The timing of Fuchs’ recruitment by Peierls could not have been more dramatic, as it coincided with three momentous developments in the course of the war. First, this was a key moment for science. The invention of radar had helped win the Battle of Britain in 1940; now the race to develop an atomic bomb could decide not just a battle but also the outcome of the war, and determine subsequent global hegemony. Second, the timing was hugely significant for global politics, as the Soviet Union, formerly an ally of Nazi Germany, was invaded by Germany in June and the following month became allied with the United Kingdom. For Fuchs, this confluence would be seminal. Third, political pressures within the United Kingdom put development of an atomic weapon in the balance. In Whitehall, radar was still perceived as the leading research field for the war effort. Attitudes to an atomic bomb were mixed. If the project was believed in at all, it was as a long-term hedge. The fear that Hitler might get there first spurred the scientists, many of whom were émigrés and saw the race against the Nazis as a personal cause.
Fuchs also arrived when the project was moving forwards rapidly. In March 1941 Peierls had had a brainwave: the idea of wrapping a layer of iron around the lump of U235 in order to reflect some of the escaping neutrons back into the uranium and thereby increase the number of fissions. This promised to reduce the amount of U235 required for a weapon, but the design of such a device would first need careful evaluation by theorists.
The immediate goal of MAUD scientists was to enrich uranium and eventually isolate U235. Peierls and another émigré, Franz Simon of Oxford University, had sketched a first outline design of a practical diffusion plant, while Simon’s team had built a half-scale model of a single stage. Peierls had made estimates of the plant’s manpower and costs, and the contract with Metropolitan Vickers for the twenty-stage plant had been agreed.
Fuchs’ arrival as Peierls’ assistant helped to release the master’s creativity. Peierls’ vision was clear: the first task must be the production of the raw material – U235 – for the weapon and in sufficient quantity to make at least one bomb. The immediate goal was therefore to separate U235 from natural uranium. This would require the uranium to be in a gas, for which the only practical substance was uranium hexafluoride, a mixture of uranium and fluorine. The first assignment that Peierls set Fuchs when he joined in May 1941 was: How can one ensure that when a gas passes through a diffusion plant, uranium isotopes are filtered out rather than those of other elements?
Fuchs solved this problem. His report that summer kept the variety of gas secret, as it referred only to elements ‘A’ and ‘X’, with no mention of uranium or fluorine. He examined the hypothetical case of ‘an element X in a compound with element A, where A has two isotopes, A1 and A2’. The question that he now had to solve was ‘How far [does] the existence of two A isotopes affect the efficiency of separation of the X isotopes?’ As Fuchs had not yet signed the Official Secrets Act, Peierls seems to have given him this abstract problem without revealing the identities of the elements A and X. Fuchs had almost certainly deduced their identities, however. To see why, recall what was already in the public domain at that time.
Fission had been discovered in 1938, and the idea of a chain reaction had attracted wide attention among scientists. In 1939 the ‘Great Dane’ – the atomic theorist Niels Bohr – had published his insight that the rare isotope U235 is key to the phenomenon, but from that moment research in nuclear physics became secret. Even so, the implication of Bohr’s observation would have been clear to any competent physicist presented with a problem about separating isotopes, especially within a top-secret war programme. That the element ‘X’ was uranium would have been obvious to Fuchs; perhaps the only unknown for him would have been the nature of the compound, but as uranium hexafluoride is the only practical gaseous compound of uranium, he might, with a little more effort, have been able to deduce this also.
Peierls was impatient to press ahead even before Fuchs was formally approved. On Wednesday 11 June 1941 he wrote to a colleague who was working on the secret project at Cambridge: ‘I propose to be in Cambridge next Sunday afternoon and Monday … I shall bring Fuchs with me, who has now started work here, and I would like him to meet [theorists in your team] as their problems are similar in some ways from the mathematical point of view.’
It is unrealistic to suppose that Fuchs could have discussed mathematics with any of the Cambridge team without both parties knowing the context of the physical problems that the mathematics was intended to solve. Mathematics can provide a set of tools that enable analysis of a physical problem, but there is little point in forging ahead without first ensuring that the tools are the right ones, and that first requires a deep understanding of the physical processes themselves. In the circumstances Peierls’ decision to take Fuchs with him to Cambridge was natural. To defeat Hitler was the urgent task; Fuchs was fully committed to that end and his abilities were critical to Peierls’ capacity to move forward. Peierls was a scientist, totally focused on that goal. There was every reason for him to believe that Fuchs’ motivation was the same. Signing documents was important for the mandarins, but in the meantime there was work to be done. One of Peierls’ fundamental beliefs was that research was most effective when ‘guided by the necessity to get the best answer in the shortest possible time rather than by questions of formal organisation and prestige’. As if to reinforce his sense of urgency, five days later the German army invaded Russia, which now became allied with the United Kingdom in the war against the Nazis.