FEYNMAN: I am Feynman.
DIRAC: I am Dirac. [Silence]
FEYNMAN (admiringly): It must have been wonderful to be the discoverer of that equation.
DIRAC: That was a long time ago. [Pause]
DIRAC: What are you working on?
FEYNMAN: Mesons.
DIRAC: Are you trying to discover an equation for them?
FEYNMAN: It is very hard.
DIRAC (concluding): One must try.36
Dirac’s reticence had surprised even his former student Abdus Salam, sitting next to him: from the conversation, Salam concluded that Feynman and Dirac had not previously met. One explanation for Dirac’s behaviour, strange even by his standards, is that he did not recognise Feynman: Dirac had an unusually poor memory for faces, which is why he rarely remembered physicists he had met only once, even if their characters were as memorable as Feynman’s.
Dirac was convinced that the best way to understand strongly interacting particles was to describe their behaviour with equations, just as he had done when he discovered the electron equation. But most theoreticians were not now thinking along those lines: some were exploring new types of field theory; others gave up all hope of finding equations to describe the particles’ motion and sought only to describe in broad terms what can happen when they interact. In this approach, a ‘scattering matrix’ gives, for every possible initial state of the particles, the likelihood that it will lead to each of the possible final outcomes. Dirac rejected it as ‘a façade’.37
Apart from the strongly interacting particles, experimenters had also discovered another family in the subatomic zoo. The first hint had arrived from experiments on cosmic rays in 1946, when Carl Anderson identified a particle later to be called the muon. It was some two hundred times as heavy as the electron and unstable, but in other respects it bore a close resemblance to the electron: it had the same spin and did not feel the strong interaction. But there was one crucial difference: in 1962, experimenters showed that the muon is associated with its own variety of neutrino, different from the familiar neutrino linked with the electron. All four particles – the electron, the muon and their neutrinos – appeared to have no constituents and to be part of a family, later known as leptons, following Leon Lederman’s introduction of the term, taken from the Greek word for something small and delicate, leptos.
The arrival of new particles normally did nothing to excite Dirac – he still had not come to terms with the photon and electron. But in late 1961, Dirac broke his rule of not working on new problems until he had solved the ones already on his plate: he tried to understand the muon, which he believed might simply be an excitation of the electron. He abandoned the usual image of the electron as a point particle and pictured it as a spherical bubble in an electromagnetic field: ‘One can look upon the muon as an electron excited by radial oscillations,’ he suggested. Dirac described the bubble using a relativistic theory whose equations described its motion in space-time. It was a sublime piece of applied mathematics but most physicists ignored it, apparently because its account of the electron was so unconventional: it gave a geometric account of a particle usually assumed to have no size and paid no attention to its spin. Nor did the theory’s predictions do much to win over doubters – Dirac calculated that the mass of the first quantum excitation of his electron accounted for only a quarter of the measured mass of the muon.
Dirac first presented his theory of ‘the extended electron’ to his colleagues at the Institute for Advanced Study in Princeton on the warm autumn afternoon of 16 October 1962. Oppenheimer was sitting in the front row, his deep-blue eyes still alert and penetrating, his complexion as fragile as an eggshell.38 Still a master inquisitor, after making one of his smart comments, usually at the speaker’s expense, he would sometimes turn round and survey the audience, to check that everyone had appreciated it. When Dirac was the speaker, however, Oppenheimer was on his best behaviour.
An hour after Dirac’s audience had dispersed, at 6.30 p.m., President Kennedy met his officials in the White House to discuss urgent intelligence reports: the Soviets were building secret missile bases in Cuba, ninety miles from Florida and therefore potentially a threat to the USA.39 Six days later, Kennedy went public with the intelligence, announcing a naval blockade of Cuba and demanding that the Soviets remove the missiles. Khrushchev angrily refused to back down. Oppenheimer’s scorpions were staring straight into each other’s eyes.
The tension dropped on 28 October, when the Soviets agreed to remove the missiles in return for concessions from the Americans; it seemed to many – including Dirac, watching the crisis unfold on his television in Princeton and possibly wondering whether he was about to see his third world war – that humanity had been lucky to survive. The planet seemed to be at the mercy of its Dr Strangeloves.
Bohr lived just long enough to see the Cuban missile crisis. Three weeks later, after Sunday lunch at home with his wife Margrethe, he went upstairs for a nap and died of heart failure. In a letter of condolence to Margrethe, Dirac said that he was ‘excessively sorry’ to hear of ‘the loss of one of my closest friends’ and recalled his first stay with the Bohrs in Copenhagen in 1926: ‘I was greatly impressed by the wisdom that Niels showed, not only in physics but in all branches of human thought. He was the wisest man I knew, and I did my best to absorb some of the wisdom he imparted.’40
This was the latest of a series of blows to Dirac, who was seeing his closest colleagues die off one by one. In Princeton, von Neumann had died in 1957, followed by Veblen in 1960. And only eleven months before Bohr’s death, Dirac had written the obituary in Nature for Schrödinger, who had died in his Vienna home of heart disease. In his article, Dirac went out of his way to defend Schrödinger’s apparent welcoming of Nazism in May 1938: ‘He was forced to express his approval of the Nazi regime, and he did this in as ambiguous a way as he could.’41 Many of those who had read Schrödinger’s article joyfully pledging support for ‘the will of the Führer’ will not previously have noticed that it contained many ambiguities. But, as Heisenberg and Kapitza had seen, Dirac could not be faulted on his loyalty.
Until 1962, Dirac had shown no interest in publicly discussing his recollections of the beginnings of quantum mechanics. But that year, when he turned sixty, he changed his mind. He agreed to be interviewed by the American philosopher of science Thomas Kuhn, a former student of Van Vleck. Kuhn persuaded Dirac to help compile the archive for the history of quantum physics. Kuhn knew that Dirac was nervous of talking to strangers in unusual environments, so he held the first interview in Wigner’s home in Princeton, with Wigner present and often chipping in with tactfully phrased questions to draw him out. During the forty-minute session, Dirac spoke quietly and clearly, often sounding tentative and mildly amused that anyone would be interested in what he would have to say.
For almost forty years, Dirac had hardly spoken a word to his physicist colleagues about his upbringing, but Kuhn and Wigner heard childhood memories pour out of him, including a torrent of domestic detail.42 About ten minutes into the interview, Dirac began to talk about his brother. It is clear from Wigner’s delicately phrased questions and from his mild incredulity at Dirac’s responses that the two men had scarcely broached the subject in the thirty-five years they had known each other. During this part of the interview, Dirac speaks as gently as usual, but each of his carefully articulated words seems to bear a heavy burden of sadness and regret, especially when he responds to Wigner’s question about why Felix took his own life:
I suppose he was just very depressed. And, well … that kind of life where we were brought up without any social contacts at all must have been very depressing to him as well as to me and having a younger brother who was brighter than he was must have depressed him also quite a lot.43
Dirac left much unsaid, but Kuhn and Wigner were wise not to press him; if they had, he would almost certainly have clammed up and perhaps even refused further interviews.
Privately, Dirac was in no do
ubt why his brother killed himself. Dirac told Kurt Hofer that he was sure his father was primarily responsible for the tragedy: Charles had denied Felix a normal upbringing, forced him to speak French against his will and crushed his ambition to be a medical doctor.44 But, even after decades of reflection, Dirac could not understand the depth of his father’s grief after Felix’s suicide: his father was still a mystery to him and still, as he told his closest friends, the only person he had ever ‘loathed’.45
Three months after the interview, Kuhn wrote to thank Dirac for his participation and informed him that his taped disclosures about Felix’s death would be removed from the published version and ‘filed separately for future use’.46 The material was made public only after Dirac’s death.
In 1962, Dirac was about to enter the final stage of his career in Cambridge. His family circumstances were changing rapidly: his daughter Mary was preparing to emigrate to the USA; Monica had gone off to university ‘to discover the Beatles’. Shortly before leaving, Monica had been thrown out of the house by her mother, just as she ejected Judy in her teenage years.47 Now Judy and her family were settled in the USA and Gabriel was pursuing his academic career in Europe.
Dirac imagined that he would spend the rest of his life at home in Cambridge, tending his garden and working in his study. But Manci had other plans.
Notes - Chapter twenty-six
1 Enz (2002: 533).
2 Dirac probably heard the news through the grapevine in Cambridge before the news was published. One of the first accounts of the experiment was published in the Guardian on 17 January 1957.
3 Shanmugadhasan (1987: 56).
4 Dirac raised the issue of left–right symmetry in quantum mechanics in the Ph.D. examination of K. J. Le Couteur in 1948, see Dalitz and Peierls (1986: 159).
5 On 25 August 1970, Dirac gave a piece of paper to the physicist Ivan Waller bearing the message: ‘The statement that I do not believe there is any need for P and T invariance occurs in Rev Mod Phys vol 21 p 393 (1949). I never followed it up. PAM Dirac.’ Waller archive, RSAS. See also Pais (1986: 25–6).
6 Polkinghorne (1987: 229).
7 Seven years later, in 1964, when two experimenters at Princeton University confirmed that some quantum processes that involve the weak interaction are not symmetric when time is reversed, most physicists were once again shocked. But not Dirac: he had also foreseen that possibility in the two paragraphs of his 1949 relativity paper.
8 The ‘wrong horse’ quote is from a round-table discussion at the Fermilab Symposium in May 1980, Brown and Hoddeson (1983: 268). The ‘complete crushing’ quote is from Dirac’s talk at the Argonne Symposium on Spin, 26 July 1974, see ‘An Historical Perspective on Spin’ Lecture notes, pp. 3, Dirac Papers, 2/29/3 (FSU).
9 Taubman (2003: 302).
10 ‘The Soviet Crime in Hungary’, New Statesman, 10 November 1956, p. 574.
11 Interview with Tam Dalyell, 9 January 2005. Dalyell recalls that his meeting with Dirac took place in either 1971 or 1972.
12 Letter from Dirac to Kapitza, 29 November 1957, Dirac Papers, 2/4/12 (FSU).
13 The connection with the anniversary was pointed out in the New Statesman in 26 October and 9 November 1957.
14 Interview with Monica Dirac, 1 May 2006.
15 Dirac often told his daughter Mary that he would like to travel to the Moon. Interview with Mary Dirac, 10 April 2006.
16 Newhouse (1989: 118).
17 Newhouse (1989: 118).
18 The other two physicists at lunch with Dirac were Peter Landshoff and John Nuttall. Interview with Peter Landshoff, 6 April 2006.
19 Letter from Dirac to Walter Kapryan, 19 July 1974, Dirac Papers, 2/7/6 (FSU).
20 I thank Bob Parkinson and Doug Millard for their advice on the reasons why space rockets were launched vertically rather than horizontally.
21 Interview with the Revd. Sir John Polkinghorne, 11 July 2003.
22 Interview with the Revd. Sir John Polkinghorne, 11 July 2003. Dirac once asked ‘What is a rho meson?’, a particle then well known to almost all particle physics researchers.
23 Interview with the Revd. Sir John Polkinghorne, 11 July 2003.
24 Interview with Monica Dirac, 7 February 2003. In 1967, Dirac’s parking rights were further constrained, and, again, Manci was outraged. Letter from R. E. Macpherson to Dirac, 2 November 1967, Dirac Papers, 2/6/3 (FSU).
25 Interview with John Crook, 1 May 2003.
26 After the Christmas vacation of 1959, Gabriel urged his mother to stop telling Dirac ‘I will leave you’ in front of them. Letter from Gabriel to the Diracs, 13 January 1960, Dirac Papers, 1/8/12 (FSU).
27 Interview with Stanley Deser, 5 July 2006.
28 Letter to Dirac from Manci, 10 April 1954 (DDOCS).
29 Interview with Monica Dirac, 7 February 2003.
30 Hardy (1940: 87). See, for example, letters to Dirac from Gabriel, 22 September 1957 and 8 October 1957, property of Barbara Dirac-Svejstrup.
31 Interview with Mary Dirac, 21 February 2003.
32 Dirac told Gamow in 1961 that he began his work on general relativity in the hope of finding a connection between the theory and neutrinos, but that the project had failed. Letter from Dirac to Gamow, 10 January 1961, LC, Gamow archive.
33 The word ‘graviton’ appears to have been used for the first time in print by the Soviet physicist D. I. Blokhintsev in the journal Under the Banner of Marxism (Pod znamenem marxisma): Blokhintsev (1934). See Gorelik and Frenkel (1994: 96).
34 ‘Physicists Offer New Theories on Gravity Waves and Atomic Particles’, New York Times, 31 January 1959.
35 Deser (2003). I am grateful to Sir Roger Penrose (interview 20 June 2006) and Stanley Deser (interview 5 July 2006) for advice on Dirac’s contribution to general relativity.
36 Pais (1986: 23) and Salam (1987: 92).
37 Dirac describes the theory in this way in the notes for the talk he gave on 8 October 1970, ‘Relativity Against Quantum Mechanics’, Dirac Papers, 2/28/19 (FSU). See also Dirac (1970).
38 This description of Oppenheimer is based on the one given by Stephen Spender in Journals 1939–83. See also Bernstein (2004: 194).
39 Anon. (2001: 109–34).
40 Letter from Dirac to Margrethe Bohr, 20 November 1962, NBA. Margrethe’s reply, dated 19 December 1962, is in Dirac Papers, 2/5/9 (FSU).
41 Nature, 4 February 1961, pp. 355–6; see p. 356.
42 Interview with Dirac, AHQP, 1 April 1962, pp. 5–7.
43 Interview with Dirac, AHQP, 1 April 1962, p. 5 (text from the original tape).
44 Interview with Kurt Hofer, 21 February 2004.
45 In my interviews with Leopold Halpern and Nandor Balázs, respectively on 18 February 2003 and 24 July 2002, they both noted that Dirac said he had ‘loathed’ his father – an extremely strong word for him to use.
46 Letter from Kuhn to Dirac, 3 July 1962, Dirac Papers, 2/5/9 (FSU). Dirac subsequently gave four more interviews with Kuhn in 7 Cavendish Avenue, Cambridge, on 6, 7, 10 and 14 May 1963.
47 Interview with Monica Dirac, 30 April 2006.
Twenty-seven
[Some critics] act as if Flaubert, or Milton, or Wordsworth were some tedious old aunt in a rocking chair, who smelt of stale powder, was only interested in the past, and hadn’t said anything new for years. Of course, it’s her house, and everybody’s living in it rent free; but even so, surely it is, well, you know… time?
JULIAN BARNES, Flaubert’s Parrot, 1984
By the mid-1960s, Dirac was spending most of the week working at home. At the department he looked increasingly out of place: ‘He was irrelevant,’ his young colleague and former student John Polkinghorne remembers.1 Other Cambridge physicists thought the same but followed the scientists’ unwritten code of chivalry: when great researchers go to seed and speak out against modern trends in their subject, they should be ignored and even mocked in private, but be heartily praised in public for their past achievements.
Outside the univers
ity, too, Dirac cut the lonely figure of a misfit from another age, uncomfortable with the new popular culture and its irreverence. It was unthinkable to him that serious critics could treat a painting of a soup tin as a mainstream work of art and that many of the defining songs of a generation were written by cheeky, working-class Liverpudlians who could not read music. What, Dirac wondered, was he to make of a group whose lead vocalist claimed to be a walrus?2
Dirac was beginning to fear old age and the prospect of being effectively abandoned by his colleagues: all the signs were that Batchelor was going to bundle him out of his Lucasian Chair at the statutory retirement age of sixty-seven. The threat led Dirac to make a brief venture into the poisonous netherworld of university politics in the spring of 1964, when he joined Hoyle and a few others to seek Batchelor’s removal after his first five-year stint as head of their department. Outmanoeuvred, they failed miserably.3 With no wish to be part of Batchelor’s empire, and with his child-rearing responsibilities behind him, Dirac – encouraged by Manci – resumed his travels and spent even more time in his garden, trimming his immaculate lawn, pruning his roses and growing far more vegetables than Manci needed for her larder. His bookshelves heaved with horticultural magazines and books, making his study look as if it belonged not to a research physicist but to a landscape gardener.4 He still did research but knew that he had next to no chance of coming up with a radically new idea. He was enduring the fate of all ageing theoretical physicists: his spirit was outliving his imagination.
The Strangest Man Page 54