by Ray Monk
A few months later, in August 1943, Bohr wrote again to Chadwick announcing a change of mind. ‘In view,’ he told Chadwick, ‘of the rumours going around the world, that large scale preparations are being made for the production of metallic Uranium, and heavy water to be used in atomic bombs, I wish to modify my statement as regards the impracticability of an immediate use of the discoveries in nuclear physics.’
What had changed his mind? Jeremy Bernstein has suggested (persuasively, I think) that the cause of Bohr’s volte-face was a visit he received in Copenhagen from the German physicist J. Hans D. Jensen in the summer of 1943. Jensen had been urged to speak to Bohr by Heisenberg, who, realising that his own visit to Copenhagen had been something of a disaster, thought Jensen – well known among physicists to be politically left-wing – might soften Bohr’s attitude towards the German atomic programme. Much had happened in the two years that separated the two visits. In September 1941, when Heisenberg had visited Bohr, there was every reason to think that the Germans might win the war, and some reason, among the German physicists associated with what was officially known as the ‘Uranium Research Programme’, to think that the Nazis might be ahead of the Allies in the race to build an atomic bomb.
At an early stage, the Nazi bomb project had abandoned any attempt to build a bomb from uranium-235. The effort involved in separating uranium isotopes on an industrial scale was more than the wartime economy of Nazi Germany could cope with, especially as nobody on the Nazi side had realised what Frisch and Peierls realised – namely that the critical mass of U-235, using fast rather than slow neutrons, was surprisingly small. As was revealed after the war, Heisenberg’s thinking about critical mass was fundamentally flawed. On his calculations, a bomb would require about one ton of pure U-235, and obtaining such a quantity was clearly out of the question. What Heisenberg and the other German physicists had realised at a fairly early stage, however, was that plutonium would be just as good as U-235 in a fission bomb and that it could be produced relatively easily in a nuclear reactor using unenriched uranium and slow neutrons.
For most of the war, therefore, the Nazi atomic project concentrated on building a reactor. The design of this reactor underwent several changes, but at an early stage it was decided not to use graphite as a moderator, as Fermi had done in Chicago, but rather to use heavy water. Heavy water differs from ordinary water in that its molecules consist not of two atoms of ordinary hydrogen and an atom of oxygen (HO), but rather of two atoms of deuterium and an atom of oxygen (DO or 2HO), deuterium being the isotope of hydrogen the nucleus of which has a neutron as well as a proton. It is indeed possible to build a reactor using heavy water as a moderator, and several such reactors have in fact been constructed; the first to go critical was built by the Allies in Argonne, Illinois, in 1944. The problem, however, is that such reactors need many tons of heavy water (the one at Argonne used 6½ tons), which, though nothing like as difficult to obtain as uranium-235, is not easy to produce.
With the occupation of Norway in 1940, the Germans acquired the first and largest heavy-water production plant in the world, the Vemork plant at Lake Tinn, about eighty miles west of Oslo, which produced about twelve tons a year. The supply of heavy water from Vemork to the German atomic-bomb project, however, was successfully interrupted by a series of Allied attacks on the plant, most notably a commando raid in February 1943, a bombing raid in November 1943 and, finally, the sinking, in February 1944, of a ship loaded with heavy water that the Nazis were attempting to transfer to Germany. Heisenberg had estimated that a reactor built with the purpose of producing plutonium would need about five tons of heavy water. Thanks to the Allied operations in Norway, the German bomb project received in total during the war no more than three tons. Meanwhile, as part of the Canadian contribution to the Manhattan Project, a plant in Trail, British Columbia, was, from 1943 onwards, producing six tons a year.
In the face of the huge technical and theoretical problems that stood in the way of designing and building an atomic bomb, and in the light of the deteriorating economic and military situation of Nazi Germany as the war went on, the German bomb project was scaled down at exactly the time when the Allied project gained its irresistible momentum, namely in the first half of 1943. When Heisenberg came to Copenhagen in September 1941, Nazi Germany had an atomic-bomb programme based on the plan of building a heavy-water nuclear reactor that would produce enough plutonium to build a bomb; by the time Jensen visited Copenhagen in the summer of 1943, it had been conceded by the Nazis that there was little chance of nuclear energy having any direct military use for them and the sole purpose of what was left of their atomic programme was to build a reactor for industrial purposes. In May 1943, Heisenberg gave a lecture to engineers and military officers in which he outlined a possible design for such a reactor. His design used plates of uranium, three tons of it, immersed in one and a half tons of heavy water. When Jensen visited Bohr he explained this design and emphasised that the intention was to use it for civil rather than military purposes.
Bohr evidently took from his conversation with Jensen only the information that the Germans were pressing ahead with the utilisation of fission energy, without taking seriously, or perhaps without believing, the assurances that the intention was to build only a reactor, not a bomb – hence his remark to Chadwick that ‘large-scale preparations are being made for the production of metallic Uranium, and heavy water to be used in atomic bombs’. As is shown by the rest of his letter to Chadwick, Bohr’s knowledge and understanding of atomic-bomb physics at this time were fairly rudimentary and indeed, in some important respects, flawed and confused. He clearly knew nothing at all about plutonium and evidently believed that bombs could be made using slow neutrons and heavy water. The differences between an atomic reactor and an atomic bomb were obviously still not clear in his mind.
When he left Denmark to go to Britain, Bohr took with him a drawing of the reactor Jensen had described to him, apparently believing it to show the design of the Nazi atomic bomb and therefore thinking it had great military significance.fn52 Bohr and his wife escaped Denmark by boat to Sweden and then by plane to Britain, arriving in Croydon, near London, on 5 October 1943. He was met from the plane by Chadwick, who took him to the Savoy Hotel in London, where he brought Bohr up to date on the developments in the Tube Alloys project: the Frisch–Peierls memorandum, the MAUD report and the Manhattan Project. That evening, Bohr officially became a member of the Tube Alloys project, and therefore part of the British ‘brains and resources’ that the Quebec Agreement had stipulated should be shared with the US.
So it was that Bohr and his son, Aage, who was by this time a notable physicist in his own right and who had followed his parents to London, sailed to America at the end of November 1943 as part of the British mission to join the Manhattan Project. They arrived in New York on 6 December, and then went to Washington to meet General Groves, after which they travelled with Groves by train to New Mexico, where Oppenheimer, with great and evident delight, welcomed them. After giving ‘Nicholas Baker’ and his son ‘James’ (as Niels and Aage Bohr were code-named) time to settle in, Oppenheimer convened a meeting of some of his most senior scientists, including Bacher, Bethe, Serber and Teller, to discuss the drawing that Bohr had brought with him and which he had already discussed with Groves. ‘It was clearly a drawing of a reactor,’ Hans Bethe later recalled, ‘but when we saw it our conclusion was that these Germans were totally crazy – did they want to throw a reactor down on London?’ The following day, Oppenheimer was able to write to Groves telling him that what was depicted in the drawing Bohr had brought with him from Denmark ‘would be a quite useless military weapon’.
Though Bohr had much to learn and very little to teach about the physics of the atomic bomb, he was so revered and so inspirational that having him at Los Alamos seemed to lift the spirits of all the scientists there. On 17 January 1944, after Bohr had left Los Alamos for Washington, Oppenheimer wrote to Groves to say that he hoped Bohr�
�s collaboration with the project would continue, ‘since it has been of great help to us and is likely to be so throughout the year’:
By word and deed Dr Baker has done everything he could to support this project and to indicate that he is sympathetic not only with its purposes and general method of procedure, but with the policies and achievements of the project’s overall direction. I should like to make it quite clear that the effect of his presence on the morale of those with whom he came in contact was always positive and always helpful, and that I see every reason to anticipate that this will be true in the future.
‘Bohr at Los Alamos was marvellous,’ Oppenheimer said years later. He ‘took a lively technical interest’ in what was going on and talked to many people, but his real function there, Oppenheimer said, was that:
he made the enterprise which looked so macabre seem hopeful and he spoke with contempt of Hitler, who with a few hundred tanks and planes had hoped to enslave Europe; he said nothing like that would happen again and his own high hope [was] that the outcome would be good and that in this the role of objectivity, friendliness, cooperation that science had established would play a helpful part – all this was something that . . . we wished very much to believe.
By giving the project his blessing, Bohr, in the minds of many of the scientists at Los Alamos, gave it a legitimacy and a prestige that it did not have before, and this renewed their enthusiasm for the task and their willingness to put up with the otherwise uncongenial military situation in which they found themselves. That, presumably, is what the official history of Los Alamos means when it states that Bohr’s influence ‘was to bring about stronger and more consistent cooperation with the army in the pursuit of the common goal’. Regarding the technicalities of building a bomb, Bohr, despite his interest in the work being done at Los Alamos, realised that he had little to contribute. ‘They didn’t need my help in making the atom bomb,’ he is reported to have told a friend after the war. What he did have to contribute – and in this respect he exerted an enormous influence on Oppenheimer’s own thinking – were some wide-ranging thoughts on the politics of the bomb, which, had they been adopted, might have had a profound impact on the subsequent history of the world during the second half of the twentieth century.
When, almost as soon as he arrived in England, he was brought up to speed by Chadwick on the progress that had been made in designing and building a bomb, Bohr was disconcerted to discover how little thought had been devoted, either in England or in the United States, to the political implications the bomb would have for the post-war world. On his second night in England Bohr dined with Sir John Anderson (later Lord Waverley), who was then Chancellor of the Exchequer and the Cabinet minister responsible for Tube Alloys. Anderson was unusual among politicians in having a reasonably good grasp of the science behind the bomb, having studied science at the University of Leipzig, where he wrote his dissertation on the chemistry of uranium. Oppenheimer had a great deal of respect for Anderson, whom he described as ‘a conservative, dour and remarkably sweet man, who was very congenial in his spirit to Bohr and was a good friend to him’. It was Anderson who invited Bohr to join the Tube Alloys project and then to go to Los Alamos as a member of the British mission.
Because of the universal esteem in which he was held, Bohr, though in many ways a simple and unassuming man, was given access to people at the very top of the social and political order. While he was in Washington, before he went to Los Alamos, a reception in his honour was held at the Danish embassy, at which he was able to renew his acquaintance with Felix Frankfurter, a Supreme Court associate and a close personal friend of the President. Though there was little opportunity on this occasion for extended discussion, Frankfurter invited Bohr to have lunch with him the next time he was in Washington.
When he left Los Alamos at the end of January 1944, Bohr went to Washington to take up this invitation. By this time he had thought very seriously about the post-war situation and had had what he regarded as a revelation about the ‘complementarity’ of atomic bombs – a revelation as important, he believed, as his earlier epiphany regarding the complementarity of subatomic particles. Just as electrons are at one and the same time waves and particles, so, Bohr now believed, atomic bombs were at one and the same time the greatest danger to mankind and the greatest boon. Atomic bombs could put an end to civilisation and human life itself, or, precisely because of that, they could bring an end to war. What was needed, Bohr felt, was a spirit of cooperation and, above all, openness. If the power of atomic bombs was made clear to everybody, Bohr reasoned, there would be at least the possibility of cooperation and therefore the possibility that this terrible weapon could turn out, because of its very terribleness, to be the best thing mankind had ever invented.
Bohr’s view was therefore the exact opposite of the policy that the US had adopted ever since the discovery of fission. Where that policy had been based on the attempt to prevent the Soviets from acquiring the ‘secret’ of the bomb, Bohr believed that the best thing would be to consult the Soviet Union about the dangers to humanity posed by the development of such powerful weapons, and to treat the problem of controlling such weapons as one that demanded international cooperation rather than competition. In this way, he believed, those weapons would force upon the countries of the world a fundamental change in international relations, one that would make war itself obsolete.
Astonishingly, Frankfurter was sympathetic to Bohr’s ideas and, even more astonishingly, thought President Roosevelt would be responsive to them. He thus offered to arrange a meeting between Bohr and Roosevelt. In a private memorandum he wrote about a year later, Frankfurter says that, when he mentioned Bohr’s ideas to Roosevelt, the President ‘shared the hope that the project might bring about a turning point in history’. The atomic bomb, Roosevelt told Frankfurter, ‘worried him to death’, and he ‘was very eager for all the help he could have in dealing with the problem’. He was therefore keen to meet Bohr, but, he said, he would not discuss this crucially important issue behind the back of his ally Winston Churchill, and so, before he met Bohr, he wanted Bohr to meet Churchill.
At the beginning of April 1944, therefore, Bohr, accompanied by Aage, flew to London for a meeting with Churchill. Shortly before, Anderson had written a memorandum for Churchill outlining Bohr’s ideas and recommending that the Soviet Union be informed about ‘this devastating weapon’. He went on to propose that it be invited ‘to collaborate with us in preparing a scheme for international control’. On his copy of the memorandum Churchill had written beside the word ‘collaborate’ the uncompromising reaction: ‘On no account’.
Churchill kept Bohr waiting for over a month and did not see him until 16 May. In the meantime, Bohr received via the Soviet embassy an invitation to go to the Soviet Union, ‘where everything will be done to give you and your family a shelter and where we now have all the necessary conditions for carrying on scientific work’. He was also told by a Soviet official that they knew he had been in America and was asked directly what information he had received about the war work of American scientists, a question to which Bohr responded with bland generalities.
After his warm reception by Anderson and Frankfurter, Bohr’s meeting with Churchill was a bitter disappointment. The meeting lasted a bare thirty minutes, most of which was taken up with Churchill’s vehement dismissal of the idea of sharing information about the bomb with the Soviet Union. Bohr left the meeting under no doubt that his ‘revelation’ would, if Churchill had anything to do with it, have no influence whatsoever on shaping Allied policy in the post-war period. This rebuff was something about which he remained angry for the rest of his life. ‘It was perfectly absurd to believe that the Russians cannot do what others can,’ he later said. ‘There never was any secret about nuclear energy.’ Churchill, for his part, dismissed Bohr from his mind – remarking to Frederick Lindemann (now Lord Cherwell), who had accompanied Bohr to Downing Street: ‘I did not like the man when you showed him to me, with his
hair all over his head’ – and turned his attention back to the preparations for D-Day.
These landings took place on 6 June 1944, and by the time Bohr left England an Allied force of several hundred thousand men was advancing through France. Back in Washington, he was urged by Frankfurter to put his ideas down in writing in the form of a memorandum for the President. This led to a meeting with Roosevelt in August, in which Roosevelt expressed sympathy for Bohr’s ideas and suggested that Churchill could be won round. After Roosevelt and Churchill met in September, however, the opposite happened: Roosevelt came round to Churchill’s view on the matter, the two of them agreeing not only that ‘the suggestion that the world should be informed regarding tube alloys, with a view to international agreement regarding its control and use, is not accepted’, but also that: ‘Enquiries should be made regarding the activities of Professor Bohr and steps taken to ensure that he is responsible for no leakage of information particularly to the Russians.’
‘The President and I are much worried about Professor Bohr,’ Churchill wrote to Cherwell on 20 September, citing as grounds for concern Bohr’s unauthorised discussions with Frankfurter and his contacts with the Soviet Union. ‘It seems to me,’ Churchill declared, ‘Bohr ought to be confined or at any rate made to see that he is very near the edge of mortal crimes.’ In the event, Churchill was dissuaded from actually locking Bohr up, but that was the end of Bohr’s personal contacts with the leaders of the Western world. After recounting this story in his lectures on Bohr, Oppenheimer remarks: ‘This was not funny, it was terrible and it shows how very wise men, dealing with very great men, can be very wrong.’
