Diana Preston - Before the Fallout

Bohr also refuted Heisenberg’s suggestion that the news that atom bombs were possible had stunned him into silence, insisting that the possibility of nuclear weapons had been “obvious” to him for a while. The reason he had not spoken was twofold. First, a “great matter for mankind was at issue in which, despite our personal friendship, we had to be regarded as representative of two sides engaged in mortal combat.” Second, he was dismayed to learn “that Germany was participating vigorously in a race to be the first with atomic weapons.”

Further unsent letters written in subsequent years reveal a softening of Bohr’s attitude toward Heisenberg—some affection still remained. A draft letter to Heisenberg congratulating him on his sixtieth birthday ended with “fondest greetings and warmest wishes for many happy years.” However, Bohr continued to agonize over Heisenberg’s motives. He asked Heisenberg to clarify on whose authority he had come to Copenhagen, writing, “I have often wondered from which official police agency permission was given to talk to me about a question which was surrounded by such great secrecy and held such great dangers.” He wanted Heisenberg to tell him “what purpose lay behind” his visit. It was “quite incomprehensible” to him how Heisenberg could claim to have suggested to Bohr “that German physicists would do all they could” to prevent an atomic bomb. On the contrary, “you [Heisenberg] informed me that it was your conviction that the war, if it lasted sufficiently long, would be decided with atomic weapons and I did not sense even the slightest hint that you and your friends were making efforts in another direction.”

Over the years many suggestions have been made about Heisenberg’s true purpose. Some have claimed that Heisenberg was trying to discover what Bohr knew about the Allied program. Bohr later told Oppenheimer that Heisenberg and von Weizsacker had come “less to tell what they knew than to see if Bohr knew anything that they did not.” It certainly appears that, during his visit to Copenhagen in March 1941, von Weizsacker had been fishing for information. He reported to the Nazi authorities on his return that “concerning the more technical questions,” Bohr “knew a great deal less than we.”

Others have suggested that Heisenberg was trying to pass messages about the German program to the Allies. According to von Weizsacker, this was true. In a letter in 2002, he wrote, “We hoped that Bohr could tell colleagues in England and the USA that we were no longer working on a bomb.” Elizabeth Heisenberg, in her book Inner Exile about life with her husband, similarly suggested that Heisenberg “saw himself confronted with the spectre of the atomic bomb, and he wanted to signal to Bohr that Germany neither would nor could build a bomb. That was his central motive. He hoped that the Americans, if Bohr could tell them this, would perhaps abandon their own incredibly expensive development. Yes, secretly he even hoped his message could prevent the use of an atomic bomb on Germany one day. He was constantly tortured by this idea.”

A particularly intriguing dimension is the story that at their meeting Heisenberg gave Bohr a drawing, which, later in the war after his escape from Denmark, Bohr sent to the United States. Robert Oppenheimer, Edward Teller, and Hans Bethe puzzled over the sketch, which showed a boxlike structure with sticklike objects projecting from the top. Bethe recalled, “As far as we could see, the drawing represented a nuclear power reactor with control rods. But we had the preconceived notion that it was supposed to represent an atom bomb. So we wondered: ‘are the Germans crazy? Do they want to drop a nuclear reactor on London?’” After further study he and Teller concluded that it “was clearly a drawing of a reactor.” Some have claimed that Heisenberg gave the sketch to Bohr as proof that the Germans were working on peaceful, not military, applications of nuclear power. However, Aage Bohr, in whom his father confided, always maintained that Heisenberg gave Bohr no such thing. If the drawing was indeed of German origin, it must have been provided by someone else, although the risks would have been enormous. Otherwise, it may have represented Bohr’s own interpretation of German thinking based on his discussions with Heisenberg and others. To heighten the mystery, despite extensive searches in the archives in Denmark, the United States, and the United Kingdom, no trace of the sketch can now be found.

According to Hans Bethe, the Copenhagen meeting was doomed to fail: “It was impossible that the two of them could understand each other. Heisenberg knew about plutonium and was convinced it was the key to the whole business. Bohr didn’t know about plutonium. Therefore there was a technical misunderstanding.” In Bethe’s view, practical considerations also intervened; “Bohr was much better at speaking than listening and he mumbled.”

No one will ever know the full truth of what happened and why. Perhaps Heisenberg himself did not know exactly what he was trying to achieve. Perhaps he felt intuitively that to talk to his father figure, Bohr, could clear his mind as it had previously on scientific questions. An English officer after the war recorded private conversations between Heisenberg and von Weizsacker and reported that “they seem to consider international physics as being almost synonymous with work under the leadership of Niels Bohr.” Aage Bohr suggested that, as well as respect for his father, affection also played a role: “Undoubtedly one of the reasons why Heisenberg went to Copenhagen was to see if there was anything he could do for his Danish physicist friends living as they were in an occupied country. Heisenberg had a strong sense of loyalty towards them.”

However, Heisenberg certainly did not appreciate the sensitivities of the situation. Although eager in his memoirs to portray his readiness to put himself in others’ shoes, he was not good at it in reality. He could not place himself in Bohr’s position as a half-Jewish citizen of a country occupied by a brutal regime standing for everything Bohr despised. He also had an ability to ignore unpleasant truths and unconsciously to twist them into a more palatable form.

According to Hans Bethe’s account of a postwar discussion with Heisenberg, the latter believed passionately that Germany should win the war: “He said he knew that the Germans had committed terrible atrocities against the populations on the Eastern Front—in Poland and Russia—and to some extent in the west as well. He concluded that the Allies would never forgive this and would destroy Germany as a nation—that they would treat Germany about the way the Romans had treated Carthage. This, he said to himself, should not happen; therefore, Germany should win the war, and then the good Germans would take care of the Nazis.” Bethe found it “unbelievable that a man who has made some of the greatest contributions to modern physics should have been that naive.”

Heisenberg’s reasons for visiting Copenhagen were undoubtedly complex, quite possibly confused, and quite likely a combination of the various motives alleged. He may have convinced himself that building an atom bomb before the war was likely to end was currently beyond the capability of any country. However, to protect the Germany he still loved, he had to be sure that Britain and the United States were not making faster progress. Until he knew whether Germany was at risk, he could not decide how he, and his fellow scientists, should act.

Whatever his motives, Heisenberg clearly never got the chance to say all he wanted because Bohr became so agitated. All he achieved was convincing Bohr that Germany was actively pursuing the atomic bomb.