Quantum Man: Richard Feynman's Life in Science Page 18

  Gell-Mann first became well known following his proposal in 1952 that the new mesons, whose production was so strong but whose decay was so weak, behaved this way because some quantity associated with the new particles was conserved in the strong interaction. He called this strange new quantity, perhaps not inappropriately, strangeness. The conservative editors of the Physical Review, however, where he first published his ideas, felt this new name was inappropriate in a physics publication, and refused to use it in the title of his paper on the subject.

  Gell-Mann argued as follows: Because strangeness is conserved, the new particles have to be produced in pairs, particles and antiparticles, with equal and opposite values of this new quantum number. The particles themselves would then be absolutely stable, because to decay into non-strange particles would violate this conservation law—changing the strangeness number by one—if the strong force was the only one operating. However, if the weak force, the one responsible for the decay of neutrons and the reactions that power the sun, did not respect this conservation law, then the weak force could induce decays of these new particles. But because the force was weak, the particles would then survive a long time before decaying.

  As attractive as this idea was, success in physics does not involve merely postdiction. What would it predict that could allow it to be tested? Indeed, this was the immediate reaction among many of Gell-Mann’s colleagues. As Richard Garwin, a brilliant experimentalist who had played a key role in the development of the hydrogen bomb, put it, “I don’t see what use it could possibly be.”

  The leap came when Gell-Mann realized that this strangeness quantum number could be used to classify sets of existing particles, and he even made a stranger prediction. He predicted that a neutral particle called the K-zero should have an antiparticle, the anti-K-zero, which was different from itself. Since most other neutral particles, like the photon, are equivalent to their antiparticles, this proposal was unusual, to say the least. But ultimately it proved to be correct, and the K-zero–anti K-zero system has served as a remarkable laboratory for probing new physics, cementing Gell-Mann’s reputation among the then-rising new generation of particle physicists.

  It was after his introduction of strangeness, and the death of Fermi, that Gell-Mann began to receive offers to work outside of Chicago. He wanted to go to Caltech to work with Feynman, and Caltech encouraged the decision by matching competing offers and making Gell-Mann, at age twenty-six, the youngest full professor in its history. The hope was that this would not be the first history-breaking event that Gell-Mann, along with Feynman, might bring to the university.

  Feynman and Gell-Mann had a remarkable partnership involving intellectual give-and-take. The two of them would argue nonstop in their offices, a kind of friendly argument or, as Gell-Mann would later call it, “twisting the tail of the cosmos,” as they tried to unravel the newest mysteries at the forefront of particle physics. It had an impact on their students and postdocs as well. I remember when I was a young researcher at Harvard, working with Sheldon Glashow, a former student of Schwinger’s and a Nobel laureate. Our meetings were punctuated with a mixture of arguments and laughter. Glashow had been a postdoc at Caltech with Gell-Mann, and I expect was strongly influenced by the style of discussion he witnessed there, of which I, and hopefully my students, have become further beneficiaries. The partnership between Feynman and Gell-Mann also was an uneasy marriage of opposites. Gell-Mann was the very epitome of the cultured scientist, and Feynman was not. Gell-Mann was, by nature, judgmental of people and their ideas, and always worried about intellectual priority. Feynman had no patience for physics nonsense or pomposity and appreciated talent, but if he got scooped, as mentioned earlier, what he cared most about was whether he had been right or wrong, not who ultimately got credit. It was an interesting partnership, which, given the difference in character and style, was bound to run into trouble eventually—but not right away.

  Nevertheless, this was a time when both scientists were near their creative peaks. Gell-Mann was just beginning to revolutionize the world of elementary particles, and Feynman had just completed his own revolution in quantum mechanics. When they began to work together, another vexing physics problem had arisen, also related in part to the new strange particles that Gell-Mann had been classifying. This problem was far more puzzling than the merely extra-long lifetimes that Gell-Mann’s theory explained. It had to do with one of the most common, and commonsense, symmetries of nature that characterize the physical world.

  At some point in our childhood we all learn to tell the difference between right and left. It’s not easy, and Feynman used to tell his students that sometimes even he had to look at the mole on his left hand in order to be sure. That is because the distinction between left and right is arbitrary. If we called everything that we call left, right, and right, left, then what would change except the names? The real question is whether “left” and “right” are indeed merely human semantic constructs, or whether nature has a more fundamental way to distinguish them.

  Think of it another way, along the lines that Feynman once described. If we were in communication with aliens on another planet, how would we tell them the difference between right and left? Well, if their planet had a magnetic field like earth’s and orbited their star in the same direction as the earth does, we could have them take a bar magnet and align its north pole to point north, and then left could be defined as the direction in which the sun sets. But they would say, “Yes, we have a bar magnet, but which end is north?”

  We could go on and on like this and convince ourselves that terms like left and north are arbitrary conventions we have invented, but that they have no ultimate meaning in nature. Or do they? Noether’s theorem tells us that if nature doesn’t change if we reverse right and left, there should be a quantity, which we call parity, that is conserved, that doesn’t change no matter what physical processes are taking place.

  This doesn’t mean all individual objects are left-right symmetric, however. Look at yourself in the mirror. Your hair may be parted one way, or your left leg may be slightly longer than your right one. Your mirror doppelganger, however, has its hair parted in the other direction, and its right leg is longer. The things that remain identical, like a sphere, for example, when we flip right and left are said to have even parity, and those that change are said to have odd parity. What Noether’s theorem tells us is that both even-parity and odd-parity objects would nevertheless obey the same laws of physics in a mirror world. The associated conservation law tells us that even-parity objects do not spontaneously turn into odd-parity objects. If they did, we could use this spontaneous transformation to define an absolute left or right.

  Elementary particles can be classified by their parity properties, usually associated with the way they interact with other particles. Some have even-parity interactions and some have odd-parity interactions. Noether’s theorem tells us that a single even-parity particle cannot decay into a single odd-parity particle plus an even-parity particle. It can, however, decay into two odd-parity particles because if one particle heads out to the left and one to the right, if the identity of the particles is also interchanged under such a parity switch at the same time as the directions of the particles are flipped as a result of interchanging left and right, then the outgoing configuration would look identical afterward—that is, it would have even parity, as the original particle had.

  So far so good. However, physicists discovered that the decay of strange mesons called K-mesons—whose long lifetimes Murray had explained via strangeness—nevertheless was not obeying the rules. Kaons, as they are also called, were observed to decay into lighter particles called pions, but sometimes they would decay into two pions and sometimes into three pions. Since pions have odd parity, a state of two pions has different reflection properties than a state of three pions. But then it would be impossible for a single-type particle to decay into the two different configurations, be
cause that would mean sometimes the initial particle would have even parity and sometimes odd parity.

  The simple solution was that there must be two different types of kaons, with one type having even parity and decaying into two pions and one type having odd parity and decaying into three pions. The problem was that these two types of kaons, which physicists had dubbed the tau and the theta, otherwise looked completely identical. They had the same mass and the same lifetime. Why should nature produce two such identical but different particles? Various exotic new symmetries might be invented that would give them the same mass, and Gell-Mann and others had been pondering such possibilities, but to also produce the same lifetime seemed impossible, because the generic quantum probability to decay into three particles is much less than that to decay into two particles, all other things being equal.

  This was the situation in the spring of 1956 when Feynman and Gell-Mann began to work together at Caltech, and both attended the major particle physics conference of its time, called the Rochester Conference, which was then still being held in Rochester, New York. There they heard about compelling new data that once again made the tau and theta look like identical twins.

  The situation became so difficult to justify that some physicists privately began to wonder whether the tau and theta were distinct. At the conference, Feynman was rooming with a young experimental physicist, Martin Block. Records indicate that in the Saturday session near the end of the meeting, Feynman got up and raised a question for the experts that he attributed to Block, that perhaps the two particles might actually be the same, and that the weak interactions might not respect parity—that nature might, at some level, distinguish right from left.

  Murray Gell-Mann was later described as having teased Feynman mercilessly afterward for not having had the courage to ask the question in his own name, so I contacted my old friend Marty Block and asked what really transpired. He confirmed that he had asked Feynman why parity couldn’t be violated by the weak interaction. Feynman had been tempted to call him an idiot until he realized that he could not come up with an answer, and he and Marty debated the issue each night during the conference until Feynman suggested that Marty bring up this possibility at the meeting. Marty said no one would listen to him, and asked Feynman to raise it for him. Feynman ran the idea past Gell-Mann to see if he knew of any obvious reasons why this could not be possible, and he didn’t. So Feynman, in his traditional way, was giving credit where credit was due, rather than avoiding ridicule by raising an outrageous and potentially obviously wrong possibility.

  Feynman’s question received a response from a young theorist, Chen Ning (Frank) Yang, who, according to the official report, answered that he and his colleague Tsung-Dao Lee had been looking into this issue but had not reached any conclusions. (Block told me that the report was incorrect, that he remembered that Yang had responded that there was no evidence for such violation.)

  When Feynman and Gell-Mann had discussed Block’s question at the meeting, they realized they couldn’t come up with a good empirical reason why the breaking of parity symmetry in weak kaon decays would be impossible. If the weak interaction violated parity, where else might it show up in particle physics? The weak interaction itself was not well understood. As mentioned earlier, Fermi had come up with a simple model of the prototypical weak decay, the decay of the neutron into a proton, called beta decay, but no unified picture of the different known weak decays had yet emerged.

  Even as the two theoretical giants Feynman and Gell-Mann mulled over this strange possibility, and various other physicists at the conference felt strongly enough about the question to make bets, the two younger physicists Lee and Yang, both former colleagues of Gell-Mann’s from Chicago, had the courage and intellectual temerity to return home and seriously explore all of the data then available to see if parity violation in the weak interaction could be ruled out. They discovered there were no experiments that could definitively answer the question. More importantly, they proposed an experiment involving beta decay itself. If parity were violated, and neutrinos were polarized so that they were made to spin in a certain direction, then parity violation would imply that electrons, one of the products of their decay, would preferentially be produced in one hemisphere compared to the other. They wrote a beautiful paper on their speculation that was published in June of 1956.

  The possibility seemed crazy, but it was worth a try. They convinced their colleague at Columbia University, Chien-Shiung Wu, an expert in beta decay, to back out of a European vacation with her husband in order to perform an experiment on the decay of neutrons in cobalt 60. This was a different era from today, where the time between theoretical proposals in particle physics and their experimental verification can be decades apart. Within six months, Wu had tentative evidence not only that the electrons were emerging from her apparatus asymmetrically, but also that the asymmetry seemed about as large as was physically possible.

  This convinced another Columbia experimental physicist, the future Nobel laureate Leon Lederman, whom Lee had been trying to get to conduct a similar experiment on pion decay, to perform the experiment. Again, with a speed that is almost unfathomable to physicists who grew up a generation later, Lederman and his colleague Richard Garwin reconfigured their apparatus on a Friday, after a lunchtime faculty session to discuss the possibility, and by Monday they had the result, within a day of the completion of Wu’s experiment. Parity was indeed violated maximally. God, to paraphrase the baseball analogy expressed by the doubting theorist Wolfgang Pauli, was not a “weak left-hander,” but a strong one.

  The result produced a sensation. That so sacrosanct a symmetry of the world around us is not respected at a fundamental level by one of the four known forces of nature sent tidal waves throughout the physics world, with reverberations felt in all of the media as well. (Lee and Yang apparently had one of the first modern physics press conferences at Columbia to announce the experimental confirmation of their proposal.) In one of the quickest such developments in the history of physics, Lee and Yang shared the Nobel Prize in 1957 for their suggestion, made just one year earlier.

  WHY DID FEYNMAN not follow up on his question at the Rochester meeting? Once again, he had found himself close to the answer to a vital question in physics but had not pushed through to the conclusion. This tendency might reflect a character trait that would come back to haunt him: He didn’t want to follow other physicists’ leads. If the community was fixated on a problem, he wanted to steer clear and keep his mind open so he could work out puzzles as he liked to, from start to finish. Moreover, he hated to read the physics literature, something that was essential to the work that Lee and Yang had done.

  But it might also be that Feynman had bigger fish to fry, or so he thought. Recognizing that the weak interactions violated parity was one thing, but it was not the same as coming up with a new theory of nature. This was something that Feynman yearned to do. He had long felt his work on QED was merely a technical kluge, not really fundamental, not like the equation his hero Dirac developed.

  What attracted Feynman more was the possibility of coming up with a theory to unify all the different observed weak interaction phenomena, involving decays of very different types of particles, like neutrons, pions, and kaons, into a single picture. Fermi had produced a rough but beautiful, if ad hoc, model for beta decay, but the data on the weak decays of different particles was inconclusive and eluded unification. The central questions thus became: Was there a single unified weak interaction describing all of these processes, and if so, what was its form?

  Feynman’s sister, who was also a physicist, berated him for his timidity regarding parity nonconservation. She knew that just a little work, combined with the diligence to write it up, would have made all the difference. She urged him to not be so intellectually laissez-faire regarding his ideas about the weak interaction.

  Nowadays, the violation of reflection symmetry in the weak inte
raction is most easily displayed by a simple statement: the exotic particles called neutrinos, the products of beta decay so-named by Fermi, which are the only known particles to interact solely by the weak interaction, are “left-handed.” As I have described, most elementary particles carry angular momentum and behave as if they are spinning. Objects that spin one way would be observed to be spinning in the opposite direction if viewed in a mirror. All other known particles can be measured to behave as if they were spinning either clockwise or counterclockwise, depending on the experiment. However, neutrinos, the elusive weakly interacting particles, maximally violate mirror symmetry, at least as far as we know. They only spin one way.

  Tsung-Dao Lee was actually alluding to this implication when he was describing his work with Yang at the 1957 Rochester Conference, and it grabbed Feynman’s attention. Back in the early days, when Feynman was trying to first reproduce Dirac’s equation as an undergraduate with Ted Welton, he had missed the boat, coming up with a simpler equation that didn’t properly incorporate the spin of the electron. Dirac’s equation had four different components, to describe the two different spin configurations of electrons and of their antiparticles positrons.

  Feynman now realized that by using his path-integral formalism, he could naturally come up with an equation that looked like Dirac’s but was simpler. It had only two components. This excited him. He recognized that if history had been different, his equation could have been discovered first, and Dirac’s equation derived from it later. Of course, his equation ended up having the same consequences as Dirac’s—his equation described one spin state of the electron and one for its antiparticle, and there was another similar equation that described the other two states—so it was not really new. But it did offer a new possibility. For neutrinos, which appeared to have only one spin state, his equation would, he felt, be more natural.