Inside the Centre: The Life of J. Robert Oppenheimer (78 page)

Though much research still needed to be done, certain features of the design of the implosion bomb were fixed in the autumn of 1943. Its maximum size, for example, was determined by the size of the bomb bay
in the B-29 bomber that would be used to deliver it: five by twelve feet. Also, it was realised from the beginning that an implosion bomb could not be long and slender, but would have to be round and large, hence the name ‘Fat Man’. Apparently, the hope was that anyone listening covertly to discussions about modifying B-29s to accommodate ‘Thin Man’ and ‘Fat Man’ would interpret them as referring to plans to transport, respectively, President Roosevelt and Prime Minister Churchill.

Directing the programme to design and build ‘Thin Man’, which lasted from March 1943 to July 1944, had been a demanding job, but its demands paled before those required to complete ‘Fat Man’ by the deadline of the summer of 1945. This latter was a truly gargantuan task that involved, among other things: mastering, and in some cases, inventing new mathematical techniques to describe and predict the behaviour of shock waves; determining, by experiment and observation, the right shape for the explosive charges that would be used to implode the fissionable material; inventing a method of initiating the chain reaction in an implosion device; developing a new branch of physics (the hydrodynamics of implosion);
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and designing and constructing a kind of bomb that no one before the war had even envisaged.

Moreover, for Oppenheimer, it involved maintaining smooth personal relations with, on the one hand, military and security men, and, on the other, prickly, obsessive scientists with large and easily bruised egos. Seth Neddermeyer was not an easy man, and he had developed an intense obsession with making his version of implosion work. In effectively side-lining him, while keeping him working on the programme under the leadership of someone else, Oppenheimer demonstrated rare tact, sensitivity and understanding.

In March 1944, still thinking of implosion as a means of making a uranium bomb with as little uranium as possible, Oppenheimer wrote to Groves that the ‘prime objective’ of the laboratory for the coming year was ‘to bring to a successful conclusion the development of the implosion unit with U-235’. The arrival of the British mission at the end of 1943 and the beginning of 1944 gave Oppenheimer the opportunity to pick yet more brains for hints on how to solve the problem of squeezing a ball of uranium without deforming it. Chief among these fresh brains was that of Rudolf Peierls, who, when implosion research received its new impetus, was still attached to the Columbia University group in New York, but who in February 1944 came to Los Alamos for a visit. ‘At that time,’ Peierls writes in his autobiography, ‘the laboratory was urgently required
to obtain numerical solutions to the equations for the implosion.’ As it happened, the equation required ‘was of the same form as that for the blast wave in air, for which I had done my numerical experiments . . . I therefore came just at the right time to explain the step-by-step method by which the equation could be solved and the limits of the size of the steps.’ Doing it step-by-step was important, because these calculations were to be done on IBM punch-card machines, the ancestors of modern computers. After the February visit Oppenheimer wrote to Groves confirming the usefulness of his discussions with Peierls, with whom, he reported, he had gone ‘into the technical aspects of the British methods in considerable detail’. Oppenheimer was now ‘planning to attack the implosion problem along these lines with the highest possible urgency’.

Developing implosion was both a theoretical and an experimental matter. Indeed, as had been shown during von Neumann’s visit in the autumn of 1943, implosion raised theoretical questions interesting enough even for Edward Teller, who had steadfastly refused even to pretend to be stimulated by what he regarded as the merely engineering problems raised by the gun-assembly method. After his visit to Los Alamos, von Neumann kept up a correspondence with Teller, and in January 1944, Teller was appointed head of a small group of the Theoretical Division, devoted to solving the mathematical and theoretical problems raised by implosion. After Peierls’s visit in February, the work of this group was centred on using the mathematical techniques developed by the British.

The effort – indeed, the perceived need – to solve the problems of implosion was soon considered sufficiently important for Bethe, in March 1944, to reorganise the entire Theoretical Division to meet ‘the great and increased urgency of the implosion program’. In accordance with this reorganisation, Teller was put in charge of group T-1, the responsibilities of which were officially described as ‘Hydrodynamics of Implosion, Super’. Oppenheimer and Bethe both thought it was obvious that the first of these was the more important. Teller, however, whose interest in implosion in January seems to have waned by March, thought otherwise, and from the spring of 1944 onwards spent almost all his time working on the Super. The short section of his autobiography that he devotes to his abandonment of work on implosion presents a somewhat unclear picture of why, exactly, he lost interest in it. After describing ‘Johnny’ von Neumann’s visit to Los Alamos and his conversations with him about ‘fast’ implosion, Teller seems keen to convey the importance of those discussions and to highlight his own role in them:

But, having stressed his role in providing the guiding ideas of the implosion problem, Teller recounts how, after ‘Johnny’ had left, Bethe called him into his office and told him: ‘I want you to take charge of solving the equations that will be needed to calculate implosion.’ This, Teller says, was a task he was reluctant to take on because it ‘seemed far too difficult’: ‘Not only were other people more capable than I of providing such work, but I also suspected that a job that formidable might not be completed in time to have any influence on a bomb that could be used during this war.’

As a reason for concentrating on the Super – which stood no chance whatsoever of being completed in time to influence the outcome of the war – this seems strikingly unconvincing. Besides, if Teller had originated the central ideas that inspired the renewed interest in implosion, who did he think could possibly be more capable of seeing it through to completion?

Interspersed with his account of how he helped to establish and then backed away from the rejuvenated implosion programme, Teller provides some personal reflections that, one suspects, are more to the point in explaining why he abandoned it. Though he enjoyed Bethe’s company, Teller says, ‘as physicists we approach problems differently’. Whereas Bethe was a brick builder, Teller was a brick layer; he liked to
build
things, rather than provide other people with the tools: ‘I much prefer (and am better at) exploring the various structures that can be made from brick, and seeing how the bricks stack up.’ Then, apparently apropos of nothing, Teller confesses that when Oppenheimer told him that he had appointed Bethe as head of the Theoretical Division, ‘I was a little hurt.’ ‘I had worked on the atom bomb project longer than Bethe. I had worked hard and fairly effectively on recruiting, and on helping Oppie organize the lab during the first chaotic weeks.’

In other words, it seems to be implied, Teller quit working on implosion because he could not bear the thought of being consigned to working
on a mere brick-building task, working under another brick builder. He wanted to be working not on small mathematical tasks in the service of a goal set by someone else, but on large tasks in which he could pursue his own vision. He wanted to be the boss, not the under-labourer – especially if it turned out that the particular under-labouring task he had been set was one that someone else, namely Peierls, was better at than he.

On 1 May 1944, Oppenheimer wrote to Groves, asking if, as a matter of the ‘greatest urgency’, Teller (‘who is, in my opinion and Bethe’s, quite unsuited for this responsibility’) could be relieved of his role in the implosion programme and replaced by Peierls. But even now, and despite the tone of his letter to Groves, Oppenheimer did not completely fall out with Teller. On the contrary, he actively encouraged him to devote his energies to the Super and even – at a time when he and the entire laboratory were stretched to the limit – made time to see him for an hour’s discussion every week.

Peierls came to Los Alamos to take up leadership of the implosion theory group on 3 June 1944. Soon afterwards Oppenheimer gave a party for Lord Cherwell, Churchill’s scientific advisor, but somehow forgot to invite Peierls. When he came to Peierls’s office the next day to offer abject apologies for this unintended slight, Oppenheimer told him: ‘There is an element of comfort in this situation: it might have happened with Edward Teller.’

Shortly before Peierls’s arrival, a major breakthrough in the theory of implosion was made by another member of the British mission, James Tuck, who had worked previously on armour-piercing shells. Drawing on that work, he came up with a means of solving the problem of creating the smooth, symmetrical, inward-moving spherical shock wave that implosion required, which had, in practice, until this point proved impossible to create. The best one could do was simulate such a spherical shock wave by placing jets of explosive energy in a spherical arrangement. This, however, created a series of diverging and converging shock waves, the physics and mathematics of which were at this time only imperfectly understood. What Tuck, building on von Neumann’s suggestion of shaped charges, envisaged was a way of arranging the explosive charges so that the waves of energy produced by them converged. The arrangement Tuck had in mind called for a series of ‘lenses’, analogous to optical lenses. Just as an optical lens forces waves of light to converge on a target, so Tuck’s lenses would force waves of explosive energy to converge, thus increasing their force and enabling the ‘fast implosion’ envisaged by von Neumann to be realised. Such an arrangement was, however, as David Hawkins puts it in his official history of Los Alamos, ‘a completely untried and undeveloped method, which no one wished to employ unless it became absolutely necessary to do so’. After Segrè’s shattering news about spontaneous fission in
reactor plutonium, it looked as if it might indeed be necessary to employ this untried method. The laboratory needed to take risks. After all, in the summer of 1944, a mere twelve months before the deadline for producing the bomb, there was, as Hawkins puts it, ‘not a single experimental result that gave good reason to believe that a plutonium bomb could be made at all’.

In August 1944, then, the entire laboratory was reorganised to reflect the central importance of the effort to solve the many problems that stood in the way of making an implosion bomb. Stanislaw (‘Stan’) Ulam, the Polish mathematician and friend of Teller’s who arrived in Los Alamos at the end of 1943, has recalled with amusement the ‘fascination with organizational charts’ that he found at Los Alamos:

As a result of the August 1944 reorganisation, those charts became more complicated as divisions multiplied, new groups were added and hundreds more men were recruited. Oppenheimer calculated that, if the laboratory were to solve the problems of implosion in time to have a plutonium bomb ready for the summer of 1945, he would need an additional 600 men. These, of course, would not all have to be distinguished physicists, chemists or ballistics experts. What was needed was relatively unskilled labour to carry out the experiments and make the observations and measurements that were required to solve the many scientific problems that remained unsolved. Why did Oppenheimer need so many men? The answer lies in the sheer number of questions raised by implosion. For example, in order to understand the nature of implosion itself, and to know what was needed to achieve the smooth symmetrical shock wave it required, literally thousands of experiments – analogous to, but far more sophisticated than, those conducted by Neddermeyer and his team on stove pipes – needed to be carried out.

In September 1944, the theoretician Robert Christy suggested that the metal to be imploded should be a solid ball, rather than the hollow sphere envisaged by Neddermeyer. After this had been accepted, the problem of implosion was tackled by experiment after experiment in which the implosion of a solid sphere of metal, usually cadmium, was attempted and measurements taken to see how close each successive attempt had come to producing that elusive uniform, symmetrical shock wave. The methods used to make these measurements were many and varied, and some were invented at Los Alamos. For example, Robert Serber came up with a novel
idea that became the basis of what was called the ‘RaLa method’. The idea was to put a radioactive substance – the one chosen was an isotope of lanthanum (La-140), called Radiolanthanum (hence ‘RaLa’) – at the centre of the metal sphere being imploded. The gamma rays emitted by this radioactive source would be absorbed by the metal in proportion to its density, so the density changes in the metal as it was imploded could be measured by recording the intensity of the gamma radiation before, during and after the detonation. In this way, they could see how close they were to achieving a uniform, symmetrical shock wave. Other methods of measuring what happens when a lump of metal is imploded were devised using X-rays and photographs.

In addition to these meticulous experiments and observations concerned with the nature of implosion, much experimental work had to be carried out in order to design and manufacture the shaped explosive charges, providing answers to such questions as: what material should the explosive charge be made of? how, exactly, should it be shaped? and how should the shaped charges, the ‘lenses’, be arranged around the plutonium core? As a result of these experiments, a design was arrived at that was far more complex than the gun-assembly bomb originally planned. The weapon that was now envisaged looked like an enormous football, at the centre of which was a 3½-inch solid sphere of plutonium with a hole for the initiator, around which was a uranium tamper with a diameter of nine inches, in turn surrounded by thirty-two explosive charges, all carefully shaped into ‘lenses’, concentrating the shock wave at the centre of the sphere. In all, ‘Fat Man’ would be fifty-four inches wide and weigh nearly 5,500 pounds.