“My God! This young boy was claiming that this theory was renormalizable!” Salam recalled, in a 1984 interview with Robert Crease and Charles Mann.
It cut me to the quick! Both of us considered ourselves the experts on renormalizability, wrestling for months with the problem—and here was this slip of a boy who claimed he had renormalized the whole thing! Naturally, I wanted to show he was wrong—which he was. He was completely wrong. As a consequence, I never read anything else by Glashow, which of course turned out to be a mistake.16
Glashow, however, was not easily discouraged, and despite any embarrassment he may have felt at having mistakenly claimed to have solved the renormalization problem, he persisted in searching for links between electromagnetism and the weak force. In this effort he was encouraged by Gell-Mann if by few others. (“What you’re doing is good,” Gell-Mann recalled having told Glashow, over a seafood lunch in Paris, “but people will be very stupid about it.”)17 In 1961, Glashow produced a paper, “Partial-Symmetries of Weak Interactions,”18 that called attention to “remarkable parallels” between electromagnetism and the weak force, depicted them as linked by a broken symmetry, and predicted the existence of the W and Z force-carrying particles—later known as the W+, W−, and Z°. These hitherto undetected particles were to play an important role in experimental tests of the unified electroweak theory, but Glashow was unable to predict their masses, which left the experimenters with nothing to go on. Glashow and Gell-Mann then wrote a paper demonstrating that all the symmetries evinced in what are known as noncommutative or Cartan groups correspond to Yang-Mills gauge fields. Their efforts to identify a gauge symmetry group that would embrace both the strong force and Glashow’s protounified electroweak forces, however, came to naught. Glashow, discouraged, set aside his work on electroweak unified theory.
Meanwhile, in 1959, Salam and Ward had, like Glashow, arrived at insights about links between the weak and electromagnetic forces, but had, like Glashow, met with an indifferent response from the scientific community, and likewise grew discouraged. “A broken symmetry breaks your heart,” said Salam.19
The situation then brightened, thanks to new insights into the mechanism of spontaneous symmetry-breaking first presented by Yoichiro Nambu, Jeffrey Goldstone, and others and culminating in work published by Peter Higgs in 1964 and 1966. This research demonstrated that symmetry-breaking events could create new kinds of force-carrying particles, some of them massive. (The particles envisioned by Yang-Mills gauge theory had been massless.) If the particles that carry the weak and electromagnetic forces were related by a broken symmetry, these new tools might make it possible to estimate the masses of the W and Z particles characteristic of the unified, more symmetrical force from which the two forces were thought to have arisen.
Weinberg in particular was captivated by the concept of spontaneous symmetry breaking. “I fell in love with this idea,” he said in his Nobel Prize address in 1979, “but as often happens with love affairs, at first I was rather confused about its implications.”20 Initially he tried to apply the new symmetry-breaking tools to the strong force. This worked well insofar as global symmetries were concerned—specifically, Weinberg found that he was able to make successful predictions of the scattering of pi mesons—but when he sought to extend the technique to local symmetries, the results were disappointing. “The theory as it was working out was making nonsensical predictions that didn’t look like the strong interactions at all,” Weinberg recalled in a 1985 interview. “I could fiddle with it and make it come out right, but then it looked too ugly to bear.”21 The worst problem was that the particle masses predicted by the breaking of the symmetry group Weinberg was contemplating did not match those of the particles involved in the strong interactions.
But then, in Weinberg’s recollection, “at some point in the fall of 1967, I think while driving to my office at MIT, it occurred to me that I had been applying the right ideas to the wrong problem.”22 The particle descriptions that kept bobbing up out of his equations—one set massive, the other massless—resembled nothing in the strong force, but fit perfectly with the particles that carry the weak and electromagnetic forces. The massless particle was the photon, carrier of electromagnetism; the massive particles were the Ws and Zs. Moreover, Weinberg found, he could calculate the approximate masses of the Ws and Zs. Here, finally, was an electroweak theory that made a verifiable prediction. Salam independently reached a similar conclusion the following year—testimony, Weinberg said, to “the naturalness of the whole theory.”23
With that, the work that would win the 1979 Nobel Prize in physics was complete. Yet little heed was paid to it at first. Weinberg’s paper, the first complete statement of the electroweak theory, was cited not once in the scientific literature for four full years after it appeared. The main reason was that the theory had not yet been shown to be renormalizable. Once that was accomplished—in 1971, when its dolorous infinities were scotched in a heroic effort by the Dutch physicist Gerard’t Hooft—interest in the electroweak theory intensified, and the focus of attention turned to the question of testing the theory through experiment. This called upon those embodiments of big science, the particle accelerators.
Accelerators are to particle physics what telescopes and spectrographs are to astrophysics—both an exploratory tool for finding new things and a supreme court for testing existing theories. Their operating principle is based on Einstein’s E = mc2. One accelerates charged particles to nearly the speed of light by propelling them along an electromagnetic wavefront created by pulsing electromagnets, then smashes them into a target, creating tiny explosions of intense power. New particles condense from the tiny fireball, like raindrops precipitating in a storm cloud, and are recorded by surrounding detectors as they come reeling out. The original detectors were photographic plates; later these were replaced by electronic sensors coupled to computers.
Engaged in the race to test the electroweak theory were researchers at two of the world’s most powerful accelerators—CERN, the European center for nuclear research near Geneva, and Fermilab, named after the physicist Enrico Fermi, on the Illinois plains west of Chicago. Both are proton accelerators.* The protons come from a little bottle of hydrogen gas, small enough for a backpacker to carry, that contains a year’s supply of atoms. Computer-controlled valves release the gas in tiny puffs, each scantier than a baby’s sigh but each containing more protons than there are stars in the Milky Way galaxy. The gas enters the electrically charged cavity of what is called a Cockroft-Walton generator.† The field strips the electrons away from the hydrogen atoms and sends the protons speeding down a tunnel and into a pipe the size of a garden hose that describes an enormous circle—three miles in circumference in the case of Fermilab. The protons are accelerated around the ring by pulses sent through surrounding electromagnets, while focusing magnets gather them to a beam thinner than a pencil lead. When they reach a velocity approaching that of light—at which point, thanks to special relativity effects, their mass has increased by some three hundred times—they are diverted from the ring and slammed into a stationary target inside a detector. Their tracks, subjected to yet another magnetic field in the detector, betray their charge and mass and thus their identity.
Though similar in design, the CERN and Fermilab accelerators exemplified rather different styles of doing big science. Fermilab, built under the direction of the American physicist and sculptor Robert Wilson, was conceived and executed as a work of art, an embodiment of the aesthetics of science. A Wilson sculpture, a looming set of steel arches titled “Broken Symmetry,” was erected at the main entrance. The accelerator tunnel, buried underground, was delineated, for purely aesthetic purposes, by an earthwork berm. Within the ring buffalo grazed; swans swam in the waters employed to cool the electromagnets. The administration building, a sweeping, convex tower, was set against the berm like a diamond on an engagement ring; Wilson modeled it on the proportions of Beauvais Cathedral in France. As he recalled his reasons for th
is decision:
I found a striking similarity between the tight community of cathedral builders and the community of accelerator builders. Both of them were daring innovators, both were fiercely competitive on national lines, but yet both were basically internationalists…. They recognized themselves as technically oriented; one of their slogans was Ars sine scientia nihil est!—art without science is nothing.24
Wilson defended the aesthetics of his creation—which, it should be added, was completed under budget—by drawing further parallels between art and science:
The way that science describes nature is based on aesthetic decisions. Physics is very close to art in the sense that when you examine nature on a small scale, you see a diversity in nature, you see symmetries in nature, you see forms in nature that are just utterly delightful. Eventually, in the way that one looks at sculpture or art, people will also begin to look at those great simple facts.25
CERN, for its part, looked about as aesthetically unified as a Bolshevik boiler factory. Its administration building, slapped together from prefabricated plastic panels and aluminum alloy window frames that bled pepper-gray corrosives in the rain, called to mind less Beauvais Cathedral than the public housing projects of suburban Gorky. Its laboratories were scattered across the landscape, as haphazardly as the debris from a trucking accident, on a plot of land that straddled the French-Swiss border outside Geneva. The prevailing style was late Tower of Babel, with scientists switching from French to German to English in mid-sentence while lunching at the laboratory cafeterias, one of which accepted only French currency and the other only Swiss. Yet for all its air of disorder, CERN worked every bit as well as Fermilab, and by the early 1970s was beginning to surpass it.
It was in this fevered context that the two laboratories raced to test the predictions of the electroweak theory. The new force-carrying particles postulated by the electroweak theory, the W+ W-, and Z°, were massive, meaning that it would take a lot of energy to bring them into existence in an accelerator collision. In 1971, no accelerator could yet summon up sufficient energy to create W and Z particles, if they existed. In the meantime, however, the experimentalists could hope to discern the existence of the Z indirectly, by identifying the effects, in accelerator collisions, of “neutral currents.” This consisted of searching through thousands of accelerator events for evidence of the few neutral current interactions in which the Z° would have played a role. Encouraged by Weinberg’s estimation that such events “are just on the edge of observability,” a team working under the experimental physicist Paul Musset at the CERN accelerator began staying up nights, examining thousands of photographs of particle interactions. After a year’s work they were finally rewarded when the myopic Musset, who scrutinized the particle tracks with his nose almost touching the print, discerned a kink in the recorded path of a particle that gave away its identity as a pion rather than a muon, indicating that it had emerged from a neutral current reaction. Salam learned of the result shortly after arriving at Aix-en-Provence, where he was to attend a physics conference. He was lugging his suitcase to a student hostel near the train station when a car stopped next to him. Musset looked out and said, “Are you Salam?” Salam said yes. “Get into the car,” Musset said. “I have news for you. We have found neutral currents.”26
This was welcome news to Salam, Glashow, and Weinberg, but it nevertheless fell short of fully vindicating the electroweak theory, for other theories also predicted the existence of neutral currents. The Weinberg-Salam theory surpassed its predecessors in predicting the mass of the carriers of the electroweak force—about 80 GeV for the Ws and 90 GeV for the Z. (A GeV is one billion electron volts; in this context, it is convenient to express mass in terms of energy.) The Ws and Zs were known collectively as intermediate vector bosons. To produce enough intermediate vector bosons to make their detection likely would require a particle accelerator with a minimum energy of some 500 to 1,000 GeV.
Neither accelerator could reach this level, but both were hurriedly being souped up to approach it, by means of a daring new technique involving the collision of protons, not with a stationary target, but with an oncoming stream of antiprotons. The universe, so far as we can tell, contains only trace amounts of antimatter—this in itself is one of nature’s more intriguing broken symmetries —but antimatter can be created in accelerator collisions, and by the 1970s accelerator engineers were beginning to talk of collecting the antiprotons they created and then colliding them with protons coming the other way. Since matter and antimatter particles annihilate each other when they meet, the result would be to greatly boost the effective power of the accelerator.
Fermilab approached the problem methodically. They would first install new magnets to increase the power of the accelerator to 1,000 GeV (equal to one teraelectron volt, or one TeV), and only thereafter take on the more hazardous business of trying to make and store antimatter. CERN proceeded in a more intrepid fashion, going for a matter-antimatter collider right away. Wilson, with his customary gentility, wished them well: “May they reach meaningful luminosity and may they find the elusive intermediate boson,” he wrote. “We will exult with them if they do.”27 CERN officials, with equal courtliness, described the Fermilab plan as “a project of great vision being attacked with courage and enthusiasm.”28 But behind the pleasantries raged a fierce competition between rival teams of the world’s brightest and most egocentric scientists and engineers.
Of these, few were brighter—and none more egocentric—than Carlo Rubbia, the driving force behind the CERN effort. Born in northern Italy in 1934 of Austrian parents, Rubbia was by nature an internationalist (“I have an accent in every language,” he said) who felt at home cajoling and browbeating the scores of scientists who made up his enormous research teams, among them Italian, French, English, German, and Chinese researchers, a Finn, a Welshman, and a Sicilian. A driven man, Rubbia traveled ceaselessly, flying from CERN to Harvard to Berkeley to Fermilab to Rome so incessantly that friends who monitored his progress calculated that he had a lifetime average velocity of over forty miles per hour. (“Ah,” he said, settling into his seat one morning, “my first flight of the day!”) Massive and energetic and constantly in motion, he resembled nothing so much as a human proton: Like Rutherford, who told his tailor, “Every year I grow in girth, and in mentality,” Rubbia ballooned in size until, by 1984, he was boasting that his form now approached the perfection of a Platonic sphere.
Rubbia’s hopes of winning a Nobel Prize rested on a conception concocted by an austere CERN engineer named Simon van der Meer. Van der Meer was convinced that one could make antiprotons (albeit at a rate of only one hundred-billionth of a gram of them per day) and keep them in storage until enough had accumulated to collide them with protons in significant numbers. Storing anti-matter would be a tricky business, akin to the old conundrum of how to bottle a universal solvent: If an antiproton made contact with a particle of ordinary matter, both would instantly annihilate. Van der Meer proposed to handle the problem by constructing an antiproton accumulator, a small ring in which the antiprotons could be kept circling for days, suspended in a vacuum in an electromagnetic field. To keep the antiprotons concentrated in tight, secure bundles, Van der Meer proposed a technique called stochastic cooling—stochastic meaning statistical, and cooling meaning reducing random motions among the particles. As little clumps of antiprotons whirled around the storage ring, sensors would detect the drift of those that strayed, and computers would then send a correcting message across the ring to adjust the magnets on the opposite side to correct for the drift. Since the antiprotons were moving at close to the speed of light, the computation would have to be done very quickly, whereupon the message would be sent speeding across the diameter of the storage ring just in time to configure the magnets before the antiproton bundle arrived via the longer, roundabout route. Once a sufficient supply of antiprotons had been collected and concentrated, they could be released into the main ring, accelerated to terminal velocity,
and steered into a headlong collision with bunches of protons coming the other way.*
An accelerator speeds subatomic particles—in this case, protons—around a ring, then diverts them to a target inside a detector.
The building of a proton-antiproton collider fed via stochastic cooling represented one of the most audacious endeavors in an age of high technology. Van der Meer himself considered the idea so radical that he originally did not even try to publish it. Many accelerator experts predicted that stochastic cooling would not work, and that if it did, the matter and antimatter bundles would blow each other up the first time they collided, rather than producing the repeated collisions—some fifty thousand of them per second—that would be required to flush out the intermediate vector bosons. (The accelerator would only just get into the energy range of the intermediate vector bosons, and the physicists had to rely upon quantum probabilities to deliver up a detection event.) There were snickers in the audience when Rubbia first proposed constructing an antimatter collider; when he brought up the idea at Fermilab he was invited to leave; and when he and two colleagues submitted a paper on it, the editors of the Physical Review Letters, a leading journal, refused to publish it. But Rubbia kept pushing, despite the high stakes—one hundred million dollars to build the antimatter accumulator and to modify the accelerator, plus another thirty million dollars to build the detectors—and he made it a habit to project an air of robust assurance, keeping his reservations to himself. “Let’s be serious,” he said later. “If we had spelled out these doubts before the project was launched, nobody would have given us the money for it. … I was scared stiff the beam wouldn’t work.”29
Coming of Age in the Milky Way Page 33
