Interview with the Spiegel Magazine,

Published in No. 28 on July 3, 1967

Spiegel: Professor Heisenberg, do you think that your role in the German atomic project – as the leading theoretician who, so to speak, has pointed the mathematical way to atomic energy for the Germans – is portrayed accurately in the SPIEGEL series by the British historian David Irving?

Heisenberg: Let me start by stating that I never had complete organizational authority – as for instance Oppenheimer had in the United States. I was, like others, supposed to work out the theoretical foundations - even if only to know what possibilities the other side, the United States, had -, and to advise those people who did the experiments. In addition, I was head of the Kaiser–Wilhelm-Institut in Berlin-Dahlem, starting in 1942. But I did not, for instance, have to make decisions concerning any experiments done outside of my institute.

Spiegel: This probably became apparent in the experiment of 1940 of Professor Bothe in Heidelberg. Bothe was to clarify in an experiment whether pure carbon – graphite – would be suitable as a moderator in a nuclear reactor, and in this measurement an error occurred. It was Irving’s opinion that this error became a significant obstacle in the German atomic bomb project.

Heisenberg: On theoretical grounds, we surmised that a nuclear reactor could be built. The measurement of Bothe seemed to indicate that it could not. Nevertheless, we had no reason to doubt the validity of Bothe’s measurement, since theory provided only an estimate and not an exact calculation, because the state of theory of the atomic nucleus at that time would not yet have made that possible.

Spiegel: Did Bothe’s erroneous result decide the race for the atomic bomb as Irving assumes?

Heisenberg: There never was a race for the atomic bomb; instead, there were, on the German side too, efforts to build nuclear reactors. Naturally, Bothe’s measurement influenced that, but one must also not overestimate this influence. It is true: Had we known it can also be done with graphite, we could have spared ourselves all the efforts with respect to heavy water as a moderator. On the other hand: We knew that it could be done with heavy water. And, production of heavy water was technically not too difficult.

Spiegel: Would it have been simpler in Germany with graphite?

Heisenberg: The industry would have had to produce large amounts of very pure graphite instead of heavy water. Technically that would have been simpler, certainly cheaper and faster also.

Spiegel: If German physicists had used graphite instead of heavy water, would they have, during the war, achieved a chain reaction in a reactor, the prerequisite for building an atomic bomb?

Heisenberg: Probably, but in the big picture of the atomic energy developments it would still have changed only very little. Let’s assume we had in 1943/44 – anything earlier was highly unlikely - managed to finish a critical reactor. Then every agency in charge of armament that wanted to contemplate atomic bombs would have had to argue: If one now wants to produce plutonium this way, one will have to increase the expenses at least by a factor of a hundred, to invest a lot of manpower, take it away from other weapons productions, and so on. And that was really impossible in the war situation then.

Spiegel: Irving claims that German physicists up until 1942 were ahead or at least even with American atomic research.

Heisenberg: That is not quite right. In Germany we knew what Americans also knew, namely that and how atomic energy in principle can be used and technically applied. Thus I would say: Somewhat equal in knowledge, not equal in what already at that time had begun, or had been prepared in terms of organization in the US.

Spiegel: German physicists knew from their calculations how many kilograms of Uranium 235 are needed to build an atomic bomb – and these numbers, it appeared after the war, agreed with those on the American side?

Heisenberg: Most numbers of this kind, not just regarding the size of the atomic bomb agreed almost completely, except for the aforementioned Carbon measurement. On both sides physics, of course, looks the same, and on both sides work was done properly. This is why then – roughly during the fall of 1941 –there came this shocking realization of the physicists, probably also on the American side: it is really possible, one can build atomic bombs. But simultaneously with the shock there came for us also the recognition: It is possible – but only with a tremendous technical effort, and that, thank God, we can not at all afford in Germany. Looking at the tremendous effort, we had hoped the Americans too might opt out, as they would probably win the war faster without an atomic bomb.

Spiegel: You mean, if these resources used for the atomic bomb had instead been invested in other weapons of war?

Heisenberg: Quite right – it turned out to be true, and one can not stress this point enough - Americans used a rather significant portion of their total armament resources on these atomic works. If instead they had invested everything into, let’s say, airplanes and tanks, then Germany would probably have lost the war even sooner. What would have happened in the American war against Japan, I can not presume to judge.

Spiegel: How come that the in 1939 rather strong initial interest by the Ministry of Ordnance in Berlin took such a turn later on? Isn’t it astonishing how the Ministry of Ordnance hurriedly collected the German atomic physicists in 1939?

Heisenberg: The interest of the official German agencies from the beginning was quite uneven. When, in 1938, Hahn and Strassmann in Germany discovered Uranium fission, the whole world reported on it extensively with huge banner headlines. American physicists, I have been told, who were gathered at a conference and were informed about Hahn’s discovery, immediately sat down with their instruments and measured day and night and then, still at the conference, confirmed: Hahn is right. Uranium fission was considered in the newspapers a discovery of the highest rank. But in Germany nobody heard about it except the physicists.

Spiegel: Hahn’s success was essentially silenced.

Heisenberg: In Germany, all the newspapers were officially sleeping, and probably had to sleep, essentially only specialists knew about this matter. And then in the summer of 1939 Dr. Siegfried Flügge, an assistant to Otto Hahn at the time, wrote in the journal "Die Naturwissenschaften" a paper in which he mentioned the chain reaction and the possibility of substances of the utmost explosive power. It was this paper that stirred up considerable excitement. The Ministry of Ordnance might have argued: If in America they report on this subject with such emphasis, then it is obviously not completely unreasonable and we have to look into this.

Spiegel: In spite of this, the project was not initiated in a big way. Irving sees the reason for this, at least partially, in the German leadership’s not being informed, or being informed only incompletely.

Heisenberg: I don’t believe one can describe it that way.

Spiegel: Irving reports that Hitler was officially informed about the atomic project only once, namely by the Minister of armaments, Speer, after you and other physicists had presented a talk to the armaments people in 1942. Mr. Speer confirmed this to us these days.

Heisenberg: I would say that in this frequently mentioned meeting in the Harnack House in June of 1942, the highest ranking leaders in the armament industry were totally correctly informed about everything. We told them that we principally saw a path towards an atomic bomb but we also immediately added that such a development would certainly take several years. By the way, if I remember correctly, at the time we were asked by the Field Marshall General, Milch, how long in our opinion it would take the Americans to finish a nuclear reactor and an atomic bomb. Our careful estimate was: Even if the Americans worked on this with full effort, they would not finish their reactor before the end of 1942 – which was half a year down the road then – and an atom bomb most likely not before the end of 1944. Actually, we believed it would take very much longer.

Spiegel: Your prognosis, at least in respect to the nuclear reactor, was correct.

Heisenberg: There is just one point that Irving doesn’t see right: The German leadership was – contrary to Irving’s opinion – in the summer of 1942 quite logical and consistent in not ordering an attempt to produce atomic bombs. Irving states: The Germans knew they could build reactors using heavy water, that such a reactor would generate plutonium from which they could build bombs. But then he continues: It seems strange that after the meeting in 1942 – where all this had been presented to the top armament leadership– nothing happened, no financial resources were requested and the government did not order a program. But in reality this was quite understandable because then the war situation was stressed to such a level that there was no breath left for such long-term developments.

Spiegel: And yet the V1 and V2 projects were cranked up.

Heisenberg: Supposedly there was an order by the Führer – which I myself have never seen in writing, however, – according to which all developments that did not lead to military applications within half a year were actually forbidden. It is true that the V1 and V2 took more time too, but the people from Peenemünde could tell the leadership: In one to two years we will be there. But we physicists knew for certain and could state with a clear conscience: in under three or four years nothing could be done. And that we were right in that respect and that the German leadership assessed the situation correctly in this instance was confirmed: Even though the Americans had a much bigger potential in scientists, engineers and material, not a single air raid, and an industry that could work 24 hours a day in total light conditions – despite all these possibilities the Americans did not end up finishing the bomb before the end of the war with Germany.

Spiegel: It seems clear that German atomic research after the meeting in the Harnack House was dragging.

Heisenberg: It was like treading water. Irving describes it quite correctly. But allow me perhaps to add here a critical comment to his otherwise very reliable report: Irving is an historian who took pains to study sources, documents, and pictures, to interview people still living - and he has done this with extreme care. Wherever facts are concerned, Irving can, based on his thorough research, report reliably and accurately. It is different with the psychological side: when it is about the motives, Irving gets into difficulties. He would have to possess an intimate knowledge of the psychological situation in a totalitarian State engaged in warfare – and this he can not have and can’t even acquire after the fact, say via conversations. And there he did not quite avoid the danger of filling in the resulting gap with stereotypes. One favorite stereotype that will probably influence the opinions for some time to come is the very " race for the atomic bomb"…

Spiegel: … between America and Germany.

Heisenberg: Irving is unconsciously presupposing this scenario as a given. But that’s not how it really was. Certainly, there were people in Germany who asked themselves: Isn’t it possible, after all, to build something like an atomic bomb. Obviously, the German leadership too had to think about those possibilities. But there was never a genuine effort, ordered from above and financed, in which, like in any arms project, thousands of people would have participated – that never existed.

Spiegel: Irving cites a number of facts that, in his opinion, acted as a brake on the development, among them, to give some examples, the reservations the NS-leadership harbored toward research in the natural sciences…

Heisenberg: All that played a role, of course. There existed up front a gaping mistrust between the national socialist leadership on one side and the atomic physicists on the other which went both ways.

Spiegel: Probably the problem with race too…

Heisenberg:…definitely played a role.

Spiegel: You were not supposed to believe in Einstein but make use of him.

Heisenberg: Yes, all this nonsense. But the decisive point was that in the summer of 1942 a different decision was made in Germany than in America. The decision in America said: now we know that atomic bombs can be built, therefore top effort toward this goal. In Germany, on the other hand, it was: this would go beyond the German industrial potential anyway, and the war will most likely be over before such weapons would play a role – thus we are not going to do it at all.

Spiegel: Under a different system, would simple curiosity not have tempted you to realize such a project, similar to what Oppenheimer once stated, that all things ‘technically sweet’ must be tempting to a physicist?

Heisenberg: It goes without saying that every physicist is under this temptation. The question is always what the other components of one’s conscience, so to speak, contribute. I once wrote in a letter to Mr. Bethe, whom I knew well, that I never would support the thesis brought forward in some newspapers or in the book by Robert Jungk that the German physicists had been morally superior to the Americans. I would always maintain that the situation in which we were was vastly different from that of our American colleagues, and that any rational person in our situation had to react the way we did. You can never second guess yourself what you would have done, if …The only thing you can say is what you are doing in a particular situation in which you have to make a decision.

Spiegel: Through the book of Robert Jungk, particularly, the public impression had been created that a moral decision had ultimately lead to, let’s say, a "Go slow" of the German physicists. Now it appears, however, as if you had never been faced with such a decision.

Heisenberg: In such areas there are never one hundred percent truths, everything is so immensely complicated and intermingled. Without question, it was convenient for us that after careful examination of all the factors, we could honestly say that in the time available during the war we could not build atomic bombs.

Spiegel: On the other hand, there is your visit with Niels Bohr …

Heisenberg: .. which does fit in exactly with this.

Spiegel: If you allow us to recall, in October of 1941 you visited the Danish physicist and Nobel-laureate, Niels Bohr, in Copenhagen and hinted at the suggestion that German and American physicists might secretly come to agree not to build the bomb. Wasn’t that an attempt to instigate a moral decision?

Heisenberg: The history of this was that we in Germany suddenly saw a path to an atomic bomb that, at least in principle, was technically feasible and this situation itself was quite horrific to us. We had unlimited trust in Bohr as one of the leading atomic physicists; his human advice was bound to matter a lot to us. Unfortunately though, in this conversation we were not really able to communicate.

Spiegel: After the war, did you talk with Niels Bohr about this visit once more?

Heisenberg: In the year 1947, a British officer who was in charge of us in Göttingen, flew me to Copenhagen, and we then spent an entire evening at the country house of Niels Bohr in Tisvilde talking about this event. We both had the impression that this talk in 1941 had turned out rather ill fated. Bohr told me in 1947 that he became so extremely shocked about my statement that now we knew one could build atomic bombs. Bohr had not kept up with this side of physics then and apparently had only through me at that visit learned that it is possible to make atomic bombs. And this had excited him to such an extent that he no longer heard or understood what else I was telling him. Later in 1943, in America, Niels Bohr only reported: The Germans know that one can make atomic bombs….

Spiegel: .. and with that ended up accelerating the development in the USA?

Heisenberg: No. The decision in America by that time had long since been made.

Spiegel: You found out about the consequences of this decision, for all practical purposes, only in 1945 while you were interned at Farm Hall and heard an announcement on radio about the deployment of the American atomic bomb over Hiroshima. How, in retrospect, did you evaluate the role of the atomic physicists in this event?

Heisenberg: I have learned that scientific discoveries lend themselves not only to positive but also to terrible consequences. We have to take pains to avoid such consequences in the future.

Spiegel: Professor Heisenberg, we thank you for this conversation.

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