Inside the Centre: The Life of J. Robert Oppenheimer (18 page)

What Oppenheimer would have known Rutherford for, and what most people today still know him for, is his development, in 1911, of the planetary model of the atom, which pictures an atom as consisting of a positively charged nucleus around which orbit negatively charged electrons. This was a revolutionary way of picturing the atom, which it took
imagination, intuition and a willingness to be led by experimental evidence to conceive. Rutherford’s predecessor at the Cavendish, J.J. Thomson, had paved the way by being the first person to demonstrate that an atom was not the indivisible, hard ball previously imagined, when in 1897 he discovered the existence of particles, later called electrons, that were
part of
an atom. These tiny ‘subatomic’ particles, Thomson showed, were negatively charged, which, given that atoms themselves were neutral, meant that the other part of the atom had to be positively charged. As Thomson pictured it, an atom was made up of a positively charged mass into which electrons were embedded. On this ‘plum pudding’ model, the positive charge was understood to be evenly spread throughout the atom. In a series of ingenious experiments conducted at Manchester, Rutherford demonstrated that this could not be so and that the positive charge of an atom was, rather, concentrated in a tiny ‘nucleus’. To appreciate
how
tiny this nucleus is, if an atom were the size of a golf course, then the nucleus would be the size of just one of its holes. According to Rutherford’s model, the electrons orbit this nucleus like planets around the sun. An atom is less like a plum pudding and more like a solar system.

Rutherford’s theory was significantly refined in 1913 by the man whom Oppenheimer would come to admire more than anyone else in the world: the Danish physicist Niels Bohr. Bohr had arrived in Cambridge in 1911 to work with J.J. Thomson, but was disappointed to discover that Thomson, then in his fifties and past his best as a scientist, was uninterested in him or his work. After meeting Rutherford when he came to give a paper at Cambridge, Bohr decided he would rather work with him than with Thomson and so transferred from Cambridge to Manchester. At Manchester, under Rutherford’s benign and sympathetic guidance, Bohr devoted himself to the theoretical problems raised by Rutherford’s model of the atom. At the heart of these problems was the fact that, according to the laws of physics as they were then understood, the atom as pictured by Rutherford would be inherently unstable. Newton’s laws of motion tell us that an object moving in a circle undergoes acceleration, and Maxwell’s theory of electromagnetism tells us that a charged particle, like an electron, will lose energy in the form of electromagnetic radiation as it accelerates (it will, in other words, emit light). Very quickly, the electron’s energy would be completely dissipated and, unable to continue its orbital motion, it would collapse into the atomic nucleus. As this did not, in fact, happen, something had to be wrong, either with Rutherford’s model or with classical physics – or, as it turned out, with both.

Bohr’s daring solution, outlined in a series of three papers published in 1913, was to jettison classical physical laws and replace them with what is now called ‘old quantum theory’, but which Oppenheimer, as a student, would have known simply as ‘quantum theory’. On the ‘Rutherford-Bohr
model’, as it was henceforth known, the electrons in an atom are held in their orbits by the electromagnetic attraction between the negatively charged electrons and the positively charged atomic nucleus. In order to understand the behaviour of electrons in their ‘static orbits’, Bohr maintained, we need to appeal, not to the laws of classical physics, but rather to the quantum theory developed by Planck and Einstein in their attempts to understand the behaviour of the ‘photons’ of light. Electrons do not, as classical theory would suggest, lose energy as they orbit the nucleus, so long as they stay in the same orbits. They absorb or emit energy only when they move from one orbit (or, as Bohr would now have it, ‘stationary state’) to another, and when they do that, they move, not in a continuous motion, but in jumps or ‘quanta’, the mathematics of which is centred on
h
, Planck’s famous constant, which Oppenheimer had spent so much time pondering at Harvard. Furthermore, according to Bohr, electrons are only ‘permitted’ certain states; namely those allowed for in the mathematics of quantum theory. The properties of chemicals are determined by the number of electrons they have, which gets larger and larger as one goes through the periodic table, from the lightest element, hydrogen, which has just one electron, to the heaviest, uranium, which has ninety-two. As more electrons are added, they are allotted places in the ‘stationary states’ around the nucleus. If an electron jumps from one state to another, then (and only then) does it emit light. Otherwise, it stays in its state, suffering no energy loss. Why electrons are only permitted certain states Bohr could not explain, but the hypothesis that they were so restricted turned out to be too powerful to ignore; it allowed an extraordinarily neat account of the molecular structure of chemicals to be given that provided a physical explanation of the entire periodic table.

A detailed account of the Rutherford–Bohr model of the atom, and of the ‘old quantum theory’ of which it forms a fundamental part, is given in William C. McC. Lewis’s
Quantum Theory
, the third volume of
A System of Physical Chemistry
, one of those works that Oppenheimer claimed to have read in his first year at Harvard. Another one of those works, Arnold Sommerfeld’s
Atombau und Spektrallinien
, was entirely devoted to that model, Sommerfeld’s account of which became accepted not only as the definitive statement of Bohr’s theory, but also as a significant improvement upon it, so that the model came to be called the ‘Bohr–Sommerfeld model’.

Having, at least purportedly, read Lewis and Sommerfeld, and having heard Rutherford’s name mentioned in all the physics courses he took at Harvard, as well as in his conversations with Bridgman and Kemble, it is not surprising that Oppenheimer would regard the Cavendish Laboratory as being at, or at least near, the ‘centre’ of new developments in physics. What
is
a little curious, however, is that Oppenheimer should have wanted
to pursue research in
experimental
rather than theoretical physics. After all, in his letter to Fergusson from Harvard in the spring of 1925 he had remarked, in relation to some work he had been doing with Bridgman: ‘that brief excursion convinces me that my genre, whatever it is, is not experimental science’. Given this realisation, and the fact that it had been
theoretical
physics that had fascinated him since his first year at Harvard, why did he apply to work in a laboratory with a man known throughout the world as an experimentalist? In an effort to explain this later in life, Oppenheimer said that he did not know that one could make a living as a theoretical physicist. This is difficult to believe. After all, he was taught by Edwin Kemble, whom he surely knew to be a theoretical physicist, even if he had not realised that many, perhaps most, of the physicists he had read or read about – Bohr, Einstein, Sommerfeld, and so on – were also theoretical physicists. It seems that he decided he wanted to be at Cambridge because that is where many of his Harvard friends had gone, and, having decided that, he was led by the reputation of Rutherford and the Cavendish, and by Cambridge’s pre-eminence in experimental physics and its comparative neglect of theoretical physics, to attempt to overcome his ineptitude for experimental work.

It would turn out to be a grievous error of judgement. For one thing, it put Bridgman in a difficult position when it came to writing Oppenheimer a reference recommending him to Rutherford. While Bridgman could say in all honesty that Oppenheimer had a ‘perfectly prodigious power of assimilation’, ‘a high degree of originality’ and ‘much mathematical power’, he felt obliged to point out that ‘his weakness is on the experimental side’:

Bridgman also felt obliged to add: ‘As appears from his name, Oppenheimer is a Jew, but entirely without the usual qualifications of his race. He is a tall, well set-up young man, with a rather engaging diffidence of manner, and I think you need have no hesitation whatever for any reason of this sort in considering his application.’

In the context of the anti-Semitism that plagued American academic life in the 1920s, and in particular the ongoing controversy at Harvard about Lowell’s desire to impose quotas on Jewish students, Bridgman’s remarks are entirely understandable and clearly well intentioned. However, they were, as far as Rutherford was concerned, entirely unnecessary and possibly offensive in their assumption of a background prejudice against Jews who were
not
tall, ‘well set-up’ and diffident. It is certainly not true
that there was no anti-Semitism in British academic life, but Rutherford himself was, as far as one can tell, entirely free from it. Raised on a farm in New Zealand, he was emphatically not a typical Cambridge professor; his outlook was robustly and resolutely egalitarian, without snobbery or racism. In the 1930s, he became the first president of the Council for Assisting Refugee Academics, which was formed to help Jewish academics forced to flee Nazi Germany. That Oppenheimer’s name betrayed a Jewish ancestry would have been of no concern whatsoever to him.

However, that Oppenheimer’s weakness as a physicist was ‘on the experimental side’ would surely have persuaded Rutherford that he was not an ideal candidate for a postgraduate research position at the Cavendish. Accordingly, in the letter that Oppenheimer received on his return to New York from New Mexico, Rutherford wrote that, as he already had so many ‘excellent applicants’, he could not take Oppenheimer on as a research student, at least for a time. While his place at Christ’s College was still open, then, Oppenheimer would have to suffer the indignity of being, for a while at least, an undergraduate student. Later in the academic year, Oppenheimer was told, he could be accepted as a graduate student, so long as the Cambridge authorities were, in the meantime, convinced that he had some aptitude for original work, whether experimental or practical.

The registrar at Christ’s College, responsible for graduate admissions, was the famous explorer Raymond Priestley, who had been a member of first Shackleton’s and then Scott’s expeditions to the Antarctic, the latter of which was the subject of Priestley’s best-selling book,
Antarctic Adventure
. In the First World War, Priestley was decorated with the Military Cross for his part in the celebrated capture by the Allies of the Riqueval Bridge, which he wrote about in another popular book,
Breaking the Hindenburg Line
. After the war he was elected a Fellow of Christ’s and devoted himself to academic administration. After receiving Rutherford’s rejection, Oppenheimer wrote to Priestley, telling him that he would be taking up his offered place, but that he ‘should like to be admitted to the University as a research student as soon as possible’. ‘If admission cannot be granted at once,’ Oppenheimer added, ‘I should be quite willing to wait a term.’

As soon as he arrived in Cambridge on 16 September 1925, before setting off on his walking holiday with Fergusson, Oppenheimer wrote again to Priestley, formally applying to be considered as a graduate student, reading in physics, analysis and physical chemistry, in preparation for undertaking ‘as soon as it seems advisable’ a research problem in physics. He was particularly interested, he told Priestley, in the theory of electronic conduction, especially those aspects of it ‘which can give an indication of the laws of force to which the motion of electrons is subject’. He would,
he said, ‘be very glad of an opportunity for further experimental work, and, if possible, for critical advice in the corresponding theoretical problems’. In support of his application for graduate status, Oppenheimer wrote out a list of all the relevant courses he had taken at Harvard and submitted his degree, his grade cards and his references. ‘If any more definite information is required,’ his letter to Priestley ends, ‘I shall be glad to try to give it. I am twenty-one years old.’

Having thus made his case to be admitted as a doctoral student, rather than the undergraduate he officially still was, Oppenheimer set off on his walking holiday in Cornwall with Fergusson. For the reasons anticipated by Smith, this tour with Fergusson, instead of easing Oppenheimer gently into English social life, seems to have left him feeling intimidated, awed and firmly persuaded that he would never be accepted by ‘the tutors & the dukes’. Fergusson, during his two years in England, had become associated with the very set of artists, writers and intellectuals that Oppenheimer had read about in Aldous Huxley’s
Crome Yellow
, and who met at Ottoline Morrell’s Oxford home, Garsington. This included, but was not restricted to, the Bloomsbury Group of Virginia Woolf and John Maynard Keynes. As Oppenheimer put it in a letter to Smith, Fergusson ‘knows everyone at Oxford; he goes to tea with Lady Ottoline Morrell, the high priestess of civilized society & the patroness of Eliot & Berty;
^fn17
& he is a member of a congress of litterateurs who meet every summer at Pontigny to talk about “Mysticism & Literature” & other such modest things’.

The meetings at Pontigny that Oppenheimer mentions here were a series of annual colloquia that took place every year between 1910 and 1939 at the former Cistercian abbey at Pontigny in Burgundy. They were organised by the philosopher and literary critic Paul Desjardins, who owned the abbey. ‘To be invited to Pontigny,’ writes a biographer of one of the regular attendees, Prince Mirsky, ‘was to be recognised as a member of European intellectual aristocracy.’ Though the leading figures at Pontigny were French intellectuals, such as André Gide and Charles Du Bos, there were also many connections between the Pontigny colloquia and the Bloomsbury/Morrell Group. Roger Fry and Lytton Strachey (both members of the Bloomsbury Group and regular invitees to Garsington), for example, were habitual attendees at Pontigny.