This time-machine application is really a problem for the astros. Under normal circumstances, we particle physicists would be amused and flattered but unconcerned about how accelerators mimic the early universe. In recent years, however we've begun to see the link. Farther back in time, where the energies are considerably higher than 1 TeV—the limit of our present accelerator inventory—lies a secret that we need. This earlier, hotter universe contains a vital clue to the lair of the God Particle.
Accelerator as time machine—the astrophysics connection—is one view to consider. Another connection comes from Robert Wilson, the cowboy accelerator-builder, who wrote:
Familiarly enough, both aesthetic and technical considerations were inextricably combined [in the design of Fermilab]. I even found, emphatically, a strange similarity between the cathedral and the accelerator: The one structure was intended to reach a soaring height in space; the other is intended to reach a comparable height in energy. Certainly the aesthetic appeal of both structures is primarily technical. In the cathedral we see it in the functionality of the ogival arch construction, the thrust and then the counterthrust so vividly and beautifully expressed, so dramatically used. There is a technological aesthetic in the accelerator, too. There is a spirality of the orbits. There is an electrical thrust and a magnetic counterthrust. Both work in an ever upward surge of focus and function until the ultimate expression is achieved, but this time in the energy of a shining beam of particles.
Thus carried away, I looked into cathedral building a bit further. I found a striking similarity between the tight community of cathedral builders and the community of accelerator builders: Both of them were daring innovators, both were fiercely competitive on national lines, but yet both were basically internationalists. I like to compare the great Maître d'Oeuvre, Suger of St. Denis, with Cockcroft of Cambridge; or Sully of Notre-Dame with Lawrence of Berkeley; and Villard de Honnecourt with Budker of Novosibirsk.
To which I can only add that there is this deeper connection: both cathedrals and accelerators are built at great expense as a matter of faith. Both provide spiritual uplift, transcendence, and, prayerfully, revelation. Of course, not all cathedrals worked.
One of the glorious moments in our business is the scene in a crowded control room, where the bosses, on this special day, are at the console, staring at the screens. Everything is in place. The labor of so many scientists and engineers for so many years is now about to hatch as the beam is traced from the hydrogen bottle through the intricate viscera ... It works! Beam! In less time than you can say hooray, the champagne is poured into Styrofoam cups, jubilation and ecstasy written on all faces. In our holy metaphor I see the workmen lowering the last gargoyle into place as priests, bishops, cardinals, and the requisite hunchback stand tensely around the altar to see if it works.
One must consider the aesthetic qualities of an accelerator as well as its GeVs and other technical attributes. Thousands of years hence, archaeologists and anthropologists may judge our culture by our accelerators. After all, they are the largest machines our civilization has ever built. Today we visit Stonehenge or the Great Pyramids, and we marvel first at their beauty and at the technological achievement of building them. But they had a scientific purpose as well; they were crude "observatories" for tracking astronomical bodies. So we must also stand in awe of how ancient cultures were driven to erect grand structures in order to measure the movements of the heavens in an attempt to understand and to live in harmony with the universe. Form and function combined in the pyramids and Stonehenge to allow their creators to seek scientific truths. Accelerators are our pyramids, our Stonehenge.
The third finale has to do with the man Fermilab is named for, Enrico Fermi, one of the most famous physicists of the 1930s, '40s, and '50s. He was Italian by birth, and his work in Rome was marked by brilliant advances in both experiment and theory and by a crowd of exceptional students gathered around him. He was a dedicated and gifted teacher. Awarded the Nobel Prize in 1938, he used the occasion to escape from fascist Italy and settle in the U.S.
His popular fame stems from heading up the team that built the first chain-reacting nuclear pile in Chicago during World War II. At the University of Chicago after the war he again gathered a brilliant group of both theoretical and experimental students. Fermi's students from both his Rome period and his Chicago period dispersed around the world, winning top positions and prizes everywhere. "You can tell a good teacher by how many of his students win Nobel Prizes," goes an ancient Aztec saying.
In 1954 Fermi gave his retiring address as president of the American Physical Society. With a mixture of respect and satire, he predicted that in the near future we would build an accelerator in orbit around the earth, making use of the natural vacuum of space. He also cheerfully noted that it could be built with the combined military budgets of the United States and the USSR. Using supermagnets and my pocket cost estimator, I get 50,000 TeV for a cost of $10 trillion, not including quantity discounts. What better way to return the world to sanity than by beating swords into accelerators?
Interlude C
HOW WE VIOLATED PARITY IN A WEEKEND ... AND DISCOVERED GOD
I cannot believe God is a weak left-hander.
—Wolfgang Pauli
LOOK AT YOURSELF in a mirror. Not too bad, hey? Suppose you raise your right hand, and your image in the mirror also raises its right hand! What? Can't be. You mean left! You'd clearly be in a state of shock if the wrong hand went up. This has never happened with people, as far as we know. But an equivalent act did occur with a fundamental particle called a muon.
Mirror symmetry had been tested in the laboratory over and over again. The scientific name for mirror symmetry is parity conservation. This is the story of an important discovery, and also of how progress oftentimes involves the killing of an exquisite theory by an ugly fact. It all started at lunch on Friday and was over by about 4 A.M. the following Tuesday morning. A very profound conception of how nature behaved turned out to be a (weak) misconception. In a few intense hours of data taking, our understanding of the way the universe is constructed was changed forever. When elegant theories are disproven, disappointment sets in. It appears that nature is clumsier, more ponderous, than we had expected. But our depression is tempered by the faith that when all is known, a deeper beauty will be revealed. And so it was with the downfall of parity in a few days of January 1957 in Irvington-on-Hudson, twenty miles north of New York City.
Physicists love symmetry because it has a mathematical and intuitive beauty. Symmetry in art is exemplified by the Taj Mahal or a Greek temple. In nature, shells, simple animals, and crystals of various kinds exhibit symmetrical patterns of great beauty, as does the almost perfect bilateral symmetry of the human body. The laws of nature contain a rich set of symmetries that for years, at least before January 1957, were thought to be absolute and perfect. They have been immensely useful in our understanding of crystals, large molecules, atoms, and particles.
THE EXPERIMENT IN THE MIRROR
One of these symmetries was called mirror symmetry, or parity conservation, and it asserted that nature—the laws of physics—could not distinguish between events in the real world and those in the mirror.
The mathematically appropriate statement, which I'll give for the record, is that the equations describing the laws of nature do not change when we replace the z-coordinates of all objects with −z. If the z-axis is perpendicular to a mirror, defining a plane, this replacement is exactly what happens to any system when it is reflected in the mirror. For example, if you, or an atom, are 16 units in front of a mirror, the mirror shows the image as 16 units behind the mirror. Replacing the coordinate z with −z creates a mirror image. If, however, the equations are invariant to this replacement (for example, if the coordinate z always appears in the equation as z2), then mirror symmetry is valid and parity is conserved.
If one wall of a lab is a mirror and scientists in the lab are carrying out experiments, then their mirror im
ages will be carrying out mirror images of these experiments. Is there any way of deciding which is the true lab and which is the mirror lab? Could Alice know where she is (in front of or behind the looking glass) by some objective test? Could a committee of distinguished scientists examining a videotape of an experiment tell if it was carried out in the real or the mirror lab? In December of 1956 the unequivocal answer was no. There was no way a panel of experts could prove they were watching the mirror image of the experiments being conducted in the real laboratory. At this point a perceptive innocent might say, "But look, the scientists in this movie all have their buttons on the left side of their coats. It must be the mirror view." "No," the scientists answer, "that is just a custom; nothing in the laws of nature insists that buttons be on the right side. We have to put aside all human affectations and see if anything in our movie is against the laws of physics."
So before January 1957 no such violations had been seen in the mirror-image world. The world and its mirror image were equally valid descriptions of nature. Anything that was happening in the mirror space could in principle and practice be replicated in the laboratory space. Parity was useful. It helped us classify molecular, atomic, and nuclear states. It also saves work. If a perfect human stands, disrobed and half concealed by a vertical screen, by studying the half that you do see, you can pretty much know what is behind the screen. Such is the poetry of parity.
The "downfall of parity," as the events of January 1957 were later described, is a quintessential example of how physicists think, how they adapt to shock, how theory and mathematics bend to the winds of measurement and observation. What is far from typical about this story is the speed and relative simplicity of the discovery.
THE SHANGHAI CAFE
Friday, January 4, 12 noon. Friday was our traditional Chinese-lunch day, and the faculty of the Columbia University Physics Department gathered outside the office of Professor Tsung Dao Lee. Between ten and fifteen physicists trooped down the hill from the 120th Street Pupin Physics Building to the Shanghai Café on 125th and Broadway. The lunches started in 1953, when Lee arrived at Columbia from the University of Chicago with a fairly new Ph.D. and a towering reputation as a theoretical superstar.
What characterized the Friday lunches was uninhibited noisy conversations, sometimes three or four simultaneously, punctuated by the very satisfactory slurping of winter melon soup and the sharing out of the dragon meat phoenix, shrimp balls, sea cucumbers, and other spicy exotica of northern Chinese cuisine, not yet trendy in 1957. Already on the walk down, it was clear that this Friday the theme would be parity and the hot news from our Columbia colleague C. S. Wu, who was conducting an experiment at the Bureau of Standards in Washington.
Before entering into the serious business of lunch discussion, T. D. Lee carried out his weekly chore of composing the lunch menu on a small pad offered by the respectful waiter-manager. T. D. composes a Chinese menu in the grand manner. It is an art form. He glances at the menu, at his pad, fires a question in Mandarin at the waiter, frowns, poises his pencil over the pad, carefully calligraphs a few symbols. Another question, a change in one symbol, a glance at the embossed tin ceiling for divine guidance, and then a flurry of rapid writing. A final review: both hands are poised over the pad, one with fingers outstretched, conveying the blessings of the pope on the assembled throng, the other holding the stub of a pencil. Is it all there? The yin and the yang, the color, texture, and flavor in proper balance? Pad and pencil are handed to the waiter, and T. D. plunges into the conversation.
"Wu telephoned and said her preliminary data indicated a huge effect!" he said excitedly.
***
Let's return to the laboratory (the real world as She made it) with one wall a mirror. Our normal experience is that whatever we hold up to the mirror, whatever experiments we do in the lab—scattering, production of particles, gravity experiments like Galileo's—all the mirror-lab reflections will conform to the same laws of nature that govern in the lab. Let's see how a violation of parity would show up. The simplest objective test of handedness, one we could communicate to inhabitants of the planet Twilo, employs a right-handed machine screw. Facing the slotted end, turn the screw "clockwise." If the screw advances into a block of wood, it is defined as right-handed. Obviously the mirror view shows a left-handed screw because the mirror guy is turning it counterclockwise, but it still advances. Now suppose we live in a world so curious (some Star Trek universe) that it is impossible—against the laws of physics—to make a left-handed screw. Mirror symmetry would break down; the mirror image of a right-handed screw could not exist; and parity would be violated.
This is the lead-in to how Lee and his Princeton colleague Chen Ning Yang proposed to examine the validity of the law for weak-force processes. We need the equivalent of a right-handed (or left-handed) particle. Like the machine screw, we need to combine a rotation and a direction of motion. Consider a spinning particle—call it a muon. Picture it as a cylinder spinning around its axis. We have rotation. Since the ends of the cylinder-muon are identical, we cannot say whether it is spinning clockwise or counterclockwise. To see this, place the cylinder between you and your favorite antagonist. While you swear it is rotating to the right, clockwise, she insists that it is rotating to the left. And there is no way to resolve the dispute. This is a parity-conserving situation.
The genius of Lee and Yang was to bring in the weak force (which they wanted to examine) by watching the spinning particle decay. One decay product of the muon is an electron. Suppose nature dictates that the electron comes off only one end of the cylinder. This gives us a direction. And we can now determine the sense of rotation—clockwise or counterclockwise—because one end is defined (the electron comes off here). This end plays the role of the point of the machine screw. If the sense of spin rotation relative to the electron is right-handed, like the sense of the machine screw relative to its point, we have defined a right-handed muon. Now if these particles always decay in such a way as to define right-handedness, we have a particle process that violates mirror symmetry. This is seen if we align the spin axis of the muon parallel to our mirror. The mirror image is a left-handed muon— which doesn't exist.
***
The rumors about Wu had begun over the Christmas break, but the Friday after New Year was the first gathering of the Physics Department since the holidays. In 1957 Chien Shiung Wu, like me a professor of physics at Columbia, was quite a well-established experimental scientist. Her specialty was the radioactive decay of nuclei. She was tough on her students and postdocs, exceedingly energetic, careful in evaluating her results, and much appreciated for the high quality of the data she published. Her students (behind her back) called her Generalissimo Mme. Chiang Kai-shek.
When Lee and Yang challenged the validity of parity conservation in the summer of 1956, Wu went into action almost immediately. She selected as the object of her study the radioactive nucleus of cobalt-60, which is unstable. The cobalt-60 nucleus changes spontaneously into a nucleus of nickel, a neutrino, and a positive electron (a positron). What one "sees" is that the cobalt nucleus suddenly shoots off a positive electron. This form of radioactivity is known as beta decay, because the electrons, whether negative or positive, emitted during the process were originally called beta particles. Why does this happen? Physicists call it a weak interaction, and think of a force operating in nature that generates these reactions. Forces not only push and pull, attract and repel, but are also capable of generating changes of species, such as the process of cobalt changing to nickel and emitting leptons. Since the 1930s a large number of reactions have been attributed to the weak force. The great Italian-American Enrico Fermi was the first to put the weak force into a mathematical form, enabling him to predict many details of reactions such as that which occurs with cobalt-60.
Lee and Yang, in their 1956 paper called "The Question of Parity Conservation in the Weak Force," selected a number of reactions and examined the experimental implications of the possibility t
hat parity—mirror symmetry—was not respected by the weak force. They were interested in the directions in which the emerging electron is ejected from a spinning nucleus. If the electron favored one direction over another, that would be like dressing the cobalt nuclei in buttoned shirts. One could tell which was the real experiment, which was a mirror image.
What is it that differentiates a great idea from a routine piece of scientific work? Analogous questions can be asked about a poem, a painting, a piece of music—in fact, gasp and choke, even a legal brief. In the case of the arts, it is the test of time that ultimately decides. In science, experiment determines whether an idea is "right." If it is brilliant, a new area of research is opened, a host of new questions are generated, and a large number of old questions are put to bed.
T. D. Lee's mind worked in subtle ways. In ordering a lunch or in commenting on some old Chinese pottery or on the abilities of a student, his remarks all had hard edges, like a cut precious stone. In Lee and Yang's parity paper (I didn't know Yang that well), this crystalline idea had many sharp sides. To question a well-established law of nature takes a lot of Chinese chutzpah. Lee and Yang realized that all of the vast amount of data that had led to the "well-established" parity law was irrelevant to that piece of nature that caused radioactive decay, the weak force. This was another brilliant, sharp edge: here, for the first time to my knowledge, the different forces of nature were permitted to have different conservation laws.
Lee and Yang rolled up their sleeves, poured perspiration on their inspiration, and examined a large number of radioactive decay reactions that represented likely candidates for a test of mirror symmetry. Their paper provided laboriously detailed analyses of likely reactions so dumb experimentalists could test the validity of mirror symmetry. Wu devised a version of one of these, using the cobalt reaction. The key to her approach was to make sure that the cobalt nuclei—or at least a very good fraction of them—were spinning in the same sense. This, Wu argued, could be ensured by running the cobalt-60 source at very low temperatures. Wu's experiment was extremely elaborate, requiring hard-to-find cryogenic apparatus. This led her to the Bureau of Standards, where the technique of spin alignment was well developed.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 33
