To the Editors:

One of the more contentious issues involving Werner Heisenberg’s visit to Niels Bohr in Copenhagen in September of 1941 has been whether or not Heisenberg gave Bohr a drawing of what turned out to be a reactor [see Thomas Powers, “The Unanswered Question,” NYR, May 25, 2000; and “Heisenberg in Copenhagen,” Letters, NYR, October 15, 2000, and February 8, 2001]. Bohr’s son Aage, who was his father’s confidant at the time, has categorically denied that his father was given such a drawing, while others such as Thomas Powers are persuaded that Aage Bohr was wrong and that indeed such a drawing was given to his father.

Before I describe what seems to me to be the definitive answer to this dilemma, let me review the agreed-upon facts. There is no dispute that when Bohr and his son arrived in the US in December of 1943, Bohr had such a drawing. He showed it to General Groves, who was in charge of the atomic bomb program and it alarmed Groves, who, like Bohr, thought it might be the design of a German nuclear weapon, to such an extent that he insisted that Robert Oppenheimer convene a meeting at Los Alamos to discuss it as soon as Bohr arrived there. On the thirty-first of December there was such a meeting and the drawing was passed around. Oppenheimer also stated that the drawing was connected with Heisenberg, although none of the people who were at the meeting, with whom I later talked, were sure that Heisenberg had drawn it. The original drawing seems to have disappeared. But it was sufficiently detailed so that Hans Bethe and Edward Teller were able to analyze its physics and to conclude that it was indeed a reactor which could never explode like a bomb.

Upon reading their report it was clear to me that indeed the design was that of one of Heisenberg’s experimental reactors. But if the drawing did not come from Heisenberg directly, where did it come from? I made the conjecture that some other physicist with a connection to the German program must have visited Bohr and was the source of the drawing. Indeed, I discovered that there was such a physicist, Hans D. Jensen, who visited Copenhagen in both 1942 and 1943, and I conjectured that it was Jensen who made the drawing for Bohr or at least told Bohr enough about the project so that Bohr could make his own drawing.1 (Incidentally, Jensen shared the Nobel Prize for Physics in 1963 for his work in nuclear physics. During the war, while not one of the central figures, he did take part in the German program. He died in 1973.) What I wish to do here is to explain why I am now convinced that this conjecture is correct. In this I am indebted to Klaus Gottstein and Helmut Rechenberg, who were associated with Heisenberg and discussed with him and with C.F. von Weizsäcker, who accompanied Heisenberg to Copenhagen, their recollections of the visit.

First some dates. Bohr was able to communicate via the underground with the British physicist James Chadwick, who was deeply involved with the British nuclear program. Chadwick wrote to Bohr on January 25, 1943, inviting him to come to England. Bohr’s response is undated, but after declining the invitation he explained to Chadwick that, in his view, the practical use of nuclear energy was in the foreseeable future impossible. As he put it, “Above all I have to the best of my judgement convinced myself, that in spite of all future prospects any immediate use of the latest discoveries of atomic physics is impracticable.” But by some months later he had changed his mind. He wrote Chadwick a second undated letter in which he said that he was now persuaded that solid pieces of uranium placed in “a large tank of heavy water” could produce nuclear energy.

This, grosso modo, was Heisenberg’s design. The letter itself is quite curious. Bohr analyzes the reactor in somewhat the spirit of Bethe and Teller, concluding while if it exploded, although it would produce an explosion substantially greater than a chemical explosion, it would probably not be worth making as a weapon, because of its complexity. But then he ends up with the statement that “the situation, however, is of course quite different, if it is true that enough heavy water can be made to manufacture a large number of eventual atom-bombs.” He must have felt that this was enough of a danger so that when he arrived in the United States the following December, he called it to the attention of Groves. This was the reactor design whose drawing Bethe and Teller analyzed. We do not know the exact date of Bohr’s letter, but we do know that it was first discussed by Chadwick at a meeting on September 10, where he noted that he had been in communcation with Bohr “within a month or so.” He must have received this letter in the late summer.2 What had occurred between the two letters to cause Bohr to change his mind? On the fifth of May, Heisenberg gave an invited lecture on his program which was sponsored by the German air force academy.3 Copies of this lecture were distributed to the audience. Rechenberg conjectures that Jensen had one of these copies and described its contents to Bohr when he made his second visit to Bohr in the summer of 1943. Hence the drawing. This explanation seems totally reasonable to me and resolves all the contradictions.


What is interesting about this particular lecture of Heisenberg is what he did not say. He did not tell his audience that an important use of his reactor would have been to produce what came to be called plutonium as an explosive. This is something that he had spelled out to a different audience in February of 1942.4 I do not know if Jensen knew about this but he certainly did not tell Bohr. Until Bohr left Copenhagen he knew nothing of this. If he had, his warning to Chadwick would have been much more dire, and the people at Los Alamos truly alarmed.

Jeremy Bernstein

New York City

Thomas Powers replies:

Heisenberg’s sketch of a reactor—if it was Heisenberg’s—has the same significance as his report to Bohr that Germany was embarked on a bomb program: both were classified military information, and passing either or both to Bohr was strong evidence that Heisenberg had his own political agenda. I believe the sketch was drawn by Heisenberg for the same reason that Jeremy Bernstein used to believe it—because that is what Hans Bethe told us that he had been told by Bohr. Bethe of course was not at the celebrated meeting in Copenhagen in September 1941, and therefore could not know that Heisenberg made the drawing. But what Bohr told him—about that Bethe made himself very clear to me in several phone conversations and numerous letters. I assume he spoke just as clearly to Bernstein.

But now Bernstein has changed his mind because Bohr’s son Aage, who accompanied his father to Los Alamos in 1943, told me in a letter in 1989 that “Heisenberg certainly drew no sketch of a reactor during his visit in 1941″—something Bernstein first learned in my book Heisenberg’s War: The Secret History of the German Bomb (1993). Why Aage and Bethe disagree on this point I do not know; Aage did not explain further. But what Aage says doesn’t change what Bethe says, and I continue to accept Bethe’s extended and detailed account of the whole episode, with its explicit claim that Bohr told him Heisenberg drew the sketch, rather than Aage’s very brief note saying Heisenberg drew no sketch.

Bernstein, on the other hand, is now convinced that someone else must have made the drawing and he “conjectures” that it was done by another German physicist, Hans Jensen, who also visited Bohr in Copenhagen in 1942 and 1943 and may have given him the drawing then; or perhaps Jensen only described a reactor in detail sufficient for Bohr to have made his own drawing. Bernstein further reports that Helmut Rechenberg, keeper of the Heisenberg archives in Munich, “conjectures” that Jensen could have learned about the reactor after obtaining a copy of a lecture given by Heisenberg on May 5, 1943. The reader should note that no one has ever claimed that Jensen actually had a copy of Heisenberg’s lecture, or that he gave Bohr such a drawing—not Bohr, not his son Aage, not Heisenberg, not Heisenberg’s friend Carl Friedrich von Weizsäcker, and most emphatically not Jensen. So this conjecture, which “seems totally reasonable to [Bernstein] and resolves all the contradictions,” is conjecture from first to last, unsupported by any factual claim. I agree with Bernstein that Jensen could have attended Heisenberg’s lecture, could have come away with a copy of his remarks, could have given Bohr a drawing of a reactor, or described one to him; but the mere fact that he could have done all these things cannot be said to establish that he did.

What I find most surprising about Bernstein’s latest conjecture is the fact that he was so quick to abandon all faith in the version of events given to him by Bethe, about whom he wrote a whole book, in favor of a single line in my book from a man with whom he has never discussed this episode, either in person or by letter. The real question to be addressed here is why Bethe and Aage Bohr disagree, and the person to ask is Aage himself. Something about Heisenberg’s visit to Copenhagen in 1941 deeply angered the elder Bohr, but he never explained what it was. Heisenberg, harshly criticized whenever he tried to explain his motives during the war, eventually quit trying, but in comparison to Bohr he was a fount of candor and detail—indeed, most of what we know about the visit comes in one way or another from him.


Aage Bohr is probably the last person living who could usefully expand on the record to help explain what was discussed by the two men, what Heisenberg said or wanted, why the elder Bohr remained so angry, and why he chose to say nothing of the reasons for his anger. In the course of Aage Bohr’s account we might establish who made the drawing, or we might not. Compared to the larger question—how these two physicists dealt with the moral issues posed by nuclear weapons—the question of who made the drawing recedes in importance. If Bernstein wants to carry this inquiry forward I would like to suggest that he set his conjectures aside for the time being, write a letter to Aage Bohr, and urge him to speak candidly on this subject.

This Issue

September 20, 2001