Having been awarded the Royal Society’s Baker Medal, Dirac had to prepare a special lecture, where he presented some of his new thinking on quantum physics. In the early afternoon of 19 June 1941, when Dirac arrived at Burlington House, he saw that central London had suffered surprisingly little in the Blitz; most of the damage had been done in the City and the East End. Giving the lecture was in keeping with the spirit of the hour – Londoners were going about their business as usual, and that included attending lectures about matters of no practical importance.
Dirac rose to the podium at 4.30 p.m. to describe why he was so unhappy with the current state of quantum mechanics: why is it, he wondered, that the first version – set out by Heisenberg and Schrödinger – is so beautiful whereas the relativistic version is so diseased? 51 It might be possible, he showed, to remove one of the pathologies of the relativistic theory – negative-energy photons – using a technical device later dubbed the ‘indefinite metric’. Although not a panacea, the technique demonstrated to the standing army of quantum physicists that Dirac was still one of their generals. Even Pauli was impressed and wrote to Dirac to say so.52
Dirac’s conclusion to the lecture was that the ‘present mathematical methods are not final’ and that ‘very drastic’ improvements were needed. He knew, however, that they were unlikely to be made at a time when most of the best scientific brains were working on top-priority projects for the military. Only rarely did the scientists on opposing sides communicate. One such encounter took place in late September 1941, when Heisenberg travelled to Nazi-occupied Denmark to see Bohr (who knew nothing of the Anglo-American project to build a nuclear bomb) in a fraught meeting that was remembered and interpreted quite differently by the two men.53 The playwright Michael Frayn dramatised their discussions six decades later in Copenhagen, a metaphor for the uncertainty principle: the more the intentions of the participants at the meeting are probed, the murkier they appear to be. Although it will never be possible to know precisely what the two men said, one consequence of their meeting is now clear: their friendship was damaged beyond repair.
Dirac, in touch with neither Bohr nor Heisenberg, knew nothing of the meeting. When it took place, he was in Cambridge, preparing for the new term, no doubt anxiously reading the news of the Nazis’ invasion of the USSR, which had begun when Hitler unilaterally broke the pact with Stalin three months before. Kapitza was now in Hitler’s sights. On 3 July, a few days after the pact collapsed and Stalin joined the Allies, Kapitza sent Dirac a telegram, one of the few communications that Dirac received from him during the war:
In this hour of stress when our two countries fight against a common enemy I want [sic] send you a friendly word. The united strength of all men of science will help the victory over the treacherous enemy who by brutal force destroyed the liberty and crushed the freedom of scientific thought in Germany and is trying to do the same in all the world. My greetings to all friends united in their will for fighting to complete victory for the freedom of all people for the freedom of scientific thought so dear to our two countries.54
Later during the conflict, Dirac was moved to similarly grand words in a rare letter to Kapitza. After offering his ‘hearty congratulations’ to Kapitza on his second Stalin Prize, Dirac wrote that he hoped ‘that the great Hitler menace which now darkens this world will soon be obliterated’.55
Flo was also thinking about Kapitza and his compatriots: ‘Those plucky Russians are putting up such a grand fight!’, she wrote to her son. By the summer of 1941, Bristol appeared to have seen the worst of the bombing; about 1,200 people had been killed.56 She was ailing and desperate to stay at 7 Cavendish Avenue, where Manci was struggling to cope after her maid and cook had departed. In early October, Flo arrived with her luggage and hatbox, having declared that she wanted to help with the housework, though her doctor wrote privately to Dirac: ‘I want you to see that she does not do extra work’ as ‘her heart is overstrained and she is rather run down’.57 She stayed longer than the month she had planned, working under Manci’s direction as a kitchen maid and house cleaner, helping the servants and Mary’s nurse. Soon after the Americans entered the war, following the bombing of Pearl Harbor on 7 December 1941, Flo wrote to one of her neighbours: ‘Paul says it will take two years to conquer the Japs.’ But she was homesick and tired of being Manci’s charlady: ‘I really am afraid I will be quite ill if I stay on. Manci imposes on me too much.’58
Flo never sent the note as, four days before Christmas, she had a fatal stroke. Dirac seems to have taken her death with his usual almost-inhuman stoicism: his sliver-thin vocabulary of emotions did not include conventional expressions of grief. Manci saw no tears. Yet he knew better than anyone the tragedy of her unfulfilled life: the suicide of her first-born; her servitude during a sham marriage and its horrible final years, when she was like a rabbit domiciled with a bear. Dirac knew that his mother had her flaws: she was absent-minded and disorganised, selfishly determined to keep her younger son to herself. But Dirac knew that life had not been generous to his mother and that he had been her greatest love.
Her funeral took place two days after Christmas.59 Dirac threw away most of her belongings but not the Christmas card on which she had written her feelings about Manci. He kept that among his papers.
Notes - Chapter twenty-two
1 Bowyer (1986: 51).
2 This was one of Manci’s favourite expressions about how the British treated her. Interview with Mar yDirac, 21 February 2003.
3 Boys Smith (1983: 44).
4 Cambridge Daily News, 2 September 1939, p. 5.
5 Cambridge Daily News, 1 September 1939, p. 3. I am grateful to my mother, Joyce Farmelo, for her recollections of her time as an unhappy evacuee and her other wartime experiences.
6 E-mail from Mary Dirac, 5 March 2006.
7 ‘Cambridge During the War; the Town’, Cambridge Review, 27 October 1945; ‘Cambridge During the War; St John’s College’, Cambridge Review, 27 April 1946. See also ‘Thoughts Upon War Thought’, Cambridge Review, 11 October 1940.
8 Barham (1977: 32–3).
9 Letter to Dirac from his mother, 26 January 1940, Dirac Papers, 1/4/10 (FSU).
10 Manci spent the final months of her pregnancy in the Mountfield Nursing Home in London. Information about Mary’s birth from her baby book. Further clarification in an e-mail from Mary Dirac, 16 January 2006.
11 Letter to Dirac from Manci, 20 February 1940 (DDOCS). Manci’s exact words are ungrammatical: ‘I never felt as much that she has nor heart nor feelings whatsoever as yesterday.’
12 Peierls (1985: 150, 155).
13 Rhodes (1986: 323).
14 Facsimiles of the memos are in Hennessy (2007: 24–30).
15 Peierls (1985: 155).
16 The earliest extant letter about this, from Peierls to Dirac, is dated 26 October 1940, AB1/631/257889, UKNATARCHI.
17 Rhodes (1986: 303–7); Fölsing (1997: 710–14).
18 Letter to Aydelotte from Veblen and von Neumann, 23 March 1940, IAS Archives Faculty Series, Box 33, folder: ‘Veblen–Aydelotte Correspondence 1932–47’. The words omitted, marked by the ellipsis, are ‘There are considerable deposits of uranium available near Joachimsthal, Bohemia, as well as in Canada.’
19 Letter to Adyelotte from Veblen, 15 March 1940: IAS Archives General Series, Box 67, folder: ‘Theoretical Physics 1940 Proposals’.
20 Cannadine (1994: 161–2).
21 Letter from Manci to Crowther, 28 June 1941, SUSSEX.
22 Barham (1977: 54); Bowyer (1986: 51).
23 Letter to Dirac from his mother, 27 June 1940, Dirac Papers, 1/4/10 (FSU).
24 Letters to Dirac from his mother, 16 August and 31 August 1940, Dirac Papers, 1/4/10 (FSU).
25 Letter to Dirac from his mother, 12 May 1940, Dirac Papers, 1/4/10 (FSU).
26 Letter to Dirac from his mother, 21 June 1940, Dirac Papers, 1/4/10 (FSU).
27 Letter from Dirac to Manci, 27 August 1940 (DDOCS).
28 Letter
from Dirac to Manci, 23 August 1940. Four days later, he wrote to her: ‘I am sorry to be away from you these days, but do not think there is any real danger in Cambridge’ (DDOCS).
29 Gustav Born later recalled that Dirac on this vacation was ‘a twinkling-eyed, kindly, distant man’, happiest when on his own. Interview with Gustav Born, 12 February 2005.
30 ‘The ladies do the cooking, and the men take it in turns to do the washing up,’ Dirac told Manci: letter, 23 August 1940 (DDOCS).
31 Letter from Dirac to Manci, 2 September 1940 (DDOCS).
32 Letter to Dirac from Manci, 8 September 1940 (DDOCS).
33 Letter from Pryce to Dirac, 18 July 1940, Dirac Papers, 2/3/10 (FSU).
34 Letter from Dirac to Manci, 21 January 1940 (DDOCS).
35 Letter from Gabriel to Dirac, 30 August 1945, and another undated later in the same month, Dirac Papers, 1/8/12 (FSU).
36 Letter to Dirac from his mother, 31 August 1940, Dirac Papers, 1/4/10 (FSU).
37 Letter from Peierls to Oppenheimer, 16 April 1954, LC, Oppenheimer archive.
38 The first part of this quotation is from the letter Dirac wrote to Manci on 18 December 1940; the second and third parts are from the letter he wrote to her the next day.
39 Letter to Dirac from Manci, 22 December 1940 (DDOCS).
40 Werskey (1978: 23); see also the foreword by C. P. Snow to Hardy (1940: 50–3).
41 Letter to Dirac from Hardy, May 1940, Dirac Papers, 2/3/10 (FSU).
42 Attendance register of Tots and Quots in 1940, Zucherman archive, wartime papers, SZ/TQ, EANGLIA.
43 Letter from Crowther to Dirac, 15 November 1940, Dirac Papers, 2/3/10 (FSU).
44 Brown (2005: Chapter 9).
45 The first letter to Dirac, from Peierls, in connection with war work is dated 26 October 1940, UKNATARCHI.
46 Bowyer (1986: 181). Manci often spoke of Judy’s role in the firefighting (e-mail from Mary Dirac, 23 April 2006). Manci refers to an earlier near-miss on 15 February 1941 in her letter to Crowther on 17 February 1941, SUSSEX.
47 Dirac often referred to Crowther as ‘the newspaper man’. See, for example, letter from Dirac to Manci, 4 May 1939 (DDOCS).
48 The spy was Jan Willen der Braak. ‘The Spy Who Died Out in the Cold’, Cambridge Evening News, 30 January 1975.
49 Letter from Harold Brindley, 7 August 1939, STJOHN; Dirac refers calmly to discussions with Eddington in a letter to Peierls, 16 July 1939, Peierls archive (BOD).
50 Letter from Pryce to Dirac, 11 June 1941, Dirac Papers, 2/3/11 (FSU).
51 The time of the lecture is recorded in the Royal Society’s Meeting Notices. Afternoon tea began at 3.45 p.m.
52 Letter to Dirac from Pauli (then at the Institute for Advanced Study), 6 May 1942, Dirac Papers, 2/3/12 (FSU).
53 Bohr did not find out about the project until he escaped occupied Denmark in autumn 1943: see Bohr (1950).
54 Telegram to Dirac from Kapitza, 3 July 1941, Dirac Papers, 2/3/11 (FSU).
55 Letter from Dirac to Kapitza, 27 April 1943, Dirac Papers, 2/14/12A (FSU).
56 Penny (2006: ‘Fatalities in the Greater Bristol Area’).
57 Letter to Dirac from Dr Strover, 2 October 1941, Dirac Papers, 2/3/11 (FSU).
58 Letter from Flo Dirac to her neighbour Mrs Adam, written shortly before Christmas 1941, Dirac Papers, 1/2/1 (FSU).
59 Flo was buried in the Borough Cemetery (now the City Cemetery) in grave space 7283.
Twenty-three
There is no room now for the dilettante, the weakling, for the shirker, or the sluggard. The mine, the factory, the dockyard, the salt sea waves, the fields to till, the home, the hospital, the chair of the scientist, the pulpit of the preacher – from the highest to the humblest tasks, all are of equal honour; all have their part to play.
WINSTON CHURCHILL, speech to the Canadian Parliament,
30 December 1941, later broadcast on the BBC
To Dirac’s neighbours, it appeared that the war had little impact on his life: he remained another professor going quietly about his business, his civic duties involving nothing more than an occasional night on fire watch at the Cavendish.1 But none of his neighbours knew that he spent most of 1942 and 1943 working on nuclear weapons. Even Manci had only a vague idea of what he was doing: she told the people she knew in Cambridge that he was working on ‘decoding’.2
Most leading scientists did more to support the military than Dirac. Patrick Blackett was one of several of Dirac’s friends who took his place at the top table of the Government’s scientific advisers and attended dozens of interminable policy meetings. He joined his former Cavendish colleagues Cockcroft and Chadwick on a special committee set up to consider the implications of Frisch and Peierls’ prediction of the small amount of uranium needed to make a bomb.3 They consulted Dirac, but he had no wish to be part of the proceedings.4
By August 1941, Churchill authorised the manufacture of a nuclear weapon, following the advice of the committee and approving comments from his friend and chief scientific adviser, Frederick Lindemann.5 The British Government allocated the resources its scientists requested to begin to build the bomb and set up the ‘Tube Alloys’ project, a name chosen to be dull enough to escape the attention of prying eyes and ears. Blackett, the one dissenting voice on the committee, believed that the British could not build the bomb alone: the project would be successful only if it were pursued in collaboration with the Americans. He would soon be proved right. Blackett was no happier in his other dealings with the Government. He was one of the pioneers in the use of science to inform decisions about the management of the war; for example, in weighing the risks and benefits of different military strategies.6 The hard-headed application of this new discipline of ‘operational research’ brought Blackett and his colleagues, including Bernal, into disagreements with the military and the politicians, who both preferred to take decisions with their hearts as well as their heads. Blackett insisted that Churchill’s policy of aerial bombing enemy civilians – supported by the military and the public – was ineffective, the misguided result of a failure to identify the enemy’s key industrial and military targets. It would be better to bomb the enemy’s fleet of U-boats, he told an unmoved Lindemann. Churchill persevered with his policy and kept his scientific committees at a distance: for him, ‘Scientists should be on tap, not on top.’7
Like many mathematicians, Dirac was invited to work at the Government’s research station in Bletchley Park. In late May 1942, he was approached by the ancient-history scholar Frank Adcock, who had been charged with recruiting the best Cambridge brains. Adcock wrote to Dirac, ‘There is some work concerned with the war which is itself important and would, I believe, be of interest to you. I am not free to say just what the work is.’8 When Dirac asked to know more, a Foreign Office official wrote to clarify: ‘The work would be a full-time job [nominally nine hours a day] and would require you to leave Cambridge.’9 With Manci four months pregnant, this was too much disruption for Dirac to contemplate, so he never did work in the huts of Bletchley Park with Max Newman and Newman’s former student Alan Turing.10 This would have been one of the most intriguing collaborations of the war.
In Cambridge, Dirac supervised graduate students and gave his quantum-mechanics lectures to about fifteen students on Tuesday, Thursday and Saturday mornings. In 1942, his audience included Freeman Dyson, an exceptionally talented student, then nineteen years old.11 Dyson was disappointed: in his view, the course lacked all sense of historical perspective and made no attempt to help students tackle practical calculations. Not one to suffer in silence, Dyson amused his fellow students by bombarding Dirac with questions, sometimes catching him off-guard and once causing Dirac to end a lecture early so that he could prepare a proper response.12 Almost twenty years before, the young Dirac had pressurised Ebenezer Cunningham in one of his lecture courses; now it was Dirac’s turn to be shown the drawn sword of youth.
By early 1942, Dirac was thinking more about technology than quantum mec
hanics. He was a consultant to the Tube Alloys project and worked closely with Rudolf Peierls. One of the first reports that Dirac wrote for him concerned another way of separating a mixture of isotopes, using a simple method that involves injecting the mixture into the base of a hollow cylinder spinning rapidly about its long axis. The centrifugal force generated by the rotation causes the heavier isotope to move towards the outer rim and the lighter one to accumulate closer to the central axis, thus effecting a separation. When Dirac sent his report to Peierls in May 1942, he wrote that he had ‘written up [his] old work’ and did not mention its provenance.13 It is clear from the manuscript that Dirac wanted to investigate the motion of the gases in the tube, to find how far up the spinning cylinder the injected gas will reach. Using classical mechanics, he found that the device would be a stable source of separated isotopes and calculated that, if the cylinder had a radius of one centimetre and rotated almost five thousand times a second, its length should be about eighty centimetres. This confidential report, declassified in 1946, proved to be seminal for the designers of centrifuges. Dirac’s calculations provided the theoretical underpinning of the countercurrent centrifuge, invented three years earlier by the American scientist Harold Urey. This technique was not used during the manufacture of the first nuclear bombs – other methods made less onerous engineering demands – but later became the nuclear engineer’s preferred choice as it gives a particularly efficient way of separating uranium isotopes.
The Strangest Man Page 46