ELECTROWEAK THEORY

the theory that describes both the electromagnetic force and the weak nuclear force. Superficially, these forces appear quite different. The weak force acts only across distances smaller than the atomic nucleus, while the electromagnetic force can extend across substantial stretches of space (e.g., as observed in thunderstorms), weakening only with the square of the distance. Moreover, within the nucleus, the weak force is some 1,000,000 times weaker than the electromagnetic force. Yet, one of the major discoveries of the 20th century has been that these two forces are different facets of the same, more fundamental force. (See also fundamental interaction.) The electroweak theory arose principally out of attempts to produce a self-consistent theory for the weak force, in analogy with quantum electrodynamics (QED), the successful quantum theory of the electromagnetic force developed during the 1940s. The two basic requirements for the theory of the weak force are, first, that it should be gauge invariant (i.e., it should behave in the same way at different points in space and time) and, second, that it should be renormalizable (i.e., it should not contain nonphysical infinite quantities). During the 1960s Sheldon Glashow, Abdus Salam, and Steven Weinberg independently discovered that they could construct a gauge-invariant theory of the weak force, provided that they also included the electromagnetic force. To mediate the interactions, the new theory predicts the existence of four massless messenger particles, two charged and two neutral. The short range of the weak force indicates, however, that it is carried by massive particles. This implies that the underlying symmetry of the theory is hidden, or broken, by some mechanism that gives mass to the particles exchanged in weak interactions but not to the photons exchanged in electromagnetic interactions. The assumed mechanism involves an additional interaction with an otherwise unseen field, called the Higgs field, that pervades all space. In 1971 Gerard 't Hooft proved that the unified electroweak theory proposed by Glashow, Salam, and Weinberg was renormalizable, and the theory gained full respectability. Later, experiments revealed the existence of the weak messengers, the neutral Z particle and the charged W particles; the masses of these particles were as predicted by the theory. See also standard model. Christine Sutton