Thanks for invitation, in particular to Brian Schwartz, who have done an impressive job of putting this together. I’m happy to be here.

I am not going to give you a a media-show. I’ll show you just one – boring – overhead [containing text: "Niels Bohr Archive – www.nba.nbi.dk"]. Most of you may actually know this web address as the way to get to the release from the Niels Bohr Archive of Bohr’s notes and draft letters on Heisenberg’s 1941 visit to Copenhagen. My reason for reminding you of it is to emphasize that the release constitutes just a tiny tiny bit of the Archive’s resources. Among our holdings are Niels Bohr’s scientific, political and other papers, the papers of several of Bohr’s colleagues, a large photo collection (can be seen at our website) and collections of films and tape recordings. We are about to complete the publication of the Niels Bohr Collected Works and have recently moved to new quarters at the Niels Bohr Institute which includes a new research library. We also offer seminars and symposia, including an international symposium in September last year – in which Brian Schwartz and many others participated – dealing with the general relationship between drama and the history of science taking Michael Frayn’s "Copenhagen" as the point of departure. We do have other things to offer than the recent release, so do come and see us!

The Archive is an independent institution with some (not enough!) financial support from the Danish state. The board of directors includes a representative from the Bohr family – presently Vilhelm Bohr who will talk after me. Vilhelm has been an invaluable support in instigating and effecting the recent release of papers, and I – we – should thank him for that.

When Brian asked for a title of my talk, I called it "The Bohr–Heisenberg Meeting from a Distance." I confess that I did this mainly to provide a non-committal title and to keep the options open. At that time, not long ago, I was up to my neck – and higher – in the practicalities of the release, and I honestly didn’t know how to conceptualize the experience. The subsequent enormous interest in the documents by historians and the media alike has made it even more difficult to secure the peace of mind needed to prepare a talk such as this.

There may also have been more subtle, perhaps even unconscious, reasons for inventing this particular title. I may have equated the geographical distance between Copenhagen and Washington with my psychological need for a conceptual distance from the matters at hand. That, of course, was not a very astute consideration, for where else than here – with the present contributors and audience – would it be more impossible to escape such concerns?

But it has slowly occurred to me that it is indeed just here that my title may be particularly appropriate, if given a different meaning, in which the requirement of distance does not reflect my own personal and egotistical need in my own overworked situation, but indeed the need of all of us to sit back and consider what the release is all about.

We can all (I suppose!) agree that the expectations of the media were very much exaggerated. How, after all, as several newspapers implied, could Bohr’s private notes of his recollections sixteen years or more after the fact be expected to resolve conclusively the question of Heisenberg’s intentions and even why the atomic bomb was not developed in Nazi Germany? This suggestion seemed to me too preposterous to deserve comment. To be fair, there have also been doubts: a Danish newspaper asked how we could know that Heisenberg never received final versions of the letters and an American journalist called me in the middle of the night asking how we could be sure that the documents were not fraudulent and authored by someone else than Bohr. All in all, it is my impression that after the release, the media coverage has been fairly balanced, naturally with exceptions.

Before the release, the expectation among historians was also high, beginning with one historian’s public announcement of the existence of one of the documents. Curiously, this announcement was made at the symposium organized by Brian Schwartz and Harry Lustig in connection with the Broadway production of "Copenhagen." Insisting on the importance of the document, this historian refused to reveal anything about its content, which had been shown to him in confidence by a member of the Bohr family back in 1985.

After the release, other historians too have insisted upon the importance of the papers. A notable case in point is the 15 February issue of the Munich-based Sueddeutsche Zeitung ("The South-German Newspaper"), which includes invited articles by several of the historians – all American – present here today. However, the new documents do not seem to have affected any of these historians’ very different views on Heisenberg’s actions and intentions with regard to the German atomic bomb project and what transpired at the 1941 meeting between Bohr and Heisenberg. I find this an intriguing example of the relationship between evidence and interpretation in the history of science.

The high expectations – of the media and the historians alike – have of course been fueled by Michael Frayn’s play "Copenhagen," which is indeed the very occasion for this symposium. Even the timing of the release – originally scheduled for the year 2012, fifty years after the death of Niels Bohr – was prompted to some extent by the interest caused by Frayn’s play. So regardless of many historians’ criticism of the historical veracity of the play, and of its function as a mediator of the history of science to a wider public, there are reasons to be grateful to Frayn for adding to the discussion and provoking the release of new evidence.

It is not the time and place here to state unequivocally which historical truths about the 1941 meeting can and cannot be stated on the basis of the newly released documents. On the other hand, I cannot dodge this question altogether. With regard to substance, I am of the opinion that the documents largely confirm the statement published about the meeting in an article that Niels Bohr’s son Aage Bohr wrote back in 1964. Originally published in Danish, the article was published in English in 1967. According to both this article and the released documents, it was Bohr’s definite impression that Heisenberg simply informed him – in general terms – of the existence of a German atomic bomb project and his own involvement in it. Although Bohr was naturally alarmed by this information, there is no basis, neither in Aage Bohr’s article nor in the documents themselves, for concluding that Heisenberg brought up the question of the atomic bomb as part of some kind of spying mission for the German government.

On the other hand, in the released documents Bohr is adamant that Heisenberg also did not give the impression that he visited Copenhagen on an idealistic mission to establish some kind of agreement among physicists on both sides of the war not to develop atomic bombs. Aage Bohr emphasizes the same point in his article. Indeed, it was such a claim that Bohr understood Heisenberg to make in his letter to the author Robert Jungk, and it was upon reading Jungk’s book containing an extract of this letter that Bohr felt compelled to write the first of several unsent letters to Heisenberg. Bohr had not received the impression at all that Heisenberg sought to prevent the development of an atomic bomb. Quite to the contrary, he had sensed that Heisenberg was rather proud of his involvement in the German project and was indeed confident that the Nazis was on their way to a quick victory – even without the atomic bomb.

I do find Bohr’s statements in his several draft letters and notes remarkably consistent, and have no doubt that they reflect an honest attempt to describe what happened in the 1941 meeting with Heisenberg. Indeed, his conviction seems so strong that I find it hard to believe that he could have misunderstood Heisenberg to the extent that it has been claimed. To me, moreover, the fact that the drafts and notes were not intended for publication constitutes an added assurance of the veracity of Bohr’s claims, in that he wrote the material not to convince others, but to clarify his own thoughts. But at the same time, it should not be forgotten that the documents were written sixteen years or more after the meeting they seek to understand, in an entirely different historical context, partly on the request of people with the contemporary historical situation on their minds. Bohr himself seems quite aware of this, as the draft letters are not formulated as a final statement, but rather as a plea to Heisenberg for a shared discussion to reach a better understanding.

When interpreting the released documents, it is important to realize that they constitute a reaction to what Bohr considered Heisenberg’s misguided description of the 1941 meeting, not to the meeting itself. A good starting point for understanding the documents, then, might be not to ask what light they shed on the 1941 meeting but rather to situate them in the time in which they were written. What were Bohr’s interests at the time? What were Heisenberg’s interests? Why did Heisenberg phrase himself as he did in the letter to Robert Jungk, the publication of which incited Bohr’s reaction in the first place? Is there something in the contemporary historical situation, as Bohr understood it, that can help explain why Bohr never sent the letters? Only after a serious approach to such questions, I think, can we begin to ask what light the documents can and cannot shed on what transpired in 1941.

With regard to Bohr’s concrete act of writing these drafts and notes, we need to know more about the actual circumstances. In this respect, it is interesting to note that when being asked by the United Press at the end of January 1958 about his reaction to Jungk’s book, Bohr was quoted as providing the following response: "When I get something to say about this topic, it will take place in the form of a publication that I am writing myself." That this was not simply a ploy to stave off the media (which in any case would have been quite uncharacteristic of Bohr) is clear from his correspondence almost four years later with his close colleague, the Belgian physicist Léon Rosenfeld.

Toward the end of October 1961, Niels Bohr, who was travelling at the time, wrote to Rosenfeld, asking him to come to Copenhagen to help him write the introduction to a Festschrift in celebration of Heisenberg’s birthday, which was due shortly. Rosenfeld’s response (found in Bohr’s administrative correspondence and now transferred to the growing supplement to his scientific correspondence) was uncharacteristically blunt: "I have again carefully considered the unhappy Heisenberg matter, and I did not find much reason that now, at the last moment, you should abandon the very reasonable decision made last summer in Tisvilde: namely, that first and foremost one must see to it that any ambiguity with regard to the actual events at H[eisenberg]’s visit to Copenhagen during the war is removed by way of a public account from your side; only then would you be able, without the danger of misunderstanding, to write about your scientific relations with H[eisenberg], which after all cannot be separated from your personal relationship with him." In spite of Rosenfeld’s strong wording, Bohr did write the introduction to the Heisenberg Festschrift with the help of a Danish assistant, having another assistant write a letter to Rosenfeld that he had done so. What Rosenfeld may not have quite appreciated is that the absence of Bohr’s name in the Festschrift would in itself have constituted a very strong statement. In any case, this incident helps explain why even Bohr’s seemingly innocent drafts of letters for Heisenberg’s 60^th birthday, which is part of the release, were a sensitive matter.

In spite of the references both in early 1958 and in late 1961 to Bohr’s preparing a published account of his experiences during the war, including Heisenberg’s visit, such a publication never came to be. Nor is there a manuscript for such a publication in the Niels Bohr Archive. Indeed, the material constituted by the release comprises the only description from Bohr’s side of Heisenberg’s visit to Copenhagen in September 1941. Thus, not only did Bohr not publish anything about the 1941 meeting; he even decided not to open the matter in his correspondence with his old friend Heisenberg. Why?

For one thing, Bohr was known for his tendency to write and rewrite anything he was working with – publications, funding applications, reports, letters – before he was fully satisfied. He did this most often (as in the case of the current release) in close collaboration with some "helper" – as his closest collaborators liked to call themselves. Although part of the explanation, however, this is hardly the full one, for whenever Bohr worked on anything for so long, it usually reached some kind of completion.

In one of the released documents, Bohr recalls that he had told Heisenberg his recollection of the 1941 visit already on two occasions several years earlier. Perhaps, then, Bohr considered it futile to make his point all over again after he felt that Heisenberg had publicly contradicted it. If so, this must have been a painful decision, especially as Bohr had no desire to break off their unusually close friendship, which may have been affected by the 1941 meeting, but certainly not destroyed, as some have indicated. After the war Bohr was supportive of Heisenberg on many occasions and they continued to correspond and meet both privately and at conferences.

Another motivation may have had to do with the importance in Bohr’s mind of retaining unity in the international physics community. This was an important concern for Bohr in its own right. Moreover, both Bohr – as we have heard earlier today – and Heisenberg were heavily involved, each in their own way, with the question of nuclear balance and the spreading of nuclear weapons, and the real or conceived disunity that might result from a personal debate with Heisenberg might be detrimental for these efforts, in part by hurting Heisenberg’s prestige. These questions loomed much larger for Bohr than the after all comparatively trivial matter of the personal meeting back in 1941.

While not resolving conclusively the question of why the Germans did not develop the atomic bomb, or even of what precisely transpired in Copenhagen in September 1941, the released documents open new questions for historical research, not only with regard to 1941 but, even more, about the time in which the documents were created.

In the short run, the publication of the release on the internet may have added to the current hype about their importance. In the longer run, their continued easy availability will hopefully allow for more reflection about their complex implications.