Inside the Centre: The Life of J. Robert Oppenheimer (34 page)

The first to benefit from this was Harvey Hall, with whom Oppenheimer published a major two-part article called ‘Relativistic Theory of the Photoelectric Effect’, which was received by the
Physical Review
on 7 May 1931. The photoelectric effect is the name given to the emission of electrons when metal is exposed to light of a certain frequency. The phenomenon has enormous importance in the development of physics because it was in an attempt to explain it that Einstein put forward the proposal that light is made up of particle-like ‘quanta’, upon which quantum physics was built. The specific subject of the Hall/Oppenheimer article was the application to the observations of this phenomenon of Dirac’s theory of the electron. This was also the subject of Hall’s PhD thesis, which was submitted and passed in the summer of 1931, making Hall Oppenheimer’s first PhD student to complete his doctorate.

In the late 1940s, the San Francisco office of the FBI, looking for dirt on Oppenheimer, found an employee of the University of California, a ‘very reliable individual’ (in fact, it was Oppenheimer’s colleague Leonard Loeb, who formed an intense dislike of Oppenheimer), who claimed that it was ‘common knowledge’ at Berkeley that Oppenheimer had ‘homosexual tendencies’ and that he was ‘having an affair with Hall’. Rumours get repeated and thus persist, but in this case there is very little substantiation. Hall was not, as far as anyone knows, homosexual. He married in 1934, had two sons and a daughter, and was to remain with his wife, Mary, for sixty-nine years (he died in 2003, at the age of ninety-nine). Evidence that Oppenheimer was homosexual, or even that he was believed to be so, is also scarce. David Cassidy, in his biography of Oppenheimer, quotes a letter from Robert Millikan to Richard Tolman from 1945, in which Millikan claims that at various times both Pauling and Lawrence had expressed doubts over ‘the character of [Oppenheimer’s] influence on younger associates’, but, apart from being third-hand hearsay, it is not
at all clear what exactly (other than a vague sense of moral impropriety) is being suggested here.

Most physicists, when considering the collaboration between Hall and Oppenheimer, have been more concerned about the sloppiness of their mathematics than the supposed looseness of their morals. Quoting with approval a remark made by one of Oppenheimer’s later students, Robert Serber, that Oppenheimer’s ‘physics was good, but his arithmetic awful’, Abraham Pais has drawn attention to the serious ‘carelessness’ in the work on the photoelectric effect that Oppenheimer published with Hall. A central claim in their paper was that experimental results showed that something was wrong with the theory of quantum electrodynamics as so far developed. In particular, they claimed that observations of photoelectric phenomena had revealed energies of electrons far greater – twenty-five times greater – than were predicted by the Dirac equation, and that therefore there must be some error in the theory based on this equation. In fact, as Pais, points out: ‘The error was his.’ Oppenheimer and Hall had simply miscalculated.

At the root of the problem was not only Oppenheimer’s legendarily erratic mathematics, but also his almost obsessive conviction, and determination to prove, that there was something wrong with the Dirac equation and the theory of quantum electrodynamics built upon it. Serber has remarked that this determination created a ‘fundamental barrier to Oppenheimer’s success in making progress with the difficulties of quantum electrodynamics’ – a good illustration of which is Oppenheimer’s short paper ‘On the Theory of Electrons and Protons’, which he published in the
Physical Review
as a ‘letter to the editor’ in the spring of 1930.
^fn27
The paper deals with an acknowledged problem in Dirac’s theory of electrons, which is that the Dirac equation allows for solutions that attribute negative energy to electrons. Dirac referred to these negative-energy states as ‘holes’ and suggested that they might represent the place of
positively
charged particles. As the only positively charged particle then known was the proton, Dirac suggested that the negative-energy states are actually occupied by protons.

Oppenheimer, however, showed that these positive charges in Dirac’s theory could not have the mass of a proton (which is much bigger – about 2,000 times bigger – than that of an electron), but must rather have the same mass as an electron. In other words, the theory demands the existence of a hitherto unknown particle: a positively charged electron, or what is now known as a ‘positron’. But because he was convinced that the theory
was wrong, Oppenheimer did not draw from his arguments the obvious conclusion, namely that positrons must exist. He thought he had found not evidence for the existence of positrons, but rather another reason for thinking something was amiss with the Dirac equation.

Dirac accepted Oppenheimer’s argument about the mass of the ‘anti-electron’, but, having faith in his famous equation, drew the conclusion that Oppenheimer’s scepticism prevented him from drawing, and announced in a paper written in the spring of 1931 ‘a new kind of particle, unknown to experimental physics, having the same mass and opposite charge to an electron’. Later in the year, during a lecture at Princeton, Dirac insisted that these anti-electrons ‘are not to be considered as a mathematical fiction; it should be possible to detect them by experimental means’. When, shortly later, experimental evidence for the existence of positrons was announced, it was Dirac, not Oppenheimer, who got the credit for having correctly predicted it.

During his attendance at the Ann Arbor summer school in 1931, Oppenheimer was able to renew personal contact with the European physicists, including Wolfgang Pauli, who arrived full of talk about yet another ‘new kind of particle, unknown to experimental physics’, which he called the ‘neutron’. It was an unfortunate choice of word, since ‘neutron’ had already been used by Rutherford for something very different from what Pauli had in mind. In 1920, Rutherford had suggested that the nuclei of atoms heavier than hydrogen contained not only protons, but also neutral particles of a similar mass, to which he gave the name ‘neutrons’. He proposed this as a way of making sense of observational data concerning the mass and electrical charge of various nuclei. For example, a helium nucleus (an alpha particle) has four times the mass of a proton, but only twice the charge, which would make sense if, instead of being made up of four protons, the helium nucleus consisted of two protons and two neutrons. Attempts to discover this neutral particle, however, proved unsuccessful, although at the very time that Pauli was talking about
his
neutron at Ann Arbor, moves were afoot at the Cavendish that would shortly result in experimental confirmation of Rutherford’s.

Pauli’s ‘neutron’ was put forward to solve a very different problem. This ‘neutron’ was much smaller than Rutherford’s and was something that Pauli thought must exist in order to explain beta radiation. The distinction between alpha and beta radiation had been made by Rutherford in 1897, and subsequently it was discovered that alpha radiation consists of alpha particles – that is, helium nuclei – while the much more penetrative beta radiation consists of streams of electrons, which are emitted from a decaying nucleus.
^fn28

The problem that Pauli sought to solve arose out of experimental observations that showed that beta radiation did not always have the same energy; rather, there was a continuous energy spectrum in beta decay, with electrons being emitted with a range of energies from near-zero upwards. If we are to understand beta radiation as the decay of a nucleus with a given and fixed mass, then the electrons that are emitted ought to be emitted with the
same
energy in every case, otherwise energy is not conserved – the energy of the decayed nucleus plus the electron is
not
equal to the energy of the original nucleus. Some mass or energy has gone missing. So, in the face of the observed fact of the continuous spectra of beta emissions, either what was usually considered a fundamental physical principle – the law of the conservation of energy – had to be abandoned or beta radiation could not be understood as simply the emission of electrons; something else had to be going on.
^fn29

In response to this problem, Bohr, among others, was prepared to abandon conservation of energy, but for Pauli this was too great a step, and, in order to preserve conservation of energy, he suggested what he called a ‘desperate way out’: ‘To wit, the possibility that there could exist in the nucleus electrically neutral particles, which I shall call neutrons . . . The mass of the neutrons should be of the same order of magnitude as the electron mass . . . The continuous beta-spectrum would then become understandable from the assumption that in beta-decay a neutron is emitted along with the electron, in such a way that the sum of the energies of the neutron and the electron is constant.’ In other words, Pauli’s ‘neutron’ would supply the missing energy: the energies of the electron, the decayed nucleus
and
the ‘neutron’ would equal the energy of the nucleus before decay.

Pauli’s remarks quoted above were made in December 1930 in a letter to colleagues attending a conference on radioactivity, where the main topic of discussion was the problem of the continuous beta spectrum. He was evidently tentative about the proposed new particle (which he later called
‘that foolish child of the crisis of my life’
^fn30
), since he did not publish anything about it in the period between writing the above letter and his attendance at the Ann Arbor summer school in 1931. Nor did he present a paper about it at Ann Arbor. He did, however, talk a great deal about it, both in private conversations and in seminars. Among those listening intently to him were Oppenheimer and J. Franklin Carlson (‘Frank Carlson’ to everyone who knew him), who, since Hall’s graduation the previous year, was now the student with whom Oppenheimer worked most closely. As a result of listening to Pauli’s discussions, Carlson and Oppenheimer left Ann Arbor with an idea of how Pauli’s hypothetical new particle could furnish the topic for both future joint research and for Carlson’s PhD thesis.

After Ann Arbor, Oppenheimer spent some time at Perro Caliente with Frank, and then went to New York to visit his parents, before returning on 10 August to Berkeley. From there he wrote to Frank, who was still in New Mexico. In Michigan, Oppenheimer had bought Frank a second-hand car, a Packard Roadster that he called Ichabod, possibly after the Old Testament character, or possibly with reference to Robert Browning’s poem ‘Waring’, about a departed friend, of which verse six begins:

Frank, Oppenheimer wrote, could collect Ichabod from the Packard dealership in Ann Arbor, where he had left it to be repaired. What state the car was in when Oppenheimer bought it is not known, but after he had used it to drive to Ann Arbor, it was in urgent need of repair. Summer-school participants remember Oppenheimer’s arrival in Ichabod: everybody heard a loud crunch as the rim of one of its wheels hit the gravel, and graduate students rushed out to change the flat tyre.

Frank was then about to start his second year as a physics student at Johns Hopkins University, and it seems that the plan was for him, once he had collected the car, to drive to New York to see their parents, before heading off for Baltimore. Ella Oppenheimer had recently been diagnosed with leukaemia. ‘I am afraid you will find mother pretty weak and miserable,’ Oppenheimer warned Frank. ‘The reports have not been
very encouraging.’ He added that he intended to go to New York at Christmas: ‘I have a long vacation, and shall plan to spend most of it with her.’

In the event, Ella’s condition worsened rather more quickly than had been anticipated and Oppenheimer was forced to fly to New York midway through the semester. On 6 October 1931, Oppenheimer received a telegram from his father: ‘Mother critically ill. Not expected to live.’ Denise Royal, in her 1969 biography of Oppenheimer, quotes ‘a friend’ who saw Oppenheimer shortly after he received the telegram and remembers the agony on his face: ‘He had a terribly desolate look. “My mother’s dying. My mother’s dying,” he repeated over and again.’

Before this, Oppenheimer, together with Frank Carlson, had written a short notice, another ‘letter to the editor’ of the
Physical Review
, announcing a new line of research that would, they promised, be developed in a subsequent article. The aim of the research was to investigate whether the ‘neutron’ discussed by Pauli at Ann Arbor might hold the key to an ongoing scientific mystery: the nature of cosmic rays.

The suggestive name ‘cosmic ray’ had been coined by Robert Millikan at Caltech in the 1920s, but the phenomenon of very penetrative radiation occurring high in the earth’s atmosphere had been identified and studied in the first few years of the twentieth century. Millikan became fascinated by these ‘rays’ and was the first to prove that they entered the earth’s atmosphere from outer space (hence ‘cosmic’). In the 1920s and ’30s, Millikan was involved in several controversies regarding the composition of cosmic rays, most notably with Arthur Compton, who held that they consisted primarily of protons. Millikan, on the other hand, thought they consisted of photons – that is, they were not particles at all, but pure electromagnetic radiation. At stake in this dispute, at least from Millikan’s point of view, was something deeper and more general than a mere scientific disagreement. For Millikan, indeed, the issue had
religious
significance.

For both Compton and Millikan, and for everybody else interested in cosmic rays, the intriguing thing about them is their extraordinary energy, which in the 1920s and ’30s was measured at up to 100 million electron volts (since then, energies far higher than that have been detected). There were two ways such energy could be released: either heavy atoms were decaying and releasing protons and electrons as they transformed into lighter elements, or light atoms were fusing with other light atoms to form heavier elements, releasing gamma radiation as they did so. In other words, only two things would produce such energetic rays: the decay of matter or the creation of it. Millikan was religiously committed to the latter view: cosmic rays, he believed, were the ‘birth cries’ of new atoms created by God to counter the effects of decay, and, as such, it was
important for him to believe – and to convince others to believe – that they were made up of photons.