Richard Feynman
Page 20
Any disappointment Feynman may have felt in 1957 at his failure with the problem of superconductivity was, though, far outweighed by the joy he experienced that summer by making another Nobel-quality contribution to physics, in a completely different field again, the theory of the weak interaction.
Feynman had always been impressed by the beauty and power of Dirac’s mathematical description of the electron, and had hankered after making a similar discovery. Such fundamental discoveries are very rare in physics; one of the few examples comparable to Dirac’s equation for the electron would be Maxwell’s equations of electromagnetism. So Feynman knew that this was a dream that would probably never be fulfilled. But he came close in 1957 – sufficiently close to satisfy himself that he had made a significant contribution – with his version of the theory of beta decay, the weak interaction process in which a nucleus (or an individual neutron) spits out an electron.
Feynman’s close involvement with the theory of the weak interaction began at a conference held in Rochester, New York, in April 1956, and lasted just about eighteen months. He had other things on his mind during that period – his divorce took place in the summer of 1956, and he was in the midst of his series of epic papers on superfluidity. But his attention was caught by a curious problem involving two types of particles, then known as theta and tau, which had first been discovered in cosmic rays. The puzzle was that in almost, but not quite, every way the theta and tau were identical – they had the same mass, and though the particles were unstable they each had the same lifetime, and so on. There was just one difference. When the theta particle decayed (through the weak interaction) it disintegrated into two particles in the family known as pions, while when the tau particle decayed it produced three pions. While the set of three pions had an amount of a property called parity equal to –1, the set of two pions had an amount of parity equal to +1. Assuming that no parity had been lost in the decay process, that meant that the theta and tau particles themselves had different parity, and so must really be different particles.
This notion of ‘parity conservation’ was a cherished belief of physicists, because it is related to the way things are reflected in a mirror. If parity is conserved, it means that nature, at a fundamental level, does not distinguish between left and right. If parity were not conserved, though, that would mean that the laws of physics would be different (perhaps only slightly different, but different) in Alice’s looking-glass world. Several people were struggling with this problem, trying to find a way to allow the theta and tau to be the same particle while preserving parity conservation, in the period immediately before and after the 1956 Rochester meeting. They included a team of two Chinese-born American physicists – Chen Ning Yang (known to his friends as Frank) at the Institute for Advanced Study in Princeton and Tsung Dao Lee (known as T. D.) at Columbia University – and, working on his own, Murray Gell-Mann, born in New York City in 1929, who had recently (in 1955) become a professor at Caltech, and would spend many years in the office next but one to Feynman, separated by the office of their secretary.
Feynman’s roommate at the 1956 Rochester meeting was Martin Block, an experimenter who felt diffident about challenging the cherished ideas of the theorists in public, but who, as Feynman recalled in Surely You’re Joking, asked him one evening if it would really be so bad if parity were violated – if the theta and tau really were the same particle. Feynman thought this was a good question, and urged Block to ask the experts. But Block demurred, insisting that nobody would listen to him, and asking Feynman to pose the question:
So the next day, at the meeting, when we were discussing the tau–theta puzzle, Oppenheimer said, ‘We need to hear some new, wilder ideas about this problem.’
So I got up and said, ‘I’m asking this question for Martin Block: What would be the consequences if the parity rule was wrong?’2
Lee answered the question, but using complicated jargon which neither Feynman nor Block really understood. At least one other experimenter discussed with Feynman the possibility of carrying out an experiment to search for parity violation in other particle interactions, but didn’t actually do the experiment. But Block later told Feynman that he had travelled home from the conference in the same plane as Lee, and had used the opportunity to press the case, arguing that at least the possibility was worth investigating.3 Feynman went back to California to sort out his divorce and carry on his work on liquid helium. But Lee and Yang, who had already been working on the parity problem and may have been stimulated further by the discussions at the 1956 Rochester meeting, published a paper later that year looking at the whole situation of parity violation in weak interactions, discussing the theoretical implications, and suggesting experiments that could be carried out to test the idea. By the end of the year, Chien Shiung Wu, another researcher at Columbia University, had carried out one of the experiments proposed by Lee and Yang and had shown conclusively that parity is sometimes violated in weak interactions. Less than a year after that, in the autumn of 1957, Lee and Yang received the Nobel Prize for their work, one of the quickest such awards ever made. (Although Alfred Nobel actually specified that his prizes should be given for work carried out in the previous year, the rule is almost always broken.)
But although everybody knew, by the end of 1956, that parity was not conserved, and that therefore the theta and tau particles were the same thing (now called the kaon) decaying in two different ways, nobody had a satisfactory theory to describe such peculiar behaviour. The following April, at another of the annual Rochester meetings, Feynman took advantage of the opportunity to stay with his sister, Joan, who had completed her PhD in solid state physics and was living in nearby Syracuse. On this occasion, she was able to repay him handsomely for some of the sound advice he had given her, many years before, that had set her on the road to that PhD.
Richard had a copy of the paper Lee was to present to the 1957 Rochester meeting, and complained to Joan that he couldn’t understand it.
‘No,’ she said, ‘what you mean is not that you can’t understand it, but that you didn’t invent it. You didn’t figure it out your own way, from hearing the clue. What you should do is imagine you’re a student again, and take the paper upstairs, read every line of it, and check the equations. Then you’ll understand it very easily.’4
Sounds familiar? Remember when Joan was 14, and Richard told her how to cope with the astronomy book he gave her – ‘you start at the beginning and you read as far as you can, until you are lost. Then you start at the beginning again, and you keep working through until you can understand the whole book.’5
Richard took his sister’s advice, and found that what he had thought to be difficult and incomprehensible was indeed ‘very obvious and simple’, once he got to grips with it. So much so, that he realized some old work he had done in another context could be applied to these problems, and made new predictions about the outcome of experiments involving the weak interaction. In a typical Feynman blitz, he worked through everything the same night, solving in his own way problems that others had been puzzling over for months. The theory of weak interactions that he came up with (which, of course, was based on the path integral approach) didn’t quite work; while it made some clear-cut predictions, in other cases, including the archetypal example of neutron decay itself, it was still a bit messy. Nevertheless, it was progress. The next day, Feynman was able to persuade one of the scheduled speakers at the meeting, Ken Case, to give up five minutes of his time to allow Feynman to give a quick outline of his ideas to the conference. ‘Then’, as Feynman put it, ‘I went to Brazil for the summer.’6
Nobody else worked like this. He had made a vital breakthrough, worked out the implications in a few hours and managed to summarize his discovery in five minutes. Then, instead of writing it up for publication, he went to Brazil. But Feynman was never worried about priority, or being beaten by other scientists, whether they were Euclid or Lee and Yang. Very often, he never bothered to publish his own work. Many
times, a colleague would visit his office at Caltech to ask Dick’s advice about a problem, only to find that he had solved it himself, long ago, and never even mentioned it to anyone. More than that, as Murray Slotnick had been disconcerted to discover at the 1949 meeting of the American Physical Society, usually Feynman had solved a much more general version of the problem.
This combination of skill with the physics and complete indifference to publication extended far outside the fields in which Feynman made his name. The astrophysicist Willy Fowler, who also worked at Caltech, had a favourite Feynman anecdote from the time, in the early 1960s, when quasars had just been discovered.7 Fred Hoyle gave a seminar at Caltech suggesting that quasars might be supermassive stars, and was nonplussed when Feynman (an expert in quantum theory and superfluid flow, but not, as far as anyone was aware, in gravitational theory) stood up to say no, that was impossible, such a star would be gravitationally unstable. It turned out that Feynman had worked out a thorough treatment of the stability of supermassive stars, including a full account of the effects described by the General Theory of Relativity, years before, and essentially simply for his own amusement. According to Fowler, it ran to over a hundred pages of work, work which any astrophysicist would have been proud to have done, but he had simply never bothered to publish, having satisfied himself (the only audience he really wanted to impress) that it was right.
In fact, Fowler’s anecdote gives a slightly distorted picture of the truth, because it was no secret that Feynman was interested in gravity – the surprise was how far that interest had taken him by the early 1960s, when quasars were discovered. He had actually attended one of the first conferences on the role of gravitation in physics, held at the University of North Carolina, Chapel Hill, in January 1957 (this was the occasion, described in Surely You’re Joking, when he arrived late for the meeting, discovered that there were two campuses in North Carolina, and found his way to the right one by asking the cab driver if he had noticed the destination of a group of people ‘talking to each other, not paying attention to where they were going, saying things to each other like “G-mu-nu. G-mu-nu.”’ The cab driver recognized the description of the physicists immediately, and took Feynman to the right campus8). So Feynman was actively involved in gravitational research even before the 1957 Rochester meeting where, at Joan’s behest, he got to grips with Lee’s paper.
After the Chapel Hill meeting, Feynman worked on gravitation for four or five years, trying to find a way to develop a quantum theory of gravity. He was especially interested in gravitational radiation, and was one of the first people to argue strongly that ‘gravitons’, the gravitational counterparts to photons, must exist. The search for gravitational radiation has as yet proved fruitless, but a new generation of detectors should be able to detect bursts of radiation from collapsing stars in the early part of the 21st century; appropriately, Caltech is one of the leading centres of this search today. But Feynman’s own investigations of quantum gravity ran into a brick wall in the early 1960s. In July 1962, he attended a conference in Warsaw where he described the work he had done, and this work appeared in the proceedings of the conference, published in 1964.9 Although progress has been slow in this field, Feynman’s work (especially his use of the Lagrangian formalism) is still relevant today, as we shall see in Chapter 14. What really is remarkable, though (and this is surely the point of Fowler’s story) is that Feynman carried out this work alongside his other investigations, including developing his theory of the weak interaction.
The big problem with Feynman’s embryonic theory of the weak interaction, as he acknowledged at the 1957 Rochester meeting, was that it didn’t work for neutron decay. It didn’t work in a quite specific way, involving the types of virtual particles involved in the interactions. These particles, although ephemeral, are an essential ingredient in all modern theories of particle interactions (including quantum gravity!), and their own properties affect the way in which those interactions, including the weak interaction, take place. Some of the properties they possess, related to their spin and parity, were dubbed A, V, S and T (shorthand for ‘axial’, ‘vector’, ‘scalar’ and ‘tensor’, but the names don’t really matter). Feynman’s new description of the beta decay of neutrons said that it must involve V and A interactions, but the published experimental results on beta decay said that the process involved S and T interactions.
If he had stayed around and looked into this discrepancy with the experimenters, he might well have resolved it in the spring of 1957. But while Feynman was in Brazil, other physicists continued to puzzle over the problem. At the University of Rochester itself, home of the annual high-energy physics meetings, Robert Marshak (who had founded the Rochester gatherings in 1950) and his student George Sudarshan were coming round to the view that maybe beta decay could involve V and A interactions, after all; Murray Gell-Mann was thinking along similar lines at Caltech, and the three of them discussed the implications when Marshak and Sudarshan visited Caltech in July 1957, with Feynman still away in Brazil. Sudarshan, indeed, had already spent a lot of time on the problem before the April 1957 Rochester meeting – but as a student, he wasn’t allowed to give a presentation there, and his supervisor, Marshak, was preoccupied with giving a major paper on another topic. Somehow, neither of them mentioned their work on the weak interaction in the discussion periods at that meeting.
On the way back from Rio, Feynman travelled via New York, and stopped off at Columbia University hoping to discuss the latest experimental results on the problem of the weak interaction with Wu. She wasn’t there, but a colleague brought Feynman up to date with the situation – basically, it was still a mess. By the time he got back to Caltech, Gell-Mann was away on vacation, but Feynman went to talk the problem over with the experimenters. They agreed that the situation was hopelessly confused. ‘It’s so messed up,’ they told him, ‘Murray says it might even be V and A.’10
Feynman was electrified. If beta decay involved V and A interactions, not S and T, his theory was right after all! He calculated everything again, and it worked. At first, it seemed that a certain number calculated in accordance with his theory disagreed with experiment by 9 per cent; then he discovered that the number printed in the textbooks was wrong, and had since been revised by 7 per cent, in the right direction. The discrepancy was really only 2 per cent, pretty good for anything involving particle physics. In another all-night session, he calculated away, buoyed up by the euphoric feeling that he had made one of the truly fundamental discoveries in physics:
I felt that it was the first time, and the only time, in my scientific career that I knew a law of nature that no one else knew. Now, it wasn’t as beautiful a law as Dirac’s or Maxwell’s, but my equation for beta decay was a bit like that. It was the first time that I discovered a new law, rather than a more efficient method of calculating from someone else’s theory.11
This is a rather self-deprecating way of referring to QED, given that in order to make his contribution there Feynman had found a completely new way to formulate quantum theory (and classical theory!) from first principles, but for whatever reasons the equation for beta decay was the discovery that he himself was most impressed by. ‘Now,’ he thought, ‘I have completed myself.’ And, just for once, he was sufficiently fired up to write the discovery up for publication immediately.
But things weren’t quite that simple, and for once – the only time that it really mattered to him – Feynman’s laid-back approach to the question of establishing priority was to cost him dear. Soon, Gell-Mann returned from vacation intending to write up his own version of the V and A theory of the weak interaction, and was somewhat miffed to find that Feynman had picked up what Gell-Mann regarded as his own ball and run off with it.
Pouring oil on potentially troubled waters, and anxious to avoid two rival papers on the same discovery by different authors coming out of Caltech at the same time, the head of the physics department at Caltech, Robert Bacher, urged Feynman and Gell-Mann to produce a
joint paper, which they did. It was received by the Physical Review on 16 September 1957, and published in 1958 in less than six pages of the journal. It was a clear step forward in physics, in a sense giving the ‘equation of the neutrino’ in the way that Dirac had provided the ‘equation of the electron’. It soon became (and remains) a widely quoted classic – much to the chagrin of Sudarshan and Marshak, who had written up their own version of the idea back in July, presented it at a meeting in Italy in the autumn of 1957, but only got into print in a journal (also the Physical Review) after Feynman and Gell-Mann. The result was that their work was unjustly seen as a ‘me too’ exercise. This came as a severe blow to Sudarshan, who was a young researcher who had just completed his first major piece of work, and realized he was unlikely to do anything as significant in the rest of his career; he never overcame the bitterness he felt about the way credit was apportioned to Feynman and Gell-Mann. In all fairness, though, the rule in science is that credit generally goes to the person who publishes first, and Sudarshan and Marshak had ample opportunity to get something into print between the 1957 Rochester conference and the end of that year; even when they did publish, their work was not so complete or elegant as the Feynman and Gell-Mann version. Feynman himself, though, always tried to give Sudarshan due credit, being careful always to refer to the work of Sudarshan and Marshak, as well as the Feynman and Gell-Mann paper, when discussing the theory of the weak interaction.12
But there was another lesson that Feynman learned from the experience of his work on the weak interaction. Why, if the interaction really involved V and A instead of S and T, had everybody been so certain that it was S and T? It turned out that all the experts had been quoting, some second or third hand, from one experiment. On advice from Robert Bacher, Feynman went to the library and looked up the paper which everybody quoted when saying that the weak interaction was S and T. He discovered that the conclusion was based on the positions of the last two data points on the edge of a graph, based on experimental measurements, plotted in that paper, ‘and there’s a principle that a point on the edge of the range of the data – the last point – isn’t very good, because if it was, they’d have another point further along …’13 Until then, he had ‘never looked at the original data … Had I been a good physicist, when I thought of the original idea back at the Rochester Conference, I would have immediately looked [it] up … Since then I never pay any attention to anything by “experts.” I calculate everything myself.’