Volume 28, Number 3 July 1999

By Jeremy Bernstein, AIP Press, Woodbury, New York. 1996, ISBN 1-56396-258-6

By Paul Laurence Rose, University of California Press, Berkeley, 1998, ISBN 0-520-21077-8

More than 50 years after the end of World War II, controversy continues to swirl around Werner Heisenberg and his role in the German nuclear project. The project was a failure: the Germans made no serious effort to develop an atomic bomb and even their attempt to build a nuclear reactor did not succeed, although by the war’s end they were very close. The controversy involves two questions, one technical and the other moral: how well did Heisenberg understand the physics of a nuclear explosion, and what was his attitude toward working on a project that could create an atomic bomb for Hitler? The books under review shed new light on both questions.

Heisenberg was not a Nazi but he considered himself a patriotic German and declined repeated invitations to flee to the West. He readily accepted the call to lead the German nuclear project even though, as he claimed in a 1965 interview, the possibility of making atomic bombs "created a horrible situation for all physicists, especially for us Germans…because the idea of putting an atomic bomb in Hitler’s hand was horrible [1]."

The German project at first progressed at about the same pace as that of the Western Allies. By early 1942, however, Heisenberg had decided that making a bomb within the short time frame imposed by the Nazi leadership would be impossible. "We were happily able to give the authorities an absolutely honest account of the latest development, and yet feel certain that no serious attempt to construct atomic bombs would be made in Germany," he explained [2]. He was spared the moral dilemma of deciding what to do if the bomb seemed achievable. His account might, to be sure, be self-serving. If he had tried his best to build a bomb for the Nazis and just failed, he would hardly have admitted it publicly after the war. But the physics community, with a few exceptions, generally accepted his explanation.

At the end of the war Heisenberg and nine other German scientists were interned at Farm Hall in Britain, where their conversations were secretly recorded. British authorities declassified those conversations only in 1992; the raw transcripts were published under the title Operation Epsilon. A new edition with commentary and additional background material by Jeremy Bernstein was published in 1996. Bernstein’s contributions add immeasurably to the value of the book, which is required reading for anyone seriously interested in the subject.

The most fascinating parts of the Farm Hall transcripts are the conversations that took place just after the Hiroshima bombing was announced. The Germans at first speculated that the weapon was not a true atomic bomb. After confirming that it was, they tried to figure out how the bomb had been made and conducted a post mortem on their own project, including some recriminations and apportioning of blame.

The transcripts contain a wealth of information about Heisenberg’s understanding of bomb physics. They show that he understood the difference between a reactor and a bomb and that he did realize that fast neutrons are essential for an explosion. But they also reveal some serious misconceptions. Heisenberg apparently believed that several tons of pure ²³⁵U are needed to make a bomb. He made the astonishing disclosure that he had never calculated the critical mass. "Quite honestly I have never worked it out, as I never believed one could get pure 235," he told Otto Hahn. He thereupon performed a crude calculation, based on a random-walk argument, which contains a basic error and yields a critical mass of one ton, far greater than the correct value of about 50 kg[3]. A week later, he presented a more sophisticated derivation, which yields a much lower critical mass. Bernstein gives a lucid analysis of the entire episode.

Confusion persists over Heisenberg’s ideas concerning the critical mass. When Hahn reminded him that he "used to say that one needed 50 kg of 235 in order to do anything," Heisenberg replied equivocally: "I wouldn’t like to commit myself for the moment." When asked at a 1942 briefing for Army officials how big an atomic bomb would have to be in order to destroy London, Heisenberg replied "as big as a pineapple." And a February 1942 German Army report, whose authorship is unknown but which Mark Walker attributes to Heisenberg, contains the statement that "10-100 kg of material" might be sufficient for a bomb.

What is one to make of all this? My best guess is that, although Heisenberg never made a detailed calculation of the critical mass, he believed all along that it is very large. His Farm Hall calculation sounds like a reenactment of one previously performed, not like something concocted on the spur of the moment. If he had on some earlier occasion calculated a much smaller critical mass, he would have remembered it. The smaller numbers cited might refer to the amount of uranium that must fission in order to produce an appreciable weapon yield, a quantity quite different from the critical mass. Another possibility is that the smaller figures refer to plutonium rather than uranium.

During the days that followed, the German scientists discussed the reasons for the failure of their own project as well as the morality of making a bomb. Those discussions formed the genesis of the "Lesart" (German for "version") which attributes moral superiority to the Germans. Carl von Weizsacker put it most directly: "History will record that …the peaceful development of the uranium engine was made in Germany under the Hitler regime, whereas the Americans and the English developed this ghastly weapon of war." The Lesart was promoted by a number of writers, notably the Swiss journalist Robert Jungk[4].

Thomas Powers went even farther, asserting that "Heisenberg did not simply withhold himself, stand aside, and let the project die. He killed it [5]." Powers’ evidence is flimsy and his assessment is not widely shared by scholars. Heisenberg himself never publicly claimed to have sabotaged the project [6].

The Lesart is a fatuous argument. When the German scientists initiated their project it was not at all clear that making a bomb would prove to be impossible. The Germans were aware, moreover, that a reactor produces plutonium, which can be used as fuel for a nuclear explosive; hence reactor research cannot be characterized as being purely peaceful. Finally, as the Farm Hall transcripts reveal, several of the German scientists regretted that they had failed to make a bomb. Only Hahn and Max von Laue are unequivocally repelled by atomic weapons. Heisenberg is circumspect on the subject.

Sam Goudsmit, the leader of the mission that followed the invading armies in 1945 to investigate the status of the Nazi bomb program, was incensed at the arrogance of the Lesart. I knew Goudsmit quite well (as did Bernstein) and can attest to the depth of his feelings. He was convinced that the German scientists were willing participants in the Nazi project, whose failure he attributed to scientific blunders and the repressive Nazi administrative system. Goudsmit expressed these views forcefully in his book Alsos, published just after the war, as well as in several magazine articles. There followed an acrimonious exchange of letters between himself and Heisenberg; the two had been close friends before the war.

The historian Paul Lawrence Rose is Goudsmit’s spiritual heir. His book contends that Heisenberg’s account of the German project is consistently and intentionally misleading, an attempt to cover up his scientific blunders and to protect his reputation. Rose uses the Farm Hall transcripts skillfully to support his argument. His book is meticulously documented and makes a strong case, although his argument seems somewhat overstated. Rose’s antipathy toward Heisenberg is apparent from the first page; his book reads more like a polemic than a scholarly work. He is scathingly critical of practically everyone who has written on the subject. A more objective approach would have made his argument more credible. His book is nonetheless a valuable contribution to the debate and deserves careful study. Ultimately, each reader must be the judge of Heisenberg’s true feelings.

1. Thomas Powers, Heisenberg’s War: the Secret History of the German Bomb, Alfred A Knopf, 1993, p. 584.

2. Werner Heisenberg, Physics and Beyond: Encounters and Conversations, Harper & Row, 1971, p. 180.

3. Heisenberg’s calculation in fact leads to an even greater critical mass–about 13 tons. His value 1 ton was a numerical error.

4. Robert Jungk, Brighter than a Thousand Suns, Harcourt Brace Jovanovich, 1958.

5. Powers, op cit, p. 479.

6. In Biography of an Idea, Ruth Nanda Anshen quotes Heisenberg as writing to her in 1970 that "Dr. Hahn, Dr. von Laue, and I falsified the mathematics in order to avoid development of the bomb by German science." No such letter appears in Heisenberg’s files and Nanshen’s own copy is inexplicably missing; both Hahn and Laue were dead by that time. The story does not ring true: the only mathematics that might have been falsified would have been a calculation of the critical mass, which, as noted above, Heisenberg never performed. Hahn was not a theorist and understood very little about the chain reaction, and Laue had no connection with the nuclear project. Paul Rose is nonetheless convinced that the letter is genuine.