For once, Dirac was willing to share his ideas and briefed Tamm on his magnetic monopole theory, suggesting that he use the new theory to calculate the energy values and quantum waves that describe an electron in the vicinity of a monopole. Apart from when he was asleep, Tamm worked non-stop for three and a half days and finished just in time for Dirac to include his results – less exciting than Dirac had hoped – in the paper. In college, Tamm fraternised easily with the dons, including a few who had become friends with Dirac, having broken through his crust of reserve. Among them were the mathematician Max Newman and the Cavendish experimentalist John Cockcroft, both five years older than Dirac.10 The Yorkshire-born Cockcroft was a trained engineer and a natural manager, intensely focused to the point of near silence and with a flair for helping Kapitza and his other colleagues to solve technical problems. He was ‘a sort of scientific dogsbody of genius’, Crowther said.11
Only four days after Tamm arrived, Dirac organised a breakfast in his room to talk about Russia with Tamm and the classicist Martin Charlesworth. Dirac’s gyp will have delivered the food, probably plates of bacon, eggs and fried bread, served with a pot of tea, toast and marmalade. The three men talked for four and a half hours.12 Dirac wanted to learn about the Soviet economy, but he was uneasy when there was any sign that Tamm might present his Marxist views in public, as he showed when Tamm told him that he had been invited to speak on ‘Higher Education in the Soviet Union’ in London. Dirac remarked pointedly to him that he hoped the talk would be on education, not politics.13
From the tone of the letters he wrote to his wife in Moscow, Tamm was surprised that so many Cambridge dons were interested in the Soviet experiment. When he had lived in Britain eighteen years before, the university was known for its conservatism, but around the time he arrived there this time, the Marxist Bernal and his colleagues had established a nucleus of left-wing thought and activity among the academics.14 As Dirac will have heard, it was standard Marxist practice to praise the successes of the Soviet Union and not to dwell on its failures, but to draw attention to the millions of victims of unemployment and imperialist wars and the economic waste that could allegedly be prevented by a properly planned cooperation. 15 The comments Tamm makes in his letters give the impression that Dirac was then no more than an interested observer of the Marxist proselytisers; his passion was physics, though he was now more relaxed about taking time off to pursue other interests. After lunch, Dirac would often drive Tamm out into the countryside, sometimes pausing by a roadside tree so that Tamm could teach Dirac the elements of rock climbing and help him to overcome his fear of heights; in return, Dirac taught Tamm to drive and even helped him pass the recently introduced driving test.
In late June, near the end of Tamm’s visit, he and Dirac headed north to the more challenging terrain of Scotland, where they spent a week in the mountains of the Isle of Skye with the industrial chemist James Bell. An expert climber, he had been a friend of Tamm’s since their student days in Edinburgh and was a close follower and sceptical supporter of the Soviet experiment, steering a moderate course between Soviet propaganda and the anti-Soviet articles in the British press.16 Skye provided just the kind of scenery and company Dirac loved, and his vacation gave him an excuse to delay his return to Bristol.
That year, the summer days of Cambridge did not have their usual languor. They were rudely interrupted by a political frisson whose unlikely source was the Science Museum in London, the location of the second International Congress on the History of Science and Technology.17 For a few days in early July 1931, a red flag flew over South Kensington. Such gatherings usually attracted no attention, but this one was special: it was attended by a high-powered Soviet delegation that included Nikolai Bukharin – formerly one of Lenin’s closest associates, now a colleague of Stalin’s – and by several leaders of the Soviet scientific community, notably Boris Hessen. A few weeks before, Stalin had announced the end to almost eighteen months of political warfare between the Soviet state and its intelligentsia, so this conference offered an opportunity to present the Soviet outlook on science and technology in a favourable light. Bukharin had been the darling of the Bolshevik Party but had been pilloried in 1929 when he opposed forced collectivisation of farming and the crash industrialisation of the economy. A year later, he was sacked as the editor of Pravda, but remained loyal to Stalin and gave a full-throated presentation of the Marxist view of science to his audience in the museum. Bukharin stressed the historical context of science and the influence of social and economic conditions on scientific development, dismissing the traditional emphasis on the achievements of outstanding individuals, such as Newton and Darwin. The Soviets knew the right way forward, Bukharin concluded – by developing science as part of a unified plan for the whole of society:
The building of science in the U.S.S.R. is proceeding as the conscious construction of the scientific ‘superstructures’: the plan of scientific works is determined in the first instance by the technical and economic plan, the perspectives of technical and economic development. But this means that thereby we are arriving not only at a synthesis of science, but at a social synthesis of science and practice.18
At the end of Bukharin’s lecture, there was silence, followed by coughs and shufflings. But the talk was a success: it was reported in several British newspapers and magazines and made an indelible impression on many of the delegates. Desmond Bernal called the gathering ‘the most important meeting of ideas […] since the [Bolshevik] Revolution’.19 Dirac did not participate in the meeting but will have heard about it from Tamm, who accompanied the Soviet party to visit Marx’s grave in Highgate Cemetery, and from Kapitza, who organised a lunch in their honour at Trinity College.20
That MI5 was carefully monitoring Bukharin’s activities during his visit to Britain would not have surprised Kapitza, but he would surely have been taken aback if he had known that, since January, Special Branch had been opening, checking and sometimes copying mail sent to him from Moscow and Berlin. Armed with folders bulging with vaguely incriminating reports – all of them scientifically inaccurate, sometimes to the point of illiteracy – MI5 were concerned that he had access to sensitive military information and suspected ‘that he may be sending [it] abroad’.21 The search revealed nothing and the government warrant to intercept his mail was suspended on 3 June. But MI5 kept its tabs on him.
Dirac was shortly to travel to the United States for another hiking vacation and a sabbatical term in Princeton, but he was duty-bound to visit Bristol first. He disliked confrontations, so he must have been steeling himself in late July as he prepared to spend a week in 6 Julius Road.22 Everyone was even more unhappy than they had been when he had last seen them, as Dirac knew from his mother’s letters. Betty, unable to afford to run her car, sold it for a knockdown price. Charles, bitter that he was being forced to retire, consoled himself by spending the evenings with his friends Mr and Mrs Fisher at their bungalow in Portishead. Flo, suspicious that Mrs Fisher was one of his mistresses, was hoping he would leave to set up home with her or his girlfriend in the Esperanto group: ‘I can’t help it anyhow, he is tired of me and likes someone younger.’23
Dirac thought his family home was a disgrace – it was in a state of seedy disrepair, as his father refused to have maintenance work done and his mother disliked housework more every year.24 According to Flo, the atmosphere inside was toxic, thick with resentment. She despised Charles, and it would not be surprising if he were upset that she had exploited their marital problems by thickening the wedge between him and his son. It would have been out of character for Dirac to do anything other than to keep his head down and to depart after putting in a token appearance. He did just that, driving back to Cambridge after a few days to give a talk. But he could not escape quite so easily: on the day before the seminar, another harrowing letter from his mother arrived:
19 July 1931
My dear Paul,
I don’t know if this will surprise you but your father & I a
re going to part (as his own father & mother did.)
It is his own idea; he says he has hated me for 30 years. I know I could never please him but didn’t know it was quite so bad as that.
He will give me £1 a week or more (it will have to be more) & I am to clear out.
I don’t mind, if I have never pleased him. I sent one of his lady friends away when you were just born because she came in every night & he took her home to Bedminster & returned nearly 12 P.M. She has kept in with him ever since & he says he wishes he had married her. She is a nurse now & I suppose will come & look after him.
Otherwise, he sits in the waiting room at Zetland Road with Mrs Fisher from Portishead & she comes up here pretty often, or he is always out. Betty says she will stay with him as they are both after his money.
I am going to see a lawyer Fred [my brother] knows, to-morrow morning & will get it settled before he leaves school on Friday or he may clear out.
Do you know of a tiny cottage or bungalow near the sea up your way? It would be a complete change & I love the sea. I expect Louie or Nell would come along occasionally & I should not meet anyone I know.
If you could find me a tiny place anywhere I should be so grateful. I wouldn’t interfere with you in the least but you could come & see me in your car whenever you had time.
We are not having any row about it – it is not dignified so you need not stay away if you care to come along earlier. I’ll post this while they are at Church.
With love from Mother25
Dirac could now understand a scene that had haunted him since he was a child: his parents bawling at each other in the kitchen while he, Felix and Betty were locked outside in the garden. The phrase ‘he has hated me for 30 years’ probably struck home in Dirac’s mind, constantly in search of numbers to process: as he was only twenty-nine years old, she had, in effect, told him that he had not been conceived in a loving relationship, let alone raised in one.
Flo did not wait for her son’s advice. She went straight to her lawyer, who advised that Charles could not legally throw her out unless she was with another man, otherwise he would lose his pension. As soon as she was alone, she wrote to Dirac: ‘[Charles and I] don’t speak, but never did much, but I guess it better to stick to Betty. Two of us ought to manage him.’26
Ten days after he received his mother’s most recent letter, on 31 July 1931, Dirac sailed from Liverpool to North America, then in the tightening grip of economic depression. He took his mother with him for the first part of the journey, apparently to give her a short break from the acrimony in 6 Julius Road (she appears to have returned home immediately).27 After another long hiking vacation with Van Vleck, in the Glacier National Park, Dirac arrived in Princeton – a little over an hour’s drive from both New York and Philadelphia – then stirring after the long torpor of the summer vacation.28 The mathematician Malcolm Robertson, who arrived there at the same time, later remembered being overwhelmed when he drove through the town for the first time at dusk:
This was my first glimpse of the charming college town that was to play such a large part in my life, and a joyful and exhilarating experience it was indeed. I have never forgotten that first encounter, and my feeling of excitement and awe at the lovely stately homes among the old trees, the magnificent university campus with both new and old stone buildings, acres of well-kept lawns, and even a lake and a peaceful golf course.29
Soon after Dirac arrived there at the end of August, he was given a handsomely appointed office in Fine Hall, home of the university’s mathematics department, the newest building on the campus. It was largely the initiative of the tweed-suited Princeton mathematician Oswald Veblen, who oversaw every detail of the building’s opulent design, right down to the locations of the electrical sockets.30 Almost a third of its budget for internal decorations had been allocated to rugs woven from seamless Scottish chenille. Throughout the new building, there was other evidence of his Anglophile tastes, with a firm nod to the ambience in Göttingen: the hall’s faux Oxbridge architecture and furnishings, its freshly varnished oak-panelled walls, even the ritual of taking afternoon tea. In the common room used for special occasions, Veblen had arranged for Einstein’s aphorism Raffiniert ist der Herr Gott, aber boshaft ist Er nicht (God is cunning, but He is not malicious) to be engraved in German on the rim of the huge stone fireplace.31
On the morning of Wednesday 1 October, Dirac walked to Fine Hall from his lodgings near the town centre through the blaze of red and orange foliage, dried-out leaves crackling underfoot. A few hours later, for the first time in his career, he was to co-present a seminar, and with the least likely of his colleagues, Wolfgang Pauli. For Princeton University’s physicists, walking to the hall through the connecting corridor, and other faculty members, crossing the campus in the biting chill of the late afternoon, this was an exciting start to the new academic term, an opportunity to see two of the subject’s luminaries talking about some of their freshest ideas. The occasion was, Pauli wrote to Rudolf Peierls, ‘a first national attraction’.32
Each speaker was going to present what amounted to a prediction of a new particle: Dirac presented the monopole, Pauli another hypothetical particle, later called the neutrino. The event marked the dawn of a new culture in physics, in which theory could pre-empt experiment. The figures and demeanours of the two speakers contrasted comically. Dirac was thin as a reed, distant and serene, with the smooth and unblemished skin of a young man but, incongruously, with a pronounced stoop. The overweight Pauli was two years Dirac’s senior but his waistline made him look older. When sitting, he looked like a judge deep in reflection, his arms folded over his belly, his bulbous torso rocking rhythmically back and forth. At the seminar, he probably looked troubled and in some pain, having broken his left shoulder when he fell downstairs a few months before, the worse for drink.33
Many in the audience will have read about Dirac’s prediction, but Pauli’s had not appeared in an academic journal, though attentive readers of the New York Times read about it in an article published a few months before.34 Pauli had first proposed the existence of his new particle in a private letter to a meeting of experts on radioactivity. 35 There, he tentatively suggested that the existence of the particle could explain the problem that Bohr had identified with energy conservation when a radioactive nucleus ejects an electron. The essence of the problem was that electrons from these nuclei did not all have the same energy; rather, the electrons had a continuous range of energies. Pauli put forward a ‘desperate’ explanation for this spectrum of energies: the electron in each radioactive decay was ejected with another particle – hitherto undetected – so that the two particles shared their total energy in proportions that varied from one decay to the next. According to Pauli’s theory, the new particle should have no electrical charge, the same spin as the electron and only a tiny mass. Few of Pauli’s peers liked the idea: for Wigner it was ‘crazy’, for Bohr it was implausible and Dirac thought it was simply wrong.36 Pauli later described the neutrino as ‘that foolish child of the crisis of my life’, referring to his troubled psychological state. His problems had begun earlier in the year, following a series of tragedies – the suicide of his mother three years before, the remarriage of his father to a woman Pauli loathed, and the ending of his brief first marriage, when his wife had the impertinence to leave him for a scientific mediocrity (‘such an average chemist’).37
The next day, Pauli left Princeton to return to Europe, but Dirac stayed to give a six-lecture course on quantum mechanics, ending with a presentation of his hole theory. In the closing few minutes, he affirmed more clearly than ever in public that anti-electrons should be detectable because:
[they] are not to be considered as a mathematical fiction; it should be possible to detect them by experimental means.38
Dirac repeated his suggestion that the idea could be tested experimentally by arranging for pairs of ultra-energy photons to collide: if the theory were correct, in some of these collisions the photons would
disappear and an electron would appear with an anti-electron. But he was pessimistic. So far as he could see, it would not be feasible for experimenters to test the idea in the next few years.
He did not realise that the solution to his problem lay in the columns of the New York Times. Dirac read it regularly and must have seen the articles on the investigations of cosmic rays being carried out by Millikan, who had given them their catchy name in 1925. The rays had been discovered in 1912 but were still a mystery: all that was known for sure was that they had extremely high energy, typically thousands of times higher than particles ejected from atomic nuclei on Earth.39 Millikan developed a religion-based theory of the cosmic rays and, by 1928, regarded it as ‘fairly definite’ that they were the ‘signals broadcast throughout the heavens […] the birth cries of infant atoms’, clear evidence for divine benison.40
Dirac must have known that high-energy cosmic rays could produce anti-electrons if the rays collided with other particles on Earth. Yet it seems that he was never much interested in these particles, perhaps because he was influenced by modish opinion in the Cavendish Laboratory in the mid-1920s, when no one there studied the rays. Rutherford’s deputy James Chadwick had sighed when he came across another of Millikan’s research articles on cosmic rays: ‘Another cackle. Will there ever be an egg?’41 But that was six years before, and by the autumn of 1931 the attitude to the rays at the Cavendish was changing. The first of its scientists to latch on to their importance was Blackett, who was at a crossroads in his career, casting around for a new research topic.42 This subject must have had a special appeal to the independent-minded Blackett as it would distance him from Rutherford, whose ego was becoming overweening.
The Strangest Man Page 28