Copenhagen (16 page)

To me, I have to say, those immediate and unprepared reactions suggest quite strongly that the first part of Powers’s thesis, at any rate, is right, and that there
had
been the ‘fatal lack of zeal’ that he diagnosed. Perhaps Gerlach’s claim, unchallenged by the others, that they had actually ‘sabotaged the war’ suggests at the very least a consciousness that quite a lot of stones had been left unturned.

*

But do the transcripts support Powers’s contention that Heisenberg ‘cooked up a plausible method of estimating critical mass which gave an answer in tons, and that he well
knew how to make a bomb with far less, but kept the knowledge to himself?

One preliminary point needs to be cleared out of the way first: the question whether Heisenberg understood an even more fundamental point, the difference between a reactor (which is operated by slow neutrons in natural uranium, or some other mixture of U238 and U235) and a bomb (which functions with fast neutrons in pure U235 or plutonium). Goudsmit, who plainly had access to the transcripts when he wrote his book at Alsos, seems to have thought they supported his view that Heisenberg didn’t. Before the transcripts were published Rose shared Goudsmit’s dismissive view.

But, according to the transcripts, what Heisenberg tells Hahn that same night, when Gerlach, their Nazi coordinator, has retired to sob in his room, and they are finally alone together, is that ‘I always knew it could be done with 235 with fast neutrons. That’s why 235 only [presumably = “only 235”] can be used as an explosive. One can never make an explosive with slow neutrons, not even with the heavy water machine [the German reactor], as then the neutrons only go with thermal speed, with the result that the reaction is so slow that the thing explodes sooner, before the reaction is complete.’

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 for Hahn he ‘gets it wrong at every level’—he does the arithmetic wrong, and is in any case doing the wrong arithmetic. ‘Knowing how scientists work,’ says Bernstein, ‘I find it implausble that he ever did the calculation correctly before. One can imagine even a Heisenberg forgetting a number—he was, in any case, not very good with numbers—but it is very difficult to imagine his forgetting a general method of calculation, a method that once led him to a more reasonable answer.’

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.

*

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, as the first and more plausible part of Powers’s thesis suggests? If so, you might wonder whether this reluctance was a state definite enough to be susceptible of explanation. Heisenberg was trapped in a seamless circle which explains itself: he didn’t try the calculation because he didn’t think it was worth doing—he didn’t think it was worth doing because he didn’t try it. The oddity, the phenonomenon that requires explaining, is not this non-occurence but its opposite—the escape of Frisch and Peierls from that same circle. It seems almost like a random quantum event; in which case, of course, it is no more explainable than its not happening.

After the war, certainly, Heisenberg was not just passively reluctant about any military application of nuclear power, but very actively so. In the 1950s, when there was a proposal to arm Federal Germany with nuclear weapons, he joined forces with Weizsäcker and others to fight a vigorous campaign that entirely and permanently defeated it.

There is also one small piece of evidence about his attitude during the war that Powers rather curiously doesn’t
comment on: the question of the cyclotron.

At the crucial meeting between Heisenberg and Speer in 1942, which seems finally to have scuppered all possibility of a German bomb, Heisenberg is reported to have emphasised the need to build a cyclotron. A cyclotron could have been used, as the cyclotrons in America were, for isotope separation, the great sticking-point in the German programme. In the account of this meeting in his memoirs Speer says: ‘Difficulties were compounded, Heisenberg explained, by the fact that Europe possessed only one cyclotron, and that of minimal capacity. Moreover, it was located in Paris and because of the need for secrecy could not be used to full advantage.’ Powers mentions this, but does not go on to the obvious corollary: that if Speer’s recollection is accurate, then Heisenberg was plainly lying, because he knew perfectly well that there was a second cyclotron to hand—at Bohr’s institute in Copenhagen. This would suggest that his apparent anxiety to lay his hands on a machine that might actually separate some U235 was not quite what it seemed. Or, at the very least, that he placed Germany’s war aims below his desire to protect Bohr’s institute.

Perhaps Speer is simply wrong. It seems uncharacteristic of Heisenberg to have risked such a blatant falsehood, and he makes no mention of it in his own accounts of the meeting. All the same, when he went back to Copenhagen in 1944, after Bohr had fled, to adjudicate a German proposal to strip the institute of all its equipment, presumably including the cyclotron, he seems to have contrived to leave it even then still in Danish hands.

*

One of the forms of indeterminacy touched upon in the play is the indeterminacy of human memory, or at any rate the indeterminability of the historical record. There are various examples which I left out, for fear of making the play even more tangled than it is. Some, such as the difficulties about the amazingly realistic figure for the critical mass that von
Ardernne recollected being given by Heisenberg and Hahn in 1941, I have already mentioned in this Postscript. There were others. A minor one concerns whether there were two ships sent to load the Jews of Copenhagen for deportation, as some witnesses recall, or a single one (named as the Wartheland). A more significant point of dispute is the drawing which Heisenberg did or didn’t make for Bohr during their meeting in 1941.

According to Hans Bethe, who was one of the team at Los Alamos, Heisenberg drew a rough sketch to show Bohr the work that was being done in Germany. Bohr evidently took it to Los Alamos with him when he went, because Bethe (and others) recall it being passed around at a meeting there. Bethe told Powers that Bohr believed it represented a bomb; but the consensus of opinion at the meeting was that it was a reactor. However, Aage Bohr, Niels’s son, a physicist himself (and another Nobel prizewinner), who was with his father in Copenhagen during Heisenberg’s visit, and with him again in Los Alamos, was absolutely insistent that there was no drawing.

If the story is true it might help to explain Goudsmit’s insistence, in the teeth of the evidence from Farm Hall, that Heisenberg couldn’t tell the difference between a reactor and a bomb. It would certainly cast doubt on Heisenberg’s recollection that the entire discussion with Bohr in 1941 took place during the walk, and that Bohr broke off the conversation almost as soon as it was broached. It seems improbable to me that Heisenberg would have risked putting anything down on paper, and if even so he had then I can’t see why he didn’t seize upon it after the war, to support his claim that he had hinted to Bohr at the German research on a bomb. I suppose it’s possible that Bohr made the sketch himself, to illustrate to his colleagues at Los Alamos what he thought Heisenberg was getting at, but the truth of the matter seems to be irretrievable.