The result was a sensation, and not only among physicists – it even featured prominently on the front page of the New York Times. But the observation was no surprise to Dirac.2 He had foreseen the possibility that parity symmetry might be broken, in the introduction to the review of relativity he wrote in 1949. There, he considered whether quantum descriptions of nature would remain the same if the positions of the particles are reversed in a mirror (a left–right swap) and, separately, if time runs backwards instead of forwards. In his conclusion, he took the unusual step in a technical article of using a personal pronoun: ‘I do not believe that there is any need for physical laws to be invariant under these reflections [in space and in time], although all the exact physical laws of nature so far known do have this invariance.’
Dirac had realised that although the laws of gravity and electromagnetism had left–right symmetry and time-reversal symmetry, the laws of other fundamental interactions may not have this property. No leading physicist had remembered reading these words, and even Dirac himself forgot that he had written them.3 After 1949, he was aware of the possibility of quantum asymmetries in space and time but apparently said nothing about it, except once during a cross-examination of a Ph.D. student.4 A few years later, when he heard colleagues talk of the shock of parity violation, he would calmly draw attention to this passage in his paper.5 To students who asked him about it, he said simply, ‘I never said anything about it in my book.’6 He knew, however, that he could not expect many plaudits for his contribution: the winners-take-all rule of scientific conduct entitled Lee and Yang to take the credit for fully appreciating the importance of the breaking of parity symmetry.7 Theirs was one of the great discoveries of the modern era.
The death of Pauli had removed from the fraternity of senior theoreticians the one member Dirac disliked. Although they did not overtly compete with one another, undercurrents of rivalry swirled beneath their superficial rapport. Their approaches to theoretical physics were different, as Pauli was a conservative analyst, while Dirac was a revolutionary intuitionist. But that need not have divided them. Most of Pauli’s peers thought that his scabrous insults were a small price to pay for the high quality of his insights. But Dirac demurred; he often went out of his way to remind lecture audiences that Pauli ‘very often bet on the wrong horse when a new idea was introduced’, including the time he ‘completely crushed’ the idea of spin when it first hatched.8 Nor, it appears, could Dirac forgive Pauli’s pitiless strafings. When Pauli stood over him, damning hole theory, demanding that he recant, perhaps Dirac could see the ghost of his father?
Dirac’s daughters never saw him show much interest in politics except perhaps when he watched the television news, with the inscrutability of a sphinx. Manci was quite different: she closely followed international events and had strong opinions about many of them, which she spent afternoons discussing on the telephone with friends. In November 1956, she and her family – including her brother Wigner – looked on sadly when Soviet tanks and troops crushed the uprising in Hungary against its government, a puppet of Moscow, and killing some twenty thousand Hungarians. Landau condemned Khrushchev and his Politburo as ‘vile butchers’.9 In the UK, the New Statesman, usually a moderate critic of the Soviet Union, denounced the invasion as ‘loathsome’, ‘indefensible’ and ‘unforgivable’.10 Soon, the Communist Party haemorrhaged, and the hard-left core of Cambridge academics was reduced to an ineffectual rump, including Bernal, one of the few whose loyalty to the cause was undiminished. Dirac appears to have said nothing about the Hungarian invasion even to his closest friends: by the mid-1950s, he appears to have lost every vestige of his youthful idealism. He took the rare step of giving vent to this distaste when he first met Tam Dalyell, an Eton-educated Tory who switched allegiance to the Labour Party in 1956 after the disastrous British invasion of Egypt, following the nationalisation of the Suez Canal. Dirac indicated that he welcomed the maverick Dalyell’s change of political heart, but added pointedly, ‘I don’t like politicians.’11
Yet Dirac was still following reports from the Soviet Union. ‘We’re all very excited by the sputniks,’ he wrote to Kapitza at the end of November 1957.12 Dirac had first heard about the launch of the artificial satellite, apparently to mark the fortieth anniversary of the Bolshevik Revolution, on the morning of 5 October.13 That evening, he and Monica went to the back garden of 7 Cavendish Avenue shortly after dusk hoping to see the twinkling satellite pass over in the night sky.14 Newspaper reports of the orbiting ‘Red Moon’, a beach-ball sized sphere girdling the Earth in ninety-five minutes, made front-page headlines for a week, and Dirac wolfed the reports down.15 Sputnik’s success transformed the West’s view of Soviet technology from condescension to fearful admiration. For Americans, the Sputniks were frightening wake-up calls, even more disturbing after the attempt to launch their own satellite in early December ended in fiasco, when it exploded a few seconds after lift-off (one jeering journalist suggested that it should have been called ‘Stayputnik’).16 The Sputnik missions demonstrated that the Soviets were well on the way to developing intercontinental ballistic missiles and to launching a human being into space. The missions panicked the media and politicians into believing that the Soviet Union – which many Americans believed was a backward, agrarian country – was way ahead of the USA in science education. Edward Teller went on television to pronounce that ‘The United States has lost a battle more important and greater than Pearl Harbor.’17 Life magazine pointed out that three in four American high-school students studied no physics at all. As a result of all this pressure, President Eisenhower ordered a renaissance in school science and, between 1957 and 1961, Congress doubled federal expenditure on research and development, to $9 billion. An unlikely beneficiary of this largesse was high-energy physics: a new generation of subatomic particle accelerators were, in a sense, the Sputnik’s progeny.
Dirac was as interested in the technology of space flight as in any scientific benefits it might bring. He watched television footage of the launches with the same enthusiasm that he had shown when observing from the back garden of 6 Julius Road the launches of some of the first aeroplanes. But he was puzzled: why were the space rockets launched vertically rather than horizontally? So far as he could see, the challenge of propelling a rocket into space is much the same as that of launching a heavily loaded aeroplane, and vertical take-off is extremely inefficient as much of the fuel is used before the rocket is clear of the launch pad; it would therefore be best to launch the rocket horizontally, at high speed. Dirac was fascinated by this question. In May 1961, soon after the Americans put an astronaut into space – less than a month after the Soviets had beaten them to it – Dirac took aback his two fellow diners over lunch at St John’s College by sitting not in his habitual silence but, instead, talking about rocketry non-stop for almost an hour.18
In the coming decades, he followed reports of the Soviet and American space programmes and attended specialist meetings on them at the Royal Society. Even after talking with several experts, he remained unconvinced that the rockets were being launched in the most economical way, so he took the unusual step of asking NASA for an explanation.19 Its officials informed Dirac that he was wrong because he was underestimating the importance of the ‘drag’ effect of the atmosphere on a space rocket and the performance of the rocket’s engine, which improves with altitude.20 Such rockets are launched vertically so that they can climb quickly, enabling them to reach altitudes where the inhibiting aerodynamic pressures on the rocket are much lower than they are at ground level. As the air thins with height, the engine’s exhaust can impart greater thrust. These advantages together make it much more economical to launch the rockets vertically, as several experts explained to Dirac, though it seems that he never quite believed them.
Since Dirac’s arrival in Cambridge in 1923, his working environment had hardly changed. But, towards the end of the 1950s, there was a concerted drive in the Cambridge science departments to manage themselves more e
fficiently, partly so that they could compete more successfully with other international centres of science and, indeed, with other parts of the university. In Dirac’s bailiwick, the leader of the drive was George Batchelor, an Australian-born mathematician with an uncompromising manner that made clear the extent of his ambition to anyone who doubted it. Then in his late thirties, Batchelor was an expert in fluid mechanics, the branch of applied mathematics concerned with the flow of gases and liquids, a subject for which Dirac had little time – he regarded it as the small fry of theoretical physics. Nor did he like Batchelor, one of the few people who could bring out the snob in him; their colleague John Polkinghorne recalls that Dirac once offended the rhino-skinned Batchelor by dismissing George Stokes, one of the pioneers of fluid mechanics, as ‘a second-rate Lucasian professor’.21
From the beginning of the autumn term in 1959, Dirac officially worked in the Department of Applied Mathematics and Theoretical Physics, headed by Batchelor. Polkinghorne admired Batchelor as an effective, congenial leader, but Dirac and his colleague Fred Hoyle – now a top-flight cosmologist and a popular broadcaster – both declined offices in the new department and disliked virtually every change he wanted to make. One of the proposed changes was to adopt a more communal approach to research, a notion that could not have been more inimical to Dirac, who looked like a refugee from another age on the rare occasions he attended the new social gatherings. In seminars, he often appeared to be catching up on his sleep but would sometimes give the lie to that by asking a pertinent question. But he would also embarrass senior colleagues by showing how little he knew about the latest research discoveries, even about new particles familiar to greenhorn students.22
Although Dirac was not one to stand on his dignity, he was stung when Batchelor ejected him from the office he had occupied for some twenty-five years and ‘volunteered’ him to give additional lectures. Having been wounded by a series of such slights, he snapped when an officious parking attendant in the Cavendish told him that he had no right to leave his car there. John Polkinghorne recalls Dirac’s response: ‘He was furious. He told the attendant that he had parked there for twenty years.’23 He accepted Batchelor’s executive decision, but Manci was less compliant and wrote a scathing letter to the Vice Chancellor, who wrote back soothingly and then forgot about her.24 The authorities no longer felt obliged to keep Dirac happy, and he knew it.
Perhaps in part because of his unhappiness at work, Dirac’s marriage was for the first time under strain. The wife of one of the Fellows at St John’s briefly caught sight of this when Manci light-heartedly accosted her outside Woolworth’s: ‘Let’s go for a coffee – he hasn’t spoken to me for a week and I’m so bored.’25 Stories like this did not surprise the Diracs’ acquaintances in Cambridge as most of them had never fully understood how such different people could be happy together. But this happiness was partly an act. Behind their front door, her attitude towards him swung from one extreme to another: one day, she would throw her arms round him and enquire coquettishly whether he loved her; the next, she would tell him angrily: ‘I’d leave if I had somewhere to go.’26 Such threats left Dirac unmoved. According to one story, she once snapped at him when he was eating his dinner, ‘What would you do if I left you?’ only for him to reply – after a half-minute pause – ‘I’d say “Goodbye dear”.’27
Although he sometimes gave the impression that his research had dried up, Dirac was still thinking hard about his physics. When he gave Manci the signal that he was at work, she ordered the girls to be quiet: Monica would retire to her room, while Mary switched off the gramophone, endlessly blaring out the soundtrack of Oklahoma! Now in their teens, the girls had realised that their father was a distinguished scientist and that he was exceptionally quiet and self-effacing. 28 ‘I was lucky,’ he told Monica. ‘I went to good schools, I had excellent teachers. I was in the right place at the right time.’29
Gabriel, recovered from his illness, was acutely aware of his stepfather’s status: his surname drew amused comments from his mathematical colleagues and did him no harm at all. Dirac was close to Gabriel and went out of his way to promote his career, often exchanging letters with him to chew over chess problems they had read in newspapers (G. H. Hardy had described such problems as ‘the hymn tunes of pure mathematics’30). Judy and her family – by the summer of 1960, she had three children – were more distant, and she was in one long fight with her mother, who had all but lost patience with her. As many family friends confirm, Manci was a much better wife than a mother, always supportive and loyal to her husband but often insensitive to her children. It seems that Mary suffered most from her mother’s tongue: Manci repeatedly browbeat her, told her she was ‘ugly’ and also ‘lazy’, a word she used to describe everyone in the family who did not earn a wage, including Dirac’s sister Betty.31 No one, least of all Dirac, dared to remind Manci that she had yet to do a day’s paid work.
By the late 1950s, Mary was back at home and working in Cambridge, contemplating emigration; Monica was preparing to study geology at university. The girls were rapidly becoming independent, and the Diracs wanted to make the most of their new freedom by travelling even more. For someone so friendly, Manci had surprisingly few friends in Cambridge – she was close only to Sir John Cockcroft’s wife Elizabeth – and she was continually planning trips to see her family and friends abroad, the further from Cambridge the better. Dirac felt much the same way: an outsider in his own department and resentful of Batchelor’s machinations, he preferred to be where he was appreciated. The result was that, in the dozen years before his retirement in 1969, the Diracs were away from Cambridge almost as much as they were there.
Soon after the neutrino was discovered, Dirac had the idea that the particle’s existence might be explained by Einstein’s general theory of relativity.32 This was at the back of his mind in September 1958, when he began another sabbatical at the Institute for Advanced Study in Princeton, intending to develop a new version of Einstein’s theory based on his favourite way of setting out fundamental theories, using Hamiltonians to describe the interactions. His aim was to find a general classical description of every basic type of field – electromagnetic, gravitational and so on – preparing the ground for their quantisation.
Although his project failed, his method of analysing the general theory of relativity gave new insights into gravity. He described some of them in his lecture at the annual meeting of the American Physical Society, held in New York in the grip of a bitterly cold spell, at the end of January 1959. Always averse to large gatherings, Dirac was probably not looking forward to his stay as he walked the two blocks from Penn Station to the huge, overheated New Yorker hotel, to join the five thousand delegates, most of them in a starched white shirt and tie, sleeves rolled up. Without Dirac’s scientific celebrity, he would have been just another of the meeting’s invisible men, but his renown made his attendance one of the talking points in the bars and lounges. Many of the audience arrived early after lunch to secure a seat in the huge ballroom, between the imitation Ionic columns reaching to the ceiling, and below the three giant chandeliers decorating the room like cheap jewellery.
Dirac began his talk by making it clear that he was not going to comment on the particle physics in fashion but about the electromagnetic and gravitational interactions, both known for centuries but still not fully understood. Everyone in the audience knew that Maxwell’s field theory of electromagnetism predicted the existence of electromagnetic waves, including visible light, and that the energy of the field comes in quanta, known as photons. By a similar token, Einstein had shown that the general theory of relativity predicts the existence of gravitational waves. Dirac announced that his study of the gravitational field’s energy indicated that it is delivered in separate quanta, which he called ‘gravitons’, a long-neglected term first introduced a quarter of a century before in the journal Under the Banner of Marxism.33 After Dirac reintroduced the name, it stuck. These particles will be much harder to detect
than photons, he pointed out, but experimenters should lose no time in beginning the hunt for them. He gave the impression to the New York Times journalist Robert Plumb that this was an important prediction; the next day, Plumb’s report appeared on the front page: ‘[Dirac] believed that his postulation at this time was in the same category as his postulation of positive electrons a quarter of a century ago.’34
Dirac did not succeed in quantising the general theory of relativity, but his Hamiltonian method turned out to be his most influential contribution to the theory.35 His approach, and similar techniques developed independently by other physicists, enabled Einstein’s equations to be conveniently set out in a comparatively simple form, especially in situations when gravitational fields change rapidly. This excursion by Dirac into relativity theory looked odd to most physicists. In the late 1950s, the development of the general theory of relativity was a cottage industry by comparison with the industrial scale of particle physics. Relativity was an unfashionable subject for theorists, and Dirac was one of the few who thought it important to develop it and to find a single theoretical framework to understand gravity and electromagnetism. The main topic at the conference was the strong interaction and the particles that feel it, including the newly discovered mesons. One of the leaders in the field was Feynman, who met Dirac again in the autumn of 1961 at the Solvay meeting, where they had another of their Pinteresque exchanges:
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