Something happened—some terrible offense was given which could never be recalled—during the wartime visit of the German physicist Werner Heisenberg to the man who probably meant most to him in the world, the Danish physicist Niels Bohr. It would be forgotten now, certainly Michael Frayn never would have written a play about it, if the offense had not somehow involved Heisenberg’s role as a leader of the German effort to invent atomic bombs. But the bomb was part of it and scientists and historians have been arguing about what happened ever since.
Here is what is known: in September 1941 Heisenberg traveled to Copenhagen, where he told Bohr that in Germany a research effort was underway to develop bombs using the principle of atomic fission. Some kind of misunderstanding ensued. In despair Heisenberg told his wife and close friends that the conversation had gone astray, Bohr became too angry to continue, the meeting ended abruptly. Bohr’s wife and friends later confirmed that indeed he was angry, so angry that the old friendship and intimate working relationship could never be restored. They did not see each other for many years, until Heisenberg came again to Denmark in 1947. There, in Tisvilde, where Bohr had a house in the country, they tried to sort out the earlier conversation but could agree on little—not even where it took place: on an evening walk, as Heisenberg remembered, or in Bohr’s study in his home? “After a while,” Heisenberg wrote in a memoir, “we both came to feel that it would be better to stop disturbing the spirits of the past.”
There is no evidence that the two men ever broached the subject again during the remaining fifteen years of Bohr’s life, but plenty of other people did, then and later. The 1941 meeting was minutely analyzed by American and British intelligence authorities after Bohr’s escape to Sweden in 1943, one jump ahead of the Germans. Rumor of Heisenberg’s visit spread through the scientific grapevine even before the war was over, and its meaning was hotly debated afterward. Why had Heisenberg come to Copenhagen? What made Bohr so angry?
The interest in these questions wasn’t idle. The German military had placed Heisenberg in charge of theoretical work on the feasibility of atomic bombs during the first weeks of the war and he remained a principal director of uranium research until the last shots were fired. When the war ended he was in southern Germany working on a small experimental nuclear reactor which never achieved a self-sustaining chain reaction. It was a tiny program without scientific or military significance. Bohr, meanwhile, had gone on from Sweden to Britain and the secret American laboratory at Los Alamos in the high desert of New Mexico. There he had alarmed officials with reports of Heisenberg’s progress toward a bomb, had established an intimate friendship and excellent working relationship with J. Robert Oppenheimer, director of the American laboratory, and …