by Ray Monk
His first lecture in this course attracted about forty students, among whom was Carl D. Anderson, then a PhD student and later a very eminent physicist. ‘I didn’t know what Oppenheimer was talking about,’ Anderson has recalled. ‘He, in those days, was not a good lecturer. He paced back and forth, and wherever he happened to be at that instant, he would write some squiggles on the blackboard – part of an equation – and they were scattered all over at random.’ Within a few weeks Anderson was the only student still registered for the course. When he, too, went to see Oppenheimer to ask him for permission to drop the course, Oppenheimer pleaded with him to stay – without Anderson, he would have no course, and without a course, he would have no official position at Caltech. As an inducement, Oppenheimer promised Anderson that, if he remained on the course, he would be guaranteed to get an A, on which basis Anderson remained.
In addition to his ‘course of sprouts’, Oppenheimer was persuaded by Richard Tolman to give some extra evening lectures on Dirac’s quantum electrodynamics. These were open to anyone who wanted to come, but were intended chiefly for academic members of staff. In the event, the first of the proposed series was attended by about a dozen people. Again, Carl Anderson was present and remembers that, after Oppenheimer had talked for about two hours, Richard Tolman got up and said: ‘Robert, I didn’t understand a damn word you said tonight, except . . .’ And then he went up to the blackboard and wrote an equation. ‘That’s all I understood.’ In reply, Oppenheimer told him that he had got that equation wrong. ‘And,’ says Anderson, ‘there was never a second meeting of this attempt on Oppenheimer’s part to tell various people, mostly faculty, what Dirac’s theory was all about.’
During this first spring at Pasadena, Oppenheimer was visited by his parents. The previous year Julius had sold his share of the family business. Whether by luck or judgement, it is impossible to say, but Julius had thus protected the family fortune from the effects of the Wall Street Crash of October 1929. So little impact did the crash have on his family, and so little interest did he show in politics at this time, that Oppenheimer later recalled that he did not even know the crash had happened until long after the event, when he was told about it by Ernest Lawrence. In March 1930, Julius and Ella, their fabulous wealth still intact, came out west to visit their son. ‘We had a delightful evening at the Tolmans,’ Julius wrote to Frank from Pasadena. ‘Tomorrow afternoon we are going there for tea and shall meet a number of the professors and some other of Robert’s friends, and on Friday we are going with Mrs Tolman to Los Angeles to hear the Tchaikovsky concert.’ Robert, Julius wrote, was ‘very busy with conferences, lectures, and his own work, but we manage to see him a short time daily.’
Julius was unhappy about the state of Oppenheimer’s car, an old Chrysler, and so, ‘against severe protest’, insisted on buying him a new one, which ‘he is most delighted with . . . he has reduced his speed about 50% from what he used to drive, so we hope no further accidents will occur’. The recklessness of Oppenheimer’s driving was legendary. In a previous letter to Frank, he himself had written: ‘From time to time I take out the Chrysler, and scare one of my friends out of all sanity by wheeling corners at seventy. The car will do seventy-five without a tremor. I am and shall be a vile driver.’ The accident Julius mentions is possibly the occasion on which Oppenheimer crashed his car while trying to impress and scare his passenger, the writer Natalie Raymond (‘Nat’ as she was known to her friends, one of whom described her as ‘a dare-devil, an adventurer’), by racing a train. She was knocked unconscious and, at first, Oppenheimer thought she was dead. Her compensation was to be presented by Julius with a Cézanne drawing and a small painting by the French artist Maurice de Vlaminck.
The day after Julius wrote to Frank, Oppenheimer also wrote to him. Oppenheimer’s letter is one of the most interesting he ever sent his brother, containing as it does a series of reflections on what Rabi recognised as Oppenheimer’s central problem: identity. Frank had written to him expressing a fear characteristic of his age (he was then seventeen), namely that the Frank his older brother had known had disappeared. Oppenheimer responded with warmth and reassurance. ‘It is not easy,’ he told Frank, ‘to believe that the Frank I know is completely vanished; and I should be very very sorry if that were so.’ Nevertheless, he paid Frank the compliment of treating the issue he had raised – the question of personal identity – with complete seriousness. ‘I think,’ he wrote, ‘that you do overestimate the inconstancy and incoherence of personal life’:
for I believe that throughout the variations – and they are wild enough, God knows – there is, there should be, and in mature people there comes more and more to be a certain unity, which makes it possible to recognize a man in his most diverse operations, a kind of specific personal stamp.
Oppenheimer was, evidently, inclined to take philosophical questions very seriously indeed, for a reason he spelled out to Frank: ‘The reason why a bad philosophy leads to such hell is that it is what you think and want and treasure and foster in the times of preparation that determines what you do in the pinch, and that it takes an error to father a sin.’ The letter ended with the affectionate plea: ‘Don’t you go and change too much, now; because I think you were pretty damn nice before.’
As he did at Berkeley, Oppenheimer lived, while he lectured at Caltech, in the faculty club. His friends in Pasadena included Richard Tolman, whom he already knew, and the Danish physicist Charles C. Lauritsen. Tolman and his wife, Ruth, became especially close friends (it was they who introduced Oppenheimer to Natalie Raymond), and he often dined at the Tolmans’ home, as did his parents when they were in Pasadena. Contact with people like Tolman and Lauritsen, people who were closely in touch with recent developments in physics, was one of the main reasons Oppenheimer was reluctant to give up his position at Caltech.
However, although it was at Caltech that Oppenheimer thus stayed in touch with current research, it was at Berkeley that he hoped to build his peculiarly American school of theoretical physics. To accomplish this, he knew that he would have to attract to Berkeley more able students than the ones he had inherited, and so, at the many conferences and meetings he attended, he kept an eye out for possible recruits. One of the most promising recruiting grounds in this respect was the summer school at Ann Arbor, which, together with a restorative few weeks at Perro Caliente, became one of the annual fixtures of his summers. The very first graduate student to begin a PhD thesis under Oppenheimer’s supervision had gone to Berkeley as a result of having attended the summer school at Ann Arbor. This was Melba Phillips, originally from Indiana, who, while a master’s student at Battle Creek College in Michigan, had attended the summer school and been inspired by a course given by Edward Condon on quantum mechanics. On Condon’s recommendation, she applied to Berkeley and found herself in the autumn of 1930 being assigned Oppenheimer as her PhD supervisor.
By that time, Oppenheimer already had three PhD students, but they had begun their research work under another supervisor. They were Harvey Hall and J. Franklin Carlson, both of whom had started under William Howell Williams, and Leo Nedelsky, who had been working with Samuel Allison. All three flourished under Oppenheimer and went on to have successful careers in physics. Oppenheimer devoted considerable energy to his PhD students, working closely with them and making sure that when they left Berkeley they had significant publications to their name. To achieve this, he developed a practice of publishing joint papers with his PhD students, and during the 1930s a good proportion of his work consisted of such joint publications.
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 Einst
ein 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 energi
es 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:
