Inside the Centre: The Life of J. Robert Oppenheimer (118 page)

Much of Oppenheimer’s time during these years was spent giving public lectures to large audiences, often to commemorate a death or an anniversary. On 2 February 1956, he gave an address to the American Institute of Physics on the occasion of its twenty-fifth anniversary. The talk, published in
Physics Today
, was entitled ‘Physics Tonight’, and sought to give an impression of the ‘wonderfully diverse and varied set of enterprises’ in which physicists were involved. To illustrate this diversity he discussed three examples; one each of, respectively, the physicist as discoverer, the physicist as citizen and the physicist as teacher. Predictably, under the heading of ‘physicist as discoverer’, he discussed ‘what is called in the trade particle physics’, drawing attention to its chaotic state in what he assumed was a transient stage of its development. ‘In some ways,’ he said, with what almost seems like nostalgia, ‘this field may remind us of the quantum theory of atoms as it was in the earlier years of this century; but we have not found that single key to the new physics that Planck discovered at the turn of the century, nor anything analogous to Bohr’s postulates.’ He was, however, confident that ‘physics tonight’ could look forward to a bright new morning:

Turning to the physicist as teacher, Oppenheimer’s advice was a little vague, if not completely vacuous. ‘We must make more humane what we tell the young physicist, and must seek ways to make more robust and more detailed what we tell the man of art or letters or affairs, if we are to contribute to the integrity of our common cultural life.’ What he means by this, or, indeed, whether it means anything at all, seems to be open to question.

Equally opaque are his comments on the physicist as citizen, which seem designed to point out only a lack of clarity:

The specific issues that Oppenheimer listed in ‘Physics Tonight’ as ‘the special problems that at the moment seem most pressing of solution’ were not ones readily comprehensible to a general audience and reflected the fact that on this occasion he was talking to fellow physicists. In Oppenheimer’s words, those issues were ‘the relation of the τ-meson [tau-meson] and the θ-meson [theta-meson]; why the antiproton interacts with such a large cross section with nuclei; whether we can understand the scattering of pions in S states’. In fact, these were exactly the issues that dominated the sixth Rochester Conference, which was held on 3–7 April 1956. It was, says Pais, ‘a historic meeting, for several reasons’. For one thing, it was the first Rochester meeting at which Soviet scientists participated – an extraordinary gesture given that in the summer of 1956 there was no sign of a thawing in the Cold War; quite the opposite, in fact. It was also the first meeting at which the participants had a chance to discuss the issues that Oppenheimer mentioned in ‘Physics Tonight’, issues that raised, as Oppenheimer implied, fundamental questions.

On the second day Oppenheimer gave a public address to an overflow audience on his favourite topic, the ‘sub-nuclear zoo’, drawing particular
attention to one of the puzzles he had mentioned in his
Physics Today
article, and the fundamental question that it raised. The puzzle was that two heavy mesons, the tau-meson and the theta-meson, seemed to have identical masses and identical lifetimes, yet opposite parities. The notion of ‘parity’ can be understood in terms of a mirror-image. If you look in the mirror, left becomes right and right becomes left; or, to put it another way, spatial coordinates have been ‘flipped’. If they are then flipped again, they go back to how they were, which is called a ‘rotation’. A rotation has a parity of 1, a flip has a parity of –1.

Returning to tau-mesons and theta-mesons, these particles puzzled physicists because there seemed to be fairly compelling grounds for believing that they were, in fact, the same particle, and equally compelling grounds for thinking they were not. The reason for thinking they were the same particle was simply that they had exactly the same mass and exactly the same lifetime, which, if they were different particles, would be an amazing coincidence. On the other hand, they seemed to be different with respect to what happened to them when they underwent beta decay. As explained earlier, when a neutron undergoes beta decay, it emits an electron and a neutrino, and what remains is a proton. Another way of saying this is that its beta-decay products are a proton, an electron and a neutrino. The tau-meson and the theta-meson have different beta-decay products.

That a single particle can decay in two different ways would not be particularly puzzling, but what did puzzle scientists was that, if these two were the same particle, then what they thought was a fundamental law of nature – the conservation of parity – would in this case not be upheld. When a tau-meson undergoes beta decay, it produces three pions (as the ‘Yukawa particle’ ended up being called), two positive and one negative. The theta-meson, on the other hand decays into two pions, one positive, the other neutral. A pion has a parity of -1 (a flip), which means that a tau-meson has a parity of 1 (three flips, one for each of its pions), and the theta-meson -1 (two flips, or a rotation, so ending up the same). Assuming the law of the conservation of parity, therefore, the tau-meson and the theta-meson had to be, despite appearances, different particles.

It was in connection with this puzzle that Oppenheimer uttered two remarks that were savoured by those present as being comically characteristic of him, in that they combined apparent profundity with utter unintelligibility. The first of these was: ‘The τ-meson will have either domestic or foreign complications. It will not be simple on both fronts.’ The second was: ‘Perhaps some oscillation between learning from the past and being surprised by the future of this tau–theta dilemma is the only way to mediate the battle.’ Both remarks were repeated again and again by the delegates at the conference, who delighted in their
ambiguity and the fact that, as Robert Crease has said, they ‘hinted at a rising wave of possibly revolutionary physics without advancing the problem’. In order to make sense of the experimental findings regarding the tau- and theta-mesons, the theorists had either to say that the two were – despite having the same mass and the same lifetime – different particles, or else they had to say that a principle that had been assumed to be a fundamental law of physics – the conservation of parity – was actually no such thing. On the way back from Rochester, Yang and Pais bet John Wheeler a dollar that the two were different particles. As it turned out, Yang had put himself into a win–win situation here, since he was soon to be involved in an attempt to
prove
that parity had been violated. If he succeeded, he would lose the bet and owe Wheeler a dollar; he would, however, also have made a Nobel Prize-winning contribution to physics.

Two months after the sixth Rochester Conference, Yang sent Oppenheimer an article that he and Lee had written, in which they made a bold suggestion. What they suggested was that, though parity conservation had been experimentally demonstrated with regard to
strong
interactions, such as those between nucleons, there was no such experimental data with regard to weak interactions, such as those associated with beta-decay. As the tau-meson was distinguished from the theta-meson by means of their beta-decay products, then, suggested Yang and Lee, if it turns out that the law of parity conservation does not hold in weak interactions, there would be nothing to prevent one from concluding that they were in fact the same particle. They also suggested some possible experiments that might settle the issue. When this article was published in the October 1956 issue of the
Physical Review
, the authors thanked Oppenheimer, among others, for ‘interesting discussions and comments’. In fact, Oppenheimer’s comment was to suggest – as if their proposal were not bold enough – that fundamental conceptions of space and time might have to change in order to make sense of the tau–theta puzzle.

One possible experiment suggested by Yang and Lee was to look for violations of parity in beta decay in the release of electrons from a radioactive substance such as cobalt-60. Another possible experiment was to look for violations of parity in the decays of pions and muons, other examples of weak interactions. A team of experimentalists led by Chien Shiung Wu at Columbia took up the challenge laid down by Yang and Lee, and by the end of 1956 had demonstrated beyond all doubt that they were right: the conservation of parity did not hold for weak interactions. The tau–theta puzzle had been solved: they were the same particle. Wheeler won his dollar, and Yang and Lee were awarded the 1957 Nobel Prize in physics.

In January 1957, shortly after the results came in, Yang cabled Oppenheimer, who was then in the Virgin Islands, to tell him: ‘Wu’s experiment yielding large symmetry.’ Oppenheimer replied: ‘Walked through door. Greetings.’ The allusion in Oppenheimer’s telegram is explained in Yang’s Nobel Prize speech, in which he said:

The excitement generated by the breakthrough of Yang and Lee was reminiscent of that which accompanied the breakthroughs of the 1920s and ’30s. On 16 January 1957, the
New York Times
had it as its front-page story under the heading: ‘Basic concept in physics is reported upset in tests. Conservation of parity in nuclear theory challenged by scientists at Columbia and Princeton Institute.’ The excitement was shared by Oppenheimer, who declared: ‘No one today knows where this discovery will lead . . . something has been found whose meaning only the future will reveal.’

In the spring of 1957, Oppenheimer – now fifty-three – gave the William James Lectures at Harvard, an annual series of talks somewhat akin in terms of prestige to the BBC’s Reith Lectures. Oppenheimer’s overall title was ‘The Hope of Order’. Among those present was Jeremy Bernstein, who remembers:

Bernstein was at this time coming to the end of a two-year appointment and had applied to the institute for a fellowship. He was, he recalls, ‘truly amazed – and absolutely thrilled – when I received a letter of acceptance . . . not long after this letter arrived, there was Oppenheimer giving a lecture at Harvard’:

After the lecture, Bernstein decided to go onto the stage to introduce himself. To begin with, Oppenheimer ‘looked at me with what I distinctly remember as icy hostility’, but when Bernstein told him that he would be joining the institute that autumn, ‘his demeanor completely changed’:

The lectures were never published, but the accounts of them that appeared in the
Harvard Crimson
indicate that they covered the same ground as the 1953 Reith Lectures. When interviewed by the local television station, Oppenheimer remarked: ‘I believe in the popularization of science. I don’t think I do it terribly well. But we must know that it is as impossible as it is essential. It has those two inescapable sides, I think.’

One senses that, as he spent more of his time popularising physics, he felt himself increasingly removed from the cutting edge of the subject. When Bernstein arrived at the institute in the autumn of 1957, he was surprised to be told, immediately upon telling the secretary who he was, that Oppenheimer wanted to see him right away. As soon as he walked into Oppenheimer’s office, he recalls, Oppenheimer greeted him: ‘What is new and firm in physics?’ While Bernstein was wondering how to reply, the phone rang. ‘It’s Kitty,’ Oppenheimer told him after hanging up. ‘She has been drinking again.’

The Princeton physicist Sam Treiman remembers that every Tuesday Oppenheimer hosted a lunch in his office for a group of six or so physicists, including Yang, Pais, Dyson and Treiman himself. Oppenheimer, he recalls, ‘attached great importance to the lunches, often calling me a day in advance to remind’. Treiman was not so convinced about the scientific value of these meetings, in which, he says, the participants ‘overdrank the sherry, and just rambled on about current developments in physics . . . The conversation was never highly technical. It had more to do with who’s in, who’s out, what are the best bet, etc.’