Nuclear fission was discovered in Germany in 1938. Heisenberg remained in
Germany during
World War II, ostensibly to help rebuild German science after the extensive brain drain that occurred in the 1930s as a result of Nazi policies banning Jews from government jobs, which led to the expulsion of Jewish physics professors from the state universities. Heisenberg by all accounts was loyal to Germany, but not the Nazi regime. The
Kaiser Wilhelm Institute for Physics (of which he was the Director) was appropriated by the
Nazi Heereswaffenamt (Army Ordnance Board). He belonged to a team led by
Walther Bothe to develop one of Germany's many
nuclear weapon/nuclear power programs, but the extent of his cooperation in the development of weapons has been a subject of much controversy. Heisenberg's work consisted of various efforts to create sustained fission reactions. A rival atomic bomb project was led by
Kurt Diebner for Heereswaffenamt, who, with
Paul Harteck worked on
uranium enrichment and a uranium-based atomic bomb. Neither team was successful before the end of the war, because of various factors including complications from various invasions toward the end of the war and lack of funding from the government.
There has been speculation that Heisenberg had moral qualms and tried to slow down the project. Heisenberg himself may have attempted to paint this picture after the war, and Thomas Powers' book
Heisenberg's War and
Michael Frayn's play
Copenhagen adopted this interpretation. This is because during a June 1942 meeting with
Albert Speer, the minister for Nazi munitions, Heisenberg did not champion the project in a way which got it much attention or funding (which
Samuel Goudsmit of the
Alsos project interpreted as being partially because Heisenberg himself was not fully aware of the feasibility of an atomic bomb). At best, he tried to hinder the German project; at worst, he was just unable to create an atomic bomb.
The debate about Heisenberg's views on the use of atomic weapons is centered on the period from 1939–1942, during which time Germany made a decision not to pursue a nuclear weapons programme. During this period, several events give insight into Heisenberg's role in that decision. At various points evidence during the period suggested that Heisenberg was deliberately steering Germany's research efforts toward developing nuclear energy, rather than nuclear weapons. Some evidence suggests that Heisenberg attempted to communicate these views to the Allies. For example, in April 1941 a German Jewish physicist,
Fritz Reiche, arrived in the United States bearing a message from Heisenberg's colleague and friend
Fritz Houtermans which was relayed to American officials in the following handwritten note:
:"a reliable colleague [Houtermans] who is working at a technical research laboratory asked him [Reiche] to let us know that a large number of German physicists are working intensively on the problem of the uranium bomb under the direction of Heisenberg, that Heisenberg himself tries to delay the work as much as possible, fearing the catastrophic results of a success." (Thomas Powers,
Heisenberg's War: The Secret History of the German Bomb.)
Next, there was Heisenberg's visit with an old friend Niels Bohr in occupied
Copenhagen in September 1941, the purpose of which has been the subject of great debate. Further, German scientist Hans Peter Jensen visited Niels Bohr in Copenhagen during 1943, of which Bohr wrote that Jensen
:"talked [about] efforts to increase the production of
heavy water in Norway and hinted in this connection that the German physicists were only considering general technical energy generation."
Finally, in May 1943, the German spy Erwin Respondek passed a report to Sam Woods, an American consular official in Zurich, that
:"the Kaiser Wilhelm group [where Heisenberg was chief of theoretical work in Berlin] purposely raised 'difficulties' to slow down work on the project." (Thomas Powers,
Heisenberg's War: The Secret History of the German Bomb.)
According to some Heisenberg critics, the German war efforts stalled in 1940 not because of moral qualms, but because Heisenberg had made a gross overestimate of the "critical mass" of fissionable material (Uranium 235) required for a bomb.
An estimate of this amount was crucial to the decision about proceeding with a serious nuclear weapons program because of the enormous difficulty and expense of separating the
U235 from the
U238 that makes up the vast bulk of natural
uranium and the length of time it would take to develop a reactor capable of transmuting the uranium into
plutonium.
According to some critics, Heisenberg had miscalculated the "critical mass" by not taking into account the "
drunkard's walk" trajectory of the slow
neutrons emitted, thereby overestimating the amount needed as being in the order of
tons, not
kilograms as was in fact the case.
However, the contention that Heisenberg had wrongly determined in 1940 that a uranium bomb was not technically feasible is at odds with other evidence. First, during the 1941 visit with Bohr, Heisenberg stated that
:"in the preceding years [Heisenberg] had devoted [him]self almost exclusively to the question and were quite certain that it could be done," and that he "felt certain that the war, if it lasted sufficiently long, would be decided with atomic weapons."
According to Bohr's later notes,
:"Heisenberg said explicitly that he did not wish to enter into technical details but that Bohr should understand that he knew what he was talking about as he had spent 2 years working exclusively on this question."
It is unclear why Heisenberg would report to Bohr in 1941 that his research efforts had led him to conclude that a usable nuclear weapon was feasible if, in fact, a miscalculation in 1940 had led him to conclude that it was not feasible.
Second, after the war, Heisenberg and other German physicists were taken by the British to
Farm Hall, where their conversations were monitored. The transcripts, however, are ambiguous and subject to debate. At points, it appeared that Heisenberg had miscalculated the
critical mass of uranium required for an atomic bomb—covert eavesdropping revealed that, on hearing of the Allied
bombing of Hiroshima, he was at first convinced it was a
propaganda trick, so sure was he that the critical mass was impracticably large. Some historians have questioned the reliability of the transcripts, as Heisenberg probably knew he was being monitored.
Indeed, there are indications that Heisenberg had made the correct calculation earlier. In June 1942, Heisenberg answered a question about the size of the fissionable core of a bomb by holding his hands to suggest something the size of a football or pineapple, which would have been roughly right. Indeed, after presenting the "incorrect" calculation to the Farm Hall scientists (including those sympathetic to the Nazi regime), one of Heisenberg's confidants,
Otto Hahn, questioned Heisenberg's remark that "tons" of U-235 were needed for a bomb, "But tell me why you used to tell me that one needed 50 kilograms of 235 in order to do anything. Now you say one needs two tons."
Later, Heisenberg told
Hahn,
:"Quite honestly I have never worked it out as I never believed one could get pure 235. I always knew it could be done with 235 with fast neutrons. That's why 235 only can be used as an explosive. One can never make an explosive with slow neutrons, not even with the heavy water machine [the German nuclear reactor], as then the neutrons only go with thermal speed, with the result that the reaction is so slow that the thing explodes sooner, before the reaction is complete."
Ultimately, upon seeing the reports of the bombing of Hiroshima, Heisenberg told his friend,
von Weizsäcker
:"I was absolutely convinced of the possibility of our making an uranium engine [reactor] but I never thought that we would make a bomb and at the bottom of my heart I was really glad that it was to be an engine and not a bomb. I must admit that."
Whatever the cause, it is clear that on
4 June, 1942, Heisenberg met with German Minister
Albert Speer concerning possible uses of Heisenberg's nuclear research, and particularly its potential suitability for the development of nuclear weapons. Notwithstanding Heisenberg's September 1941 report to
Bohr that he felt certain nuclear weapons could be constructed and powerful enough to conclude the war if it lasted long enough, during this meeting with Speer he highlighted the technical difficulties and vast time and materials required to separate the uranium needed for the project.
It was this meeting, and Speer's report on it to
Hitler, that effectively scuttled any military applications for his work, and limited Heisenberg's work during the remainder of the war to theoretical uses of nuclear energy. As Speer wrote,
:"Difficulties were compounded, Heisenberg explained, by the fact that Europe possessed only one
cyclotron, and that of minimal capacity. Moreover, it was located in
Paris and because of the need for secrecy could not be used to full advantage."
Curiously, albeit perhaps tellingly, Heisenberg did not mention the cyclotron in
Copenhagen as a possible source for enriching uranium.
Heisenberg (1946) also proposed an enhancement of
Kolmogorov's model of
turbulence, to explain the mechanism of transfer of energy from large to small eddies.
