Werner and Elisabeth Heisenberg, Göttingen, Germany, circa 1946

Heisenberg Family Archives

Werner and Elisabeth Heisenberg, Göttingen, Germany, circa 1946

Almost as soon as World War II ended in Europe, and with redoubled intensity after the bombing of Hiroshima, physicists all over the world began to ask how close the Germans had come to making an atomic bomb. But it was not clear whom to ask. Everything to do with development of the bomb was cloaked in secrecy and ten of the leading scientists involved in German atomic research had gone missing. One of them, Otto Hahn, the first to explain the fission process that made bombs possible, was on November 15, 1945, awarded the Nobel Prize in chemistry for his discovery, but the prize committee, it turned out, had no idea where Hahn was.

Among the few who did know were leading scientists who had developed the American bomb at Los Alamos in New Mexico. Many of them were Jews by Nazi standards who had fled Hitler’s Germany, including the physicists Hans Bethe and Victor Weisskopf, who had feared at the beginning of the war that the great German theoretical physicist Werner Heisenberg would build a bomb for Hitler. In 1942, learning that Heisenberg was going to give a scientific talk in Zurich, Bethe and Weisskopf had proposed an American operation to kidnap Heisenberg in Switzerland and even offered to take part themselves. This episode, improbable as it sounds, has been well documented elsewhere1 and after many twists and turns the original proposal led to Heisenberg’s detention in southern Germany in May 1945.

By November, Heisenberg, Hahn, and the other German scientists were being secretly held and closely monitored at a British country house called Farm Hall. The man who had the most to do with putting them there was the Dutch-born physicist Samuel Goudsmit, scientific director of an intelligence group called the Alsos mission. Goudsmit’s task was to track down the Germans who had been working on nuclear fission during the war and to answer the basic question—how close did the Germans get?

For the first year or two after the war pretty much everything Bethe and Weisskopf knew about the answer to the basic question came from information supplied by Goudsmit, a colleague in the Manhattan Project. The answer was not close at all. The Alsos mission found nothing that ever posed a threat to the Allies—instead there was a scattered program of small-scale, poorly funded research efforts that centered on an experimental reactor hidden in a cave in southern Germany that Heisenberg mistakenly hoped would soon be successful. When he first talked to Heisenberg in May 1945, Goudsmit had been privately scornful of German efforts that had achieved so little, and he dismissed Heisenberg’s attempts to explain the history of that little as the excuse-making of a scientist guilty of crude errors about the physics and technical construction of bombs. Freed by the British in early 1946, Heisenberg insisted for a time that it was all more complicated than that—the Germans, he said, had been spared the moral dilemma of whether to build a bomb because the job was just too big for Germany in wartime.

American officials discouraged talk of moral dilemmas, and lost interest in the basic question once Goudsmit and the Alsos mission had established that the Germans had no bombs or prototypes, no working reactors or stockpiles of plutonium and uranium-235, no community of scientists with bomb-making expertise who might work for the Russians. But Bethe and Weisskopf were different. As scientists, as pre-war friends of Heisenberg, and even as fellow bomb-makers, Bethe and Weisskopf might have been expected to ask more questions of Heisenberg, but as soon as the basic question was answered—not close at all!—their curiosity died.

In 1948, Bethe visited Heisenberg in Göttingen, in the British zone, where Heisenberg and his colleagues were again doing physics. Their talk was friendly enough, Bethe told me, but he held his own curiosity in check and let the moment pass without asking any of the questions that might have helped explain the German “failure.”

Weisskopf did likewise. In the fall of 1948 he exchanged many letters with Goudsmit, who was still trying to sort out why Heisenberg “failed,” but Weisskopf had tired of the whole discussion. “Let’s stop this useless prying into the past,” he wrote Goudsmit, adding that he wanted to tell Heisenberg the same thing. “I would make only one point, but this very strongly: stop prying into the past.”

Heisenberg was certainly the man to ask but his death in 1976 closed that door for good. The mystery of Heisenberg’s “failure” was deepened by the fact that surviving German war records provided no explanation why the program to build a bomb had been radically scaled back—essentially shut down—in 1942. Germany had begun with many advantages—first-rate scientists, the technical expertise and big economy needed for a huge industrial project, access to uranium ore in Czechoslovakia, and above all the enthusiasm of the Heereswaffenamt, the German army’s division for weapons development, which had a bomb project underway on the first day of the war, three years before the Americans got moving. The best explanation—indeed the only explanation—for the about-face in 1942 came from Hitler’s economics czar, Albert Speer, who told the German magazine Der Spiegel in 1967 that Heisenberg had convinced him that building a bomb was too big a project for Germany in wartime:


We had asked Heisenberg to put together a list with material and financial demands, and we even encouraged the gentlemen. But their demands were so ridiculously tiny—a few million marks—that we got the view that the development was very much at its beginning, obviously the physicists themselves didn’t want to put much into it.

For a historian the question inevitably then becomes: Why did Heisenberg give this advice, which ended the bomb program? By the early 1990s, when I knew him, Weisskopf had come to regret his many lost opportunities to “pry into the past” in the simplest, most obvious way—by asking Heisenberg why things went as they did.

“We never talked,” Weisskopf told me in an interview in 1991. “I blame myself most for this. I never went to him and said, ‘I have as long as you like—tell me what happened.’”

What happened was observed by Elisabeth Schumacher, who was twenty-two when she met Heisenberg at a musical evening in Leipzig in January 1937 and married him three months later. Over the following nine years, whenever they were separated by Heisenberg’s scientific travels or the war itself, Elisabeth and her husband exchanged more than three hundred letters that survived the fighting. Both Heisenberg and his wife later wrote accounts of the war years,2 but their letters, filled with the worries and hopes of ordinary family life, offer a quieter, more intimate picture of the years when Heisenberg ran the program that was going nowhere. Husband and wife both knew that the German secret police were free to open and read their letters at will, and tried to avoid dangerous ground.

Heisenberg had already drawn the attention of the Gestapo for teaching the theories of Albert Einstein, derided by Nazi scientists as “Jewish physics,” and the danger was underlined by the fate of friends and acquaintances who got into political trouble as the war unfolded. Now published in English as My Dear Li, the letters make no attempt to explain things to outsiders, and certainly not the wartime deliberations of Heisenberg and his closest friends as they considered what to say to Speer or other officials. The reader must expect no grand revelations, but will find an intimate record of the tenor of Heisenberg’s moods and preoccupations as the war gradually closed in around German scientists.

When Elisabeth died in 1998, twenty-two years after the death of her husband, she left the letters to her oldest daughter, Anna Maria, in two neatly wrapped bundles. Anna Maria edited the letters, which have now been translated into English by her sister-in-law, Irene, wife of Heisenberg’s son Jochen, a professor emeritus of physics at the University of New Hampshire.

The first of the letters was written shortly after Heisenberg met his future wife in Leipzig, where he won her heart, according to Anna Maria, with his performance of a Beethoven piano piece. They were soon married, but scientific business and later the war often called Heisenberg away. When they were apart they wrote, often daily, touching more or less substantially on all the events in which historians have taken the most interest.

Prominent among them are Heisenberg’s trip to America in the summer of 1939; his role as chief of research on nuclear fission at the Kaiser Wilhelm Institute in Berlin; his trip to Copenhagen to talk to Niels Bohr in September 1941; his meeting with Albert Speer in June 1942; his withdrawal with the reactor research team to southern Germany in 1943; and his months incarcerated along with other German scientists at Farm Hall in Britain after the war’s end in Europe.

Readers coming to these letters will read them as Elisabeth did, with an eye for what is between the lines. Like her, we know what Heisenberg was concerned not to write—that he was working on nuclear fission, that some German officials hoped for mighty results, that he had longed to speak with Bohr in Copenhagen about the deep questions raised by the possibility of atomic bombs. Knowing how Heisenberg revered Bohr, and what he hoped to gain from talking to his old friend, lends impact to his quiet account of their first conversation in Bohr’s house on September 16, 1941, which “quickly veered,” he wrote Elisabeth a few days later, “to the human concerns and unhappy events of these times.”


Exactly what was said by Heisenberg, and how it was taken by Bohr, are difficult to establish. But it seems clear from the available evidence, which is not scant, that Heisenberg wanted to raise the question of atomic bombs, which he knew were possible, but that Bohr angrily ended the conversation as soon as he grasped the implication of what Heisenberg was trying to say—that Germany had an active bomb program. From that moment Bohr would listen to nothing further that Heisenberg had hoped to add. (After leaving Copenhagen Bohr eventually went to the US, where he worked as a consultant on the atomic bomb project in Los Alamos.)

It was past midnight when Heisenberg returned to his hotel, where he described what had happened to his traveling companion, the physicist Carl Friedrich von Weizsäcker. Heisenberg was older by a decade, but the two had been close friends for many years. Before meeting Elisabeth, Heisenberg had been in love with Weizsäcker’s sister, Adelheid. Heisenberg, Weizsäcker, and a third scientist, Karl Wirtz, were leaders of an effort to guide nuclear research past two dangers—a complete shutdown, which would condemn young physicists to military service and the brutal war in Russia, or takeover by Nazi extremists who might think an atomic bomb could still give Hitler a complete victory. A distraught Heisenberg told Weizsäcker that Bohr “has not understood anything that I said. It has gone astray.”

Werner Heisenberg and Niels Bohr, Copenhagen, 1934

Fermilab/US Department of Energy

Werner Heisenberg and Niels Bohr, Copenhagen, 1934

Bohr was angry and Heisenberg in despair. It is this conversation, which the two men heard so differently, that is the subject of Copenhagen, Michael Frayn’s play, which has many interesting things to say about the relationship of Bohr and Heisenberg, but not about the German bomb program. In late 1941 in the months following the meeting between Bohr and Heisenberg in September, the bomb part of the research effort was barely alive, and it expired the following June after the meeting between Heisenberg and other leading researchers with Albert Speer.

In his letter to Elisabeth, written on the train from Berlin to Leipzig the day after the meeting with Speer, Heisenberg said he found the whole experience “exceedingly strange and unnerving; suddenly, I will no longer have to concern myself at all with the whole Lenard-Stark clique [a coterie of German physicists who accused Heisenberg of pursuing “Jewish” physics], and can push through almost anything I find important.” That ended one of Heisenberg’s fears—that the program would be shut down completely.

A few days later, worrying that he had put too much down on paper, Heisenberg wrote Elisabeth to ask if she had safely received his letter about the Speer conference. “It would make me uncomfortable if it were lost and had fallen into the wrong hands.” This touch of apprehension helps to describe Heisenberg’s wariness; his position protected him in some ways but left him terribly exposed in others.

What the letters reveal are glimpses of Heisenberg’s inner life, like the depth of his relief after the meeting with Speer, reassured that things could safely tick along as they were; his deep unhappiness over his failure to explain to Bohr how the German scientists were trying to keep young physicists out of the army while still limiting uranium research work to a reactor, while not pursuing a fission bomb; his care in deciding who among friends and acquaintances could be trusted.

As the war approached its end Heisenberg’s world grew ever smaller. He snatched moments when he could take time for his own work on theoretical problems but admitted that “it was kind of crazy” to do so. In January 1945 he wrote Elisabeth, “I am trying to keep the institute functioning on the smallest scale; under all circumstances, one must avoid having it be shut down and then having the people taken away.”

During the final weeks of the war Heisenberg kept a diary, included with the letters in My Dear Li, which describes his dangerous three-day trek by foot, train, and bicycle to rejoin his family in Urfeld. On May 2, ten days after he arrived home, Heisenberg was taken into custody by the American Colonel Boris Pash, commander of the intelligence unit that included Samuel Goudsmit. Then followed nearly eight months of comfortable Babylonian captivity, in which he was treated well but told nothing. Most of that time he and nine other German scientists, including Weizsäcker, were held at Farm Hall. Heisenberg was permitted to write home only two or three letters before his detention ended—one of them, as it happened, on August 6, the day Hiroshima was bombed.

As soon as Heisenberg and the others were told of the bombing later that day, there began an intense, extended discussion of the science involved in making a bomb, of the reasons for the German “failure,” and of the moral questions raised by a weapon of such great power. Many of their discussions were secretly recorded by the British and eventually, more than forty years later, published as what are called “the Farm Hall tapes.” It is clear from the tapes that the Germans had no blueprint for the making of an atomic bomb, and in that sense did not know how to do it. The ten scientists of course all knew one another, but they had rarely worked together during the war; they held varying political views, none of their differing programs had achieved any clear success, and all—Heisenberg and Weizsäcker included—were worried how their actions might be judged by their fellow Germans after peace returned.3

In one of his first letters to Elisabeth after returning to Germany in January 1946, Heisenberg tried to sum up lucidly why the German program had turned out the way it did. He quoted a paragraph by his friend Weizsäcker, saying it “describes this problem exactly right.” Weizsäcker had also written to his wife after the long silence of Farm Hall:

You have probably been asked about our supposed work on the atomic bomb…. As far as our work during the war is concerned: we were spared the difficult moral decision whether we should build an atomic bomb. The technical and organizational means available to us in Germany would not have permitted at all the effort America had to put forth in regard to the problem. We confined ourselves to the preliminary work on the lesser effort of building a machine [i.e., a reactor] capable of producing heat, along the same paths that America apparently used as well in its pursuit. At the end of the war, we were close to a success.

For Elisabeth’s eyes alone, Heisenberg added a further comment about the development and use of atomic bombs. “I know many of the British and American colleagues who have worked on it, some of them are my pupils,” he wrote, “and they have my sympathy, because their names are now tied to this atrocity.”

Heisenberg’s judgment was unknown to the builders of the bombs that destroyed Hiroshima and Nagasaki. Without question it would have infuriated them, deeply troubled as many already were by the enormity of what they had done. This question—was it right?—still has no accepted answer. It is one of history’s ironies that Heisenberg, who built no bomb, was the first atomic physicist to be asked to justify himself. What the international scientific community wanted to know was whether Heisenberg had behaved responsibly and decently, or had been only a “good German,” doing what he was told and looking the other way. In one of the first issues of the Bulletin of the Atomic Scientists, the American physicist Philip Morrison, who had been active in American intelligence efforts, wrote that “it will never be possible to forgive” the German physicists—not for trying to build a bomb, Morrison stressed, since the Americans and the British had of course done that; but for trying to build a bomb for “the cause of Himmler and Auschwitz.”

Later, when critics protested that his charge was too broad and tendentious, Morrison narrowed his indictment more specifically to Heisenberg himself, not naming but unmistakably referring to him as “a famous German physicist in Göttingen today…who could live for a decade in the Third Reich, and never once risk his position of comfort and authority in real opposition to [Hitler’s regime].”

It was not just Heisenberg who was being held to a rigorous standard by Morrison, but German science, and by extension science itself. With his attack on Heisenberg, Morrison was stumbling into the middle of an argument about the moral responsibility of scientists that began with the development (led by the German chemist Fritz Haber) of poison gas for battlefield use in World War I. The horrors of gas warfare were multiplied by atomic bombs, leading some critics to argue that scientists, fascinated by what worked and pressed by their governments to develop fresh horrors, always found it impossible to say no.

If Heisenberg had been pliable and did what he was told to protect his own “comfort and authority,” as Morrison charged, then it would be fair to say that he and the majority of his colleagues had failed a crucial moral test. But if Heisenberg had sought and found a way to say no—if he had deliberately convinced Hitler’s government to abandon hope for an atomic bomb as taking too long to make and being too big, too expensive, and too uncertain for Germany in wartime—then Heisenberg’s example would suggest that even in the midst of the bitterest of wars scientists could act morally, and remain faithful to a core of common human values.

Morrison’s attack on Heisenberg had the effect of closing down serious consideration of Heisenberg’s record. Questions about moral responsibility remained muddy in the first years after the war, but time gradually clarified them. What Heisenberg did, and why he did it, slowly emerged as one of the most uncomfortable questions of the nuclear age.

It is the moral questions about atomic bombs that make the history of the German program such a touchy matter. Heisenberg, Weizsäcker, and others were careful never to claim that the German scientists had actively opposed the Heereswaffenamt’s hope to produce a bomb during the war. “They never had to make a moral decision,” Heisenberg told a New York Times reporter in December 1948, “and this for the reason that they and the Army agreed on the utter impossibility of producing a bomb during the war.”

But how did the army know it was utterly impossible? That is what Heisenberg and his colleagues told the army, and that is what Albert Speer said they told him in 1942, and that is why the program was cut back to an insignificant research effort. Explaining what happened to the German bomb program comes down, in the end, to explaining what Heisenberg did, and why he did it.

In the first years after the war the ongoing argument about Heisenberg raised an infinity of painful questions for Bethe and Weisskopf and the thousands of their colleagues who had helped to build the atomic bombs that destroyed two Japanese cities in 1945. The subject is just as painful now, and perhaps explains why biographers of Heisenberg and historians of German bomb-making efforts have been reluctant to address, or even to cite, evidence that suggests the bomb-makers had a hidden agenda. I will cite two examples that go to the heart of the matter.

In April 1941 the German mathematician Fritz Reiche, one of the last Jews to escape Berlin safely before the border was closed, brought with him a secret message from a friend and colleague of Heisenberg’s, which Reiche passed on to a physicist he knew at Princeton, Rudolf Ladenburg. A few days later, Ladenburg reported Reiche’s message to the head of the Uranium Committee in Washington, a predecessor to the Manhattan Project. The source of the message, Ladenburg wrote, was a “reliable colleague” in Germany who wanted to warn the Americans

that a large number of German physicists are working intensively on the problem of the uranium bomb under the direction of Heisenberg, that Heisenberg himself tries to delay the work as much as possible, fearing the catastrophic results of a success. But he cannot help fulfilling the orders given to him, and if the problem can be solved, it will be solved probably in the near future. So he gave the advice to us to hurry up if the USA will not come too late.

A second example of rarely or never-mentioned evidence is a letter published by the Frankfurter Allgemeine Zeitung on July 7, 1990, describing a conversation about atomic bombs with Carl Friedrich von Weizsäcker in early 1944. The author of the letter was Max Himmelheber, a German inventor and wartime flier, who had met Weizsäcker in Berlin before the war. In September 1940 Himmelheber’s Messerschmitt was shot down over Britain where he spent the next three years as a prisoner of war, passing the time by writing a paper on “Limits of Technology.”

In his paper Himmelheber argued that every major technical achievement had been discovered but one—“the match” needed to release the energy locked inside the atom. In late 1943, still not fully recovered from his wounds, he was exchanged through Sweden and after his arrival in Germany he sent his paper to Weizsäcker, who took an interest and invited Himmelheber to visit him at the University of Strasbourg, where Weizsäcker was teaching. In his 1990 letter Himmelheber said he was “shaken” when Weizsäcker told him, “We have found the match.” Himmelheber continued:

[Weizsäcker] asked me if I had heard the names [Otto] Hahn and [Fritz] Strassmann and proceeded to tell me about their successes. Herr von Weizsäcker told me also that he, Heisenberg, and other colleagues had been summoned [in September 1939] by a high-level office of armaments. (Today I know that it was the Herreswaffenamt.…) And now Weizsäcker told me the following story which made a deep impression: because of this summons, the most intimate circle of atomic physicists around Hahn and his discovery had convened in advance of the meeting. Speculation was that they would be asked whether they could build an atomic weapon…. Von Weizsäcker told me that the group had agreed that such a dreadful weapon should, under no circumstances, become available to the world, and that it was their duty to refuse collaboration on such a project on account of ethical considerations. Such outright refusal could obviously not be voiced because it would be viewed as sabotage, if not treason. Therefore, Weizsäcker related, the assembled group agreed to take the position not to deny outright the possibility of building an atomic bomb, but, in view of the present war situation, to make the point that it could not be implemented within a realistic time frame….

These revelations made a deep impression on me. I could hardly grasp the fact that, only a few weeks after my return from England, I was privy to knowing of a resistance group against Hitler and an all-out war, a group, consisting of most prominent personalities who, in some sense, had the wherewithal to substantially influence the fate of humanity.4

These letters potentially cast the standard history of the German bomb in a very different light, suggesting that Heisenberg and his colleagues found a silent way to make a moral decision, not that they were spared one.5

In the thirty years that remained to him, whenever Heisenberg was asked to explain the German “failure” to build a bomb, his answer was what he had written to Elisabeth in letters before, during, and just after the war—that he had stayed in Germany because it needed him, and he had been spared the difficult moral decision of whether to build a bomb by the impossible immensity of the task—exactly what you might expect to hear from a man who didn’t want to do it, and found a way to say no.