Military victory for Hitler’s Germany probably never appeared more certain than it did in the early fall of 1941, when Werner Heisenberg, on a Sunday evening in mid-September, boarded the night train from Berlin to Copenhagen. Forward elements of the German army were pressing on Moscow, the Americans had not yet entered the war, German armies occupied most of the rest of Europe, and Britain was trapped on its island. “News from the Russian front has been pretty bad,” wrote one of Winston Churchill’s advisers, Alexander Cadogan, in his diary later that week, while Heisenberg was visiting the institute of his old friend and mentor, the physicist Niels Bohr. “Everything is pretty murky, and how exactly we are going to win this war, I should like someone to explain.”
The prospect of victory for Germany, of course, brought despair to its enemies, victims, and new subjects. Nazi sympathizers and collaborators were openly active in Denmark, Jewish refugees in Bohr’s institute lived in terror of sharing the fate of the Jews of Poland, and even in his own office Bohr worried about informers and microphones. He refused to have anything to do with the occupation authorities and he would make no exception for Heisenberg’s lecture at the German Scientific Institute on the evening of Friday, September 19.
But Heisenberg himself had been a friend for twenty years; together they had practically invented modern physics and, so far as Bohr knew, Heisenberg came on no official business. As a result, over the week of Heisenberg’s visit Bohr saw him several times, once when it was just the two of them, alone in Bohr’s office. Like Bohr, Heisenberg also feared informers and microphones. The German hesitated to say what was on his mind; the Dane hesitated to respond. Bohr had already been deeply offended when Heisenberg, over lunch with members of Bohr’s institute, predicted that the Germans were bound to win the war, and the private meeting did not go well either. In some way—how and why was unclear—the subject of atomic weapons was raised. Neither recorded the conversation while it was still fresh. Brief as it was, the private meeting seemed to close the door on intimacy forever.
Over the years Heisenberg tried to reopen that door without success. In August 1947 he visited Bohr at his summer house in Tisvilde but the two men repeatedly failed even to agree on where the 1941 conversation took place—Bohr said it was in his office at the institute, Heisenberg remembered that they had been out walking. “After awhile,” Heisenberg wrote in a memoir published in 1971, “we both came to feel that it would be better to stop disturbing the spirits of the past.”
But in the mid-1950s Heisenberg tried again. This time he raised the subject of the 1941 meeting with Bohr indirectly, in a letter to the Austrian journalist and historian Robert Jungk, whose book on the origin of atomic weapons, Brighter …