Inside the Centre: The Life of J. Robert Oppenheimer (60 page)

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.

Thus, if you fuse together nuclei of elements lighter than iron,
or
fission nuclei heavier than iron, the result will be the creation of nuclei that have a greater mass defect than the ones you started with, and thus a tremendous release of energy in accordance with the equation E = mc
²
. The amount of energy released per fusion of, say, hydrogen is less (by
about one-tenth) than the amount of energy released per fission of uranium, but, because the nuclei are so much lighter (by about one-fiftieth) and therefore there are more of them in any given quantity of material, the energy release
per kilogram
will be far greater in fusion than in fission.

It had been assumed that a fusion bomb was an impossibility because of the tremendous heat that would be required to get the nuclei moving energetically enough to fuse together. To get a fusion reaction going, one would have to reproduce something similar to the conditions that prevail inside the sun. What Fermi mentioned casually to Teller over lunch was the possibility that such heat might, after all, be created: by fission. At Chicago, Teller, together with the young physicist Emil Konopinski, set to work on a report on the possibility of a fusion bomb and concluded that, as Teller later put it, ‘heavy hydrogen [deuterium or tritium] actually could be ignited by an atomic bomb to produce an explosion of tremendous magnitude’.

When he was invited to the meeting organised by Oppenheimer and Serber, Teller asked that Konopinski should also be included, and, when Bethe arrived in Chicago to accompany him to Berkeley, he found that Teller’s mind was racing far ahead of the issue they were being collected together to think about. ‘We had a compartment on the train to California, so we could talk freely,’ Bethe remembered. ‘Teller told me that the fission bomb was all well and good and, essentially, was now a sure thing. In reality, the work had hardly begun. Teller likes to jump to conclusions. He said that what we really should think about was the possibility of igniting deuterium by a fission weapon – the hydrogen bomb.’

Apart from Bethe, Konopinski, Teller and van Vleck, Oppenheimer had also invited Felix Bloch from Stanford and Richard Tolman from Caltech. So, with Serber, Nelson, Frankel and himself, that made ten. According to one account, the meeting began with an attempt by Oppenheimer to bring the contributors face-to-face with the fact that what they were doing was planning to build a bomb of hitherto unimaginable power. To help them to visualise what this might entail (and presumably to overcome any lingering squeamishness there might be about the fact that they were engaged in the design of an explosive), Oppenheimer drew their attention to some details of a large explosion that had occurred in 1917 in the harbour of Halifax, Nova Scotia. The explosion was caused by a collision between two ships, one of which was carrying 5,000 tons of TNT, and resulted in the deaths of up to 2,000 people and the destruction of an area of almost one square mile. No one knew how powerful the atomic bomb would be, but the best guess was that it would be several times more powerful than the Halifax explosion (in fact, the Hiroshima bomb was three times and the Nagasaki bomb four times more powerful,
though the number of people killed in each case was more than twenty times the number killed in Halifax).

With everybody’s mind thus focused, Serber explained what had been done so far, both by Breit’s team and, in the preceding few months, by Oppenheimer’s. Nelson and Frankel then gave their critical-mass calculations and, remembers Serber; ‘Everybody agreed that it looked under good control from a theorist’s point of view.’ Bethe’s recollections confirm Serber’s impression. ‘The theory of the fission bomb was well taken care of by Serber and two of his young people,’ he remarked later. They ‘seemed to have it well under control so we felt we didn’t need to do much’.

With all the ‘luminaries’ apparently agreeing with his view that the fission bomb was essentially ‘now a sure thing’, Teller turned the discussion away from fission and towards fusion. As Serber remembers it, what Teller was proposing was ‘a detonation wave in liquid deuterium set off by being heated by the explosion of an atomic bomb’. In his autobiography Serber describes how, when Teller mentioned this idea, ‘everybody forgot about the A-bomb, as if it were old hat, something settled, no problem, and turned with enthusiasm to something new’.

Everyone present realised that if the ‘Super’ (as they began calling it) could be made to work, it would be many times more powerful than an atomic bomb. In an atomic bomb, one kilogram of uranium would explode with the force of (roughly) 15,000–20,000 tons of TNT; in a thermonuclear, hydrogen bomb, one kilogram of deuterium would explode with the force of 80,000–100,000 tons. Moreover, deuterium is relatively cheap and plentiful. Twenty-six pounds of it would not be difficult to acquire, and that, potentially, could make a bomb equivalent to about one million tons of TNT.

That was startling enough, but, recalls Serber:

While Bethe was looking at the numbers, Oppenheimer – who took the apocalyptic scenario presented by Teller more seriously than either Serber or Bethe – made a long-distance call to Compton to tell him that his group had ‘found something very disturbing’. Compton asked how soon Oppenheimer could come to Chicago to see him and talk about it. The following day, came Oppenheimer’s reply. And so, early the next morning, Oppenheimer took the train to Chicago, where Compton met him in his car. As they drove back to Compton’s house, Oppenheimer
recounted the discussion that his group had been having about fission, fusion and the possibility of global catastrophe, which, as Compton writes, ‘could not be passed over lightly’.

With Compton, Oppenheimer agreed there could be only one answer to the crisis, which was, in Compton’s words: ‘Oppenheimer’s team must go ahead with their calculations. Unless they came up with a firm and reliable conclusion that our atomic bombs could not explode the air or the sea, these bombs must never be made.’

By the time Oppenheimer got back, Bethe had done the figures and discovered, as he put it, ‘some unjustified assumptions in Teller’s calculations’. Bethe, in fact, never took seriously the idea that they could destroy the earth’s atmosphere and was surprised that Oppenheimer had thought it worth troubling Compton with, ‘but then Oppie was a more enthusiastic character than I was. I would have waited until we knew more.’

With the apocalyptic worry disposed of, the group got back to discussing bomb physics, again concentrating on the ‘Super’. What came as a pleasant surprise to the members of the group, even to those who knew Oppenheimer well, was what an extraordinarily capable chairman he showed himself to be. Oppenheimer had never previously organised
anything
– he had never, for example, served as chairman of his department at Berkeley – and yet, here he was, in charge of nine of the country’s most distinguished physicists, revealing himself to be an able leader who commanded the respect of everyone present.

‘The conference didn’t exactly end,’ remembers Serber, ‘it sort of fizzled out. After a week people began to leave, some stayed on a couple of weeks longer.’ For everyone involved, it had been a memorable series of discussions, Oppenheimer’s handling of which had been a revelation. ‘As Chairman, Oppenheimer showed a refined, sure, informal touch,’ Teller later said. ‘I don’t know how he had acquired this facility for handling people. Those who knew him well were really surprised. I suppose it was the kind of knowledge a politician or administrator has to pick up somewhere.’ It was crucial to the success of the meetings, however, that these political and administrative gifts went hand-in-hand with the kind of deep insight into both science and scientists that was required to get the best out of the participants. ‘A spirit of spontaneity, adventure and
surprise prevailed during those weeks in Berkeley,’ Teller remarked, ‘and each member of the group helped move the discussion toward a positive conclusion.’ These sentiments were echoed by Bethe, who recalled: ‘The intellectual experience was unforgettable.’

By the time the conference had ‘fizzled’ to an end, Oppenheimer’s own reputation and position within the US bomb project had been transformed from that of a useful, but not essential advisor to that of an indispensable leader and facilitator.

His report from the meeting of the ‘luminaries’ was received and approved by the S-1 committee towards the end of August 1942. Its central message was that an atomic (fission) bomb could indeed be built, but that it ‘would require a major scientific and technical effort’. Such a bomb would need more U-235 than some previous estimates had suggested – about 66 pounds – but its power would be something like 150 times greater than had previously been thought – that is, equivalent to about 100,000 tons of TNT. The report also touched on the possibility of the ‘Super’, saying that a 66-pound fission bomb could, in principle, be used to initiate a fusion explosion in liquid deuterium, two or three tons of which would explode with the force of 100 million tons of TNT, completely destroying an area of 360 square miles.

In the light of the conclusions drawn by Oppenheimer, the S-1 committee submitted a report to Bush, summarising the findings of the ‘luminaries’ and claiming that enough fissionable material for an atomic-bomb test could be obtained by March 1944. ‘We have become convinced,’ the report stated, ‘that success in this program before the enemy can succeed is necessary for victory. We also believe that success of this program will win the war if it has not previously been terminated.’

By the end of August 1942, Bush was giving it as his opinion that ‘nothing should stand in the way of putting this whole affair through to a conclusion’. To him, it was clear that what was now required was strong leadership. The same thought had occurred to General Brehon B. Somervell, who was in charge of the section of the army that included the Engineering Corps, and Somervell knew just the man to provide that strong leadership.