FUNDAMENTAL INTERACTION

in physics, any of the four basic forcesgravitational, electromagnetic, strong, and weak. All the known forces of nature can be traced to these fundamental interactions. Gravitation and electromagnetism were recognized long before the discovery of the strong and weak forces because their effects on ordinary objects are readily observed. The gravitational force acts between all objects having mass; it causes apples to fall from trees and determines the orbits of the planets around the Sun. The electromagnetic force is responsible for the repulsion of like and attraction of unlike electric charges; it explains the chemical behaviour of atoms and the properties of light. The strong and weak forces were discovered by physicists in the 20th century when they finally probed into the core of the atom. The strong interaction binds the protons and neutrons of the atomic nucleus together in spite of the intense repulsion of the positively charged protons for each other. The weak interaction manifests itself in certain forms of radioactive decay and in reactions between the lightest subatomic particles (i.e., electrons, muons, and their associated neutrinos). The four forces are often described according to their relative strengths. The strong force is regarded as the most powerful force in nature. It is followed in descending order by the electromagnetic, weak, and gravitational. Despite its strength, the strong force does not manifest itself in the macroscopic universe because of its extremely limited range. It is confined to an operating distance of about 10-13 cmabout the diameter of a proton. When two particles that are sensitive to the strong force pass within this distance, the probability that they will interact is high. The range of the weak force is shorter. Particles that are affected by this force must pass within 10-17 cm of one another to interact, and the probability that they will do so is low even at that distance. By contrast, the gravitational and electromagnetic forces operate, at least in theory, at an infinite range. That is to say, gravity acts between all objects of the universe, no matter how far apart they are, and an electromagnetic wave, such as the light from a distant star, travels undiminished through space until it encounters some particle capable of absorbing it. For years physicists have sought to show that the four basic forces are simply different manifestations of the same fundamental force. The most successful attempt at such a unification is the electroweak theory, proposed during the late 1960s by Steven Weinberg, Abdus Salam, and Sheldon Lee Glashow. This theory, which incorporates quantum electrodynamics (the quantum field theory of electromagnetism), treats the electromagnetic and weak forces in a unified manner, postulating the existence of a more basic electroweak force that is transmitted by four gauge bosons. One of these is the photon of electromagnetism, while the other three are associated with the weak forcethe electrically charged W+ and W- and the neutral Z0 particles. Unlike the photon, these weak gauge bosons are massive. In the 1970s investigators formulated a theory for the strong force that is similar in structure to quantum electrodynamics. According to this theory, the strong force is transmitted by gauge bosons dubbed gluons. Like photons, gluons are massless and travel at the speed of light. But they differ from photons in one important respect: they carry what is called colour charge, a property analogous to electric charge. Gluons are able to interact together because of colour charge, which at the same time limits their range. Investigators are seeking to devise comprehensive theories that will unify all four basic forces of nature. So far, however, gravity remains beyond attempts at such unified field theories.