Copenhagen - Michael Frayn [46]
The calculation of the critical mass is not the only thing that Heisenberg got wrong that night. Even when he revealed to Hahn that he understood how the critical mass could be reduced by the use of a reflective shield he suggested a material, carbon, that would have had the opposite effect to the one intended. Carbon is a good moderator for a reactor, and Heisenberg’s proposing it for the ‘tamper’ in a bomb, says Bernstein, ‘shows he was thinking like a reactor physicist, which, for the last two years, he was.’
These were of course Heisenberg’s first thoughts off the top of his head in the wake of Hiroshima. A week later, with the help of what few details the newspapers had given of the two bombs, Heisenberg offered all his fellow-internees a lecture in which he presented a complete and considered account of how the Allies had done it. The inclusion in the lecture of quite fundamental matters, argues Powers, together with the questions which his hearers asked, make it clear that it was all news to everyone present except his closest associates. ‘What the Farm Hall transcripts show unmistakably,’ he says, ‘is that Heisenberg did not explain basic bomb physics to the man in charge of the German bomb program [Gerlach] until after the war was over.’ They ‘offer strong evidence that Heisenberg never explained fast fission to Gerlach.’ At the end of the lecture, says Powers, ‘the German scientists, given a second chance, would have been ready to start building a bomb.’
Bernstein sees the lecture very differently. He demonstrates that Heisenberg’s exposition is still marred by quite fundamental misconceptions. Heisenberg now seems to have ‘the first inkling’ of how to calculate the critical mass (though he still does the arithmetic wrong), but is not much nearer to the practicalities of building a bomb than his audience. What the novelty of a lot of this material suggests to Bernstein is simply that communications between the different sections of the German project were very poor.
As a non-scientist I can’t offer any opinion on the physics. To my eyes, I have to say, Heisenberg does seem to have come a remarkably long way in a week—if, that is, he was starting more or less from scratch. And he surely must have been. It’s really not plausible that he hadn’t recollected more by this time if he actually had done the work. The conclusion seems to me inescapable: he hadn’t done the calculation. If he had kept the fatal knowledge of how small the critical mass would be from anyone, as Powers argues, then it was from himself.
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In the end, it seems to me, your judgment of Heisenberg comes down to what you make of his failure to attempt that fundamental calculation. Does it suggest incompetence or arrogance, as his detractors have claimed? It’s possible. Even great scientists—and Bernstein agrees that Heisenberg was one of them—make mistakes, and fail to see possibilities that lesser men pick up; Heisenberg accepted that he had made a mistake in the formulation of uncertainty itself. And I think we have to accept Bernstein’s judgment that, although he was the first person to be able to grasp the counter-intuitive abstraction of quantum mechanics, he was not so good at the practicalities of commonsense estimates and working arithmetic.
Or does the failure suggest something rather different? An unconscious reluctance to challenge the comforting and convenient assumption that the thing was not a practical possibility? Comforting and convenient, that is, if what he was trying to do was not to build a bomb. Is it all part of a general pattern of reluctance,