I read with great interest in the July 2000 issue of Physics Today the articles by David C. Cassidy (page 28) and Hans A. Bethe (page 34) on Werner Heisenberg and the German uranium project and by Gerald Holton (page 38) on Heisenberg and Albert Einstein, as well as letters by Gustav Born and Alvin M. Weinberg (page 74). I have also read Michael Frayn’s play Copenhagen, regarding the 1941 meeting between Niels Bohr and Heisenberg.

I discussed that visit with Heisenberg himself, and also asked Carl Friedrich von Weizsäcker about it. Weizsäcker had accompanied Heisenberg to Copenhagen, and talked with him immediately after he returned in despair. Heisenberg was my director at the Max Planck Institute for Physics from 1950 to 1971. I was associated with Weizsäcker from 1974 to 1980 at his Starnberg institute. In early 2000, at a colloquium in honor of Weizsäcker’s 88th birthday, I asked him again about the Heisenberg–Bohr visit.

Heisenberg was worried that a nuclear bomb might be built and used, and wanted Bohr’s opinion about whether the small international community of nuclear scientists could agree not to work on the bomb. But he did not get his message across.

Heisenberg told me that he used very involved language with Bohr so that, if necessary, he could have given the Gestapo a harmless interpretation. The reactor project was a military secret, and telling a foreigner about it was treason, punishable by death. Unfortunately, Bohr understood only that Heisenberg worked on fission and knew in principle how a nuclear fission bomb could be made, and that, somehow, he wanted to get Bohr involved. Bohr ended the conversation.

In 1980, Edward Teller and I discussed the misunderstanding. Teller said that Bohr had been shocked and dismayed about the Nazis, was himself in personal danger, and apparently did not listen carefully enough. Bohr had spent his life teaching complementarity and the necessity of using imperfect language to express the truth, so Teller would assign blame primarily to Bohr for the misunderstanding.

However, by a well-meant but insensitive remark Heisenberg may have contributed to the confusion. He knew that one official at the German legation in Copenhagen was a trustworthy anti-Nazi. He seems to have suggested that Bohr contact this person if he should ever have difficulties with German authorities in Denmark. Bohr may have misunderstood the comment as a suggestion that he collaborate with the Germans. This interpretation is supported by a report that Bohr gave to the Russian physicist Evgenii Feinberg after the war.

Cassidy is wrong when he suggests that Heisenberg went to Copenhagen to convince Bohr that German victory was inevitable. That he wanted Bohr to use his influence to prevent Allied scientists from “building a bomb that could be used against Germany” may be partially true. The full truth is probably that Heisenberg wanted to talk with his friend about avoiding altogether the construction of nuclear bombs. Bohr was isolated in occupied Denmark, so Heisenberg cannot have hoped to learn anything from him about Allied operations.

Cassidy’s comment that Heisenberg could have turned for counsel to Max Planck and Max von Laue is beside the point. Both Planck and von Laue were respected older colleagues who had also chosen to stay in Germany. But neither worked on fission, and neither had as close a relationship with Heisenberg as Bohr had.

True, Heisenberg and Weizsäcker thought that German domination of Europe would be a lesser evil than Soviet domination. Auschwitz was not yet known, but Stalin’s purges, prison camps, and massacres were. Nevertheless, Heisenberg did not consider it his task to express this opinion. He was closely watched by Nazi authorities when traveling, and statements doubting Germany’s superiority and final victory would be subject to severe punishment. After surprising initial German victories, Heisenberg may have doubted his earlier convictions that Germany would lose the war. His hope, and that of other non-Nazi Germans, was that, after victory, the German army would remove the Nazi system.

Heisenberg made several trips during the war, but most of these were not at his own initiative. In most cases he was asked by German authorities to accept invitations to give lectures. After earlier accusations that he represented “Jewish physics,” these official invitations brought him and modern physics a level of esteem.

It is unfair to say that Heisenberg’s determination to stay in Germany “arose not only from his personal attachment to the German nation and culture, but also from his misguided belief that if he personally could survive in Germany … then so, too, would decent German science.” Like Planck and von Laue, he felt responsible for the students who represented the future of German science. He did not want to desert them.

Highly misleading is Cassidy’s statement that “Heisenberg returned home intent on continuing fission research. He had already resigned himself to the march of events. …” Heisenberg did return to fission research. But he believed that construction of a nuclear bomb by war’s end was impossible. Of course, it cannot be said that he refrained from bomb construction for moral reasons; the moral question simply did not arise, because a bomb did not seem feasible.

Cassidy gives the impression that Heisenberg’s “bomb work” was thwarted by lack of support from the German army. In fact, such decisions were in the hands of the munitions and armaments minister, Albert Speer, who offered support for the reactor project.

The resolution of the “mystery” of Heisenberg’s trip to Copenhagen, as described here, is different from Cassidy’s assumptions. Heisenberg was relieved that fate had kept him from being involved in bomb construction. He never claimed moral superiority, but he regretted reproaches of moral culpability that were occasionally directed at him because he had remained in Germany.