Inside the Centre: The Life of J. Robert Oppenheimer (74 page)

When Bohr read this account, he was appalled at how little it accorded with his own memory of that meeting, and so he wrote, but did not send, a letter to Heisenberg, repudiating it. ‘Personally,’ he wrote, ‘I remember every word of our conversations, which took place on a background of extreme sorrow and tension for us here in Denmark.’ What made a
particularly strong impression on him, he told Heisenberg, was ‘that you and [Heisenberg’s colleague, Carl von] Weizsäcker expressed your definite conviction that Germany would win and that it was therefore quite foolish for us to maintain the hope of a different outcome of the war’. He also remembered Heisenberg giving him the ‘firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons’:

Years later Oppenheimer, evidently basing his account on what he had heard from Bohr, said that Bohr had thought that Heisenberg and Weizsäcker came to Copenhagen ‘less to tell what they knew than to see if Bohr knew anything that they did not’.

After his visit from Heisenberg, Bohr stayed in Denmark for another two years, during which time the situation for Danes hostile to the Nazis became ever worse. In the summer of 1943, the semi-autonomy enjoyed by Denmark throughout its occupation by Germany came to an abrupt end when, enraged by the Danes’ refusal to obey an order to declare martial law, the Nazis reoccupied Copenhagen. Soon afterwards it became very clear that Danish Jews – even those who were internationally renowned, Nobel Prize-winning physicists – were no longer safe. In the autumn of 1943, Bohr received a warning that he was about to be arrested by the Gestapo, whereupon he made plans to escape to Britain with his family.

Earlier that year, in January, Bohr had received, through clandestine
means, a letter from James Chadwick, urging him to leave Denmark and promising him a warm welcome in Britain ‘and an opportunity of service in the common cause’. Realising that this was an effort to enlist him for the Allied attempt to build an atom bomb, Bohr replied turning the offer down. Not only, he told Chadwick, did he feel it his duty ‘to help resist the threat against the freedom of our institutions and to assist in the protection of the exiled scientists who have sought refuge here’, but also ‘I have to the best of my judgment convinced myself that, in spite of all future prospects, any immediate use of the latest marvellous discoveries of atomic physics is impracticable’. However, he did not rule out a change of prospects, circumstances or mind in the future, and then, he promised Chadwick, ‘I shall make an effort to join my friends and I shall be most thankful for any support they might be able to give me for this purpose.’

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 (H
₂
O), but rather of two atoms of deuterium and an atom of oxygen (D
₂
O or
²
H
₂
O), 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’:

‘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:

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.