by Philip Ball
Heisenberg later claimed that he had long known the critical mass to be quite small. In 1948 he told Samuel Goudsmit that in his meeting with Albert Speer and other officials in Berlin in June 1942 he was asked how large a bomb would have to be to destroy a city, and answered ‘about the size of a pineapple’. ‘This statement’, he stressed to Goudsmit, ‘of course caused a surprise especially with the known physicists and it has therefore remained in the memory of several participants.’ This was the only time Heisenberg claimed to have a definite and accurate estimate of the critical mass before his calculation at Farm Hall, and it’s not clear what reasoning lay behind it.
Paul Lawrence Rose argues that Heisenberg in fact made a severe overestimation of the critical mass in 1940, based on a misunderstanding of the physics involved, and that only after challenged by Hahn at Farm Hall did he work through the theory properly. Rose believes that Heisenberg subsequently evaded the issue because he did not want to admit his mistake, and that Heisenberg’s post-war story intentionally obscured the truth that he and his colleagues would have developed a bomb had they not been deterred by this misapprehension that the critical mass was unattainably immense.
Rose asserts that a failure to distinguish the physics of bombs from that of reactors (which cannot generate a nuclear explosion, although an uncontrolled chain reaction can lead to excessive heat production and meltdown) led the Germans to believe that one could make a kind of hybrid ‘reactor-bomb’. Indeed Heisenberg speculated, on hearing the news of Hiroshima, that perhaps the Allies had dropped such a chimeric device. And according to one version of the Copenhagen encounter, Heisenberg gave Bohr a drawing of the German bomb design when they met, yet when Bohr passed this on to the scientists at Los Alamos they thought it looked more like a reactor than a bomb. This story was told to Bernstein by Hans Bethe, who worked at Los Alamos. But Bohr’s son Aage has always strongly denied that Heisenberg gave his father such a sketch, and indeed it seems rather unlikely given the caution that Heisenberg displayed about disclosing information in their meeting. Nonetheless, the ambiguous bomb/reactor drawing does seem to have turned up at Los Alamos from some source or other.
Other historians think Rose has exaggerated the physicists’ theoretical shortcomings. Rainer Karlsch and Mark Walker say that the patent application filed by Weizsäcker in 1941 for making a plutonium bomb ‘makes it crystal clear that he did indeed understand both the properties and military applications of plutonium’. Walker says that ‘as far as they got, [the German scientists’] understanding was comparable to what the Americans and émigrés did’. He considers the accusation that the German physicists lacked technical competence to be another of the myths told about the German bomb.
Be that as it may, faced with the evidence from Hiroshima that a bomb could have been made within just a few years, Heisenberg and colleagues needed to explain why they had let their leaders believe otherwise, without having to admit that this was because of any technical error. In his 1947 Nature paper Heisenberg placed the onus for the funding decision on the Nazi government. He argued that the scientists had known perfectly well that a bomb could have been made with uranium-235 or plutonium, and that their research was on a par with that in the United States until 1942, when Albert Speer allegedly decided to withdraw most of the support and put it instead into the rocket programme. This, Heisenberg claimed, suited them fine: they could get on with research on the peaceful uses of nuclear energy that would be needed once the war was over:
We could feel satisfied with the hope that the important technical developments, with a peacetime application, which must eventually grow out of [Hahn’s and Strassmann’s] discovery, would find their beginning in Germany, and in due course bear fruit there.
That is far too tidy a picture. For one thing, if the German scientists knew as much as the Allies and were on the same footing until 1942, how come Enrico Fermi’s reactor in Chicago went critical that year while the Germans never succeeded even in building a working reactor during the entire war? The Germans pleaded disruption by bombing raids, and lack of adequate government support. But the latter, at least, does not match the fact that nuclear research enjoyed a rather high priority for resources—greater, for instance, than aircraft production. In the 1950s Speer, under arrest for war crimes, said that he hoped Heisenberg would not try to place the blame for the failure of the uranium work on him. But evidently that had already happened.
Another false trail in the ‘official’ German story is the question of the neutron moderator. Walther Bothe was made a scapegoat for the failure of the reactor project because his ‘erroneous’ measurements of neutron absorption in graphite had excluded it from consideration in favour of heavy water, shackling the efforts to the travails of heavy-water production. Heisenberg in particular laid the blame on Bothe for why the uranium research was retarded. This was not only unjust but untrue. While Bothe’s studies indeed seemed to indicate that graphite wouldn’t work, Wilhelm Hanle found subsequently that the excessive absorption seen by Bothe was caused by impurities in the material—very pure graphite should work just fine. However, the cost of such purification was deemed by Army Ordnance to be too high.
In the end, the relevance of technical matters such as the critical mass to the question of why the Germans did not make a bomb is unclear. Their lack of progress in separating uranium isotopes, and the fact that they never quite managed to make a uranium reactor that could sustain a controlled fission chain reaction, meant that even a very modest estimate for the critical mass would have seemed unattainable during the war. It is not clear that the funding decisions of the German authorities ever hinged on fine details of reactor or bomb design and engineering, about which even the scientists seemed very vague. Rose’s suggestion that all depended on what Heisenberg did or didn’t know ironically echoes the physicist’s own grandiose belief that the uranium project, if not all German physics, depended on him alone.
In the post-war years, Heisenberg and Weizsäcker oscillated between suggesting that they were passively ‘spared the decision’ of whether to make a bomb because of a lack of funding, and that they actively manipulated the situation so that there was no prospect of them ever having to face the dilemma. Weizsäcker even claimed in 1993 that he had participated in the research hoping that those with the technical knowledge of such an awesome weapon would become so indispensable to the Nazis that they might be able to influence Hitler’s policies. Did he truly think he might, then, prevail on Hitler to close the concentration camps? Weizsäcker himself seemed to sense how implausible this sounds, stressing that it was a ‘dreamy wish’ if not indeed a little crazy. In any case, how did he think such a motive would carry force unless the physicists had been able to demonstrate that they could fulfil their promise and liberate nuclear energy?
But it never seemed greatly to matter to Heisenberg and Weizsäcker whether these stories were rigorous, consistent or plausible. It was enough that they should confer some degree of moral impunity. The evidence now excavated from the war years undermines these fictions. So were Heisenberg and Weizsäcker sincere but self-deceiving, or actively attempting to mislead? Paul Lawrence Rose, inclined to believe the very worst of these men at all times, considers their stories a fantasy concocted to preserve their dignity, reputation and ‘honour’—the latter being understood in the distinctly German sense of one’s inner integrity, rather than (as others might see it) the moral orientation of one’s actions. Mark Walker, on the other hand, argues that it was not so much, or not entirely, self-interest that shaped their accounts, nor even fear of being denounced as Nazi stooges, but the fervent wish to preserve the reputation of German science. There seems in any event to be more behind them than a selfish attempt to appear blameless and untainted by Nazi corruption.
Can we, in the end, say that the German scientists tried to make a bomb or not? As Walker has argued, it isn’t a good question, precisely because it sounds as though it should have a simple answer. The words are deceptively
ingenuous. What, for example, do we mean by ‘tried’? The physicists knew it should be possible, and they undertook the initial stages of the programme, such as developing techniques for separating isotopes. They were aware that if a ‘uranium machine’ could be made to work then it would produce at least one new fissile element. ‘Explosives’ featured repeatedly, if not ubiquitously, in their appeals to the Nazi leaders. But neither the scientists nor their leaders regarded an atomic bomb as a significant priority, for none of them believed it could be done in short order. The physicists did not argue a strong case for creating a bomb because they lacked the conviction that they could achieve it in the near term.*6 A misunderstanding of the physics involved might have played a part in that, but it was probably not the determining factor. The German government did not lack the funds—the Peenemünde rocket programme cost a comparable amount to the Manhattan Project—but mercifully they, too, had insufficient faith.
Documents confiscated from the KWIP by the Russians and recently returned to the Max Planck Society hint that in fact the Germans did build an atomic bomb, after a fashion. These papers seem to indicate that Kurt Diebner’s group at Gottow produced two small ‘nuclear’ explosions in Thuringia, eastern Germany, in March 1945, killing hundreds of prisoners of war and concentration-camp inmates conscripted as slave labourers. Let alone anything else, if this dramatic claim is true then it implicates those physicists directly in war crimes.
It is perfectly possible that non-nuclear explosions occurred during the uranium research, especially because water may react with uranium to produce flammable hydrogen gas. In June 1942 one of Heisenberg’s prototype reactors at Leipzig, developed with Robert Döpel, was destroyed in a hydrogen explosion from which they were both lucky to escape unscathed. But the Gottow detonation seems to have been no accident. The accounts point to an attempt to trigger either fission of isotope-enriched uranium or, still more extraordinarily, nuclear fusion of deuterium, with a blast using conventional explosives to set off a concentric spherical device with a uranium or plutonium core. It’s certainly the kind of wild, desperate act that the last days of the war provoked. If it happened, Heisenberg and Weizsäcker seem to have known nothing about it, although allegedly Walther Gerlach was aware of and approved the tests. Yet even Diebner appears not to have believed that there had been any precursor to the Hiroshima bomb: at Farm Hall he exclaimed that ‘We always thought we would need two years for one bomb.’
Whitewash
In 1946 Heisenberg was permitted to return to a shattered Germany, where he settled in Göttingen and established the Max Planck Institute for Physics—in effect a reconstituted KWIP, now officially bearing the name that Debye had chosen. Göttingen was designated by the Allies as the hub of science reconstruction in what would soon become West Germany. Weizsäcker was made director of theoretical physics at Heisenberg’s institute. He became a pacifist, campaigned for nuclear disarmament, and founded a centre for ‘science and peace research’ in Hamburg. In 1984 his brother Richard became president of the Federal Republic of Germany and presided over the reunification. In 2009, two years after Carl’s death, the National Academy of Sciences Leopoldina, Germany’s oldest scientific society, inaugurated the Carl Friedrich von Weizsäcker Award for ‘scientific contributions to socially critical questions’. Despite this sincere concern for science’s social role, on his wartime activities Weizsäcker never clearly and unequivocally voiced a word of regret.
In September 1946 the Kaiser-Wilhelm-Gesellschaft, having been briefly dissolved, was resurrected as the Max-Planck-Gesellschaft.*7 Planck’s name, it was thought, would expunge any association with the Nazi regime, an aspiration reinforced by the appointment of Laue as president. Whether Planck himself would have accepted that symbolic role, we do not know. He too settled in Göttingen, not only elderly now but broken in spirit. He had suffered terrible personal tragedies: having lost his first son in the First World War, the life of his second had been taken by the Nazis following the plot to assassinate Hitler. Both of his daughters had died in childbirth before Hitler came to power.
Planck died in 1947. Posterity recognizes his goodness, but it has become increasingly clear how inadequate that alone was for navigating the challenges Planck faced. Historian Dieter Hoffmann, who is seldom sparing in his criticisms of the German physicists of that era, said on the fiftieth anniversary of Planck’s death that ‘he stands for professional excellence and the sustained search for truth, for scientific and personal integrity, for humanity and truthfulness, humility and modesty’. But to his biographer John Heilbron, Planck was a man catastrophically and tragically betrayed by his own beliefs:
What he did during the Nazi period was to act in accordance with a world view that allowed him no escape from his situation with his honor intact.
The rebuilding of German physics began by salvaging its reputation. That demanded more myths. In 1946 the vice chairman of the DPG, Wolfgang Finkelnburg, wrote in the society’s journal Physikalische Blätter that
physicists have a right to know how, despite all of the difficulties and with much courage, the Executive Committee of the German Physical Society did everything in its power in the years after the last physicists’ conference in 1940 to represent to the [National Socialist] party and the [Reich Education] ministry a clean and decent scientific physics and to prevent worse events than already occurred. I believe that this fight against party physics may be regarded as a heroic chapter of the real German physics, because—although led actively by only a few—it was effectively and morally supported by the predominant majority of physicists.
When we are compromised by our actions, the urge to evade blame and to construct a story that we can live with is one that everyone can recognize. Yet Dieter Hoffmann is harsh but not unfair when he says that the DPG’s chairman Carl Ramsauer and his colleagues contributed to the broader conspiracy of silence in post-war Germany about the realities of the Third Reich. There was no ‘heroism’ in the DPG’s activities, he says, but only—at best—damage limitation. The ‘formula of exoneration’ applied by Ramsauer and others, Hoffmann says—the notion ‘that they had done everything possible for science, and implicitly only for noble causes’—leaves out entirely ‘any consideration that they had conducted their science within and for a criminal regime that they had supported and worked for to gain personal and professional advantages’.
Finkelnburg’s statement illustrates how the Deutsche Physik movement (now labelled ‘party physics’) was exploited retrospectively as a way to distinguish the ‘clean and decent’ majority of physicists from the Nazis. In effect it served as a receptacle for any taint of collaboration with the regime. Finkelnburg does not mention that after 1940 Deutsche Physik was all but finished anyway, nor that this was never the ‘official’ physics of the National Socialist leadership, who had tended to regard the physicists’ battles with bemusement. No, what German physics now needed were scapegoats. That was true throughout German society, of course: it became common practice to load collective guilt for Hitler’s regime on to a few ‘true Nazis’. But in physics this went one stage further: in the narratives presented by Heisenberg and others, those scientists who had most enthusiastically supported the National Socialist agenda were also the least competent. Thus German science could be redeemed not only politically but also professionally.
This fractionation of scientists into ‘Nazis’ and ‘non-Nazis’ can be witnessed even at Farm Hall, where the physicists began to turn on each other. It was obviously compromising now to have been a party member, as Bagge and Diebner were. Both tried to excuse themselves by arguing that they had never held real sympathy for the National Socialists. Diebner said that this membership was purely a matter of expedience—he’d hoped it would improve his job prospects once Germany had won the war—and he listed his various acts of ‘opposition’. His colleagues weren’t impressed; some said they could not in good conscience sign a declaration of their anti-Nazi stance during the war if Diebner was a
lso a signatory. The younger Bagge claimed rather pathetically that his mother had enlisted him in the National Socialists without his knowledge, a task impossible even for the most Machiavellian of mothers.
The issue was something of a red herring in any case. With the exception of Laue, Heisenberg and Hahn, all the others had belonged to a National Socialist organization, and Heisenberg had given his services to the government willingly enough. Major Rittner witnessed such nationalistic chauvinism amongst the detainees that he felt compelled to allude in a report in September to ‘the inborn conceit of these people, who still believe in the Herrenvolk’. Laue, he said, is the only possible exception.
Very few Germans in any walk of life suffered repercussions as a result of being alleged Nazi sympathizers. Many of the scientists—the physicists especially—did have genuinely valuable intellectual wares to trade for their rehabilitation, and this sort of barter was acceptable to both the Americans and the Soviets (Robert Döpel at Leipzig was one of the nuclear physicists who went east). Even the ‘Aryan physicists’ Stark and Lenard escaped serious recrimination. They were called before the denazification tribunal, but Lenard was deemed too frail to stand trial. Stark was at first classified as a ‘major offender’, the most serious category of the five-point ranking, and was sentenced to six years of forced labour. But an appeals court commuted the classification to ‘lesser offender’ (the third category), and ordered Stark simply to pay a 1,000-mark fine.
That was a typical pattern. The post-war trials were notoriously ineffectual, since it was extremely difficult and time-consuming to investigate any allegation thoroughly, let alone to prove it. Literally millions of cases were simply dropped. Many who supported the regime had little difficulty in obtaining the so-called Persilscheine or whitewash certificates. The most vociferous of Nazis in the universities were dismissed without compensation, while others who had doubtless helped the regime were eased into early retirement. Pascual Jordan, for example, a party member whose enthusiasm for National Socialism was such that its ideology has been said even to have seeped into his physics, was issued a whitewash certificate by Heisenberg, who attested that he had ‘never reckoned with the possibility that [Jordan] could be a [true] National Socialist’ (rather inviting the question of what it would take to convince Heisenberg of that).*8