Superforce: The Search for a Grand Unified Theory of Nature
Perfect Symmetry: The Search for the Beginning of Time
O amazement of things—even the least particle!
—Walt Whitman, “Song at Sunset”
Theoretical physicists are in a state of high excitement these days, and for good reason. New discoveries in particle physics, combined with brilliant theoretical invention, suggest that they are on the verge of nothing less than explaining everything.
Well, not exactly Everything, but everything possible for physics to explain. More precisely, they believe they are close to constructing a unified field theory that will describe exactly how the universe, almost instantly after the big bang, acquired all the particles and forces that allowed it, some 15 billion years later, to grow into the universe we know. A few adventuresome theorists think they may soon be able to explain how the primeval explosion itself was caused by a random quantum fluctuation of Nothing.
One of the two books under review is by the American physicist Heinz Pagels, whose previous book The Cosmic Code is one of the best introductions to quantum mechanics I have ever read. The other volume is by the British physicist Paul Davies, author of many earlier books that are models of science writing for laymen. Both books are admirable up-to-the-minute accounts of the search for what Pagels calls the Holy Grail. Those who work in “the shadowy world of fundamental physics,” Davies writes in his first paragraph, are about to complete their long quest “for a prize of unimaginable value—nothing less than the key to the universe.”
Is it really true that physics may be nearing the end of a road, “going for broke,” as Pagels puts it? Of course, there will remain the infinite problems on what Pagels calls “the frontier of complexity”—such trifles as explaining how the basic forces and particles manage to get together and write books about themselves—but there may be nothing more to learn on a bedrock level. The situation will be something like that of plane geometry. All its theorems are implied by its axioms, but the number of theorems yet unknown is infinite. A unified field theory would in no way be the end of science or technology. There will be endless inventions to make, endless worlds out there in space to explore. It would only mean the end of the search for fundamental laws.
Unfortunately, physicists have been in previous states of euphoria about reaching the end of the road. In the late Twenties everything seemed wonderfully simple. Maxwell’s field equations had explained electromagnetism. Einstein’s field equations had explained gravity. And Einstein was hard at work on a theory to unify these two forces. All matter was made of atoms that contained just two kinds of particles: protons in the nucleus and electrons whirling around in paths described by probability waves. “Physics as we know it will be over in six months,” said Max Born, one of the great architects of quantum mechanics.
In 1932 nature began to look shaggy again. A new particle, the neutron, was found hiding in the nucleus. Paul Dirac’s theoretical work implied that the electron had an antiparticle twin exactly like it but with a positive charge, and, sure enough, in 1932 the positron was found. Then came the deluge. Dozens of entirely unsuspected particles began to turn up as physicists started clashing particles together at high speeds in the new accelerators—particles that had, in Robert Oppenheimer’s words, an “insulting lack of meaning.” When it appeared later that protons and neutrons were made of smaller particles, the physicist Murray Gell-Mann, drawing on Finnegans Wake, called these “quarks” (“three quarks for Muster Mark!”).
Two entirely new forces arrived on the scene. Neutrons and protons were found to be held together in the nucleus by a “strong force,” much stronger than gravity or electromagnetism but operating at extremely small distances. In radioactive decay, when a neutron decays into an electron, proton, and an antineutrino, the process was found to be controlled by a “weak force.”
Slowly, over the next four decades, the hundreds of newly discovered particles began to make sense. It now appears that everything material is made from combinations of six kinds of quarks and six particles called leptons. The four forces, or “interactions,” as physicists prefer to say—gravity, electromagnetism, the “strong” and the “weak” forces—are transmitted by a third set of more ghostly particles called bosons.
The leptons are pointlike particles that are influenced by the weak force. For example, when radioactivity takes place, and a neutron decays into a proton, the weak force creates two leptons, an electron and an antineutrino. Leptons have no known interior structure, but they have mass and a curious quantum property called spin, to which I will soon return. The electron is the most important lepton because it is part of all atoms, and because it carries electrical charge. (Electric currents are produced by moving electrons.) The muon is the funniest lepton. It has been called a fat electron because it is just like an electron in every respect except that it weighs more than two hundred times as much. “Consider the muon,” Columbia University’s Isidor I. Rabi once began a lecture. “Who ordered that?” The tauon, discovered in 1976, is even odder. It is a fat muon more than 3,500 times as massive as the electron. When the three particles are involved in certain interactions, each is associated with its own kind of neutrino.
Neutrinos, which make up the other three leptons, are as close to nothing as a particle can get. They have no electric charge, no known mass. Their only property seems to be spin. Quantum spin is something like the spin of a top but much more mysterious. (How can a point spin?) All three neutrinos are left-handed in the sense that when they move away from you their spin is counterclockwise. Each of the six leptons has a twin antiparticle of opposite spin (and opposite charge if not a neutrino), so if antiparticles are considered, there are twelve leptons.
The six quarks come in six types or “flavors”: up, down, charmed, strange, top (or truth), and bottom (or beauty). These words have no connection with their ordinary meanings. They are simply colorful terms for properties of quarks that can only be described by mathematical expressions. All quarks are believed to be pointlike, though some theorists have speculated that they are made of smaller particles, and a few think there may even be an infinity of sublevels. Each quark has mass, spin, a fractional electric charge of plus two-thirds or minus one-third, and a different kind of charge called color. It differs from an electric charge in that it applies to a new property of matter that comes in three varieties: red, green, and blue. Of course the quarks do not have colors in the usual sense, but the color names are useful because of analogies between the mixing of quark colors and the mixing of ordinary colors.
Each quark has its antiquark of opposite charges and spin. The heavier a particle, the harder it is to produce in an accelerator, and for this reason the heaviest quark, the top quark, was the last to be detected. Carlo Rubbia, heading a team of scientists at CERN (The European Laboratory for Particle Physics, in Geneva) announced evidence for the top quark late in 1984.
Quarks join together in only two ways, pairs or triplets, to make composite particles called hadrons. Hadrons are influenced only by the strong force. It binds together the quarks inside them and also binds them to one another. Quark doublets (each consisting of a quark and an antiquark) form hadrons called mesons. The triplets (three quarks) are the baryons, of which the most important are the proton (two up quarks and one down) and the neutron (two down, one up). Every possible combination of doublets and triplets is a hadron that has been experimentally verified.
Particles that carry the four forces go by various names: bosons, exchange particles, interaction particles, carrier particles, virtual particles, ghost particles. They can be thought of as particles that zip rapidly back and forth between the “real” particles. The photon carries the electromagnetic force, the graviton (not yet detected) carries gravity. The weak force has three carriers: a positively charged W particle, a negatively charged W particle, and a chargeless Z particle. A 1984 Nobel prize went to Rubbia and Simon van der Meer for their observations at CERN of all three particles. The strong force that glues together the quarks inside the hadrons, and the hadrons to each other, is carried by eight kinds of bosons appropriately called gluons. (Pagels uses the term gluon for all exchange particles, and color gluons for carriers of the strong or color force.)
Every force is described by a field such as the magnetic field around a magnet or the gravity field around the earth. Every field has its carrier particle, and every particle, carrier or otherwise, has its field. Pagels’s chapter on fields is especially useful in making clear the overriding importance of fields and their symmetries.
Symmetry is that property of a structure which remains the same if you perform a certain operation on it. For example, the letter H has 180 degree rotational symmetry because if you turn it upside down it doesn’t change. It also has left-right symmetry because it is unaltered by mirror reflection. It has glide symmetry because it stays the same if you slide it along the page. The letter F has glide symmetry but lacks rotational and reflection symmetry.
Fields share all the symmetry properties of their particles. A particle is simply a property of a field. Newton’s atoms were like hard little billiard balls, but in quantum mechanics such atoms have totally dematerialized. For a crude analogy, think of a sheet of paper filled with hundreds of parallel creases. Imagine that another set of parallel creases, at right angles to the first set, glides across the page. At spots where the creases intersect, little bumps move across the page. Particles resemble those bumps. They are created by the movements of fields. “The world according to this view,” Pagels writes, “is a vast arena of interacting fields manifested as quantum particles flying about and interacting with each other.”
In our analogy the fields were made of paper. What are quantum fields made of? The question is meaningless. They are not made of anything. They are irreducible in the sense that they can’t be reduced to something more fundamental. They are pure mathematical concepts. They just are.
The first great modern unification theory was James Clerk Maxwell’s joining of magnetism and electricity. They were thought to be independent forces until Maxwell’s field equations (field equations describe how a field changes in space and time) combined them. Gravity and inertia were similarly considered different until Einstein showed them to be manifestations of a single field. Einstein spent the last part of his life vainly trying to unify gravity and electromagnetism. Nobody can blame him for failing because the data he had at hand were too scant.