In 1992 some of us fancied that the newly released Farm Hall transcripts would settle the Heisenberg atomic-bomb debate once and for all. But for the Heisenberg apologists, the truth does not set them free. It becomes merely one more obstacle to be circumvented, spun, distorted, suppressed and denied, a challenge to their powers of invention and fantasy. Ten years on, history is repeating itself with the new Bohr letters. No reader with an open mind can miss the self-evident truth about Heisenberg that they expose. But the spin has already been applied by the defence team. The letters are only one man’s point of view; they weren’t sent; they are coloured by post-war experience; they are ambiguous. And behind all these objections looms the big question – What is truth, what is intention? Mr Frayn does indeed ask about truth, but like jesting Pilate, he will not stay for an answer. The new postscript that is circulating concedes that perhaps the author should have said more about Heisenberg’s 1941 praise of the Nazi conquest of Europe, but otherwise it is largely unrepentant. So I shall try to pinpoint the key aspects of Mr Frayn’s struggle with truth in three areas.

1. Did Heisenberg really understand how to make an atomic bomb?
2. The visit to Copenhagen in 1941.
3. The problem of Frayn’s "subtle revisionism".

1. DID HEISENBERG UNDERSTAND THE BOMB SCIENTIFICALLY? THE CRITICAL MASS CALCULATIONS.

First of all, the scientific problem of the atomic bomb. Did Heisenberg really understand it during the war as he claimed to Bohr?

I want to stress here that even if we abstract all the post 1945 evidence, there is adequate documentary proof of wartime origin to establish the truth of the matter. First, we have the secret German reports – nearly 400 of them. Several key reports are discussed for the time in my book. Second, there is an official survey of the matter compiled by Heisenberg’s team in January 1942 for the Army Weapons Research Office that was in charge of the German atomic project. The only analysis of this to appear, incidentally, is that to be found in my book. My book also includes a third source, Bohr’s wartime correspondence on the bomb with Chadwick and British intelligence. And finally there are the Farm Hall transcripts of the German scientists’ discussions of August 1945.

From these sources it emerges that Heisenberg conceived three classes of atomic bomb.

1. The pure U235 bomb.

2. An exploding reactor-bomb

3. The plutonium bomb.

The last of these was glimpsed as a long-term possibility, with the plutonium being produced over a long period – longer than the war could last, thought Heisenberg - by the operation of uranium reactors. Reactors were therefore the chosen path of development since they would provide not only power and motors, but also nuclear explosive. Since Heisenberg aways arrogantly assumed that his theories and experiments were ahead of the Allied ones, he had no reason to fear Allied progress on a plutonium bomb.

As to pure U235, Heisenberg grasped that this kind of bomb had to depend on a fast-neutron reaction. But how much U235 was required as the critical mass for a nuclear explosion? By December 1939 Heisenberg had finished a detailed technical investigation of fission which is vague on the crucial point of critical mass, but implies that it would be huge. In early 1940, indeed, Heisenberg presented his colleagues with a back of the envelope calculation that indicated it to be several tons in mass, as Karl Wirtz told the British scientist Charles Frank in the garden of Farm Hall in 1945. As it happens, we know the details of this calculation, because it is gone over three times by Heisenberg in the Farm Hall transcripts. It employs a brilliant random walk technique to show that in order for a chain of 80 generations of fissions to be completed, a sphere of a radius yielding 15 tons or so of U235 is needed. (SLIDES OF NO. 1 CRITICAL MASS CONCEPTS; AND NO. 2 FARM HALL /80). (An explanation of this matter can be found in the article Jonothon Logan on critical mass in American Scientist, May 1996). The fallacy in this calculation, however, is that it relates to a maximum critical mass, in which Heisenberg can be sure that the original fissioning neutron and its 80 step chain are all still contained within the critical mass before its expansion breaks off the chain reaction. In reality, the reaction is so rapid that new chains are constantly forming and reaching 80 generations in a critical mass that is much smaller, what we might call a minimum effective critical mass. This was the calculation that Frisch and Peierls made in March 1940 and which yielded a small critical mass of only a few pounds, rather than tons of U235. For the British, such an amount might seem prodigiously difficult to produce, but it was small enough to be worth a try. For Heisenberg, though, the mass of tons ruled out any possibility of a uranium bomb, and this was the reason he did not press its case with Speer in 1942 when Hitler’s arms minister offered him a blank cheque. (SLIDE NO. 3 OF 1942 MEETING). At Farm Hall, however, after hearing of Hiroshima, Heisenberg decided to re-think out the whole problem and within a week had come up with a new calculation – the "diffusion equation" - which was comparable to that of Frisch and Peierls, and which he presented to a seminar of astonished colleagues who were clearly hearing it for the first time. We have therefore two calculations by Heisenberg, the random walk one of 1940, and the diffusion equation of 1945.

In the play, Frayn practices a sleight-of-mind on the audience concerning this key issue. He claims that Heisenberg never did a calculation until 1945; and he suppresses the random walk calculation of 1940 while artfully alluding to one of its features, the 80 fission steps:

(PASSAGES FROM FRAYN’S COPENHAGEN).

BOHR: So you needed to calculate the figure for pure 235?

HEISENBERG: Obviously.

BOHR: And you didn’t?

HEISENBERG: And that’s why…you spent the entire war believing that it would take not a few kilograms of 235, but a ton or more…And to make a ton of 235 in any plausible time…

HEISENBERG: Otto Frisch and Rudolf Peierls actually did the calculation. They solved the diffusion equation…They did the calculation.

BOHR: So Heisenberg, tell us this one simple thing: Why didn’t you do the calculation?…

HEISENBERG: ….Because I wasn’t trying to build a bomb.

BOHR: Why are so confident that it’s going to be so reassuringly difficult to build a bomb with 235? Is it because you’ve done the calculation?

HEISENBERG: The calculation?

BOHR: Of the diffusion in 235. No, it’s because you haven’t calculated it…You hadn’t consciously realized there was a calculation to be made.

In his new postscript Mr Frayn still refuses to admit there was indeed a 1940 calculation; "The central argument [of the play] turns (he says) on Heisenberg’s confession …that he had not attempted the calculation" before 1945". And Frayn goes on to avow that "by my count there are something like 35 speeches devoted to establishing this…to asking why he hadn’t attempted this". Mr Frayn simply refuses to understand that Heisenberg had indeed attempted a calculation before 1940, and that this was the reason why the German team did not make a determined effort to extract the small amount of U235 needed for a bomb. He therefore has to invent all kinds of speculative explanations as to why Heisenberg had failed to attempt the calculation.

This leaves the reactor-bomb. Here we have two main sources. The first is a report prepared by Heisenberg’s assistant, Paul Mueller, and circulated to the uranium project obviously with his approval in 1940. (SLIDES NO. 5a AND 5b OF MUELLER G-50). This is an analysis of the amount of moderator and other substances required to render an atomic reactor thermally unstable by reaching a temperature of 800 degress at which point Heisenberg theorized it would explode – not a true nuclear explosion but still a messy event. It would be delivered perhaps by ship, and one might note that Leo Szilard took out a patent for such a device. Indeed so did the German team in 1942. We know this from an indiscretion of Wirtz’s at Farm Hall, and I have also found many traces of it in the German papers, though the patent itself was lost when the secret patents were destroyed by the Germans in 1945. I am fairly sure, however, that a copy of it still exists in a private German collection. The patent is also listed in the official survey of the project finished in early 1942, the Army Weapons Office Report which evaluates the reactor- bomb. (SLIDES NO. 6a and NO. 6b OF HWA REPORT, P. 43, AND CONCLUSION). From these sources, however, one gets the strong impression that the reactor-bomb was regarded as a theoretical possibility but highly impractical weapon. As I shall show in a moment, Niels Bohr was of the same opinion.

2. THE COPENHAGEN 1941 VISIT

Why did Heisenberg visit Copenhagen? The trigger for the visit was the appearance in early summer 1941 of a Swedish newspaper report of American efforts to create an atomic bomb that would destroy a city. Carl-Friedrich von Weizsaecker, intimate friend of Heisenberg, nuclear physicist, and son of the German State-Secretary Ernst, immediately brought this report to his father’s attention and the wheels turned quickly with Carl-Freidrcih himself writing to the High Command of the Wehrmacht and other officials. SLIDE NO X1 OF WEIZSAECKER’S 1941 LETTER By the late September Weizsaecker and Heisenberg were in Copenhagen, conversing with Niels Bohr. This was an official mission that went well beyond any merely scientific interest. It was approved clearly as an intelligence foray at the highest levels. Bohr’s contacts with the Allies were suspected by Heisenberg and so the visit was intended to sound him out as to an Allied breakthrough and, on a scientific level, to make sure that Bohr had not developed any insight into the nature of a bomb that might have eluded the Germans, though Heisenberg clearly found that almost impossible to conceive. This is the psychological motivation for Heisenberg’s opening gambits with Bohr – first, an assertion that nuclear weapons could not be achieved during the duration of the present war, and, when Bohr failed to rise to the bait, Heisenberg’s boasting that he had been working on the matter intensively and knew everything there was to know about it. Bohr’s silence must have really got under Heisenberg’s skin to provoke this outburst. Alas, the Gestapo report, like so many other relevant documents, is not to be found for the present, though there prospects of it turning up in due course. But thankfully Mr Frayn’s new postscript now admits that there was indeed a "spying" element to the mission. We owe to Arnold Kramish’s fascinating book of 1986 The Griffin much information on the intelligence aspects of the mission as well as striking protraits of the two German scientists involved.

It is this intelligence element that explains one of the most striking features of the new Bohr letters: the repeated question to Heisenberg: Who authorized his visit? How could he come and speak about such dangerous and secret subjects to Bohr? It seems to me from the phrasing of the letters that Bohr knew perfectly well by 1958 who had authorized the visit and was trying to force Heisenberg to an admission on this critical point.

A second major feature of the Bohr letters is their confirmation of Heisenberg’s sermonizing on the desirability of a Nazi victory (something he liked to tell colleagues about in other occupied countries as well as Jewish refugee friends after the war! – a graphic illustration of the depth of Heisenberg’s conviction on this point).

But perhaps the most astonishing aspect of the new Bohr materials is something they do not explicitly mention. Having been told of Heisenberg’s work on atomic bombs, just why did Bohr not become seriously alarmed at the possibility that Nazi Germany might be getting such a weapon? Though Bohr avers they did not discuss details of atomic bombs, we have to understand that Bohr shared Heisenberg’s firm confidence that an atomic bomb was not really feasible. As my book shows, in 1939-40 Bohr had concluded like most scientists that the critical mass of a U235 bomb would be enormous and out of the realm of possibility. Had he realized that only a few pounds of U235 were needed, Bohr would have been considerably more worried by Heisenberg’s boasting. But what of the reactor-bomb? Here again Bohr was confident that this was not feasible. But in 1943, approached by British intelligence, he decided to go over the problem in detail, and the result was two letters to Britain in which he again reassured himself that the reactor-bomb was not on. (SLIDE NO. 7 OF BOHR LETTERS TO CHADWICK) Then on arriving in Britain himself in October 1943, Bohr was amazed to hear of Allied progress on the U235 bomb and of the small critical mass. Could he also have been wrong about the feasibility of the reactor-bomb? Bohr worried about this and in December 1943 his concern resulted in a discussion with several physicists in Oppenheimer’s office at Los Alamos at which he produced the drawing of a possible reactor-bomb. (Whether this was Heisenberg’s drawing or Bohr’s own from 1941, or a quite new one by Bohr, is of marginal importance; it is the concept that is important). The outcome was that Bethe and Teller were asked to produce an evaluation which they finished a few days later and which laid Bohr’s fears about a German reactor-bomb to rest. (SLIDE NO. 8 OF BETHE-OPPENHEIMER REPORT). This report incidentally was first noted in Richard Rhodes’ invaluable book on The Making of the Atomic Bomb). One important question: I am not sure whether Bohr’s statement in the new letters that he had no contacts with the Allies was a lapse of memory or whether he still felt bound by British secrecy requirements.

Thus we have a context for the Copenhagen visit which explains its motives and what was in the minds of the protagonists, even if we do not know the actual words that were used. It was an intelligence mission by Heisenberg, and both men had already reassured themselves that neither a U235 nor a reactor-bomb were feasible. Both men thought they understood the atomic problem completely, and both were wrong. Bohr was to be disabused of his mistake in London in October 1943, but it took until August 1945 for the scales to fall from Heisenberg’s eyes.

3. FRAYN’S REVISIONISM.

Mr Frayn’s new postscript finds it extremely difficult to make sense of "the subtle revisionism" which I had earlier detected in his play. He cites only my suggestion that the play seems to adopt the antisemitic implication of Heisenberg and others that it was two German Jews, Frisch and Peierls, who were responsible for the invention of the atomic bomb. This is of course factually true, but the psychological implication, which should be readily accessible to someone so interested in the epistemology and irony of intention, is that that it was Germany’s innocent victimized Jews who presented the world with this accursed weapon. As Heisenberg says twice near the end of the play, Frisch and Peierls "actually did the calculation"; Frayn doesn’t dare to put in Heisenberg’s mouth the comment "they were Jews", but he mischievously entrusts it to the decent Margrethe who would have understood it in a different way from Heisenberg. Still, a New York audience receives this like a punch to the stomach. As to the real life Heisenberg and Weizsaecker, they were fully alive to this little irony which is a recurrent subtext in the thought in some circles in Germany, scarcely ever daring to be voiced in public, though often enough thought and privately expressed, I can assure you. What now gives this matter more bite is that in the last few months a deeply unpleasant vein of antisemitism has burst out in British elite and cultural circles that has produced perhaps a cruder revisionism than Mr Frayn would countenance.

The "epistemology of intention", as Mr Frayn calls it, is really just a mask for a key aspect of this subtle revisionism. In an interview in The New York Times of 9 February 2002, Frayn insists on how the supposedly moral man, Bohr, "rightly or wrongly, did actually contribute to the death of many people through the Allied atomic bomb program" whereas the supposedly immoral Nazi compromiser, Heisenberg never "in fact, killed anyone". By the way, this is not really true: As a youth in a radical paramilitary unit in 1918, Heisenberg guarded one leftist prisoner the night before his execution , and, more relevantly, he was aware of the use of women concentration camp prisoners as slave-labor for the manufacture of the uranium metal plates in his reactors, a highly toxic process. Thus, he contributed if indirectly to the deaths of a number of people.

Still, this apart, there might be an irony of sorts, but it’s a banal and cheap irony, which perhaps intentionally swallows one of Heisenberg’s own repeated evasions of the central moral issue. For the Allied scientists, working on the atomic bomb presented a moral issue that most of them quickly resolved. But that’s a universal problem of science and government. The German scientists had a quite different, specific moral problem : should they work on a bomb for Hitler? Frayn’s play deals with the universal problem, not the specific German one, and thus serves the purpose of Heisenberg’s own revisionism.

I’ve made plain elsewhere what I take to be the main lineaments of Mr Frayn’s revisionism. His denigration of Niels Bohr as an almost callously indifferent figure, even to his own family, his portrait of an honourable "German" Heisenberg, the conformity of his play’s irony of intentions to the current British revisionism that sees Churchill’s decision to fight on against Hitler in 1940 as a catastrophic error since what that good intention to save British power actually produced was the decline of British power. This is the subtext underlying Mr Frayn’s wondering aloud in a British Guardian interview two years ago just what the RAF’s successful fight against the Luftwaffe in 1940 actually achieved in the end. What it achieved was the defeat of evil. But I suspect that in Mr Frayn’s moral universe, evil is something that’s too beset by the irony of intention to be taken seriously.