The God Particle: If the Universe Is the Answer, What Is the Question?

by Leon Lederman

  Which doesn't mean that science and the Eastern religions have suddenly discovered a lot in common. Still, if the religious metaphors offered up by the authors of texts comparing the new physics to Eastern mysticism help you in some way to appreciate the modern revolutions in physics, then by all means use them. But metaphors are only metaphors. They are crude maps. And to borrow an old expression: we must never mistake the map for the territory. Physics is not religion. If it were, we'd have a much easier time raising money.

  6. ACCELERATORS: THEY SMASH ATOMS, DON'T THEY?

  SENATOR JOHN PA STORE: Is there anything connected with the hopes of this accelerator that in any way involves the security of this country?

  ROBERT R. WILSON: No sir. I don't believe so.

  PASTORE: Nothing at all?

  WILSON: Nothing at all.

  PASTORE: It has no value in that respect?

  WILSON: It has only to do with the respect with which we regard one another the dignity of men, our love of culture. It has to do with, are we good painters, good sculptors, great poets? I mean all the things we really venerate and honor in our country and are patriotic about. It has nothing to do directly with defending our country except to make it worth defending.

  We have a tradition at Fermilab. Every June 1, rain or shine, at 7 A.M., the staff is invited to jog the four miles around the main ring of the accelerator on the surface road, which doubles as a jogging path. We always run in the direction that the antiprotons accelerate. My last unofficial time around the ring was 38 minutes. The current director of Fermilab, my successor John Peoples, put up a sign his first summer in the job, inviting the staff to run on June 1 with "a younger, faster director." Swifter he was, but neither of us was fast enough to beat the antiprotons. They complete the circuit in about 22 millionths of a second, which means that each antiproton laps me about 100 million times.

  The Fermilab staff continues to be humiliated by the antiprotons. We get even, though, because we get to design the experiments. We steer the antiprotons head-on into protons racing just as fast in the opposite direction. The process of getting particles to collide is the essence of this chapter.

  Our discussion of accelerators will be a bit of a departure. We've been racing through century after century of scientific progress like a runaway truck. Let's slow down the pace. Here we'll talk not so much about discoveries or even physicists but about machines. Instruments have been inextricably tied to scientific progress, from Galileo's inclined plane to Rutherford's scintillation chamber. Now an instrument takes center stage. One cannot understand the physics of the past several decades without understanding the nature of the accelerator and its accompanying array of particle detectors, the dominant tools in the field for the past forty years. By understanding the accelerator, one also learns much of the physics, for this machine embodies many principles that physicists have labored centuries to perfect.

  I sometimes think about the tower at Pisa as the first particle accelerator, a (nearly) vertical linear accelerator that Galileo used in his studies. However, the real story starts much later. The development of the accelerator stems from our desire to go down into the atom. Galileo aside, the history begins with Ernest Rutherford and his students, who became masters of the art of exploiting the alpha particle to explore the atom.

  The alpha particle is a gift. When some naturally radioactive materials spontaneously disintegrate, they shoot out these heavy, energetic particles. An alpha particle typically has an energy of 5 million electron volts. An electron volt (eV) is the amount of energy a single electron would receive if it crossed from the case (negative) of a 1-volt flashlight battery to the battery's terminal (positive). By the time you finish the next couple of chapters, the electron volt will be as familiar as the inch, the calorie, or the megabyte. Here are four abbreviations you should know before we go on:

  KeV: thousand electron volts (K for kilo)

  MeV: million electron volts (M for mega)

  GeV: billion electron volts (G for giga)

  TeV: trillion electron volts (T for tera)

  Beyond TeV we resort to powers-of-ten notation, 1012 eV being one TeV. Beyond 1014, our foreseen technology runs out, and we are in the domain of cosmic ray particles, which bombard the earth from outer space. The numbers of cosmic ray particles are small, but their energies go all the way up to 1021 eV.

  In particle physics terms, 5 MeV isn't very much. Rutherford's alphas barely managed to break up the nucleus of a nitrogen atom in perhaps the first on-purpose nuclear collision. And only tantalizing hints of what was to be learned emerged from those collisions. Quantum theory tells us that the smaller the object being studied, the more energy you need—equivalent to sharpening the Democritan knife. To cut the nucleus effectively we need energies of many tens or even hundreds of MeV. The more the better.

  IS GOD MAKING THIS UP AS SHE GOES ALONG?

  A philosophical digression. As I will describe, particle scientists went along cheerfully building ever more powerful accelerators for all the reasons any of us sapiens does anything—curiosity, ego, power, greed, ambition ... Every so often, a group of us in quiet contemplation over a beer would speculate about whether God Herself knows what our next machine—for example, the 30 GeV "monster" that was nearing completion at Brookhaven in 1959—will produce. Are we just inventing puzzles for ourselves by achieving these new, unheard-of energies? Does God, in Her insecurity, look over the shoulder of Gell-Mann or Feynman or one of Her other favorite theorists to find out what to do at those huge energies? Does She call together a committee of resident angels—Reb Newton, Einstein, Maxwell—to suggest what 30 GeV should do? This point of view is occasionally encouraged by the jumpy nature of theoretical history—as if She is making it up as we go along. However, progress in astrophysics and cosmic ray research quickly assures us that this is just Friday-evening-before-the-sabbath nonsense. Our colleagues who look up tell us with assurance that the universe is very much concerned with 30 GeV, with 300 GeV, indeed with 3 billion GeV. Space is awash with particles of astronomical (ouch!) energies, and what is today a rare and exotic happening at an infinitesimal collision point on Long Island or Batavia or Tsukuba was, just after the birth of the universe, an ordinary, everyday, garden-variety happening.

  And now back to the machines.

  WHY SO MUCH ENERGY?

  The most powerful accelerator today, Fermilab's Tevatron, produces collisions at about 2 TeV, or 400,000 times the energy created by Rutherford's alpha particle collisions. The yet-to-be-built Superconducting Super Collider is designed to operate at about 40 TeV.

  Now 40 TeV sounds like a great deal of energy, and it is indeed when invested in a single collision of two particles. But we should put this into perspective. When we strike a match, we involve about 1021 atoms in the reaction, and each process releases about 10 eV, so the total energy is roughly 1022 eV, or about 10 billion TeV. At the Super Collider there will be 100 million collisions per second, each one releasing 40 TeV, for a total of 4 billion TeV—not too different from the energy released in lighting a match! But the key is that the energy is concentrated in a few particles rather than in the billions and billions and billions of particles contained in any speck of visible matter.

  We can look at the entire accelerator complex—from the oil-fired power station through the electrical power lines to the lab where transformers ship the electrical energy to magnets and radio-frequency cavities—as a giant device for concentrating, with extremely low efficiency, the chemical energy of oil into a measly billion or so protons per second. If the macroscopic quantity of oil was heated so that each of the constituent atoms had 40 TeV, the temperature would be 4 × 1017 degrees, four hundred thousand trillion degrees on the Kelvin scale. The atoms would melt into their constituent quarks. Such was the state of the entire universe less than a millionth of a billionth of a second after creation.

  So what do we do with all this energy? Quantum theory demands more and more powerful accelerators to study smal
ler and smaller things. Here's a table showing the approximate energy one needs to crack open various interesting structures:

  ENERGY (approximate) SIZE OF STRUCTURE

  0.1 eV Molecule, large atom, 10−8 meters

  1.0 eV Atom, 10−9 m

  1,000 eV Atomic core, 10−11 m

  1 MeV Fat nucleus, 10−14 m

  100 MeV Nuclear core, 10−15 m

  1 GeV Neutron or proton, 10−16 m

  10 GeV Quark effects, 10−17 m

  100 GeV Quark effects, 10−18 m (more detail)

  10 TeV God Particle? 10−20 m

  Note how predictably the required energy goes up as the size goes down. Note also that you need only 1 eV to study atoms, but 10 billion eV to begin to study quarks.

  Accelerators are like the microscopes used by biologists to study ever smaller things. Ordinary microscopes use light to illuminate the structure of, say, red corpuscles in blood. Electron microscopes, beloved by the microbe hunters, are more powerful precisely because the energy of the electrons is higher than that of the light in an optical microscope. The electrons' shorter wavelengths allow biologists to "see" the molecules from which a corpuscle is constructed. It is the wavelength of the bombarding object that determines the size of what you can "see" and study. In quantum theory we know that as the wavelength gets shorter the energy increases; our chart simply demonstrates the connection.

  In 1927, Rutherford, in an address to Britain's Royal Society, expressed the hope that one day scientists would find a way to accelerate charged particles to energies higher than that provided by radioactive decay. He foresaw the invention of machines capable of generating many millions of volts. There was a motivation for such machines beyond pure power. Physicists needed to be able to hurl more projectiles at a given target. Alpha sources provided by nature were less than bountiful: fewer than a million particles could be directed toward a 1-square-centimeter target per second. A million sounds like a lot, but nuclei occupy only one hundredth of a millionth of the target area. You need at least a thousand times more accelerated particles (a billion) and, as mentioned, a lot more energy—many millions of volts (physicists weren't sure how many millions)—to probe the nucleus. This seemed like a daunting task in the late 1920s, yet physicists in many laboratories began to work on the problem. What ensued was a race to create machines that would accelerate the requisite huge number of particles to at least one million volts. Before we discuss the advances in the technology of accelerators, we should talk about some basics.

  THE GAP

  The physics of particle acceleration is simple to explain (watch out!). Connect the terminals of a DieHard battery to two metal plates (also called terminals), positioned, say, a foot apart. This arrangement is called the Gap. Seal the two terminals into a can from which the air is removed. Organize the equipment so that an electrically charged particle—electrons and protons are the prime projectiles—can move freely across the gap. A negatively charged electron will gladly rush toward the positive terminal, gaining an energy of (look at the label on the battery) 12 eV. Thus the Gap produces an acceleration. If the positive metallic terminal is made of wire screen instead of a solid plate, most of the electrons will pass through it, creating a directed beam of 12 eV electrons. Now an electron volt is an extremely small amount of energy. What we need is a billion-volt battery, but Sears doesn't handle such an item. To achieve high voltages requires moving beyond chemical devices. But no matter how big the accelerator, whether we're talking about a 1920s Cockcroft-Walton device or the fifty-four-mile-around Super Collider, the basic mechanism remains the same—the Gap, across which particles gain energy.

  The accelerator takes normal, law-abiding particles and gives them extra energy. Where do we get the particles? Electrons are easy. We heat a wire to incandescence, and electrons pour out. Protons are easy, too. The proton is the nucleus of the hydrogen atom (hydrogen nuclei have no neutrons), so all we need is commercially available hydrogen gas. Other particles can be accelerated, but they must be stable—that is, have long lifetimes—because the acceleration process is time consuming. And they must be electrically charged, since the Gap obviously wouldn't work on a neutral particle. The leading candidates for acceleration are protons, antiprotons, electrons, and positrons (anti-electrons). Heavier nuclei such as deuterons and alpha particles can also be accelerated, and they have their special uses. An unusual machine under construction on Long Island in New York will accelerate uranium nuclei to billions of electron volts.

  THE PONDERATOR

  What does the acceleration process do? The easy but incomplete answer is that it speeds up the lucky particles. In the early days of accelerators, this explanation worked fine. A better description is that it raises the energy of the particles. As accelerators got more powerful, they soon were able to achieve speeds close to the ultimate: the velocity of light. Einstein's 1905 special theory of relativity asserts that nothing can travel faster than light. Because of relativity, "velocity" is not a very useful concept. For example, one machine may accelerate protons, say, to 99 percent of the velocity of light, while a much more expensive one, built ten years later, can achieve 99.9 percent of the velocity of light. Big deal. Go explain this to the congressman who voted all that dough just to achieve another 0.9 percent!

  It's not speed that sharpens the Democritan knife and yields new domains of observation. It's energy. A 99-percent-of-the-velocity-of-light proton has an energy of about 7 GeV (the Berkeley Bevatron, 1955), whereas a 99.95 percent proton has 30 GeV (Brookhaven AGS, 1960), and a 99.999 percent proton has 200 GeV (Fermilab, 1972). So Einstein's relativity, which rules the changes in velocity and energy, makes it silly to talk about speed. What is important is energy. A related attribute is momentum, which, for a high-energy particle, can be considered directed energy. Incidentally, the particle being accelerated also gets heavier because of E = mc2. In relativity a particle at rest still has the energy given by E = m0c2, where m0 is defined as the "rest mass" of the particle. When the particle is accelerated its energy, E, and hence its mass increase. The closer to the velocity of light we get, the heavier the object becomes, and consequently the more difficult it is to increase its speed. But the energy keeps going up. Conveniently, a proton's rest mass is about 1 GeV. So the mass of a 200 GeV proton is more than two hundred times that of the proton resting comfortably in the hydrogen gas bottle. Our accelerator is actually a "ponderator."

  MONET'S CATHEDRAL, OR THIRTEEN WAYS OF LOOKING AT A PROTON

  Now, how do we use these particles? Simply said, we cause them to make collisions. Since this is the core process by which we learn about matter and energy, we must go into detail. It's okay to forget the various particulars about the machinery and how the particles are accelerated, interesting as these may be. But remember this part. The whole point of the accelerator is the collision.

  Our technique of observing and eventually comprehending the abstract world of the subnuclear domain is similar to how we comprehend anything—a tree, for example. What is the process? First, we need light. Let's use sunlight. The photons from the sun stream toward the tree and reflect off leaves and bark, twigs and branches, and some fraction of these photons is collected by our eyeball. The photons, we can say, are scattered by the object toward the detector. The lens of the eye focuses the light on the retina at the back of the eye. The retina detects the photons and sorts out the various qualities: color shade, intensity. This information is organized and sent to the on-line processor, the occipital lobe of the brain, which specializes in visual data. Eventually, the off-line processor comes to a conclusion: "By Jove, a tree! How lovely."

  The information coming to the eye may be filtered through spectacles or sunglasses, adding to the distortion that the eye has already introduced. It's up to the brain to correct these distortions. Let's replace the eye with a camera, and now, a week later, with a greater degree of abstraction, the tree is seen projected in a family slide show. Or a video recorder can convert the data provid
ed by scattered photons into digital electronic information: zeroes and ones. To enjoy this, one plays it through the TV, which converts the digital information back to analog—a tree shows up on the screen. If one wanted to send "tree" to our scientist colleagues on the planet Ugiza, the digital information might not be converted to analog, but it could convey, with maximum precision, the configuration that earthlings call a tree.

  Of course, it's not so simple in an accelerator. Different kinds of particles are used in different ways. Still, we can push the metaphor for nuclear collisions and scattering another step. The tree looks different in the morning, at noon, in the setting sun. Anyone who has seen Monet's numerous paintings of the entrance to the cathedral at Rouen at different times of the day knows what a difference the quality of light makes. What is the truth? To the artist the cathedral has many truths. Each shimmers in its own reality—the hazy morning light, the stark contrasts of the noontime sunshine, or the rich glow of the late afternoon. In each of these lights a different aspect of truth is exhibited. Physicists work with the same bias. We need all the information we can get. The artist employs the sun's changing light. We employ different particles: a stream of electrons, a stream of muons or neutrinos—at ever-changing energies.

  Here's how it works.

  What is known about a collision is what goes in and what comes out—and how it comes out. What happens in that tiny volume of the collision? The maddening truth is that we can't see. It's as if a black box covers the collision region. The inner mechanistic details of the collision are not observable—are hardly even capable of being imagined—in the spooky, shimmering quantum world. What we do have is a model for the forces at play and, where relevant, for the structure of the colliding objects. We see what goes in and what comes out, and we ask if the patterns are predictable by our model of what is in the box.