Most physical theories aim at explaining apparently different phenomena by simple sets of laws. The best example is Newton’s theory of gravitation. Apparently unrelated phenomena, such as the apple falling from the tree and the Moon circling Earth, are explained by a unique force: gravity. Gravity is a force that all masses exert on each other. It is an extremely weak force and can only be experienced when at least one of the masses is very large, such as Earth.
When scientists started experimenting with electricity and magnets they discovered other forces that seemed to act independently from gravitation: the magnetic force and the force of attraction or repulsion between electrically charged bodies. The two might be thought of as reciprocals, since electric charge produces magnetism and magnetism produces charge. When he developed his electromagnetic theory, James Clerk Maxwell unified these forces into one: the electromagnetic force. Charge and magnetism become two aspects of a single underlying reality.
In this century the study of the atomic nucleus has revealed two other forces in addition to gravity and electromagnetism. The strong force holds the protons and neutrons in the atomic nucleus together, and the weak force is responsible for certain slow nuclear processes, such as some types of radioactive decay.
With these four forces physicists can describe the whole range of known physical processes, and therefore these forces are called fundamental forces. The gravitational and electromagnetic forces are long-range forces because they act over the entire universe, while the weak and strong forces only act over very short distances. The forces act by means of particles, which are said to carry or to mediate the forces. Gravity and electromagnetism are mediated by massless particles, which is why they affect suitable bodies at infinite distances. The strong force was described by Yukawa’s theory of 1935 as acting through a medium-mass particle, or meson [Greek word meaning “middle” or “intermediary”]. This explains why it is only effective at short distances.
(In a theory called quantum chromodynamics, the mesons of the strong force are a manifestation of an underlying color force, although this does not change anything about the way the strong force operates.) In science, the fewer laws to explain phenomena the better.
Although the four forces seem to act independently of each other, physicists have been searching for a way to put them into a single theory, so that each is a different manifestation of some underlying reality. Theories that would achieve this are called unification schemes. Albert Einstein, for example, was convinced that it would be possible to unify gravitation and electromagnetic forces. He spent much of his later years trying to find such a theory, but without success.
In 1967 Steven Weinberg, Abdus Salam, and Sheldon Glashow formulated a theory called electroweak theory that unifies the weak force and the electromagnetic force. According to this theory, the weak forces in atomic nuclei are mediated by three particles with even larger masses than the particles transmitting the strong force. The carriers of the weak force are the W+, W-, and Z0 particles with charges, respectively, of +1, –1, and zero. Masses must be high because the force operates at short distances only.
In the Weinberg-Salam-Glashow theory, however, at very high energies there is just one electroweak force; but at lower energies, the high-mass W and Z particles and the massless photons separate, becoming carriers of two different forces. Experimental verification of the existence of the high-mass particles eluded high-energy physicists until 1983, when scientists at CERN (Centre Européen de Recherche Nucléaire), near Geneva, Switzerland, under the direction of Carlo Rubbia, discovered the W and Z particles in head-on collisions in a huge proton-antiproton collider.
Theories that would unify the electromagnetic force, the strong force, and the weak force are called grand unification theories, or GUTs. Theoretical physicists have been somewhat successful in developing GUTs, but there are several competing theories and no evidence as to which one might be correct. It is generally believed that it will be much more difficult to find a theory that unifies the four fundamental forces. A theory that would unify gravity with the other forces –– a theory sometimes called supergrand unification, or SuperGUT, a.k.a. the Theory of Everything –– still has to be worked out. A promising beginning has been made with the development of superstring theories that emerged during the 1980s.