How the Universe Works
The ultimate goal of physics is a theory that explains all matter and energy. Today physicists may be close to achieving that goal, in large part because of the efforts of Sheldon L. Glashow
BY ROBERT P. CREASE AND CHARLES C. MANN
FOR MORE THAN TWO DECADES SHELDON L. Glashow, a physics professor at Harvard University, has sat in a succession of messy offices expending pencils, paper, and malodorous cigars in an attempt to make sense of the dizzying variety of subatomic particles and forces that have been discovered in the past fifty years. It is a convivial task, and one that Glashow claims is a form of play not a jot more useful (for the moment, at least) than painting pictures of soup cans or writing short stories. He spends much of his time arguing with colleagues and friends, cheerfully exhorting graduate students on to greater effort, expounding ideas at seminars: swimming in the great choppy sea of discourse that engenders modern science.
The object of so much talk is to draw within one theoretical picture all matter and energy, from the hottest supernovae to the whirring fragments of the atom. This goal is known as “unification,”and in a sense it defines physics as a whole. Whether they knew it or not, all of the physicists who ever lived spent their days laying down bricks in the road to unification. History is littered with discarded notions of how to put it all together; Einstein looked for such a theory for years, and failed completely. In view of this dismal record, the progress achieved in physics since the Second World War is remarkable. Within that relatively short period scientists have taken greater strides toward unification than all of their predecessors put together. Glashow’s work has been central to that effort.
Glashow is fifty-one years old. A tall man with a wide face and an innocent expression, he has a fondness for practical jokes, funny names, and silly bets. His thick graying hair, which falls in a tangle over his forehead, needs less to be combed than to be subdued; his glasses are often askew; he speaks with a pure New York inflection untouched by his twenty-two years in Harvard Yard. He is an excellent orator, given to bold declarations and amusing metaphors, but for all his eloquence he is unsure of the public’s ability to understand current physics even in outline.
In the fall of 1973 Glashow and one of his colleagues, Howard M. Georgi III, came up with a theory that could account for every known form of matter and for every form of energy except one. At first physicists dismissed as implausible at best what Glashow and Georgi called their “Grand Unified Theory,” but over the years, as the compelling clarity of its picture of nature has become widely understood, many have changed their minds and others have proposed variations. Today Grand Unified Theories (or GUTs) compete for the place in physics that the theory of evolution occupies in biology. Just as evolution explains the descent of species, GUTs attempt to explain the descent of matter and, in large part, of energy.
The most startling implication of the GUTs is that the universe is mortal. According to these theories, over a long, long period—so long that it would take at least thirty-one digits to write the number of years—the essential constituents of matter will fall apart, leaving the cosmos vacant of everything but light. If this prediction is true, then humanity is located on a time line in a universe that is destined to vanish.
Scientists in the United States, Japan, India, and elsewhere are testing the GUTs, and although the results are inconclusive, most theorists are convinced that a roughly correct picture of how the universe works at its most basic level has been drawn. The picture is incomplete, but Glashow and his colleagues believe that their work points the way to a full unification. “The outlines are there,” Glashow says, his gestures nearly concealed behind an oracular veil of cigar smoke. “There are a few things we don’t know yet, but we’ve killed off a whole lot of fundamental questions that nobody knew how to answer for a long time.”In other words, the GUTs are not the end of physics, but they could be the beginning of the end.
Not all scientists agree with that assessment. Skeptics argue that unification will be as elusive, finally, as the Grail. The late Wolfgang Pauli, a fat, ferociously sardonic physicist who sometimes signed his letters “The Wrath of God,” dismissed Einstein’s unsuccessful search for unity with the apothegm “Let no man join together what God hath put asunder.” Julian Schwinger, Glashow’s mentor and a recipient of the 1965 Nobel Prize for Physics, speaks of unification as a “fad"—a “grand illusion” that is not “a theory in the usual sense but an aesthetic and emotional glow about how things would work if only we could compute them.” Schwinger says, “It’s nothing more than another symptom of the urge that afflicts every generation of physicists— the itch to have all the fundamental questions answered in their own lifetimes.”
“The Grand Unified Theories are amusing and beautiful, and have a lot of interesting consequences, but they are still speculative,” Murray Gell-Mann, a professor of physics at the California Institute of Technology, cautions. (An impatient and enormously erudite man, Gell-Mann is himself the recipient of what he refers to as “one of those Swedish prizes.”) He is quick to stress, however, that the GUTs are based on a solid foundation: the great advances achieved by physicists in the 1950s and 1960s. Neither this earlier work nor Glashow’s leading role in it ts as well known to the public as Gell-Mann thinks it should be.
“It’s difficult to come up with anyone else who’s been so terribly important to the whole process and gotten so little recognition for it,” Gell-Mann says. “I suppose it’s partly because of his style. Some people in our field are terribly pompous and can’t stand the thought of having a wild man like Shelly become a spokesman. He will get up at a conference and say, ‘Well, this is a really ugly idea, and I’m sure it’s all wrong, but what the hell. I’m on the program, so here it is.’ And the cigars—I’m sure they hate the cigars.
“ The thing about him, though, is that at any one time there are only a few physicists who have the right taste. Shelly has splendid taste, and much of what he has said about physics has turned out to be fight—that is, when it hasn’t been completely off the wall.”
A HUNDRED YEARS AGO THE SCIENCE Of PHYSICS split into two branches-one experimental and one theoretical—and there has been less communication between them than might be supposed. Experimenters try to measure the properties of matter, and in doing so they strengthen or destroy the models that theorists construct. One group tends to outpace the other. A theoretical insight will leave experimenters with nothing to do but confirm predictions to a dozen decimal places, or an unanticipated experimental discovery will lay waste to years of theoretical work.
At the end of the nineteenth century many physicists were convinced that their picture of the universe was essentially complete. Atoms, which had recently been shown to exist, were thought of as little spheres resembling infinitesimal billiard balls; two forces, electromagnetism and gravity, arranged these balls into molecules and clumped the molecules together into planets. So certain were some theorists of their knowledge that John Trowbridge, the head of the Harvard physics department, urged gifted students to select another field, because, he said, there was nothing of importance left to do in his discipline.
By 1932, the year Glashow was born, experimental discoveries had demolished the complacency of the theorists several times over. The atom had been discovered to consist of a cloud of negatively charged electrons surrounding a nucleus that is composed of positively charged protons and uncharged neutrons. The nucleus had been found to display such peculiar features that theorists were able to explain them only by hypothesizing the existence of two new forces.
For one thing, theorists wondered why the electrical repulsion of the protons within the nucleus didn’t cause it to fly apart. Something must hold it together, they reasoned. Because ordinarily that something would have to be about a hundred times more powerful than electromagnetism, it came to be known as the “strong force.”
Moreover, the nucleus had been discovered to be the locus of radioactivity, one rare form of which proved so resistant to explanation that theorists finally threw up their hands and said that it had to be caused by yet another force, and an extremely feeble one at that. It its effects were almost always swamped by those of the electromagnetic and strong forces, then that would explain why they were not often observed. This force theorists called, logically enough, the “weak force,” and ordinarily it is trillions of times less powerful than electromagnetism.
The behavior of the strong and weak forces appears to be considerably different from that of electromagnetism or gravity. The strong and weak forces are felt only over extremely short distances—on the order of a hundred trillionths of an inch. But within that small realm they play a grand role indeed, causing particles to change many of their properties and even to come into or pass out of existence. (Because “force” sounds like simple pushing and pulling, physicists also refer to forces as “interactions.”)
Today electromagnetism, the strong and weak interactions, and gravity are thought to be the basic forces of the universe, from which all others—muscle power, the pounding of the tides, the sudsing action of detergent, and nuclear explosions, to mention a few—derive. Electromagnetism and the strong and weak interactions directly influence the busy comings and goings of the subatomic world, and therefore are known as elementary-particle forces. The fourth force—gravity—is odd man out. It has almost no effect on individual atoms, but its slow, patient pull keeps the planets in orbit and gathers the stars into galaxies.
The discovery, in the first twenty years of this century, of the unexpected world within the atom engendered a half-dozen branches of physics, with names like quantum mechanics, subatomic physics, cosmic-ray physics, nuclear physics, high-energy physics, and particle physics. In recent years all or part of these subdisciplines have been subsumed into what is known as “quantum field theory.” The spawning ground of the GUTs, quantum field theory is conceivably the most outré—and certainly the most glamorous—area of physics. With its arcane vocabulary and metaphysical resonances, it has attracted considerable public attention, for it promises to put the house of reality in order.
The quantum realm was conceived early in this century by Max Planck and Albert Einstein, who arrived at the conclusion that light consists of tiny units and is not the uninterrupted flow that common sense suggests. Just as a beach, which looks smooth and continuous from afar, from close up can be seen to be composed of grains of sand, so ordinary light is composed of countless “grains” of light. Planck called these particles quanta, from the Latin word for “how much.” And physicists soon realized that all forms of energy, not just light, come in quanta.
The idea of quantized energy took theorists a step away from the conventional view of the world. They went a reluctant step further when experimental data forced them to marry the idea of quanta to a much older notion—that of the field. A field is a region of space in which certain quantities are precisely defined at every point. For instance, the pattern of arcs that iron filings form around a magnet illustrates the magnet’s field. Each point along each arc is subject to a force of a particular strength and direction. The alignment of each filing indicates the direction of the field at that point, and the density of the filings indicates the field’s strength. Fields can be defined for such diverse domains as temperature, sound, and matter. Pauli, Werner Heisenberg, P. A. M. Dirac, and others spent their time during the late 1920s showing that both subatomic particles and energy quanta can best be described as fields and that the hurly-burly of subatomic affairs should be thought of as a set of interactions among various types of fields.
A CORNERSTONE OF QUANTUM FIELD THEORY IS Heisenberg’s famous “Uncertainty Principle,” which he formulated in 1927. Like many scientific laws, the Uncertainty Principle can be stated in several ways, one of which is as follows: When you shine a light to look at something, you bounce light off it. Because a quantum of light—a “photon”—is, in a manner of speaking, the same size as a subatomic particle, when you look at such a particle the two will knock each other around. (To picture this, imagine trying to find out where somebody is by bouncing a bicycle off him.) This interaction between particles and photons, Heisenberg discovered, means that many of a particle’s characteristics—including such important ones as its position, velocity, and energy—can never be specified simultaneously. The act of measuring one interferes with the measurement of the others.
The Uncertainty Principle marked a fundamental break between quantum field theory and classical physics. Many physicists, including Einstein, were dismayed by the idea that anything should be inherently unknowable. Their dismay increased as the full implications of Heisenberg’s work became apparent.
According to the Uncertainty Principle, the energy of a field over any given period cannot be determined exactly—a small but irreducible margin of error is unavoidable. Paradoxically, the less time one spends measuring, the greater this margin of error becomes. It is therefore conceivable that within an interval on the order of trillionths of a trilliconth of a second a field might contain a vast amount of energy that escapes detection.
This logic has unnerving consequences. According to Einstein’s equation E = mc2, mass and energy are two forms of the same thing. Thus the Uncertainty Principle dictates that if any small area can contain undetectable energy, then, according to Einstein’s equation, it can contain undetectable matter. For all physicists know, apparently empty space is filled with particles that pop in and out of existence beyond their reach. And worst of all, the Uncertainty Principle shows that this bedrock uncertainty about the contents of a field is not just an unfortunate defect of the tools available to physicists. Mathematically there is no difference between this margin of error in measurement and an actual random fluctuation in the energy (or matter) measured. Thus, at least in theory, because any space might harbor particles for a short time, it must do so.
The Uncertainty Principle exposed a frightful chaos in the lowest order of matter. The spaces around and within atoms, previously thought to be empty, were now supposed to be filled with a boiling soup of ghostly particles. Small wonder that many theorists, including Einstein, were appalled. From the perspective of quantum field theory, the vacuum contains random eddies in the field of space—time: tidal whirlpools that occasionally hurl up bits of matter only to suck them down again. Since these bits virtually don’t exist, they have been named “virtual particles.”
Far from being a bizarre anomaly, virtual particles are a central feature of quantum field theory. Shortly after the Uncertainty Principle demonstrated their existence, Heisenberg, Pauli, and others postulated that electromagnetism—the phenomena of light, electricity, and magnetism—itself consists of nothing more or less than charged particles batting virtual photons around among themselves, for all the world like a group of jugglers exchanging tenpins. It is an odd picture of the origins of electricity and magnetism, this churning that now attracts, now repels, the charged snippets of matter in atoms. Today physicists think it lovely, but at first they were dismayed.
Those skeptical of this description of electromagnetism were buoyed by the March 1, 1930, issue of the journal Physical Review, which contained an article by a brilliant twenty-five-year-old physicist named J. Robert Oppenheimer. Oppenheimer, who would later become the head of the program to build the atomic bomb, tried to calculate precisely what would happen to a single electron if it were surrounded by a swarm of virtual photons, as field theorists had described. This task involved adding up the total number of ways in which an electron can interact with a quantum of light that appears out of nowhere. Because the number of possible interactions is infinite, the answer proved difficult to calculate. In fact, Oppenheimer couldn’t come up with an answer that made sense. Therefore, he stated that the theory as a whole did not make sense, because one couldn’t get meaningful answers out of it. Oppenheimer believed that he had shown that the theory was not, to use modern terminology, “renormalizable,” which was tantamount to saying that it was worthless.
Throughout the 1930s Oppenheimer, Pauli, Heisenberg himself, and most of the other luminaries of physics were convinced that the infinities were nature’s way of telling scientists that they were barking up the wrong tree, and that quantum field theory would have to be junked in favor of something else. Theorists searched fruitlessly for alternatives until shortly after the war, when Willis Lamb, an experimental physicist who was then at Columbia University, jolted the physics community with the news that he had actually measured the effects of virtual photons on the orbit of electrons—exactly the quantity that Oppenheimer, using pencil and paper, had tried and failed to calculate. Spurred in part by Lamb’s feat, two theorists—Julian Schwinger, then at Harvard, and Richard Feynman, then at Cornell University—realized that if they juggled the equations of quantum field theory around and rephrased certain terms, some of the infinities could be made to cancel out others, leaving a finite residue. (Similar work had been done by the Japanese physicist Shinichiro Tomonaga, but postwar conditions prevented him from playing a role in the final formulation of the theory. For this work Feynman, Schwinger, and Tomonaga shared the Nobel Prize for Physics in 1965.)
The renormalized quantum field theory of electromagnetism is called “quantum electrodynamics,” or QED. The most exact scientific theory ever produced, QED is one of the major intellectual triumphs of this century. The theory encompasses, among other things, all of chemistry, which has as its base the principles governing the electrical forces that bind atoms into molecules.
Aglow with the thrill of discovery, Feynman, Oppenheimer, Schwinger, and others tried to apply the mathematical language of QED to the strong and weak interactions—a crucial first step toward unification. Their efforts ended in failure. They came up with a plausible theory of the weak interaction that could account roughly for most of the known data, but despite calculations that covered acres of blackboard space, they could not renormalize it. They devised a theory of the strong interaction which they could renormalize, but experimental findings disproved it utterly. To make matters worse, a discovery by the British experimenters G. D. Rochester and G. C. Butler called the entire enterprise into question. In 1947 Rochester and Butler came across two new particles exhibiting a trait that had not been seen before. If the methods of QED could truly be extended to the strong and weak interactions, then these particles should have perished as they were born, pulled apart by the strong force that governed their creation. Instead they mysteriously clung to life until the weak interaction finally caused them to fall to pieces. At the age of twenty-three Gell-Mann made a name for himself by suggesting that their unexpected persistence was not a variant of properties already known but in fact a new property that identified a new class of matter. He called this property “strangeness,” and the particles “strange particles.” Strangeness was another wrench thrown into the works of quantum field theory, and for the second time theorists began to wonder if any part of the theory—any at all—was correct.
A T THE EXTREME NORTHERN TIP OF MANHATTAN IS A little side street, Payson Avenue, which runs uphill for a long block beside Inwood Park, facing the grass and trees is a row of small two-story houses, one of which—number 65—was Glashow’s boyhood home. Glashow says, “It was a glorious place to grow up—in a real house, next to a park, in an intellectually active half-Jewish, half-Irish neighborhood.”
Glashow’s father, Lewis Gluchovsky, had come from the town of Bobruvsk, in White Russia, where the family had owned a Turkish bath house (“the Republic baths of the city.”Glashow says). When he arrived here, the immigration authorities changed his name; shortly thereafter Gluchovsky/ Glashow became a plumber in Manhattan. He married another immigrant, Bella Rubin. They had two boys, aged fourteen and eighteen, when the third, Sheldon Lee, was born. One of Glashow’s brothers became a doctor, the other a dentist. Sheldon decided at the age of eight or nine that he wanted to be a scientist.
“It was the beginning of the Second World War. People were interested in those days in identifying airplanes and understanding how bombs are dropped, and I got curious about the ballistic problem. My brother explained to me that if a plane drops a bomb and doesn’t take some evasive action, the bomb is going to blow up right underneath where the plane is going to be. That of course struck me as a very odd fact, because the assumption is that when the plane drops the bomb, the plane is moving forward and the bomb is not.
“I had a little notebook, this black-and-white speckled notebook that kids still have. My brother would write little lessons—two or three or five such lessons-about things he had learned. Not long before, Harold Urey had discovered heavy water, and Lrey was his professor in school, so he was a little turned on about it, and he communicated that to me.”
Lewis Glashow was not entirely pleased by his son s interest in science. “My father told me that science would be all right, but why didn’t I do it in my spare time and go into medicine, like my brothers,”Glashow says. “I differed with him slightly about that. He relented when my brothers came back safely from the war. When I entered high school, he helped me build a chemistry lab in the basement, where I loved to perform long and dangerous experiments.”
Glashow’s passion for science was not directed to physics until he entered the Bronx High School of Science, in 1947. After classes the boys in the Science Fiction Club— there were no girls—would cluster around an old laboratory table littered with hospital-surplus Bunsen burners and the tracery of test-tube racks. The preferred topics of discussion included the contents of a magazine called Astounding Science Fiction and the ideas of L. Ron Hubbard and Immanuel Velikovsky. Two of Glashows best friends in the club were Gerald Feinberg and Steven Weinberg. (Feinberg became a distinguished physicist and has written several respected books. Glashow and Weinberg shared the Nobel Prize for Physics in 1979.) All three contributed to the Science Fiction Club’s magazine ETAOIN SHRDLU; the name is a term from typography.
The founder of Bronx Science, Morris Meister, was a pioneer of fast-track education and a tireless advocate of the notion that if bright, science-oriented students are brought together, certain ill-defined but nonetheless valuable learning processes will occur. As far as Bronx Science is concerned, he seems to have been right. Since the school’s founding, in 1938, it has produced three winners of the Nobel Prize for Physics—more than any other high school in the world ever has, and more in those forty-six years than most countries, including France and Italy.
Considering that statistic, it is surprising to learn that the physics taught to Glashow, Feinberg, and Weinberg was of dubious merit. Feinberg says that the instructor who taught his introductory section “for all we could tell did not believe in the existence of atoms.” Students interested in pursuing advanced physics were offered a choice, between automotive physics, in which an old CurtissWright airplane engine was soberly taken apart and put back together each semester (a good course “for those who realize that the automobile is here to stay,” the 1950 Yearbook states), and radio technology, in which the class assembled shortwave kits of the sort advertised on the back cover of Boys’ Life. Glashow took neither.
Despite the modest course offerings, a kind of underground science took place outside the classroom. Glashow persuaded his mother to buy him college outline texts whenever she dragged him downtown to shop; at lunch Feinberg often led excited conversations about quantum mechanics. Most important was the give-and-take among the club’s members; the boys competed with one another to reach the most complete understanding of the latest scientific discoveries, but they also helped stragglers to catch up. Feinberg says, “After the meetings Shelly and I would spend hours on the phone talking about science. It drove our parents crazy. We were sure we were going to find wonderful things.”
After graduation Feinberg chose to stay in New York City and go to Columbia. Glashow and Weinberg went upstate, to Cornell. Although Cornell has become an internationally important center for physics, in the early 1950s it had the nickname “Moo U" and was known chiefly for agricultural science. Cows grazed on the lawn, and campus lore had it that professors were not allowed to eject dogs that might stray into their classrooms. Although some prominent physicists were at Cornell when Glashow and Weinberg entered, in the class of 1954, their courses were restricted to graduate students. Those who taught undergraduates regarded most forms of collaboration among students as dishonest, and after Bronx Science Glashow, at least, found that attitude hard to take. He says, “There was a style mismatch. Some of us were thrown out of the school for cheating. We would collaborate on their dumb problem sets and they didn’t like that, for example. One of us would show up with six copies of homework—sort of identical homework—and the teacher would simply ask if one of us wanted the 100 percent credit or if we wanted to divide it up.”
In his senior year Glashow was exposed for the first time to quantum field theory. He was the only undergraduate in the class, and the subject baffled him. At exam time the professor, Silvan Schweber, was nonplussed by the fact that although he could give all of the graduate students pass-fail grades, he had to give an undergraduate a numerical one. Glashow recalls: “So he said, ‘How about an 85?' Which was not too low, not too high. I wanted to try for something better, so I said, ‘No, give me an exam.’ Schweber asked me a bunch of questions, and I got every one of them wrong. At the end he said, ‘How about an 85?’
I said, ‘Great!’ ”
Despite his trials, Glashow learned some physics, mostly in the same way he had learned in high school—by shooting the breeze with his friends. He argued with Weinberg, dashed off letters to Feinberg, and dropped down to New York City on weekends to hear lectures. Perhaps the most important material he absorbed came from one Harold V. McIntosh, a graduate student who had decided to pursue physics at Cornell after he had gotten a degree from the Colorado School of Mines. Brainy and eccentric, McIntosh had a little coterie of enthralled undergraduates whom he introduced to his current obsession: the mathematical theory of groups. Glashow says, “He was always coming up to me with problems involving vibrating bedsprings and jiggling ozone atoms and things like that, which he claimed group theory was capable of solving. Actually, what I learned from him was as relevant as any course I took.” (The last Glashow heard, McIntosh was teaching physics in Mexico.)
Glashow chose to do his graduate work at Harvard, where he hoped to write his thesis under Schwinger, one of the heroes of the effort to renormalize QED. At first Glashow found Harvard no better than Cornell. “Nothing is quite as dull as being here,” he wrote to Feinberg. He saw Schwinger rarely and considered the classes he took insufferably tedious.
By Glashow’s second year, however, he was taking courses from the master, an experience that he found both electrifying and peculiar. A charismatic teacher, Schwinger would lecture at lightning speed, covering the blackboard with equations, many of which he expressed in his own idiosyncratic system of symbols. Regardless of the descriptions in the catalogue, Schwinger’s advanced graduate courses were always about whatever Schwinger was working on at the moment. He would sometimes begin class with the news that what he had said the day before was wrong. Glashow loved him.
A shy man, Schwinger had a reputation for aloofness outside of class. His students claimed that all they ever saw of him was the flash of his florid silk tie as he ducked into the men’s room to avoid them. (“He suffers from an uncertainty principle,” Glashow says.) Nevertheless, Glashow, at the beginning of his third year, asked Schwinger to be his thesis adviser. So did eleven other students. To Schwinger’s consternation, he found all of them waiting in his office one day. Thinking to test their abilities individually and avoid dealing with them as a pack, he devised an enormously complicated problem and instructed the multitude to work on it and return, one by one, with their solutions.
“So of course we collaborated,” Glashow says. “The twelve of us came back a few days later, all at the same time, having solved the problem. He was happy with the solution—it was elegant—but not so happy with the complete failure of his scheme to sort out the masses. So he then came up with a different problem for each of us and in that way, against his will, got us all started on our theses.
The question for Glashow was based on a hunch. As early as 1941, even before Schwinger helped to renormalize QED, it had occurred to him that it might be possible to show that the weak force and electromagnetism were aspects of the same phenomenon. Schwinger says, “I mentioned this to Oppenheimer, and he took it very coldly. After all, it was an outrageous speculation.” Although Schwinger turned his attention elsewhere, the idea stayed with him. In 1956, almost casually, he gave the task of checking it out to Glashow.
AT HALF PAST FOUR ON THE AFTERNOON OF DECEMber 10, 1979, Glashow entered the auditorium of the Stockholm Concert House to a flourish of trumpets. Directly behind him were his old classmate Weinberg and a physicist named Abdus Salam, who was wearing full Pakistani formal regalia, including shoes with toes that curled several inches in the air. (Salam teaches at the International Centre for Theoretical Physics, in Trieste, and the Imperial College of Science and Technology, in London.) In a ceremony that took several hours, the three were awarded a Nobel Prize, not for any Grand Unified Theory but for a theory that made the GUT possible—one incorporating the weak and electromagnetic interactions. This “electroweak” theory grew out of the Ph.D. thesis that Glashow began working on in 1956; it came together in 1971. Glashow and Georgi formulated their GUT in 1973—six years before Glashow won the prize for work that he considers an intermediate step toward that theory.
A Nobel laureate is obliged to give a speech before the week of festivities is over. Glashow’s was mainly an attempt to convince any remaining doubters that the recent work in particle physics constituted a fundamental advance. He opened by recalling the time when Schwinger had started him on his career.
“In 1956, when I began doing theoretical physics, the study of elementary particles was like a patchwork quilt,”he said. “Electrodynamics, weak interactions, and strong interactions were clearly separate disciplines, separately taught and separately studied. There was no coherent theory that described them all.”
“Things have changed,” Glashow told his audience. “Today we have what has been called a standard theory of elementary particle physics, in which strong, weak, and electromagnetic interactions all arise from a [single] principle. It is, in a sense, a complete and apparently correct theory, offering a qualitative description of all particle phenomena”— an enormous achievement, tossed off in a phrase—“and precise qualitative predictions in many instances. . . . The theory we now have is an integral work of art: the patchwork quilt has become a tapestry.”
A physical theory, like a tapestry, is woven within a frame. Newton could not have put together the laws of mechanics if he had not first invented a language in which to write them: calculus. Similarly, Einstein could not have understood the odd geometry of general relativity if a sickly German named Georg Friedrich Bernhard Riemann had not first dreamed up an apparently impractical branch of mathematics in which parallel lines could meet: Riemannian geometry. And Glashow could not have written his doctoral thesis if he had not happened to come across a beautiful but mostly empty loom known as gauge field theory.
Modern gauge field theory was created largely at Brookhaven National Laboratory in 1954, by the physicists Chen Ning (“Frank”) Yang and Robert Mills. In a short paper published that year in Physical Review Yang and Mills built the frame upon which modern quantum field theory has been woven. At first their ideas were dismissed by all but a few as pure mathematics, without any physical significance. Now, partly as a result of Glashow’s work, the drive toward unification has boiled down to a quest for a YangMills gauge field theory powerful enough to account tor every particle and force in existence.
The basic idea of the paper had occurred to Yang in 1947 and 1948, when he had been a graduate student at the University of Chicago. He had recently come from China, where his father was a mathematician, and he was fascinated by the use of different formal methods to describe the same thing from varying perspectives. In particular, he was intrigued by the region of mathematics known as gauge theory, which describes the relationship between physical forces and the symmetries that underlie them.
Used loosely, “symmetry” means harmony or balance. Physicists and mathematicians, however, define the term more precisely. They say that something is symmetrical if one or more of its aspects is indifferent to a change. A rubber ball can be turned freely and its appearance won’t be altered; therefore, as a physicist might say, the ball is “symmetric about its shape.”In gauge symmetry a change in one aspect of a phenomenon is precisely compensated for by a change in another aspect, so that a quantity related to both aspects remains the same.
This kind of compensation can be likened to the drawing of contour lines on a topographic map. Say that the mapmaker has decided to let the distance between any two contour lines correspond to a ten-yard rise or fall in elevation. (The farther apart the lines, the flatter the terrain.) If a volcano changes the lay of the land, the position of the lines on the map will have to be changed as well, to compensate for the newly created steepness. The gauge field has the role in particle physics that the mapmaker has in cartography; it preserves an unchanging relationship between two or more changing phenomena.
The simplest example of a gauge theory is QED (the theory of electromagnetism). In this case, what always remains the same is the net electric charge of a particle or group of particles.
Just as someone who has never played chess must watch a game for a while before he can pick out the usually untouched kings as the most important pieces on the board, it often takes scientists a while to discern the central aspect of any phenomenon. Physicists had known since the 1930s that electromagnetism could be described as a gauge field, but most had left it at that, it occurred to Yang when he was in graduate school that such a description might be essential to understanding electromagnetism, and also that there might be other kinds of gauge fields. Five years later he mentioned the idea to Mills, his office mate at Brookhaven. According to Mills, “He and I just asked ourselves, ‘Here is something that occurs once. Why not again?’”
To find a gauge symmetry like the one for electric charge, Yang and Mills reached into the bag of terms that physicists use to describe subatomic particles and seized upon “isotopic spin.” Coined by Heisenberg in 1932, the term refers to a way of classifying the proton and the neutron. These two particles are identical in almost every respect; electric charge is the only important difference. Heisenberg wondered if theorists might make headway by considering the proton and the neutron to be different states of the same particle, in somewhat the same way that the isotopes carbon-14 and carbon-12 are slightly different versions of the element carbon. To describe this relationship formally, he proposed that the two particles be thought of as spinning like tops in some imaginary space. If the axis of spin points up, the particle has a positive charge and is a proton; if the axis points down, it has no charge and is a neutron. Heisenberg expressed this relationship by saying that the proton and the neutron should be assigned the same value—the proton plus and the neutron minus—of the hypothetical entity that he called isotopic spin.
All of Heisenberg’s theorizing would have been only a game if experimenters had not discovered that nature was playing too. Just as the net amount of charge is constant in electromagnetic interactions, isotopic spin turned out to be constant in strong interactions of all kinds. Thus Yang and Mills hypothesized that because a particle’s charge is preserved by a gauge field (i.e., electromagnetism), isotopic spin might be preserved by some other gauge field. And they asserted, just to see what would happen, that the gauge field for isotopic spin is governed by the interactions of certain virtual particles, which they called “vector bosons.” (The term refers to aspects of these particles that play no role in this discussion.)
Yang and Mills hoped that their equations would work out such that these vector bosons, which nobody had ever observed, would prove to be associated with the strong interaction. Gauge fields would then be a language appropriate not only to electromagnetism but also to the strong force. They would catch both in one net.
When Yang and Mills did the mathematics, the equations came out right if vector bosons had electric charge but no mass. This was most discouraging. A massless particle, such as the photon, is easy for experimenters to make and a charged particle, such as the electron, is easy for them to detect. Yang and Mills could not understand why massless charged particles, if they existed, had not alreadybeen discovered. (“That was the embarrassment of it,” Glashow says. “ This lovely theoretical idea ended up predicting these massless charged particles that could not possibly exist!”) Even though nature didn’t seem to be cooperating, Yang and Mills thought that their idea was so beautiful that they went ahead and published it.
BECAUSE YANG AND MILLS HAD COME UP WITH A wholly new way to look at the forces among elementary particles, their work interested many physicists. Yang and Mills in effect were telling their colleagues: “Look at particles. See if you can find any unchanging properties, and try to imagine a gauge field that might account for them. Work out the properties of that fictional field and its associated virtual particles. Are they anything like the real world?”
One of the physicists who paid attention was Schwinger. Characteristically, he rederived the whole mechanism in his own way, and in his own notation. But instead of using a Yang-Mills gauge field to look at the strong interaction, as most of his colleagues were doing, Schwinger considered the weak one. And in October and November of 1956 he dazzled his students, including Glashow, with a series of public lectures purporting to explain the weak force in terms of gauge theory.
Out of the lectures came a paper entitled “A Theory of the Fundamental Interactions,” which was published in the journal Annals of Physics in November of 1957. The most important part of Schwinger’s theory had to do with particles called “leptons,” which are unique in that they respond to the weak force but not to the strong. By 1956, when Schwinger was lecturing about his idea, three types of leptons had been discovered: the familiar electron; the muon, which is exactly like the electron except that it is 200 times heavier; and the neutrino, a particle so small that it may have no mass. All are as impervious to the strong force as a tree stump is to a magnet. Schwinger conjectured the existence of two types of neutrino—one associated with the electron and one with the muon.
Schwinger needed the first speculation to make a second bit of speculation come out right. He postulated that the weak force is carried among the leptons by three particles: the photon and two hypothetical vector bosons, which are now called W+ and W - . Although most physicists regard his work as being based on that of Yang and Mills, Schwinger came to his ideas from a different path.
There had to be two vector bosons to account for the way in which weak interactions convey electric charge. When the weak force transmutes an uncharged neutron into a positive proton and a negative electron, a virtual particle is emitted, and in its decay an electron is created. (Any such emission and subsequent decay is called a “current.”) Schwinger argued that when the weak force transfers a positive charge, the current is carried by a positive version of the vector boson, the W+ , whereas when the weak force transfers a negative charge, the current is carried by a negative version of the vector boson, the W He further hypothesized that the familiar photon is the intermediary when no charge is involved.
Schwinger told his graduate students that this picture of the weak force had far-reaching consequences. Because the photon and the Ws make up a triplet, Schwinger said, electromagnetism and the weak interaction should be treated as part and parcel of the same phenomenon: that is, he had unified them. But he was more cautious in print, as is proper. He never used the word “unification” in the paper, but referred instead to the weak force as “a partner of the electromagnetic field.”
There was no compelling reason for his suppositions; Schwinger merely wanted to demonstrate that his pet notion could be used to construct a coherent theory. In addition, he found the result aesthetically pleasing. His colleagues did not. When Schwinger went to New York City to visit his former teacher Isidor I. Rabi, at Columbia University, Rabi told him bluntly, “Everybody hates that paper.” Upset by the reaction, Schwinger was further distressed to learn that the experimental data he had relied on were wrong. (As it turned out shortly thereafter, only part of those data were invalid, and Schwinger was much closer to a successful electroweak theory than he or anyone else knew. Abdus Salam says, “If those experiments hadn’t been wrong, he might have gotten the entire thing then and there.”) Schwinger gave up in disgust and turned to other areas of physics.
Before he did, though, he asked Glashow to look into the possibility of a connection between the weak and electromagnetic interactions—to see if a connection could still somehow be established. “It was a vague request,” Glashow says. “He asked me to think about it. And that’s what I did for two years—think about it.”
Today Glashow’s thesis—the product of those two years of thought—sits on his bookcase, stuck in a black spring binder on a neglected top shelf. He has to hunt for it, wading through the stacks of preprints and journals that clutter his office, when he wants to show it to visitors. On a bulletin board beside the door are many strata of notes and memos, including letters from a physics student in Beijing who claims to have discovered antigravitation (“Is most important result!”), newspaper advertisements explaining that Transcendental Meditation can harness the power of the Grand Unified Field, and a number of clippings about the Soviet physicist Andrei Sakharov, in whose defense Glashow has been active. “Here it is!” Glashow says, standing on a wobbly stool. He hefts the volume appreciatively. “These things are always long, to show you have lots of bright ideas, and filled with tons of calculations— student showboating. Mine is a complete parade of crazy digressions. I haven’t looked at this in years. But there’s one part I’m still proud of. Here, wait—it’s in the appendix.”
This is no surprise. Graduate students traditionally hide their most radical statements in footnotes and appendices, where they can be disavowed if necessary. Glashow sits at his desk wreathed in cigar smoke and shakes his head at the maunderings of his youthful self. When he finds the page, he sticks a finger into the air.
“Yes. ‘It is of little value to have a potentially renormalizable theory of beta processes [another name for weak interactions] without the possibility of a renormalizable electrodynamics.’ That’s because the Ws are charged, so you had to get into QED to talk about them. Here we go. ‘We should care to suggest [his voice rises here; his pleasure in the younger Sheldon Glashow is apparent] that a fully acceptable theory of these interactions may only be achieved if they are treated together.’ ”
He snaps the binder shut, kicking up a little cloud of dust. “That’s the basic idea of the electroweak theory right there—that the weak interaction would never make any sense by itself, because it’s always mixed up with electromagnetism. So you have to look at them together. It sounds simple, but that kind of realization is a big deal. Before Newton could work out his theory of gravitation, he first had to realize that the force that moves the planets around the sun and the force that drops apples on people’s heads are the same thing. So, in a certain sense, that sentence won me the Nobel Prize. It took thirteen more years”—until 1971—“to show that I was right, but damn it, I was.”
WHILE WORKING ON HIS THESIS GLASHOW WON A National Science Foundation Fellowship, a prestigious award that entitles its recipient to study abroad. Glashow spent two years, from 1958 to 1960, at the Niels Bohr Institute for Theoretical Physics, a center established after the First World War by the eminent scientist for whom it is named, and affiliated with the University of Copenhagen. The Bohr Institute every year invites a few young physicists to participate in the discussions held there. Glashow attended lectures, gave talks, met the great Bohr, and in general lived the life of which he had dreamed years before at Bronx Science.
At the institute Glashow expanded the appendix of his thesis into a paper, “The Renormalizability of Vector Meson Interactions,” which he submitted to the journal Nuclear Physics in the fall of 1958. (Maddeningly, physicists at that time used “vector boson” and “vector meson” interchangeably. Today they are careful to distinguish between the two, and hereafter this article will correct any outdated usage.) Glashow was referring to what are now called vector bosons. He wanted to prove that tying electromagnetism and the weak force together would produce a coherent and renormalizable theory. By November, when he submitted his paper, he thought he had it.
Pleased with himself, he went to England in the spring of 1959 to present his work. “I lectured in London or somewhere,”he says. “ They all sat there smiling very politely, told me that they liked what I was doing, and went home to tear it apart.” Abdus Salam was in the audience; for years after, he says, he didn’t trust Glashow’s work. For Glashow the episode was embarrassing and annoying—a nightmare. (“They don’t make graduate students that dumb anymore,” he groans.)
The reaction to the London lecture forced Glashow to make a choice more dependent on his personality than on the rules of scientific inquiry. Young theorists have to come up with bold ideas if they are to acquire reputations, yet their papers cannot be half-baked. Glashow’s idea had been bold, but it was wrong. If he kept going, he risked being labeled an eccentric; if he went on to something else, it would be an admission of defeat. Glashow chose not to abandon unification; instead he advocated an even wilder, more speculative extension of his original position.
His decision to hold to unification was more than simple muleheadedness, however. Glashow has that lucky intuition—the right taste, as Gell-Mann says—that is a gift for selecting the correct road in advance of, and sometimes even against, the data and stubbornly following it heedless of scorn or, worse, indifference. He threw himself into the sequel, turning over his ideas with the bemused care of a raccoon washing a shiny ring.
In March of 1960, after Glashow gave a talk in Paris, Gell-Mann, whom Glashow had never met, invited him to lunch. Gell-Mann was then in the middle of an extraordinary decade during which he dominated particle physics, and Glashow was more than happy to be his guest. They went to a seafood restaurant. Glashow mentioned having an aversion to fish; Gell-Mann explained that serious people like seafood, that he knew that Glashow was a serious person, and that therefore he would order fish for both of them. (“Murray’s all the things people say he is—arrogant, intolerant, and so on—but I love him,” Glashow says. “I’ve been terribly lucky, working right off the bat with two great physicists—Julian, and then Murray.”)
Over the entrée they talked about vector bosons. GellMann said that for several years he too had been thinking about ways to reconcile the weak and electromagnetic forces. He urged Glashow to press on, and suggested that they work on field theory together at the California Institute of Technology the next fall. “What you’re doing is good, but people will be very stupid about it, he told Glashow.
“Partial-Symmetries of Weak Interactions,”the paper that established Glashow’s claim to a Nobel Prize, was published in the journal Nuclear Physics in November of 1961— one month before the author’s thirty-first birthday. The paper has the dry, confident tone that characterizes the best scientific prose. “At first sight,” Glashow begins,
there may be little or no similarity between electromagnetic effects and the phenomena associated with weak interactions. Yet certain remarkable parallels emerge with the supposition that the weak interactions are mediated by unstable bosons.
Next Glashow faces up to a problem that had first arisen in the work of Yang and Mills and that was still unsolvable: how much these W bosons weigh.
The mass of the charged intermediaries must be greater than [zero], but the photon mass is zero—surely this is the principal stumbling block in any pursuit of the analogy between hypothetical vector [bosons] and photons.
It is a stumbling block we must overlook.
That is, Glashow simply plants a “don’t know flag on the question and turns to the rest of the theory. He argues that the theoretical problems can be resolved if one assumes not three virtual particles but four.
In order to achieve a . . . theory of weak and electromagnetic interactions, we must go beyond the hypothesis of only a triplet of vector bosons, and introduce an additional, neutral vector boson, Zs.
He confesses, “The reader may wonder what has been gained by the introduction of another neutral vector [boson].”
What had been gained was this: the neutral partner of the W + and W“— the Zs (the current term is Z°, or “zeezero”)—allowed Glashow to separate the weak and electromagnetic interactions and to deal mathematically with each in turn. In Glashow’s first scheme the photon had worked overtime—for both electromagnetism and the weak force—with the result that the two tasks had interfered with each other in the theory. His second model was much neater.
In this model the weak and electromagnetic forces within the atom are like two children with an elaborate Lionel train set, each at a separate control panel: they frantically throw the switches, toot the whistle, and turn the throttle without consulting each other. The motion of the train is the result of the actions of both, and it depends from one moment to the next on what each is doing. So it is with subatomic particles. Surrounded by a haze of photons and vector bosons, the movements of particles are a synthesis of the actions of both.
The contribution of each force to the total interaction is expressed in a ratio of so much weak to so much electromagnetic, which Glashow describes in the paper. This ratio is customarily known as a “mixing angle,” because of a principle from high school trigonometry that most students immediately forget: Any ratio or percentage can be expressed as a trigonometric function of some angle.
Today, to Glashow’s chagrin, the mixing angle of the forces is sometimes called the “Weinberg angle. It acquired this name because seven years after Glashow’s paper was published Steven Weinberg independently reinvented it. No one had paid much attention to Glashow’s work; it had appeared in an unimportant journal, and if that was not enough to guarantee its obscurity, there was the fact that it was written using Schwinger’s idiosyncratic symbols. In an effort to promote Glashow’s work GellMann reported on it in 1960 at the Rochester Conference, an important annual meeting of physicists. “I gave the first clear description of his ideas,”Gell-Mann says. “ The reaction was very unenthusiastic.”
The reason for the lack of interest was that by adding a Z to the W particles to make the model look like a gauge theory Glashow had created a new problem having to do with weak neutral currents. Physicists had looked at millions of weak interactions and were certain that weak neutral currents did not exist. Not once, in hundreds of careful experiments, had they seen a weak interaction that might mark the passage of an uncharged vector boson.
Hundreds of experiments and millions of observations are enough to establish a fact with rocklike firmness. Unfortunatelv, in this case nature was playing a trick on physicists. The weak interactions of most types of particles are usually stifled by the strong and electromagnetic interactions. But strange particles can decay only by means of the weak interaction, so physicists had concentrated on strange particles in order to observe this interaction, for reasons that Glashow would not unravel for a decade, strange particles happen to be just about the only kind in which weak neutral currents do not occur; thus the years of experimental research did nothing to disprove his theory.
At the time, of course, Glashow did not know this. Like his colleagues, he believed that the data on strange particles held true for all forms of matter. To save his idea he invented a reason why the Z0 had never shown its face. He asserted that it must be a lot heavier than its brethren the W+ and the W —. To create a Z° experimenters would have to bring to bear much more energy than any existing accelerator could generate. No wonder the particle had never been seen. (Such theoretical gambits infuriate experimenters, by the way.)
Alas, this strategy threw the baby out with the bath water. In order to explain how weak neutral currents could exist and not be observed, Glashow had hypothesized a Z° so heavy that it would hardly ever put in an appearance. Thus the paper concluded, “Our considerations seem without decisive experimental consequence.”
The problem of the mass of the vector bosons and the question of weak neutral currents dogged Glashow’s electroweak theory from the beginning, and these difficulties were not resolved for years.
IN 1960, AFTER GLASHOW HAD FINISHED HIS PAPER ON weak interactions, he went to Pasadena to work with Gell-Mann. One of the true enfants terribles of twentieth-century science, Gell-Mann had entered Yale at the age of fifteen and received his doctorate at twenty-one. A polymath, he is sometimes said by his colleagues to have no particular talent for physics but to be so smart that he is a great physicist anyway. Although only three years older than Glashow, he had been on the scene almost ten years longer. By 1960 he had the run of the California Institute of Technology and was spewing out ideas at a dizzying rate. He is stocky and moves with a bullet-like determination, his shoulders thrust forward, his gaze fixed ahead. Notorious for his drive and his asperity, he is also quick and generous with praise. Like Glashow, he loves conversation. The two enjoyed each other’s company and soon began to collaborate on an ambitious article that they hoped would point the way to unification.
The result—“Gauge Theories of Vector Particles”—appeared in the journal Annals of Physics in September of 1961. It is a curious document. Gell-Mann calls the first half “classic, a landmark,” and the second half “a mess.”
The first half sets up a kind of grammar for quantum field theory—a catalogue of possibilities. Drawing upon group theory (Glashow’s introduction to it at Cornell, under the tutelage of Harold McIntosh, had not been a waste of time), the two physicists noted that the sets of hypothetical virtual particles described by Yang and Mills could be considered as a particular type of group. Moreover, all possible variations on this type of group had already been laid out, nearly thirty years before, by the French mathematician Elie-Joseph Cartan. Cartan had given the groups names. For example, Cartan’s group U(l) corresponded to the gauge field for electromagnetism— QED—and his group SU(2) X U(l) to Glashow’s electroweak theory. (SU stands for “special unitary” group, and U for “unitary.”)
Glashow and Gell-Mann showed that every one of the groups that Cartan had defined corresponded to a YangMills gauge field. The difficulties arose in the second half of the paper, in which Glashow and Gell-Mann tried to do something practical with the classifications borrowed from Cartan. They proposed a theory of the strong force to go along with Glashow’s electroweak theory and expressed them both in terms of Cartan’s groups. With this trick they sought to treat the strong and electroweak forces as YangMills gauge fields. They hoped to explain those forces in the same language as electromagnetism, and link them to the only theory in elementary particle physics that by 1960 had been confirmed: QED. But the attempt was like a game of basketball and a game of hockey played in the same arena at the same time. Gell-Mann says, “We wanted to make a Yang—Mills theory for the weak interaction and one for the strong interaction. But to do this we had to have them both in the same space. And they fought each other. It was terrible. They would get mixed up with each other in some ghastly way, and the weak interactions would turn strong. It was bad, it was sick. Finally I said the hell with it. This one, I said, we’re not going to solve now.”
Glashow abandoned the effort too. He was discouraged not only by their failure to put the strong and electroweak forces into one theoretical framework but also by the still unresolved problems of the mass of the W and Z particles and the lack of experimental evidence for weak neutral currents. For almost a decade he concentrated on the strong force, consigning all his earlier work to one of the dusty piles of paper in his office.
By the early 1960s gauge field theory was in all but complete disrepute, because of the severe difficulties encountered by everyone who tried to use it. Yet Gell-Mann, Glashow, and a few others pressed on with it. During the same time that Gell-Mann and Glashow were collaborating on their paper, Gell-Mann completed an article on his own—one that created a sensation in the physics community when he circulated it in January of 1961. Although the paper was based on field theory, it had the unexpected effect of distancing physicists even further from the subject; it would take years for anyone to see how GellMann’s ideas could be used to write a gauge field theory.
The article was an attempt to bring some order to the profusion of new particles that experimenters had found in particle accelerators during the previous decade. By 1958 the jumble of subatomic particles had become so unwieldy that scientists at the University of California at Berkeley began to publish an almanac to keep track. The first one was nineteen pages long and discussed sixteen particles; by 1960 enough additional particles had been discovered to warrant a second, considerably larger issue, with handy reference cards for scientists to carry in their wallets. (The most recent edition, published in April of 1984, runs to 304 pages and refers to about 200 particles, not counting their various avatars and states.)
Also by 1960 a rudimentary classification scheme for particles, based on weight, had been adopted. It consisted of three categories: baryons, mesons, and leptons. “Baryon” is derived from a Greek word that means “heavy one,”and refers, appropriately enough, to such large, massive particles as protons and neutrons. “Meson is derived from the Greek word for “middle one,” and is used for particles lighter than baryons but heavier than leptons. (“Lepton,” roughly translated, means “light one.”) Baryons and mesons are affected by the strong force; leptons are not.
As dozens of new particles came on the scene, theorists realized that their categories would have to be refined. Many strategies were tried, but in his paper Gell-Mann suggested the best one. In short, he argued that SU(3)— another of Cartan’s special unitary groups—could be the basis for a kind of periodic table of baryons and mesons. He showed that similarities among these particles—that is, almost all the particles that had been discovered by 1960—could be used to group the particles into families. Because the proton and the neutron belonged to a family with eight members, Gell-Mann named his new scheme the “eightfold way,” in a joking homage to the teachings of the Buddha, (To his considerable annoyance the phrase has helped to foster the suspicion that physics has something to do with Eastern mysticism.) Just like the periodic table of the previous century, this classification of particles was of tremendous heuristic value, not only because it organized known particles but also because it suggested where new ones might turn up. For example, right away Gell-Mann asked experimenters to search for a particle that would complete one of his families. After two years of intense effort, it was found.
At first neither Gell-Mann nor anyone else could explain why particles were so much at home within SU(3). Then, in 1963, Gell-Mann proposed a reason. He guessed that baryons are really combinations of three smaller particles, to which he gave the name “quarks.” (Gell-Mann explains that he chose this silly-sounding name in order to avoid “pretentious scientific terminology.”) He called the three quarks “up,” “down,” and “strange,” and also proposed a corresponding triplet of anti-quarks: “anti-up,”“antidown,” and “anti-strange.” Baryons, he said, are made of three quarks; a proton, for instance, consists of two ups and a down clamped together by the strong force. The lighter mesons are composed of one member of the first trio (a quark) and one of the second (an anti-quark).
How did this explain why the eightfold way successfully orders particles? Gell-Mann argued that the quarks and anti-quarks that make up baryons and mesons divvy up electric charge. The up quark has a charge of +2/3, whereas the down and strange quarks each have a charge of — 1/3. Thus the proton—two ups and a down—has a total charge of ( + 2/3) + ( + 2/3) + ( — 1/3) = + 1. The particles in the baryon families of SU(3) consist of every combination of three units of +2/3 or — 1/3; the result is always an integer.
But quarks explain more than SU(3)’s usefulness. GellMann soon realized that quarks make sense out of strangeness and isotopic spin. A particle’s strangeness is given by the number of strange quarks inside it; a strangeness of 0, for instance, means that no strange quarks are inside. Easy! And isotopic spin is determined by the number of up and down quarks in the particle; a proton, which has more ups than downs, spins on an upward axis. The values of both strangeness and isotopic spin are constant in strong interactions because, as it happens, the strong force cannot change one kind of quark into another. It can shuffle them around like cards in three-card monte, or hold them together in an iron grip, but it cannot alter the proportions of up. down, and strange.
Gell-Mann suggested that although the weak force cannot hold quarks together, it might get them to change types, or “flavors.” And that trait would account for the long lives of strange particles, which can decay into ordinary particles only when the feeble weak force finally asserts itself and turns a strange quark into an up or a down.
Up, down, strange: just three quarks explained phenomena that theorists had been puzzling over for years. Nevertheless, “quarks went over like a lead balloon,”as Gell-Mann says. The reason was that the fractional electric charges +2/3 and - 1/3 contradicted one of the most thoroughly established rules in physics. Over the half-century during which anyone had been looking, a particle with, say, two-thirds the charge of a proton had never been spotted. Such particles simply could not exist.
Gell-Mann replied that quarks had never been seen because they cannot be seen. They are always chained together, like the hero and the heroine in the movie The Thirty-Nine Steps. This, too, did not go over well. Experimenters did not like the notion of something that in principle could not be found with the aid of their machines. Their frustration was not assuaged by Gell-Mann’s serene statement that the failure to see fractionally charged quarks meant that “they exist but are not ‘real.’ ”
Glashow adored quarks. Hypothetical particles perpetually locked inside all the particles in the nucleus was just the sort of loony supposition he thrives on, and he was determined to play with it.
IN THE SPRING OF 1964 GLASHOW RETURNED TO THE Bohr Institute. Bohr had died, but little else had changed. The institute was still full of bright young theorists talking up their ideas in tiny offices. One of these theorists was James Bjorken, who had arrived from Stanford a few months earlier. Soon Glashow hit him with his latest pet notion: namely, that there might be a fourth quark. He called it “charm.”
Charm excited Glashow, because it arranged the universe in an elegant pattern. By then the existence of four leptons—the electron, the muon, and Schwinger’s two types of neutrino—had been accepted. If a fourth quark could be summoned into existence, Glashow reasoned, then the subject matter of physics could be divided into two families.
Everyday matter consists of just electrons, their neutrinos, and combinations of up and down quarks: “the runts of the litter,” Glashow calls them. At high energies, however, such as those generated by particle accelerators, another family of particles appears: muons, muon neutrinos, charm, and strange quarks, which are fatter, heavier counterparts of the everyday particles. Quarks are assembled into protons, neutrons, and so forth by the strong force, whereas leptons heed only the call of the weak. BY means of the weak force, all but one member of the heavier family—the muon neutrino—decay into everyday matter. As far as scientists know, the muon neutrino is stable—it does not decay. Glashow was convinced that these two families of particles, together with the virtual particles that transmit forces, accounted for all the building blocks of the cosmos. (The picture today is only slightly different. In the 1970s physicists discovered a third family, arranged just like the other two but with still heavier members.)
Glashow spent weeks thrashing around to find some justification for charm other than his certainty that God could not have been stupid enough to decree four leptons and only three quarks. He failed to come up with any, but he managed to convince Bjorken that they should send the idea off all the same. The paper appeared in the journal Physics Letters in August of 1964.
As it turned out, charm worked more magic than Glashow or Bjorken knew. When incorporated into SU(2) x U(1)—the group corresponding to the electroweak theory that Glashow had conceived in 1960—charm explained why experimenters had never observed weak neutral currents.
In many cases, the greater the number of ways in which a particle can decay, the more readily its decay will occur.
I his much is a familiar part of our world. For example, if two doors on a bus are open rather than one, twice as many people can exit at any given time. But in the quantum world such “exits” can interfere with one another, and opening an additional “door” may have the paradoxical effect of reducing rather than increasing the frequency with which particles decay.
So it is with strange particles—the kind that physicists had always used to study the weak interactions. According to the SU (2) x U(1) theory, strange particles should often emit Z°s and turn into ordinary particles. But experimenters had never seen any such event, let alone the number likely if SU(2) x U(l) were correct. Charm reconciled Glashow’s lovely theoretical model to this ugly experimental fact. By a quirk of mathematics (and nature), the probability that a strange particle will emit a Z0 and become an ordinary particle and the probability that it will emit a Z° and become a charmed particle are almost equal. It is as if the strange particle stands irresolute between two equally tempting options and cannot bring itself to choose either. (Glashow wrote later, in an article published in Scientific American: “As it happened, a sign in the equation that defines [the charmed reaction] is negative, and the two interactions cancel each other.”) Thus, although weak neutral currents should be able to occur in strange particles, they never do. Experimenters would have to look at other particles for evidence of such currents.
Unfortunately, when Glashow and Bjorken wrote their paper they didn’t see the power of charm, “There it is in black and white, and we missed it!” Glashow says, smiting his brow theatrically. “If you read my 1961 paper with Gell-Mann, you’ll say, “These people say the theory doesn’t work and something is missing.’ And if you read my 1964 paper with Bjorken, you’ll exclaim, ‘By God, that’s it! A fourth quark!' But did we notice it? No. Not for six years.”
In 1964 Glashow was not even thinking about weak neutral currents. He was preoccupied by how poorly quarks fitted into field theory. If no theorist could come up with a field that would hold quarks together in a way that would explain why they were never detected in experiments, it was difficult to understand how adding a fourth quark would help matters. Troubled, Glashow gave up on charm, as he had earlier given up on SU(2) x U(l), because he didn’t know how to show that it was right. For the next seven years he published inconsequential papers, threw others away, and grew cranky. In 1966 he became a full professor at Harvard, and arrived on campus to find other members of the physics department in the same funk.
HOWARD GEORGI, A PROFESSOR OF PHYSICS AT HARvard and Glashow’s collaborator on the Grand Unified Theory, says, “It really was an extraordinarily dull period in theoretical physics. Hardly anybody knew what they were doing—not even Steven Weinberg, who came up with one of the most important keys to the whole enterprise.”
Weinberg, Glashow’s friend in high school and college, had received his Ph.D. from Princeton. Shortly after Glashow went to teach at Harvard, Weinberg took a job nearby, at the Massachusetts Institute of Technology. A solitary thinker, Weinberg has a special gift for working with the loftiest general principles of physics. Instead of playing with specific models, the way Glashow does, Weinberg tends to chew over the ideas and conditions that shape those models. He had spent a good deal of time in the early 1960s working out an idea called “hidden symmetry” or “broken symmetry,” which has to do with the way utterly different things can come from one unobservable source.
Systems with hidden symmetry can be likened to the male fiddler crab, which has one large and one small front claw. The big claw is a threat signal—the male crab’s equivalent of a clenched fist. It is waved to scare off other males. Despite their different appearance, the two claws are generated by the same genetic material; their real symmetry is hidden from view.
In Weinberg’s scheme, although vector bosons—the particles that transmit the weak force—should, like photons, be massless, in the conditions of the real world the symmetry between vector bosons and photons breaks, and vector bosons acquire mass. Exactly how the symmetry breaks is hardly clear. It seems to involve a mechanism featuring a special, never-seen particle—the Higgs boson—that appears, breaks the symmetry, and quickly goes away. Despite the ungainliness of such a process, Weinberg was delighted. His paper on the subject appeared in Physical Review Letters in November of 1967. (Ten years later he wrote, in an article published in the magazine American Scientist. “Nothing in physics seems so hopeful to me as the idea that it is possible for a theory to have a high degree of symmetry which is hidden from us in ordinary life.”)
Weinberg realized that adding symmetry-breaking to a variation of Glashow’s model of the electroweak force, SU(2) x U(l), would explain neatly how vector bosons could fit into a gauge theory and still have mass. Yang and Mills had shown that the virtual particles associated with a gauge theory could not have mass, but experimenters had pointed out that if such massless particles existed, they would long since have been produced in laboratories. Glashow had been unable to resolve the contradiction, and had simply asserted that vector bosons acquire mass in some unknown way. In his paper Weinberg argued that Glashow’s W and Z particles get their mass through spontaneous symmetry-breaking and the action of the Higgs boson.
The paper was elegant and the accomplishment major, but nobody, not even Glashow, gave it much notice. A year later, as part of a grab-bag article inventorying possible ways to unify the weak and electromagnetic interactions, Abdus Salam published an almost identical model. He incorporated hidden symmetry into a theory similar to Glashow’s SU(2) X U(l), which he had created with a colleague, John Ward, in 1964. And his paper was neglected too. Those physicists who happened upon Weinberg’s or Salam’s model did not believe that the theory was renormalizable; further, the models still predicted weak neutral currents, and such currents still had not been observed. Neither Weinberg nor Salam saw that charm would do the trick.
A more egregious example of missed connections occurred in 1970, when Glashow, working with two Harvard postdoctoral fellows, John Iliopoulos and Luciano Maiani, at last realized that charm solved the problem of weak neutral currents, but failed to link it to SU(2) X U(1).
Glashow, Iliopoulos, and Maiani argued that charm must exist in order for SU(3)—Gell-Mann’s description of particles responsive to the strong interaction—to be reconciled with the weak interaction. It is difficult to overstate the audacity of such an argument: Glashow and company were contending that a new state of matter had to exist to make two as yet unproven theories fit together in an aesthetically pleasing way.
Glashow presented the new work on charm to an audience at MIT that included Weinberg. He recalls, “After the talk, Steve said, ‘What you’ve done is very interesting,’ but he never sensed the relevance to his paper. We weren’t aware of Steve’s paper either. We didn’t refer to it—not out of malice but out of sheer ignorance. There we stood, each with half of the puzzle solved, and nothing happened.” Glashow takes a disgusted pull on his cigar. “What on earth could I have been thinking about?”
IN 1971, AFTER ALMOST FIFTEEN YEARS OF WORK, GLAshow was unable to point to a single major success. In a sense, he had bet his career that gauge theory would pan out; most physicists were convinced that it would not. Even Weinberg, unable to renormalize his own electroweak model, seemed to be throwing in the towel.
Depressed, Glashow went to a conference in Marseilles, which John Iliopoulos also attended. They stayed on for a few months and busied themselves with an attempt to renormalize gauge field theory—a purely mathematical task that had stumped physicists since Yang and Mills had written their paper, in 1954. Glashow’s forte as a physicist is not mathematics—“I think in pictures, not equations,” he observes—and he found the work little to his liking. He says, “There we are, laboriously canceling out infinities, when this guy Veltman comes up to us. And he says, ‘My young student has solved the problem of renormalizabiiity of gauge theories. All the work you ve done for the past year is a waste of time!’ And he was right. His student had spontaneously re-created the whole SI (2) X U(1) theory, and renormalized it to boot.”
The Dutch physicist Martinus Veltman was one of the few who had clung unswervingly to quantum field theory. He had holed up at the University of Utrecht, in Holland—the only particle physicist in residence—and had taught himself the peculiar mathematical techniques that he thought would be necessary to vindicate the theory. In 1969 he acquired a graduate student named Gerhard ‘t Hooft. “I gave him a piece of the problem,”Veltman says. He gave it to the right person, for ‘t Hooft used Veltman’s mathematical tools to construct two brilliant papers. The first demonstrated how Yang—Mills fields could be made renormalizable; this work had already been done by Veltman and others, but few physicists knew or cared, because it did not resolve the mass problem. 1 he second paper added spontaneous symmetry-breaking, solving the mass problem, and an SU(2) X 1(1) model of the weak interactions to Yang—Mills, and showed how the whole thing could be renormalized.
Incredibly, when ‘t Hooft developed his ideas, he had read nothing by Glashow, Weinberg, or Salam, and only some of the literature on spontaneous symmetry-breaking. He reinvented the entire body of work (except charm) on his own. Realizing that his student had come up with something important, Veltman quickly alerted the physics community.
When Glashow returned to the United States in the fall of 1971, physicists at Harvard and MIT gathered to hear about’t Hooft’s proofs, and at first they were baffled. But as weeks passed, the realization spread to departments all over the country that a breakthrough had finally occurred. Not only SU(2) x U(l) but also every other gauge model based on Cartan’s groups suddenly made mathematical sense. The physicists at Harvard immediately became advocates of gauge theories; the lights of Lyman Hall burned late into the night. Glashow was jerked out of his doldrums, to embark on the most productive period of his career. He churned out paper after paper exploring the implications of ‘t Hooft’s work.
(He also managed to find time to court the woman who eventually became his wife. According to Glashow, he met her in September of 1971, at the home of the mathematician Daniel Kleitman. Glashow was asked to help Kleitman’s sister-in-law, Joan Alexander, shuck corn. She was twenty-eight, recently divorced, and had two small children; Glashow was thirty-eight, and fond of children. In February they became engaged, and in May they married. Besides Joan’s children, they now have two of their own.)
During the early 1970s SU(2) X U(l) and charm received unexpected support. First, neutral currents were discovered—in 1973, shortly after experimenters had stopped looking for them exclusively in strange particles and had turned to neutrinos. Their existence was hard to understand unless one accepted charm, as Glashow did. Second, a successful gauge field theory of the strong force was put forward by Weinberg, on the East Coast, and several physicists on the West Coast, including Gell-Mann. It said that the strong force, like electromagnetism, is associated with a charge. The charge is not electric, but has to do with a quality that has been named “color” (though it has nothing to do with ordinary colors), To underline the new theory’s similarities to QED, Gell-Mann christened it “quantum chromodynamics,’or QCD—chroma being the Greek word for “color.” (Although QCD is quite different from Gell-Mann’s earlier SU(3) theory, which it supplants, it is described by the same mathematical group. Confusingly, physicists have used the term SLT(3) for both that earlier theory and QCD; hereafter, in this article, it shall refer only to QCD.)
Field theorists had been slow to accept quarks, because it was hard to imagine a Yang-Mills field confined enough to keep quarks always glued together into mesons and baryons. The gauge field of QCD works backward: the farther apart quarks are within a particle, the harder it is to separate them. Ultimately the effort required becomes so great that if one tries to tug loose a single quark, the energy that must be brought to bear creates a new quark and an anti-quark. The anti-quark bonds to the quark that has been freed, creating a meson; the new quark pops right back into the original particle, leaving it with the same number of quarks as before.
“The whole process is rather hard to visualize,” Glashow says, laughing. “It’s ... ah, a bit counterintuitive.”
Why experimenters had not seen weak neutral currents when they looked at strange particles but did see them when they looked at neutrinos was one of the most pressing problems in physics in the mid-1970s. Many scientists had hoped that a theory of quarks and the strong force would somehow solve the puzzle. They were disappointed. QCD had nothing to say about weak neutral currents. But charm did, Glashow claimed. Moreover, the fourth quark would provide a bridge between QCD and the electroweak theory, confirming both and paving the way to unification. Without charm, Glashow said, the theories could not be reconciled, either with each other or with experimental results.
“A lot of people were doubtful about the whole thing,”Nicholas Samios, the director of Brookhaven, says. “It came together very fast, and out of left field. Shelly said that SU(3) and SU(2) X U(l) had to fit together to make SU(3) X SU(2) x U(l). And he said that the way to join them was charm.”
In April of 1974 Glashow spoke before a conference of meson specialists that was held at Northeastern University, in Boston. In a speech titled “Charm: An Invention Awaits Discovery” he explained to the assembled experimenters that by virtue of the mechanism described in the Glashow-Iliopoulos-Maiani paper, a fourth quark existed, even though there was still no evidence for it. He set down his reasoning on a blackboard, cheering up some in the crowd when he botched the mathematics the first time through. Undeterred, Glashow challenged the meson physicists in the room: they were rightfully the ones to discover charm, because they were concerned with quarks, he said; but because they weren’t looking for it, charm would be found by “outlanders”—other sorts of experimenters. By the time of the next meson conference, Glashow said, “there are just three possibilities: One, charm is not found, and I eat my hat. Two, charm is found by (meson specialists], and we celebrate. Three, charm is found by outlanders, and you eat your hats.”
Two months later Iliopoulos threw down the gauntlet before the Seventeenth Annual International High Energy Physics Conference. An excellent orator, Iliopoulos delivered a tub-thumping defense of SU(3) x SU (2) x U (1). There are, he asserted, two fundamental classes of matter: quarks and leptons. There are four particles in each class, shuffled neatly into two families. Electromagnetism, the weak force, and the strong force are covered by two gauge theories consistent with each other, the whole being described in a grand synthesis called SU (3) x SU (2) x U(l). The synthesis demands that charm exist, Iliopoulos said, and he bet the assembled physicists bottles of fine wine that the fourth quark would be found before they met again. At stake, he said, was more than Glashow’s model. At stake was the validity of quantum field theory.
ALTHOUGH GLASHOW AND ILIOPOULOS DID NOT KNOW it at the time, their bets were safe. In fact, physicists at universities in Tokyo and Yokohama, led by Kiyoshi Niu, had found evidence of charmed particles three years before. Unfortunately, their paper was published as a letter to the editor in a journal that, though published in English, has few readers outside of Japan. (One American scientist calls it “a graveyard of great ideas.”) Moreover, although the Japanese knew that their discovery was important, they had never heard of charm, and they referred to their finding as a “new particle”—the “X.” Niu and his colleagues had found the X by analyzing the highly energetic bursts of particles created by cosmic rays; most European and American physicists considered this method so antiquated and unreliable that they paid no attention to Niu’s efforts to publicize his result. Hence Niu and his team played no role in establishing the validity of charm, and they got none of the credit for its discovery.
Nor did a second team of experimenters, at Brookhaven, that was turning up evidence of charm even as Glashow spoke. The first definitive evidence was published seven months later by two different teams—another at Brookhaven, and one at Berkeley and Stanford. When all was said and done, it took one more year and a fifth sighting before charm was accepted.
The laborious vindication of charm is an example of the difficulty of proving ideas in quantum field theory. Subatomic particles cannot be seen directly, because of the Uncertainty Principle, which rules out the possibility of “seeing” a particle in a particular place at a particular time. Moreover, most subatomic particles are so unstable that they must be created by huge machines, called particle accelerators, after which they live only minute fractions of a second before mutating into something else. The difficulty is magnified in the hunt for quarks, because quarks never appear by themselves, solitary and easily distinguishable; they are always part of a crowd.
finding charm was thus much harder than looking for the proverbial needle in the haystack. In this case, experimenters first had to make the needle, after which, of course, it vanished. Furthermore, they were looking for just a piece of the needle—a piece that they could never see directly.
Confronted with such obstacles, experimenters have developed a variety of ingenious mechanisms to persuade particles to reveal their doings. These devices do not perform the impossible task of making particles visible; rather, they get them to leave visible traces.
A particle accelerator (or “atom smasher,” as it is sometimes called in newspapers) is a big machine that breaks atoms apart and sends their protons or electrons flying. The stream of particle “bullets” thus created is shunted out of the accelerator through various channels into special components known as detectors. There the protons or electrons slam into other particles or into the nuclei of atoms. The pieces fly all over the place, and scientists try to measure their trajectories for clues to what is going on.
( The whole process has been likened to firing a gun at a watch to see what is inside.) Because accelerators house more than one detector, a number of groups can perform experiments simultaneously. Detectors have become increasingly sophisticated over the years. By the early 1970s the most sensitive design was a chamber in which a particle traveling through a liquid leaves a wake of minute bubbles, which are recorded on film.
The Hrst sighting of charm in the United States was made in a bubble chamber by a team of scientists at Brookhaven headed by Nicholas Samios. In 1974 Brookhaven had just finished building one of the largest bubble chambers in the world. Ten feet tall, the bright silver-colored chamber, ringed with wire and filled with liquid hydrogen, pointed skyward like a fat rocket. Surrounding the vat was a network of shiny cooling pipes and insulation to keep the hydrogen cold. A platform about fifteen feet above the ground allowed scientists access to three cameras mounted at portholes at the top. At the base of the detector a hydraulic jack rammed a thousand-pound piston in and out once every two-and-a-half seconds, creating the pressure at which particles will leave bubbles.
“It was a brutal thing,” Robert Palmer, a member of Samios’s team, says. “Standing on top of the machine while it was in operation was a terrifying experience. The piston moves with incredible force, and it’s heavy. The whole assembly was bolted to the ground, but despite that the thing would jump every time—”
“—and the ground would shake,” Samios cuts in. “When you came to work and drove in the parking lot, you could always tell when the experiment was running. You could feel the vibrations a hundred yards away.”
Samios designed an experiment to create neutrinos from one of the streams of protons generated by the laboratory’s accelerator, fire these neutrinos into the chamber, and watch for collisions between them and hydrogen nuclei. Such collisions would be extremely rare; neutrinos are affected only by the weak force, and can therefore slip through matter like greased pigs, touching nothing and being touched by nothing. In the time it takes to read this sentence, thousands of neutrinos from the sun will pass through the page and continue into the earth without a trace.
Brookhaven approved the procedure in August of 1973. Glashow, who was then a consultant at the laboratory, proposed to Samios shortly thereafter that the new experiment would be a good way to look for charm. A speeding neutrino might interact weakly with a proton still in the chamber—that is, a W + would pass from one to the other. Upon absorbing the W + , one of the down quarks inside the proton would change into a charmed quark, and for a fraction of a second—10 13 seconds, according to the theory—a charmed baryon would come into existence. The charmed baryon would not survive long enough to be photographed. But when it decayed (and the charmed quark changed again, this time into a strange quark), it would become a strange baryon, called a “lambda.” The lambda, too, would eventually disintegrate into other particles, but it would hang around long enough to be detected.
Samios would be able to tell that the lambda was the artifact of a charmed baryon, rather than of the neutrino’s initial collision, because, as it happens, neutrinos cannot produce lambdas directly. Glashow explained that if Samios could find an event in which a neutrino had created a lambda, this would constitute strong evidence of charm’s existence.
The experiment started in January of 1974. Except for occasional breaks for maintenance, from then until March of 1975 the particle accelerator shot a billion neutrinos into the bubble chamber every two-and-a-half seconds. Twenty-four hours a day, seven days a week, the three cameras atop the detector snapped pictures, capturing on 70mm film whatever was inside.
By the end of the run Samios’s team had taken more than 500,000 pictures. According to Glashow’s theory, interactions between a neutrino and a proton should have been recorded on about 2,000 pictures—or less than one percent. The problem now was to find the right ones, a remarkably time-consuming process.
Brookhaven hired nonscientists to do the job. Mostly housewives with families who needed extra income, they worked round the clock, three shifts a day, at specially constructed projectors that flashed the images onto white tabletops. The scanners, as the women were called, copied the tracks of potentially interesting events into sketchbooks and measured the various angles and lengths.
When a picture taken inside a bubble chamber is blown up on a screen, the result can be astonishingly lovely: white lines skirl across a gray field, intersecting, branching out, and looping, like the epicycles in a Ptolemaic drawing of the heavens. Milda Vitols, who supervised the scanners, showed them how to decipher the tangle. Each type of particle leaves its own characteristic track—or “signature”—in the general snarl, and with experience anyone can learn to read these signatures. Electrons, which are quite light, produce faint tracks as they zip through the hydrogen at nearly the speed of light. Protons, which are almost two thousand times more massive than electrons, and which travel more slowly at the energies that Samios had employed, leave lines like thick pen strokes across the picture. Being electrically neutral, a lambda would not leave tracks, but its decay would yield two particles whose tracks would be visible. Vitols told the scanners to look for two rays appearing out of nowhere and shooting off in a V. (This would be the decay products of the lambda.) If the scanners saw a V, she said, they should look to see if the vertex pointed back to the vertex of another set of tracks. (This would be the locus of the initial collision between the neutrino and the proton.) If they found a second group of lines near the first, Vitols said, the scientists would be very excited, because the configuration would mean that their quarry, the lambda, had glided silently between those two points for the brief flicker of its existence.
On May 30, 1974, Helen LaSauce, a former switchboard operator and the mother of three, advanced her scanning machine to frame 6,907 of roll 27 and spotted an obvious collision. A pair of tracks parted from one vertex, and close by five tracks led away from another. It took Samios, Palmer, and the other members of the team seven months to exclude every possible source of experimental error—a task that was complicated by their inexperience with the new bubble chamber and by the continuing lack of a similar event with which to compare the one that LaSauce had found. Even when they had finished, they were reluctant to assert that they had discovered a new state of matter on the basis of one event in an untried detector, so they waited a bit longer—in vain, it turned out—to see whether at least one more had occurred.
WHILE SAMIOS DOUBLE-CHECKED HIS INCONCLUsive result, two other groups of experimenters came up with definitive evidence of charm. Whereas Samios had found a charmed baryon, these teams came up with mesons made from charmed quarks. One team, headed by Samuel C. C. Ting, of MIT, drew protons from the same accelerator at Brookhaven that Samios and Palmer had used. The other, headed by Burton Richter, followed a different procedure at the Stanford Linear Accelerator.
Ping says that when he proposed his experiment, in January of 1972, he had never heard of charm. In fact, he is proud—to the point of arrogance, his detractors say—of carrying on his work without regard to what the theorists are doing. “There are two kinds of experimenters,” he says. “One does what the theorists tell them to. The others follow their own ideas. I am of the second kind. Now, I’m happy to sit down and eat Chinese dinner with theorists. But to spend your time doing what they say to do is wasted effort.”
Ting has spent much of his career studying pairs of electrons and positrons, which are easily controlled in experiments and which yield useful information about the characteristics of other, related particles. (A positron is the positively charged counterpart of an electron.) Ting used the Brookhaven accelerator to smash protons into atomic nuclei, because this process often produces showers of mesons. Although some types of mesons decay too quickly to be directly observed, they can be studied by means of the products of their decay: electron—positron pairs. Ping wanted to see if new varieties of mesons could be created at higher energies than had been examined before. Early in the fall of 1974 he was told that the machine was producing many electron—positron pairs at such higher energies—about 3.1 billion electron volts, or three GeV. (An electron volt is a small unit of energy used by particle physicists, and one billion electron volts equals one GeV.) After weeks of cross-checking, Ting was finally prepared to announce that he had come across a new particle. He named it the “J particle,” because “J” is the symbol in physics for the electromagnetic current.
In the meantime, Richter’s team found the same particle by yet another method. Their experiment used a new accelerator that causes direct collisions between electrons and positrons; it also employed a new detector to review the resulting debris. Usually such debris consists of muons, but at the right energies various types of mesons can be produced, which make themselves known by the artifacts of their decay. When Richter’s team discovered a surge in the number of these artifacts, they knew that they had discovered a new meson. They christened it the “psi.”
The particle that Ting and Richter discovered is now called the “J/psi" by everyone except Ting and Richter, who refer to it as the “J” and the “psi,” respectively. The J/psi electrified the physics world, for it wasn’t immediately clear how the new meson fit into any of the models. Ting had found 242 events, and he was told that theorists had as many ways to explain the new particle. In December, a month after the discovery, he met Glashow at Brookhaven and heard that the J/psi was probably made up of a charmed quark and a charmed anti-quark. It was the first time that Ting had seriously considered the fourth quark, and he liked the idea, but he also liked some of the 241 others. Impatient with theoretical pettifogging, he turned to other work.
Richter, on the other hand, was aware of charm, but he also knew that if charmed quarks existed, they should combine with other, noncharmed quarks (specifically, antiquarks) to form mesons. A year and a half went by before one of Richter’s collaborators, Gerson Goldhaber, of Berkeley, found the mesons.
That data convinced even the most diehard skeptics: charm had been confirmed, by Glashow’s “outlanders.” In Paris, Iliopoulos received his wine. And at the next meson conference the scientists ate candy Mexican hats. “They were extremely peppery—not a pleasant candy,” Roy Weinstein, the physicist who presided over the meeting, says. “But everyone ate them with relish, and agreed that Shelly’s prediction had been magnificent.”
T THE SAME TIME THAT THE EXPERIMENTERS WERE confirming SU(3) X SU(2) x U(l), the prospect of a Nobel Prize began to creep into the minds of the physicists involved. Established through a posthumous bequest by the Swedish explosives manufacturer Alfred Nobel, the prizes are administered by Swedish academics who have a reputation for being extremely cautious. (For example, no Nobel citation has ever mentioned quarks explicitly, evidently because the Swedish Royal Academy is reluctant to accept the idea.) By the terms of Nobel’s will, no award in a given category may be shared among more than three people, and no discovery may be honored twice.
Many, if not most, prizewinners have an ambivalent attitude toward the Nobel, which brings unwanted attention even as it gratifies the ego and the purse. (A single winner can receive $150,000 or more.) Few physicists would say publicly that one of their colleagues who had won a Nobel had not merited it, but most would agree that the award process has slighted some deserving scientists. Selecting the Nobel prizewinner is relatively easy when one person makes a discovery with little or no help, as Planck, the winner in 1918, did when he proposed the idea of the quantum. The sudden coming together of SU(3) x SU(2) x U(1) in the early 1970s, however, was a completely different situation. It represented the abrupt and largely unexpected fusion of the work of many researchers. Moreover, because this fusion vindicated field theory, which had been out of favor for years, few of the prior steps— SU(2) x U(l), for example—had been awarded prizes.
There was considerable scuttlebutt about who the prizewinners should be. Yang and Mills had started gauge field theory in 1954, but Yang had won the prize in 1957 for something else. Schwinger, too, had already received a Nobel, for demonstrating how to renormalize QED. The academy had also given one to Gell-Mann, not for quarks but for “discoveries concerning the classification of elementary particles.” Glashow had first put SU(2) x U(1) together, but his formulation had been incomplete; Salam and John Ward had produced the same model later. Several British and Belgian theorists had developed the principles of broken symmetry, which Weinberg had subsequently added to SU(2) X U(1). A similar notion had been presented by Salam the following year, but it was buried in an article that Salam had not had time to prepare fully. Later,’t Hooft had renormalized the theory with Veltman’s help—a great accomplishment, but one that some scientists felt was in the realm more of mathematics than physics. The physicists Oscar W. Greenberg, Moo-Young Han, and Yoichiro Nambu had had the first inklings of QCD; collaborating with Gell-Mann, the German scientist Harald Fritzsch had also made contributions, as had others, such as, again, Weinberg. This list does not even include the experimenters. In the charm experiments alone, there had been Kiyoshi Niu and his team, who had, after all, seen the first charmed event even though they had not recognized it as such; Samios and his team, who had been the first to find charm and call it that but who had published fourth; Ting and Richter, each with his own roster of collaborators; and Gerson Goldhaber, whose results had convinced the physics community that the fourth quark exists.
The Swedish Royal Academy of Sciences laid to rest some of the speculation when it gave the prize in 1976 to Ting and Richter—not for the discovery of charm, of course. The judges cited only their “pioneering work in the discovery of a heavy elementary particle of a new kind.”The award was not universally popular. It is easy to find experimenters who dislike Ting, despite his eminence. And it is equally easy to find people who will say that a member of Richter’s team somehow learned of Ting’s discovery and duplicated it before it could be announced. (Both Richter and Ting say that this did not occur.)
After 1976, discussion concentrated on the theoretical side, and the field narrowed. From the first, Weinberg was a likely candidate. Although his 1967 paper had been ignored initially, he had been lucky enough to have it extensively discussed by ‘t Hooft. Salam had been less fortunate. His work had been similar, but it had been published a year later in the proceedings of an unimportant Swedish conference. Moreover, the article consisted simply of a transcript of the lecture Salam had given, and the correct electroweak model, with symmetry-breaking, was not stressed. (Salam explains that he was too busy setting up a new institute of physics in Trieste to revise and develop his work.) For a while physicists had referred to the “Weinberg model” of weak interactions, but then Salam had sent a letter to a number of scientists asking them to call it the “Weinberg-Salam model,” and Weinberg had gracefully accepted the change. When the Nobel committee followed its annual practice of asking prominent physicists and former prizewinners for their recommendations, at least two are said to have made sure that the committee knew of Glashow’s contributions. Others argued that ‘t Hooft should get a share.
The politicking had personal consequences, the most severe of which was the unraveling of the decades-long friendship of Glashow and Weinberg. The subject is so sensitive that few physicists are willing to discuss it. Weinberg himself, who is now at the University of Texas at Austin, politely but firmly refused our requests for an interview in connection with this article.
“Steve and Shelly are complete opposites,” someone who knows both well says. “Steve is a royalist, Shelly a revolutionary anarchist. Steve works best by himself. Shelly works best with others. He’s a futzer. He arrives in the morning with four or five wild ideas, most of them wrong, and expects other people to tear them apart. Steve is sensitive and private, while Shelly is gregarious and tends to view a person like Steve as an anthill to poke sticks down. It would be surprising if a tension hadn’t developed between them sooner or later.”
By the late 1970s the two had become openly hostile. Weinberg wrote a review paper that managed to discuss the history of charm without once referring to his former classmate. Annoyed, Glashow took to telling people that he was the originator of the Weinberg angle and that its correct name was the “weak mixing angle.” (“It does begin with the same letter,” he says.) To the uninitiated this sort of slight might seem trivial, but scientists rely on references to their work in articles and footnotes to sustain their reputations. Glashow reports with some delight that when he spoke at Stockholm in 1978, a year before he won the Nobel, a wizened member of the Swedish Academy appreached him and said (here he does a surprising, rather good imitation of a decrepit Scandinavian academic): “Jaa . . . thees Weinberg enkle, is not same as yoor enkle from sixty paper, ja?" The speech that Glashow had just given had included an exploration of the possibility that gauge theory might be wrong. Talking to an audience of Nobel Prize-giving Swedes about a topic that could jeopardize the prize for everyone involved is classic Glashow (“He proposes ideas just to get other physicists annoyed,” Georgi says, laughing.) Weinberg is said to have called Glashow the night before to argue that the speech was irresponsible.
When the 1979 Nobels were announced, the prize for physics was bestowed in equal shares upon Glashow, Salam, and Weinberg, “for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including inter alia the prediction of the weak neutral current.” The award for the electroweak theory was interpreted by most as a seal of approval for SU(3) x SU(2) x U(l) as well, because the confirmation of charm’s existence had proved that SU(2) X U(l) does not make sense without SU(3).
The prize somewhat eased the tension between Glashow and Weinberg, and in the final footnote of the printed version of Glashow’s award speech he thanked his “high school friends Gary Fein berg and Steven Weinberg for making me learn too much too soon of what I might otherwise have never learned at all.”
PHYSICS TEXTBOOKS PRESENT IDEAS IN A CLEAR, LOGIcal way, and move from simple premises to their more complex corollaries. But physicists work in a different way, and their handiwork—the science—advances in fits and starts. At any given time thousands of ideas are discussed, and hundreds of experiments are performed, but only a few seem truly significant a few years later. It is often impossible to predict when an article is published whether or not it will be important years later; revolutionary approaches can be lost in the thickness of conference proceedings and rediscovered by scientists when the conference is only a dim memory; brilliant experiments may lie unnoticed in journals and Ph.D. theses; good ideas may be put forward and forgotten, or derided and dismissed because the field is not ready to absorb them.
Amid the tumult of the discovery of charm, a paper that used SU(3) x SU(2) x U(l) as a base to go far into the unknown had been published to little serious notice; it was so radical that most physicists had simply scoffed at it. In 1973, months before Iliopoulos’s impassioned proclamation of SU(3) x SU(2) x U(l) and the discovery of the J/ psi, Glashow and Howard Georgi had produced the first Grand Unified Theory. Like SU(2) x U(l), the GUT was ignored for years. To believe that it was plausible, one had to believe in the standard model. To believe in the standard model, one had to believe in charm. And at the beginning of 1974, when the first GUT was published, the believers in charm included Glashow, Georgi, and barely enough other scientists to fill a small room.
Georgi is thirty-seven now—“getting senile,” he says— but in 1973, when he collaborated with Glashow on the GUT, he was a third-year postdoctoral fellow at Harvard. Glashow thought their talents were perfectly complementary. Sitting in his office he would spin out one idea after another, and snarl in mock fury as Georgi shot them down. “He comes at you in the morning with ten ideas he’s had since the day before,” Georgi says. “You have to tell him what’s wrong with them, which means you have to have this kind of automatic computer in your brain to spit back responses. But if you can’t figure out what’s wrong with an idea right away, then you go to work on it.”
Although to outsiders the collaboration can seem anything but harmonious, Glashow and Georgi have enjoyed it immensely. In 1973 and 1974 alone they wrote four important papers together, one of which set out the first Grand Unified Theory.
When SU(2) x U(l) was coming together, Gell-Mann says, he was fond of needling his colleagues by informing them that it was not a fully unified theory. “It’s a mixing,” he says. “That’s why you have a mixing angle, because the electromagnetic and weak forces are not really unified. A truly unified theory would show how both the weak and the electromagnetic forces are different aspects of the same interaction.”
The notion that the weak and electromagnetic forces could be directly unified occurred to Glashow on the day, in October of 1973, when he first heard about SU(3), the gauge theory of quantum chromodynamics. He saw immediately the rough outline of the theory that he was looking for. He needed to find yet another group—one large enough to contain both SU(3) and SU(2) x U(1). Such a group would permit him to put quarks and leptons into one family and would allow lor a unified description of the strong, weak, and electromagnetic forces. The obvious difficulty was that these forces didn’t look as it they could be unified, inasmuch as the strong force is a hundred times more powerful than electromagnetism, and both are enormously stronger than the weak interaction. Somehow the differing strengths would have to be reconciled.
The relative strength of a force between two objects depends in part on such variables as their masses and the distance between them. Above all, it depends on the “coupling constant,” an abstract number whose physical basis need not be explained here. Each force in the universe has a coupling constant, which has been measured by experiment and describes that force’s absolute strength.
To produce a truly unified theory of the strong, weak, and electromagnetic forces, one would have not only to find a group to contain all three but also to link them by means of a single coupling constant. Glashow laid all this out for Georgi, who is an expert model builder, and the two sat down to work out a theory.
Georgi remembers the day well. True to form, he and Glashow spent the afternoon arguing furiously about how to proceed. Unable to hammer out a solution, each left Lyman Hall in some distress. Later that evening Georgi sat down at his desk at home to work on the problem further.
“I first tried constructing something called the SO(10) model, because I happened to have experience building that kind of model,” Georgi says. “It’s a group in ten dimensions. The model worked—everything fit neatly into it. I was very excited, and I sat down and had a glass of scotch and thought about it for a while.
“Then I realized this SO(10) group had an SU(5) subgroup. So I tried to build a model based on SU(5) that had the same sort of properties. That model turned out to be very easy too. So I got even more excited, and had another scotch, and thought about it some more.
“And then I realized this made the proton, the basic building block of the atom, unstable.” If the proton was unstable and would eventually fall apart, so would all atoms—and thus all matter, Georgi realized. If the model on his scratch pad was true, then the universe would ultimately disintegrate. He says, “At that point I became very depressed and went to bed.”
The next morning he arrived in Glashow’s office with “some good news and some bad news, he recalls. The good news was SU(5), of course, and the bad news was proton decay. His colleague didn’t take the bad news hard. “It wasn’t shattering,”Glashow says. “I mean, we know the sun will burn out in a few billion years. This is known. It’s a fact. Spaceship Earth and all that—poof! That matter falls apart a long, long time afterward is scarcely an upsetting idea. It’s bad enough as it is.”
Glashow was troubled by the question of why the protons in the universe had not already decayed, given SU(5)’s implication that they could have. The two men raced upstairs to the Harvard physics library to look up what experimenters had found to be the minimum lifetime of the proton. The figure was 10 27 years, or “1" followed by twenty-seven zeros—trillions of times the present age of the cosmos. Georgi and Glashow sat down to figure out how to make the theory predict that protons would hold out that long.
“My immediate reaction was that we had to make the virtual particles that bring about proton decay very heavy,” Glashow says. “That meant they wouldn’t appear very often. This we could do thanks to Steve, who had taught us in his 1967 paper how to give the intermediate vector particles a big mass. Only in our case they had to have a much bigger mass—a thousand trillion times bigger than anything that had ever been seen.” In fact, the proton-decay particle was later calculated to have a mass of almost a billionth of a gram—nearly heavy enough to be weighed on a scale.
Glashow loved the idea of monstrous subatomic particles. “It was another way for him to throw rocks at the establishment,” Georgi says. “Nobody we knew had ever even talked about elementary particles that heavy. Not even within twelve orders of magnitude of that.
GEORGI AND GLASHOW SENT SU(5) TO PHYSICAL Review Letters in January of 1974. They had decided not to be shy about what they were doing, and had given the paper the most imposing name they could think of: “Unity of All Elementary-Particle Forces.”Only three and a half pages long, the paper linked nearly all of the significant discoveries in subatomic particles that had been made in the past quarter-century. The mathematical tools it employed came from group theory and Yang and Mills; articles by Gell-Mann, Glashow, ‘t Hooft, Salam, Weinberg, and others were cited. Quarks, symmetry-breaking, and gauge fields all played a role.
Georgi and Glashow opened with a fanfare as brassy as that in any recent scientific paper:
We present a series of hypotheses and speculations leading inescapably to the conclusion that SU(5) is the gauge group of the world—that all elementary particle forces (strong, weak, and electromagnetic) are different manifestations of the same fundamental interaction involving a single coupling strength. . . . Our hypotheses may be wrong and our speculations idle, but the uniqueness and simplicity of our scheme are reasons enough that it be taken seriously.
They then proceeded to lay out the ground rules, acknowledging their sources. (The italics are Georgi and Glashow’s.)
Our starting point is the assumption that weak and electromagnetic forces are mediated by the vector bosons of a gauge-invariant theory with spontaneous symmetry break-ing. A model describing the interactions of leptons using the gauge group SU(2) x U(1) was first proposed by Glashow, and was improved by Weinberg and Salam, who incorporated spontaneous symmetry breaking.
In the next paragraph they introduced what was then little more than a favorite hypothesis of Glashow’s: charm.
To include [baryons and mesons] in the theory, we must use the Glashow-Iliopoulos-Maiani . . . mechanism and introduce a fourth quark . . . carrying charm.
In the fourth paragraph they invoked QCD. They now had the entire standard model, which was then years from being demonstrated in experiment.
Thus, we see how attractive it is for strong, weak, and electromagnetic interactions to spring from a gauge theory based on the group F— SU(3) x SU(2) x U(l). Alas, this theory is defective in one important respect: It does not truly unify weak and electromagnetic interactions. The SU(2) x U(l) gauge couplings describe two interactions with two independent coupling constants; a true unification would involve only one.
Next Georgi and Glashow hunted through the list of groups that Cartan had compiled decades before, and came up with three that could provide the basis for a truly unified electroweak theory. These three had already been used in models made by Weinberg and, later, Georgi and Glashow. None was found to be reconcilable with SU(3). This allowed the writers to conclude, with some pleasure, “We see we cannot unify weak and electromagnetic interactions independently of strong interactions.” They then declared themselves forced to consider the “outrageous possibility” that a single group could account for all three. Methodically they set down all of the groups of sufficient size. As it happens, there are nine. One by one, these were eliminated, except the last, which was, of course, SU(5)— the basis for the model that Georgi had created over shots of scotch a few months before.
The last half page of the paper was devoted to characterizing SU(5) and its implications, which are multifarious. To start, saying that forces are equivalent implies a similarity among the particles with which they interact. Therefore, within this Grand Unified Theory (as well as within each variant that has been proposed since) quarks and leptons are kith and kin. Particles within a family can decay into one another, and thus Georgi and Glashow had to postulate the existence of a brand-new, never-before-seen force that would allow for just that possibility. They named this force the “superweak,” and suggested that it is mediated by a new set of extremely heavy vector bosons, which Glashow has since nicknamed “ponderons,” or “vector basketballs.” These particles weigh too much to be emitted often, but when a quark does emit such a particle, that quark turns into a lepton. If the quark is inside a proton, the proton will fall apart. The pieces of the proton will, in turn, eventually decay into photons, which will ultimately traverse an utterly empty space. Because protons are a nonrenewable resource, the universe will end as it began, with light. Eventually the light will dissipate, and the cosmos will settle into a long darkness. Obeat lux.
In their paper Georgi and Glashow, afraid of the reaction that this implication would provoke, refrained from mentioning it until the end: “Finally, we come to a discussion of superweak interactions and . . . superheavy vector bosons. In addition to mediating [some] bizarre interactions . . . they make the proton unstable.”
“Unity of All Elementary-Particle Forces” appeared in February of 1974, and initially met with little favor. Most theorists didn’t even notice it in the flood of models that had been provoked by’t Hooft’s proofs. Some of those who did pointed out that the SU(5) paper failed to provide any clear reason for supposing that the coupling constants could be made equal. But a reason was provided a few months later, in a paper that Georgi wrote with Weinberg and Helen Quinn, who, like Georgi, was a postdoctoral fellow at Harvard.
“Hierarchy of Interactions in Unified Gauge Theories” was published in Physical Review Letters in August of 1974. In it the authors pointed out that the coupling constants of electromagnetism and the weak force are not true constants at all, but vary with the energy of the interactions. In fact, as the energy of the interactions increases, the values of the “constants” change. At 1014 GeV and higher, the coupling constants converge. More important, at these high energies the coupling constant of the strong force dips down, and ultimately it joins that of the other two. Where the coupling constants merge, the three forces reveal their unity. (Hopes for a complete unification rest on the belief that the gravitational coupling constant must behave in a similar way.)
Now, 1014 GeV is a lot of energy for one particle to have. It corresponds to a temperature of about 1028 degrees Fahrenheit—one so extreme that it has not been reached in the universe since the first fraction of an instant after the Big Bang. Thus, whereas Georgi and Glashow had shown that uniting the strong, weak, and electromagnetic forces is theoretically possible, Georgi, Quinn, and Weinberg showed that these forces had actually been united only at the beginning of time, after which they had separated like curds and whey. When Georgi, Quinn, and Weinberg plugged the conjoined coupling constant into SU(5), moreover, they found that the average lifespan of protons is roughly 1031—ten quadrillion quadrillion—years.
SU(5) COVERS BOTH THE BEGINNING AND THE END OF existence. And the model provides clues for hooking gravity into it as well, for total unification. Since the confirmation of charm and weak neutral currents, theorists have been scrambling to decipher these clues, and they have produced various hypotheses known as “supersymmetry,” “supergravity,” and “quantum gravity.” All use Georgi and Glashow’s GUT as a starting point, in the same way that SU(5) uses SU(3) x SU(2) x U(l).
The result is a ladder of theories. Firmly on the bottom is SU(3) x SU(2) x U(l), whose predictions have been confirmed (“to the point of boredom,”Georgi says) by Ting, Richter, and others. The Wand Z particles were discovered at CERN, an international physics laboratory in Geneva, Switzerland, last year, but the theory was so well established by then that the event was—for theorists, at least—an anticlimax. The GUTs proposed by Georgi and Glashow and other physicists, which fully unite the strong, weak, and electromagnetic forces, are the next rung on the ladder. Although as yet unconfirmed, these theories are considered compelling by most physicists, finally, at the top of the ladder, in the theoretical stratosphere, are supersymmetry and its cousins, which are organized according to a principle somewhat different from SU(5), though, like that model, they put apparently different particles together in groups. Supersymmetry groups are large enough to include gravity, but are so speculative that many experimenters doubt that they can ever be tested.
“Do you remember Antaeus, from Greek mythology?" Maurice Goldhaber, a physicist at Brookhaven, asks. The brother of Gerson Goldhaber, whose discovery clinched the case for charm, Maurice is skeptical of the current trend toward very speculative theories. “Antaeus was the strongest person alive, invincible as long as he was in contact with his mother, the earth. Once he lost contact with the earth, he grew weak and was vanquished. Theories in physics are like that. They have to touch ground for their strength.”
Nevertheless, it is easy to see why physicists are attracted to such speculation. The emerging view of the universe is compellingly simple. Although the models differ, the broad thrust of each is the same, and all promise to describe not only the relationships among the types ot forces and matter in the cosmos but also the history of the cosmos.
On the far side of the Big Bang is a mystery so profound that physicists lack the words even to think about it. Those who are willing to go out on a limb guess that whatever might have been before the Big Bang was, like a vacuum, unstable. Just as there is a tiny chance that virtual particles will pop into existence in the midst of subatomic space, so there seems to have been a tiny chance that the nothingness would suddenly be convulsed by the presence of a something.
This something was an inconceivably small, inconceivable violent explosion, hotter than the hottest supernova and smaller than the smallest quark, which contained the stuff of everything we see around us. The universe consisted of only one type of particle—maybe only one particle— that interacted with itself in that tiny, terrifying space. Detonating outward, it may have doubled in size every 10 35 (one divided by 10 35) seconds or so, taking but an instant to reach literally cosmic proportions.
Almost no time passed between the birth of the universe and the birth of gravity. By 10 43 seconds after the beginning the plenum was already cooler, though hardly hospitable: every bit of matter was crushed with brutal force into every other bit, within a space smaller than an atomic nucleus. But the cosmos was cool enough, nonetheless, to allow the symmetry to break, and to let gravity crystallize out of the unity the way snowflakes suddenly drop out of clouds. Gravity is thought to have its own virtual particle (the graviton), and so the heavens now had two types of particles (carriers of forces and carriers of mass), although the distinction wasn’t yet as clear as it is in the universe today.
At 10 35 seconds the strong force, too, fell out of the grand unified force. Less time had passed since the Big Bang than it now takes for a photon to zip past a proton, and yet the cosmos was beginning to split. Somewhere here, too, the single type of mass-carrying particle became two—leptons and quarks—as another symmetry broke, never to be complete again. The universe was the size of a bowling ball, and 1060 times denser than the densest atomic nucleus, but it was getting colder and thinner rapidly.
One ten-billionth of a second after the Big Bang the firmament reached the Weinberg—Salam—Glashow transition point, and the tardy weak and electromagnetic forces broke away. All four interactions were now present, as well as the three known families of quarks and leptons. The basic components of the world we know had been formed.
“Let me draw the whole picture for you,” Glashow says, putting a line in white chalk from one end of the blackboard to the other. “This represents the universe from the beginning to the end of time.”After thinking a moment, he draws a second line in purple chalk; it is just a bit shorter. “We live in the fortunate era—the era in which there is matter. Matter first appeared 10 38seconds or so after the Big Bang, and will all disappear maybe seconds from now.” He hunts around in the tray below the board, finally happening upon a piece of brown chalk, which he uses to plot a line considerably shorter than the first two. “Within the fortunate era of matter there is a somewhat shorter period in which atomic nuclei exist, because the universe is cool enough to permit them to form. And within that”—a red line now, barely a foot long—"an even tinier domain in which there are atoms. He makes a dot near one end of the red line. “Then you have a brief ten billion years or so when things are palatable on Earth.
“After that, eventually it all winds down. We won’t be bothered when, say, ten percent of the protons go. If we re still around, we may not even notice. But when ninetynine percent go, you won’t have enough left to make a person, and it will be unpleasant. After this point there’s a long period of decline, and a very boring period it is, too. This is not a view we could have gotten without Grand Unified Theories. We may not like it, but we have no choice.”
Some physicists, in fact, do not like it, nor do they like the general effort that has brought so many in their field around to Glashow’s view. Glashow’s former teacher Julian Schwinger is one. He says, “Unification is the ultimate goal of science, it’s true. But that it should be unification now—surely that’s the original definition of hubris. There are a heck of a lot of energies we haven’t gotten to yet. Grand unified schemes make the implicit assumption that what happens in them is not fundamentally different from what we know—an assumption about how nature works that’s in conflict with what we’ve spent the last century learning.”
WHETHER GLASHOW OR SCHWINGER IS CORRECT IS A matter for experimenters to determine. The Grand Unified Theories make two further predictions, one of which is proton decay—an idea that Glashow phrases as the question “Are diamonds forever?”
Maurice Goldhaber has been charting the lifetime of the proton for some thirty years. He says, “They used to say the proton was immortal. But I asked myself, how do they know the proton has a guardian angel watching over it? This is not a question for theorists. It’s really a question for experimenters. You cannot just tell your children that the proton is stable. You have to go find out. There are two ways to do it. You can watch one proton for quadrillions of years to see if it decays, or you can watch quadrillions of protons for one year to see if any of them decay.” The more protons that remain stable as you watch, the longer it must take for the last proton to disappear. In 1953 and 1954 Goldhaber began to gather a lot of protons in one place to see if they would decay. Eventually he and other experimenters succeeded in proving that, on the whole, protons are good for at least 1027 years—the lifetime that Georgi and Glashow found in a book in Lyman Hall.
Today Goldhaber is at work on a proton-decay experiment with scientists from the University of California at Irvine, the University of Michigan, and Brookhaven. Located in a salt mine half a mile deep by the shore of Lake Erie, the experiment consists, as Glashow puts it, of “a big swimming pool lined with plastic garbage bags.” The swimming pool is filled with 8,000 tons of water so pure that it would dissolve the oil out of your skin if you stuck your hand in it. (Eight thousand tons of water hold about 1034 protons.) It took two years to dig the hole, which measures seventy feet on each side, seal it with polyethylene, and line it with more than 2,000 photodetectors. On the day the experiment began, Goldhaber toasted the collaboration with the words “To the proton—may it live forever! But if it has to die, let it die in our arms!”
For almost two years, experimenters have been watching the tank. So far they have found no sure sign of proton decay; the minimum lifespan is now 1032 years and climbing. Other, smaller tests have turned up what might prove to be evidence of proton decay, but, as Goldhaber says, “Not every candidate is elected.”
Glashow is unperturbed. “I’m convinced the theory is sound. We can fiddle some more with the proton lifetime, if the thing comes to that. But the essential framework of the GUTs simply explains too many things for it to be fundamentally wrong. We have the outlines of a full theory, and a few experimental results aren’t going to change that. Besides, it’s a mistake for theorists to take too seriously the first batch of experimental results.”
Although Glashow is sanguine about the ultimate result of the proton-decay experiments, he is much less happy about the second prediction implicit in the Grand Unified Theories—namely, that proton decay is the only major new phenomenon that experimenters will find. According to SU(5), all the important aspects of the strong, weak, and electromagnetic interactions are understood, and thus the immense range of energy that lies between the 1,000 GeV that accelerators today can provide and the 1014 GeV needed to confirm the GUTs should contain no surprises.
That vacant expanse—in which the sole landmark is proton decay—has come to be known as “Glashow’s desert.” Proud though Glashow is of the giant step toward unification that SU(5) represents, he and most of his colleagues believe that the desert does not exist, and that therefore the model is wrong—or wrong in part, anyway. SU(5) is successful as a prototype, in other words, but it will probably be replaced by another model.
Already experimental evidence may have put a dent in SU(5). Late last March two groups of researchers at CERN stunned the physics world when they announced that the same experiments that had turned up the W and Z particles had also happened upon bizarre subatomic events that apparently are not predictable by either SU(5) or the standard model. In mid-May, Carlo Rubbia, a physicist at Harvard who leads one of the teams at CERN, gave a talk at a centennial celebration of the Harvard physics laboratory in which he described still other anomalies so peculiar that some physicists immediately dubbed them “Zen events.”
These discoveries have yet to be confirmed, because the heavy demands on CERN’s accelerator wall not allow Rubbia to use it again until the end of September.
Although physicists don’t yet know what the new findings mean, these seem to be associated with the Z°. When Rubbia and his colleagues examined the subatomic collisions that produce Z°s, they found that these particles decay by shooting out photons in a way that the standard model does not seem to explain. When the teams at CERN looked closely at the data, they also discovered unexpected sprays of subatomic particles (“jets,” in the jargon). These may indicate that the Z°s are created by the decay of heavy particles that, according to Georgi, “are not on any current map.”
There was another interesting result. When protons and anti-protons collided at the unusually high energies Rubbia was employing, the subatomic debris created should have flown out in several directions, but instead, in a few instances, it flew out in only one. It is specifically this behavior that physicists have in mind when they jokingly reler to “Zen events,” because it is suggestive of the Buddhist conundrum about the sound of one hand clapping. “A large amount of energy seems to disappear,” Rubbia says. His sharp and obvious pleasure in being on the trail of something unknown is evident in his voice. “It is completely unaccounted for! Its disappearance could be the sign of a neutral, weakly interacting particle of a kind we’ve never seen.”
Rubbia continues, “We don’t know if we’re seeing one phenomenon or aspects of several different phenomena. I can only tell you this: the probability that it is an error or background—or, better, that it is irrelevant—is, according to the standard laws of physics, extremely small. I could be wrong. In this kind of stuff, there is always room for error. But we have a clean, strong signal, not one of these muddy is-it-or-isn’t-it messes, You can see the damn things.”
The news from CERN has invigorated theorists. Before the findings were even published, physicists were discussing Rubbia’s surprises at congresses and seminars, gossiping about them in corridors, and scribbling speculations based on them on napkins in laboratory cafeterias. Theorists are riveted by the possibility that the results at CERN are a signpost pointing the way at least to a confirmed Grand Unified Theory, if not, someday, unification itself.
Although Gerald Feinberg is excited by the results, he cautions, “It’s still too early to say. These events could look like things that are not in the standard model but really be compatible with it, because nobody has explored the implications of the standard model thoroughly enough. People are still learning about it. Georgi, too, thinks the findings are unlikely to challenge the basic validity of SU(3) x SU(2) x U(l), but hopes they will ultimately help physicists go beyond this theory.
Glashow, for his part, is delighted by the anomalies. “Physics has gotten interesting again,” he says cheerfully. “I appear to have been proven decisively wrong about the desert, thank God. I don’t think it will happen, but there’s the exciting possibility that thirty years of work will go down the tube, and that everything we know is totally wrong. I myself have written two papers alleging to explain why strange things should happen with Z°s. I submitted one of them just yesterday to Physics Letters. It should be out in August sometime. In it I claim that what we’re seeing is the first manifestation of a fifth force of nature—the ‘smelly force.’ It has to do with a previously unknown property I call ‘odor.’ Odor goes along with color, right? I note that the O(18) group, which I’m pushing, predicts that there could be hundreds of odoriferous particles—or maybe I said ‘odorous.’ Anyway, I’m really talking structure.”
He chuckles as he anticipates the reaction of his colleagues to this idea. (Rubbia, in fact, upon being told that he might be the discoverer of the smelly force, guffawed and said, “Shelly’s got a million ideas, doesn’t he?" Rubbia paused for a moment, and then said, “On the other hand, come to think of it, there might be something to it.”)
“Listen to this,” Glashow says, grabbing the manuscript of his odor paper. “It’s highly technical, most of it, but here’s the good part. I say, ‘An example of a Grand Unified Theory producing effects such as we describe is 0(18) with [all particles that are not bosons] belonging to a single 256-dimensional representation. At low energy, the effective field theory can be based on the subgroup 0(5) x SU(3) x SU(2) X U(l), with 0(5) and SU(3) unbroken and SU(2) X U(l) spontaneously broken to electrodynamic U(l). This is not, of course, the whole story. . . . The 0(18) model also involves twenty-six colorless but odorous leptons.’
He breaks off. “I was thinking about ending it with the words ‘No desert here,’ but I decided the editors would argue too much about it. Anyway, there’s a lot of other good ideas people have been tossing around. Howard Georgi and I are working on another paper that I think is really interesting. You see, if you look at the configuration of these events, it’s clear that. . . .”
For another half hour Glashow, like the Walrus and the Carpenter, talks of many things—of the seventh quark and the fourth neutrino; of sliding constants and expanding groups; of the symmetries of odor and how to spot the Higgs boson; of color, flavor, and mass; and of many other signs and wonders that nature has in store for those who walk the road to unification.