She returned the letters he had written in Russian, each one marked with a grade and with his errors neatly corrected in red ink. Hoping that he would approve of her cutting down on her smoking, she asked how many ti
mes each day he would like her to think of him; he worried that her memories of him were even slightly harmful to her. They were like cooing teenagers, each desperate not to offend the other and constantly seeking forgiveness. When Rho apologised if she had appeared to be insolent, Dirac reassured her that he was not in the least upset and that, in any case, he ‘was not expecting Russian women to be as boring as English ones’.12 Impatient to see each other again, it would not be long before their wish was fulfilled.
In the meantime, Dirac continued to learn Russian with a woman teacher who gave him hour-long lessons on Saturday mornings in Cambridge. Her name was Lydia Jackson, a Russian émigré poet known as Elisaveta Fen before her ill-fated marriage to Meredith Jackson, a Fellow at St John’s. Romantic and strong-willed, she felt out of place in Cambridge – no place for assertive women, she thought – and made a living by teaching the language of her homeland. At a gathering of one of London’s literary circles, she introduced George Orwell – probably one of her lovers – to the woman who became his first wife.13 Jackson liked to talk about the Soviet Union with Dirac, and, by her tantalisingly vague account, it seems that she was more sceptical than he was about Stalin’s regime.14 He rarely spoke about science but did once exchange a few words with her about mathematics: she thought it was a human invention, while Dirac maintained it had ‘always existed’ and had been ‘discovered’ by humans. ‘Doesn’t that mean that it was created by God?’ she asked. He smiled and conceded, ‘Perhaps animals knew a little mathematics.’15
Her familiarity with Dirac is clear from her letters to him. In one, she commends him for being down to earth, not one of his most lauded qualities: ‘I know that you are not as absent-minded as all professors and mathematicians are supposed to be: there must be quite a large chunk of an engineer still in you.’ After referring teasingly to a spot of nude bathing she had done in a pond on Hampstead Heath, she gives him some stout advice for the sabbatical he was about to take in Princeton:
The Strangest Man Page 36