When Otto Hahn, Werner Heisenberg, and Carl Friedrich von Weizsäcker returned to Germany from Farm Hall, they needed to justify their wartime work on uranium without appearing to have betrayed the German war effort. Hahn was aided by his Nobel Prize and his presidency of the Max Planck Society. Hahn used his prestige to systematically defend German science and repress its nazification, contributions to the war effort, and participation in war crimes. Heisenberg and Weizsacker helped create the legend of Copenhagen: they had supposedly traveled to occupied Denmark in order to persuade Niels Bohr to help them forestall all nuclear weapons. This legend was popularized by the author Robert Jungk, but denied, at least privately, by Bohr. When the West German chancellor Konrad Adenauer and his Defense Minister Franz Josef Strauss suggested that the Federal Republic should have its own nuclear weapons, Hahn, Heisenberg, and Weizsäcker joined fifteen other prominent German scientists to issue the Göttingen Declaration, rejecting West German nuclear weapons and refusing to participate in the development of such weapons. Weizsäcker subsequently refined his stance on nuclear weapons.

When Germany invaded Poland, German officials set up a research project into the possible military uses of uranium fission. A nuclear reactor, what the Germans called a “uranium machine,” needed a moderator to slow down neutrons. Water, heavy water, and pure carbon in the form of graphite were all considered, but water would need uranium in which the percentage of isotope 235 is increased, and it did not appear feasible for German industry to produce graphite with sufficient purity. Paul Harteck and his collaborator Wilhelm Groth first tried to separate the uranium isotopes with a separation tube. When this failed, the two physical chemists turned to centrifuges. Scientists in Berlin, Heidelberg, and Leipzig began experiments on the behavior of materials when bombarded with neutrons and on model nuclear reactors. At first the materials needed were scarce but Germany captured the Norsk Hydro in Norway, the largest heavy water producer in the world, and the defeat of Belgium brought with it tons of uranium compounds. From the start of the war through to the autumn of 1941, this research had low priority and made modest progress. At this stage of the war, powerful new weapons did not appear needed.

By the time Walther Gerlach took over the uranium project in late 1943, it was clear that Germany could not build atomic bombs before the end of the war, which was now going very badly for Germany. Work continued under Paul Harteck on isotope separation and heavy water production. Small but steady improvements were made with the centrifuges, but only very small samples with low levels of uranium 235 enrichment were ever achieved. Heavy water production failed, despite great efforts. The work on model nuclear reactors continued, culminating in a final experiment using a lattice of uranium cubes immersed in heavy water. This came close but fell short of achieving a self-sustaining nuclear fission chain reaction. During this period the scientists were focused first and foremost on the survival of themselves and their families. Both Gerlach and Werner Heisenberg sought to facilitate this by continuing to suggest to powerful members of the Nazi elite that their research might lead to an unexpected breakthrough and win the war.

At the end of the war ten German scientists were interned in a country house named Farm Hall in Britain. With one exception, all had worked on the wartime research project on the economic and military applications of nuclear fission. There were microphones hidden in the walls and the Germans’ conversations were recorded, excerpted, translated, and transcribed, including in particular their reactions to the surprising and shocking news of Hiroshima. The Germans discussed four basic questions among themselves: Did they know how to build an atomic bomb? Could the Germans have built these weapons? Did the Germans try to make atomic bombs? Had they been Nazis?

The author of this book, Mark Walker, interacted with two of the main protagonists, Carl Friedrich von Weizsäcker and Robert Jungk, without initially understanding that he had thereby become himself a historical actor in the very history he was writing.

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.