Inside the Centre: The Life of J. Robert Oppenheimer
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So immersed was Oppenheimer in his collaborative work with Lawrence on bomb-related questions that, even though he was not officially employed on war work, he wrote to Robert Millikan on 20 March 1942, resigning his part-time post at Caltech in order to give his full attention to war-related research. ‘New and compelling reasons,’ wrote Oppenheimer, ‘have arisen for my leaving Berkeley as little as possible.’ Six days later, Lawrence wrote to Conant, suggesting that Oppenheimer’s involvement in the work of the S-1 committee be made official, or, as Lawrence put it, urging ‘the desirability of asking Oppenheimer to serve as a member of S-1’:
I think he would be a tremendous asset in every way. He combines a penetrating insight into the theoretical aspects of the whole program with solid common sense, which sometimes in certain directions seems to be lacking, and I am sure that you and Dr Bush would find him a useful adviser.
With Lansdale’s report from Berkeley still fresh in his mind, Conant was in no hurry to bring a suspected security risk on board. Conant was, however, unable to tell Lawrence who he should and should not appoint to the Rad Lab and thus was unable to prevent Lawrence from inadvertently creating one of the biggest security nightmares of the entire US bomb project. For, in involving Oppenheimer in his work, Lawrence was also opening the door to Oppenheimer’s students and, in this way – despite Lawrence’s own political conservatism and hostility to left-wing political activity – the Rad Lab acquired a reputation as a hotbed of radicalism. Lawrence had already taken on Frank Oppenheimer, who, after Pearl Harbor, was put in charge of building the 184-inch cyclotron. According to the historian Gregg Herken, Lawrence’s ‘boys’ – the other Rad Lab scientists – remembered Frank ‘nervously chain-smoking, pacing back and forth on the wooden latticework that rose above the big magnet’.
Lawrence’s plan was to persuade the government, via the S-1 committee, to invest hundreds of millions of dollars in a large-scale industrial plant of ‘Calutrons’ (as he was calling the modified cyclotrons) in order to produce the U-235 required for the bomb. It was a plan beset with all kinds of problems, some of which required a better understanding than they had at this time of the physics behind the electromagnetic separation of isotopes, which is where Oppenheimer and his students came in.
Thus it was that in the early summer of 1942 two of Oppenheimer’s students, Stanley Frankel and Eldred Nelson, were working on the ‘theoretical’ problem of how to improve the focusing of the Calutron beam. Their paper on the subject was shown by Oppenheimer to another of his students, Rossi Lomanitz. ‘Uranium was never mentioned,’ Lomanitz later said. ‘It didn’t need to be.’ Lomanitz was one of a group of Oppenheimer’s students who were active in radical politics and, quite probably, members of the Communist Party. Among the other members of this group were Lomanitz’s flatmate, David Bohm, and their close friend Max Friedman, both of whom were recruited by Oppenheimer into the Rad Lab over the spring and summer of 1942. A fourth member, Joe Weinberg, was considered too radical to be employed on such security-sensitive work.
The 184-inch Calutron was switched on for the first time on 26 May 1942, by which time Oppenheimer, though he had not yet received security clearance, was playing a pivotal role in the work of the S-1 committee. Without actually becoming a member of the committee, as Lawrence had proposed, Oppenheimer was appointed as a consultant, with special responsibility for investigating the physics of fast-neutron collisions. His predecessor in this position was Gregory Breit, a physicist at the University of Wisconsin (it had been Breit who suggested that Joe Weinberg transfer from Wisconsin to Berkeley to work with Oppenheimer), who had been involved in the uranium project from the beginning. Breit had a reputation for being a difficult man to work with, partly because of his almost obsessive concern with secrecy. As one of the editors of the Physical Review, Breit had used his influence to persuade physicists to impose upon themselves a voluntary ban on publishing anything that might have military value for the duration of the war. When the S-1 committee was formed, Breit was asked by Compton to act in a consultancy role as the head of a small group advising the committee on the physics of fast neutrons and the related question of the design of the bomb. Breit’s official title, which Oppenheimer took delight in inheriting, was ‘Co-ordinator of Rapid Rupture’.
It is not entirely clear when Oppenheimer was asked to take over from Breit. In his autobiography Robert Serber recalls that ‘a few weeks after Pearl Harbor’ – so, around Christmas time 1941 – he received a phone call from Oppenheimer, who told him that he was in Chicago and wanted to come to Urbana to talk to Serber about something. When he arrived, the two went for a walk in the countryside: ‘There, alone in that rural setting, he told me that he was going to be appointed to head the weapons end of the atomic-bomb project, to replace Gregory Breit in that position.’ Oppenheimer wanted Serber to come to Berkeley as his assistant on the project. Serber agreed to come, but, he says, could not leave Urbana until the end of that semester and so did not arrive in Berkeley until the end of April 1942.
Clearly, however, Oppenheimer could not replace Breit until Breit left, which he did not do until May. It seems possible that, from December 1941 until May 1942, Oppenheimer and Breit were working alongside one another. One of Breit’s tasks in his role as ‘Co-ordinator of Rapid Rupture’ was to hold a series of seminars in order to allow the exchange of ideas among members of his small group. For a while, at least, these seminars were held at Chicago, with both Oppenheimer and Breit taking part. Samuel Allison, a member of Breit’s group, recalls:
Breit was always frightened something would be revealed in the seminars. Oppenheimer was frightened something would not. I backed Oppenheimer and challenged Breit to cut the censorship. He accused me of being reckless and hostile to him. I failed. The seminars became uninformative.
‘Breit was a terrible choice,’ another member of the Met Lab stated. ‘He was actually capable of turning a technical problem into a fist fight.’ Nuel Pharr Davis, in his book on Oppenheimer and Lawrence, records a power struggle between Oppenheimer and Breit over leadership of Breit’s group:
Compton, who had become impressed with something firm and bold in Oppenheimer’s manner, gave Breit no backing. Breit realized that Oppenheimer stood for the new climate of opinion. He tentatively suggested Oppenheimer visit him on his home grounds at Wisconsin to thresh their difficulties out, and he hinted that Oppenheimer might like to make Wisconsin his base so that they could work closely together. But when the time came to give a definite invitation, Breit simply could not do it. He turned in his resignation to Compton and got completely out of the fission project on June 1, 1942.
One of the few pieces of documentary evidence relating to Breit’s replacement by Oppenheimer is a letter Breit wrote to Briggs on 18 May 1942. ‘I do not believe that secrecy conditions are satisfactory in Dr Compton’s project,’ Breit wrote:
Within the Chicago project there are several individuals strongly opposed to secrecy. One of the men, for example, coaxed my secretary there to give him some official reports out of my safe while I was away on a trip . . . The same individual talks quite freely within the group . . . I have heard him advocate the principle that all parts of the work are so closely interrelated that it is desirable to discuss them as a whole.
That individual was Enrico Fermi, who, it will be remembered, took a great deal of persuading back in 1939 that secrecy should be imposed. There was evidently no way that Fermi and Breit could continue to work together on the same project, and, as Fermi was entirely indispensable, it was Breit who left, providing Oppenheimer with his first official position as part of the US atomic-bomb project.
Strictly speaking, Oppenheimer was still not officially allowed to know anything about the project, not even the fact of its existence, since he did not yet have security clearance. On 28 April 1942, he filled out a government security questionnaire, but his application for clearance took more than a year to be approved. Meanwhile he not only continued to have access t
o the deliberations of the people charged with providing the US with its first atomic bomb, but also to play an increasingly central role in shaping those deliberations.
At about the same time that Oppenheimer filled out his security form, Robert and Charlotte Serber arrived in Berkeley. As Serber recalls, the day after they arrived:
I went down to Oppie’s office in Le Conte Hall where he had accumulated a number of British documents concerning bomb design. I remember there was a paper on critical mass and something on efficiency, I don’t remember. The papers were rudimentary but were really quite helpful in getting us started.
Seven months after Oliphant’s visit, it seems, the MAUD report was still serving a role as inspiration.
Compton, it will be remembered, had been given six months from December 1941 in which to make the case for investing hundreds of millions of dollars in the atomic-bomb project, and that time was coming to an end. At a meeting of the S-1 committee on 23 May 1942, it was decided to recommend going ahead with what were now five methods of providing fissionable material for a bomb – centrifuge, gaseous diffusion, electromagnetic separation and two different methods of producing plutonium – at an estimated cost of $500 million. On 17 June, Compton’s suggestions were approved by President Roosevelt, who also recommended transferring the project from civilian to military control.
From that moment on, the US no longer had a research project led by scientists with the aim of investigating the possibility of building an atomic bomb; it had an engineering project run by the US army with the aim of actually building an atomic bomb. And security now was no longer a matter of voluntary agreements; it was something imposed on the project by a team of 300 members of the US army’s Counter Intelligence Corps, under the able leadership of (the newly promoted) Captain John Lansdale. Three days after the President had given his approval to Compton’s recommendations, the project was discussed at the highest possible political level, when, at the second Washington conference, Roosevelt and Churchill agreed that America and Great Britain should cooperate with each other in their joint effort to beat the Nazis in the race to produce the world’s first atomic bomb.
At this point, Oppenheimer’s part in the joint project was still the fairly minor one that he had inherited from Breit. However, the discussion group he led had two main tasks – to investigate fast-neutron collisions and to think about bomb design – and the second of these was hardly a peripheral concern. In effect, Oppenheimer, though his application for security clearance had not yet been approved and he was not yet a member of the S-1 committee, was now in charge of designing the bomb.
His main accomplice in this crucial task was Serber, who within a month or so of arriving at Berkeley achieved more in getting the bomb designed than Breit’s team had managed in the previous five months. He was helped by having at his disposal the combined talents of Stanley Frankel and Eldred Nelson, the two graduate students of Oppenheimer’s who had helped to improve the beam of the Calutron. They were still employed by Lawrence at the Rad Lab, but, so long as, in Serber’s words, ‘I didn’t take up so much time that Ernest’s requirements suffered’, Serber had them as his assistants. He assigned them the task of improving the calculation of critical mass and was surprised when they came back to him with a formula that allowed an exact solution to the problem, ‘provided, of course, one knew all the physical constants, such as the value of the cross sections and the number of neutrons per fission’. As far as those difficulties were concerned, no more theory was necessary; all that was required were the results of further experiments.
While Nelson and Frankel were calculating critical mass, Serber looked into the problem of efficiency. When a lump of uranium goes critical, not all the material will fission, because the uranium will expand with the heat and be blown apart by the explosion before most of it fissions. So the problem of efficiency is: how much of the uranium in a bomb will actually fission and therefore be converted into explosive energy? Given the extreme difficulty of separating U-235 from natural uranium, the question of efficiency was very important, since the more efficient the bomb, the less enriched uranium would be required.
The basic design of the uranium bomb had already been laid out by Frisch and Peierls in their memorandum of 1940: two subcritical pieces of uranium would be brought together to form one supercritical piece. The problem with this design was that, for it to work, the two pieces would have to be brought together very quickly, otherwise a stray neutron would most likely set off a chain reaction before the two pieces were in place, causing the bomb to ‘fizzle’. To avoid this, the fissionable materials had to be extremely pure, and the ‘gun’ firing the two pieces together had to be extremely fast. This meant that Oppenheimer’s team had to investigate and solve two sets of questions, one involving the chemistry of uranium and plutonium and the other involving firearms and explosives.
Faced with these kinds of problems, Oppenheimer’s instinct was the exact opposite of Breit’s; whereas Breit had wanted above all to protect the secrecy of the discussions among his group, even if that meant inhibiting those discussions, Oppenheimer wanted above all to encourage those discussions, even if that meant compromising a little on security. In July 1942, therefore, he and Serber decided to host a meeting at Berkeley of, as Oppenheimer put it, ‘luminaries’ – top-level physicists whose expertise might be brought to bear on the problems facing them.
The man Oppenheimer wanted most urgently involved in this meeting was someone who, up to this point, had not been involved in the bomb project at all, and who had, in fact, remained deeply sceptical that an atomic bomb could possibly be built. That man was Hans Bethe, widely regarded at this time as the leading nuclear physicist in the world. Bethe’s review articles of the late 1930s were seen as being so authoritative they had become known as ‘Bethe’s Bible’. His work on stellar energy, which was eventually (in 1967) to win him the Nobel Prize, was well known to Oppenheimer, as it was to most physicists, providing as it does a profound and fundamentally important analysis of how nuclear fusion lies at the heart of the energy produced by stars.
Having been removed from his post at Tübingen because he was partly Jewish, Bethe had, since 1935, been at Cornell, where he was to remain for the rest of his career. Though he badly wanted to contribute to the Allied war effort, he had refused to have anything to do with the atomic-bomb project because he thought it extremely unlikely to succeed. ‘Separating isotopes of such a heavy element [as uranium] was clearly a very difficult thing to do,’ he later said, ‘and I thought we would never succeed in any practical way.’ To help enlist Bethe, Oppenheimer approached John H. van Vleck, a professor of physics at Harvard, and asked him to convince Bethe that his participation was necessary.
Though not yet fully convinced that the project would be successful, Bethe agreed to come to the meeting at Berkeley organised by Oppenheimer and Serber. On the way, he stopped at Chicago to pick up his old friend Edward Teller, who had also been invited. At Chicago, Teller explained to Bethe the progress that had been made at the Met Lab and, in particular, the progress made with the project led by Fermi and Szilard to create plutonium in a nuclear reactor. At what was to become the famous rackets court at Stagg Field, Bethe saw the ‘tremendous stacks of graphite’ that Fermi and Szilard had amassed as part of what would be the world’s first nuclear reactor.fn44 ‘I then,’ he remembered, ‘became convinced that the atom-bomb project was real and that it would probably work.’
For his part, Teller was so convinced the fission bomb would work that he had lost interest in it as a theoretical problem. Much more interesting to him was the possibility, first mentioned speculatively to him by Fermi one day over lunch, of a fusion bomb. Just as the fission of heavy elements releases great amounts of energy, so does the fusion of lighter elements. In fact, fusion – if it could be achieved – offers much greater yields of energy than fission.
The individual nucleons that make up a nucleus have a greater total mass than the nucleus itself.
In combining to make up a nucleus, they lose some of their mass. This is called ‘mass defect’. The missing mass is converted into the energy required to hold the nucleons together – that is, it becomes what is called ‘binding energy’. In both fission and fusion, nuclei with comparatively low binding energies are converted into nuclei with high binding energies – that is, elements with comparatively high mass per nucleon are converted into elements with comparatively low mass per nucleon. As Frisch and Meitner were the first to realise, this missing mass is released as energy, potentially as a massive explosion.
It sounds contradictory that both the fusion of lighter elements and the fission of heavier elements release energy. One might expect that, if energy is released by the process of fission, it would be absorbed by the process of fusion. The explanation for this lies in what is known as the ‘curve of binding energy’. Not all elements have the same binding energy. Neither does the difference go up or down in continuous proportion to the mass of the element. Rather, the binding energy starts off small for the lightest elements, such as hydrogen, helium and lithium, and then increases until one gets to iron (atomic number 26, with a mass of 56), then it decreases again.
Thus, while it is true that the collected mass of the individual nucleons that make up a uranium nucleus will be greater than the mass of the nucleus itself – just as the collected mass of the individual nucleons that make up a helium nucleus will be greater than the nucleus itself – it is also true, as noted in the previous chapter, that the collected mass of the separated pieces of the split uranium nucleus (say, barium, krypton, plus two neutrons) will have a slightly smaller combined mass than that of the original nucleus. The reason for this is to be found in the curve of binding energy, which shows that the mass defect (binding energy) for barium and krypton is greater than that for uranium, so those nuclei have a correspondingly lower mass per nucleon either than the nucleons considered individually or than the nucleons combined into a uranium nucleus.