Quantum Physics

984 Chapter 30 Nuclear Energy and Elementary Particlestronic television. In this method, called inertial electrostatic confinement, positivelycharged particles are rapidly attracted towards a negatively charged grid.Some of the positive particles then collide and fuse.An international collaborative effort involving four major fusion programs iscurrently under way to build a fusion reactor called the International ThermonuclearExperimental Reactor (ITER). This facility will address the remaining technologicaland scientific issues concerning the feasibility of fusion power. Thedesign is completed, and site and construction negotiations are under way. If theplanned device works as expected, the Lawson number for ITER will be about sixtimes greater than the current record holder, the JT-60U tokamak in Japan.30.4 ELEMENTARY PARTICLESThe word “atom” is from the Greek word atomos, meaning “indivisible.” At onetime, atoms were thought to be the indivisible constituents of matter; that is, theywere regarded as elementary particles. Discoveries in the early part of the 20thcentury revealed that the atom is not elementary, but has protons, neutrons, andelectrons as its constituents. Until 1932, physicists viewed these three constituentparticles as elementary because, with the exception of the free neutron, they arehighly stable. The theory soon fell apart, however, and beginning in 1937, manynew particles were discovered in experiments involving high-energy collisions betweenknown particles. These new particles are characteristically unstable and havevery short half-lives, ranging between 10 23 s and 10 6 s. So far more than 300 ofthem have been cataloged.Until the 1960s, physicists were bewildered by the large number and variety ofsubatomic particles being discovered. They wondered whether the particles werelike animals in a zoo or whether a pattern could emerge that would provide abetter understanding of the elaborate structure in the subnuclear world. In thelast 30 years, physicists have made tremendous advances in our knowledge of thestructure of matter by recognizing that all particles (with the exception of electrons,photons, and a few others) are made of smaller particles called quarks.Protons and neutrons, for example, are not truly elementary but are systems oftightly bound quarks. The quark model has reduced the bewildering array of particlesto a manageable number and has predicted new quark combinations thatwere subsequently found in many experiments.30.5 THE FUNDAMENTAL FORCES OF NATUREThe key to understanding the properties of elementary particles is to be able todescribe the forces between them. All particles in nature are subject to four fundamentalforces: strong, electromagnetic, weak, and gravitational.The strong force is responsible for the tight binding of quarks to form neutronsand protons and for the nuclear force, a sort of residual strong force, bindingneutrons and protons into nuclei. This force represents the “glue” that holdsthe nucleons together and is the strongest of all the fundamental forces. It is avery short-range force and is negligible for separations greater than about10 15 m (the approximate size of the nucleus). The electromagnetic force, whichis about 10 2 times the strength of the strong force, is responsible for the bindingof atoms and molecules. It is a long-range force that decreases in strength as theinverse square of the separation between interacting particles. The weak force is ashort-range nuclear force that tends to produce instability in certain nuclei. It isresponsible for beta decay, and its strength is only about 10 6 times that of thestrong force. (As we discuss later, scientists now believe that the weak and electromagneticforces are two manifestations of a single force called the electroweakforce). Finally, the gravitational force is a long-range force with a strength onlyabout 10 43 times that of the strong force. Although this familiar interaction isthe force that holds the planets, stars, and galaxies together, its effect on elementary