Inside the Centre: The Life of J. Robert Oppenheimer (55 page)

After Schwinger’s year as an NRC fellow expired in the summer of 1940, Oppenheimer immediately appointed him as Schiff’s replacement as research associate. In that role Schwinger stayed for just a year, during which the focus of his and Oppenheimer’s research interests was the attempt to understand the particle that Oppenheimer in those days still called the ‘mesotron’ (it was not until after the war that he began calling it the ‘meson’). As we now know, and as mentioned in the previous chapter, what Oppenheimer called the ‘mesotron’ was, in fact, two very different particles: the mu-meson (or muon), which is a component part of cosmic rays, and the pi-meson (or pion), which is the carrier of the strong nuclear force. In the period from 1939 to 1942 almost all of Oppenheimer’s published work, and a good deal of the work undertaken by his graduate and postdoctoral students, was devoted to solving the puzzles of the ‘mesotron’, most of which arose from the mistaken assumption that the mu-meson and pi-meson were the same thing.

In trying to explain how Oppenheimer exerted such an inspirational influence on his students, Edward Gerjuoy writes: ‘I feel Oppie did his physics, talked about his physics, lived his physics, with an unusual passion, which had to inspire students; in any event it sure inspired me.’ As an example of this passion, Gerjuoy describes Oppenheimer’s absorption in the problem of the ‘mesotron’:

At about the same time that he co-wrote his letter to the editor with Schwinger, Oppenheimer put his name to a long article, co-written with Robert Serber and Hartland Snyder, called ‘The Production of Soft Secondaries by Mesotrons’, in which they analysed the ‘soft component’ of cosmic rays as being made up of electrons and gamma rays that were released in mesotron ‘showers’. Their conclusion was the familiar one: that the standard quantum theory is sufficient to explain the emissions of
electrons and gamma rays up to a certain energy, but that ‘the problem of extending the formulae above these critical energies probably goes beyond the framework of the present theory’. That ‘probably’, together with the speculative suggestion of the breakdown of the theory, was exactly the kind of thing to which Schwinger had so strongly objected.

Schwinger himself spent much of his time at Berkeley puzzling over the ‘mesotron’ – as his biographers correctly note, ‘everybody at Berkeley was talking about mesons’ – and, in addition to his joint work with Oppenheimer, wrote papers on the subject with William Rarita, a physicist from Brooklyn College who was then on a sabbatical visit to Berkeley, and with Herbert Corben, an Australian, who after studying at Cambridge had come to Berkeley on a postdoctoral fellowship. In a letter to the editor of the
Physical Review
published in March 1941, called ‘On the Spin of the Mesotron’, Oppenheimer referred to this work of Schwinger, Rarita and Corben, and to a paper by Robert Christy and Shuichi Kusaka, and to yet another on the subject by Eldred Nelson, also a graduate student of his. All three of these students – Christy, Kusaka and Nelson – wrote their PhD theses on mesotrons. The general impression of these publications is that almost all the finest minds at America’s greatest centre of theoretical physics were engaged in trying to understand the huge discrepancy between the observed properties of the particles that make up cosmic rays with the theoretical calculations based on standard quantum electrodynamics. Here is the clearest instance yet of Serber’s remark that Oppenheimer’s progress was hindered by his almost obsessive conviction that the standard theory was wrong; if he had trusted that theory a little more, it would surely have occurred to him that the discrepancies were due to the misidentification of the cosmic-ray particle (the muon) with the Yukawa particle (the pion).

In June 1941, Oppenheimer and Schwinger sent another jointly written paper, ‘On the Interaction of Mesotrons and Nuclei’, to the
Physical Review
, this time concentrating on the ‘mesotron’ as the carrier of the strong nuclear force – that is, the pion. The paper was, according to Schwinger’s later recollection, essentially written by him, Oppenheimer simply adding his name to it after it had been written. Indeed, he implied, it could not have been written by Oppenheimer, involving as it did quantum-mechanical treatments of meson fields, the mathematics of which was beyond Oppenheimer’s competence. Oppenheimer, Schwinger said, was ‘adequate technically to deal with the semi-classical treatment of spin’, but ‘He was not adequate, or at least he never attempted to follow or join in, with the quantum treatment, which was more elaborate.’ ‘Well,’ he added, with more than a hint of condescension, ‘he was trying to keep his hands in lots of different topics and it is very difficult to work intensively on all these subjects.’

A few days after that paper was sent off, Schwinger and Oppenheimer both presented several papers at an American Physical Society meeting in Pasadena, although not with each other. One of Schwinger’s papers was with Edward Gerjuoy, and two of Oppenheimer’s papers (both on mesotrons) were jointly written, one with Christy and the other with Nelson. After that, Schwinger left California to take up a place at Purdue University. Despite the fact that, as Schwinger later put it, ‘I still did not quite know how to act in the face of His Majesty’, he and Oppenheimer parted on good terms. Neither, however, seemed particularly sorry to part; Schwinger was not offered, nor did he apply for, another year as Oppenheimer’s research associate. He left with his admiration for Oppenheimer still intact, but tempered somewhat by what he saw as Oppenheimer’s loss of creativity due to his acceptance of the role of organiser and manager, rather than that of a single-minded research physicist. Oppenheimer, Schwinger later said, ‘very much insisted on displaying that he was on top of everything, which he very often was’, but, inevitably, in striving to be on top of
everything
, Oppenheimer skirted over the details of particular subjects, and, for Schwinger, the details were everything. Oppenheimer’s grasp of specific topics, Schwinger recalled, ‘became more and more superficial, which I regretted very much. It was a lesson to me, never to lose completely your touch with the subject, otherwise it’s all over.’ Oppenheimer, he thought, ‘could pull it off better than most people’:

Schwinger’s comments on Oppenheimer are perceptive. As he was possibly the first person to realise, the summer of 1941 marked the end of Oppenheimer’s time as a creative scientist and the beginning of an entirely different phase in his life.

Schwinger’s recollections of Oppenheimer having lost touch with his subject chime with Oppenheimer’s letter to the Uehlings in May, in which he struck a melancholy note about the future of ‘physics in our sense’. As we have seen, however, that letter conveys a sense of Oppenheimer feeling out of touch not so much with theoretical physics as with ‘all the men active in physics [who] have been taken away for war work’. And, after all, he was right to feel that important work was being done from which he was excluded. By the summer of 1941, much progress had been made on the physics of fission and its possible application to explosives, which Oppenheimer would have known nothing about. Most crucially, an
unexpected answer had been provided to one of the questions Oppenheimer had first raised when the discovery of fission had been announced: what is the critical mass of uranium? The question of critical mass could be put like this: given that neutrons are released in fission and that a chain reaction is therefore possible, how large would a lump of uranium have to be in order to sustain a chain reaction long enough to produce a massive explosion? In a small amount of uranium, the neutrons released by fission would escape from the surface before they had initiated another fissure. The question that arises, then, is: how large would a piece of uranium have to be in order for the neutrons to set up a fission chain reaction rather than escape from the surface?

One answer to that question with which Oppenheimer
would
have been familiar was published by Rudolf Peierls in October 1939. Peierls was a German Jewish physicist whom Oppenheimer had met in Zurich and who had been in England since 1933. Since 1937 Peierls had been professor of physics at the University of Birmingham. In the
Proceedings of the Cambridge Philosophical Society
for October 1939 he published a formula for calculating critical mass and applied it to a simplified case of natural uranium fissioned by unmoderated fast neutrons. The answer he obtained was that the critical mass was several tons – too much for a practical weapon – a result that confirmed what Bohr had already said: an atomic bomb was not a realistic proposition.

By the time Peierls’s paper was published, he had been joined in Birmingham by another German Jewish refugee physicist, the co-discoverer of fission, Otto Frisch. Frisch had also been thinking about critical mass and had asked himself a question that, remarkably, no other physicist had yet asked. ‘One day in February or March 1940,’ Peierls later recalled, ‘Frisch said, “Suppose someone gave you a quantity of pure 235 isotope – what would happen?”’ In order to calculate accurately the critical mass of pure uranium-235, Frisch and Peierls needed what theoretical physicists call ‘the numbers’ – namely, the basic facts established by experiment and observation. In this case, one of ‘the numbers’ was already well known – the number of neutrons released per fission – but for much of the rest they had to guess. They did not know, for example, the fission cross-section for uranium-235 (that is, how likely it was that a neutron hitting a uranium-235 nucleus would cause it to fission), but from Bohr and Wheeler’s work they felt able to assume that
every
neutron that hits a nucleus would produce fission (this turned out not to be quite right, but it was close enough). Further informed guesses allowed them to calculate how quickly the chain reaction would go through the uranium, how many ‘generations’ of fission would take place before the uranium expanded too much for further fission to take place, and how much energy would be released.

The result staggered them. Far from being measured in tons, as Bohr and every other physicist had previously calculated the critical mass of natural uranium fissioned by slow neutrons to be, Frisch and Peierls calculated the critical mass of pure uranium-235 fissioned by fast neutrons to be about one kilogram. In fact, as we now know, because all the relevant ‘numbers’ have been determined by laboratory experiment, it is rather more than that, being about fifteen kilograms. Nevertheless, as Frisch and Peierls were the first to realise, it is a matter of kilograms, not tons. And the energy release from that relatively small lump of uranium would be enormous. Frisch and Peierls calculated it to be equivalent to several thousand tons of TNT. The problem, of course, is that the separation of U-235 from natural uranium is difficult – so difficult that most people who had considered it did not regard it as a practical means of making a bomb. In the light of the calculations made by Frisch and Peierls, however, it looked considerably more practical. To be sure, an expensive industrial plant would have to be built, but, as Peierls remembers himself and Frisch saying to each other: ‘Even if this plant costs as much as a battleship, it would be worth having.’

For the second time in just under two years Frisch found himself one of only two people in possession of a shattering piece of information. Realising that what had occurred to them might also occur to scientists working for the Nazis (Heisenberg, for one, was more than capable of doing the same calculations), Frisch and Peierls quickly wrote up their analysis as a two-part report – the first part, ‘Memorandum on the Properties of a Radioactive “Super-bomb”’, stating their conclusions in non-technical terms, and the second, ‘On the Construction of a “Super-bomb”; Based on a Nuclear Chain Reaction in Uranium’, providing the technical details. As Jeremy Bernstein has said: ‘What is impressive about these papers is their absolute clarity.’ No one who read them could fail to be convinced that, if a fairly small lump of the 235 isotope could be separated from natural uranium, a bomb of awesome power could be constructed. Frisch and Peierls even explained how such a bomb could function: two subcritical lumps of uranium-235 could be brought together, thus forming a critical mass. ‘Once assembled,’ they remarked, ‘the bomb would explode within a second or less, since one neutron is sufficient to start the reaction and there are several neutrons passing through the bomb every second, from the cosmic radiation.’ This was, essentially, the design of the bomb that exploded over Hiroshima some five years after it was conceived by Frisch and Peierls (though, in the Hiroshima bomb, a neutron initiator – a mixture of polonium and beryllium – was used, rather than relying on passing cosmic rays).

Frisch was at this time still classed as an enemy alien, and Peierls had only just received British citizenship; neither would be a candidate for
active participation in the British war effort. So they gave their memorandum to Mark Oliphant, the head of the physics department at Birmingham, who had been responsible for recruiting both of them, and who, after adding a covering note declaring ‘I am convinced that the whole thing must be taken rather seriously’, sent it to Henry Tizard, an Oxford-trained chemist who served as the civilian chairman of the British government’s Committee on the Scientific Survey of Air Defence. Tizard then set up a separate committee, consisting entirely of people who had learned their physics at the Cavendish: Oliphant, Chadwick, Cockcroft and, as chairman, G.P. Thomson. The committee met for the first time on 10 April 1940 and, according to Oliphant, was immediately ‘electrified by the possibility’ of an atomic bomb.