The New York Review of Books

To the Editors:

Jeremy Bernstein’s review of the Victor Weisskopf memoirs [NYR, April 11] generously acknowledges my 1984 paper on Heisenberg’s misunderstanding of the scientific principle of the atomic bomb. Though the lecture itself was never published, it has now been expanded into a book which I hope will appear in the not too distant future. I am also glad to say that my connection with the University of Newcastle NSW ceased in 1985 and that my current base is the University of Haifa.

Paul Lawrence Rose
Department of History
York University
Ontario, Canada

To the Editors:

Jeremy Bernstein’s review of Victor Weisskopf’s autobiography, The Joy of Insight: Passions of a Physicist is generally admirable, but it should be noted that his description of the nuclear fission chain reaction in a reactor is not quite correct. Uranium 238 can not be fissioned by absorbing a slow neutron—neutrons with energies above about 1.4 million electron volts are required. Since very few of the neutrons released in the fission process have this much energy, in a reactor U-238 does not contribute to the reaction to any great extent. Uranium 235, on the other hand, readily fissions when absorbing a neutron of any energy whatsoever. This possibility allows it to absorb low-energy neutrons far more readily than U-238. Thus neutrons are deliberately slowed in a reactor to insure that U-235, despite its rarity (only 0.7% of natural uranium, or about 3% in the enriched fuel used in most power reactors), can absorb enough of the neutrons to allow the chain reaction to continue.

This said, I would like to second and amplify Bernstein’s judgement that the failure of physicists in Nazi Germany, and especially Werner Heisenberg, to develop nuclear weapons was due more to technical misjudgement than to moral scruples. By Heisenberg’s own admission he dismissed from the outset the possibility of producing pure U-235 in significant quantities, and the entire German scientific community missed the possibility of creating Plutonium 239, the other route to a bomb. Thus Heisenberg never had a reason to carry out the simple calculation that led Frisch and Peierls in England to conclude a pure U-235 bomb was feasible. Instead, he embarked on a program to produce a reactor capable of going into a condition now called “prompt critical,” in which the chain reaction builds up in a few thousandths of a second.

Such a device could in fact have produced an explosion of sorts. Indeed, the Chernobyl accident began with just such an explosion, when a small segment of the reactor went prompt critical. But it would have been far too heavy, and far too feeble, to constitute a practical weapon. For that purpose the reaction must build up in less than a millionth of a second, which can only happen in a compact assembly of pure fissionable isotopes.

Far too many western intellectuals have embraced the myth that German scientists never seriously intended to present Hitler with a new and terrible weapon, but were instead engaged in a subterfuge designed to preserve German physics, protect young scientists from conscription, and provide an energy source for Germany’s postwar reconstruction. This myth originated in Heisenberg’s Physics and Beyond and was promulgated in Robert Jungk’s Brighter than a Thousand Suns. Though it may correctly represent Heisenberg’s thinking after 1943, when the war was clearly lost and his project in the doldrums, it is clear that prior to this time he tried very hard (with limited success) to interest the Nazi regime in a vigorous nuclear weapons program.

There is no doubt that Heisenberg deplored the Nazi regime both for its excesses and its anti-intellectualism. But like many of his comrades in the Youth Movement (Jugendbewegung) of the Weimar era, he was beguiled by a sense of romantic nationalism and German particularism to active support for the German war effort.

Three other factors contributed to Germany’s failure to develop the bomb. One was that in the critical year of 1940, when a “crash program” might logically have begun, the German high command assumed the war would be over by 1943. A second was that prior to the defeat at Stalingrad the Nazi regime did not assert the centralized planning and control of German industry that a Manhattan-style project would have required. Finally, there was little coherence in the nuclear program, which was dispersed among many laboratories under funding provided by three separate government agencies. Thus, in this case at least, a totalitarian regime proved less adept than more democratic ones at marshalling resources for a technical project of strategic importance.

Robert H. March
Professor of Physics
University of Wisconsin
Madison, Wisconsin

To the Editors:

Jeremy Bernstein’s long digression on Werner Heisenberg in his review of Victor Weisskopf’s recent memoir, The Joy of Insight, is unfair to both Weisskopf and Heisenberg. Bernstein appears convinced that Heisenberg had something to be ashamed of at the end of the Second World War—at the very least, some foolish scientific mistakes, and possibly a serious effort, later denied, to make an atomic bomb for Hitler.

Bernstein’s account leans heavily on the Dutch-born, Jewish physicist Samuel Goudsmit, who ran an intelligence effort for General Leslie Groves, head of the Manhattan Project which built the American atomic bomb. Goudsmit’s book and papers are essential for anyone trying to figure out what Heisenberg really did during the war, but what Goudsmit wrote must be read with care: for a time after the war he blamed Heisenberg personally for the death of his parents in a concentration camp.

Bernstein also cites a paper by the historian Paul Lawrence Rose, but Rose, too, appears to have gathered his information mostly from Goudsmit’s files. I have not seen Rose’s paper, but evidently it includes a reference to Heisenberg’s estimate of how much Uranium 235 would be required for a bomb, which was very much inflated. Rose and Bernstein think Heisenberg was simply in error, but I think something a good deal more complicated was going on. There is good evidence Heisenberg told some officials the figure was on the order of two tons. The most interesting thing about this “mistake” is that Heisenberg knew, and in the fall of 1941 told Otto Hahn, that the true figure was only a few kilograms. If American officials had been convinced in 1942 that a bomb would need two tons of U-235 there would have been no Manhattan Project.

Weisskopf, like many other émigré scientists who left Germany in the 1930s, was fearful on the outbreak of war in 1939 that Heisenberg would build an atomic bomb. Intelligence information reaching the British, and later the Americans, seemed to confirm the existence of an ambitious German bomb program. The fear of Heisenberg reached almost incredible intensity. But the Allied capture of German scientists and their research documents in 1945 made it clear the Germans had long abandoned all hope of a bomb, and spent the last three years of the war working on an experimental reactor. Even this work achieved little. What more could the Allies have asked?—no bomb, no reactor, not even any warnings to officials that the Allies might be up to something. So why did Heisenberg’s scientific work during the war, which posed no threat whatever, later excite so much suspicion and anger among Allied scientists—many of them old friends of Heisenberg from the 1920s and 1930s?

The history of this episode is fabulously complex. My own attempt to sort it out will eventually fill a fat book, and includes much new information which Bernstein has had no chance to see. But Bernstein dismisses Weisskopf’s version as if the case was closed. He says for example that Heisenberg “began working on a German atomic bomb with enthusiasm in 1939….” I know of no evidence whatever to support the words I have emphasized. In September 1939 Heisenberg, a member of an Alpine reserve unit, was called up and assigned to work on the German bomb program. Whether or not Heisenberg worked on the bomb with enthusiasm—that is, whether he really tried or wanted to build a bomb—is the substance of the argument about his wartime role.

Those who suspect Heisenberg of trying to whitewash his own role generally claim he made a fundamental error in conceiving a bomb, and thought a weapon would work on the same principles as a reactor. Goudsmit was convinced of this for a time and passed on the belief to General Groves, who stated it as fact in his memoirs.

The principal cause of this misunderstanding must be laid at the doorstep of Niels Bohr, Heisenberg’s close friend and collaborator of the 1920s and 1930s. In the fall of 1941 Heisenberg went to see Bohr in Copenhagen—”one of the strangest episodes in Heisenberg’s wartime career,” as Bernstein fairly says. There Heisenberg drew a sketch for Bohr, who took it with him when he fled Denmark in September 1943. At Los Alamos on the last day of the year Bohr discussed this sketch with Hans Bethe and several other scientists (Weisskopf was one of them) working on the American bomb. Bohr told Bethe and the rest of the group that this was Heisenberg’s sketch of a bomb. Bohr himself thought it represented a feasible plan for a bomb. Indeed, the group had been assembled by Oppenheimer at Groves’ request specifically to consider Bohr’s fear that Heisenberg could build a bomb on the principles contained in Heisenberg’s sketch.

A glance was enough. What Heisenberg had drawn was a reactor—Bethe and the others saw this immediately. So naturally, as Bethe told Bernstein a dozen years ago, “our conclusion was that these Germans were totally crazy—did they want to throw a reactor down on London?” It was a natural response, but the mistake was Bohr’s—not Heisenberg’s. Goudsmit conceded as much long before he died, and no other scholar who has been through the records of the German bomb program has any doubt Heisenberg fully understood the difference between a bomb and a reactor.

It was Weisskopf years ago who first told me about Heisenberg’s visit to Bohr during the war. One or two brief accounts had found their way into print and scientists had been wondering about it for years, but I’d never heard of it. I was astounded: was it really true that a leader of the secret German atomic bomb program had told Niels Bohr what the Germans were up to? My astonishment only increased when I learned from Bernstein’s book about Hans Bethe that Heisenberg had drawn a sketch of a reactor for Bohr. I discussed this at great length with Bethe, who remains much puzzled by the whole episode.

Anyone hoping to understand Heisenberg’s role during the war must come up with a satisfactory explanation of this visit to Bohr. The very first step is to recognize what this conversation represents. Heisenberg was a principal leader of German nuclear research during the war. His work had a high military priority, was funded either by the army or by the Reich Research Council under Goering which ran war-related research, and it was secret. Niels Bohr was a citizen of an occupied country, known to be in contact with Allied scientists through the American embassy in Copenhagen, and a Jew besides, at least from the Nazi point of view. Heisenberg told Bohr there was a German bomb program, and he drew for him a sketch of a reactor—the device most likely to make fissionable material for a bomb. The files of the German bomb program and intelligence organizations have been open since the war; there is no evidence whatever that Heisenberg was authorized to tell Bohr these things. In other words, Heisenberg was engaged in what can only be called treason—and not some vague, iffy, technical sort of treason, but a betrayal of secrecy which went to the heart of the matter, and, in fact, soon reached the desks of Allied intelligence officers. And that’s just for starters.