Niels Bohr and Atomic Weapons

This is a much abbreviated account of lectures given between August, 1963 and May, 1964, at the Brookhaven National Laboratory, at the University of California, at the California Institute of Technology, and at Los Alamos, New Mexico. The major deletions involve technical aspects of the bomb, and above all a summary of Bohr’s work over three decades on atomic and nuclear problems; for a review of these, vide N. Bohr; The Rutherford Memorial Lecture; Proc. Lond. Phys. Soc., 78: 1083-1115 (1961).

When in 1939 Bohr left the United States for Denmark, he did not expect that explosive application of the fission process lay close at hand. His Institute in Copenhagen, his baroque nineteenth-century home at Carlsberg, were now a very different world. For years these had been a refuge for colleagues from Germany, and then Austria. When Fermi came up to get his Nobel Prize in Stockholm, he did not go back to Italy but stopped in Copenhagen; then he came to this country. From Russia there were also refugees: Charlotte Houtermans, whose husband was in prison in Russia until the Molotov-Ribbentrop Pact, and Placzek, Weisskopf, many others. Thus Bohr had, in addition to his deep devotion to Denmark, which had kept him in Copenhagen twenty years earlier when he had been pressed to come to England, also a sense of responsibility for his wards.

The Institute was closed in 1940. The so-called Director of the dead Institute was a man who had tried to enter the living one; but Bohr had been a little too canny for that. Heisenberg and Weissacker came over from Germany, and so did others. Bohr had the impression that they came less to tell what they knew than to see if Bohr knew anything that they did not; I believe that it was a standoff.

Then in ‘43 it became far too dangerous to Bohr’s freedom and life. He had been in touch with the Danish underground, and through them with the British Secret Service; he had a letter from Rutherford’s student Chadwick, then director of the Cavendish Laboratory, encouraging him to come to England. So in the last days of September, he escaped one night in a small boat to Sweden; three weeks later he was flown to England in the bomb bay of an unarmed Mosquito. They gave him an oxygen mask and a headgear with ear phones; but the Royal Air Force was not used to such great heads as Bohr’s, and he was unconscious.

But once in England and recovered, he learned from Chadwick what had been going on. To Bohr the enterprises in the United States seemed completely fantastic. Today, of course, they may seem old hat; but it made a deep impression on him that there was to be a great diffusion plant separating uranium isotopes in Oak Ridge, that uranium atoms would be made to fly through a vacuum to concentrate the lighter ones, that there were plutonium-producing reactors abuilding in Hanford, that …

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