Inside the Centre: The Life of J. Robert Oppenheimer (43 page)

Almost certainly, the first contact that either of the Oppenheimer brothers had with someone happy to call themselves a communist was when, in the spring of 1936, halfway through Frank’s first year at Caltech, they met a twenty-four-year-old graduate student of economics at Berkeley called Jacquenette Quann. ‘Jackie’ (as everyone called her) was a working-class French-Canadian woman, who worked as a waitress and babysitter to pay her way through university. While an undergraduate, she had joined the Young Communist League, attracted to it not through any intellectual commitment to Marxist-Leninism, but rather through its involvement with practical issues, such as the rights of workers and the threat of fascism, about which she was concerned. She came into the Oppenheimers’ lives quite by accident one evening when she was babysitting for Wenonah Nedelsky, the estranged wife of Oppenheimer’s student Leo Nedelsky. Robert, accompanied by Frank, went to visit Wenonah and the two of them met Jackie, whose plain-speaking exuberance quickly won Frank over. Within a short time he and Jackie were lovers, and that summer he invited her to Perro Caliente. On 15 September 1936, they were married.

Oppenheimer did not approve of his younger brother’s rush into matrimony. ‘He tried to put us off from getting married,’ Jackie later said. ‘He was always saying things like “Of course, you’re much older than Frank” – I’m eight months older actually – and saying that Frank wasn’t ready for it. Later he used to refer to me as “the waitress my brother has married”.’ In a formal statement he wrote at the time of his security hearing in 1954, Oppenheimer wrote tersely: ‘My brother Frank married in 1936. Our relations thereafter were inevitably less intimate than before.’ Under cross-examination, he elaborated on this a little, adding that not only were relations between him and Frank less intimate after Frank’s marriage, but they were also ‘occasionally perhaps somewhat more strained’. More expansive is a statement quoted by Peter Michelmore in his 1969 book,
The Swift Years: The Robert Oppenheimer Story
, the source of which Michelmore does not give. Frank’s ‘defection’, Michelmore writes, ‘hurt Robert deeply, for he wrote petulantly of his brother’s marriage, “It was an act of emancipation and rebellion on his part against his dependence on me. Our early intimacy was never again established.”’

Apart from his evident anxiety at losing the most intimate, most important relationship he had, another worry Robert had about Jackie’s influence on Frank was that it was bad for his physics. Frank, Robert later said, ‘worked fairly well at physics but he was slow. It took him a long time to get his doctor’s degree. He was very much distracted by his other interests.’ It is sometimes assumed that something similar happened to Robert Oppenheimer – that after he began to take an interest in politics his work in physics lost some of its earlier intensity. In fact, the opposite is true: the very best physics he ever wrote was produced precisely during the period of his political awakening.

During the period 1935–8 – while he was in his early thirties – the focus of that work was provided by Oppenheimer’s continued interest in cosmic rays. During the 1930s, there were two reasons for a physicist to be interested in cosmic rays: first, they were an interesting and puzzling phenomenon in their own right, presenting physicists with the challenge of saying what they were made of and how they originated; and second, their tremendous energy allowed physicists their only opportunity (until the advent of particle accelerators many times more powerful than Lawrence’s early cyclotrons) of seeing whether physical theories, such as quantum electrodynamics, successfully held up when used to measure and predict the behaviour of particles travelling at something approaching the speed of light, which is when relativistic effects become relevant.

For these reasons, the study of cosmic rays became the focus in the 1930s for some of the most interesting experimental physics and some of the leading theoretical work; the experimentalists would take off on
adventurous expeditions to far-flung corners of the world to measure radiations at high altitudes, and the theorists would use the information thus obtained to test the validity of theories and to inspire new insights into the make-up of the physical world, which with every step forward seemed to be more complicated and stranger than anybody had imagined.

Oppenheimer was well placed to contribute to this work, since some of the most important observations of cosmic rays were being undertaken by two experimentalists at Caltech: Carl Anderson, the discoverer of the positron, and his colleague Seth Neddermeyer. In a paper he published at the end of 1934 entitled ‘Are the Formulae for the Absorption of High Energy Radiations Valid?’, Oppenheimer paid tribute to the work of these two in a footnote that read: ‘Such clarity as there is in this account of the experimental situation I owe entirely to Dr Anderson and Mr Neddermeyer, who have with great patience explained to me just what the evidence is, what it indicates, and how little it proves.’

At the beginning of that paper Oppenheimer notes that the observations of cosmic rays made by Anderson and Neddermeyer have ‘made it possible to extend our knowledge of the specific ionization and energy loss of electrons from particles of a few million volts on up to a few billion’. Despite the progress made at Berkeley by Lawrence’s Radiation Laboratory, it would be some time before that kind of energy could be created artificially. With regard to what those observations of such extraordinary energies reveal about the accepted formulae for calculating high-energy radiation, Oppenheimer remarks that it is ‘possible to do justice to the great penetration of the cosmic rays only by admitting that the formulae are wrong, or by postulating some other and less absorbable component of the rays to account for their penetration’. It is a dichotomy reminiscent of that which Oppenheimer had earlier posed in relation to positively charged electrons: either Dirac’s theory of the electron was wrong, or such particles had to exist. And just as in this earlier case, Oppenheimer missed out on an important advance in physical theory by choosing the wrong side of the dichotomy, saying that the theory was wrong, rather than insisting that this ‘less absorbable component’ of cosmic rays had to exist. For, as would be revealed in the ensuing years, this ‘less absorbable component’ was yet another new particle.

During 1935, Oppenheimer’s intellectual energies, as we have seen, were directed towards the questions that arose from the artificial creation of radioactive isotopes, questions that gave rise to the paper he and Melba Phillips wrote in the summer of 1935, which introduced the ‘Oppenheimer–Phillips process’. In turning from the analysis of what happens when a deuteron splits into a proton and a neutron to the consideration of cosmic rays, Oppenheimer may have thought that he was, temporarily at least, leaving nuclear physics behind. However, nuclear physics and cosmic-ray
physics were about to come together in an unexpected way. In the early part of 1935, an article appeared in an obscure journal that remained completely unknown to people researching cosmic rays and would not have seemed relevant to their research even if they had known of it. Nevertheless, that article was to play a major role in the subsequent development of cosmic-ray physics, to provide a theory that is still accepted today in fundamental nuclear physics, and to change the subsequent course of particle physics.

The article in question was entitled ‘On the Interaction of Elementary Particles I’ and appeared in the
Proceedings of the Physical and Mathematical Society of Japan
, which had received it at the end of November 1934. Its author was a Japanese theoretical physicist called Hideki Yukawa, who had come up with a novel theory to answer a fundamental question in nuclear physics: what holds the particles in a nucleus – the protons and the neutrons – together? Clearly, protons and neutrons are not held together by electrostatic forces, since neutrons do not have any charge. Nor can they be held together by gravity, as the gravitational force is very many orders of magnitude too weak to account for the binding energies observed. Yukawa put forward the bold suggestion that there is a hitherto-unknown basic physical force – now known as the ‘strong nuclear force’ – that exerts a pull between the protons and the neutrons in the nucleus. He further hypothesised that there must be a hitherto-unknown particle, which would have a mass somewhere between an electron and a proton, that would carry the force, in much the same way that, in quantum electrodynamics, electromagnetism is carried by the photon. Yukawa even speculated that this new particle ‘may also have some bearing on the shower produced by cosmic rays’. American university libraries did not, as a rule, subscribe to
Proceedings of the Physical and Mathematical Society of Japan
, but Yukawa sent Oppenheimer a copy of it. For about eighteen months after its publication Oppenheimer might well have been the only English-speaking scientist to have read it.

As it turned out, cosmic-ray research would indeed confirm Yukawa’s hypothesis of a particle bigger than an electron but smaller than a proton, and, thus confirmed, that hypothesis would in turn provide the solution to the puzzle about the penetrative power of cosmic rays that had prompted Oppenheimer to talk about their having a ‘less absorbable component’. However, as would become gradually clear (it took about twelve years, beginning in 1935) amid much confusion and controversy, the penetrative particle in cosmic rays is
not
the carrier of the ‘strong nuclear force’ –
that
is another particle somewhat like it. How confused the initial picture of these particles was can be gleaned from their changing nomenclature: to begin with, before it was realised they were
different, they were called ‘mesotrons’, which was changed to ‘mesons’ for reasons of linguistic probity (the Greek word for ‘middle’ being ‘
mesos
’ rather than ‘
mesotros
’); then, to distinguish them from each other, the one that is a component of cosmic rays was called a ‘µ-meson’ (mu-meson), and the one that is the carrier of the strong nuclear force was called the ‘π-meson’ (pi-meson). Then it was decided that mesons are by definition carriers of the strong nuclear force and therefore that the mu-meson is not a meson at all. It was accordingly renamed the ‘muon’, while the other was renamed the ‘pion’. Playing a leading role in both the creation and the clearing up of these confusions, and thus being there at the birth of what has grown into the (for most people) utterly bewildering new discipline of particle physics, were Oppenheimer and his students.

Much of the key observational evidence of cosmic rays that led to the discovery of the ‘mesotron’ was collected by Anderson and Neddermeyer at the summit of Pikes Peak in the Rocky Mountains, where they went in the summer of 1935. There, at an altitude of about 14,000 feet, they set up the equipment they had brought with them and took thousands of photographs of extremely high-energy cosmic-ray collisions. Without knowing anything about Yukawa’s article, Anderson wrote to Millikan at Caltech from the top of Pikes Peak to say he thought he had evidence of a particle intermediate in mass between electrons and protons. He was a cautious man, however, and did not want to publish this result until he was completely sure of it. It was therefore not until the summer of 1936 that Anderson and Neddermeyer published a scholarly account of their trip to Pikes Peak in the
Physical Review
. Entitled ‘Cloud Chamber Observations of Cosmic Rays at 4,300 Meters and Near Sea Level’, their article reproduced some of their more dramatic photographs, taken both at Pikes Peak and in Pasadena, and in a very modest and hesitant way tried to make sense of them. Without actually declaring that they had found a new particle, they gave good reasons why the penetrative particle they had photographed could not be either a proton or an electron. They also made the important announcement that their observations refuted something that had been widely believed by physicists, namely that the theory of quantum electrodynamics broke down when applied to particles of extremely high energy. They had observed particles at more than one billion volts, they recorded, and the theory had stood up very well.

It was this last aspect of the article that most interested Oppenheimer. He had been saying for years that the theory broke down at high energies, but he did not seem to mind being proved wrong on that account. On the contrary, he seemed delighted with the findings reported by Anderson and Neddermeyer, not least because they allowed him to pick a theoretical fight with Heisenberg, a fight he felt confident of winning. Heisenberg had
recently been drawn into the analysis of cosmic rays, thinking that he had a new insight that would shed light on their nature. The insight in question was one derived from recent work published by Enrico Fermi on the subject of beta decay. Taking up the issue that had prompted Pauli to suggest the neutrino (a name conferred on the as-yet-undiscovered particle by Fermi), Fermi had proposed a completely new analysis of beta radiation that bore a striking analogy to Yukawa’s new analysis of nuclear forces. The analogy was no accident; Fermi’s theory was published at the beginning of 1934, about ten months before Yukawa’s, and was one of the main inspirations for Yukawa’s theory. Like Yukawa, what Fermi proposed was the introduction of a new basic force into physics – what is now called the ‘weak nuclear force’ – to explain beta emissions. This new force would act upon electrons, neutrinos and nucleons (protons and neutrons) and would explain the process whereby a neutron decays into a proton, emitting as it does so beta radiation (electrons) and neutrinos.

In a paper that he published in June 1936, Heisenberg suggested that this new force field postulated by Fermi held the key to understanding cosmic rays. In particular, he believed that the phenomenon that had earlier attracted the attention of Oppenheimer – the ‘showers’ of electron/positron production – might be explained by Fermi’s new field. At the root of Heisenberg’s analysis was the belief that the accepted theory of quantum electrodynamics broke down at the extremely high energies observed in cosmic radiation. As Anderson and Neddermeyer had shown this belief to be false, the motivation for Heisenberg’s theory had disappeared. This was pointed out in a confrontational manner by Oppenheimer in a paper called ‘On Multiplicative Showers’, which he co-wrote with Frank Carlson (for whom he had not yet managed to find an academic position and who therefore remained at Berkeley and Caltech), and sent to the
Physical Review
at the end of 1936. ‘It would seem,’ declared Oppenheimer and Carlson, that Heisenberg’s theory ‘is without cogent experimental foundation; and we believe that in fact it rests on an abusive extension of the formalism of the theory of the electron neutrino field’.