Science Matters (19 page)

The main effort in fusion research today is in a project called ITER (International Thermonuclear Experimental Reactor), which is being built in the town of Cadarache in southern France. It is a major international collaboration, involving the United States, Japan, the European Union, Russia, and many other countries. The goal is to build a machine in which powerful magnets hold a gas of hydrogen isotopes together while they are heated to 100 million degrees. The machine is designed to deliver 500 megawatts of power (about half of what a large conventional power plant or fission reactor would deliver), and would be the first demonstration of our ability to get more energy out of a fusion reactor than we put in. The start-up of the machine is scheduled for 2016.

D
EEP UNDERGROUND
at the border between Switzerland and France, near the city of Geneva and in the shadow of the Alps, a new machine is probing the structure of matter at levels never before achieved. Completed in 2008, the Large Hadron Collider (LHC) is the largest high-tech construction project ever completed. In a circular tunnel 27 kilometers (about 20 miles) around, bunches of protons are accelerated in opposite directions. When they have reached energies corresponding to trillions of volts, they are allowed to collide head-on. For the briefest of moments, the energy at the collision site reaches a level not seen since the first fraction of a second in the life of the universe. In the debris of those collisions, scientists hope to answer an old question: of what is the world made?

Discovering the nature of matter is like peeling layers off an onion. Atoms are constructed from electrons and nuclei. Nuclei, though composed principally of neutrons and protons, are complex places where hundreds of different kinds of elementary particles whiz around, being created and absorbed each instant. And
these particles themselves are not truly “elementary,” but are made from things that are more elementary still.

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:

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.

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. Current 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.

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.

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.