What Really Happened in Copenhagen?: Letters from Heisenberg and Bohr

The following is an excerpt from a 1957 letter from Heisenberg to author Robert Jungk, who was in the process of writing Brighter Than a Thousand Stars, a book about the German atomic bomb project:

“In my memory which may, of course, deceive me after such a long time, the conversation roughly unfolded the following way. … 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. … 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.”

Dr. Gerald Holton, a Professor of History of Science at Harvard University points out that “There are significant parts of Heisenberg’s letter which Jungk chose not to print …” and that Heisenberg qualifies the letter by saying that his memory may deceive him. Thus the version Bohr read and responded to in the letter below was not a complete representation of Heisenberg’s report.

The following unsent letter from Niels Bohr to Werner Heisenberg was found in Bohr’s personal copy of the 1957 edition of Jungk’s Brighter Than A Thousand Sunsafter his death. It was released to the public in 2002.

“I have seen a book, Brighter Than a Thousand Sunsby Robert Jungk … I think that I owe it to you to tell you that I am greatly amazed to see how much your memory has deceived you … I also remember quite clearly our conversation in my room at the Institute, where in vague terms you spoke in a manner that could only give me the firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons and that you said that there was no need to talk about details since you were completely familiar with them and had spent the past two years working … on such preparations. I listened to this without speaking since [a] great matter for mankind was at issue in which, despite our personal friendship, we had to be regarded as representatives of two sides engaged in mortal combat. That my silence and gravity, as you write in the letter, could be taken as an expression of shock at your reports that it was possible to make an atomic bomb is a quite peculiar misunderstanding, which must be due to the great tension in your own mind. From the day three years earlier when I realized that slow neutrons could only cause fission in Uranium 235 and not 238, it was of course obvious to me that a bomb with certain effect could be produced by separating the uraniums. … If anything in my behaviour could be interpreted as shock, it did not derive from such reports but rather from the news, as I had to understand it, that Germany was participating vigorously in a race to be the first with atomic weapons.”6_24