Heisenberg's Principles More Certain

Historians, physicists, and theater-goers soon will learn what German physicist Werner Heisenberg really said to his Danish mentor Niels Bohr during their famous conversation in Copenhagen in 1941. And the news isn't good for those who would like to believe that Heisenberg tried to subvert the Nazi atomic bomb effort. Heisenberg told Bohr about the work and expressed no misgivings about participating in it, a British newspaper reports, citing a secret letter from Bohr to Heisenberg.

When the two Noble laureates met, Heisenberg talked about nuclear fission--this much has been known. The exchange so disturbed Bohr that he ended his friendship with Heisenberg and never spoke of the encounter in public; for decades historians and physicists have speculated about what exactly Heisenberg said. Some argued that he revealed Germany's ambition to make an atomic bomb and tried to coax Bohr into helping. Others contended that he tried to tell Bohr obliquely that physicists from all nations should agree not to work on such a bomb, but that Bohr misunderstood. The debate intensified 3 years ago with the production of the play Copenhagen by writer Michael Frayn, which suggests Heisenberg may have even tried to sabotage the Nazi bomb effort.

However, in an unposted letter to Heisenberg, kept secret until now, Bohr wrote that Heisenberg told him of the Nazi effort and his willing participation in it. Finn Aaserud, director of the Niels Bohr Archive in Copenhagen, hinted at the letter's contents to The London Times on 6 January. "Essentially the letter shows that he told Bohr that it was possible that the war could be won with atomic weapons, indicating that he was involved in such work," Aaserud told The Times. The archive will publish the letter in February.

After the end of World War II, Heisenberg said that he had tried only to ask Bohr, who died in 1962, whether physicists "have the moral right to work on the practical exploitation of atomic energy?" But Heisenberg, who died in 1976, probably obscured his role as head of German wartime nuclear research to distance himself from the Nazis, and also to explain why German scientist failed to make a bomb, says David Cassidy, a historian of science at Hofstra University in Hempstead, New York, and author of Uncertainty: The Life and Science of Werner Heisenberg. "He had to prove to his German colleagues that he wasn't incompetent," Cassidy says. "On the other hand, he had to prove to his foreign colleagues that he wasn't a Nazi producing weapons for Hitler."