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By Gerald Holton
It may be an advantage that, although I knew Bohr and Werner 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 (see http://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 when 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 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 Danish edition; 1958 English edition, pages 102-104).
In Bohr's first and most detailed document , on which I was first consulted in 1985, he starts by offering Heisenberg an excuse, namely, that Heisenberg's memory might have "deceived" him when he wrote to Jungk. In fact, Heisenberg had started his own letter with a disclaimer: "As far as I remember, although I may be wrong after such a long time...." In the most controversial parts of his letter, he wrote, "This talk probably started with my question whether or not it was right for physicists to devote themselves in wartime to the uranium problems...." [emphasis mine] In stark contrast, Bohr writes, "Personally, I remember every word of our conversations."
Bohr's first document denies outright that Heisenberg had tried to obtain information from him on details concerning the development of atomic weapons, contrary to what some still like to believe, since Heisenberg had said he was "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 is no longer in doubt since the release of the Farm Hall papers.
Even C.F. v. Weizsaecker agreed that had been the purpose of the team's work in his autobiography, Bewusstseinswandel (Munich: Carl Hanser Verlag, 1988, page 365).
Bohr then explains his reaction, which was 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. Even so, Bohr offers Heisenberg again a way out, writing that Heisenberg's description (in his letter to Jungk) of Bohr as being simply shocked by that news was a "misunderstanding... due to the great tension in your mind." Bohr then repeats that his own memory of the conversation was clear, and that his interaction with Heisenberg had been the subject of "thorough discussion" with others. Such discussions must have been numerous, but Bohr's draft refers only to one: "During conversations with Moller, Heisenberg and Weizsaecker 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 writes that he finds it "incomprehensible" that Heisenberg later claimed 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 at the center of some of the revisionist writings, such as the book by Thomas Powers, and part of Michael Frayn's Tony- award-winning play, Copenhagen, in which the Heisenberg character is even made to utter 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."
Perhaps to give Heisenberg another 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 he 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 that 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 (Munich: Carl Hanser Verlag, 1993), had bewailed that he had allowed himself to be used to "propagate a legend," and now "felt betrayed." Bohr would never have used such language, no matter how deeply hurt he may have felt. In fact, one may speculate that 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, 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.
Gerald Holton is Mallinckrodt Professor of Physics and Professor of the History of Science, Emeritus at Harvard University. A slightly different version of the above article was distributed at the March 2, 2002 symposium, "The Copenhagen Interpretation: Science and History on Stage," held at the Smithsonian Institution's Museum of Natural History in Washington.
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