by Philip Ball
What did Heisenberg expect and hope to achieve by meeting with Bohr? Some say he was attempting to sound out his former mentor about Allied work on nuclear weapons.*3 Others say that Heisenberg sought Bohr’s approval for his nuclear research, or his opinion on the likelihood of harnessing uranium fission. Heisenberg himself implied later that he wanted to discuss with Bohr the idea of securing international scientific cooperation over the uses of nuclear energy, and in particular to engineer an embargo on bombs. Weizsäcker insisted on this view, writing as late as 1991 that ‘the true goal of the visit by Heisenberg with Bohr was . . . to discuss with Bohr whether physicists all over the world might not be able to join together in order that the bomb not be built’. This self-serving claim had been uncritically reported by Robert Jungk in 1956. David Cassidy, a leading expert on the nuclear age and on Heisenberg in particular, suggests that the possibility of making trans-uranic elements by nuclear transmutation had made the shadow of an atomic bomb suddenly loom alarmingly, driving Heisenberg to seek moral guidance from a father figure.
Whatever his motivation, the fact is that Heisenberg accepted an invitation to lecture on his work in astrophysics and cosmic rays during a conference at the German Cultural Institute in Copenhagen, and he travelled there with Weizsäcker, who had worked at Bohr’s institute in 1933–4.
What also seems unambiguous is that Heisenberg and Weizsäcker alienated their Danish peers with their insensitivity to the hardships of occupation. As Bohr recalled, ‘Heisenberg and Weizsäcker sought to explain that the attitude of the Danish people towards Germany, and that of the Danish physicists in particular, was unreasonable and indefensible since a German victory was already guaranteed and that any resistance against cooperation could only bring disaster to Denmark.’ Some accounts say that Heisenberg even called the war a ‘biological necessity’. Lise Meitner meant it as no compliment when she told Hahn after the war that ‘[Heisenberg’s] appearance in Denmark in 1941 is unforgettable.’ She was not there, but in June 1945 she wrote to Debye’s former colleague Paul Scherrer describing what she’d heard about the event. This second-hand account may be somewhat exaggerated, but it offers a striking picture of the impression that Heisenberg left:
I have heard very peculiar things about him from young Danish colleagues, about when he came to Copenhagen in 1941 together with W[eizsäcker] to stage a German physical conference and absolutely refused to see the unfairness of it. He was completely infatuated with the chimera of a German victory and set forth a theory of superior men and nations over which Germany was meant to rule.
Heisenberg sensed the hostility he aroused in Copenhagen, and was bemused by it. ‘It is amazing, given that the Danes are living totally unrestricted, and are living exceptionally well’, he wrote to his wife, ‘how much hatred or fear has been galvanized here.’ Why, they had even refused to attend his talk in the Cultural Institute, simply because the institute had previously hosted ‘a number of brisk militarist speeches on the New Order in Europe’. Weizsäcker forced that issue by taking the head of the Cultural Institute, without an appointment, to see Bohr, who had no wish to be put in such a situation.
Looking back in 1948, Heisenberg recalled that their crucial discussion in Copenhagen had occurred while they were walking through the expansive Faelledpark in the centre of the city. Here he said that he had asked Bohr about the morality of working on nuclear energy, but with reference only to reactors, since he claimed still to believe that bombs would not be feasible until well after the war was over. If this was indeed what was on Heisenberg’s mind at that time, it would be somewhat surprising, since there is not a single known indication that he or any other of the German physicists thought about such moral issues during that period, or indeed until confronted with Hiroshima. One rather strained interpretation is that Heisenberg hoped to signal to Bohr, without saying it explicitly, that the Germans were far away from making an atom bomb, so that Bohr might use his contacts to convey this information to the Allies and perhaps avert any attempt by them to do so. In other words, this was a covert effort to keep the world free of such weapons of destruction.
‘Because I knew that Bohr was under surveillance by German political operatives’, Heisenberg later told Jungk, ‘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 any more 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. I then asked Bohr once more whether, in view of the obvious moral concerns, it might be possible to get all physicists to agree not to attempt work on atomic bombs, since they could only be produced with a huge technical effort anyhow. But Bohr thought it would be hopeless to exert influence on the actions in the individual countries, and that it was, so to speak, the natural course in this world that the physicists were working in their countries on the production of weapons.
Could it have been a suspicion that his account would be challenged that led Heisenberg to add a mitigating comment? In any event, he qualified his statements to Jungk thus:
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 . . . Even now, as I am writing this conversation down, I have no good feeling, since the wording of the various statements can certainly not be accurate any more, and it would require all the fine nuances to accurately recount the actual content of the conversation in its psychological shading.
He was right to anticipate contradiction. Bohr never quite forgave Heisenberg for his conduct on that visit, although the reserved and cordial Dane did manage to resume polite social relations after the war. Yet when Bohr saw Heisenberg’s account of their meeting in Jungk’s book, he was so upset by its egoistical nature that he wrote an uncharacteristically angry letter that he redrafted several times but was never able to bring himself to send. Bohr was particularly incensed by Heisenberg’s claim that he had dissuaded his fellow German physicists from trying to make a bomb, and that he had suggested to Bohr the idea of an international boycott on research towards nuclear weapons. He had said that Bohr seemed ‘slightly frightened’ by the idea that such weapons could be made—as though this had never occurred to the Danish physicist until then.
‘Personally, I remember every word of our conversations, which took place on a background of extreme sorrow and tension for us here in Denmark’, Bohr wrote in his unsent letter:
In particular, it made a strong impression both on Margrethe [his wife] and me, and on everyone at the institute that [you] expressed your definite conviction that Germany would win and that it was therefore quite foolish for us to maintain the hope of a different outcome of the war and to be reticent as regards all German offers of cooperation. 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.
In another draft, he added ‘you informed me that it was your conviction that the war, if it lasted sufficiently long, would be decided with atomic weapons, and [I did] not sense even the slightest hint that you and your friends were making efforts in another direction’.*4
Bohr remained in Copenhagen for a perilously long time. Having a Jewish mother, he was officially ‘non-Aryan’, although at first the Germans were relatively lenient with the Danish Jews in order to maintain the fiction that they were in the country at the invitation of the Danish government. But in 1943 their exemption from the concentration camps was terminated, and in the early autumn the Nazis began to deport prominent Danish Jews. Bohr was tipped off about his own impending arrest at the end of September, and in early October he escaped by boat to Sweden. Fearing that he might be assassinated there by German agents, the British flew him from Stockholm to England. At the end of the year he flew to Los Alamos, where he contributed little to the technical work—‘They didn’t need my help in making the atom bomb’, he attested—but immensely to morale. ‘He made the enterprise seem hopeful’, Robert Oppenheimer, the scientific leader of the Manhattan Project, later wrote.
After the Copenhagen visit Heisenberg continued to give scientific talks throughout the territories occupied by Germany, to the satisfaction of the National Socialist leaders. According to the Dutch physicist Hendrik Casimir, during a subsequent visit to Holland in 1943 he claimed that the German domination of Europe was justified thus:
History legitimizes Germany to rule Europe and later the world. Only a nation that rules ruthlessly can maintain itself. Democracy cannot develop sufficient energy to rule Europe.†5
If it were not Germany, he warned, then Europe would be run by the Soviet Union, which would be far worse. The prospects for the German war were by then looking very dim, but Heisenberg’s own prestige was never greater.
While there is now no definitive way to discover what was really said in Copenhagen, we can certainly make inferences about Heisenberg’s role in wartime German science. He was in Copenhagen on behalf of the conquering power, and he anticipated that the Danish scientists would be reassured in seeing it represented by a friendly face. In other words, he felt that his personal status would somehow negate all the indignities of occupation: the same kind of grandiosity that left Heisenberg convinced that he must remain in Germany come what may, since only he could rebuild German science after the war. He had become lost in self-regard.
Saving face
Why didn’t the Germans build an atomic bomb? Would they have done so if they could?
After the war, Heisenberg was determined to show that he and his colleagues had been in command of their situation and had engineered its outcome. ‘From the very beginning’, he wrote in a description of German nuclear research published in Nature in 1947 (a translation of a piece that first appeared the previous December in Naturwissenschaften), ‘German physicists had consciously striven to keep control of the project.’ This, he said, was made possible by the convenient fact that making a bomb was neither impossible (in which case the question of whether they should would not have arisen) nor easy (in which case they certainly couldn’t have prevented it):
The actual givens of the situation, however, gave the physicists at that moment in time a decisive amount of influence over the subsequent events, since they had good arguments of their administrations—atomic bombs probably would not come into play in the course of the war, or else that using every conceivable effort it might yet be possible to bring them into play.
In 1968 Heisenberg offered a somewhat less triumphalist version of this story:
Obviously we were not fully aware of the extent of the danger, but then in the first two years the following became evident: it was possible to build nuclear reactors relatively easily with moderate means, i.e. certainly in the period of a few years available to us, that is, that particular reactor we knew would work, namely a reactor using natural uranium and heavy water. It was also clear that an explosive is produced in such a reactor [plutonium] from which atom bombs could be made; but luckily it was also clear then that this would involve an enormous technological investment lasting many years. Therefore we could report these results with complete honesty and a good conscience to the government agencies, and the consequence was—just as we had hoped—that the government decided to make no effort to construct atom bombs, but that we received certain—albeit modest funds [sic]—to continue work on the design of a reactor, precisely on a heavy-water reactor.
This is a carefully constructed tale. It emphasizes that the physicists were totally competent: they knew the reactor would work, they knew bombs could be made from it, they knew that would be extremely difficult (although, given the relative ease of separating plutonium, Heisenberg overplays this technical challenge). They provided the Nazi leaders with honest information, albeit tailored to secure continued funding without committing them to a bomb. Moreover, by stressing the modesty of those funds, Heisenberg can explain why they didn’t get as far as the Manhattan Project. This judicious management of the situation meant that, as he put it in Nature, they were ‘spared the decision’ of whether to work on a bomb for Hitler. Such statements by Heisenberg after the war made no mention of his explicit promise to the authorities in 1942 that a ‘uranium machine’ could produce a powerful explosive.
Thus Debye’s half-humorous account to Warren Weaver of how the German physicists considered that they were duping the authorities into funding fundamental research was, for Heisenberg, the full reality of the matter:
The official slogan of the government was ‘We must make use of physics for warfare.’ We turned it around for our slogan: ‘We must make use of warfare for physics.’
Ours was the noble goal of knowledge, Heisenberg is implying, which we pursued by a clever ruse. The story places the scientists at the helm, reduces their political leaders to a bunch of credulous fools, makes their state-funded military research almost an act of resistance, and separates pure, untainted science from the nasty realities of the Nazi regime.
But how much of it can we believe?
The suggestion by Heisenberg and his colleagues that they purposely slowed the pace of research to keep a terrible weapon out of Hitler’s hands was furiously disputed by Samuel Goudsmit. He insisted that the scientists would certainly have made a bomb if they had been able, but that they were prevented from doing so both because of the incompetence of their political management and because they themselves did not understand how to do it. He wanted Heisenberg to acknowledge what Nazi rule had cost German science, and thus to show how science can flourish only in a free society. But he also wished to see some recognition in the German physicists of their own arrogance and complacency, thinking that they alone could solve the problems of harnessing nuclear fission. Heisenberg, however, refused to accept that the best German scientists—not Nazi pawns like Diebner—were anything other than highly competent. The two men entered into protracted and sometimes intemperate correspondence, but neither seemed willing to recognize the real obstacle to the German bomb: that the Nazis were never sufficiently persuaded of its feasibility to allocate resources on the scale of the Manhattan Project. Given that, Heisenberg was right to say that they were spared the ultimate moral decision. All the same, Goudsmit put his finger on the crucial point, which applied not just to Heisenberg but to almost all of the German physicists: he ‘fought the Nazis not because they were bad, but because they were bad for Germany, or at least for German science’.
Heisenberg was stung by Goudsmit’s imputation that the Germans hadn’t made a bomb because they never figured out how—an idea promoted also by Leslie Groves in Now It Can Be Told. Their sense of intellectual superiority had suffered badly from the discovery that the Allies had been so far ahead of them, and while they insisted that the reason was solely the level of funding provided by the respective state leaders, that suspicion remained: had the Germans not argued more strongly for support
because they were mistaken about the magnitude of the task?
A key aspect of this question relates to the amount of material needed to make a bomb: the critical mass above which a fissile substance would develop a spontaneous, runaway chain reaction. In the report to Army Ordnance in 1942 the critical mass of uranium-235 or plutonium was estimated as ‘presumably around 10–100 kilograms’. Yet even this vague figure seems not to capture the extent of the uncertainty. The estimates were probably made by Heisenberg, whose statements on the matter give a very confusing picture. When he heard about the Hiroshima bomb at Farm Hall, Heisenberg reacted with disbelief because he could not believe that the Allies could have produced several tons of pure uranium-235. To this, Hahn responded, ‘I thought that one needed only very little “235” . . . if they have, let us say, 30 kilograms of pure “235”, couldn’t they make a bomb with it?’ Heisenberg replied that ‘it still wouldn’t go off’: he didn’t believe it was enough. ‘This statement’, comments Jeremy Bernstein, ‘shows that at this point Heisenberg has no idea how a bomb works.’
If Heisenberg had forgotten his earlier estimates of the critical mass, Hahn had not. ‘But tell me why you used to tell me that one needed 50 kilograms of “235” in order to do anything’, he demanded. ‘Now you say one needs two tons.’ Heisenberg was evidently disconcerted, and he dissembled: ‘I wouldn’t like to commit myself for the moment.’ He went away and made a better calculation, deciding that indeed only a few tens of kilograms would suffice.