The Strangest Man

by Graham Farmelo

  There is a difference of opinion among theoretical physicists about the probability of reaching practical results at an early date. This, however, is a well-known stage in the pre-history of every great invention. The tremendous importance of the utilisation of atomic energy, even if only partially successful, suggests that the matter should not be left in the hands of the European gangsters, especially at the present juncture of world history.18

  Aydelotte responded by helping Szilárd with his search for funding. The prime responsibility of Aydelotte and Veblen, however, was the Institute for Advanced Study, and they dreamt of setting up a wartime haven for the most eminent quantum physicists, including Bohr, Pauli, Schrödinger, Dirac and even Heisenberg.19 But when the war intensified, it became unthinkable for most of them to concentrate on anything other than the war. The pursuit of the fundamental laws of physics was set aside.

  In April 1940, the Nazis overwhelmed Norway and Denmark and launched a blitzkrieg on Belgium, Luxembourg and the Netherlands a few weeks later: the Phoney War was over. Dirac’s sister Betty and her family were now living in an occupied country. Joe, like all the other Jews, lost much of his freedom: he was subjected to a curfew, forbidden to ride in trams or cars and forced to wear a yellow star when outside his house. A month before, the German forces had conquered Denmark unopposed and had invaded Norway, swatting aside the British Government’s naval campaign to repel them. Chamberlain was forced out of office and replaced by Churchill – the man regarded by many as a belligerent class warrior soon became the saviour of his country and the embodiment of bulldog spirit, a national hero.20 The Diracs gathered round their radio to listen to his broadcasts and to reports of his speeches. Three days after he entered 10 Downing Street he told the House of Commons in his first speech as Prime Minister that the aim was ‘Victory – victory at all costs, victory in spite of all terror; victory, however long and hard the road may be; for without victory, there is no survival.’ Manci was star-struck: she sent Churchill a note consisting of just two words – ‘God’s blessings’ – after a broadcast he had made a few days after the Luftwaffe dropped its first bombs on Cambridge on 18 June 1940.21

  At 11.30 p.m. on that night, the air-raid sirens began to wail, and the Diracs scurried down to the shelter of their cellar. Moments before midnight, they heard a Heinkel bomber dive low overhead and, after a piercing whistle, a huge explosion when the plane dropped two high-explosive bombs about a mile away. Ten people were killed, a dozen were injured, and a row of Victorian houses was laid waste.22 The following night, the bombers struck Bristol for the first time, targeting the British Aeroplane Company’s factory in Filton. Dirac’s mother was desperate to speak to her son but, with no telephone, the best she could do was to write to him:

  The awful raiders pay a midnight call every night. The first was a downright shock on Monday. I flew down with all my dressing gowns, collected all the green cushions from the big chairs & made myself warm & comfortable propped against the kitchen door […] To my surprise I got intensely angry at their cheek & impudence in disturbing my night’s rest & daring to visit our Island in such a manner.23

  Choosing not to take drams of whisky and play poker with her neighbours in their cellars, Flo spent most nights alone, crouched in the cupboard under the stairs with cotton wool in her ears, trying to sleep during the hours of ‘fireworks’.24 At five in the morning, when the sirens and steamers in the docks roared their ‘all clear’, she went up to Betty’s room to catch up on her sleep. Flo was lonely, sick with rheumatism and gout, anxious about her family and disappointed that her son was such a poor correspondent: ‘I am sure you can spare five minutes for a few lines if you try very hard.’25

  By August 1940, the ‘Battle of Britain’ was underway. The Luftwaffe was pummelling London and fighting over the skies of England with the Royal Air Force, helped by the early warnings made possible by the new radar technology. Despite the widespread fear of an imminent Nazi invasion, daily life in Britain continued normally. Food and everyday supplies were in the shops, the trains and buses were running, and there were queues outside cinemas showing Gone with the Wind.26 It was a summer of almost uninterrupted glorious weather, and the more prosperous Britons, including Dirac, saw no need to forgo their annual vacation. Dirac and Gabriel took a four-week break in the Lake District, renting a cottage in Ullswater with Max Born and his family – his wife, their nineteen-year-old son Gustav, their daughter Gritli and her new husband Maurice Pryce, a theoretical physicist at the University of Liverpool.27 The outdoor life, primitive facilities and the prospect of communal cooking were not for Manci, who remained in Cambridge with Judy, baby Mary and her nurse, after Dirac had assured her that the danger of air raids in Cambridge had been exaggerated (‘you should not let the air raid warnings worry you, dear’).28

  While Gabriel stayed in the cottage, his head buried in a book, Dirac and Pryce headed off early to the mountains with a vacuum flask of hot tea and a packed lunch. With Pryce and Gustav Born, Dirac climbed the highest peak in England, Scafell Pike, rowed on the lakes, climbed up several rock faces and followed some of the paths trodden by Wordsworth, who had lived in nearby Grasmere.29 At night, the party dined on the balcony, overlooking a lake as still as a pond: it scarcely seemed possible that they were in a country fighting for its life until they switched on their radio and heard the news from London.30

  Barely four days after Dirac’s vacation began, Manci was in the cellar with Mary and Judy, following the first of several air raids. ‘I am very sorry to be away during these air raids,’ Dirac wrote to his wife, though he was not worried enough to return home.31 Feeling abandoned and dejected, Manci dropped her usual affectionate tone when she wrote to him:

  I know very well that you never do or did what people happened to ask you for. So I am not asking you anything; it is but a question. Would you return to Cambridge if I was not here? Because if you would not, then do not come home please.32

  As usual, her wrath soon abated. Dirac was habituated to her outbursts and fended them off by remaining silent. It was a singular marriage, not one most people could endure, but it was working.

  Dirac’s climbing partner Maurice Pryce – formerly a colleague of Dirac and Born in Cambridge – was studying isotope separation with the Liverpool team and had recently asked Dirac’s advice about his centrifugal jet method.33 But it seems that Dirac did not think seriously about developing the method until several months later. This delay is surprising, as many of his peers were talking urgently of the need to develop a nuclear weapon ahead of the Nazis. Perhaps part of the explanation for his tardiness is that he was preoccupied with his stepchildren, constantly quarrelling and consuming more of his energy than he would have liked.34 Gabriel, then an introverted fifteen-year-old, was developing into a talented mathematician. Encouraged by Manci, he revered his stepfather as a hero, looked to him for advice and even copied his handwriting, down the last detail of the curl on the capital D. Judy, two years his junior, was growing into an attractive young woman and quite different from her brother: she was lazy, headstrong and not at all frightened of provoking her mother. Manci’s high-handedness sometimes alarmed Dirac, who privately warned Gabriel that he should not take too much notice of her tantrums.35

  Dirac agonised about his sister and her family, behind enemy lines. She had written to him from Amsterdam via the Red Cross mail service on 3 July to report that she was safe, and the letter took three months to arrive. Shortly after he read it, Dirac heard that Dutch citizens would be fined £15,000 if they were caught listening to British radio transmissions. He was also concerned about his mother, who occasionally visited Cambridge but spent most of her time alone in 6 Julius Road, going out only occasionally to the shops, the cinema and to volunteer for the emergency canteen service. Bristol was the fourth most heavily bombed city in the UK (after London, Liverpool and Birmingham): almost every night, the planes attacked the city and, though Julius Road was two miles from the worst of the attacks, Flo wa
s in fear of her life. She went to bed early and tried to sleep through the seven-hour barrages, until the sirens blasted the ‘all clear’ signal into the dawn.36

  These were among the darkest days of the war. Peierls in Birmingham was one of many who believed that the fight against Hitler was then ‘hopeless’, as he recalled fourteen years later.37 Although Germany had failed to win the Battle of Britain, the war was going its way, as Hitler well knew: he told his ally Mussolini in October 1940 that the war had been won.

  In mid-December, Dirac’s mother was admitted to a nursing home, suffering from concussion, after a stone had fallen on her when she was out walking. Dirac rushed to Bristol and, between visits to Julius Road, walked around the bombed-out heart of the city. At the Merchant Venturers’ College, he saw that many of the buildings he had known since he was a child had been pulverised into smouldering piles of rubble. Several of the homes on his route had been bombed out, their once-private spaces now embarrassingly on show for all to see. ‘The middle of Bristol is terribly damaged […] most of the best shopping areas are in ruins […] and many beautiful churches have gone,’ he wrote to Manci.38 She was too angered by being left alone to feel much sympathy:

  You know that envy is not in me but I am a little revolted that you had to go, and have to stay. After all 60 years ought to have been enough for anybody to make friends […] she is only interested in people as far as what she will be able to talk about them.39

  Unmoved, Dirac helped his mother to return home and stayed with her until she could resume her routine, returning to Cambridge shortly before the year’s end. All over the UK, the New Year celebrations were subdued, for the country was pinned to the wall.

  Most scientists in Britain had put themselves at the service of their country but, as usual, Dirac did not swim with the shoal. In peacetime, he was part of the mainstream of physics but always one step from it, so that his individuality was not constrained. He now had the same relationship with the scientists working for the military: he supported them but only to an extent that neither his daily routine nor his intellectual independence was compromised. One of the first invitations to participate in war work that Dirac received had come, surprisingly, from the mathematician G. H. Hardy, who was contemptuous of the applied mathematics involved in war work as unworthy of ‘a first-rate man with proper personal ambitions’.40 He wrote to Dirac in May 1940, asking him to join a team of twelve mathematicians to code and decode messages at the Civil Defence offices in St Regis, in the event of a Nazi invasion.41 Dirac appears to have declined, probably because he would not consider moving from Cambridge and because teams, to him, were anathema.

  The journalist Jim Crowther did not, however, stop trying to involve his retiring friend in public affairs: in mid-November 1940, he tried to persuade Dirac to attend a meeting of the Tots and Quots dining club, an informal gathering of academics who were interested in exploring how their expertise might be useful to society (the name of the club is a reference to the Latin quot homines, tot sententiae: ‘so many men, so many opinions’). Its twenty-three members in 1940 – including Bernal, Cockroft and Crowther – were often joined by guests, such as Frederick Lindemann, H. G. Wells, the philosopher A. J. Ayer and the art historian Sir Kenneth Clark.42 The location of the club’s political centre of gravity, well to the left, was reflected in the outcome of their debates, most of them held over a few bottles of wine and an indifferent meal in London’s Soho. The meeting Crowther wanted Dirac to attend, on Saturday, 23 November 1940, was scheduled to discuss Anglo-American scientific cooperation and was to take place in Christ’s College, Cambridge. Crowther knew the best way to encourage Dirac to attend: ‘It would be quite unnecessary for you to join in the discussion if you did not wish to.’43 Crowther succeeded, and Dirac listened to a wide-ranging discussion about ways of promoting scientific cooperation with American scientists, until shortly after midnight. Bernal opposed the suggestion that British research projects should be transferred to the United States, arguing that the best way forward was to promote personal contacts between British and American scientists. It was important, he stressed, not to give up too easily on preserving the independence of British science.44

  The record of this special Tots and Quots meeting makes no mention of any contribution from Dirac. So far as records show, he attended no other social gathering of scientists during the war.

  At about the time of the meeting, Dirac began to think again about his method of separating mixtures of isotopes.45 Seven years earlier, he had demonstrated that the technique might work; he now turned to a theoretical analysis of the process, to help engineers investigate ways of separating a mixture of 235U and 238U. His original idea was to deflect a gaseous jet of the mixture through a large angle, so that the heavier and therefore slower-moving isotopes would be deflected less than the lighter ones, and the two components would separate. He tried to find a general theory of all processes that might separate isotopic mixtures in this way, aiming to deduce the conditions that would most effectively separate them. To solve the problem, he had to use all his talents: the mathematician’s analytical skills, the theoretician’s penchant for generalisation and the engineer’s insistence on producing useful results.

  He gave his first account of the theory in a confidential, three-page memorandum. Dirac wrote it for Peierls and his colleagues, probably in early 1941, between the incessant bombing raids, and typed it at home. He wrote the paper in his usual spare style but taking care to highlight the most important conclusions so that they would be clear even to engineers allergic to complicated mathematics. The memo does not focus on his own jet separation method but concerns every conceivable way of separating isotopes in a liquid or gaseous mixture by causing a variation in the isotopes’ concentration. The separation might be achieved, for example, by subjecting the mixture to a centrifugal force or by carefully arranging for the temperature to change across the container. To make the calculations tractable, he made the reasonable assumptions that the fluid mixture contains only two isotopes (each made of simple atoms) and that the concentration of the lighter one is small compared with the concentration of the other. In a short calculation, he derived a formula for what he called the ‘separation power’ of the apparatus, a measure of the minimum effort needed to cream off a given amount of the lighter isotope. He found that every part of such an apparatus, irrespective of how it is built, has its own maximum separation power, and he showed how to calculate it.

  Dirac often drove to Oxford to talk with the experimenters who were developing ways of separating isotopes, under the impish Francis Simon, another German refugee physicist. Dirac surprised many of the experimenters by participating vigorously in their meetings and by making practical suggestions about the design of their apparatus. During these discussions, he conceived several other ways of separating isotopes, each of them based on his original centrifugal jet stream method.

  The Oxford group built one of Dirac’s designs, and it worked, but his method was less efficient than the competing technique of gaseous diffusion, which exploits the fact that the atoms of two isotopes in equilibrium and with the same energy have different average speeds: the lighter, swifter atoms are more likely to diffuse through a membrane than heavier ones, enabling the mixture to be separated. Consequently, at this stage in the development of nuclear energy, resources were diverted to gaseous diffusion, and Dirac’s idea was set aside.

  Late at night on 9 May 1941, a bomb fell opposite the Diracs’ home, damaging two houses and causing small fires that Judy helped the fire fighters to extinguish.46 This was the most frightening moment for the Diracs in the worst year of bombing in Cambridge, and it was relentless where they lived, close to the strategic target of the railway station. But the Diracs’ everyday life was much the same as it was before the war. Part of this routine involved welcoming visitors; Dirac was determined not to follow his father’s example of virtually barring the family home from others, apart from paying students. One of the most f
requent visitors to 7 Cavendish Avenue was Jim Crowther, ‘the newspaper man’.47 A one-man clearing house of information about the activities of leftist scientists, he was a favourite of Manci’s, who entertained him and his wife Franciska as royally as rationing allowed: she could stretch to a cup or two of tea, but biscuits and cakes were luxuries. After one get-together, Crowther lent her Somerset Maugham’s On Human Bondage to help her improve her English and her understanding of British foibles. Still worried that people in Cambridge thought of her as an outsider, she even sensed disquiet that she might be an enemy agent. Suspicions of aliens intensified in the town in the spring of 1941, when an innocent-looking Dutch seller of second-hand books in Sidney Street was unmasked as a spy. When he heard that military intelligence was on to him, he broke into an air-raid shelter on Jesus Green and shot himself.48

  During the Diracs’ conversations with the Crowthers, Dirac heard Crowther’s bulletins on the scientists’ war work, delivered with his subtle political colouring, though almost certainly without the political edge that he reserved for conversations with more committed colleagues. Crowther knew that this was time well spent: Dirac would never commit himself to the cause of the left, but he was a powerful ally, if only because no other British physicist came close to his intellectual prestige.

  Although Dirac spent most of his time on war work, he was still thinking about quantum mechanics. In one project, he collaborated with Peierls and Pryce to refute accusations made by Eddington that experts in relativistic quantum mechanics, including Dirac, were persistently misusing the special theory of relativity. This disagreement had been rumbling for years: in the summer of 1939, Sir Joseph Larmor had heard that ‘Eddington has lately come to blows with Dirac.’49 Dirac, Pryce and Peierls tried to make Eddington see reason but, by the early summer of 1941, their patience had run out, and they prepared what Pryce dubbed ‘the anti-Eddington manuscript’.50 The paper appeared a year later, and Eddington’s arguments were crushed to the satisfaction of everyone except Eddington himself, who never accepted defeat.