Copenhagen - Michael Frayn [45]
Bernstein (unlike Goudsmit) reads this and what follows as showing that Heisenberg did understand the difference between a reactor and a bomb, ‘but that he did not understand either one very well—certainly not the bomb.’ Rose now seems to accept that Heisenberg’s remarks do indicate that he realised the bomb would have to be fissioned with fast neutrons (though he shows that in the past Heisenberg had been toying with the idea of some kind of vast exploding reactor).1
This same conversation between Heisenberg and Hahn, when they were alone together on that terrible night, seems to me also to resolve the question of Heisenberg’s understanding of the critical mass beyond any reasonable doubt. He takes Hahn through what he believes to be the relevant calculation and tells him that the answer is ‘about a ton.’ I can’t see any earthly reason why he should be rehearsing a fabricated calculation or a fabricated answer at this stage, in a private conversation with someone he seems to have trusted, after the German team are out of the race and in custody, and after someone else has in any case already built the bomb. If he had had the right calculation and the right answer up his sleeve all the time, now would surely have been the moment to produce them. I find it much more plausible that he was telling the simple truth when he said to Hahn just before this that ‘quite honestly I have never worked it out as I never believed one could get pure 235.’
Earlier on in the evening, it’s true, when everyone was present during the conversation immediately after the news bulletin, Hahn says to Heisenberg: ‘But tell me why you used to tell me that one needed 50 kilograms of 235 in order to do anything.’ (To which Heisenberg replies that he wouldn’t like to commit himself for the moment.) This does seem to suggest that he had made a calculation of some sort earlier, as von Ardenne claimed—though it also surely destroys once and for all the improbable proposition that Hahn had been involved in it, or had made some kind of estimate of his own. Perhaps Heisenberg had made not so much a calculation as some kind of guess or estimate. Even if was a serious calculation, it seems most unlikely that it was the right calculation, or that it was one he had adhered to.
This is made clear to me (at last) by Jeremy Bernstein. I should explain that when I first read the Farm Hall transcripts, before I wrote the play, I was using the bare uncommented text published in Britain, unaware that there was also a completely different edition published in the US, incorporating Bernstein’s detailed commentary. After the play was produced and published he was kind enough to send me it, and it illuminated a great many matters that I had not understood before. These are after all scientists talking to scientists, and they are reported verbatim with all the ellipses of spoken conversation, and with a further haze cast over the proceedings by translation. Bernstein is both a distinguished journalist and a professor of physics, and he has a long acquaintance with the history of atomic research. (He recalls being given the bare plutonium core of a bomb to hold on the Nevada test site in 1957; ‘it was slightly warm to the touch, since plutonium is marginally radioactive.’) He has a thorough understanding of the scientific issues involved, and is the ideal guide to the physics—though a slightly less percipient one, I think, to the psychology of the physicists.
I’m pleased to discover for a start that he takes the same view of Heisenberg’s admission to Hahn about never having worked out the critical mass. He believes that it has to be taken at its face value, and he asks how it can be reconciled with the figure of 50 kg recalled by Hahn. He demonstrates that when Heisenberg attempts to do the calculation