Notes on Comparing the Documents of Heisenberg and Bohr
Concerning their Encounter in 1941
Gerald Holton, Jefferson Physical Laboratory,
Harvard University,Cambridge, MA 02138

I respond to the invitation to offer some comments on Niels Bohr’s recently released documents. It may be an advantage that although I knew Bohr and Heisenberg, I was not involved with them except as an admiring scientist and interested historian of science. The current discussions about their encounter in 1941 should not deflect attention from the excellent roles these two giants played in modern science, especially because that meeting changed nothing in history except their old friendship.

As Bohr intended, his remarkable documents (which anyone interested in the matter should examine, on www.nba.nbi.dk) greatly illuminate that meeting with Heisenberg, and show how incomplete and even erroneous many previous speculations and stories are. They also show us Bohr’s mind at work, typically going again and again over the same ground–as he did in dictating his physics papers–never contradicting but constantly bringing in new details, in the hope of completing a complex report. Therefore his documents should be read with care, and then compared in detail with the portions of Heisenberg’s letter about the same event, published in Robert Jungk’s book, Brighter than a Thousand Suns (1957 in the Danish edition, 1958 in the second, English one).

In Bohr’s first and perhaps most detailed document—on which I was first consulted in 1985—he starts by offering Heisenberg an excuse, i.e., that Heisenberg’s memory might have “deceived” him when he wrote to Jungk. And in fact, Heisenberg had started his own letter with the disclaimer: “As far as I remember, although I may be wrong after such a long time….” And later, in the most controversial part of his letter, he used the word “probably” about how he recounts his talk with Bohr started; and later still: “I may have replied….” In stark contrast, Bohr writes at the beginning of his document that “Personally, I remember every word of our conversations.”

Bohr’s first document continues by denying outright that Heisenberg had tried to obtain information from Bohr on details concerning the development of atomic weapons, (contrary to what some still like to believe), since Heisenberg had said he was, according to Bohr’s recollection, “completely familiar with them and had spent the past two years working more or less exclusively in such preparation.” That such an atomic weapons program was in progress has no longer been in doubt since the release of the Farm Hall papers, and because even C. F. v. Weizäcker agreed it was the purpose of the team’s work (in his autobiography, Bewusstseinswandel (Münich: Carl Hanser Verlag, 1988).

Bohr then explains his reaction: Not anger, as some still insist, but fear. After all, the prospect offered by Heisenberg was that of a successful and energetic pursuit by the German team to make an atomic bomb, at the very time when Hitler’s armies were having their greatest successes. And that prospect was, as Bohr writes there, “a great matter for mankind.” Even at that point, Bohr, to whom Heisenberg had been for so long a kind of successful son, offers Heisenberg again a way out. Bohr writes that reporting Bohr, in Heisenberg’s letter to Jungk, to have been simply shocked by that news was a “misunderstanding…due to the great tension in your mind.” And to make sure Heisenberg understood properly, Bohr then repeats that his own memory of the conversation was clear, and the interaction between Heisenberg and Bohr had been the subject of his “thorough discussion” with others (as we also know to have been the case, from other sources). Such discussions resulting from that visit to Copenhagen of Heisenberg and von Weizsäcker must have been numerous. Bohr’s draft only refers to one: “During conversations with Møller, Heisenberg and Weizsäcker sought to explain that the attitude of the Danish people towards Germany, and that of the Danish physicists in particular, was unreasonable and indefensible since a German victory was already guaranteed and that any resistance against cooperation could only bring disaster to Denmark.”

In some of the last documents, Bohr repeats that he “carefully fixed in [his] mind” every word that was uttered on that ominous occasion, and finds it “incomprehensible” that Heisenberg should claim later (as in the letter to Jungk) to have “hinted” to Bohr that the German scientists “do all they could to prevent such an application of atomic science.” That spin of supposed moral qualms is, of course, at the center of some revisionist writings, such as the book by Thomas Powers, and part of Michael Frayn’s play Copenhagen, in which the Heisenberg character is even made to say on stage, “I understood very clearly. I simply didn’t tell the others,” and later, “I wasn’t trying to build a bomb.”

In Bohr’s last drafts, he also raises a question that had troubled him: “What authorization might have been given to you by the German government to touch upon such a dangerous question.” Indeed, in part to give Heisenberg again a ready excuse, Bohr writes: “During the course of the war, such a wise person as yourself must gradually lose faith in a German victory…. I can therefore understand that perhaps at the end you may no longer have recalled what you thought and said during the first years of the war.” Yet Bohr adds: “But I cannot imagine that during a meeting so boldly arranged as that in 1941, you should have forgotten what arrangements had been made in this regard with the German government authorities.”

Thus, when putting Heisenberg’s letter to Jungk and Bohr’s documents side by side, we see that any tortured attempt to make them seem to be somehow in accord fails, as does the idea Bohr did not “understand” Heisenberg. Rather Bohr contradicts and tries to correct every major point in Heisenberg’s published account. Earlier, Jungk, in his autobiography, Trotzdem (Münich: Carl Hanser Verlag, 1993), had bewailed that he had allowed himself to be used to “propagate a legend,” and now “felt betrayed.” Bohr would of course never have used such language, no matter how deeply hurt he may have felt. In fact, one may speculate that in addition to certain other reasons, Bohr did not send off what he had worked on for so long because even the relatively mild words in his documents seemed to him, in the end, to sound uncharacteristically strong. Still, there is some irony in the fact that Bohr, who had no reason to hide or misremember anything that happened, did not send his letter, whereas Heisenberg, having worked for years on a German nuclear program that resulted in failure, may have had at least some tendency, even if not fully conscious, to misremember when he did send his letter to Jungk.

© Copyright 2002, Gerald Holton