Did Werner Heisenberg and Carl Friedrich von Weizsäcker compromise with the Nazis? The story begins with Albert Einstein, who became a target for conservative physicists like Philipp Lenard and Johannes Stark who could not follow Einstein’s physics, and the early Nazi Party that rejected Einstein as a Jew as well as his pacifism and internationalism. When Hitler came to power, Lenard and Stark gained great influence. Stark in particular tried to accumulate power but steadily lost influence through conflicts with other Nazis. When Stark’s nemesis, the theoretical physicist Arnold Sommerfeld, was going to retire and be succeeded by Werner Heisenberg, Stark launched a vicious attack on Heisenberg in the SS newspaper. Heisenberg appealed to SS Leader Heinrich Himmler and thanks to support from the aeronautical engineer Ludwig Prandtl was eventually rehabilitated by the SS. The established physics community then launched a counterattack against the “Aryan Physics” of Lenard and Stark, which included writing Einstein out of the history of relativity theory. This was arguably Heisenberg’s greatest compromise with Nazism.

In the late 1930s scientists were puzzled by the mysterious behavior of uranium when bombarded by neutrons. Several different research groups were working on these questions, including two German chemists, Otto Hahn and Fritz Strassmann, and an Austrian, “non-Aryan” physicist, Lise Meitner. When Germany absorbed Austria in 1938, Meitner fled to Scandinavia. However, their collaboration continued, culminating in Hahn and Strassmann’s discovery that uranium had been split and, together with her nephew Otto Frisch, Meitner’s theoretical explanation of what came to be called nuclear fission. Scientists in many countries immediately began studying this phenomenon and publishing their results. By the time these publications were stopped by censors or self-censorship, it was clear that one rare isotope of uranium, 235, was easily fissionable, while the common isotope 238 could absorb a neutron and transmute into a fissionable transuranic element. This was the basis for wartime research projects on atomic bombs.

The German military catastrophe in Stalingrad began the period of “Total War.” The research into heavy water production, isotope separation, and model nuclear reactors became progressively more difficult as the war economy became more strained and the Allies began bombing Germany. Problems with heavy water production in Norway, including sabotage at the Norsk Hydro, caused the Germans to search for alternative ways to make heavy water in Germany and Italy. Although hampered by the war, progress was made with centrifuges, which had begun to slightly enrich the amount of isotope 235 in small samples of uranium. Whereas nuclear reactor experiments under the direction of Werner Heisenberg had used layers of uranium and moderator, a competing group led by Kurt Diebner began experimenting with three-dimensional lattices with better results. Because research in Berlin and Hamburg had become very difficult, if not impossible, several institutes were evacuated to towns in southwest Germany.

In 1938 nuclear fission was discovered in Germany. Like their counterparts in other countries, German scientists brought the military potential of fission to the attention of officials and began researching isotope separation and nuclear reactors, the two paths to atomic bombs. During the Blitzkrieg phase of the war, powerful new weapons were not needed, so that the research had low priority and made modest progress. When the war slowed down in the winter of 1942-1942, the uranium research was evaluated with the result that it became clear that atomic bombs could not be made in Germany in time to influence the outcome of the war. Because the Americans, who had much greater resources, were apparently working on this, the Germans continued to as well. The steadily deteriorating state of the war made research more difficult, then impossible, as the scientists were focussed on their survival. After the war, the revelations of the Holocaust, and the atomic bombing of Japan, these scientists were criticized for collaborating with the Nazis and had to justify their work. The result was the legend of Copenhagen, a claim that they had in fact been trying to forestall all nuclear weapons.

The American Alsos Mission, a scientific intelligence-gathering task force, followed behind the advancing Allied armies in the west, looking for evidence of a German atomic bomb. Its scientific leader, the physicist Samuel Goudmit, quickly determined that the Germans were far removed from building nuclear weapons but also was misled by some documents and his own prejudices, convincing himself that the Germans, including his colleague Werner Heisenberg, had not understood how an atomic bomb would work. When Goudsmit returned to the United States, he began publishing books and articles using the German uranium work as an example of how the Nazis had ruined science through political and ideological control, mistakes that America must not repeat. Heisenberg responded by defending both his scientific work and conduct under Hitler. Goudsmit criticized both Heisenberg and Carl Friedrich von Weizsäcker for compromising with the Nazis. While Goudsmit eventually reconciled with Heisenberg, he never forgave Weizsäcker. Goudsmit had lost his parents in Auschwitz, and Weizsäcker’s father, a high-ranking official in the Foreign Ministry, had been convicted of war crimes.

In 1938 nuclear fission was discovered in Germany. Like their counterparts in other countries, German scientists brought the military potential of fission to the attention of officials and began researching isotope separation and nuclear reactors, the two paths to atomic bombs. During the Blitzkrieg phase of the war, powerful new weapons were not needed, so that the research had low priority and made modest progress. When the war slowed down in the winter of 1942-1942, the uranium research was evaluated with the result that it became clear that atomic bombs could not be made in Germany in time to influence the outcome of the war. Because the Americans, who had much greater resources, were apparently working on this, the Germans continued to as well. The steadily deteriorating state of the war made research more difficult, then impossible, as the scientists were focussed on their survival. After the war, the revelations of the Holocaust, and the atomic bombing of Japan, these scientists were criticized for collaborating with the Nazis and had to justify their work. The result was the legend of Copenhagen, a claim that they had in fact been trying to forestall all nuclear weapons.

In order to determine what really happened when Werner Heisenberg and Carl Friedrich von Weizsäcker met with Niels Bohr in occupied Copenhagen in September 1941, this visit has to be placed in several contexts. By this time the German uranium research had demonstrated that atomic bombs were probably feasible, even if not for Germany during the war. The summer 1942 German offensive against the Soviet Union had not yet begun to falter, although Heisenberg was nevertheless privately very anxious about the war. The Germans alienated Bohr and his colleagues by their participation in cultural propaganda and nationalistic and militaristic comments about the war. A comparison with Heisenberg’s other lecture trips abroad shows that he acted the same way in other places. Heisenberg’s subsequent efforts in 1942 to gain support from Nazi officials by both describing the power of atomic bombs and the threat that the Americans might get them first also do not fit with an attempt at Copenhagen to forestall all nuclear weapons. Instead the best explanation for the visit is Heisenberg and Weizsäcker’s fear of American atomic bombs falling on Germany.

When the war slowed down in the winter of 1942-1942, the uranium research was evaluated with the result that it became clear that atomic bombs could not be made in Germany in time to influence the outcome of the war. The project was transferred from Army Ordnance to the Reich Research Council, the institution responsible for mobilizing civilian research for the war effort. The scientists, who were now threatened with the loss of their exemptions from frontline service, began to “sell” their research. Although they did not promise to deliver atomic bombs, they did emphasize the tremendous power of such weapons and warned that the Americans, who had much greater resources, were apparently working on this. In the meantime a model nuclear reactor experiment had produced a neutron increase, which was interpreted as proof in principle that a nuclear reactor could be built. Several influential figures responsible for armament production now took a keen interest in uranium research and the powerful Minister of Armaments Albert Speer decided to generously support the project.