Science Matters

by Robert M. Hazen

  Today we stand on the brink of discovering the ultimate “theory of everything,” which may, after a two-millennium search, provide us with a detailed understanding of how our universe is built and ordered at the most fundamental level. At the foundation of that theory lies one key idea:

  All matter is really made of quarks and leptons.

  NUCLEI AND SUBNUCLEAR PARTICLES

  Protons and neutrons are only two of the dozens of different particles found inside the nucleus of every atom. Starting in the 1950s, the frontier of physics has shifted from a study of the properties of nuclei to a study of the particles found inside them—a field called elementary particle physics, or high-energy physics. This remains the frontier today, although we know a great deal more about these particles than we did forty years ago.

  The study of the particles inside the nucleus began in the 1930s when physicists studied cosmic rays. These high-speed particles, mainly protons, are produced in stars and rain down constantly on the earth from space. When cosmic rays hit a nucleus, two things can happen: (1) they can break the nucleus apart, spewing its constituent particles out where scientists can see them, and (2) some of the kinetic energy of the cosmic rays can be converted into mass, creating new particles.

  To make a long story short, by the 1950s scientists had observed dozens of new kinds of particles coming out of collisions of cosmic rays with nuclei. All of these particles were unstable—they lived for only a short time and then decayed, like radioactive nuclei, into collections of other particles. Today scientists build huge machines called accelerators to produce beams of protons or electrons to use in place of cosmic rays. As a consequence, the roster of elementary particles has risen to several hundred.

  Amid this proliferation of “elementary” particles, some general rules of organization become clear. There are basically two kinds of particles: those involved in structure (by far the majority) and those involved in forces. The proton, neutron, and electron, all basic building blocks of the atom, are examples of the first type of particles. You can think of them and other particles like them as the bricks of the universe—the things that, put together in different patterns, constitute everything.

  The photon, the quantum of ordinary light, falls into the second class of particles. As we shall see shortly, the transfer of photons between charged objects creates the electromagnetic force that, among other things, holds electrons in their orbits. So the photon and its partners are the mortar of the universe, holding the bricks together. In the jargon of particle physics, they are called gauge particles.

  Among the “bricks,” there is a further important distinction. Some particles, like the proton and neutron, exist inside the nucleus and take part in the nuclear maelstrom. We call such particles hadrons, from the Greek for “strongly interacting ones.” Other particles, like the electron, are not normally involved in the nucleus, but remain aloof from all the activity that goes on there. We call such particles leptons, from the Greek for “weakly interacting ones.”

  Physicists have one particularly strong prejudice about nature. They believe that, deep down, nature is simple. But as more and more subatomic “elementary” particles were discovered, things seemed to be getting very complex. Physicists counted at least four families of gauge particles (the photon, and other particles called the W and Z, the gluon, and the graviton) and six different leptons (the electron, and particles called the mu meson, and the tau meson, plus the three neutrinos associated with each of these), along with hundreds of different hadrons whizzing around inside the nucleus. But hundreds of particles can’t all be elementary If they were, it would violate the central belief of physicists that nature should be simple. As patterns of regularity among the hundreds of hadrons came to light in the late 1960s, scientists realized that hadrons are not themselves elementary, but are really collections of still more elementary things. We now call these more basic building blocks quarks (the term comes from a line in James Joyce’s novel Finnegan’s Wake).

  At last particle physicists could breathe a collective sigh of relief. The idea of the quark is simple: there are only six different kinds of quarks, and different arrangements of them make all of the hundreds of hadrons, just as bricks can be combined to make an infinite variety of buildings. The proton and neutron, for example, are each made from three quarks.

  The six kinds of quarks (or “flavors,” to use the physicists’ fanciful term) come in three pairs with the following names: up and down, strange and charm, bottom and top. Particles containing all six of these quarks have been seen at accelerator laboratories in the United States, Europe, and Japan.

  So in the end everything is made from quarks and leptons. Quarks combine to make hadrons, and hadrons combine to make the nuclei of atoms. Electrons (which are leptons) attach themselves in orbit to make atoms, and atoms hook together to make all the infinite number of things we see about us.

  More recently, unified field theories of the type discussed later in this chapter have suggested that quarks themselves may be made of still more elementary objects called strings. Imagine these as tiny rubber-band-like objects with different types of vibrations corresponding to different quarks. At the moment, string theories (which involve very difficult mathematics) have not succeeded in producing clear predictions of experimental outcomes, and so are best thought of as theoretical speculations waiting to be tested.

  THE FOUR FORCES

  We all have an intuitive notion of force as a push or pull. Modern physicists, however, see forces quite differently. The current view is that every force arises from the exchange of a particle. One way to picture this exchange is to imagine two skaters approaching each other, with one skater holding a bucket of water. As they pass, the skater with the bucket throws the water at the other. Both skaters will change direction—one because of the recoil of the throw, the other because of the impact. Newton’s first law says that any change of direction results from the action of a force. It is clear that the force in this instance is due to (a physicist would say “mediated by”) the water exchanged between the two skaters. In exactly the same way, physicists picture all forces between elementary particles as being mediated by the exchange of gauge particles.

  For example, when two electrons approach each other, physicists picture what happens as follows: One electron, like the skater who throws the water, emits a photon. The other electron, like the skater who gets hit, absorbs the photon. The result: the electrons recoil from each other, and we say a force acts between them. Even the force between large objects like a magnet and a nail is thought to be generated by floods of photons being exchanged between the two pieces of metal.

  There are only four forces that operate in nature. Two are familiar from everyday experience—gravity and electromagnetism. Two others operate at the level of the nucleus. The first, called the strong force, acts to hold the nucleus together against the electrical repulsion between the protons. The other, called the weak force, is responsible for interactions like the beta decay of nuclei and neutrons. Whenever anything happens in the world, it happens because one or more of these forces is acting. The four forces differ from one another because each involves the exchange of a different kind of gauge particle.

  As two electrons approach each other, one emits a photon, which the other absorbs. This exchange results in a force between the two electrons—the electromagnetic force. This kind of illustration is called a Feynman diagram after American physicist Richard Feynman (1918–89).

  Every time you use your eyes or feel the sun’s warmth on your skin you detect photons, the gauge particles associated with the electromagnetic force. Photons have neither mass nor electric charge, and they travel at light speed.

  The strong force between quarks is mediated by a gauge particle appropriately named the gluon (it glues the quarks together). There are eight different kinds of gluons, all of them massless. Neither quarks nor gluons have been seen directly in the laboratory, although their effects have been measured. Curre
nt theory says that they cannot be isolated from the particles in which they reside.

  The weak force is mediated by two related gauge particles called the W and Z. First observed at the CERN particle accelerator in Switzerland in 1983, these particles are over eighty times as massive as the proton. The production and study of the W and Z remains a major area of effort in current high-energy physics research.

  No one has ever seen the gauge particle associated with the force of gravity, but physicists have a good idea of what it will be like. They believe that the ultimate theory of gravity requires a particle called the graviton. Physicists argue that the properties of the gravitational force require that the graviton will be massless and chargeless and travel at the speed of light, like the photon.

  The Particle Zoo

  There are so many kinds of elementary particles that sometimes it’s hard to tell the players without a scorecard. We list below a few of the terms and particles that you might run across.

  NEUTRINO: Neutrinos are nearly massless, electrically neutral particles that are often emitted during radioactive decay. They are, for example, one of the products of neutron decay. The neutrino is a lepton—it does not take part in nuclear interactions. There are three different neutrinos—one paired with the electron, and one each with the mu and tau leptons (see below).

  ANTIMATTER: For every particle, it is possible to produce an antiparticle. The antiparticle has the same mass as the particle, but is opposite in every other feature. For example, the antiparticle to the electron, the positron, has a positive electrical charge. When a particle meets its antiparticle, the two annihilate each other and all their mass is converted into energy.

  MU AND TAU LEPTONS: These particles are just like the electron, but heavier. They do not participate in nuclear reactions. The mu was discovered in cosmic ray debris in 1938, while the tau was found in 1975 at the Stanford Linear Accelerator Center. There is a type of neutrino associated with each of these, just as there is an “ordinary” neutrino associated with the electron.

  ACCELERATORS

  A large part of the cost of pursuing high-energy physics goes to build the machines called accelerators. As the name implies, these machines take particles (either electrons or protons) and accelerate them to speeds near the speed of light. These energetic particles are then directed against a target, where they collide with protons or nuclei. In the debris of these collisions, physicists search for the answers to their questions about the structure of matter.

  From machines a few feet across (in the 1930s) accelerators have grown to mammoth structures many miles in diameter. In a typical accelerator, protons are injected into a large ring lined with magnets. The magnets exert a force that keeps the positively charged protons moving in a circular path, and each time they come to a certain point in the ring, their energy is boosted. In the modern generation of machines, the effective energy is increased by sending two groups of particles around the ring in opposite directions and arranging things so they can collide head-on.

  There are four major accelerator centers you’re likely to read about:

  CERN: The European Center for Nuclear Research is run by a consortium of Western European nations. Located in Geneva, Switzerland, it has consistently enjoyed a position as one of the world’s most important centers of high-energy physics. In 2008 the Large Hadron Collider at CERN became the world’s most powerful accelerator.

  FERMILAB: The Fermi National Accelerator Laboratory, located outside of Chicago, was the world’s most powerful accelerator in the decades before the completion of the LHC. This machine’s large ring, a mile across, accommodates circulating groups of protons and antiprotons that are made to collide head-on. Fermilab pioneered the successful use of magnets made from superconducting materials and remains the largest installation of superconducting wire in the world.

  SLAC: The Stanford Linear Accelerator Center, located at Stanford University on the San Francisco peninsula, is one of the highest-energy electron accelerators in the world. The main working part is a two-mile-long tube down which electrons ride an electromagnetic wave like surfers on the ocean.

  ILC: The International Linear Collider is regarded by physicists as the machine of the future. At present it exists only on paper, but it would be the successor to the LHC. As the name implies, it would not have a circular design, but would be a linear accelerator of electrons. (For technical reasons, such a design becomes preferable at very high energies.) American physicists hope it will be built on the site of Fermilab.

  UNIFIED FIELD THEORIES

  A unified field theory is one in which two forces, seemingly very different from each other, are shown to be basically identical. In a sense both Newtonian gravity and Maxwell’s equations represent unified field theories. The first showed that heavenly and earthly gravity were identical, the second that electricity and magnetism were really the same thing. Today the term is used to refer to new theories in which two or more of the four fundamental forces are seen to be the same.

  You can visualize how seemingly different forces might be identical by thinking about the analogy of the ice skaters and the bucket of water. Suppose you had two sets of skaters—one set with a bucket of water frozen to ice, the other with a bucket of antifreeze—and suppose that the temperature in the rink was below freezing. The exchange that leads to the force might look very different—in one case it would involve a solid block of ice, in the other a fluid. We might argue that there were two different forces operating in the rink. If we raised the temperature, however, the ice would melt and we would see that the two forces were fundamentally the same—the similarity had been masked by the original low temperature.

  In the same way, physicists argue that we have four forces today only because temperatures are low. If we allow particles to collide at very high speeds, the temperature at the point of collision will go up and we should see forces unify. The theories that predict how this unification will take place are the modern unified field theories.

  The theories say that unification will take place in stages as the energy and temperature go up—two forces unifying, then a third joining those two, and finally the fourth coming in. The first unification, the one that brings together the weak and electromagnetic forces, has already been seen in the world’s great accelerators. The theories that describe this unification did quite a good job of predicting things like the masses and production rates of the W and Z particles, so we have a great deal of confidence in them.

  The next unification, in which the strong and electroweak forces come together, is described by a theory with the somewhat prosaic name of the standard model. It has been tested experimentally and come through with flying colors, so physicists have some confidence in it. The last unification, in which gravity joins the others in a single unified force, remains at the frontier of theoretical development, and will be discussed in the next section.

  FRONTIERS

  Rethinking Gravity

  Gravity is fundamentally different from the strong, electromagnetic, and weak forces. For one thing, it is much weaker than they are. This may seem like a strange thing to say, since gravity plays such a large role in our lives, but think about this: we know that a small magnet, one that can fit in the palm of your hand, can hold up a nail through the electromagnetic force, even though the entire Earth is pulling down on it with the force of gravity.

  More important, as we will see in Chapter 12, our best theory of gravity, called general relativity, describes the gravitational force in geometrical terms—it is seen as the result of the warping of space and time by the presence of matter. The other three forces, however, are described in terms of the exchange of gauge particles, as we saw in the previous section. Trying to reconcile these two different viewpoints turns out to be very difficult, but it is a task that has to be tackled if we are to unify all the forces. This final unification can be thought of as the current frontier in the millennia-old quest to understand the fundamental nature of matter. Here ar
e some terms you may read about as new data and ideas come in:

  STRING THEORY: This refers to a group of theories in which quarks are seen to be manifestations of tiny objects called strings, with different quarks corresponding to different modes of vibration.

  HIGGS PARTICLE: A particle first postulated by Scottish physicist Peter Higgs in the 1970s. The discovery of the Higgs completes the standard model. It is the particle whose interactions are supposed to explain the difference in mass between different particles.

  QUANTUM GRAVITY: A term that refers to theories that describe gravity in terms of the exchange of particles rather than in terms of the warping of space and time. Such theories may or may not involve the final unification of forces.

  SUPERSYMMETRY: A kind of symmetry in nature predicted by many string theories. If this symmetry really exists, then for each known particle there will be a heavier partner, sort of a mirror image. These supersymmetric particles are often denoted by an s, so that, for example, the partner of the electron is the selectron.

  CHAPTER TEN

  Astronomy

  THE SKY IS WONDROUS on a clear, cold, moonless night far from city lights. We marvel at the majestic sea of stars—thousands of stars, navigated by a half-dozen planets and the occasional brief meteor.

  Scientists are no different in their sense of awe and wonder, and they turn to the stars in their search for answers to questions about the meaning of it all. To our unaided eyes all the stars look like brilliant points of light, some a little brighter and others fainter, some colored with a hint of red or blue. But when we focus our telescopes skyward we see many different kinds of stars. Some are hot and dense, burning their fuel at an incredible rate. Others are cool, consuming fuel much more slowly We see stars in their infancy and stars growing old. And once in a great while we catch a glimpse of a star in its final cataclysmic hours, wracked by a massive fatal explosion. All this variety of stars tells a story: