WORLDS WITHIN THE
Source: NATIONAL GEOGRAPHIC
Vol. 167, No. 5 - May 1985
Beacon to the peculiar geography of the atom—once thought to be the smallest unit of matter—a light guide bends the paths of massless photons at a West German laboratory. Human sight depends on response to subatomic photons traveling with the energies of visible light, yet paradoxically the atom’s interior cannot be seen.
Exploration into the domain of the ultra-small is by theoretical physics, elegant mathematics, and giant machines. Within this st range territory—where the raw materials for existence are processed—matter is bound energy, particles and waves are one, and reality itself is a kind of uncertainly.
---- JOHN BOSLOUGH
IN THE SOUTHWEST CORNER of Switzerland, deep beneath tidy farms and villages, lies a maze of tunnels where it can be hotter than at the heart of the sun. The source of this awesome heat lies within a thin steel pipe encased in magnets—a particle accelerator. Operated near Geneva by CERN (the Europe-an Laboratory for Particle Physics), the accelerator is more than four miles long and one of the largest machines ever built. It produces temperatures as high as 7,000 trillion degrees ©) Celsius, comparable to conditions an instant after the super-hot explosion that created the universe itself.
Using the CERN accelerator like an immense microscope, physicists are probing the structure of the atom, an inner cosmos of subatomic particles as remote from our daily experience as the farthest reaches of space. Yet that structure may hold an explanation of how the universe was born. During the past 50 years scientists exploring the atom’s interior have solved many age-old mysteries of matter and energy. This new knowledge has brought us lasers, computers, transistors, space travel, and nuclear energy for weapons and power. “But what we’re really after is a new concept of reality,” says Leon Lederman, director of the Fermi National Accelerator Laboratory in Batavia, Illinois. “We’re after something akin to the revolution in thinking that followed Copernicus~ s announcement that the earth circles the sun.”
Lederman and other physicists are searching for the ultimate building blocks from which all things the stars, the earth, you, I, and the atom are made. Because everything in the cosmos has been composed of these particles since the primordial big bang, scientists also hope to learn the origin of the universe a goal that eluded even Albert Einstein. Today his successors, at CERN and elsewhere around the world, think they are getting close.
CERN’s doughnut-shaped particle accelerator lies 15 stories belowground, in a gracefully curving concrete cavern that twice bores through the rock of the Franco-Swiss border. It takes more than an hour to walk the four-mile circuit of CERN’s Super Proton Synchrotron, a frontier crossing that subatomic particles make 43,000 times each second without benefit of passport. During a brief shutdown for repairs, British physicist Vince Hatton led me to the accelerator’s beam line, a waist-high steel tube outfitted with alternating red and blue magnets. The air smelled of oil and machinery. The beam line looked like a snake winding through a high-tech hole on another planet.
The magnets guide a stream of protons around the beam line at nearly the speed of light (186,282 miles per second) before they collide with a beam of antiprotons whirling in the opposite direction. The impacts are so violent—like volleys of cannonballs smashing into each other—that energy is transformed into matter, creating subatomic particles that fly wildly in all directions. Most of these particles are short-lived , some last only a trillionth of a trillionth of a second before vanishing—and are rare in the universe. But as debris from a collision inside CERN’s accelerator, they occur in sufficient numbers for physicists to study them to learn what makes up atomic matter.
When running, the accelerator also produces lethally intense radiation, and steel doors operated by electronic keys keep out humans. “The system is foolproof,” said Hatton, as warning horns sounded. The machine was about to be switched on. But the vault-like exit door was locked shut. My apprehension grew until Hatton picked up a telephone. “It can’t be turned on while we’re in here,” he said, as someone half a mile away pushed a button and the door opened. Minutes later the beams were again circling the accelerator ring. In a control room, colorful computer screens blinked with graphs that charted subatomic collisions at two points along the ring. Computer operators adjusted the flow of electricity to the ring’s magnets, steering the beams to increase the collision rate. Experimenters fretted at computer terminals, brandished printouts, and haggled over operating procedures with Hatton, who oversees the accelerator centers—in the U. S., Europe, Japan, and the Soviet Union—wants to be first with a major discovery. As Nick Samios, director of the Brookhaven National Laboratory in New York, puts it: “Nobody remembers the second person to say ‘E=mc2.’” The large European accelerators, CERN and the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, West Germany, are generally better funded than their United States competitors. Survival—funding depends upon scientific breakthroughs.
So too does continued exploration of one of man’s last great frontiers, world of the atom. It is a world where matter and energy are interchangeable, where empty space is not really empty, and where gravity is overwhelmed by far stronger forces that bind together matter.
MAN HAS SPECULATED endlessly about the nature of matter. Some 2,300 years ago the Greek philosophers Democritus and Leucippus proposed that if you cut an object, such as a loaf of bread, in half, and then in half again and again until you could do it no longer, you would reach the ultimate building block. They called it an atom. The atom is infinitesimal. Your every breath holds a trillion trillion atoms. And because atoms in the everyday world we inhabit are virtually indestructible, the air you suck into your lungs may include an atom or two gasped out by Democritus with his dying breath.
To grasp the scale of the atom and the world within, look at a letter “I” on this page. Magnify its dot a million times with an electron microscope, and you would see an array of a million ink molecules. This is the domain of the chemist. Look closely at one ink molecule and you would see a fuzzy image of the largest atoms that compose it. Whether by eye, camera, or microscope, no one has ever seen the internal structure of an atom: Minute as atoms are, they consist of still tinier sub-atomic particles. Protons, carrying a positive electric charge, and electrically neutral particles called neutrons cluster within the atom’s central region, or nucleus- one hundred-thousandth the diameter of the atom. Nuclear physicists work at this level of matter. Whirling around the nucleus is a third subatomic particle, the electron, which carries a negative charge. Electric current consists of flowing electrons, point-like particles literally impossible to measure. Electrons “orbit” an atom’s nucleus according to principles governing the motion of waves. Unlike planets revolving around the sun, electrons do not follow fixed paths.
Yet the probable location of electrons can be calculated using quantum mechanics, a mathematical system developed in the 1920s to describe the weird behavior of matter and energy at the subatomic level, the world of particle physicists.
That particles can act like waves may seem bizarre. But no more so than some other oddities suggested by quantum theory: That how we probe matter affects its behavior and form; that some particles exist so briefly that they are not real but “virtual”; and that well-ordered reality—the whole of the universe—rests on chance and randomness at the subatomic level.
Besides quantum mechanics, the other concept crucial to our modern view of the atom and its parts is Einstein’s special theory of relativity. His formula, E~mc2, where E is energy, m is mass, and c is the unvarying speed of light, states that mass and energy are merely different versions of the same thing. Einstein, however, never accepted quantum mechanics. He felt that randomness could not be the ultimate reality, and he debated the point with another titan of atomic theory, Danish physicist Niels Bohr. On this point Einstein has been proved wrong, yet his special relativity theory is routinely put to work in accelerators, where energy is trans-formed into subatomic particles in a hint of how the universe may have come to be.
PARADOXICALLY, exploring the smallest things in the universe requires the largest machines on earth. As physicists have penetrated from the molecule to the atom and then to the atom’s nucleus with its protons and neutrons, they have pulled back layer after layer of matter as peeling an artichoke. To reveal each layer requires increasing amounts of energy provided by massive atom smashers. No two of these giants are alike, but there are two basic types. Some, like Stanford’s linear accelerator near Palo Alto, California, fire negatively charged electrons at atomic nuclei. Two miles long and as straight as the laser beam used to align it, the accelerator hurls electrons at 99.99 % the speed of light. But like the one at CERN, most accelerators are circular and use protons as projectiles. Protons are heavier and generate more collisions. However, collisions in an electron accelerator are easier to analyze.
Almost every American home has a primitive accelerator: the television picture tube. Inside it electricity heats a metal filament, boiling off negatively charged electrons and accelerating them through a positively charged wire grid. A magnet then steers them at the phosphorus-coated TV screen, which glows from the collisions. In most high-energy physics labs the first step in accelerating subatomic particles depends upon an accelerator, invented in 1932 by John Cockcroft and Ernest Walton. It extracts protons or electrons from atoms of hydrogen gas. In 1978 an elderly Ernest Walton visited Fermilab and inspected such an accelerator. Covered with metallic balls for discharging electrical energy, the monstrous contraption seemed fit for a horror movie, and as Walton looked on, it spat a huge bolt of lightning. “Ah,” said the delighted Walton, “the machine knows its master.”
The particles liberated by a Cockcroft-Walton accelerator are boosted to greater and greater velocity in copper chambers called radio-frequency cavities. Electric pulses fed to the cavities millions of times each second lift the particles to high energy and sweep them down an accelerator’s beam line on traveling radio waves, like surfers riding a crest. In ring-shaped machines such as the CERN accelerator, electromagnets focus the particles into a pencil-thin beam and steer it in a circle. To prevent unwanted collisions with stray atoms, oxygen and other gases are pumped from the beam line tube, leaving it nearly as airless as the moon. Particle physicists measure an accelerator’s power in electron volts; the more electron volts that an accelerator produces, the deeper it can delve into the atom. One electron volt (eV) is about the energy gained by a single electron flowing from the negative to the positive end of a flashlight battery.
Slightly greater energies can strip electrons from atoms, but it takes millions of electron volts—Me V—to probe the nucleus. Energy a thousand times higher still, in the billion eV (or GeV, for giga-electron volt) range, is needed to propel particles with enough force to shatter protons and electrons and thus create new matter. One way to boost energy is to fire two beams of particles in opposite directions around a ring, so that they slam together. This doubles the energy, giving the CERN accelerator, for instance, an energy of 630 GeV. Yet the attractive force binding together the constituents of protons and neutrons is so immense—trillions of trillions of times stronger than earth’s gravity—that even the CERN machine can only pick and poke at the atom’s inner structure.
“It’s a bit like finding out how cars work by smashing them together and seeing what falls out,” Dr. Carlo Rubbia told me at CERN, where he was on leave from Harvard. “But in particle physics, when you smash two cars together, you get 20 or 30 new cars, or even a truck or two. We’re repeating one of the miracles of the universe transforming energy into matter.”
I N 1911 the New Zealand scientist Ernest Rutherford presented a theory that overturned the belief that the atom was solid and that set the stage for today’s physics. It resulted from an experiment in which gold foil was bombarded with alpha particles, fast-moving helium nuclei. Most particles shot right through the foil, although a few—one in 8,000 or so—bounced back. Rutherford’s reasoning in solving this puzzle was perfect. Since most particles zipped through the foil, gold atoms must be mostly empty space, but with something small and hard in the middle—the atomic nucleus. His experimental method—similar to shooting bullets at a shrouded object and studying the ricochet to deduce what is inside—is still used in modern accelerators. Rutherford had understandably pictured the atom as a tiny solar system. But by the start of World War II, physicists had sharpened their picture of the atom’s structure. The atoms of all the elements then known were described as combinations of protons, neutrons, and electrons—held to be the fundamental building blocks of matter.
At the center of each atom, in this view, was a nucleus of neutral neutrons and positive protons, the number of protons identifying the element. The lightest was hydrogen, with one proton. The heaviest naturally occurring element, uranium, had 92 protons. For each proton in the nucleus, there was a negatively charged electron, gyrating around the atom’s core at a distance 50,000 times the diameter of the nucleus. If a hydrogen atom’s nucleus were the size of a tennis ball, its electron would be two miles away. Besides accelerators, particle physicists need detectors to record the collisions of subatomic particles. Among the first detectors were cloud chambers, in which particles from collisions swept through water vapor, leaving tracks of droplets that could be photographed for analysis. In modern bubble chambers, invented by physicist Donald Glaser, a liquid is used in place of water vapor. Legend has it that Glaser was inspired by watching bubbles form in a beer glass.
Detectors can be grand in scale. The Big European Bubble Chamber at CERN is a four-story steel tank holding more than 10,000 gallons of liquid hydrogen and neon chilled to minus 2430C (300 above absolute zero). I clambered atop the tank on ladders and catwalks resembling the superstructure of a battleship. On top were four cameras. Chargeless neutrinos, debris from proton collisions in the accelerator, enter the bubble chamber, collide with hydrogen and neon nuclei, and create a spray of wildly scattering high-energy particles. In their wake the liquid begins to boil, and at that instant the cameras fire to record the particles’ bubbly tracks.
Computer-controlled electronic detectors, which record signals generated by passing particles, are replacing bubble chambers. These detectors are sensitive enough to record an event shorter than the time it takes a particle to zip across a nucleus at virtually the speed of light. “We can tell exactly what particles have been caught, by the direction and length of their tracks,” Carlo Rubbia told me. He admits it’s an Alice-in-Wonderland approach, “like trying to tell the color of invisible jerseys on invisible football players by watching the movement of the ball.”
ONE OF THE MOST provocative ideas of modern physics arose from the 1928 equation of British theorist Paul Dirac, which predicted the existence of antimatter. Carl Anderson of the California Institute of Technology confirmed this idea in 1932, with his discovery of a positive electron, or positron, a particle just like an electron but with a positive rather than a negative charge. When matter and antimatter meet, they annihilate each other in a burst of radiation. Since Anderson’s discovery physicists have shown that for every type of particle there must also be an antiparticle.
This quirk of nature has led physicists to speculate about encounters between the universe and an anti-universe. In a poem, Harold Furth, now director of Princeton’s Plasma Physics Laboratory in New Jersey, imagines that Dr. Edward Teller, a creator of the hydrogen bomb, meets a Dr. Anti-Teller. In Furth’s poem, “Their right hands clasped, and the rest was gamma rays.” Antimatter poses a mystery: If particles vanish when they meet their opposites, and if every particle can have an antiparticle, why is the world made only of matter? There appears to be no more than one part in ten billion of antimatter in interstellar space. Where has all the antimatter gone?
Physicists think they know: During the first split second after the big bang, there was a small excess of matter over antimatter. Particles and antiparticles collided, annihilating each other and leaving behind only radiation and the surplus matter. These residual particles make up almost everything in the universe today: stars, galaxies, the earth, and Edward Teller. The two decades after World War II saw the blossoming of the atomic age, and to confident physicists full understanding of the atom, indeed, of the entire universe, seemed at hand. But larger and larger accelerators—most of them patterned after apalm-size cyclotron proposed in 1929 by Ernest 0. Lawrence of the University of California at Berkeley— constantly uncovered new particles. By the early 1960s dozens were known, a mélange that physicists began calling a zoo. There seemed to be no truly basic unit of matter.
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