The loudspeaker announced that the machine was about to be turned on and that all experimenters must leave the accelerator room (or get fried). We scrambled up the steep iron staircase and across the parking lot to the lab building, where the cables from the detectors were connected to electronic racks containing circuitry, scalers, oscilloscopes. Garwin had gone home hours ago, and I sent Marcel to get some dinner while I started a checkout procedure on the electronic signals arriving from the detectors. A large, thick lab notebook was used to note all relevant information. It was gaily embellished with graffiti—"Oh shit!" "Who the hell forgot to turn off the coffee pot?" "Your wife called"—as well as the necessary record of things to do, things done, conditions of the circuits. ("Watch scaler No. 3. It tends to spark and miss counts.")
By 7:15 P.M. the proton intensity was up to standard and the pion-producing target was moved remotely into position. Instantly, the scalers began registering arriving particles. I looked at the crucial row of scalers that would register the number of electrons emitted at various intervals after the muons had stopped. The numbers were still very small: 6, 13, 8...
Garwin arrived at about 9:30 P.M. I decided to get some sleep and relieve him at 6 the next morning. I drove home very slowly. I had been up for about twenty hours and was too tired to eat. It seemed as if i had just hit the pillow when the phone rang. The clock said 3 A.M. It was Garwin. "You'd better come in. We've done it!"
At 3:25 I parked at the lab and dashed in. Garwin had pasted paper strips of the scaler read-outs in the book. The numbers were devastatingly clear. More than twice as many electrons were emitted at zero degrees as at 180 degrees. Nature could tell the difference between a right-handed spin and a left-handed spin. By now the machine had come up to its best intensity, and the scaler registers were changing rapidly. The scaler corresponding to zero degrees was reading 2,560, the scaler corresponding to 180 degrees was reading 1,222. On a purely statistical basis this was overwhelming. The in-between scalers seemed satisfactorily in between. The implications of parity violation on this level were so vast ... I looked at Dick. My breathing was becoming difficult, my palms were wet, my heartbeat accelerated, I felt lightheaded—many (not all!) of the symptoms of sexual arousal. This was big stuff. I began to make a checklist: what elements could fail in such a way as to simulate the result we were seeing? There were so many possibilities. We spent an hour, for example, checking the circuits used to count the electrons. No problem. How else could we test our conclusions?
Tuesday, 4:30 A.M. We asked the operator to shut down the beam. We ran down and physically rotated the electron telescope through 90 degrees. If we knew what we were doing, the pattern should shift by a time interval corresponding to 90 degrees. Bingo! The pattern shifted as we had predicted!
6 A.M. I picked up the telephone and called T. D. Lee. He answered after one ring. "T. D., we've been looking at the pi-mu-e chain and we now have a twenty-standard-deviation signal. The law of parity is dead." T. D.'s reaction squirted through the telephone. He asked rapid-fire questions: "What energy electrons? How did the asymmetry vary with electron energy? Was the muon spinning parallel to the direction of arrival?" To some questions we had answers. Others came later in the day. Garwin began drawing graphs and entering the scaler readings. I made another list of things we had to do. At seven we started getting calls from Columbia colleagues who had heard. Garwin faded by eight. Marcel (temporarily forgotten!) arrived. By nine the room was crowded with colleagues, technicians, secretaries trying to find out what was going on.
It was hard to keep the experiment going. My breathing and sweating symptoms returned. We were the repository of new and profound information about the world. Physics was changed. And the violation of parity had given us a powerful new tool: polarized muons that were responsive to magnetic fields and whose spins could be tracked through the electron decay.
The phone calls from Chicago, California, and Europe came over the next three or four hours. People with particle accelerators in Chicago, Berkeley, Liverpool, Geneva, and Moscow swarmed to their machines like pilots rushing to their wartime battle stations. We continued the experiment and continued the process of checking our assumptions for a solid week, but we were desperately anxious to publish. We took data, in one form or another, twenty-four hours a day, six days a week, for the next six months. Data poured out. Other labs soon confirmed our results.
C. S. Wu was of course less than delighted by our clean, unequivocal result. We wanted to publish with her but, to her everlasting credit, she insisted she still needed a week to check her results.
It is difficult to express just how startling the results of this experiment were to the physics community. We had challenged—in fact, destroyed—a cherished belief, that nature exhibits mirror symmetry. In later years, as we shall see, other symmetries were also disproved. Even so, the experiment shook up many theorists, including Wolfgang Pauli, who made the famous statement "I cannot believe God is a weak left-hander." He didn't mean that God should be right-handed, but that She should be ambidextrous.
The annual meeting of the American Physical Society drew 2,000 physicists to the ballroom of the Hotel Paramount in New York on February 6,1957. People hung from rafters. Front-page articles in all the major newspapers heralded the result. The New York Times published our press release verbatim, with pictures of particles and mirrors. But none of this matched the 3 A.M. feeling of mystical euphoria when two physicists came to know a new and profound truth.
7. A-TOM!
Yesterday three scientists won the Nobel Prize for finding the smallest object in the universe. It turns out that it's the steak at Denny's.
—Jay Leno
THE 1950S AND '60S were great years for science in America. Compared to the much tougher 1990s, in the '50s anyone with a good idea and a lot of determination, it seemed, could get his idea funded. Perhaps this is as good a criterion for healthy science as any. The nation is still benefiting from the science that got done in these decades.
The flood of subnuclear structures opened up by the particle accelerator was as surprising as the heavenly objects revealed by Galileo's telescope. As in the Galilean revolution, mankind acquired new, previously unsuspected knowledge about the world. That this knowledge concerned inner rather than outer space made it no less profound. Pasteur's discovery of microbes and the invisible biological universe of microorganisms is an analogous event. The bizarre guess of our hero Democritus ("Guess?!" I hear him screeching. "Guess?!?!") was no longer even remarked upon. That there was a particle so small that it eluded the human eye was not a matter for further debate. Clearly, the search for the smallest particle called for extensions of the human eye: glasses, microscopes, now particle accelerators zooming down in quest of the true a-tom. And what we saw were hadrons, lots of hadrons, those Greek-letter particles created in the strong collisions induced by accelerator beams.
This is not to say that the proliferation of hadrons was an unalloyed pleasure. It did make for full employment, spreading the wealth so that the discoverers of new particles now made up a nonexclusive club. Want to find a brand-new hadron? Just wait for the next accelerator run. At a conference on the history of physics at Fermilab in 1986, Paul Dirac recounted how difficult it was for him to accept the consequences of his equation—the existence of a new particle, the positron, which Carl Anderson discovered a few years later. In 1927 it was counter to the ethos of physics to think so radically. When Victor Weisskopf remarked from the audience that in 1922 Einstein had speculated about the existence of a positive electron, Dirac waved his hand dismissively: "He was lucky." In 1930 Wolfgang Pauli had agonized before predicting the existence of the neutrino. He finally embraced the particle with great reluctance and only to favor a lesser evil, since nothing less was at stake than the principle of conservation of energy. Either the neutrino had to exist, or the conservation of energy had to go. This conservatism toward the introduction of new particles didn't last. As Professor Bob Dylan commented, t
he times they were a-changin'. Pioneer of the change in philosophy was theorist Hideki Yukawa, who began the process of freely postulating new particles to explain new phenomena.
In the 1950s and early '60s theorists were busy classifying the hundreds of hadrons, seeking patterns and meaning in this new layer of matter and hounding their experimental colleagues for more data. These hundreds of hadrons were exciting, but they were a headache as well. Where was the simplicity we had been seeking since the days of Thales, Empedocles, and Democritus? There was an unmanageable zoo of these entities, and we were beginning to fear that their legions were infinite.
In this chapter, we shall see how the dream of Democritus, Boscovich, and others was finally realized. We will chronicle the construction of the standard model, which contains all the elementary particles needed to make all the matter in the universe, past or present, plus the forces that act upon these particles. In some ways it is more complex than Democritus's model, in which each form of matter had its own indivisible a-tom, and the a-toms joined together because of their complementary shapes. In the standard model, the matter particles bind to each other via three different forces carried by yet more particles. All of these particles interact with each other in an intricate kind of dance, which can be described mathematically but cannot be visualized. Yet in some ways the standard model is simpler than Democritus ever imagined. We don't need a separate a-tom for feta cheese, one for kneecaps, another for broccoli. There are only a small number of a-toms. Combine them in various ways, and you can make anything. We've already met three of these elementary particles, the electron, the muon, and the neutrino. Soon we'll meet the others and see how they all fit together.
This is a triumphant chapter, for we come to the end of the road in our search for a basic building block. In the fifties and early sixties, however, we were not feeling so sanguine about finally answering Democritus's riddle. Because of the hundred-hadron headache, the prospect of identifying a few elementary particles seemed pretty dim. Physicists were making much better progress in describing the forces of nature. Four were clearly recognized: gravity, the electromagnetic force, the strong force, and the weak force. Gravity was the domain of astrophysics, for it was too feeble to deal with in accelerator labs. This omission would come to haunt us later. But we were getting the other three forces under control.
The Electric Force
The 1940s had seen the triumph of a quantum theory of the electromagnetic force. The work of Paul Dirac in 1927 successfully blended quantum theory and special relativity in his theory of the electron. However the marriage of quantum theory and electromagnetism, the electromagnetic force, was a stormy one, filled with stubborn problems.
The struggle to unite the two theories was known informally as the War Against Infinities, and by the mid-1940s it involved infinity on one side and, on the other, many of the brightest luminaries in physics: Pauli, Weisskopf, Heisenberg, Hans Bethe, and Dirac, as well as some new rising stars—Richard Feynman at Cornell, Julian Schwinger at Harvard, Freeman Dyson at Princeton, and Sinitiro Tomonaga in Japan. The infinities came from this: simply described, when one calculated the value of certain properties of the electron, the answer according to the new relativistic quantum theories, came out "infinite." Not just big, infinite.
One way to visualize the mathematical quantity called infinity is to think of the total number of integers—and then add one more. There is always one more. Another way, one that was more likely to appear in the calculations of these brilliant but deeply unhappy theorists, is to evaluate a fraction in which the denominator becomes zero. Most pocket calculators will politely inform you—usually with a series of EEEEEEs—that you have done something stupid. Earlier relay-driven mechanical calculators would go into a grinding cacophony that usually terminated in a dense puff of smoke. Theorists saw infinities as a sign that something was deeply wrong with the way the marriage between electromagnetism and quantum theory was being consummated—a metaphor we probably should not pursue, much as we are tempted. In any case, Feynman, Schwinger, and Tomonaga, working separately, achieved victory of a sort in the late 1940s. They finally overcame the inability to calculate the properties of charged particles such as the electron.
A major stimulus to this theoretical breakthrough came from an experiment carried out at Columbia by one of my teachers, Willis Lamb. In the early postwar years, Lamb taught most of the advanced courses and worked on electromagnetic theory. He also designed and carried out, using the wartime radar technology developed at Columbia, a brilliantly precise experiment on the properties of selected energy levels in the hydrogen atom. Lamb's data were to provide a test of some of the most subtle pieces of the newly minted quantum electromagnetic theory, which his experiment served to motivate. I'll skip the details of Lamb's experiment, but I want to emphasize that an experiment was seminal to the exciting creation of a workable theory of the electric force.
What emerged from the theorists was something called "renormalized quantum electrodynamics." Quantum electrodynamics, or QED, enabled theorists to calculate the properties of the electron, or its heavier brother the muon, to ten significant figures beyond the decimal point.
QED was a field theory, and thus it gave us a physical picture of how a force is transmitted between two matter particles, say, two electrons. Newton had problems with the idea of action-at-a-distance, as did Maxwell. What is the mechanism? One of the oh-so-clever ancients, a pal of Democritus's, no doubt, discovered the influence of the moon on the earth's tides and agonized over how that influence could manifest itself through the intervening void. In QED, the field is quantized, that is, broken down into quanta—more particles. These are not matter particles, however. They are particles of the field. They transmit the force by traveling, at the speed of light, between the two interacting matter particles. These are messenger partides, which in QED are called photons. Other forces have their own distinct messengers. Messenger particles are the way we visualize forces.
VIRTUAL PARTICLES
Before we go on, I should explain that there are two manifestations of particles: real and virtual. Real particles can travel from point A to point B. They conserve energy. They make clicks in Geiger counters. Virtual particles do none of these things, as I mentioned in Chapter 6. Messenger particles—force carriers—can be real particles, but more frequently they appear in the theory as virtual particles, so the two terms are often synonymous. It is virtual particles that carry the force message from particle to particle. If there is plenty of energy around, an electron can emit a real photon, which produces a real click in a real Geiger counter. A virtual particle is a logical construct that stems from the permissiveness of quantum physics. According to quantum rules, particles can be created by borrowing the necessary energy. The duration of the loan is governed by Heisenberg's rules, which state that the borrowed energy times the duration of the loan must be greater than Planck's constant divided by twice pi. The equation looks like this: ΔEΔt is greater than h/2π. This means that the larger the amount of energy borrowed, the shorter the time the virtual particle can exist to enjoy it.
In this view, so-called empty space can be awash with these ghostly objects: virtual photons, virtual electrons and positrons, quarks and antiquarks, even (with oh god how small a probability) virtual golf balls and anti-golf balls. In this swirling, dynamic vacuum, a real particle's properties are modified. Fortunately for sanity and progress, the modifications are very small. Nevertheless, they are measurable, and once this was understood, life became a contest between increasingly precise measurements and ever more patient and determined theoretical calculations. For example, think about a real electron. Around the electron, because of its existence, there is a cloud of transient virtual photons. These notify all and sundry that an electron is present, but they also influence the electron's properties. What's more, a virtual photon can dissolve, very transiently, into an e+ e− pair (a positron and an electron). In a blink of a mosquito's eye, the pair is back toget
her as a photon, but even this evanescent transformation influences the properties of our electron.
In Chapter 5, I wrote the g-value of the electron as calculated theoretically from QED and as measured by inspired experiments. As you may recall, the two figures agreed to eleven places past the decimal. Equally successful was the g-value of the muon. Because the muon is heavier than the electron, it provides an even more incisive test of the concept of messenger particles; the muon's messengers can have higher energy and cause more mischief. The effect is that the field influences the properties of the muon even more strongly. Very abstract stuff, but the agreement between theory and experiment is sensational and indicates the power of the theory.
THE PERSONAL MAGNETISM OF THE MUON
As for the verifying experiment ... On my first sabbatical year (1958–59) I went to CERN in Geneva, using a Ford Fellowship and a Guggenheim Fellowship to supplement my half-salary. CERN was the creation of a twelve-nation European consortium to build and share the expensive facilities required to do high-energy physics. Founded in the late forties, when the rubble of World War II was still warm, this collaboration of former military adversaries became a model for internadonal cooperation in science. There my old sponsor and friend, Gilberto Bernardini, was director of research. My main reason for going was to enjoy Europe, learn to ski, and dabble in this new laboratory nestled on the Swiss-French border just outside of Geneva. Over the next twenty years I spent about four years doing research in this magnificent multilingual facility. Although French, English, Italian, and German were common, the official language of CERN was broken Fortran. Grunts and sign language also worked. I used to contrast CERN and Fermilab as follows: "CERN is a lab of culinary splendor and architectural catastrophe and Fermilab is the other way around." Then I convinced Bob Wilson to hire Gabriel Tortella, the legendary CERN chef and cafeteria manager, as a consultant to Fermilab. CERN and Fermilab are what we like to call cooperative competitors; each loves to hate the other.
The God Particle: If the Universe Is the Answer, What Is the Question? Page 35
