To the Editors:
I wish to address myself to some of the technical issues that Thomas Powers brings up in connection with his response to Paul Lawrence Rose’s letter concerning Heisenberg’s wartime visit to Copenhagen [NYR, October 19, 2000]. I am in disagreement with both Powers’s and Rose’s analysis of the reasons for this meeting and have come to believe that we may never fully know the answer, although we will know more when the letter that Bohr wrote to Heisenberg, but never sent, is released in 2012—the fiftieth anniversary of Bohr’s death. Nonetheless we can get the technical details straight, which Powers has not done.
It is totally wrong to say, as Powers does, that the “random walk” method of estimating the critical mass, which Heisenberg first used at Farm Hall, is a “shorthand way.” It is an entirely incorrect way. Picture a sphere of uranium undergoing fission. In each fission roughly two neutrons are produced. The sphere must be large enough so that more neutrons are created in the fission than diffuse through its surface. The equation that determines this balance has one term that refers to the diffusion and another that refers to the buildup of the fission neutrons. What Heisenberg did was in essence to leave out this term. He later included it in a lecture he gave at Farm Hall and got more or less the correct answer.
There is no evidence that he or anyone else in the German project did this calculation correctly during the war.1 In June of 1942 Heisenberg was asked how much uranium a nuclear bomb would use that could destroy London. If one kilogram of uranium could be completely fissioned it would produce an explosion equal to about twenty thousand tons of TNT; about the size of the Hiroshima bomb. This would correspond to a sphere of uranium with a radius of a little over two centimeters—a ping- pong ball. But in a real bomb only about 2 percent of the uranium is fissioned so that about a hundred kilograms of uranium are needed, which corresponds to a radius of about eight and a half centimeters. At Farm Hall Heisenberg did not seem to have a clear idea of the efficiency question, so it is totally unclear to me what he had in mind when, in 1942, he apparently indicated that it was the shape of a pineapple. Powers also refers to Heisenberg’s discussion of “tampers”—material wrapped around the bomb to enhance the explosive effect. In Farm Hall Heisenberg was clearly thinking like a reactor physicist here. Tampers in bombs have very different requirements.
Finally I would like to comment on the famous drawing which Bohr presented at Los Alamos on December 31, 1943. I am responsible for bringing this matter to light in a profile of Hans Bethe I wrote for The New Yorker.2 Both Powers and Rose are persuaded that Heisenberg transmitted this drawing—which turned out to be a design for a reactor—directly to Bohr in Copenhagen. They insist on this despite the fact that Bohr’s son Aage, who was Bohr’s confidant at the time, has insisted that this did not happen. Of the drawing Bethe wrote me, “I am positive that there was a drawing. Niels Bohr presented it to us, and both Teller and I immediately said, ‘This is a drawing of a reactor, not of a bomb.’…Whether the drawing was actually due Heisenberg [sic] or was made by Bohr from memory I cannot tell….” I contacted every living person who was at that meeting and none of them could tell either. I think that Aage Bohr is right and that the drawing came to his father from someone else in the project—there were other visitors to Copenhagen—and not from Heisenberg.
Jeremy Bernstein
Aspen, Colorado
To the Editors:
The evidence for and against Heisenberg’s having made a serious calculation of the critical mass of an atomic bomb is, as Thomas Powers says in his reply to Paul Lawrence Rose’s letter, complex. There is a brief summary of it in my postscript to the published text of Copenhagen, together with my reasons for dissenting from Powers (and Rose), and for reaching the conclusion presented in the play (that he didn’t make it). If I’m mistaken about this, however, and Heisenberg did make it, then the bulk of the evidence seems to me to support Powers’s view that by one means or another he got the answer about right and not Rose’s that he got it totally wrong.
Some of Professor Rose’s other points I find a little mystifying. He says that Heisenberg’s trip to Copenhagen was “an intelligence foray…with the purpose of ascertaining how far the Allied project had progressed….” This suggestion is fully explored in the play, and indeed my Heisenberg entirely accepts that it was so. Professor Rose suggests that I “fantasize” Heisenberg’s fear that he was in danger of his life from the Gestapo for talking to Bohr. I can’t claim the credit for any fantasy here, I’m afraid. My Heisenberg is merely paraphrasing what the real Heisenberg claims in his memoirs: “I did not broach the dangerous subject until we took our evening walk. Since I had reason to think that Niels was being watched by German agents, I spoke with the utmost circumspection.”
Rose claims that the notion that Heisenberg “would say anything treasonous to Bohr is hardly credible.” But he also claims that an additional purpose of his conversation was “establishing whether a nuclear weapon was scientifically feasible.” Since Heisenberg knew that Bohr regarded atomic weapons as a practical impossibility in any remotely imaginable time-scale, I don’t understand how he could plausibly have done this unless, as he claims in his memoirs, he had at the very least “hinted that it was now possible in principle to build atom bombs”; and I don’t understand how any such hint, made to an enemy alien by someone working on a secret military program, could have failed to be treasonous.
Michael Frayn
London
Thomas Powers replies:
My chief worry, whenever I find myself arguing about Heisenberg in print, is that readers who happen by won’t understand what the dispute is about. At the heart of it, in my view, is an anomaly—why American
intelligence officers, in the closing months of the Second World War, discovered that not only was there no German atomic bomb, but there had not even been a big effort to build one.
This came as a surprise to the American Alsos mission at the time and it is still hard to explain. Fission had been discovered in Germany, scientists there had access to uranium ore and a Norwegian heavy water
plant, and the German military actively supported research into the feasibility of building a bomb virtually from the first day of hostilities in September 1939, a full two years before the American government took a serious interest. In a letter written to me a few years back the Italian physicist Ugo Fano described a party at the Ann Arbor, Michigan, home of Samuel Goudsmit, scientific director of the Alsos mission, in late August 1939. There two Nobel Prize–winning physicists, Werner Heisenberg, soon to return to Germany, and Enrico Fermi, a refugee from Mussolini’s Italy, were both honored guests. “At that party,” Fano writes, “[Edoardo] Amaldi drew me aside to point out its humor: ‘See Fermi, see Heisenberg, sitting in that corner. Everyone in this room expects a big war and the two of them to lead fission work on opposite sides, but nobody says!’” (Letter of September 18, 1993) Within a month Heisenberg had in effect been drafted to do theoretical work on bomb physics and he soon wrote two papers which were the basis of further German research during the war.
But the German military’s early interest in atomic bombs had died by the turn of the year 1941–1942 and in February the research effort was handed over to the Ministry of Education, where it languished till the end of the war. When an important official initiative is abandoned there usually remains a substantial paper record explaining the reasons for the turnabout, but not in this case. The only clear statement of the matter comes after the fact—from Albert Speer, Hitler’s newly appointed minister for economic mobilization, who described in his memoirs a meeting with Heisenberg and other German physicists in June 1942, when Heisenberg convinced him that developing atomic weapons was too big, too expensive, and too uncertain for Germany in wartime (Albert Speer, Inside the Third Reich, Macmillan, 1970, pp. 225ff).
That’s it—there is no other body of official paper to point to which might explain why the army dropped a program in which it had showed such a lively early interest. All the rest of the argument—not just Jeremy Bernstein’s letter above, but a shelf of books and numerous scholarly articles—finds its focus in the question which arises from the anomaly: Why did Heisenberg tell officials that a bomb was too difficult for Germany in wartime? Was he simply trying to save Germany a lot of wasted effort, or was he actively trying to hinder the development of atomic bombs?
This question should not have been difficult to answer. Heisenberg’s old friends among the Allies, many of whom had gone to Los Alamos to work on the American bomb, might have simply asked him, and my guess is that he would have answered pretty straightforwardly. But they didn’t ask, and when Heisenberg offered a tentative explanation after the war in an article in the British scientific journal Nature, he was roundly attacked. Soon he quit trying to explain himself, leaving it to historians to puzzle out what happened. To do this we must take into account all evidence of what Heisenberg did and said during the war, and in the end we must come to some conclusion about what he intended. There is really no way around this task; so far as we know the German interest in atomic bombs withered under Heisenberg’s emphasis on the difficulties, and his influence can-not be fairly judged until his motives are understood.
This brings me to Jeremy Bernstein’s letter, the latest in a series of exchanges between us on this matter, both privately and in print, going back ten years. He makes two points. The first is that I have failed to
understand how Heisenberg said he had calculated the critical mass of a bomb in a lecture delivered in August 1945 to fellow German scientists incarcerated by the British at Farm Hall in England. I warned readers at the time that I am no physicist and won’t attempt to sort out the “random walk” business here. It would be simpler to cite my source—an account by the British chief of scientific intelligence during the war, R.V. Jones, who described in print his discussion of Heisenberg’s Farm Hall lecture with his principal adviser on bomb questions, Charles Frank. Jones began with his memory of what Heisenberg had said:
