The Greatest Story Ever Told—So Far

by Lawrence M. Krauss

  If an elementary particle decays into two other particles, the wave function describing the “parity” of the final state (i.e., whether the wave function changes sign or not under left-right interchange of the particles) allows us then to assign a quantity we can call parity to the initial particle. In quantum mechanics if the force that governs the decay is blind to left and right, then the decay will not change the parity of the quantum state of the system.

  If the wave function of the system is antisymmetric under interchange of the particles after the decay, then the system has “negative” parity. In this case the wave function describing the initial quantum state of the decaying particle must also have negative parity (i.e., it would change sign if left and right were interchanged).

  Now, pions, the particles discovered by Powell and hypothesized by Yukawa, have negative parity, so that the wave function that describes the quantum state of their mirror image would change sign compared to the original wave function. The distinction between positive and negative parity is kind of like considering first a nice spherical ball, which looks identical when reflected in the mirror, and hence has positive parity:

  Versus, say, your hand, which changes character (from left to right) when reflected in a mirror and could therefore be said to have negative parity:

  These somewhat abstract considerations made the observed data on the decays of the new particles that Powell discovered perplexing. Because a pion has negative parity, two pions would have positive parity, since (−1)2 = 1. A system of three pions, however, would, by the same consideration, have negative parity, since (−1)3 = −1. Therefore if parity doesn’t change when a particle decays, a single original particle cannot decay into two different final states of different parity.

  If the force responsible for the decay behaved like all the other known forces at the time, such as electromagnetism or gravity, it would be blind to parity (it would not distinguish between right and left), so it shouldn’t change the original parity of the system after the decay, just as shining a light on your right hand will not cause it to look like your left hand.

  Since it seemed impossible for a single type of particle to decay sometimes into two, and sometimes into three, pions, the solution seemed simple. There must be two different new elementary particles, with opposite parity properties. Powell dubbed these the tau particle and theta particle—one of which could decay into two pions, and one into three pions.

  Observations suggested that the two particles had precisely the same masses and lifetimes, which was a bit strange, but Lee and Yang proposed that this might be a general property for various elementary particles, which they suggested might come in pairs with opposite parity. They called this idea “parity doubling.”

  Such was the situation in the spring of 1956 when the International Conference on High Energy Physics, held every year at the University of Rochester, took place. In 1956, the entire community of physicists interested in particle and nuclear physics could fit in a single university lecture hall, and these physicists, including all the major players, tended to gather at this annual meeting. Richard Feynman was sharing a room at the meeting with Marty Block. Being an experimentalist, Block was not as burdened by the possible heresy inherent in the suggestion that some force in nature was not blind to the distinction between left and right, and he asked Feynman if possibly the weak interaction governing the decays Powell observed might distinguish left from right. This would allow a single particle to decay to states of differing parity—meaning the tau and theta could both be the same particle.

  Block didn’t have the temerity to raise this question in the public session, but Feynman did, even though he privately thought this was extremely unlikely. Yang replied that he and Lee had thought about this, but so far nothing had come of the idea. Eugene Wigner, who would later win a Nobel Prize for elucidating the importance of such things as parity in atomic and nuclear physics, was also present, and he too raised the same question about the weak interaction.

  But to the victor go the spoils, and speculating about the possible violation of parity by a new force in nature that might distinguish left from right was different from demonstrating it. A month later Lee and Yang were at a café in New York, and they decided to examine all known experiments involving the weak interaction to see if any of them could dispel the possibility of parity violation. To their great surprise, they realized that not a single one definitively resolved the issue. As Yang later said, “The fact that parity conservation in the weak interaction was believed for so long without experimental support was very startling. But what was more startling was the prospect that a space-time symmetry law which the physicists have learned so well may be violated. This prospect did not appeal to us.”

  To their credit, Lee and Yang proposed a variety of experiments that could test the possibility that the weak interaction distinguished right from left. They suggested considering the beta decay of a neutron in the nucleus of cobalt-60. Because this radioactive nucleus has nonzero spin angular momentum—i.e., it behaves as if it is spinning—it also acts like a little magnet. In an external magnetic field the nuclei will line up in the direction of the field. If the electron emitted when a neutron in the nucleus decays preferentially ends up in one hemisphere instead of another, this would be a sign of parity violation, because in the mirror the electrons would end up in the opposite hemisphere.

  If this was true, then at a fundamental level, nature would be able to distinguish right from left. The human-created distinctions between them (i.e., sinister versus good) would not then be totally artificial. Thus the world in a mirror could be distinguished from the real world, or, as Richard Feynman poetically put it later, we could use this experiment to send a message to tell a Martian what direction is “left”—say, the hemisphere where more electrons were observed to emerge—without drawing a picture.

  At the time, this was viewed as such a long shot that many in the physics community were amused, but no one ran out to perform the experiment. No one, that is, except Lee’s colleague at Columbia the experimentalist Chien-Shiung Wu, known as Madame Wu.

  Even as we bemoan today the paucity of female physicists trained at American institutions, the situation was much worse in 1956. After all, women weren’t even admitted as undergraduates at Ivy League institutions until the late 1960s. Almost thirty years after Wu arrived from China to study at Berkeley in 1936, she noted in a Newsweek article about her, “It is shameful that there are so few women in science. . . . In China there are many, many women in physics. There is a misconception in America that women scientists are all dowdy spinsters. This is the fault of men. In Chinese society, a woman is valued for what she is, and men encourage her to accomplishments—yet she remains eternally feminine.”

  Be that as it may, Wu was an expert in neutron decay and became intrigued by the tantalizing possibility of searching for parity violation in the weak interaction after learning of it from her friends Lee and Yang. She canceled a European vacation with her husband and embarked on an experiment in June, one month after Lee and Yang had first thought of the problem, and by October of that year—the same month Lee and Yang’s paper appeared in print—she and several colleagues had assembled the apparatus necessary to do the experiment. Two days after Christmas of that year they had a result.

  In modern times particle physics experiments might take decades from design to completion, but that was not the case in the 1950s. It was also a time when physicists apparently didn’t bother to take holidays. Despite its being the yuletide, the Friday “Chinese Lunches” organized by Lee continued, and the first Friday after New Year’s Day Lee announced that Wu’s group had discovered that not only was parity violated, but it was violated by the maximum amount possible in the experiment. The result was so surprising that Wu’s group continued their work to ensure they weren’t being fooled by an experimental glitch.

  Meanwhile, Leon Lederman and colleagues Dick Garwin and Marcel Weinrich, also at Columbia, realized th
at they could check the result in their experiments on pion and muon decays at Columbia’s cyclotron. Within a week, both groups, as well as Jerry Friedman and Val Telegdi in Chicago, independently confirmed the result with high confidence, and by mid-January 1957 they submitted their papers to the Physical Review. They changed our picture of the world forever.

  Columbia University called what was probably the first press conference ever announcing a scientific result. Feynman lost a $50 bet, but Wolfgang Pauli was luckier. He had written a letter from Zurich on January 15 to Victor Weisskopf at MIT betting that Wu’s experiment would not show parity violation, not knowing that the experiment already had. Pauli exclaimed in the letter, “I refuse to believe that God is a weak left-hander,” demonstrating an interesting appreciation for baseball as well. Weisskopf, who by then knew of the actual result, was too kind to take the bet.

  Upon hearing the news, Pauli later wrote, “Now that the first shock is over, I begin to collect myself.” It really was a shock. The idea that one of the fundamental forces in nature distinguished between right and left flew in the face of common sense, as well as of much of the basis of modern physics as it was understood then.

  The shock was so great that, for one of the few times in the history of the Nobel Prizes, Nobel’s will was actually carried out properly. His will stipulates that the prize should go to the person or persons in each field whose work that year was the most important. In October of 1957, almost exactly a year from the publication of Lee and Yang’s paper, and only ten months after Wu and Lederman confirmed the notion, the thirty-year-old Lee and the baby-faced thirty-four-year-old Yang shared the Nobel Prize for their proposal. Sadly, Madame Wu, known as the Chinese “Madame Curie,” had to be content with winning the inaugural Wolf Prize in Physics twenty years later.

  Suddenly the weak interaction became more interesting, and also more confusing. Fermi’s theory, which had sufficed up to that point, was roughly modeled after electromagnetism. We can think of the electromagnetism interaction as a force between two different electric currents, each corresponding to the two separate moving electrons that interact with each other. The weak interaction could be thought of in a somewhat similar way, if in one current a neutron, during the interaction, converts into a proton, and in the other current is an outgoing electron and neutrino.

  There are two crucial differences, however. In Fermi’s weak interaction the two different currents interact at a single point rather than at a distance, and the currents in the weak interaction allow particles to change from one type to another as they extend through space.

  While electromagnetic interactions are the same in the mirror as they are in the real world, if parity is violated in the weak interaction, the “currents” involved would have to have a “handedness,” as Pauli alluded, as for example a corkscrew or pair of scissors has, so that their mirror images will not be the same.

  Parity violation in weak interactions would then be like the social rule that we always shake hands with our right hand. In a mirror world, people would always shake with their left hand. Thus, the real world differs from its mirror image. If the currents in the weak interaction had a handedness, then the weak interaction could distinguish right from left and in a mirror world would be different from the force in the world in which we live.

  A great deal of work and confusion resulted as physicists tried to figure out precisely what types of new possible interaction could replace Fermi’s simple current-to-current interaction, in which no apparent handedness could be attributed to the particles involved. Relativity allowed a variety of possible generalizations of Fermi’s interaction, but the results of different experiments led to different, mutually exclusive mathematical forms for the interaction, so it appeared impossible that one universal weak interaction could explain all of them.

  Around the time when the first experimental results on neutron and muon decay had come out suggesting that parity violation was as large as it could be, a young graduate student at the University of Rochester, George Sudarshan, began exploring the confused situation and came up with what eventually was the correct form of a universal interaction that could replace Fermi’s form—something that also required that at least some of the experimental results at the time were wrong.

  The rest of the story is a bit tragic. At the Rochester conference three months after the parity-violation discovery, and a year after Lee and Yang had presented their first thoughts on parity doubling, Sudarshan asked to present his results. But because he was a graduate student, he wasn’t allowed. His supervisor, Robert Marshak, who had suggested the research problem to Sudarshan, was by then preoccupied with another problem in nuclear physics and chose to present a talk on that subject at the meeting. Another faculty member, who was asked to mention Sudarshan’s work, also forgot. So all of the discussion at the meeting on the possible form of the weak interaction ended up leading nowhere.

  Earlier, in 1947, Marshak had been the first to suggest that two different mesons were discovered in Cecil Powell’s experiments—with one being the particle proposed by Yukawa, and the other being the particle now called a muon. Marshak was also the originator of the Rochester conferences and probably felt it would show favoritism to allow his own student to speak. In addition, since Sudarshan’s idea required at least some of the experimental data to be wrong, Marshak may have decided it was premature to present it at the meeting.

  That summer Marshak was working at the RAND Corporation in Los Angeles and invited Sudarshan and another student to join him. The two most renowned particle theorists in the world then, Feynman and Murray Gell-Mann, were at Caltech, and each had become obsessed with unraveling the form of the weak interaction.

  Feynman had missed out on the discovery of parity violation by not following his own line of questioning, but had since realized that his work on quantum electrodynamics could shed light on the weak interaction. He desperately wanted to do this because he felt his work on QED was simply a bit of technical wizardry and far less noble than unearthing the form of the law governing another of the fundamental interactions in nature. But Feynman’s proposal for the form of the weak interaction also appeared to disagree with experiments at the time.

  Over the 1950s, Gell-Mann would produce many of the most important and lasting ideas in particle physics from that time. He was one of two physicists to propose that protons and neutrons were made of more fundamental particles, which he called quarks. He had his own reasons for thinking about parity and the weak interaction. Much of his success was based on focusing on new mathematical symmetries in nature, and he had used these ideas to come up with a new possible form for the weak interaction as well, but again his idea conflicted with experiment.

  While they were in LA, Marshak arranged for Sudarshan to have lunch with Gell-Mann to talk about their ideas. They also met with an eminent experimentalist, Felix Boehm, whose experiments, he said, were now consistent with their ideas. Sudarshan and Marshak learned from Gell-Mann that his ideas were consistent with Sudarshan’s proposal, but that at best Gell-Mann was planning to include the notion in one paragraph of a long general paper on the weak interaction.

  Meanwhile, Marshak and Sudarshan prepared a paper on their idea, and Marshak decided to save it for a presentation at an international conference in Italy in the fall. Learning of the new experimental data from Boehm, Feynman decided—rather excitedly—that his ideas were correct and began to write a paper on the subject. Gell-Mann, who was competitive in the extreme, decided he too should write up a paper since Feynman was writing one. Eventually their department chairman convinced them they needed to write their paper together, which they did, and it became famous. Although the paper had an acknowledgment to Sudarshan and Marshak for discussions, their paper appeared later in the conference proceedings and could not compete for the attention of the community.

  Later, in 1963, Feynman, who tried to be generous with ideas, publicly stated, “The . . . theory that was discovered by Sudarshan and Ma
rshak, publicized by Feynman and Gell-Mann . . .” But it was too little, too late. It would have been hard in the best of times to compete in the limelight with Feynman and Gell-Mann, and Sudarshan had to live for years with the knowledge that the universal form of the weak interaction, which two of the world’s physics heroes had discovered, was first proposed—and with more confidence—by him.

  Sudarshan’s theory, as elucidated beautifully in Feynman and Gell-Mann’s paper, became known as the V-A theory of the weak interaction. The reason for the name is technical and will make more sense in coming chapters, but the fundamental idea is simple, though it sounds both ridiculous and meaningless: the currents in the Fermi theory must be “left-handed.”

  To understand this terminology, recall that in quantum mechanics elementary particles such as electrons, protons, and neutrinos have spin angular momentum—they behave as if they are spinning even though classically a point particle without extension can’t be pictured as spinning. Now, consider the direction of their motion and pretend for a moment the particle is like a top spinning around that axis. Put your right hand out and let your thumb point in the direction of the particle’s motion. Then curl your other fingers around. If they are curling in the same (counterclockwise) direction that the particle/top is spinning about the direction of motion, the particle is said to be right-handed. If you put your left hand out and do the same thing, a left-handed particle would be spinning clockwise to match the direction of your left-curled hand:

  Just as viewing your left hand in a mirror will make it look like a right hand, if you see a spinning arrow in the mirror, its direction of motion will be flipped, so that if the arrow is moving away from you in the real world, it will be moving toward you in the mirror, but the spin will not be flipped. Thus, in the mirror a left-handed particle will turn into a right-handed particle. (And so, if the poor souls in Plato’s cave had had mirrors, they might have felt less strange about the shadows of arrows flipping direction.)