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Building Hitler's Bomb

Anyone who has studied the effort to develop atomic energy in Germany during World War II immediately confronts three questions. Were German scientists trying to make a nuclear weapon? If so, how close did they come to succeeding? And if they had succeeded, would they have turned the weapon over to Hitler? All three questions have been answered in very different ways, but before proceeding to analyze them it is important to clarify what we mean by “German scientists.”

In December 1938, the German physical chemists Otto Hahn and Fritz Strassmann became the first to observe the fissioning of atomic nuclei. Bombarding the heavy element uranium with slow neutrons, they expected to come up with a nucleus of comparable mass but instead were mysteriously confronted with end products at least one of which was the relatively light nucleus barium, roughly half the weight of uranium. A month later, the Austrian-born Jewish physicists Lise Meitner and her nephew Otto Frisch—who had taken refuge in Scandinavia—correctly and momentously inferred from the Hahn-Strassmann observation that the uranium nucleus had been split in two. Neither Meitner, who had been a colleague of Hahn’s in Berlin, nor Frisch, who had been in Hamburg, can be counted among the “German scientists.”

The same goes for Rudolf Peierls, a Jew who had emigrated to England from Germany. It was a 1940 memorandum of Peierls and Frisch (by that time also in England) that persuaded the Allies an atomic bomb was a real possibility. According to Frisch and Peierls, if you could separate a relatively modest amount of the rare uranium isotope U-235 from the common uranium isotope U-238, you could generate an explosive nuclear reaction. Prior to this it was thought that tons would be required—an amount so massive that scientists like Niels Bohr had decided that nuclear weapons were a practical impossibility.

Nor can one count among “German scientists” such physicists as James Franck or Hans Bethe or, needless to say, Albert Einstein, all of them German Jews or Germans of Jewish ancestry and all of them driven from the country in the early 1930’s. Or non-Germans like John von Neumann and Eugene Wigner, Hungarian Jews who were beginning their careers in the German university system before they, too, were forced to emigrate. Or Italians like Emilio Segrè and Enrico Fermi, compelled to leave Italy because of the German-inspired racial laws. (Segrè was a Jew, Fermi’s wife was Jewish.)

Even inside Germany itself there were scientists who should not be included in the list. Strassmann himself, an avid anti-Nazi, was deprived of his livelihood during the war. And then there was the Nobelist Max von Laue, who publicly refused to deny Einstein the credit for the theory of relativity, something most other German physicists were all too ready to do. Similarly, there was Gustav Hertz, a physicist of Jewish ancestry who shared the 1925 Nobel Prize with James Franck and whose students and colleagues hid him in the Siemens industrial laboratories in Berlin for the duration of the war.

Finally, there were German scientists with acceptable racial pedigrees but unacceptable academic credentials. One of them was Manfred von Ardenne, an inventor and entrepreneur, who managed to persuade the German post office to sponsor work in nuclear physics on his estate in Berlin. In early 1941, one of his associates, Fritz Houtermans—who had been jailed by both the Soviet secret police and the Gestapo—observed that plutonium (as it came to be called) was an even better nuclear explosive than uranium. (The same discovery had been made independently by the more conventional German physicist C. F. von Weizsäcker and by the American physicist Louis Turner a few months earlier.) Von Ardenne’s group also made significant progress in separating the uranium isotopes. While their achievements were more or less ignored by the German scientific establishment, after the war the Russians thought them important enough to ship Von Ardenne, his equipment, and his colleagues east, where they helped to make the first Soviet bomb.

Who then is left?

In the fall of 1939, German Army Ordnance decided it was imperative to study nuclear fission for its possible use in weaponry. The military agency took over the Kaiser Wilhelm Institute in Berlin and began drafting physicists and chemists to work on the project. The most important “draftee” was Werner Heisenberg (1901-76), certainly one of the greatest physicists of this century and a man whose prestige in Germany was enormous.

In addition to Heisenberg, some 60 scientists from various institutions eventually joined the project. They came to call themselves the Uranverein, the Uranium Club. Clearly these are the German scientists one wants to consider—they, and the industrial infrastructure that served them. One notorious component of the latter was the Degussa company, which had taken over the Auer metallurgic company—its previous owners had been insufficiently Aryan—and used slave labor to produce uranium oxide for the Uranverein. It, too, should be counted.1



We may now return to the three questions. Was the Uranverein, first of all, trying to make a nuclear weapon? There cannot be much dispute about this, at least before early 1942, when the Army withdrew from the initiative. From then on, the project, now funded by the Reich Research Council, focused mostly on making a functioning nuclear reactor. This shift of emphasis enabled the Germans, and their apologists, to claim after the war that the project had really been aimed at the peaceful use of nuclear energy. But the claim was false. Once plutonium was discovered, reactors became weapons, by virtue of the fact that they could be used to manufacture that element. The Uranverein was very explicit about this in appealing to the government for funding. There was also a smaller program to try to design some sort of exploding reactor that—like a miniature Chernobyl—could spread radioactive material over a large area.

So to the first question my answer is yes. But how close were the Germans to their goal? To this the common answer is, not very.

Various explanations have been given for this. One is that Germany was being pressed hard by Allied bombardment. Another is that, war or no war, it lacked the industrial capacity for the job. A third is that the Uranverein was not really trying all that hard, or was even attempting to sabotage the project. My own favored explanation is none of these, but rather simple incompetence.

One must keep in mind that by early December 1942, Enrico Fermi, with an infrastructure certainly no larger than that available to the Uranverein, had succeeded in making the first functioning nuclear reactor in an abandoned squash court at the University of Chicago. This was something the Germans never achieved. The difference is that our program had Fermi while the Germans had Heisenberg. Although his ego prevented him from acknowledging it, Heisenberg was not a good engineer. If the Germans, who started first, had been able to create a self-sustaining chain reaction, the whole project would have taken on a much greater sense of possibility.

Which brings me to the last question: would the scientists have turned the bomb over to Hitler? Here, our own experience may be relevant. Once the Manhattan Project was launched in December 1941, it came under the wing of the United States Army. Scientists were drafted and sent as soldiers to Los Alamos; in the beginning there was even talk of giving them simulated ranks and putting them into uniform. After the bomb was built, the Army took possession of what it had bought and paid for. Although a few of the scientists involved tried to enter into the decision process, they had no say in what finally was done with the device. Can anyone imagine that things would have been different in Germany?



What is remarkable is that today, a half-century after the fact, the activities and intentions of the Uranverein still provoke debate. In 1993 the journalist Thomas Powers, in Heisenberg’s War, tried to argue that Heisenberg deliberately sabotaged the German project by withholding knowledge about the bomb and even attempted to pass information about it to the Allies. Powers’s book unleashed a hailstorm of protest. Now we have a book-length refutation of it in Heisenberg and the Nazi Atomic Bomb Project by the historian Paul Rose.2

How are such radical disagreements possible? After all, we are not trying to reconstruct a prehistoric civilization from a few drawings on the wall of a cave. This was a project that created a paper trail of hundreds and hundreds of documents. The principals were, at least until a few years ago, all alive and prepared to tell their stories. Let me give two examples that will show just how difficult a subject it nevertheless is.

Early in his book, Rose quotes what he refers to as a “bizarre letter,” undated but purportedly written in the spring of 1970, from Heisenberg to an American woman named Ruth Nanda Anshen. Anshen was the editor and guiding spirit of a series of books by outstanding thinkers. Two of her authors were Heisenberg and the Columbia physicist I.I. Rabi, both of whom were also on her board of editors. Here is the letter, taken directly from Anshen’s own book, Biography of an Idea (1986):

I have finished reading in your “Perspectives in Humanism” series the volume written by Professor Rabi entitled Science: The Center of Culture. I should like to review this important volume. However, I must say to you that I shall have to take exception to Dr. Rabi’s statement that “such a tremendous undertaking as Oak Ridge [where much of our work in isotope separation was performed], with huge, combined efforts of science, engineering, industry, and the Army, would have been impossible in bomb-ridden Germany. . . .”

Dr. Hahn, Dr. von Laue, and I falsified the mathematics in order to avoid the development of the atom bomb by German scientists.

When I read this letter in Rose’s book, I found it not “bizarre” but incredible. Heisenberg was always careful not to make explicit claims of this nature. He let others do that for him: principally the journalist Robert Jungk, whose 1958 book, Brighter Than a Thousand Suns, argued that German nuclear physicists had “obeyed the voice of conscience and attempted to prevent the construction of atomic bombs”; and, after Heisenberg’s death in 1976, Powers and even his widow made similar assertions. Why then would Heisenberg issue so sweeping a statement in so apparently casual a manner? Why had he not revealed this startling information in any of his own published accounts of his wartime activities? And what does “falsified the mathematics” mean? The atomic bomb did not involve a mathematical equation that one could “falsify,” but rather hundreds upon hundreds of engineering details.

And why the mention of Hahn and Laue as his co-conspirators? Hahn was not a mathematician but a physical chemist. While he disliked the Nazis, he did like his creature comforts, and there is no indication he ever risked his life for anything except helping Lise Meitner to escape Germany. (She, for her part, was so infuriated by Hahn’s generally laissez-faire attitude toward the Nazis that after the war she wrote him an exceedingly angry letter.) And as for Laue, although he was at the Kaiser Wilhelm Institute, he had nothing to do with the Uranverein, and in any case was not a nuclear physicist. In short, anyone with the slightest awareness of these matters would find Heisenberg’s claim about Laue and Hahn totally absurd—as Heisenberg would surely have known.

Rose notes another oddity. Anshen gave her papers, including all her letters, to Columbia University; this document is not among them. Still, Rose thinks it “beyond doubt that the letter was genuine.”

Upon reading Rose’s book, I decided to check the matter out. My colleague Cathryn Carson, who has studied much of the Heisenberg nachlass, confirmed that she had come across correspondence between Heisenberg and Anshen, and she approached Helmut Rechenberg, who is in charge of the Heisenberg archive in Munich (and with whom I have strongly disputed Heisenberg’s wartime activities), to let me see it. There are in fact three letters.



The first, from Anshen to Heisenberg, is in English and is dated May 30, 1970. In it she requests that Heisenberg write a review of Rabi’s book for inclusion in a new series she was editing. Heisenberg replied on June 19 in German. In this letter, a page and a half in length, he asks Anshen to inform Rabi of his reluctance to write the review on account of his disagreements with Chapter 7, on the German atomic-bomb project. He is especially upset that Rabi should have accepted the argument—first put forward by the Dutch-American physicist Samuel Goudsmit in his book, Alsos—that the Germans would have been only too happy to turn the bomb over to Hitler had they been able to build it.

It is not difficult to understand why Heisenberg should have objected to Rabi’s reliance on Goudsmit. The latter had led a mission named “Akos”—“grove,” in Greek—to follow the advancing Allied armies into Germany and capture as many of the Uranverein as possible along with their equipment. In the event, nine members of the “club,” including Heisenberg, Hahn, and von Weizsäcker, were interned in England for six months in an estate near Cambridge named Farm Hall. British intelligence had wired the place to record everything the Germans said; the transcripts, which were not released until 1992, form the most vivid record we have of the real thoughts of the Uranverein. Goudsmit clearly had access to them in writing his book, and he and Heisenberg, and later von Weizsäcker, subsequently got into a bitter exchange of letters and articles over his interpretation of what they contained.

The rest of the June 1970 letter to Anshen is Heisenberg at his self-exculpatory best. He constructs some sort of murky parallel: Rabi, he writes, “completely overlooks the fact that the German physicists had about the same kind of psychological attitude toward putting a bomb in Hitler’s hand as many Americans have today about the possibility of ending the American war with North Vietnam by dropping a hydrogen bomb on Hanoi.” He then ends by asking Anshen to take these matters up with Rabi so that they can avoid a public dispute. There is no mention of Oak Ridge or the Allied bombing of Germany; no mention of falsifying data; no mention of Hahn and Laue.

On July 9, Anshen replied, saying that she had spoken to Rabi and had decided it might be better if Heisenberg did not review his book after all. She writes: “Professor Rabi would not wish to enter into a polemical discussion with so great a physicist as you are.” Knowing Rabi as I did, I can just see him concocting this phrase—“so great a physicist as you are”—with some glee. Rabi knew his man. He had offered Heisenberg a job at Columbia in 1939, on Heisenberg’s last visit to America just before the war. As Rabi later told me, Heisenberg turned him down on the grounds that he did not want to lose his tenure in the German university system.

What should we make of all this? Unless and until someone comes up with the original of the letter that Rose quotes from Anshen’s book, I shall regard it as a chimera.



Now for the second example of how difficult it is to determine Heisenberg’s role. This is a more complicated story, and I have to take a certain responsibility for it. Beginning in November 1977, on and off for two years, I conducted a series of tape-recorded interviews with Hans Bethe that ultimately led to a three-part New Yorker profile. During the war, Bethe (who arrived in this country in 1935) had been the head of the theoretical division at Los Alamos and therefore in the inner circle of J. Robert Oppenheimer and his advisers. In the course of our conversations I asked him whether, while working on the bomb, the scientists at Los Alamos knew about the status of the German nuclear project. Such intelligence would have been a closely held secret at the time, but now Bethe shared some important information with me.

It seems that in September 1941 Heisenberg had come to Copenhagen, where he met Niels Bohr. Denmark was then an occupied country.3 The ostensible reason for Heisenberg’s visit was to take part in a conference of astronomers organized by the so-called German Cultural Institute, an outfit set up to distribute Nazi propaganda. Bohr boycotted the conference and there was some question as to whether he would see Heisenberg at all, even though in the late 1920’s and early 1930’s the two of them had hammered out together what is known as the “Copenhagen interpretation” of quantum mechanics, still in use today. It seems Bohr wanted at least to invite Heisenberg for dinner, but his wife Margrethe, who never much liked Heisenberg, objected; she felt that his whole visit was “hostile.”

Bohr evidently managed to persuade her. After dinner, he and Heisenberg had a private talk, the contents of which have become one of the most controverted aspects of Heisenberg’s entire wartime record. Of this discussion, Bohr’s son Aage, his father’s closest wartime confidant and a Nobel Prize-winning physicist in his own right, has written:

In a private conversation with my father, Heisenberg brought up the question of the military applications of atomic energy. My father was very reticent and expressed his skepticism because of the great technical difficulties that had to be overcome, but he had the impression that Heisenberg thought that the new possibilities [perhaps an implicit reference to plutonium] could decide the outcome of the war if the war dragged on.

Then what happened? According to my informant Hans Bethe, Heisenberg gave Bohr a drawing of something purporting to be the design of a German nuclear weapon. Later this drawing was “transmitted to us in Los Alamos.” I did not ask Bethe how much later, and I also did not ask him what he meant by “transmitted.” He told me that he and Edward Teller, asked to analyze the drawing, saw at once that it was a nuclear reactor. “But our conclusion was, when seeing it, these Germans are totally crazy. Do they want to throw a reactor down on London?”

My New Yorker profile of Bethe, which included this quotation, marked the first time anyone had ever mentioned such a sketch in print. Both Powers and Rose linger over the event, but they draw from it almost opposite conclusions.

Powers is convinced that, in passing classified information to Bohr, Heisenberg committed the act of a traitor—a traitor, that is, to Germany. In other words, Heisenberg as Powers portrays him is to be considered not a Nazi collaborator but a hero of the Resistance. For Rose, by contrast, in showing the drawing Heisenberg meant to intimidate Bohr, to convince him that, since the atomic bomb would play a role in the forthcoming “Pax Nazica” (Rose’s term), he should resolve to throw in his lot with the German scientists.



Both Powers and Rose have persuaded themselves that Heisenberg really did hand Bohr a secret sketch of a German nuclear weapon of some sort. But did he? In 1994, Abraham Pais, Bohr’s biographer, called me into his office at Rockefeller University. Some months earlier, Powers had asked him what he knew about the drawing. It seems that Powers had received a letter from Aage Bohr stating flatly that “Heisenberg certainly drew no sketch of a reactor during his visit in 1941. The operation of a reactor was not discussed at all.” Nor, according to Aage, did Bohr know anything about plutonium until he was briefed about the Allied project after escaping from Denmark in the fall of 1943.

I was taken aback by this news from Pais. Had I propagated a serious error? I now queried everyone I knew who had had a senior role at Los Alamos. Bethe repeated in writing what he had told me earlier; no one else seemed to know anything. Then I made contact with the late Robert Serber, one of Oppenheimer’s closest collaborators and a man noted for both his excellent memory and his extensive store of documents. Serber not only filled me in on what had happened but also sent me some corroborating papers.

When Bohr got to England in September 1943, he was briefed by the British on the Allied nuclear-weapons program. Whether he told them what he knew about the German program is uncertain. But upon arriving in the United States in early December, he met with General Leslie R. Groves, who was in charge of the Manhattan Project, and apparently showed him some kind of drawing. Groves was sufficiently alarmed to alert Oppenheimer, and on December 31, just after Bohr arrived at Los Alamos with his son, Oppenheimer called together a select group of staff members to meet them.

Serber gave me a copy of the letter Oppenheimer sent to Groves after the meeting. It lists the attendees, and I managed to contact all of those still alive: Victor Weisskopf, Robert Bacher, Aage Bohr, Teller, and Bethe, in addition to Serber himself, who recalled coming in a little late and being told by Oppenheimer that they were discussing a proposal of Heisenberg’s for a nuclear weapon and being shown the drawing, which he recognized as a reactor.

No one I spoke with could say whether the drawing was supposed to have been made by Heisenberg or was done by Bohr from memory, and the drawing itself seems to have vanished. In any case, Bethe and Teller wrote up a report showing that such a reactor could never explode like a nuclear weapon. No reactor could: this is what Frisch and Peierls had understood in 1940. A reactor operates with U-238, which only fissions when it is struck by slow neutrons, while a bomb is made of U-235 or plutonium, both of which are fissionable by fast neutrons; the entire explosive reaction in a bomb lasts only a hundredth of a microsecond.

Whether Heisenberg ever really apprehended this distinction is another subject of vehement debate. He himself said he did, and Powers agrees with him. Goudsmit and Rose say he did not, and I agree with them. The Farm Hall transcripts record a discussion among the Germans just after they first learned about Hiroshima, and it is clear to me that they lacked even a rudimentary understanding of how a nuclear weapon works. In a few days, Heisenberg figured it out and gave his fellow detainees a lecture; from their comments, it is obvious they were hearing about all this for the first time.

In reading Bethe and Teller’s report, I realized that this was not just any reactor they were analyzing but a particular design Heisenberg had clung to even though a number of junior theorists in the Uranverein had demonstrated its inefficiencies. Specifically, the design involved layers of uranium metal submerged in “heavy water”—water consisting of oxygen and heavy hydrogen, a rare isotope with one extra neutron. The Germans used heavy water to try to moderate the neutrons in their reactor designs, but it was very hard to come by and they never had enough of it. To me, this detail signified that the drawing was indeed something that had come out of the Uranverein.

But how to reconcile all this with Aage Bohr’s absolute certainty—which he conveyed to me once again in a message delivered by Pais—that no reactor was discussed when his father met Heisenberg in 1941 and that no drawing changed hands? Here I will hazard a guess: the drawing came to Bohr from someone else on some other occasion. Indeed, the notion that Heisenberg gave Bohr the drawing simply does not fit his character. Although never a Nazi, Heisenberg was a patriotic German, and both during and immediately after the war he told several people he had wanted the Germans to win. As he confided to Bethe, he was afraid that if the Allies were victorious they would level Germany and destroy German culture; but if the Germans won, the “good Germans” would take over and restore things to the way they had been before the Nazis.

What I think happened is that someone else from the Uranverein must have visited Bohr and given him the information. A likely candidate, it seems to me, is the physicist Hans Jensen, who was in Copenhagen in 1942 and did discuss the German program with Bohr. Since Bohr was then still persuaded that, in any practical sense, nuclear weapons were impossible, he probably filed Jensen’s report somewhere in his head until he was briefed in England about the Allied project. Then he recalled what he had been told and, perhaps, drew a picture. I cannot prove this, but there are some things about this history that we may never know for sure.



Postscript. When I first saw the Farm Hall transcripts in 1992, I thought they had the makings of an interesting play. This is precisely what the well-known British playwright Michael Frayn has undertaken to do in Copenhagen, which is now enjoying a successful run in London.

Frayn is not a physicist, but he has evidently read a great deal, and for the rest he has let his imagination wander. Some things—facts, names—he has gotten wrong; they are minor. But what is not minor is a bit of dialogue he has given to Bohr. Referring to Werner Heisenberg, Bohr says, “A White Jew. That’s what the Nazis called him. He taught so-called Jewish physics. And refused to stop. He stuck with Einstein and relativity, in spite of the most terrible attacks.”

Now, it is true that Heisenberg had been attacked for doing “Jewish physics,” and had even been called a “White Jew.” But in July 1938, with the help of a family connection, he was vetted and cleared by Heinrich Himmler himself, who took the occasion to suggest he dissociate himself from the “personal and political attitude of the scientists involved.” In other words, German physicists could keep relativity, but without Einstein.

This understanding of things was duly codified at a conference in the Tyrolean Alps in November 1942 attended by 30 German scientists, including Heisenberg. The summary report, written by von Weizsäcker, stated that “one must reject the imposition of the physical relativity theory into a world philosophy of relativism, as has been attempted by the Jewish propaganda press of the previous era.” The report also stated that “Einstein [had] merely followed up already existing ideas consistently and added the cornerstone.” This does not sound like sticking with Einstein; far from it.

Still, Frayn does seem to me to have captured something of Heisenberg’s moral ambiguity. His Heisenberg is neither a Resistance hero nor a simple Nazi collaborator—in Copenhagen, he does not pass the drawing on to Bohr—but something more interesting and perhaps more troubling. Frayn raises the question of why Heisenberg and, for that matter, Bohr never did the relatively simple calculation performed by Frisch and Peierls: the one that showed a bomb could be built. Certainly both of them were capable of it. The suggestion of Frayn’s play is that somewhere deep in their psyches they were held back because they did not want to know the answer.

Perhaps so. In any case, this is another thing about the history I have been recounting that will probably never be known for sure.



1 Degussa is still in business. It has a Website with a smiling face, and one of its activities has been to supply Iraq with nuclear material.

2 University of California Press, 391 pp., $35.00.

3 Denmark was not the only occupied country Heisenberg visited in the course of the war. In December 1943, he went to Krakow on the invitation of his brother’s old schoolmate Hans Frank, then enthusiastically engaged, as the governor-general of Poland, in supervising the extermination of Polish Jewry; one wonders what they talked about.


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