The atomic bomb

After the discovery of fission, Bohr was acutely aware of the theoretical possibility of making an atomic bomb. However, as he announced in lectures in Denmark and in Norway just before the German occupation of both countries in April 1940, he considered the practical difficulties so prohibitive as to prevent the realization of a bomb until well after the war could be expected to end. Even when Heisenberg at his visit to Copenhagen in 1941 told Bohr about his role in a German atomic bomb project, Bohr did not waver from that conviction.

In early 1943 Bohr received a secret message from his British colleague James Chadwick, inviting Bohr to join him in England to do important scientific work. Although Chadwick’s letter was vaguely formulated, Bohr understood immediately that the work had to do with developing an atomic bomb. Still convinced of the infeasibility of such a project, Bohr answered that there was greater need for him in occupied Denmark.

In the fall of 1943, the political situation in Denmark changed dramatically after the Danish government’s collaboration with the German occupiers broke down. After being warned about his imminent arrest, Bohr escaped by boat with his family across the narrow sound to Sweden. In Stockholm the invitation to England was repeated, and Bohr was brought by a military airplane to Scotland and then on to London. Only a few days later he was joined by his son Aage, a fledgling physicist of age 21, who would serve as his father’s indispensable sounding board during their absence from Denmark.

Upon being briefed about the state of the Allied atomic bomb project on his arrival in London, Bohr changed his mind immediately about its feasibility. Concerned about a corresponding project being pursued in Germany, Bohr willingly joined the Allied project. Taking part for several weeks at a time in the work in Los Alamos, New Mexico, to develop the atomic bomb, he made significant technical contributions, notably to the design of the so-called initiator for the plutonium bomb. His most-important role, however, was to serve, in J. Robert Oppenheimer’s words, “as a scientific father confessor to the younger men.”

An “open world”

Early on during his exile, Bohr became convinced that the existence of the bomb would “not only seem to necessitate but should also, due to the urgency of mutual confidence, facilitate a new approach to the problems of international relationship.” The first step toward avoiding a postwar nuclear arms race would be to inform the ally in the war, the Soviet Union, of the project. Bohr set out on a solitary campaign, during which he even succeeded in obtaining personal interviews with British Prime Minister Winston Churchill and U.S. President Franklin D. Roosevelt. He was unable to convince either of them of his viewpoint, however, instead being suspected by Churchill of spying for the Russians. After the war, Bohr persisted in his mission for what he called an “open world” between nations, continuing his confidential contact with statesmen and writing an open letter to the United Nations in 1950.

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Physics and Natural Law

Bohr was allowed to return home only after the atomic bomb had been dropped on Japan in August 1945. In Denmark he was greeted as a hero, some newspapers even welcoming him with pride as the Dane who had invented the atomic bomb. He continued to run and expand his institute, and he was central in postwar institution building for physics. On a national scale, he took a major part in establishing the research facility at Risø, near Roskilde, only a few miles outside Copenhagen, created in order to prepare the introduction of nuclear power in Denmark, which, however, has never occurred. Internationally, he took part in the establishment of CERN, the European experimental particle physics facility near Geneva, Switzerland, as well as of the Nordic Institute for Atomic Physics (Nordita) adjacent to his institute. Bohr left behind an unsurpassed scientific legacy, as well as an institute that remains one of the leading centres for theoretical physics in the world.