Correspondence between Werner Heisenberg & Robert Jungk, author of "Brighter Than a Thousand Suns"
An extract: Jan. 18th, 1957;
Dear Dr. Jungk!
Many thanks for your letter, asking me to write in a little more detail about my Copenhagen conversations with Bohr during WWII. In my memory which may, of course, deceive me after such a long time, the conversation roughly unfolded the following way. My visit to Copenhagen took place in the fall of 1941; I seem to remember that it was about the end of October. At that time, as a result of our experiments with uranium and heavy water, we in our "Uranium Club" had come to the following conclusion: It will definitely be possible to build a reactor from uranium and heavy water which produces energy. In this reactor (based on a theoretical work by v. Weizsäcker) a decay product of 239-uranium will be produced which just like 235-Uranium is a suitable explosive in atomic bombs. We did not know a process for obtaining of 235-Uranium with the resources available under wartime conditions in Germany, in quantities worth mentioning. Even the production of nuclear explosives from reactors obviously could only be achieved by running huge reactors for years on end. Thus we were quite clear on the fact that the production of atomic bombs would be possible only with enormous technical resources.
So we knew that in principle atomic bombs could be built, although we estimated the necessary technical effort to be even rather larger than in the end it turned out to have been. This situation seemed to us to be an especially favorable precondition as it enabled the physicists to influence further developments. For, had the production of atomic bombs been impossible, the problem would not have arisen at all; but had it been easy, then the physicists definitely could not have prevented their production. The actual givens of the situation, however, gave the physicists at that moment in time a decisive amount of influence over the subsequent events, since they had good arguments for their administrations - atomic bombs probably would not come into play in the course of the war, or else that using every conceivable effort it might yet be possible to bring them into play.
That both kinds of arguments were factually fully justified was shown by the subsequent development; for, in fact, the Americans could not employ the atomic bomb against Germany any more. In this situation we believed that a talk with Bohr might be of value. This talk then took place on an evening walk in the city district near Ny-Carlsberg. Because I knew that Bohr was under surveillance by German political operatives and that statements Bohr made about me would most likely be reported back to Germany, I tried to keep the conversation at a level of allusions that would not immediately endanger my life. The conversation probably started by me asking somewhat casually whether it were justifiable that physicists were devoting themselves to the Uranium problem right now during times of war, when one had to at least consider the possibility that progress in this field might lead to very grave consequences for war technology.
Bohr immediately grasped the meaning of this question as I gathered from his somewhat startled reaction. He answered, as far as I can remember, with a counter-question "Do you really believe one can utilize Uranium fission for the construction of weapons?" I may have replied "I know that this is possible in principle, but a terrific technical effort might be necessary, which one can hope, will not be realized anymore in this war." Bohr was apparently so shocked by this answer that he assumed I was trying to tell him Germany had made great progress towards manufacturing atomic weapons. In my subsequent attempt to correct this false impression I must not have wholly succeeded in winning Bohr's trust, especially because I only dared to speak in very cautious allusions ( which definitely was a mistake on my part) out of fear that later on a particular choice of words could be held against me. I then asked Bohr once more whether, in view of the obvious moral concerns, it might be possible to get all physicists to agree not to attempt work on atomic bombs, since they could only be produced with a huge technical effort anyhow.
But Bohr thought it would be hopeless to exert influence on the actions in the individual countries, and that it was, so to speak, the natural course in this world that the physicists were working in their countries on the production of weapons. For an explanation of this answer one has to include the following complication which, although it was not talked about as far as I can remember, but of which I was conscious, and which may also have been on Bohr's mind, consciously or unconsciously. The prospect of producing atomic bombs while at war was at the time immeasurably greater on the American side than on the German, due to the whole prior history. Since 1933 Germany had lost a number of excellent German physicists through emigration, the laboratories at universities were ancient and poor due to neglect by the government, the gifted young people often were pushed into other professions. In the United States, however, many university institutes since 1932 had been given completely new and modern equipment, and been switched over to nuclear physics. Larger and smaller cyclotrons had been started up in various places, many capable physicists had immigrated and the interest in nuclear physics even on the part of the public was very great.
Our proposition that the physicists on both sides should not advance the production of atomic bombs, was thus indirectly, if one wants to exaggerate the point, a proposition in favor of Hitler. The instinctive human position "As a decent human being one cannot make atomic weapons" thus coincided with an advantage for Germany. How far this was influencing Bohr, I cannot know of course. Everything I am writing here is in a sense an after the fact analysis of a very complicated psychological situation, where it is unlikely that every point can be accurate. - I myself was very unhappy over this conversation. The talk was then resumed a few weeks or months later by Jensen, but was equally unsuccessful. Even now, as I am writing this conversation down, I have no good feeling, since the wording of the various statements can certainly not be accurate anymore, and it would require all the fine nuances to accurately recount the actual content of the conversation in its psychological shading.. Read more: http://werner-heisenberg.unh.edu/Jungk.htm
Also see: “Copenhagen” Play Portrays Bohr and Heisenberg. It is not often that a play comes along that is based on solid research in the history of science. It is even rarer that such a play becomes a considerable public success, while at the same time receiving high acclaim from historians of science and scientists alike. Michael Frayn’s play “Copenhagen,” based on the uncertainties surrounding the 1941 meeting between physicists Niels Bohr and Werner Heisenberg in German-occupied Copenhagen, is such a play. It has already played for nearly two years in London, has been extremely well received in several other European cities, and recently opened in New York City. The play has been published as a book in England and is forthcoming in the US. The book includes a “Postscript” in which Frayn explains why he wrote the play along with a competent discussion of some of the main historical issues involved. The Postscript shows that Frayn is not satisfied with showing the play, he also encourages discussion of it: