Order came to the zoo in 1963. Murray Gell-Mann at Caltech and George Zweig at CERN independently accounted for the newly discovered particles by theorizing that they were composed of three smaller building blocks. Zweig called them aces, from the expression: “Dealer’s choice—aces are wild.” Gell-Mann named them quarks. “I liked the sound,” Gell-Mann told me. “Later I read Finnegans Wake, by James Joyce, and came upon the line, ‘Three quarks for Muster Mark!’ There were three of them, and there were three particles in the proton. I knew the name was right.” In 1969 the Nobel Committee awarded the physics prize to Gell-Mann for his work in classifying particles. Gell-Mann’s whimsical choice of the name quark set a trend. In order of ascending mass, the three quarks then known were called “up,” “down,” and “strange.” These labels do not reflect relative position or eccentricity: They distinguish quarks according to their properties, including their electric charge—always plus or minus one-third or two-thirds that of a proton.


QUARK THEORY —since confirmed by experiment-restored simplicity to nature. The new particles and the two old members of the atomic nucleus—the proton and the neutron— could be explained as combinations of quarks, bonded together according to their “color.” Color is a special property of quarks that enables them to join and form new particles. A quark’s other properties, among them its electric charge, determine its “flavor”—whether it is up, down, or strange. A proton consists of two up quarks with a positive charge of two-thirds each and one down quark with a negative charge of one-third; together they yield a single positive charge. In the same way, one up and two down quarks combine to form a neutral neutron.


In 1974 a fourth flavor of quark was produced in accelerators. It a eared as art of a meson, a high-energy embrace of quark and antiquark that lasts only an instant before ending in mutual annihilation. “Most of us had assumed there were just three quarks. We weren’t looking for more,” said Samuel Ting of MIT, in charge of an experiment at Brookhaven National Laboratory. Ting shared the 1976 Nobel Prize in Physics with Burton Richter, who had led a similar experiment at Stanford. The fourth quark’s existence had been predicted by Sheldon Glashow, a Harvard theorist who christened it a “charmed” quark. Fanciful classification continued in 1977, when Leon Lederman and a Fermilab team discovered a fifth quark called “bottom” or “beauty.”


While the up and down quarks make up protons and neutrons in our everyday, low-energy world, the other quarks exist only at extremely high energy, such as is found in the biggest accelerators. In 1984 CERN announced that colliding beams of protons and antiprotons had produced evidence of a sixth quark—”top” or “truth.” Some physicists hope that it is a final truth. However many flavors of quark there are, one oddity stands out about these subatomic particles: It seems impossible to jar loose a single quark from a proton or neutron. Quarks apparently exist only in trios or in quark/antiquark pairs. This trait, called confinement, inescapably binds single quarks together. “Remember energy is matter and matter is energy,” Sheldon Glashow told me. “When you throw energy at a proton in an effort to shake loose one quark, you create quark pairs out of the energy from the accelerator.”


Glashow and other theorists remain skeptical of the provisional announcement in 1977 that William Fairbank, an experimental physicist at Stanford, had found something that looked like a free quark. I visited Fairbank in his lab, a clutter of pipes and wiring that resembles an illegal distillery. Genial and bespectacled, Fairbank said he and colleagues had found quark-like electric charges in dust-mote-size spheres of niobium metal levitated by magnets. Fairbank believes he has detected these charges in units of one-third in at least four test balls. “This means it’s possible for fractionally charged particles, perhaps quarks, to exist free in nature,” Fairbank said. Fairbank’s presumed find has set off a worldwide rush to hunt free quarks. His supporters suggest that a few quarks escaped confinement the instant after the big bang, and that Fairbank has trapped some of these free-spirited particles—spares at the creation. But firm evidence of a solo quark remains elusive, leaving most physicists certain that lone quarks are locked forever within larger particles. Proclaims Murray Gell-Mann: “I don’t believe Bill Fairbank has found a free quark, but if he has, it’s one of mine.”


NEARLY ALL the several hundred known subatomic particles are made of quarks, bound together by what physicists call the strong nuclear force. The exceptions are called leptons, Greek for “slight.” The best-known lepton is the electron, first identified in 1897. As electric current, electrons put us in daily contact with the subatomic world and are the only leptons vital to the atom’s structure. Physicists are not sure why two other leptons even exist. Muons, discovered in 1937, have about 200 times as much mass as electrons and are the major by-product of the cosmic radiation that constantly bombards earth. Far heavier and equally furtive is the tau, a lepton discovered in 1976. Like the electron and muon, it carries a negative electric charge. The other leptons are neutrinos—”little neutral ones”—that carry no charge and are so light that their mass, if any, has so far gone undetected. Each of the neutrinos seems to couple with a heavier partner—a tau, muon, or electron. Scientists are still searching for the tau neutrino. Drasko Jovanovic at Fermilab told me that because neutrinos react only very weakly with other matter, they whip through everything. Several million neutrinos, traveling at the speed of light, are flying through your body at this instant. “Nothing stops them, not even a slab of lead as thick as the earth,” said Jovanovic. “It’s been suggested that beams of neutrinos passing through the earth be used for communication. It could work, but it would be expensive.


Meanwhile Jovanovic participates in a five-million-dollar experiment to determine if neutrinos have mass. If they do, they could account for as much as 90 % of the mass of the entire universe since, ghostlike, they fill the cosmos. Most physicists believe that the six leptons and the six flavors of quarks account for all matter, although a few theorists toy with the idea of particles more fundamental yet. The universe also contains force carriers called gauge particles.


Carlo Rubbia began capturing certain gauge particles at CERN in 1982 and 1983. “They’re little beasts we call W’s and Z’s,” says Rubbia, an animated Italian. “We’ve been on their trail for years.”Wand Z particles exist for less than a billionth of a billionth of a second and more often than not spend their brief lives within the nucleus, where they cause radioactive decay in atoms of such elements as uranium. Rubbia commands a 20-million-dollar electronic detector built specifically to find the W and Z particles. Big as a house, the detector is a vast network of coaxial cables and battleship-gray steel plates straddling a section of CERN’s main accelerator where protons and antiprotons collide. The charged particles liberated by these collisions streak through the detector’s gas atmosphere, generating trails of tiny signals. Their tracks appear on computer screens as V-shaped patterns, the arms of the V indicating where one particle has decayed into two or more other particles. We don’t actually see the particles. Their lives are too short,” Rubbia told me. “But decay products from W particles, for instance, fly predominantly forward. When we see this, we know we’ve got one.” For his pioneering work Rubbia shared the 1984 Nobel Prize in Physics with CERN colleague Simon van der Meer. The W and Z particles carry the weak behavior of atoms. The weak force breaks down each neutron in the nucleus of a radioactive atom into a proton, an electron, and an antineutrino. Other gauge particles, called photons, impart the electromagnetic force, about 100,000 times more powerful than the weak force. The electromagnetic force is responsible for keeping electrons in orbit around the nucleus, making atoms—and this magazine—seem solid. Most powerful of all—a hundred times more powerful than the electromagnetic force—is the strong force. Carried by gluons, it holds together the atomic nucleus.


Besides these three forces, the other known force at work in the universe is gravity. It is by far the weakest—the strong force is some iO~ (1 followed by 38 zeros) times more powerful. A still undetected particle, the graviton, may be the carrier of gravity, which has no meaningful role inside atoms. Without gravity, however, there would be no universe, since it binds together stars and galaxies, holds the earth in orbit, and keeps our feet planted on the ground.


MIDWAY in the seven-mile auto tunnel that cuts through the Alps between Italy and France, the air is rank with auto exhaust. In a cavern just off the smoggy roadway, shielded from cosmic radiation by the bulk of Mont Blanc, a special detector is testing whether the proton decays like most other particles, an idea that two decades ago would have been considered scientific heresy.


In the 19th century the Scottish scientist James Maxwell discovered that electricity and magnetism are two aspects of the same force. Today some physicists believe that the universe’s four forces are but manifestations of a single and deeper force. In the past two decades these scientists have proposed mathematical explanations —”grand unified theories”—of how the weak force, the strong force, and the electromagnetic force may be entities of a single underlying interaction. None of these theories yet embrace the universal fourth force, gravity. But they do predict that protons decay into other particles, because the strong force that binds a proton together and the weak force that causes radioactive decay may spring from the same basic interaction. The grand unified theories also predict that an average proton would take a million trillion trillion years or so to decay—more time than has passed since the universe began. How can short lived humans hope to measure the life span of such a durable subatomic particle? “Obviously we can’t wait billions of years to watch one proton and see if it disappears,” Italian physicist Pio Picchi told me deep inside Mont Blanc. “But we can assemble immense numbers of protons and see if one decays during a one-year period. “If protons do indeed decay, then at least one should die during the course of the year.  Picchi keeps watch on the hundred million trillion trillion protons in a 150-ton stack of iron slabs. These are fitted with thousands of detectors similar to Geiger counters, each waiting to pick up an infinitesimal burst of radiation emitted by a dying proton. So far at least one candidate event has been noted, the possible decay of a proton into a muon and another particle called a kaon.  Similar experiments in the United States, Japan, India, and France have yet to deliver results, forcing theorists to rethink their grand unified theories. But if proof is found that protons decay, it will mean that matter is inherently unstable. It would also prove what poets have said all along —that nothing lasts forever.


Besides predicting proton decay, grand unified theories attempt to trace the history of the universe back to its creation in the big bang some 15 billion years ago. Since then the cosmos has been constantly expanding. Today the visible universe is growing every second by a volume equal to that of the Milky Way galaxy.


During the 1930s astrophysicists realized that the particle reactions that other physicists were studying in cyclotrons were identical to those occurring in stars. Today physicists of both stripes have joined forces to theorize that all matter was born at the big bang. In the extreme heat of that instant all four forces, including gravity, may have been unified as one. Then, as the universe cooled, the forces split apart and their underlying unity became obscured.


NO ACCELERATOR will ever match the energy released in the big bang, and some physicists once thought it might be a waste of money to build bigger machines. However, Sheldon Glashow says: “We will find nothing if we do not look. We theorists are dependent upon experimental discoveries. Without them we are no better than medieval theologians, who endlessly debated how many angels might dance on the head of a pin.” Theorists like Glashow work in a delicate balance with experimentalists to uncover the hidden unity linking the three basic forces to gravity. Says avowed experimentalist Carlo Rubbia: “Theorists tend to forget that every time we look someplace new with a bigger machine a surprise awaits us.”   Experimentalists like Rubbia prevail for the moment at CERN, and they are building a new accelerator 17 miles in circumference. The mammoth machine will cost half a billion dollars, a remarkable investment considering that in its lifetime it will propel less than a gram of matter.


At Fermilab in Illinois, physicists have doubled up by constructing a new accelerator ring inside the tunnel housing their first machine. Eventually the newer accelerator will boost protons and antiprotons to a colossal collision energy of two trillion electron volts as they travel a circuit equal in distance to five round-trip journeys to the moon, 2,400,000 miles. Even more ambitious is the accelerator, perhaps 100 miles around, that U. S. scientists want to build. It would dwarf Fermilab’s four-mile accelerator, visible from 500 miles in space.


Perhaps the biggest obstacle to such megaengineering projects is the shrinking federal science budget. “We practically have to beg for money,” says Fermilab director Leon Lederman. CERN, a comparable laboratory, enjoys about twice as much funding as Fermilab. Lederman and other American particle physicists fear that tight budgets may cost the U. S. its traditional lead in the exploration of the atom. Money-conscious federal officials often ask Leon Lederman why the U. S. needs costly machines that cannot help solve pressing social problems. His answer never varies: “Learning about the ultimate nature of matter is of fundamental importance to the human race. It gives us a vision of ourselves, who we are, where we are going.”


Physics has always drawn powerful intellects and personalities, such as Murray Gell Mann, who with Lederman and others has set the pace of particle research for three decades. Small and intense, with steel-gray hair and penetrating eyes, Gell-Mann’s mind encompasses quarks and proton decay as easily as mushrooms or obscure languages, two of his other numerous interests. I asked him one day if physicists were not profoundly arrogant to think that they could explain the origin of the universe and everything in it by using only accelerators, telescopes, and equations. “We believe,” he said, “that our calculations are essentially correct and that we are on the edge of fully understanding the atom as well as the beginnings of the universe. It’s a little like the ant contemplating the skyscraper, isn’t it?”


And where will an understanding of the universe’s deepest secrets lead us? “There will be new technology, certainly,” Murray Gell-Mann went on. “But most remarkable will be that a handful of beings on a small planet circling an insignificant star will have traced their origin back to the very beginfling—a small speck of the universe comprehending the whole.” 





SOURCE:

Copyright @ 1985 - NATIONAL GEOGRAPHIC SOCIETY

17th and M Street N. W. Washington, D. C. 20036

Vol. 167, NO. 5, May, 1985, (pgs. 640 - 642. 653 - 663)



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