The meeting took place in September 1941 when Bohr and Heisenberg were 55 and 39, respectively. Heisenberg had worked with Bohr in Copenhagen for several years starting in 1924.
Heisenberg historians remain divided over their own interpretations of the event. Frayn's 1998 play brought more attention to what previously had been a primarily scholarly discussion. A collection of historical essays provoked by the play was published in English in 2005.[18]
Much of the initial controversy stemmed from a 1956 letter Heisenberg sent to the journalist Robert Jungk after reading the German edition of Jungk's book, Brighter than a Thousand Suns (1956). In the letter, Heisenberg described how he had come to Copenhagen to discuss with Bohr his moral objections toward scientists working on nuclear weapons but how he had failed to say that clearly before the conversation came to a halt. Jungk published an extract from the letter in the Danish edition of the book in 1956 that, out of context, made it look as if Heisenberg was claiming to have sabotaged the German bomb project on moral grounds. (The letter's whole text shows Heisenberg was careful not to claim that.)[19] Bohr was outraged after reading the extract in his copy of the book, feeling that it was false and that the 1941 meeting had proven to him that Heisenberg was quite happy to produce nuclear weapons for Germany.
After the play inspired numerous scholarly and media debates over the 1941 meeting, the Niels Bohr Archive[20] in Copenhagen released to the public all sealed documents related to the meeting, a move intended mostly to settle historical arguments over what they contained. Among the documents were the unsent letters Bohr drafted to Heisenberg in 1957 about Jungk's book and other topics.[21]
These drafts proved to be significant in several respects. First, they proved to be relatively consistent with Heisenberg's recollections of the meeting[19] given to Jungk in 1956, meaning that the course of the conversation can now be fairly well established. Bohr and Heisenberg agree that Heisenberg started the visit by stating to Bohr that nuclear weapons were now conceivable. As Heisenberg wrote to Jungk,
This talk probably started with my question as to whether or not it was right for physicists to devote themselves in wartime to the uranium problem – as there was the possibility that progress in this sphere could lead to grave consequences in the technique of the war.
Bohr confirms that by writing,
It had to make a very strong impression on me that at the very outset you stated that you felt certain that the war, if it lasted sufficiently long, would be decided with atomic weapons.
Heisenberg repeated his convictions on the technical feasibility of building nuclear weapons. As Heisenberg recalled:
He [Bohr] replied as far as I can remember with a counter-question, "Do you really think that uranium fission could be utilized for the construction of weapons?" I may have replied: "I know that this is in principle possible, but it would require a terrific technical effort, which, one can only hope, cannot be realized in this war." Bohr was shocked by my reply.
Bohr's draft letters are consistent with this:
I did not respond to this at all, but as you perhaps regarded this as an expression of doubt, you related how in the preceding years you had devoted yourself almost exclusively to the question and were quite certain that it could be done...
That point is of interest because it is at odds with the suggestion by critics that miscalculations by Heisenberg had led him to conclude erroneously that atomic weapons were not feasible.[22][23] According to Bohr's later notes, Heisenberg then told Bohr that he had not come to discuss the technical aspects of the potential weapons:
Heisenberg said explicitly that he did not wish to enter into technical details but that Bohr should understand that he knew what he was talking about as he had spent 2 years working exclusively on this question.
Unfortunately, because of Heisenberg's concerns about being monitored, his discussion of any details of Germany's nuclear efforts with someone in an occupied country would have been illegal, his remarks were cryptic. Indeed, Bohr's letters note that Heisenberg spoke "in vague terms", from which Bohr was able to form only an "impression" about Heisenberg's efforts. Bohr wrote:
I listened to this without speaking since [a] great matter for mankind was at issue in which, despite our personal friendship, we had to be regarded as representatives of two sides engaged in mortal combat. That my silence and gravity, as you write in the letter, could be taken as an expression of shock at your reports that it was possible to make an atomic bomb is a quite peculiar misunderstanding, which must be due to the great tension in your own mind. From the day three years earlier when I realized that slow neutrons could only cause fission in Uranium 235 and not 238, it was of course obvious to me that a bomb with certain effect could be produced by separating the uraniums. In June 1939 I had even given a public lecture in Birmingham about uranium fission, where I talked about the effects of such a bomb but of course added that the technical preparations would be so large that one did not know how soon they could be overcome. If anything in my behaviour could be interpreted as shock, it did not derive from such reports but rather from the news, as I had to understand it, that Germany was participating vigorously in a race to be the first with atomic weapons.[24]
That circumspect discussion, combined with Bohr's shocked reaction to it, apparently cut off the discussion between them. Thus, the Bohr letters cannot resolve the question, posed by the Copenhagen play, of what Heisenberg had tried to convey to Bohr.
Heisenberg's comment that he knew about the potential for weaponizing uranium fission, appears to counter the arguments of critics such as Rose[25] and Bernstein[26] that calculation errors in 1940 about feasibility, rather than moral scruples, led Heisenberg not to pursue building nuclear weapons.
Finally, the 1957 Bohr draft letters, written 16 years after the meeting, suggest a conflict between Bohr and Heisenberg. Heisenberg's letter to his wife, written on the eve of his departure from Copenhagen, provides no hint of a fracture. In it, he related his final evening with Bohr as very pleasant and unremarkable: "Today I was once more, with Weizsäcker, at Bohr's. In many ways this was especially nice, the conversation revolved for a large part of the evening around purely human concerns, Bohr was reading aloud, I played a Mozart Sonata (a-Major)."[27]
In a March 2006 interview[28] Ivan Supek, one of Heisenberg's students and friends, commented that "Copenhagen is a bad play" and that "Frayn mixed up some things". Supek also claimed that Weizsäcker was the main figure of the meeting. Allegedly, "Heisenberg and Weizsäcker came to Bohr wearing German army uniforms. Weizsäcker tried to persuade Bohr to mediate for peace between Great Britain and Germany and Heisenberg practically completely relied on his political judgement". Supek received these details in a confidential conversation with Margrethe, who thought he would never make them public. Supek however felt it was "his duty to announce these facts so that future generations can know the truth about the Bohr – Heisenberg meeting".
Supek's statements about Bohr's recollection of "the Bohr – Heisenberg meeting" mixes up the visit. Because Heisenberg could only visit Bohr in occupied Denmark on behalf of the German government, Heisenberg was obliged to make public lectures on behalf of the Government which were monitored by German government officials. Heisenberg tried to convey his opinions later during private discussions with Bohr. Heisenberg's letters to his wife and later to Jungk place his conversation with Bohr on Wednesday evening. Either he talked with Bohr on a walk, or at his residence. Bohr, Supek, and Heisenberg describe the meeting differently.[29]
