A Letter from Copenhagen

A letter from the German physicist Werner Heisenberg to his wife, Elisabeth, about his meeting with Niels Bohr in Copenhagen in September 1941, which is printed below, was released by the estate of Elisabeth Heisenberg on June 5. Precisely what Heisenberg said to anger his old friend Bohr has been an open question ever since Heisenberg told his wife and friends that the meeting had gone awry.

About the subject there is no question: Heisenberg told Bohr that it would be possible to build atomic bombs, Germany had a program underway, and if the war lasted long enough it might be settled by these terrible new weapons. But why Heisenberg told him this, and why it left Bohr with an anger which was never dispelled, has for decades been the subject of heated debate, and is the core question, asked and re-asked, in Michael Frayn’s remarkable play about the wartime visit, Copenhagen. “Why did he come?” asks Bohr’s wife, Margrethe, in the play. “What was he trying to tell you?”

In February of last year, Bohr’s family released the text of eleven letters concerning the wartime visit. Most were versions of a letter to Heisenberg which Bohr composed initially in 1958 and then revised again and again until his death in 1962. The letters recorded for the first time Bohr’s sense of what had been said, but they contained no new information about Heisenberg’s role in wartime Germany and left unanswered the question Bohr tried to ask in the final versions of the letter he never sent: Who authorized Heisenberg to tell Bohr of this secret German military program?

The letter bearing on the visit written by Heisenberg to his wife, Elisabeth, was composed on three separate days during the week-long visit, and then mailed on his return to Germany. The letter contains nothing about his talk of bombs with Bohr—he promises that after his return he will tell his wife “everything that happened to me”—but it does clear up a number of details about the visit, reveals something of Heisenberg’s reading of events at a critical moment, and subtly changes our understanding of the tenor of their conversations. The new letter now takes pride of place as the earliest document recording the visit.

Among the details established or nudged toward clarity are some of the most basic: Where did the two men meet—on a walk along Copenhagen’s Langlinie, as recalled by Heisenberg’s friend and colleague Carl Friedrich von Weizsäcker, who accompanied Heisenberg to Copenhagen? in Bohr’s office at his institute, as Bohr told friends? or at Bohr’s house, as others believed? Was the critical conversation held indoors (Bohr’s memory) or out, to avoid possible eavesdropping by the Gestapo? Was Margrethe present at all?

The answers to other questions are only suggested. It has long been evident that Heisenberg failed to understand how much the war had changed things between the two men; in his letter Heisenberg is unmistakably surprised by the intensity of feeling aroused in Bohr by the…

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