Letter From Werner Heisenberg to Author Robert Jungk

As far as I remember, although I may be wrong after such a long time, the conversation came about as follows. My visit to Copenhagen took place in autumn 1941, actually I think it was at the end of October. At this time we in the "Uranverein", as a result of our experiments with uranium and heavy water, had come to the following conclusion: "It will definitely be possible to build a reactor from uranium and heavy water which will provide energy. In this reactor (in accordance with theoretical work of von Weizäcker) a decay product of uranium 239 will be produced which will be as suitable as uranium 235 as an explosive in atomic bombs." We did not know a process for obtaining uranium 235 with the resources available under wartime conditions in Germany, in quantities worth mentioning. On the other hand, as the production of the atomic explosive can only be accomplished in huge reactors which have been working for years, we were convinced that the manufacture of atomic bombs was possible only with enormous technical resources. We knew that one could produce atom bombs but overestimated the necessary technical expenditure at the time. This situation seemed to us to be a favorable one, as it enabled the physicists to influence further developments. If it were impossible to produce atomic bombs this problem would not have arisen, but if they were easily produced the physicists would have been unable to prevent their manufacture. This situation gave the physicists at that time decisive influence on further developments, since they could argue with the government that atomic bombs would probably not be available during the course of the war. On the other hand there might be a possibility of carrying out this project if enormous efforts were made. Further developments proved that both attitudes were to the point and fully justified -- as the Americans for instance actually could not employ the atomic bomb against Germany.

Under these circumstances we thought a talk with Bohr would be of value. This talk took place on an evening walk in a district near Ny-Carlsberg. Being aware that Bohr was under the surveillance of the German political authorities and that his assertions about me would probably be reported to Germany, I tried to conduct this talk in such a way as to preclude putting my life into immediate danger. 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 understood the meaning of this question immediately, as I realized from his slightly frightened reaction. He 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, obviously assuming that I had intended to convey to him that Germany had made great progress in the direction of manufacturing atomic weapons. Although I tried subsequently to correct this false impression I probably did not succeed in winning Bohr's complete trust, especially as I only dared to speak guardedly (which was definitely a mistake on my part), being afraid that some phrase or other could later be held against me. I was very unhappy about the result of this conversation.

Robert Jungk, the author, writes the following disclaimer: "Heisenberg, however, says that he is unable today to recollect the precise wording of this talk. If one could interpret the content of this conversation in psychological terms, it would depend on very fine nuances indeed. As neither of the two participants possess a stenographic record, Heisenberg's notes, done from memory -- with all their reservations -- are the best existing source."