10. Translation.


Dear Heisenberg

I hereby send a reprint of a Rutherford Lecture in which I have tried to tell a little about my memories of the developments prompted by the discovery of the atomic nucleus. As you have seen from the small article I wrote for your 60th birthday and from my lecture at the opening of the 50-year anniversary Solvay Meeting, I have been much occupied in recent years with historical studies of physics, which incidentally have now been taken up by an American committee established by the Washington Academy and the Carnegie Foundation, and the intention is that in the coming years Kuhn, who is to lead the project, will have a fixed base in Copenhagen for a secretariat and an archive.

While occupying myself with such matters I have, of course, many times felt the difficulty of giving an accurate account of developments in which many different people have taken part, and I have felt this most strongly in describing what took place during the war in connection with the atomic energy projects. In the latter case there has been very keen interest from various quarters, and the governments of several countries have even begun investigations using existing archives.






Similarly, committees have been established in various countries in order to shed light on the discussions and preparations during the war that preceded the application of the results of atomic physics for military purposes, and from many quarters I have been asked in particular about the arrangement for and the purpose of the visit by you and Weizsäcker to Copenhagen in 1941.

This has been very difficult for me to answer since, as you know from our conversations after the war, I have a completely different perception of what took place during the visit than that you have expressed in your contribution to Jungk’s book.

For us in Copenhagen, who found ourselves in such a difficult and dangerous position during the German occupation, the visit was an event that had to make a quite extraordinary impression on us all, and I therefore carefully noted every word uttered in our conversation, during which, constantly threatened as we were by the surveillance of the German police, I had to assume a very cautious position. I am thinking not only of the strong conviction that you and Weizsäcker expressed concerning German victory, which did not correspond to our






hopes, but more of how, during the course of the war, your conviction had to become less strong and finally end with the certainty of Germany’s defeat. Therefore it would not be incomprehensible if it were difficult for you to keep track of how, against the background of changing circumstances, statements on the part of the Germans changed from year to year. On the other hand I remember quite clearly the impression it made on me when, at the beginning of the conversation, you told me without preparation that you were certain that the war, if it lasted long enough, would be decided with atomic weapons. 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,






but you gave no hint about efforts on the part of German scientists to prevent such a development.

It is true that, during his visits to Copenhagen in 1943 on his journeys to Norway to participate in the efforts to increase the production of heavy water, Jensen did make hints in such a direction, but because of his own mission and the constantly growing rumours of new German weapons, I necessarily had to be very sceptical and extremely cautious in my ever more dangerous existence. It was only a few months later when, in order to avoid imminent arrest by the German police, I escaped to Sweden and arrived in England that I heard about the great preparations that were under way there and in the USA and how far they had come. The question of how far Germany had come occupied not only the physicists but especially the govern-






ment and the secret intelligence service, and naturally I had to tell the latter and some members of the government about all the experiences we had had in Copenhagen and in particular about the visits by you and Weizsäcker and Jensen, which gave rise to thorough discussions about the conclusions which could be drawn about the information given during the conversations and their comparison with everything that the intelligence service had been able to obtain. The point which was put forward during these discussions, and with which all later enquiries have been particularly concerned, was how the visit had been arranged and what purpose lay behind it, as one has wondered in particular how and with what authorization such a dangerous matter, of such great political importance, could be taken up with someone in an occupied and hostile country.