There was a very particular reason why they kept pressing him to quit Germany and a very particular reason why he wouldn't, but it was almost too dangerous a subject to mention directly. The only person to approach it was Enrico Fermi. 'There is now a real chance that atom bombs may be built,' he told Heisenberg, as if Heisenberg didn't know. The possibility had occurred to most atomic physicists with Otto Hahn's discovery of uranium fission in 1938.
'Once war is declared, both sides will perhaps do their utmost to hasten this development.' Heisenberg said: 'I have the certain feeling that atomic developments will be rather slow however much governments clamour for them; I believe the war will be over long before the first atom bomb is built.' Thomas Powers attempts to show that Heisenberg was making a coded promise here: he would not help the Americans build a bomb for possible use on his homeland, which was what they wanted, but he would not build a bomb for Hitler either. The exiles, however, were unable to trust a man prepared to stay in Nazi Germany, let alone one who excused his conduct by saying, as he did to Edward Teller, 'Do you abandon your brother because he stole a silver spoon?' Once he got back on the boat they began lobbying the US government for an all-out atomic effort, quite largely for fear of Heisenberg beating them to it.
In fact Heisenberg spun out the war years with pettifogging, half-inept reactor experiments. At a crucial meeting with Speer and the Luftwaffe he explained that a uranium 235 warhead no bigger than a pineapple could destroy London, a suspiciously accurate guess at the critical mass required, but when Speer asked him to name his figure for research funds he suggested a ludicrously low 40,000 marks, which convinced the Reichsminister he was talking nothing more than blackboard theories. The German bomb project stayed in the laboratory.
Heisenberg visited Bohr in Copenhagen in 1941 and apparently tried to persuade him to use his international contacts and hold up bomb work on the other side too. Bohr, a Jew under Nazi occupation, was appalled. He took this naive gesture as an espionage ploy and never forgave his old protege.
Once he had escaped to America he worked on the bomb himself and told everyone Heisenberg was up to no good. US intelligence officers built up a certain preoccupation with 'Heisenberg's brain'. Elaborate and goofy plans were drawn up to maybe kidnap him in Germany and sort of parachute him into the Med where a submarine could like pick him up. When he gave a lecture in Zurich in 1944, a US agent, Moe Berg, former catcher of the Boston Red Sox, was sent along with a pistol and orders to kill Heisenberg if his talk revealed any working knowledge of bomb design.
At a dinner after the lecture, Heisenberg was asked if he thought Germany had lost the war. 'Yes,' he said, 'but wouldn't it be good if we had won?' Back home, having avoided assassination, he was threatened with arrest because someone had reported that 'Yes' to the Gestapo, which may show how fine a line he was treading. What the Gestapo would have made of his pax offer to Bohr, and his other assurances smuggled to America through intermediaries, one can easily, though not comfortably, imagine.
Up to his death in 1976 Heisenberg tended to obfuscate his role, claiming that he had been 'spared the moral decision' about the bomb because it was beyond Germany's means, that he never misled Speer about its feasibility, that his visit to Bohr was merely in the 'vague hope' of some agreement, that he was always a loyal German.
Powers never quite resolves the ambiguities of Heisenberg's position, perhaps because it cannot be done. He writes at excessive length, furnishing pocket biographies of every character who walks through the door, examining all the minutiae of redundant intelligence operations, losing track of the story and repeating himself at various points, but he makes a better case for Heisenberg than the man ever publicly made for himself.
Heisenberg, he argues, far from being just a dog that didn't bark, deliberately 'killed' the bomb project. He commends Heisenberg's cautious methods and draws an acute parallel with Oppenheimer, who later spoke out openly against the US hydrogen bomb and was broken by a McCarthyite committee.
Last year, when Powers must have already completed much of his research, the British government released the Farm Hall transcripts, taken from the covert recording of every word the captured German atomic scientists spoke while they were interned at a country house near Cambridge for six months in 1945. Allied intelligence wanted to make sure they had no Soviet leanings. What we want to know, of course, is what they said when they heard the BBC six o'clock news on 6 August, the day Hiroshima was bombed.
They were shocked, staggered that the thing was possible so soon, relieved they hadn't done it. Heisenberg said: 'Although we were not 1OO per cent anxious to do it . . . we were so little trusted by the state that even if we had wanted to do it, it wouldn't have been easy to get it through.' He admitted that 'we wouldn't have had the moral courage' to ask for the resources.
He went on: 'I never thought that we would make a bomb and at the bottom of my heart I was really glad . . .' There, surely, is your answer. He was a dithering compromiser who never had the know-how anyway. But, Powers points out, only two weeks later he gave the others a detailed technical lecture on how the Hiroshima bomb must have been designed and they were surprised all over again. He was way ahead of them all. Possibly 'Heisenberg's brain' really was the secret weapon the Americans feared it was, and he really did keep that dreadful secret, of how much he understood, carefully to himself. Despite Powers' endeavours, he is still keeping it.