What Bohr Remembered

1.

Military victory for Hitler’s Germany probably never appeared more certain than it did in the early fall of 1941, when Werner Heisenberg, on a Sunday evening in mid-September, boarded the night train from Berlin to Copenhagen. Forward elements of the German army were pressing on Moscow, the Americans had not yet entered the war, German armies occupied most of the rest of Europe, and Britain was trapped on its island. “News from the Russian front has been pretty bad,” wrote one of Winston Churchill’s advisers, Alexander Cadogan, in his diary later that week, while Heisenberg was visiting the institute of his old friend and mentor, the physicist Niels Bohr. “Everything is pretty murky, and how exactly we are going to win this war, I should like someone to explain.”

The prospect of victory for Germany, of course, brought despair to its enemies, victims, and new subjects. Nazi sympathizers and collaborators were openly active in Denmark, Jewish refugees in Bohr’s institute lived in terror of sharing the fate of the Jews of Poland, and even in his own office Bohr worried about informers and microphones. He refused to have anything to do with the occupation authorities and he would make no exception for Heisenberg’s lecture at the German Scientific Institute on the evening of Friday, September 19.

But Heisenberg himself had been a friend for twenty years; together they had practically invented modern physics and, so far as Bohr knew, Heisenberg came on no official business. As a result, over the week of Heisenberg’s visit Bohr saw him several times, once when it was just the two of them, alone in Bohr’s office. Like Bohr, Heisenberg also feared informers and microphones. The German hesitated to say what was on his mind; the Dane hesitated to respond. Bohr had already been deeply offended when Heisenberg, over lunch with members of Bohr’s institute, predicted that the Germans were bound to win the war, and the private meeting did not go well either. In some way—how and why was unclear—the subject of atomic weapons was raised. Neither recorded the conversation while it was still fresh. Brief as it was, the private meeting seemed to close the door on intimacy forever.

Over the years Heisenberg tried to reopen that door without success. In August 1947 he visited Bohr at his summer house in Tisvilde but the two men repeatedly failed even to agree on where the 1941 conversation took place—Bohr said it was in his office at the institute, Heisenberg remembered that they had been out walking. “After awhile,” Heisenberg wrote in a memoir published in 1971, “we both came to feel that it would be better to stop disturbing the spirits of the past.”

But in the mid-1950s Heisenberg tried again. This time he raised the subject of the 1941 meeting with Bohr indirectly, in a letter to the Austrian journalist and historian Robert Jungk, whose book on the origin of atomic weapons, Brighter than a Thousand Suns, argued that the Allies found no large-scale German program to develop atomic weapons at the end of the war for one overriding reason—German physicists had not wanted to give a bomb to Hitler. In his letter Heisenberg told Jungk the talk with Bohr in 1941 “probably started with my question as to whether it was right for physicists to devote themselves in wartime to the uranium problem….”

That wasn’t the way Bohr remembered it. To an assistant he dictated an angry letter of protest, stating flatly that he was “amazed to see how much your memory has deceived you….” But Bohr never sent the letter. Five years later, in 1962, he tried again, dictating new drafts of a letter describing his memory of their conversation, and ending each with the “hope that we can talk in greater detail about this….” But he never sent those letters either.

Why the world continues to take an interest in a sixty-year-old conversation which lasted no more than a few moments takes a little explaining. The immediate cause is of course the curiosity aroused by Michael Frayn’s long-running play Copenhagen, which attempts to sort out all the things the two men might have said to each other, if they had only once got to talking. But the play fed on the interest already aroused by decades of speculation, rumor, and one-sided report, beginning in mid-war. Bohr and Heisenberg were right to worry about loose lips; Bohr’s assistant Christian Møller, on a visit to Stockholm in early 1942, described Heisenberg’s visit to the refugee Jewish physicist Lise Meitner, who wrote openly about the impression he had made to her friend Max von Laue in Germany, saying, “I was quite saddened by what I heard.” Intense early interest was also shown by the British and American intelligence authorities who questioned Bohr repeatedly after his escape from Denmark in 1943. But Bohr could not tell them much beyond the fact that Heisenberg had raised the question of atomic weapons, thereby confirming the Allied assumption that Heisenberg would be close to the heart of any German research effort.

When the war ended Heisenberg and nine other German scientists were rounded up and jailed for six months in a British country house which had been wired for sound. Intelligence officials already knew the Germans had had no program to develop atomic bombs, but they feared the Germans might know more than they had let on about bomb design, or might have hidden stocks of uranium, or might be plotting to work for the Russians after their release. Every two weeks transcripts of their recorded conversations were quoted in reports circulated to American and British intelligence analysts, and in them, inevitably, much was said of importance for the history of the German bomb program.¹

But most significant here are several remarks concerning the German “failure,” beginning with a statement on the night of August 6, 1945, by Carl Friedrich von Weizsäcker, Heisenberg’s young friend, who had accompanied him to Copenhagen in 1941. The Germans had just learned about Hiroshima. As they were absorbing the news that the Allies had succeeded in building a bomb Weizsäcker suggested that the German “failure” was not for practical or technical reasons. “I believe the reason we didn’t do it was because all the physicists didn’t want to do it, on principle,” he said. “If we had all wanted Germany to win the war we would have succeeded.”

Later that same evening Heisenberg discussed this question alone with Otto Hahn, who would soon be awarded the Nobel Prize for the discovery of fission in 1939. Hahn had played no direct role in the bomb project but was horror-struck by the consequences of his discovery. Heisenberg’s exact words were not transcribed but the author of Report No. 4 paraphrased his remarks:

Heisenberg stated that…he feels himself that had they been in the same moral position as the Americans and had said to themselves that nothing mattered except that Hitler should win the war, they might have succeeded, whereas in fact they did not want him to win.

One of the early American readers of the Farm Hall reports was the Dutch-born physicist Samuel Goudsmit, scientific director of the Alsos mission which had been responsible for collecting the paper record of the German bomb program, and for rounding up the German scientists incarcerated in Britain. For the remainder of his life Goudsmit took a passionate interest in the reasons for the German “failure.” His own explanation changed over time but on one point he remained steadfast: the “failure” had technical, scientific, and organizational causes and had nothing to do with moral issues or any German reluctance to help Hitler win the war. After the war Heisenberg sought a gentler way to explain what happened; in an article in the British science magazine Nature, in a letter to Goudsmit personally, and in an interview with The New York Times in 1948, he claimed only that the Germans had been spared a moral decision by the magnitude of the project. Even this Goudsmit refused to accept. The cause of the failure, he insisted, was scientific blunders which the Germans hoped to disguise as moral qualms, and his view, vigorously promoted, was generally accepted by Allied scientists who had never seen the Farm Hall reports, kept secret by the British government until 1992.

The unfolding of the argument over the German “failure” has filled several fat books. But one thing is clear: Heisenberg’s remark to Hahn at Farm Hall after the war that the German scientists “did not want [Hitler] to win” would be strongly supported by evidence that he and his colleagues had actively tried to impede the bomb program in its early stages. Heisenberg’s visit to Bohr in September 1941 is thus no minor detail—what he said then might confirm, or undermine, his later claim that the Germans lacked single-minded zeal for success. The intense feelings aroused by this matter go beyond the point in question—why the Germans did what they did—to focus on a sensitive moral question: If German scientists were at the least reluctant and perhaps even refused to build a bomb for Hitler, how would Allied scientists justify the bomb that destroyed Hiroshima?

Heisenberg himself never put this question to his many old friends who had worked at Los Alamos, and it seems clear from his remark to Hahn that he would have said the Allied situation made all the difference—they had Hitler as their enemy. But Goudsmit was infuriated by any claim, however gently put, that moral considerations had anything to do with the fate of the German bomb program, and that anger persists.

2.

Recently the Bohr family, responding to the intense popular interest in the 1941 meeting aroused by Frayn’s play, decided to release eleven documents, including Bohr’s unsent letters to Heisenberg and some additional fragments.² Heisenberg over the years until his death in 1976 had described the conversation many times—in his letter to Robert Jungk, in interviews with historians, in conversation with friends and colleagues, in his memoir Physics and Beyond. But no version of the conversation in Bohr’s own words had appeared and the new documents make clear and vivid for the first time many things which had only been guessed at previously.

Clearest of all is the fact, forcefully expressed in Bohr’s first letter of 1957, that he was seriously angered by “how much your memory has deceived you in your letter to [Jungk]….” He goes on to say,

Personally, I remember every word of our conversations…. In particular, it made a strong impression both on Margrethe and me, and on everyone at the Institute that the two of you spoke to, that you and Weizsäcker expressed your definite conviction that Germany would win and that it was therefore quite foolish for us to maintain the hope of a different outcome of the war and to be reticent as regards all German offers of cooperation. I also remember quite clearly our conversation in my room at the institute, where in vague terms you spoke in a manner that could only give me the firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons….

Bohr further remembered that Heisenberg said he had been working on the problem for two years while Bohr “listened to this without speaking since [a] great matter for mankind was at issue in which, despite our personal friendship, we had to be regarded as representatives of two sides engaged in mortal combat….” If he in any sense had been shocked, it was not by the news that atomic weapons were possible, “but rather…the news, as I had to understand it, that Germany was participating vigorously in a race to be the first with atomic weapons.”

That is the heart of Bohr’s account of the meeting itself, but in succeeding drafts of the letter his memory of Heisenberg’s actual words becomes more precise, its tone is gradually softened, and its basic thrust seems to shift from the expression of anger to unanswered questions. In none of the drafts does he cite anything that he himself said, nor does he say how the conversation ended, beyond the fact that it was “very brief.” The principal contribution of the new documents is to make vivid what Bohr had “heard”—in effect, a brusque claim by Heisenberg that Bohr should “cooperate” because Germany was certain to win the war, possibly with atomic bombs built by Heisenberg himself if the fighting lasted long enough. In Bohr’s mind, it seems clear, he had folded together the remarks of Heisenberg and Weizsäcker at the institute and Heisenberg’s report on German progress in developing the atomic bomb given in their private meeting. Seen in that light, Heisenberg’s behavior would have seemed overbearing and insensitive, and it is not hard to understand why a pall should have fallen over their friendship, or why Bohr would have been angered by Heisenberg’s 1957 letter to Jungk.

But just as clearly Bohr felt some hesitation about reaching a final judgment, and he was not quite certain that what he remembered was accurate, or contained the whole of the story. He made sure to acknowledge that he understood Heisenberg urged cooperation for Bohr’s own protection, and in later drafts he cited subsequent visits to Copenhagen by Heisenberg’s colleague Hans Jensen, who worked throughout the war on developing an atomic reactor. Heisenberg, Bohr insists, “gave no hint about efforts on the part of German scientists to prevent such a development [of atomic weapons].” But he adds, “It is true that during his visits to Copenhagen in 1943…Jensen did make hints in such a direction….”

It is here that a question enters the later drafts of Bohr’s letter. After escaping Denmark, Bohr discussed the visits of all three—Heisenberg, Weizsäcker, and Jensen—with intelligence authorities, who evidently were powerfully struck by the fact that Heisenberg told Bohr about the German bomb program. Over time this raised a question in Bohr’s mind which takes on increasing prominence in later versions of his letter: “The point which was put forward during these discussions, and with which all later enquiries have been particularly concerned, was how the visit had been arranged and what purpose lay behind it, as one has wondered in particular how and with what authorization such a dangerous matter, of such great political importance, could be taken up with someone in an occupied and hostile country.”

“One has wondered…” Bohr can only be referring to himself. This is the question he wants to ask.

One may say that Bohr’s letters were finally “sent” when they were posted on the Internet by the Niels Bohr Archive on February 6, and they received a prompt response from Carl Friedrich von Weizsäcker, now eighty-nine, and living outside Munich. In an interview with the Süddeutsche Zeitung published on February 8, Weizsäcker explained more clearly than ever before why Heisenberg went to Copenhagen, and what he wanted to say to Bohr. The interviewer, Ulrich Kühne, begins by asking Weizsäcker, “Why did you and Heisenberg visit Niels Bohr in Copenhagen in 1941?”

Weizsäcker: Heisenberg wanted to speak to Bohr. We had arranged what should be discussed. After we had come to the conclusion that in Germany we were in no position to build a bomb, at least not during the war, we also didn’t want America and England to build one. Naturally we could not advise the Americans and English of this. But Heisenberg hoped that Bohr could communicate to the physicists there that the Germans would no longer work on a bomb. That was the idea…. We didn’t want a bomb falling on us.
Kühne: Did you think it possible then that the Americans could finish a bomb during the war?
Weizsäcker: We couldn’t rule it out…. And with our visit, we wanted Bohr to talk the Americans out of it. Heisenberg told me that during his walk with Bohr, he began to discuss the atom bomb, but that Bohr said at once, “I want to know nothing about it!”
Kühne: Why did Bohr cut off the discussion?
Weizsäcker: I don’t know. Heisenberg believed that Bohr was afraid he would be compelled by Heisenberg to work on a German bomb.
Kühne: Why should Bohr have believed Heisenberg that the Germans had given up working on a bomb in 1941?
Weizsäcker: Heisenberg was perhaps somewhat naive…. But I know for sure that we knew, on the way to Copenhagen in 1941 to talk to Bohr, that we could not build an atom bomb…. And so the idea was—the reason Heisenberg visited Bohr—if we now let the Americans and English know that we cannot build a bomb, they won’t believe us. But they would believe Bohr.

Weizsäcker’s version of events does not contradict Bohr’s memory of what happened, nor is the opposite true. Both simply sit uneasily among the large inventory of accounts offered over the years by Bohr’s son, Aage, and his assistant, Stefan Rozental; by Heisenberg and by his wife, Elisabeth, in a memoir of her husband, Inner Exile, published in 1984; by Weizsäcker in interviews; and by numerous others who were told of what happened, and passed it along.

The long argument over Heisenberg’s role in the German bomb program cannot be resolved by the new Bohr documents, which enrich our understanding of one small episode in the story—how Bohr felt about what he heard Heisenberg say—but, I believe, leave the rest pretty much where it was. In February 1942, a few months after the meeting in Copenhagen, the German army ended its support for nuclear research as too expensive and time-consuming to pursue in wartime. Diehard supporters of a German bomb tried to revive the program with the help of Albert Speer, the man Hitler put in charge of mobilizing the German economy, but at a meeting in Berlin in June 1942 Heisenberg stressed the magnitude of the effort and its difficulties, convincing Speer, as he told Der Spiegel in 1967, that “the physicists themselves didn’t want to put too much into it.”

If the Germans had mounted a major effort to build atomic bombs, and if their files were filled with letters and memoranda from Heisenberg warning of the dangers of Allied success and urging the authorities forward, then the argument over Heisenberg’s role would have ended long ago. The mystery is why Heisenberg has been the subject of so much suspicion and hostility when the facts seem to support his remark to Hahn at Farm Hall—that the German scientists “did not want [Hitler] to win.” The problem seems to be the question implicit in the way Heisenberg chose to explain things in public—by saying that he and his colleagues were spared the difficulty of a moral decision by the fact that an atomic bomb was too big a project for Germany in wartime. It is what Heisenberg doesn’t say that raises hackles: he was spared, but what about those who were not spared? How did they decide what to do? The question is still a sensitive one, and I believe it remains the principal source of the anger aroused by talk of Heisenberg’s role.

The importance of the new documents is that they break an unnatural silence, and do now what Bohr and Heisenberg’s other old friends should have done long ago—ask him to explain himself. Who would know better how he thought things through? Again and again in the new documents Bohr ends a letter with the “hope that we can talk in greater detail about this….” It is clear which question he wants to pose first: “what authorization might have been given to you by the German government to touch upon such a dangerous question….”

Heisenberg could have answered that no one had authorized him to do this; the visit to Bohr was something he had decided to do on his own.

The next question seems inescapable. In Copenhagen, Michael Frayn gives it to Bohr’s wife, Margrethe, in the first moments of the play: “Why did he come? What was he trying to tell you?”

If they could have once got that far, the rest would have followed naturally.