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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 there was even a secret place in New Mexico to make the bombs themselves. The English were much involved* , more than is generally known in this country. There the possibility of making a bomb had been raised, as it was in this country, by refugees from tyranny in Europe; it had been well studied especially by the physicist Rudolf Peierls. The British concluded that this needed to be explored for its potential relevance to the war and, in any case, to the future. The conviction and commitment of the British Government had a great effect in converting the American effort from a series of committees, so secret from one another that they could make little progress, into a major enterprise. It soon became clear that the British had neither the resources nor the physical security that things would go better if they worked with us in the United States.

Shortly before Bohr came to England, Churchill and Roosevelt met in Quebec, and agreed on the participation of the British in the undertaking in this country and Canada. They agreed that there would be consultation between them on political and military problems; they agreed to divide the indispensable uranium, which in part did not originally belong to either of us. This agreement had been signed when Bohr came to England, and Chadwick hoped that Bohr would come to the United States and would lend his great weight to the United Kingdom’s contribution to the undertaking.

Chadwick then asked Bohr to see Sir John Anderson, later Lord Waverly. He was Chancellor of the Exchequer, in charge of the uranium project in the United Kingdom, a conservative, dour, remarkably sweet man, who was a great friend to Bohr. He asked Bohr to help in strengthening the United Kingdom mission in the enterprise, and in the enterprise itself.

By then Bohr had had his first good look. It came to him as a revelation, very much as when he learned of Rutherford’s discovery of the atomic nucleus twenty-five years before. I shall quote short passages from Bohr, and you will know what words he used at the time. Yet I think it best that I rather baldly review the points he had in mind. I run the risk of oversimplifying by so doing; but I do so because it is easy, as history has shown, for even wise men not to know what Bohr was talking about.

First of all, he was clear that if it worked, this development would bring an enormous change in the situation of the world, and of war. The word “menace,” the word “threat,” occur over and over again. When he came to Los Alamos, his first serious question was, “Is it really big enough?” I do not know whether it was; it did finally get to be.

The second point was that he knew enough of how things were in Russia—he had close friends there, Joffe and Kapitza and Landau and many others—to be quite certain that the wartime alliance would not endure the peace as things then stood. He therefore anticipated an unheard-of arms race. He came to know about the possibility of a great increase in the power of bombs by using thermonuclear reactions, and referred to this discreetly when he wrote to Anderson, to Roosevelt, to Churchill. He expected, more I think than really has happened, that these immense enterprises of 1943 would not be so hard for a nation to undertake in ‘53 and ‘63. He wanted to try to prevent this arms race, but to do much else besides. He was clear that one could not have an effective control of atomic energy, which would permit useful application, and a free science and a free spirit of enquiry, without a very open world. He made this quite absolute. He thought that one would have to have privacy. He needed privacy, as we all do; we have to make mistakes and correct them as we learn better. But in principle everything that might be a threat to the security of the world would have to be open to the world.

Bohr knew that the Communists took quite a disdainful attitude towards speaking or revealing the truth; he understood how very much this had gone beyond the tactical duplicity recommended by Lenin to the most dangerous kind of self-delusion. In 1948, after a visit, he wrote General Marshall, the Secretary of State, “What it would mean, if the whole picture of social conditions in every country were open for judgment and comparison, need hardly be enlarged upon.”

From all this he understood that it would not be quite in character for the Soviet Union to make an open world. He felt that it was essential to attempt to engage that government by very early consultation—consultation in a hopefully cautious spirit of friendliness with an ally that had been invaded and occupied with a desperate defensive war. He hoped we would be prepared to offer full cooperation in scientific progress and industrial exploitation, if there were any, in a world in which there were adequate safeguards and, above all, in an open world. He hoped that the situation in which the Russians would find themselves, and what we would have to offer them, and the opportunity for associating themselves with a great, forward-looking change in the world, might alter the whole character of Soviet policy, and thus set a new model of international relations. In an essential and major way force would then cease to play its decisive part, and nations would exert an influence by their example, persuasion, and the extent to which they could truly contribute to the common welfare of men. He saw an example of complementarity, of which as a youth he thought and wrote so much: the complementarity between love and justice. Of all this he spoke, while still in England, to Anderson. Just a few months before his death, Waverly said to me that he had never been reconciled to the fact that Bohr’s counsel had not been followed.

Bohr came to the United States late in 1943. His cover, which was true, was that he would try to advance the cause of international scientific collaboration after the war. Officially and secretly he came to help the technical enterprise. Most secretly of all, and with Anderson’s concurrence, he came to advance his case and his cause. When he arrived in late ‘43, he saw the Ambassador of the United Kingdom, Lord Halifax, and his own Ambassador, De Kauffmann, who with great bravery and gallantry represented his nonexistent government and associated it with us in the conduct of the war. Through them he met Justice Frankfurter again. The Justice had heard in very general terms of the atomic undertaking; he listened to Bohr with growing and very deep respect. Then Bohr came with his son, Aage, his companion, his confidante, to Los Alamos.

Bohr at Los Alamos was marvelous. He took a very lively technical interest. But his real function, I think for almost all of us, was not the technical one. He made the enterprise seem hopeful, when many were not free of misgiving. Bohr spoke with contempt of Hitler, who with a few hundred tanks and planes had tried to enslave Europe for a millennium. His own high hope that the outcome would be good, that the objectivity, the cooperation of the sciences would play a helpful part, we all wanted to believe.

Early in 1944 Justice Frankfurter talked to Roosevelt about Bohr’s ideas. The President listened with great interest, and with a word of encouragement, which he asked Bohr to take back to England. At that time, Anderson had talked to the Prime Minister, seeking to extend discussion of the future of atomic energy within the British Government. This did not appeal too much to Churchill. Bohr went back in April of ‘44, with the word for Anderson of Roosevelt’s interest. Bohr learned from the First Secretary of the Soviet Embassy of a letter for him from Kapitza, who had been in Cambridge, was well loved by Rutherford, well known to Bohr, and later prevented by the Russians from leaving Russia. Kapitza wrote, asking Bohr, of whose escape to Sweden he had heard, to come to Russia, saying that things had been hard, but that now they could work again, and that Bohr would be very much at home among colleagues. Bohr concluded that the Russians were interested in practical nuclear problems. His answer was friendly; he said he had other plans to promote international cooperation after the war was won.

  1. *

    For an account of the United Kingdom enterprise, and of the relations between the United Kingdom and the United States, vide “Britain and Atomic Energy, 1939-1945 by Margaret Gowing, London, McMillan Co. Ltd., 1964, New York, St. Martin’s Press, 1964. Mrs. Gowing has had access to the official documents of the United Kingdom, and is a gifted historian. She gives a thoughtful and understanding account of what Bohr did and thought. Appendix 8 is the text of the disastrous aide-mémoire, initialled by Churchill and Roosevelt, September 19, 1944; at Hyde Park, and paraphrased here on Page 7.

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