11a. Translation.



Dear Heisenberg.

I have long been meaning to write to you on a matter about which I am constantly being asked from so many different quarters. It concerns the visit by you and Weizsäcker to Copenhagen in the autumn of 1941. As you know from our conversations in the first years after the war, we here got quite a different impression of what happened during this visit than the one you have expressed in Jungk’s book. The particular reason that I write to you is that the whole question of the atomic energy projects during the war has been made the subject of thorough studies in England based on access to government archives, including material held by the intelligence service. In this connection, I have had detailed conversations about my affiliation with the whole project, during which questions about your visit in 1941 were also brought up. I have therefore thought it most proper to try to give you as accurate an impression as possible of how we perceived the visit here.






Although we realized that behind the visit there was a wish to see how we were faring in Copenhagen in the dangerous situation during the German occupation and find out what advice you could give us, you must also have understood that it brought us, who lived only on the hope of defeat for German Nazism, in a difficult situation to meet and talk to someone who expressed as strongly as you and Weizsäcker your certain conviction of a German victory and confidence in what it would bring. Naturally, we understand that it may be difficult for you to keep track of how you thought and expressed yourselves at the various stages of the war, the course of which changed as time passed so that the conviction of German victory gradually had to weaken and finally end with the certainty of defeat.





However, what I am thinking of in particular is the conversation we had in my office at the Institute, during which, because of the subject you raised, I carefully fixed in my mind every word that was uttered. 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. I had at that time no knowledge at all of the preparations that were under way in England and America. You added, when I perhaps looked doubtful, that I had to understand that in recent years you had occupied yourself almost exclusively with this question and did not doubt that it could be done. It is therefore quite incomprehensible to me that you should think that you hinted to me that the German physicists would do all they could to prevent such an application of atomic science. During the conversation, which was only very brief, I was naturally very cautious but nevertheless thought a lot about its content, and my alarm was not lessened by hearing from the others at the Institute that Weizsäcker had stated how fortunate it would be for the position of science in Germany after the victory that you could help so significantly towards this end.





In your letter to Jungk you also mention Jensen’s visits to Copenhagen in 1943 during his journeys to Norway to participate in the efforts to increase the production of heavy water. It is true that Jensen emphasized to us that this work was only aimed at the production of energy for industrial purposes, but although we were inclined to trust his sincerity, we felt in no way certain regarding how much he himself knew about the whole effort in Germany. In those years there were often announcements from Germany of new and decisive weapons. At the meetings with Jensen, I was likewise extremely cautious as a result of the constantly growing surveillance on the part of the German police.

When I had to escape to Sweden in the autumn of 1943 in order to avoid imminent arrest and from there went to England, I learned for the first time about the already then well-advanced American–English atomic project. The question of how far Germany had come occupied not only the physicists but also the governments and the intelligence service, and I became involved in the discussions about this. I recounted all our experiences in Copenhagen, and in this connection the question was also raised about what authorization might have been given to you by the German government to touch upon such a dangerous question, with such great political consequences, with someone in an occupied and hostile country. However, the discussions had no decisive influence one way or the other, since





it was quite clear already then, on the basis of intelligence reports, that there was no possibility of carrying out such a large undertaking in Germany before the end of the war.

I have written at such length to make the case as clear as I can for you and hope that we can talk in greater detail about this when the opportunity arises.