“Absence of A-bomb: Were the Nazis duped — or simply dumb?” So asks the weekly U.S. News & World Report in a piece for its July 24-31 cover story, “Mysteries of History.” The question is being revisited now perhaps because of a recent Broadway import from London: Michael Frayn’s “Copenhagen.”

The play focuses on a murky incident that has been hotly debated since the end of the war: the meeting between the German physicist Werner Heisenberg (1901-75) and the Danish physicist Niels Bohr (1885-1962) one day in the fall of 1941, in Copenhagen. Why did Heisenberg, victorious Germany’s No. 1 scientist, go to see his former mentor and collaborator Bohr, now in an occupied land?

The question is important. Heisenberg and Bohr — both Nobel Prize-winners, Bohr in 1922, two years before Heisenberg went to study at his Institute for Theoretical Physics, and Heisenberg in 1932 — together laid the foundations of nuclear physics. And whereas Heisenberg, who worked on nuclear development under Adolf Hitler, did not produce nuclear weapons, Bohr escaped German-occupied Denmark to join forces with the Manhattan Project, which produced the atomic bombs that annihilated Hiroshima and Nagasaki.

More to the point, Bohr made it clear that, in his mind, the purpose of Heisenberg’s visit was definitely related to nuclear development, although he apparently gave differing versions of the meeting afterward.

Frayn, who bases his play on known facts where they exist, reports in the postscript to his play (Methuen, 2000): “Most anxious of all to establish some agreed version of the meeting was Heisenberg himself. He did indeed go back [to Copenhagen] in 1947 . . . and attempted to find some common ground in the matter with Bohr. But it proved to be too delicate a task, and (according to Heisenberg, at any rate, in his memoirs) ‘we both came to feel that it would be better to stop disturbing the spirits of the past.’ “

This, too, suggests that the subject Heisenberg had in mind to discuss with Bohr in the fall of 1941 concerned nuclear development. There is also the fact that he did not produce nuclear weapons during the war and, after the war, actively promoted the peaceful use of nuclear energy. So there is a possible link between the visit and what he did or did not do afterward.

But little of substance is known about the meeting, save the overall impression that it was a failure: Bohr’s wife, Margarethe, is known to have observed, “No matter what anyone says, that was a hostile visit.” This absence of information is where Frayn comes in. As Yukio Mishima once said about one of his own historical plays, “The flaw of history is that what is written is about things that happened, but not about things that did not. That’s the crack through which novelists, playwrights, poets, and other frauds slip in” — although I hasten to add that Frayn is no fraud.

In the play, Frayn makes the three “spirits of the past” — Heisenberg, Bohr and Margarethe (“now we’re all three of us dead and gone”) — try to reconstruct the meeting to answer the question: “Why did he come to Copenhagen?” The answer might throw light on what Heisenberg did or did not do with his knowledge of nuclear physics. The attempt is made through several deliberately false steps, in order to reveal the complex position in which Heisenberg the great scientific thinker and patriot might have found himself.

So, what is the answer? Heisenberg did not produce nuclear weapons probably “from lack of will to do so,” to quote the Encyclopedia Britannica. That is, not, as those who condemn Heisenberg maintain, because of incompetence. Frayn notes, “The effects of real enthusiasm and real determination are incalculable. In the realm of the just possible they are sometimes decisive.”

One dramatic scene shows Heisenberg reminding Bohr of his (Heisenberg’s) question: “Does one as a physicist have the moral right to work on the practical exploitation of atomic energy?” Bohr becomes horrified. “Because the implication was obvious,” he says. “That you were working on it.” He has misconstrued the question.

The play’s climactic moment comes when Heisenberg, who professes to have avoided making proper calculations, stops to do just that. The moment he does that, an atomic bomb explodes. (That’s what happens in the New York staging. “Copenhagen” has no stage directions.)

Of course, a single play will not erase the division of opinion. Still, Hans Bethe, an important member of the Manhattan Project, has maintained that Heisenberg “never worked on a bomb.”

Then why did he keep up his involvement in nuclear development throughout the war? “To me, in 1948, he said his main intention had been to save a few young physicists from going to the war by employing them in the uranium project,” Bethe explained. “And indeed there were several promising young German physicists whom he did save. So I believe that motive.”

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