The Strangest Man

by Graham Farmelo

  Colleagues at the institute noticed the change in Dirac’s gait. No longer lissom, he walked slowly and deliberately, as if recovering from surgery, but his vigour was returning. He spent the mornings preparing lectures for a forthcoming meeting in Ottawa, the afternoons sleeping, the early evenings on long, restorative walks round the grounds of the institute, alone except for the squirrels, rabbits and the occasional deer.42 But misfortune struck: during a visit by Judy and her baby girl, he fractured a metatarsal bone in his right foot – he was an invalid again.43 In Ottawa, for the first time in his life, he gave his lectures sitting down and looked, as he approached his fifty-third birthday, like an old man.44

  When the Diracs arrived home in Cambridge at the end of August 1955, to see their daughters for the first time in almost a year, Manci wrote a gushing thank-you note to Oppenheimer, passing on from Dirac a suggestion to help him come to terms with his tormentors. Dirac recommended Oppenheimer read the new Somerset Maugham novel, Then and Now, set in fifteenth-century Florence, about the intrigues and deceptions in the relationship between Cesare Borgia and Niccolò Machiavelli.45

  In the first seminar Dirac gave in Cambridge at the beginning of the next term, he announced to his students: ‘I have just done this work. It could be important. I want you to learn it.’ This was an extremely rare instance of Dirac publicly pointing the way ahead.46 His enthusiasm for research had been rekindled.

  Dirac’s new theory suggested that the universe might not fundamentally consist of point-like particles but of tiny, one-dimensional things that he called ‘strings’. The theory, first outlined in his Ottawa lectures, was a new approach to quantum electrodynamics that dispensed with one of the foundations of renormalisation theory that Dirac most disliked – the ‘bare electron’, the idea that the theory could be built from the fictional notion of an electron that had no surrounding field. In his new approach, he concentrated on one of the theory’s underlying symmetries, known as gauge invariance. Long familiar to theorists, this symmetry implies that the theory makes identical predictions if a quantity known as the electromagnetic potential, closely related to the electromagnetic field, is changed at every point in space-time, but only if the changes across the whole of space-time are orchestrated by a governing formula known as a gauge transformation. Dirac found a way of rebuilding quantum electrodynamics in terms of gauge-invariant quantities so that, whenever the electron features in a calculation, it is inseparable from its field. The result was a theory that gave the same results as the renormalised version but that was, for him, superior.

  Dirac disliked the concept of bare electrons so much that he wanted ‘to set up a theory in which [they] are not merely forbidden but inconceivable’.47 He found a way of doing that using the equations of his theory, by applying them to the lines of force describing the electric field of the electron, which resemble the field lines of a magnet. In the classical picture of the electron, the particle is surrounded by continuously varying lines of force: each set of lines of force is, in a sense, infinitesimally close to the next. This led Dirac to imagine a quantum version of the field and to picture the electron not as a particle but as a string:

  We may assume [that] when we pass over to the quantum theory the lines of force become all discrete and separate from one another. Each line of force is now associated with a certain amount of electric charge. This charge will appear at each end of the line of force (if it has ends) with a positive sign at one end and a negative sign at the other. A natural assumption to make is that the amount of charge is the same for every line of force and is just the [size of the charge of the electron]. We now have a model in which the basic physical entity is the line of force, a thing like a string, instead of a particle. The strings will move about and interact with one another according to quantum laws.48

  Dirac had found what he was seeking: ‘a model in which a bare electron is inconceivable, because the end of a piece of string is inconceivable without the string’. But it was only the germ of an idea, not a complete new theory. Several of his students examined it but soon set it aside, as Dirac did soon afterwards. Years later, it would transpire that he had once again been ahead of his time.

  Dirac was about to reach the low point of his career: apart from wartime, 1956 was the first year since he had begun research that he had published nothing at all.49 Now semi-detached from the physics community, he had lost touch with many of his closest friends, including Kapitza – they had not been together for almost twenty years.50 Dirac will have wanted to know how Kapitza was faring in Nikita Khrushchev’s regime, which began soon after Stalin’s death in March 1953. British newspapers had reported a new mood in the country after the Soviet public heard that Khrushchev had, in a speech to stony-faced party bosses in February 1956, denounced the personality cult of Stalin and the cruelty of his regime.51

  In the early autumn, Dirac arrived in Moscow to find it very different from the city he and Manci had seen in 1937: it was now focusing on consolidation, not revolution, and the paranoid, inwardly focused nationalism of the late 1930s had been superseded by a dread of a pre-emptive nuclear strike by the USA. Dirac found Kapitza as self-confident as he had ever been and just as full of colourful stories: in one, he told Dirac of how his arch-enemy Beria had sidelined him after he had refused to work on nuclear weapons. Kapitza believed that ‘It is a horrible thing for scientists to engage in secret war work,’ and he probably mentioned this to Dirac, who may have flinched, at least inwardly.52 While most other leading Soviet physicists had given their services to the nuclear project, Kapitza worked on ways to destroy incoming nuclear weapons using intense beams, apparently a precursor to the American Strategic Defence (‘Star Wars’) Initiative. Stalin’s good opinion had saved him from execution by one of Beria’s henchmen, Kapitza was sure. When Stalin died, Lev Landau danced for joy, but Kapitza knew his own life was in danger if Beria was the country’s next leader.53 Khrushchev outmanoeuvred Beria, but Kapitza’s life was still in peril: on what seemed to be an ordinary summer morning, towards the end of the official discussions about Stalin’s succession, Kapitza told Dirac, two state officials visited him in his small laboratory and asked for a guided tour. Their questions revealed that they knew little about science and cared even less, yet they insisted on prolonging their visit beyond its natural duration, until their departure on the stroke of noon. According to Kapitza’s account of the story, the two men had been deputed – probably by Khrushchev or his associates – to protect him from a last-minute reprisal while Beria was being arrested and taken into custody.54 A few weeks later, Beria and six of his accomplices were tried and sentenced to death; he was executed by one of Khrushchev’s three-star generals, who fired a bullet into his forehead. 55 Kapitza heard the news on Christmas Eve, a joyous moment for him.

  Dirac never tired of praising Kapitza’s refusal to work on the nuclear-bomb project. This was the story Kapitza told Dirac and everyone else, but it is almost certainly untrue. Kapitza’s letters to Stalin – published several years after Dirac’s death – make it plain that Kapitza wanted to work on the project, and he shows no hint of any moral scruples; he declined to work on the bomb only because he would not work under Beria’s heel. It is also possible that he did not command support from his colleagues, as some of them believed he was contemptuous of scientists outside his cosmopolitan circle.56 A much stronger case for Kapitza’s heroism can be made by pointing to the case of Landau, Stalin’s outspoken enemy, whom Kapitza repeatedly defended, often putting his life in grave danger.57 Hundreds of thousands of Russians were executed for showing only a fraction of Kapitza’s insubordination.

  Dirac spent most of his visit to Moscow in October 1956 sightseeing – he saw that Lenin was then sharing his tomb with Stalin – as well as reacquainting himself with his old Russian friends, including Tamm, Fock and Landau. It is surprising that Dirac was allowed to meet Tamm, as he was leading the secret project to build the hydrogen bomb (Tamm’s participation in this work may have be
en one reason why his friendship with Dirac fizzled out in the next decade).58 Landau, the permanent juvenile, was by then in the front rank of theoreticians and still flaunting his irreverence: he replaced the toilet roll in his bathroom with pages from Stalin’s autobiography.59

  Landau was in the audience of Dirac’s lectures at Moscow University, where Dirac responded to the request made to some of their guests to summarise their philosophy of physics. He wrote on the blackboard: PHYSICAL LAWS SHOULD HAVE MATHEMATICAL BEAUTY.60 In public, Landau was respectful of Dirac’s aestheticism, but in private he was cutting, once remarking to the physicist Brian Pippard, ‘Dirac is the greatest living physicist and he has done nothing of importance since 1930.’61 Overstated to the point of cruelty, this was typical Landau. He was, however, only giving voice to what many leading physicists in the mid-1950s thought but dared not say in public. Yet, as events were about to prove, Dirac’s detractors had been too hasty in writing him off.

  Notes - Chapter twenty-five

  1 The information in this section is mainly from interviews with Monica Dirac (7 and 8 February 2002) and Mary Dirac (21 February 2002 and 17 February 2006). See also M. Dirac (2003: 39–42). Information about Dirac and Betty from interview with Christine Teszler, 22 January 2004.

  2 The boarding school was Beeston Hall School in West Runton, near Cromer. E-mail from Mary Dirac, 30 October 2006.

  3 The Diracs often stayed at the Barkston Gardens Hotel, Kensington, for a week or two.

  4 Interview with Mary Dirac, 21 February 2003.

  5 Letter to Dirac from Manci, 5 September 1949 (DDOCS): ‘We can have a quiet weekend in London where the Folies Bergère is showing the full Paris show.’

  6 Professor Driuzdustades appears in Russell’s 1954 short story ‘Zahatopolk’ (see Russell 1972: 82–110).

  7 Manci and Monica often ate at the Koh-I-Noor restaurant in St John’s Street. Interview with Monica Dirac, 7 February 2003.

  8 Dalitz (1987b: 17).

  9 Interview with Monica Dirac, 7 February 2003.

  10 Interview with Tony Colleraine, 15 July 2004.

  11 Bird and Sherwin (2005: 463–5).

  12 Letter from Dirac to Manci, undated, late March 1954 (DDOCS).

  13 Szasz (1992: 95).

  14 Letter from Dirac to Oppenheimer, 11 November 1949, LC Oppenheimer archive.

  15 Szasz (1992: 86, 95).

  16 Pais often told this story. See, for example, Pais (2000: 70).

  17 It appears that Dirac was excluded from a conference as early as 1951 because of Manci’s Hungarian nationality. See interview with Lew Kowarski by Charles Weiner, 3 May 1970, AIP, pp. 203–4.

  18 The documents concerning the petition, dated 23 March 1950, are in the Bernal Papers, KV 2/1813, UKNATARCHI.

  19 McMillan (2005: 12, 199).

  20 This letter, from Dirac to Oppenheimer on 17 April, does not appear to have survived. However, Ruth Barnett, of the Institute for Advanced Study, refers to it in her letter to Dirac of 28 April 1954, Dirac Papers, 2/4/10 (FSU).

  21 McMillan (2005: 214).

  22 Letter from Dirac to Oppenheimer, 24 April 1954, IAS Dirac archive.

  23 ‘US-Barred Scientist “Not Red”’, Daily Express, 28 May 1954.

  24 ‘US Study Visa Barred to Nobel Prize Physicist’, New York Times, 27 May 1954.

  25 Letter to Dirac from Christopher Freeman, Secretary of the Society for Cultural Relations with the USSR, 26 April 1954, Dirac Papers, 2/16/9 (FSU).

  26 Pais (1998: 33).

  27 Letter from Wheeler, Walker Bleakney and Milton White to the New York Times, published in the newspaper on 3 June 1954.

  28 The name of the woman is not known for certain. Interview with Monica Dirac, 7 February 2003.

  29 Dirac Papers, 2/14/5 (FSU).

  30 After the Diracs’ stay in Mahabaleshwar, they returned to the Tata Institute in Bombay until 15 December. The Diracs then moved on to Madras and, on 20 December, travelled to Bangalore, where they spent Christmas. On New Year’s Eve, they returned to Bombay and then travelled to the Indian Science Congress in Baroda on 5 January. Four days later, they travelled to Delhi and saw the Taj Mahal shortly afterwards. The Diracs were in Calcutta from 18 January to 23 January, before returning to Delhi for a few days and then, finally, back to the Tata Institute. They left India, sailing from Bombay, on 21 February 1955.

  31 Interview with George Sudarshan, 15 February 2005. In 1955, Sudarshan was a research assistant at the Tata Institute.

  32 Dirac’s enthusiastic acceptance of the invitation to give this talk in his letter to Dr Basu, 23 June 1954, Dirac Papers, 2/4/10 (FSU).

  33 Manuscript of the talk, corrected by Dirac, is in Dirac Papers, 2/14/5 (FSU). In the published version of this presentation, many of Dirac’s finest touches are removed (Journal of Scientific and Industrial Research, Delhi, A14, pp. 153–65).

  34 Salaman and Salaman (1996: 68).

  35 Science and Culture, Volume 20, Number 8, pp. 380–1, see p. 380.

  36 Perkovich (1999: 59). India became a nuclear power in 1974, eight years after Bhabha died in a plane crash.

  37 Letter to Oppenheimer from G. M. Shrum, 4 April 1955 (Oppenheimer archive, Dirac Papers, LC). Dirac may have caught this form of jaundice, homologous serum hepatitis, from a contaminated needle during a medical examination in December 1954, Dirac Papers, 1/9/3 (FSU).

  38 Note from Manci to Oppenheimer included in Dirac to Oppenheimer, 25 September 1954 (LC, Oppenheimer archive, Dirac Papers).

  39 The Diracs sailed into Vancouver on 16 April. Letters from Manci to Oppenheimer, 15 April 1955, 22 April 1955 and other undated letters written at about the same time (LC, Oppenheimer archive).

  40 Manci often remarked on the one time she saw her husband cry. See, for example, Science News, 20 June 1981, p. 394.

  41 Interview with Tony Colleraine, 22 July 2004.

  42 Letter from Manci to Oppenheimer, 29 August 1955, Oppenheimer archive, Dirac Papers, LC.

  43 Medical report on 28 March 1955, Dirac Papers, 1/9/3 (FSU).

  44 The Diracs were in Princeton from 22 May to 30 June 1955, and they flew to Ottawa on 1 July.

  45 Letter from Manci to Oppenheimer, 29 August 1955 (LC, Oppenheimer archive).

  46 Interview with Jeffrey Goldstone, 2 May 2006.

  47 Talk on ‘Electrons and the Vacuum’ by Dirac at the Lindau conference. The manuscript, annotated by Dirac (June 1956) is in Dirac Papers, 2/27/14 (FSU).

  48 ‘Electrons and the Vacuum’, pp. 7–8.

  49 Dirac spent much of this year working on the fourth edition of The Principles of Quantum Mechanics, which was published in the following year, 1957.

  50 For an account of Kapitza’s activities between 1937–49 see Kojevnikov (2004: Chapters 5–8).

  51 Taubman (2003: Chapter 11).

  52 The quote is in a letter from Dirac to Bohr, undated, NBI. The lecture was plainly written after this visit.

  53 Dorozynski (1965: 61).

  54 Boag et al. (1990: 368). See also Knight (1993: Chapters 9 and 10).

  55 Taubman (2003: 256).

  56 Fitzpatrick (2005: 227).

  57 Dorozynski (1965: 60–1).

  58 Feinberg (1987: 185 and 197).

  59 Weisskopf (1990: 194).

  60 Dirac’s writing is still preserved on the blackboard.

  61 Landau made this remark at a conference in Moscow in 1957. Interview with Sir Brian Pippard, 29 April 2004.

  Twenty-six

  How some they have died, and some they have left me,

  And some are taken from me; all are departed;

  All, all are gone, the old familiar faces.

  CHARLES LAMB, ‘The Old Familiar Faces’, 1798

  In early December 1958, when Pauli was approaching his fifty-eighth birthday, he was looking sallow and unwell. He complained of stomach pains during a lecture at his university in Zurich in the afternoon of Friday 5 December and took a taxi home. On the following day, he went to the city’s Red Cross Hospital,
where he was admitted for tests which proved inconclusive, so doctors decided there was no alternative but to operate. A week later, a surgeon cut into the hillock of his midriff and found a pancreatic tumour so large and advanced as to be inoperable. Within forty-eight hours of the operation, he was dead.1

  The final year of Pauli’s life had not been among his happiest – a quarrel with his friend Heisenberg over an ambitious theory they were developing had turned nasty and had suppurated. But the end of Pauli’s career had also seen the seal put on one of his finest contributions to physics: during an early summer morning in 1956, he received a telegram from two experimenters in the Los Alamos laboratory to confirm that they had discovered the neutrino, the particle that Pauli had predicted, though Dirac and others had doubted that his arguments held water. Just as Pauli had foreseen, the neutrino has no electrical charge, the same spin as an electron and apparently no mass. The newly discovered particle interacts with matter primarily through the weak interaction, which is extremely feeble: of the ten thousand trillion trillion neutrinos zipping through planet Earth every second, all but a few pass straight through without deflection.

  The discovery was a triumph for Pauli but, two years later, nature put him firmly in his place when his intuition about the weak interaction was shown to be quite wrong. The story began at the Brookhaven National Laboratory in 1956, when a duo of young Chinese theoreticians – C. N. ‘Frank’ Yang and T. D. Lee (usually known as ‘TD’) – suggested what Pauli and almost all other theorists regarded as ridiculous: when particles interact weakly, nature might choose to break the perfect symmetry between left and right, the so-called parity symmetry. At a fundamental level, gravity and electromagnetism are ambidextrous: every experiment that investigates this type of interaction would give the same result if the configuration of the particles involved were swapped left to right, in their mirror image. At Columbia University in New York, experiments (suggested by Lee and Yang) to investigate whether weak interactions are left–right symmetric were carried out by two groups, one led by the aggressively confident Chien-Shiung Wu, born in Shangai, the other by Leon Lederman, a wisecracking New Yorker. The experiments each came to a climax in the bitter cold of New York in mid-January 1957, when they confirmed that Pauli had been wrong and that the suspicions of Lee and Yang were right: in weak interactions, nature does distinguish between left and right.