In response to:

The Charms of a Physicist from the April 11, 1991 issue

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.

What chiefly distinguishes Weisskopf’s account of Heisenberg’s role is generosity. He is willing to make allowances for Heisenberg’s refusal to leave Germany in the 1930s (he loved his country), for Heisenberg’s excessive caution in speaking to Bohr (“the delicacy of [his] mission”), even for predictions of German victory (fear an incautious word might follow him back to Germany). Even now a certain German rigidity in Heisenberg can make Weisskopf bristle (he is himself Austrian), but he grants the main thing—whatever ordinary human failings of which Heisenberg may have been guilty, he ran a bomb program for the Germans which could hardly have been better designed by the Allies themselves. But his old colleagues have for the most part grudgingly refused to admit as much. Weisskopf once told me that not long after the war he helped to arrange a lecture for Heisenberg at MIT, and held a party for him at his home. About half the invited guests never arrived. Later Weisskopf asked them why. Many were veterans of Los Alamos, but they gave the same explanation as the others: they didn’t want to shake the hand of the man who tried to make a bomb for Hitler.

Did Heisenberg try? Heisenberg himself always insisted he was spared the moral agony of a decision because the job was simply too big for Germany in wartime. Maybe so, and maybe not; we can’t say for sure because Germany never took up the task. A main reason it did not was Heisenberg’s advice to leave it alone. We are thus brought back to something inherently difficult to establish—what Heisenberg really thought, wanted, and intended. In the international community of physicists, Weisskopf is one of the few who have been willing to give Heisenberg the benefit of the doubt.

This ancient history still matters because it was fear of a German bomb that led to invention of the bomb. If that fear was misplaced, we ought to find it in us to admit the fact. Bernstein thinks Heisenberg tried and failed. Bernstein does not word his judgement harshly, but the case he makes is really only a softer version of the one first circulated nearly fifty years ago by Sam Goudsmit—that Heisenberg was some sort of moral weasel, a klutz when it came to bomb physics, and devoted to Germany uber alles. This is a strange conclusion to reach about a man who spared the world the horror of Hitler with a bomb. What happened begins to make more sense as soon as you recognize that Heisenberg committed a capital crime when he told the secrets to Bohr, and knew he was doing it.

Thomas Powers
South Royalton, Vermont

To the Editors:

As Secretary-Treasurer of the Struik Defence Committee in the early ’50s, I would like to clarify several things in Jeremy Bernstein’s admirable review of Victor Weisskopf’s The Joy of Insight: Passions of a Physicist. I think I owe it to the historical record of those bizarre times. In his review, Bernstein says “Weisskopf compares [Wendell] Furry’s case with that of MIT mathematician Dirk Struik, whose situation, for some unstated reason [itals. mine], he calls ‘more serious.’ ”

The more serious reason was that Struik was then charged under a state indictment with conspiracy to overthrow the government of the state of Massachusetts. Bernstein writes, “Struik had left-wing sympathies and appears, although Weisskopf does not make this clear, to have refused to sign the absurd loyalty oath to the state of Massachusetts that all teachers in the state were required to sign.” Struik, in fact, was an avowed Marxist, whose Marxist study sessions placed him in no mean jeopardy, beyond that of merely refusal to sign the loyalty oath. The Struik Defence Committee was formed spontaneously to collect funds for his legal expenses as a way of trying to correct for the unfair advantage the state had in its unlimited access to legal resources. In the event, an illustrious list of citizens rallied to this cause, a factor that fortunately helped to delay action on his case for several years.

“Five years later,” Bernstein continues, “he too won his court case.” As it happened, the Struik case never came to court. The Massachusetts law was declared unconstitutional, conflicting as it did with a federal law to the same end, as the result of Steve Nelson’s successful challenge to a similar state law in Pennsylvania.

Lee Ambrose
Silver Spring, Maryland

Jeremy Bernstein replies:

Since writing my review of Victor Weisskopf’s book The Joy of Insight, I have received a number of letters concerning the issues raised. I have also had the opportunity to discuss these matters with several colleagues. I am grateful to all of them and I would, in particular, like to single out Sir Rudolf Peierls with whom I had several long conversations, and who was kind enough to share with me some of his file of letters and documents. I have also read, with great interest, a book entitled German National Socialism and the Quest of Nuclear Power by Mark Walker (Cambridge University Press, 1989). Professor Walker, of Union College, was also kind enough to send me reprints of two of his articles. I will discuss the relevance of these when I turn to the matter of Werner Heisenberg. I would also like to thank Professor Joseph Auslander of the University of Maryland (in a letter not published here) and Ms. Lee Ambrose for their letters concerning the mathematician Dirk Struik of MIT who was suspended from that institution after being indicted by the Commonwealth of Massachussetts in 1951 for conspiracy to overthrow same. Both of these writers pointed out to me that this case never came to trial and was dropped in 1955. Professor Auslander’s letter also contains the welcome news that Dirk Struik is alive and well at age ninety-six, and still writing elegantly about the history of mathematics. More power to him!

Upon reading my colleague Robert March’s letter, I reread what I had written about U-238 and U-235 and, indeed, found it somewhat muddled. March’s comments are a useful clarification to which I would like to append the following. It is U-238, the main constituent of natural uranium, that is the essential isotope used to make plutonium by a process of nuclear transmutation. This is done either in a reactor or in an accelerator like a cyclotron. Indeed, the purpose of building the reactors at Hanford, Washington, during the war was to produce plutonium, which had first been created in a cyclotron. It is not correct, as March asserts, that the “entire German scientific community missed the possibility of creating Plutonium 239.” The use of transuranics—elements beyond uranium in the Periodic Table—was well understood within the German program. In 1940 C.F. von Weizsäcker—about whom I will have more to say shortly—delivered a report to the German Army Ordnance on the use of transuranics to make nuclear explosions. The Germans attempted to produce plutonium using a cyclotron which they took over in Paris, and they understood the use of reactors, which they were never able to build, to manufacture plutonium. With all due respect, I would suggest that my esteemed colleague read Professor Walker’s book. Hans Bethe once told me that one of the reasons that Heisenberg failed to make a reactor was because he very badly underestimated the amount of heavy water—deuterium—he would need as a moderator for neutrons. Heisenberg, as opposed to people like Fermi, was simply not especially competent at doing this kind of engineering physics.

Now I would like to turn to Mr. Powers. Before I deal with the contents of his letter I will quote, once again, the paragraph in Weisskopf’s book to which I took exception. I have interpolated several of my own comments. He writes,

Obviously, by remaining in Germany he [Heisenberg] soon became involved in the efforts of the Nazi regime to exploit nuclear energy. [It would be more correct to say that Heisenberg encouraged the regime to exploit nuclear energy], which began as soon as the possibility of a bomb became evident. The scientists on the Allied side, specifically those who were more or less opposed [Weisskopf offers no explanation of what “more or less” means in this context, nor how he knows how these scientists, whom he does not identify, were opposed to the regime, more or less] to the Hitler regime, faced very different situations. The Allied scientists were eager to work on the bomb because they trusted the policies of their leaders, Churchill and Roosevelt, and wanted to prevent the other side from developing the bomb. Those German scientists who, like Heisenberg, were not in favor of the Nazis would have been in a terrible situation if they had been called upon to devise this awesome weapon.

To me, the clear meaning of this statement is that, according to Weisskopf, an important part of the reason that the Germans failed to build the bomb was lack of “eagerness,” and that they were fortunately spared what Weisskopf thinks would have been a moral dilemma for them, of whether or not to turn such a weapon over to Hitler.

Not only do I believe that Weisskopf’s statement is false, but I also believe that it could have a pernicious effect because if read carelessly it confers on the German scientists like Heisenberg and von Weizsäcker a moral stature which, in my view, they do not have. Indeed, beginning almost immediately after Hiroshima, the German scientists began to try to rewrite history, an enterprise in which they were to find, along the way, willing coauthors. The first of these was Robert Jungk whose book Brighter Than a Thousand Suns was published in 1956 in Germany and was widely read. Here is what Jungk had to say, “It seems paradoxical that the German nuclear physicists, living under a saberrattling dictatorship, obeyed the voice of conscience and attempted to prevent the construction of atom bombs, while their professional colleagues in the democracies, who had no coercion to fear, with very few exceptions concentrated their whole energies on production of the new weapon” (Brighter Than a Thousand Suns, Robert Jungk, Harcourt, 1958, p. 105). While I doubt that Weisskopf endorses this point of view, my impression from Mr. Powers’s letter is that he does.

As to the contents of his letter, while it is quite true that the fact that Sam Goudsmit’s father was gassed in Auschwitz on his seventieth birthday along with his blind mother did not improve Goudsmit’s view of Germans, as a whole, this does not, I would argue, disqualify him as a witness to history. I saw Sam Goudsmit on an almost daily basis during the period that I worked at the Brookhaven National Laboratory, and we had many discussions about the German atomic bomb project. He certainly held people like Heisenberg and von Weizsäcker in contempt for their collaboration with the regime, but I did not have the impression that he blamed Heisenberg personally for what happened to his parents. This aside, it is totally irresponsible for Mr. Powers to say that Paul Lawrence Rose appears to have gathered his information mostly from Goudsmit’s files when he has not read Professor Rose’s paper. The relatively brief paper that was circulated had attached to it twenty-one pages of footnotes and citations. Mr. Powers would do well to read Professor Rose’s paper before he makes comments about it. It is also true that one should not read Goudsmit’s book Alsos as the definitive history of this subject. There are serious errors in the book, including the fact, which Goudsmit later recognized, that the Germans knew about plutonium. But what Goudmsit did get right were the characters of Heisenberg and von Weizsäcker. I would give Mr. Powers the same advice that I gave Professor March—read Walker’s book.

As to the question of the critical mass. Everyone agrees, as far as I can see, that Heisenberg never made, at least until after Hiroshima, a serious estimate of the critical mass. There is evidence, for example from the Farm Hall tapes (see the introduction to the 1983 edition of Alsos, Tomash Publishers, written by R.V. Jones. Jones was the British intelligence officer whose idea it was to record the captured German scientists) that the wartime calculations of Heisenberg convinced him that the critical mass was the order of tons rather than kilograms. The reason that Heisenberg never made a careful estimate of this number is, in my view, incompetence. It is the dimension of this mass that, in some sense, determines the dimension of the entire enterprise; for example how much Uranium-235 or Plutonium needs to be manufactured. There were better estimates within the German bomb project—between ten and one hundred kilograms. Professor Peierls told me that the favored number was near the high end. His estimate, with Frisch, of 600 grams was certainly too low. I find the notion, implied by Powers’s letter, that Heisenberg sabotaged the German project by deliberately telling German officials one thing, when he believed another, beyond credence. To me, it is still another example of the attempt of Heisenberg to rewrite history. It should also be said that even if Heisenberg had made a correct estimate of this mass, it is unlikely that this would have affected the project. In the beginning, the Germans felt they would win the war rapidly so that nuclear weapons would be irrelevant and later, when they began losing the war they believed that these weapons could not be made quickly enough to change the outcome. For this reason, while the bomb project was given high priority—it was always well-funded—it did not have the top priority, which meant for example that many of the young scientists were drafted and sent to the front where many of them died.

As to Heisenberg’s visit to Copenhagen in October of 1941, this meeting has to be put in context—again I refer to Walker’s book. As Walker points out, Heisenberg did not go to Copenhagen only to see Bohr. He went with von Weizsäcker to attend an astrophysics conference which had been ranged at the German Cultural Institute. These cultural institutes were set up in the occupied countries for the purpose of presenting to the occupied peoples the merits of German culture. Ernst von Weizsäcker, Friedrich von Weizsäcker’s father, who in 1947 was found guilty of complicity in the deportation of Jews to concentration camps, was state secretary in the Foreign Office. He played an active role in setting up these cultural centers, which explains why this conference of German scientists was held in Copenhagen. In the next few years, Heisenberg functioned as a kind of roving ambassador for German science and visited several of these cultural institutes. In his visit in 1943 to Holland he explained, according to eyewitnesses, how history had legitimated Germany to rule both Europe and the world. The Danish physicists boycotted the conference, but both Heisenberg and von Weizsäcker visited Bohr’s institute. During one of these visits Heisenberg made the same sort of comments he was later to make to the Dutch. He also had a private conversation with Bohr, during which matters connected with the military use of nuclear energy were discussed.

No one knows exactly what was said in these discussions, nor what Heisenberg’s true motivation was. But to put it bluntly, I find Mr. Powers’s version of these events absurd. Heisenberg was naive, arrogant, insensitive, egocentric, but he was not especially courageous and, unless I am presented with better evidence, I see no reason whatsoever to believe that Heisenberg was ever knowingly traitorous to Germany. Naively, one could say, Why not ask von Weizsäcker, he was there? The trouble with this is that from 1945 until the present day von Weizsäcker has not written anything about his wartime activities to which I would attach the remotest credence. Typical is a lecture he delivered at CERN in Geneva in January of 1988. This is what he said he did in 1939 when he first learned about fission and realized it could be used to make nuclear weapons: “I immediately went to my friend Georg Picht, who was a classical scholar and a philosopher, to discuss the question of how mankind was to survive given the fact that, if the chain reaction was possible, it would inevitably be carried out in as much as you could not stop elsewhere in the world doing it even if you did not do it yourself. If mankind was to survive, we concluded, it was necessary to abolish the institution of war. No other solution would work.” This is the same von Weizsäcker who, in 1940, was explaining to German Army Ordnance how to use transuranic elements to make nuclear bombs. I believe that the Farm Hall transcripts would show that Heisenberg and von Weizsäcker began a conscious effort to distance themselves from their wartime activities as soon as they learned of Hiroshima. To really disentangle what Mr. Powers calls this “ancient history” we must have these transcripts. They represent the raw data. A half century, or nearly, has gone by and still the British government will not release them. Why? One plausible reason is that Richard von Weizsäscker, Friedrich von Weizsäcker’s brother, is the current president of the German Democratic Republic.

This Issue

June 27, 1991