Authors: Michael Frayn
Sereny, like Rose, is markedly unenthusiastic about Heisenberg in general. She goes on to argue that Heisenberg’s claims about his intentions in meeting Bohr in 1941 ‘are now shown by Speer’s Spandau account to be false’, though quite how this is so she doesn’t explain. About what she calls ‘the facts’ of the Copenhagen meeting she is remarkably brisk. In the conversation ‘ … which Bohr subsequently reported to his associates at the Niels Bohr Institute, Heisenberg had made his political stand crystal clear. His team, he told Bohr, had gone some way towards discovering a way to produce an atom bomb. Germany was going to win the war, probably quite soon, and Bohr should join them now in their efforts.’
The idea that Heisenberg was inviting Bohr to work on the German bomb is on the face of it the least plausible out of all the possible interpretations that have been offered. It is completely at odds with what Weisskopf recalls Bohr as saying in 1948, and with what Bohr is on record as telling Chadwick at the time. In any case, the suggestion that Heisenberg thought he might be able to import someone half-Jewish into the most secret research programme in Nazi Germany is frankly preposterous.
So what is Sereny’s evidence for her account of the meeting? At this point the sense of vertigo returns, and one begins to have the feeling that one is in an Escher drawing, where the stairs up to the floor above somehow lead back to the floor one is already on, because the only reference she gives is … Powers, Heisenberg’s great champion, in
Heisenberg’s War
.
And it’s true—Powers
does
quote an opinion to this effect (and it’s the only possible source for it anywhere, so far as I know). He says he was told by Weizsäcker that some person or persons unnamed in Copenhagen, 44 years after the event, had told
him
that this is what Bohr had said he had believed Heisenberg’s intention to be. One might think that this is rather faint evidence. In any case, even if it really is what Bohr believed, it is of course not what Weizsäcker believed, or Powers either. They are reporting Bohr’s alleged belief as a possible misapprehension on his part which might have explained his anger. Indeed, Powers’s own reading of the situation is precisely the one that Sereny claims to be discredited by Speer’s remark.
*
Goudsmit gradually modified his opinion, and his final judgment on Heisenberg, when he died in 1976, was a generous one which goes some way to expunging the dismissive tone of his book: ‘Heisenberg was a very great physicist, a deep thinker, a fine human being, and also a courageous person. He was one of the greatest physicists of our time, but he suffered severely under the unwarranted attacks by fanatical colleagues. In my opinion he must be considered to have been in some respects a victim of the Nazi regime.’
Robert Jungk, one of the few authors who have ever attempted to defend Heisenberg, modified his opinion in the opposite direction. In
Brighter than a Thousand Suns
, originally published in 1956, he suggested that the German physicists had managed to avoid building nuclear weapons for conscientious reasons, and quoted Heisenberg as saying that,
‘under a dictatorship active resistance can only be practised by those who pretend to collaborate with the regime. Anyone speaking out openly against the system thereby indubitably deprives himself of any chance of active resistance.’ But Jungk later changed his mind, and described the notion of passive resistance on the part of the German physicists as a ‘myth’. He had contributed to spreading it, he said, out of an ‘esteem for those impressive personalities which I have since realized to be out of place.’
For a really spirited and sustained defence Heisenberg had to wait until Powers published his book in 1993. It is a remarkable piece of work, journalistic in tone, but generous in its understanding and huge in its scope. A little too huge, perhaps, because Powers is unable to resist being sidetracked from the main narrative by the amazing byways that he perpetually finds opening off it. I recommend it particularly to other dramatists and screenwriters; there is material here for several more plays and films yet.
His central argument is that the Allied bomb programme succeeded because of the uninhibited eagerness of the scientists to do it, particularly of those exiles who had known Nazism at first hand, and who were desperate to pre-empt Hitler; while the German programme failed because of the underlying reluctance of scientists in Germany to arm Hitler with the bomb, however strong their patriotism, and however much they wanted to profit from the possibilities for research. ‘Zeal was needed,’ he says; ‘its absence was lethal, like a poison that leaves no trace.’
But he goes further, and argues that Heisenberg ‘did not simply withhold himself, stand aside, let the project die. He killed it.’ He tries to show that at every point Heisenberg was careful to hold out enough hope to the authorities to ensure that he and and his team were left in charge of the project, but never enough to attract the total commitment and huge investment that would have offered the only real hope of success. ‘Heisenberg’s caution saved him. He was free to do what he could to guide the German atomic research effort into a broom closet, where scientists tinkered until the war ended.’
Cassidy, reviewing the book in
Nature
, described it as a good story, but insisted that ‘as history it is incredible.’ Rose dismisses it as ‘entirely bogus’ and ‘a scholarly disaster’. Powers acknowledged ruefully, in a recent letter to the
Times Literary Supplement
, that he had failed to convince any historian who had pronounced upon the matter.
The play is not an attempt to adjudicate between these differing views of Heisenberg’s personality, or these differing accounts of his activities. But it would have been impossible to write it without taking
some
view of Powers’ version of events, so here, for what it is worth, is a brief summary of the case, and of my own hesitant view of it. The evidence is confused and contradictory, and making any sense of it involves balancing probabilities and possibilities almost as indeterminable as Heisenberg found events inside the atom.
*
Some of the evidence undoubtedly appears to support Powers’s thesis in its stronger form, that Heisenberg deliberately sabotaged the project.
In the first place there are two scraps of direct testimony. One is a message brought to America in 1941 by a departing German Jewish academic called Fritz Reiche. It was from Fritz Houtermans, the German physicist who had just realised that if they could get a reactor going it would produce plutonium, and that plutonium would be a fissile alternative to the U235 that they could not separate. Reiche testified later that he had passed it on to a group of scientists working at Princeton, including Wolfgang Pauli, John von Neumann, and Hans Bethe. As Rudolf Ladenburg, the physicist who arranged the meeting, recorded it afterwards, Houtermans wanted it to be known that ‘a large number of German physicists are working intensively on the problem of the uranium bomb under the direction of Heisenberg,’ and that ‘Heisenberg himself tries to delay the work as much as possible, fearing the catastrophic results of a success.’
Rose dismisses Houtermans as a proven liar, and records that Reiche later appeared to withdraw his belief in
Heisenberg’s opposition to the project. But neither of these objections seems immediately relevant to the consistency of Reiche’s and Ladenburg’s testimony.
The second scrap of evidence is even more direct, but much more dubious. Heisenberg’s American editor, Ruth Nanda Anshen, records receiving a letter from him in 1970 in which he claimed that, ‘Dr Hahn, Dr von Laue and I falsified the mathematics in order to avoid the development of the atom bomb by German scientists.’
The letter itself has apparently vanished from the record. Rose nonetheless accepts it as beyond doubt genuine, and sees it as a yet more blatant attempt at self-justification. It is not, however, called into evidence by Powers, even though it would appear to support his case, and he mentions it only in his notes, and with the greatest reserve. Jeremy Bernstein, who seems to me the best-informed and most fair-minded of all Heisenberg’s critics, and whose book
Hitler’s Uranium Club
will be relied upon in understanding the scientific considerations that follow, dismisses it as ‘incredible’ and ‘a chimera’. It is entirely at odds with Heisenberg’s careful moderation in all his other references to the matter, and the inclusion of Hahn and von Laue in the plot is nonsensical. Hahn was a chemist, not a physicist, and, as will be plain from what comes later, had no knowledge whatosever of the relevant mathematics, while von Laue is famous as an outspoken opponent of Nazism who never worked on the German nuclear programme at all.
So much for the direct evidence, true or false. All the rest of the evidence is indirect, and relates to whether Heisenberg did actually have some understanding of the relevant physics and concealed it, or whether he failed out of ignorance. It centres on the question of critical mass. This is the amount of fissile material (U235 or plutonium) which is large enough to support an explosive chain reaction, but small enough not to explode spontaneously. An estimate of this amount was crucial to the decision about proceeding with a serious nuclear weapons programme 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. At the beginning of the war it was believed by scientists on both sides that the answer would be in tons, which put the possibility of producing it beyond practical consideration. The idea became imaginable only when two scientists working in Britain, Rudolf Peierls and Otto Frisch, did the calculation and realised quite how fast the reaction would go with fast neutrons in pure U235, and consequently how little fissile material you would need: not tons but kilograms. (The various ironies associated with this are explored in the play, and I will not repeat them here.)
Powers argues that the idea never became imaginable in Germany because Heisenberg ‘cooked up a plausible method of estimating critical mass which gave an answer in tons.’ He believes that Heisenberg ‘well knew how to make a bomb with far less, but kept the knowledge to himself.’
There is a certain amount of evidence that the German team did at one point arrive at a much lower figure for the critical mass—indeed, for one in kilograms, that bore some relation to the estimate made by Frisch and Peirls, and to the actual mass of the Hiroshima bomb (56 kg). Manfred von Ardenne, who was running an alternative nuclear programme for the German Post Office, later claimed in his memoirs that in the late autumn of 1941 he was informed independently by both Heisenberg and Hahn that they had worked out the critical mass for a U235 bomb and found it to be about 10 kilograms. This information was subsequently withdrawn by von Weizsäcker, who told him that he and Heisenberg had decided that a U235 bomb was impossible (because the heat of the reaction would expand the uranium too fast for it to continue). But Heisenberg, so far as I know, never commented on this, and von Weizsäcker, according to Bernstein, ‘essentially denied’ that any such conversation ever took place.
As Bernstein says, it is difficult to know what to make of all this—it is ‘one of several brick walls anyone who studies this subject runs into.’ I think it’s difficult to take von Ardenne’s recollection entirely literally. Hahn, as I noted
before, plainly had no understanding of the mathematics, nor of any of the other issues involved, and, as we shall see, had to have them explained to him by Heisenberg later. On the other hand (and this story has more other hands than a Hindu god), in von Weizsäcker’s report on the possibility of an American bomb programme, written in September 1941, he talked about the destructive effects of a bomb weighing 5 kg. Then again, in February 1942 a brief progress report for German Army Ordance, authors unnamed, suggested without further explanation a critical mass of between 10 and 100 kg. And at the crucial meeting with Speer at Harnack House in June 1942, when Field Marshal Milch asked him how large an atomic bomb would have to be to destroy a city, Heisenberg replied, or so he said in his interview with Irving, that it, or at any rate its ‘essentially active part’, would have to be ‘about the size of a pineapple.’
In the end, though, I believe that the crucial piece of evidence lies elsewhere, in a source that was denied to everyone who wrote about Heisenberg until recendy—the transcripts of the Farm Hall recordings. Bernstein, Powers, and Rose were the first commentators to have access to them.
Though of course they still don’t reach the same conclusions from them.
*
The story of Farm Hall is another complete play in itself. Sir Charles Frank, the British atomic physicist, in his admirably fair and clear introduction to the text of the transcripts that was published in Britain, regrets that they were not released in time for Dürrenmatt to make use of.
At the end of the war troops of the Alsos mission, to which Goudsmit was attached, made their way through what was left of the German front line and located the remains of the German reactor at Haigerloch, with the intention of finally reassuring themselves that Germany would not be able to spring some terrible nuclear surprise at the last moment. They also seized the team of scientists themselves,
making a special armed sortie to Urfeld, in Bavaria, to collect Heisenberg from his home. Hechingen, the nearby town where the team was based, and Haigerloch itself were in the French sector. The scientists were abstracted secretly, from under the noses of the French, and brought back to Britain, where they were held, under wartime laws and without anyone’s knowledge, in a former Intelligence safe house—Farm Hall, near Cambridge. The intention seems to have been partly to prevent their passing on any atomic secrets to either of our other two allies, the Russians and the French; partly to forestall any discussion of the possibility of nuclear weapons until we had completed and used our own; and partly, perhaps, to save Heisenberg and the others from the alternative solution to these problems proposed by one American general, which was simply to shoot them out of hand.