69. Heisenberg, Werner. Typed letter signed (“W. Heisenberg”), 3 pages (8.25 x 11.5 in.; 210 x 292 mm.), on separate leaves, in German, on “Max Planck Institute for Physics” letterhead, Göttingen, 3 October 1948, written to Professor Samuel A. Goudsmit, Brookhaven National Laboratory. Usual folds with staple holes at left corner.

Werner Heisenberg defends his fellow German physicists in a letter written to Samuel Goudsmit, Dutch-American physicist and critic of the German scientists of World War II.

“After the lecture General Fieldmarshal Milch asked me how large the bomb might be and whether it would be powerful enough to destroy a big city. I then answered that the bomb, that is its active part, would have the approximate size of a pineapple. Of course this statement attracted attention especially among the non-physicists, who remembered it well.”

Heisenberg writes in full: Dear Goudsmit, Many thanks for your letter. Of course it is always easier to discuss facts than it is to talk about motives. Therefore, I was happy to hear that, after reading the secret reports, you have come to the conclusion that, indeed, we knew since 1940 that one can produce atomic explosives from a pile (Weizsäcker, Houtermans) and that we also knew since 1942 that the decisive ingredient was 94 (Pu). I am concerned with these facts mainly because they touch upon the crucial contentions in your book and with which, as far as I can see, you establish the comprehensive view of your book. As the text of your book shows (p. 138/139) in the following sentences: “It was not until a full day after the first announcement of Hiroshima that Heisenberg began to understand how he and his colleagues had completely missed the basic principle of the atom bomb. It was only then that he finally came to understand that we had used uranium pile merely to produce material – plutonium – and out of this new substance had made the bomb. The pile itself was never intended to be a bomb. Heisenberg called his colleagues together and explained to them what it was all about. They were amazed, and crestfallen. It was all so simple. How could they ever have missed it? And how could they ever survive such a blow to German scientific prestige? … Heisenberg spoke to an Associated Press reporter about “Germany’s uranium pile, which I was building up to create energy for machines and not for bombs … As the world knows, the explosive, plutonium, is produced in such a uranium pile. Heisenberg’s statement is a beautiful example of how to use half-truths. It is true that the German scientists were working on a uranium machine and not the bomb, but it is true only because they failed to understand the difference between the machine and the bomb. The bomb was what they were after. And what the whole world knows about plutonium the German scientists did not know – until they were told about it after Hiroshima.” The underlined sentences are also emphasized in print and will therefore be perceived by the readers of your book as the most important statements; they are also cited in the newspapers. As I have mentioned, it is important to me that you realize that those sentences are false. The correct formulation should have been something like this: “The German physicists knew the difference between an uranium machine and an atom bomb. They also knew since 1940 that it was possible to obtain atomic explosives from the uranium burner and, since 1942, they knew that the element 94 (Pu) was involved. In any case, they knew enough about the production and manufacture of atom bombs to realize, that such a production of bombs could not succeed in Germany during the war. For this reason they were spared the moral decision of whether they should begin making atom bombs, and so they worked only with uranium machines.” You admitted in your last letter that the formulation concerning the question of plutonium was correct. But regarding the fast neutrons and, therefore, the difference between uranium burner and bomb, you seem to have reservations. In order for you to realize that you were mistaken in your book, I would like to ask you to study a secret report by Houtermans (pertaining to the issue of triggering of nuclear chain reactions, parag. III: chain reactions through nuclear fission with fast neutrons, especially Fig. I) which you might have missed; in those days I discussed the original research about this quite regularly with Houtermans. I must also repeat my question, voiced in my last letter, namely, what in your opinion was meant by our remark concerning the use of protactinium as an explosive and by my diagrams in my Luftwaffe report, if it was not the fact that we certainly knew about chain reactions with fast neutrons. I have also asked some of the people who were present at the meeting with Speer on June 6, 1942 which details they could still remember being discussed regarding the atom bomb issue. It turned out that some of the participants of that meeting, especially Miss Bollmann, a secretary from the Max Planck association, and Dr. Telschow, recalled the following incident: After the lecture General Fieldmarshal Milch asked me how large the bomb might be and whether it would be powerful enough to destroy a big city. I then answered that the bomb, that is its active part, would have the approximate size of a pineapple. Of course this statement attracted attention especially among the non-physicists, who remembered it well. Since you probably have the list of participants of that meeting there in America, you could ask some of the gentlemen in question. Finally I would like to mention that, prior to the publication of my report in Science, I naturally showed it to the involved members of the “Uranium Club” and that only after all of those people at the “Uranium Club” agreed with its contents, was it published. Because there really can not remain any doubts about this entire matter, I would like to ask if you couldn’t publish a rectification of the statements made on pp. 138/139 of your book, perhaps in one of the “Atomic Bulletins”; just those assertions in your book have been repeatedly picked up and cited by the press and have led to a broad proliferation of wrong impressions about German atomic physicists, which can really be of no interest to anyone. For the time being I would like to refrain from dealing with some of the other statements in your letter, because I feel that many of the formulations are incorrect, but I want to avoid the deepening of our differences by continuing a critical discussion. I had hoped that, after agreeing about the facts, we could also come to terms with the motives, and I am loath to relinquish that hope. One last comment: according to the latest information, Thüring and Stuart do not hold a position at a university any longer. So far, I do not know anything about Schenk. None of us would insist that the de-nazification was accomplished correctly in every instance. Many people lost their positions unjustly, and many others kept their posts equally unjustly. This problem is so hopelessly complicated that I wouldn’t know how to solve it justly. I will be happy to discuss the uranium questions with Mr. White. For the rest I agree, of course, with you and with your book that a totalitarian regime of a country harms science tremendously. I have said and written before, that I share this opinion. But in your book this notion is substantiated with false arguments and I regret that very much. With best regards, yours
W. Heisenberg.

On 4 June 1942, Heisenberg, then director of the Kaiser Wilhelm Institute of Physics, met with Albert Speer, Reichminister for Armaments and Munitions, and his top civilian and military advisors. Discussion turned to nuclear research and the atomic bomb. Heisenberg acknowledged that it would be possible to build atomic reactors (“uranium machines”). Speer then asked how nuclear physics could be used to build an atomic bomb. Heisenberg replied that the theoretical knowledge was in place but that it would take at least two years, due to the great expense and lack of such crucial resources as a cyclotron.
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