forces, fundamental

In physics, four fundamental interactions currently known to be at work in the physical universe. There are two long-range forces: the gravitational force, or gravity, which keeps the planets in orbit around the Sun and acts between all particles that have mass; and the electromagnetic force, which stops solids from falling apart and acts between all particles with electric charge. There are two very short-range forces, which operate over distances comparable with the size of the atomic nucleus: the weak nuclear force, responsible for the reactions that fuel the Sun and for the emission of beta particles by some particles; and the strong nuclear force, which binds together the protons and neutrons in the nuclei of atoms. The relative strengths of the four forces are: strong, 1; electromagnetic, 10⁻²; weak, 10⁻⁶; gravitational, 10⁻⁴⁰.

By 1971, the US physicists Steven Weinberg and Sheldon Glashow, the Pakistani physicist Abdus Salam, and others had developed a theory that suggested that the weak and electromagnetic forces were aspects of a single force called the electroweak force; experimental support came from observation at the European particle-physics laboratory CERN in the 1980s. Physicists are now working on theories to unify all four forces. See supersymmetry.