Copenhagen became the setting for one of the book’s most consequential and least settled conversations: Werner Heisenberg’s 1941 visit to Niels Bohr in German-occupied Denmark. What passed between the two physicists was never established with certainty, but Bohr’s alarm about the possibility of a German atomic weapon followed him from Copenhagen to Los Alamos and brought the disputed encounter into the Manhattan Project’s wartime deliberations.

An Encounter in Occupied Copenhagen

In September 1941, Heisenberg received special permission from the Nazi regime to attend a conference in German-occupied Copenhagen. He sought out Bohr, his former mentor and friend. Heisenberg was not himself a Nazi, but had chosen to remain in Germany and was its most eminent physicist; if the country had an atomic-bomb project, he was an obvious candidate to lead it. The visit thus joined an intimate meeting between old colleagues to the urgent question of what German scientists knew, and what they intended to do, about nuclear weapons.

Two Accounts of the Conversation

Heisenberg later maintained that he cautiously raised the uranium problem and tried to communicate that a fission weapon was possible in principle but would require an immense technical effort, one he hoped could not be completed during the war. He claimed that, under surveillance and fearing for his life, he could not explicitly say that German physicists wanted to convince the regime that such a weapon was infeasible in time for use.

Bohr did not understand the conversation that way. He heard Germany’s leading physicist say that an atomic weapon could be built and might decide the war. Alarmed and angry, he cut the conversation short. In drafts of a letter to Heisenberg that he later composed but never sent, Bohr recalled Heisenberg saying at the beginning of their talk that he was certain a sufficiently long war would be decided by atomic weapons. Bohr also wrote that Heisenberg described years of work on the question but offered no indication that German scientists were trying to prevent the weapon’s development. The two accounts leave the encounter an enduring enigma; the book does not resolve what Heisenberg meant or how much Bohr could have known of his intentions.

The City’s Wider Significance

Copenhagen matters here not through a later change to the city, but as the place where uncertainty became a lasting political and scientific fear. Bohr left the meeting afraid that Germany might end the war with an atomic weapon. When he later recounted the episode at Los Alamos, his interpretation helped focus Oppenheimer’s attention on the bomb’s consequences beyond its construction. The encounter’s unresolved meaning makes Copenhagen a point of departure for the book’s larger question: how scientists should act when the stakes are immense but the intentions of others remain uncertain.