Though marginalised in Cambridge, he was treated kindly at his favourite academic address in the USA. In the spring of 1963, Dirac heard from Oppenheimer that he had arranged for a framed photograph of him to be mounted on a wall at the Institute for Advanced Study, next to a snapshot of Einstein: ‘You two are alone on that wall.’5 This simple gesture symbolised the generosity of the American academic system, much more willing than British universities to find room for leading scholars to spend their unproductive twilight years in dignity. Mainly for this reason, Dirac spent more time in the USA. From 1962 to his retirement in 1969, Dirac visited the United States every year, for at least a couple of months, twice for almost an entire academic year (1962–3 and 1964–5).6 For much of the rest of the time, he and Manci were visiting conferences or on vacation in Europe and Israel (the USSR was no longer on their itinerary, apparently because even they could not get a visa). During these seven years, Stephen Hawking – a colleague of Dirac’s and a rising star – did not see him in the department.7
Manci had set her heart on escaping from Cambridge. Dirac disliked change and wanted to be loyal to his university but eventually agreed that it was time to emigrate, preferably to the USA. He did not have the initiative to secure a new position: that task fell to Manci, who assumed a new role as the pushy manager of a tongue-tied talent, chasing royalties and upgrades, insisting on sea-facing cabins and the room with the finest view. He was her Elvis, and she was his Colonel Parker.
Lecturing had become Dirac’s forte. Although his voice was weakening, he could be relied on to keep his audience hooked, not through wit and humour but through clarity and humility. At the podium, he looked and sounded like an elderly preacher from Bristol but had the innocence of a young lad reading an essay on Prize Day, clipping his vowels, emphasising his consonants with the force of a stab. It was often a surprise to people in the audience that such a taciturn man was so fluent, hardly ever hesitating with an ‘er’ or an ‘um’ and rarely showing a sign of even approaching a grammatical tangle. His most unnerving idiosyncrasy was a propensity to go silent in mid-sentence: when he needed to think or find the right words, he would suddenly stop talking, typically for ten seconds but sometimes for over a minute, before resuming without comment.
He presented fewer specialist talks but occasionally gave guest lectures, including a series on quantum field theory at Yeshiva University in New York in the spring of 1964. In these lectures, later recognised as classics, he developed the theory logically from its beginnings and, unusually for him, spelt out in detail the calculations that led to the prediction of the energy shift of the hydrogen atom, measured by Lamb in 1946. Although the theory and experiment agree to within experimental uncertainties, Dirac left his audience in no doubt that the theory of quantum electrodynamics is profoundly flawed: ‘If one is a research worker, one mustn’t believe in anything too strongly; one must always be prepared that various beliefs one has had for a long time may be overthrown.’8
A year earlier at Yeshiva, he gave his lecture ‘The Evolution of the Physicist’s Picture of Nature’, which he adapted into an article for the May 1963 edition of Scientific American, the only article he ever wrote for a popular-science magazine. The style and content of the talk foreshadowed dozens of similar presentations: he explained in plain, stripped-down language why fundamental physics was in crisis, drawing lessons from an often simplistic overview of the history of physics. In the article, he dwelt on one of his favourite anecdotes: Schrödinger claimed that he had discovered a mathematically beautiful relativistic version of his equation a few months before the famous non-relativistic version but did not publish the relativistic equation because it failed to account for observations on the hydrogen atom (the disagreement arose because it was not known at that time that the electron has spin). Schrödinger published his non-relativistic version only when he was sure it was in good agreement with the data, but if he had been bolder he would have been the first to publish a relativistic quantum theory. For Dirac, this story had a moral: ‘It is more important to have beauty in one’s equations than to have them fit experiment.’
Dirac suggested to his readers that ‘God is a mathematician of a very high order, and He used very advanced mathematics in constructing the universe,’ having apparently forgotten that he first encountered the God–beauty link forty years before in the writings of his colleague Sir James Jeans.9 In his positivist youth, Dirac would have regarded the link as unverifiable and therefore meaningless, but he had changed his tune: after spending decades on the terra firma of experiment-based science, he was ready to take pleasure trips on the seas of metaphysical philosophy.
The physicist in Dirac now seemed to prefer the past to the present. Uncomfortable in the company of the leading young physicists, he was most at ease when he was reminiscing with his old friends. He missed none of the triennial meetings of Nobel Laureates at Lindau, a relaxed town in southern Germany, where he talked with physicists and, with rather more reserve, to the students invited to join them. Horizon, the flagship science series of the new British television channel BBC2, made a film at the 1965 meeting, produced by Peter Loïzos. He saw that the two Nobelists most lionised by the students were Dirac and Heisenberg, who attracted swarms of admirers like Hollywood stars, and that, away from the mêlée, Dirac followed Heisenberg like a butler.
Loïzos knew it was not going to be easy to persuade Dirac to talk, as several BBC radio and television producers had asked him for interviews but had been turned down firmly.10 But Dirac agreed to be filmed in conversation with Heisenberg and the result is a unique recording of Dirac in relaxed conversation.11 Always with an agreeable smile, Heisenberg was as smartly dressed and easy-going as he had been thirty years before, but Dirac had changed rather more. His comically ill-combed hair helped to maintain his reputation for peerless dishevelment, but he was more relaxed than he had been as a young man, constantly smiling with his eyes and his mouth, speaking with a surprising assertiveness. Most striking about the encounter is that Dirac led the discussion, especially after he steered the subject towards beauty, via his anecdote about Schrödinger’s premature shelving of the relativistic version of his equation. When Heisenberg gently remarked that beauty is less important than agreement with experiment – the conventional view – Dirac took up the cudgels for aestheticism, forcing Heisenberg on to the defensive:
HEISENBERG: I do agree that the beauty of an equation is a very important point and that one can get already a lot of confidence from the beauty of an equation. On the other hand, you have to check whether it fits or whether it doesn’t. It’s only physics when it really fits with nature. But that may turn out much later.
DIRAC: And if it doesn’t fit you’d hold up publication would you? Just like Schrödinger?
HEISENBERG: I’m not sure whether I would. In at least one case I have not done so.
Smiling beatifically, Heisenberg appeared to concede the point: thirty years before, he would have persisted with the tenacity of a terrier, but his appetite for competition had been weakened by years of postwar humiliation. Delighted to have won the argument, Dirac’s face lit up with the broadest of smiles, revealing two rows of rotting teeth.
Dirac still had faith in the large numbers hypothesis, though he knew most physicists regarded it as a blot on his CV after Edward Teller had published an apparently damning refutation of it in 1948. Teller pointed out that the hypothesis implied that because the universe is expanding, gravitational forces were greater millions of years ago than they are today. Teller showed that Dirac’s idea implied that the Earth’s oceans would have boiled and evaporated away 200–300 million years ago, contrary to the geological evidence that life had existed on the planet for at least 500 million years.12 Interest in the hypothesis had flickered again in 1957, when the American cosmologist Robert Dicke demonstrated that the large numbers hypothesis is a consequence of the fact that human life occurs after stars were formed and before they die.13 If the hypothesis were wrong, as
tronomers, and all other life forms, would not exist. Dirac was unimpressed with Dicke’s reasoning and would not budge: he believed in the importance of the hypothesis ‘more than ever’.14 In November 1961, Dirac wrote his first public comment on cosmology in twenty-two years:
On Dicke’s assumption habitable planets could exist only for a limited period of time. With my assumption they could exist indefinitely in the future and life need never end. There is no decisive argument for deciding between these assumptions. I prefer the one that allows the possibility of endless life.15
Dirac’s vision of the fate of the universe was consonant with one of the articles of faith he wrote in his philosophical jottings of January 1933: ‘the human race will continue to live for ever’, a subjective assumption he had to make ‘for his own peace of mind’.16 Evidently, this most detached of theoreticians could not bear to think of a universe without human beings.
One of the few cosmologists who still believed that it was worth spending time on Dirac’s hypothesis was the vodka-swilling giant George Gamow. In 1965, he took a sabbatical in Cambridge, accompanied by his new wife Barbara, whom he had married shortly after his divorce from Rho in 1956 ‘on mental grounds’.17 The Gamows stayed at the new Churchill College, whose first Master, Sir John Cockcroft, had been chosen by the Prime Minister after whom it was named.18
One topic of discussion between Dirac and Gamow was the beauty of the ‘steady state’ theory of the universe, which says that the universe has no beginning or end, but goes on for ever like a film with an endlessly repeated plot. That summer, this was a topical question because the steady-state theory seemed to have been discredited by one of the most telling astronomical observations to have been made in decades. Two astronomers at the Bell Laboratories in New Jersey had detected an all-pervading background bath of low-energy radiation. It was only after the astronomers made their observations that they heard that just such a bath of radiation had been predicted long before by Gamow and others, using the Big Bang theory. For most cosmologists, the theory afforded a beautifully simple description of the development of the universe, compatible with the general theory of relativity and all the other great theories of science. Fred Hoyle, who had given the Big Bang theory its name in 1949 during one of his BBC radio broadcasts, was the most vocal of the diminishing number who did not give up on the steady-state theory.19 Hoyle found the idea of the Big Bang distasteful and compared the notion of the universe emerging out of nothing to a ‘party girl’ jumping out of a cake: ‘it just wasn’t dignified or elegant’.20
After one of his discussions with Dirac, Gamow wrote to ask if he had heard of a tongue-in-cheek summary of the role of aesthetics that appears to have dated from their days in Copenhagen (Gamow uses the word ‘elegant’ where Dirac would use ‘beautiful’):
Case I Trivial statement
If an elegant theory agrees with experiment, there is nothing to worry about.
Case II Heisenberg’s postulate
If an elegant theory does not agree with experiment, the experiment must be wrong.
Case III Bohr’s amendment
If an inelegant theory disagrees with experiment, the case is not lost because [by] improving the theory one can make it agree with experiment.
Case IV My opinion
If an inelegant theory agrees with experiment, the case is hopeless.21
Dirac believed that if observations agree with an ugly theory – such as quantum electrodynamics – it is little more than a coincidence. He had a fundamentalist belief in beauty, as Heisenberg found when he produced a new theory of particle physics and pressed Dirac for ‘specific criticism’, only for Dirac to give the thumbs down to the theory because its basic equation had ‘insufficient mathematical beauty’.22
Kapitza was one of the few who understood Dirac’s passion for beauty, perhaps because he had helped to foster it in their early conversations in the Cavendish and in Trinity College. Dirac may have feared that he would never again feel the thrill of Kapitza’s company in Cambridge, but he heard in the spring of 1966 that both Kapitza and his wife had secured exit visas to enable them to return for a short stay. In late April, as the Kapitzas’ arrival drew near, Dirac and Manci were like children on the eve of a royal visit, so excited that they could barely concentrate on the preparations.
By 1966, Kapitza was the Soviet Union’s most famous scientist, in the address books of most of the country’s leading artists and a licensed critic of the Government. The British Ambassador wrote in advance to Cockcroft to warn him that Kapitza was still ‘a bit of a rebel’ and suggested that ‘the public relations aspect of the visit will require rather careful watching’.23 But the Ambassador need not have worried; Kapitza was on his best behaviour, having learnt from Rutherford how to balance irreverence and propriety so that he could be seen as both close to the establishment and fiercely independent. In his interviews he was always careful to stress that he had played no part in the development of nuclear weapons and that he was as patriotic as ever, as he demonstrated in his lecture ‘The Training of the Young Scientist in the USSR’ in the Hall of Trinity College.24
When the Kapitzas visited the Diracs for lunch, Manci made a special effort in the kitchen, poaching a salmon and serving it with home-made mayonnaise and a chilled Burgundy: Mary recalled that it was the closest her parents ever came to giving a banquet.25 For just that one afternoon, the front room had the warmth of a jacuzzi – their reminiscences darted around from the summer they spent in the Kapitzas’ dacha to their days in the Cavendish, with Kapitza telling wedding-night jokes so blue that Anna left the room, leaving Dirac and Manci to giggle their way to the punchline.26
They will also have talked about Kapitza’s Club, which had ceased to exist in the spring of 1958, superseded by programmes of seminars. The Club was, however, reconvened on 10 May for its 676th meeting, so that some of its surviving members – including Dirac and Cockcroft – could meet one last time and so that Kapitza could close it.27 The venue was a smart common room in Gonville and Caius College, where the participants sipped fine dessert wines, in contrast to the meetings forty years before, when they would drink dishwater coffee. A photograph of the occasion shows Kapitza and a forlorn-looking Dirac, his left elbow leaning on the table, his left hand supporting his head. He gives the impression of being bored out of his mind.
The highlight of the meeting was a joint presentation by Dirac and Kapitza on the effect they had identified in 1933, a year before Kapitza had been detained in the Soviet Union: the possibility that electrons could be bent (diffracted) by light. When they first predicted the effect, it was impossible to observe because the available sources of light were too weak and the electron-detectors were too insensitive. But now detection looked possible, following improvements to the sensitivity of the detectors and the invention of lasers, devices that had become familiar to the public since they featured in the 1964 James Bond film Goldfinger. The barrel-chested Kapitza, standing by a blackboard and easel, pointed out that it was now odds-on that experimenters would soon observe the effect; the question was: would Dirac and Kapitza be alive to see it?28
A few days after the Kapitzas left Cambridge, Dirac switched his attention from the past to the future. He attended an entire course of lectures on modern particle physics given by the American theoretician Murray Gell-Mann, a source of many of the most productive new ideas in particle physics since the early 1950s. Then thirty-six and still at the height of his powers, he was admired for his imagination and technical brilliance but feared for his waspish tongue and disliked for his egoism, not least by Dirac.29 In the 1960s, Gell-Mann and others suggested that strongly interacting particles could be classified in mathematical patterns, and he used one of them in 1963 to predict the existence of a new particle. When experimenters detected it in the following year, it was a signal success for theoretical physics. Gell-Mann and his colleague George Zweig, working independently, also proposed that strongly interacting particles might consist of differen
t combinations of three varieties of a new type of fundamental particle that Gell-Mann called quarks (he took the word from James Joyce’s Finnegans Wake: ‘Three quarks for muster mark!’) But Gell-Mann himself was sceptical: he remarked in his lectures that quarks were probably not real particles but mathematical artefacts that help to explain the symmetries among the properties of the strongly interacting particles.30 A year later, Gell-Mann recalled that he was surprised that Dirac ‘loved’ quarks, despite their having – in Gell-Mann’s opinion – ‘many annoying properties’, including their apparently permanent confinement inside strongly interacting particles, such as protons and neutrons.31 When Gell-Mann asked Dirac why he thought quarks are so ‘marvellous’, Dirac replied that they have the same spin as the electron, the muon and the neutrino. Perhaps Dirac had seen that it was possible that all fundamental constituents of matter have the same spin – the spin of the electron. And perhaps he had sensed that it might soon be possible to set out a description of strong interactions in terms of a field theory, as he had hoped.
Gell-Mann’s lectures taught Dirac a lesson: the bottom-up way of doing theoretical physics – drawing inspiration from experimental observations – was proving much more productive than the top-down style – taking cues from beautiful mathematics – that Dirac practised and preached. Dirac privately admitted this, though he had no intention of changing his approach.32
The Strangest Man Page 55