In physics, the four fundamental interactions
believed to be at work in the physical
universe. There are two long-range forces:
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: the weak force,
responsible for the reactions that fuel the
Sun and for the emission of beta particles
from certain nuclei; and the strong force,
which binds together the protons and neutrons
in the nuclei of atoms. By 1971, Steven
Weinberg, Sheldon Glashow, Abdus Salam, and
others had developed a theory that suggested
that the weak and electromagnetic forces were
linked; experimental support came from
observation at CERN in the 1980s. Physicists
are now working on theories to unify all four
forces.