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#6012 - Werner Heisenberg Typed Letter Signed with Hand-Drawn Diagrams
On the atom bomb and Nazi government: "The factual construction of the atom bomb pre-supposed the solving of several difficult physical questions"
TLS in German, five pages, 8.25 x 11.75, Max Planck Institute for Physics letterhead, January 5, 1948. Significant letter to Professor Samuel A. Goudsmit of the Northwestern University Department of Physics, discussing reactions of uranium and scientific research in Nazi Germany; the letter itself is four pages long, and includes an additional page with hand-drawn diagrams by Heisenberg depicting chain reactions in uranium 235 and uranium in heavy water. In full (translated): "Many thanks for your letter and for your desire to clear up the differences of opinion which still remain between us. The most pressing difference of opinion between you and me seems to deal with the question whether the German physicists knew that an atom bomb functions because of the chain reaction with fast neutrons, and whether they knew that one can make atom bombs from 235U or 239Pu.
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Once more I would like to emphasize here that you obviously drew the wrong conclusions from the many data that were at your disposal, when you wrote your report about this issue and that, in any case, the true nuclear physicists in Germany (here I mean the more intimate circle of people whom you know, such as Bothe, Weitzsäcker, Wirtz, Bopp, Harteck, Houtermans, Jensen, Flügge, and perhaps a few more) knew the data in their main characteristics, but that the detailed research needed to remain undone because of the reasons which I explained in my statement. The first instance (chain reaction with fast neutrons and 235U) seemed fundamentally self-evident to us since the well-known research of Bohr and Wheeler had been published,—even though I fully realized that the factual construction of the atom bomb pre-supposed the solving of several difficult physical questions, which you did solve and which we did not touch upon because of the oft-mentioned reasons. The deliberation about the density number 239, as I wrote in my report, can be found in a secret report by Weizsäcker from the year 1940. At that time it was not clear whether the 23-min.-substance 239U, which Hahn has discovered, would result in only one or perhaps two Beta-disintegrations. Therefore, Weizsäcker did not know then whether the presumed explosive would carry the cargo 93 or 94. After the discovery of the 23-min.-substance it was also obvious that the 239U explosives, set off by sympathetic detonations, existed, and the stipulation that the substance would react like 235 at the moment of fission was evident from the Bohr-Wheeler research.
As proof of this statement about the German physicists I would like to point out one item contained in one of the secret reports which we, by chance, still have in our possession (article by Bothe about fast neutrons), where it is conjectured that pure Pa in sufficient amounts would explode by way of the chain reaction of fast neutrons. Of Pa it was known that it did not split at all when combined with slow neutrons. How, in your estimation, could the sentence about protactinium be explained, if not with the certainty that we here knew about the possibility of chain reaction with fast neutrons? Furthermore, what in your opinion is mentioned in Weizsäcker’s secret report of 1940, whose existence you can verify by asking any of the members of the U-club, if not these things about which I wrote? Of course, there is the possibility that you did not see these reports and in that case I ask you to continue in the search for that material. In this connection I would like to allude to a report which was published together with other secret matters of the Luftwaffe, probably in the spring of 1943, which contains my lecture of February 26, 1942 and which was later repeated for a panel of Luftwaffe members. In this lecture I showed two slides which (adjusted to the niveau of a Reichminister of that time) compared the proliferation of neutrons in pure 235U with that in a pile made up of pure uranium and heavy water. I am enclosing a sketch of the two slides which I reconstructed from memory as well as was possible. I don’t have to explain to you what they demonstrate. Those two slides were kept up to the end of the war in my Berlin institute; they were probably confiscated by the Russians. As far as I know they were mentioned in that report I referred to and which you will probably find somewhere in America.
I suppose, that just by chance, you have not found the reports which would give you a clear picture about the atom bomb question. Besides, I do not look upon these facts as so very important just because I think that the above-mentioned result of our work was of special scientific merit of which we should be proud; rather, on the contrary, I feel that this entire development was practically inevitable after Hahn’s discovery and after the research of Bohr and Wheeler. I think that the great accomplishments by the American and English physicists resulted mainly from the enormous effects of technical performance, in the systematic use of vast resources, which could only be provided by the strong industrial potential of America.
Only after we have agreed on the facts should we begin to talk about the political motives. Perhaps it will be beneficial to postpone the discussion of those issues to a time when it can be done orally. I do not believe that anything can be gained by talking over any of these matters publicly.
Possibly, I should address some of the points mentioned in your letter. First of all a few words about the four sentences in your letter dealing with the American opinion regarding German science under National Socialism: To 1) and 2): I have always felt that pure science suffered enormously under the National Socialist regime, first of all certainly because of the expulsion of so many able scientists from Germany; secondly, because of the inroads the absurd ideological theories made (for instance, cosmography, or 'German' physics, etc.) Occasionally I expressed my views not only in the company of trusted colleagues, which would have been quite paltry, but I stressed them repeatedly also in a number of presentations to the appropriate German government institutions at a time when it was quite dangerous to do so. In this way I tried to bring about a change. To 3) I have already replied. To 4) It would never occur to me to believe that the German physicists were different from their colleagues of the Allied Forces. But I can’t imagine that you didn’t realize that the German physicists worked under other psychological conditions than their colleagues in England and America, and that the German physicists stood in opposition to the philosophy of life of their regime in contrast to the Allied physicists who worked for one goal together with their country’s people.
Another thing you mention: the 'complacency' of the German physicists. I believe that in every meeting with higher political officials, which took place in regard to uranium projects, I energetically pointed out that the American physics set-up was much better equipped with people as well as with laboratories than was the case in Germany and that, therefore, it could be expected that the American physicists would solve the problems of atomic energy much faster than the Germans would be able to, if the latter could tackle these issues at all during the war. Again it would seem paltry of me to repeat these facts pointedly, now that there is no longer any danger involved with making such statements. Besides, you will find in the depiction of the false conclusions written in pencil that I did not intend to write a 'success story' but rather an objective account.
Perhaps I should also say something about the political attitude here, which you chose to characterize as a compromise with Nazism. During that entire time, I was never in doubt about the fact that the German regime consisted in its most official positions of fools and scoundrels. However, I also knew that, if the German people failed to undermine and ultimately abolish that political system from within, a great catastrophe would befall the country which would take the lives of millions of innocent people in Germany and in other countries. You write: 'Why didn’t you sense the hopelessness of ever being able to convince Himmler?'—I was not naïve enough to believe that we had any chance to succeed before the catastrophe broke over us. But even now I feel that I would have shirked my duty unpardonably if I hadn’t tried my utmost, at least in my small sphere of influence, to breach the delusions of the political overlords, hoping all the while that others in their positions would then act accordingly in the same manner. Himmler’s letter proves to me that such an attempt was not taken without danger ('We can’t afford to kill this man') but also that it was not entirely hopeless. By the way, I had discussed my efforts beforehand with Bohr and he emphatically supported them,—not because he had any illusions about the results, but because he correctly assumed that everything had to be tried. I have never been able to understand those people who withdrew from all responsibilities and who could assure others during some harmless dinner conversation: 'You see, Germany and Europe will be annihilated, I have said so all along.'
I think that, considering the affairs of the world as a whole, it would have been better if National Socialism could have been replaced by something saner from within instead of being abolished from the outside by force of arms. I also feel that it would have been desirable if the group of people who attempted real opposition to Hitler (about whom you can now read in several books) could have found understanding and support in foreign countries.—It is not easy to guide people toward beneficial objectives through force of arms and, right now, because of the indescribable misery, Germany is not a fertile ground for being influenced by beneficial ideas, which all seriously engaged people know to be necessary. What we need at this time in Germany is not hate-filled reckoning with the past but a quiet reconstruction and the slow beginning of humane living conditions. You know from our discussions in August 1939 that I have opted for this goal which I have attempted here to explain to you.
In other respects, be assured that the German physicists too would like to participate in any effort being made to attain a 'better understanding.' But unfortunately, even the ancient history of our discussions in Ann Arbor and Heidelberg proves how incredibly difficult it is to really understand another human being. Nevertheless, we must not despair and we must redouble our efforts toward a reconciliation." In fine condition, with minor edge paper loss.
By late 1941, the German government began to deliberate the construction of a nuclear reactor, the primary step in assembling an atomic bomb. The Reich Research Council’s House of German Research held a conference for this project on February 26, 1942, and Heisenberg delivered a speech on ‘The Theoretical Foundations for Energy Acquisition from Uranium Fission.’ His nine-page paper discussed topics such as the so-called “fast neutrons” resulting from nuclear fission that have lost little of their energy through collision; “heavy water” (deuterium oxide), used as a moderator in some nuclear reactors; and the release of nuclear power through controlled chain reactions. The included drawings reveal Heisenberg’s theoretical knowledge at the time and appear to be the only remaining record of his above-mentioned slides. While the Research Council’s influential guest list could not have understood the practical relevance of Heisenberg’s remarks to atomic bombs, his talk nevertheless spurred the Research Council to assume control of nuclear research from the army.
On May 3, 1945, Heisenberg was one of ten German scientists captured in Urfeld by Operation Alsos, which the recipient of this letter, Samuel Goudsmit, served as chief scientific advisor. In 1947, Heisenberg, now the director of the Max Planck Institute for Physics, published two articles on the failure of the German nuclear weapon project Uranverein in the journals Die Naturwissenschaften and Nature. In addition to stating that it was the lack of resources and technical support that stymied the German effort, Heisenberg insisted that their research was constricted to the investigation of peaceful uses of nuclear energy. Goudsmit dismissed Heisenberg’s claims of any sort of benevolent research, maintaining that the Uranverein’s main objective was to build weaponry, and that the scientists involved simply failed in their execution. A detailed and highly informative letter that finds Heisenberg continuing to defend his role and actions as a member of the Uranverein.