Authors: Michael Frayn
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.
They were detained at Farm Hail for six months, during which time they were treated not as prisoners but as guests. Hidden microphones, however, had been installed, and everything they said to each other was secretly recorded. The existence of the transcripts from these recordings was kept as secret as that of the prisoners. General Groves, the head of the Allied bomb programme, quoted from them in his memoirs (1962), and Goudsmit plainly had access to them, which he drew upon in his book on Alsos, but the British Government, perhaps to protect the feelings of the former detainees, some of them now prominent in post-war German science, perhaps merely out of its usual pathological addiction to secrecy, continued to block the release of the papers themselves. Even Margaret Gowing was refused access when she wrote her official history of British atomic policy in 1964, and David Irving was refused again, in spite of strenuous efforts, for
The Virus House
in 1967. The ban was maintained until 1992, when the Government finally gave way to a combined appeal from leading scientists and historians.
The German originals are lost, and the translation was plainly done under pressure, with little feeling for colloquial nuance, but the transcripts are direct evidence of what
Heisenberg and the others thought when they were talking, as they believed, amongst themselves. The ten detainees represented a wide range of different attitudes. They ranged from Walther Gerlach and Kurt Diebner, who had both been members of the Nazi party, to Max von Laue, who had been openly hostile to the regime, who had never worked on the atomic programme, and whose inclusion in the party seems on the face of it mysterious. Their conversations over the six month period reflect a similarly wide range of attitudes and feelings. The general tone is pretty much what one might expect from any group of acadmics deprived of their liberty without explanation and cooped up together. There is, as one might suppose, quite a lot of complaining, scheming, and mutual friction.
One thing, though, seems to me to emerge quite clearly: for all practical purposes German thinking had stopped at a reactor, and there had been no eagerness at all to look beyond this to the possibility of weapons. Their shocked comments in the moment of unguarded horror that followed the announcement of Hiroshima are particularly revealing. The internees had been given the news by their (almost) endlessly sympathetic and urbane gaoler-cum-host, Major Rittner, at dinner-time, but Heisenberg had not believed it until he had heard it with his own ears on the BBC nine o’clock news. ‘They were completely stunned,’ reported Rittner, ‘when they realised that the news was genuine. They were left alone on the assumption that they would discuss the position …’
‘I was absolutely convinced,’ says Heisenberg, in the conversation that followed, ‘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.’ Weizsäcker says that he doesn’t think that they should make excuses now for failing, ‘but we must admit that we didn’t want to succeed.’ Gerlach: ‘One cannot say in front of an Englishman that we didn’t try hard enough. They were our enemies, although we sabotaged the war. There are some things that one knows and one can
discuss together but that one cannot discuss in the presence of Englishmen.’
In a letter written fourteen years later von Laue complained that, during their conversations at table in the following weeks, ‘the version was developed that the German atomic physicists really had not wanted the atomic bomb, either because it was impossible to achieve it during the expected duration of the war or because they simply did not want to have it at all.’ Von Laue’s account of the elaboration of this sanitised ‘version’ (
Lesart
in German) has been seized upon by unsympathetic commentators, and contrasted with the encouraging prospects for atomic weapons that some of the physicists had undoubtedly held out to the Nazi authorities at various times during the earlier part of the war.
Well, we all reorganise our recollections, consciously or unconsciously, as time goes by, to fit our changed perceptions of a situation, and no doubt Heisenberg and his fellow-detainees did the same. But Bernstein locates the origins of the
Lesart
in those immediate reactions to the announcement of Hiroshima on the nine o’clock news. If this is so then I can only say that the team began to get their story together with quite remarkable spontaneity, speed, presence of mind, and common purpose. If they all thought as fast as this, and co-operated as closely, it’s even more surprising that they didn’t get further with the bomb.