The four different types of interaction that can occur between bodies. These interactions can take place even when the bodies are not in physical contact and together they account for all the observed forces that occur in the universe. While the unification of these four types of interaction into one model, theory, or set of equations has long been the aim of physicists, this has not yet been achieved, although progress has been made in the unification of the electromagnetic and weak interactions. See also elementary particles; gauge theory; unified-field theory.
The gravitational interaction, some 1040 times weaker than the electromagnetic interaction, is the weakest of all. The force that it generates acts between all bodies that have mass and the force is always attractive. The interaction can be visualized in terms of a classical field of force in which the strength of the force falls off with the square of the distance between the interacting bodies (see Newton's law of gravitation). The hypothetical gravitational quantum, the graviton, is also a useful concept in some contexts. On the atomic scale the gravitational force is negligibly weak, but on the cosmological scale, where masses are enormous, it is immensely important in holding the components of the universe together. Because gravitational interactions are long-ranged, there is a well-defined macroscopic theory in general relativity. At present, there is no satisfactory quantum theory of gravitational interaction. It is possible that superstring theory may give a consistent quantum theory of gravity as well as unifying gravity with the other fundamental interactions.
The weak interaction, some 1010 times weaker than the electromagnetic interaction, occurs between leptons and in the decay of hadrons. It is responsible for the beta decay of particles and nuclei. In the current model, the weak interaction is visualized as a force mediated by the exchange of virtual particles, called intermediate vector bosons. The weak interactions are described by electroweak theory, which unifies them with the electromagnetic interactions.
The electromagnetic interaction is responsible for the forces that control atomic structure, chemical reactions, and all electromagnetic phenomena. It accounts for the forces between charged particles, but unlike the gravitational interaction, can be either attractive or repulsive. Some neutral particles decay by electromagnetic interaction. The interaction is either visualized as a classical field of force (see Coulomb's law) or as an exchange of virtual photons. As with gravitational interactions, the fact that electromagnetic interactions are long-ranged means that they have a well-defined classical theory given by Maxwell's equations. The quantum theory of electromagnetic interactions is described by quantum electrodynamics, which is a simple form of gauge theory.
The strong interaction, some 102 times stronger than the electromagnetic interaction, functions only between hadrons and is responsible for the force between nucleons that gives the atomic nucleus its great stability. It operates at very short range inside the nucleus (as little as 10−15 metre) and is visualized as an exchange of virtual mesons. The strong interactions are described by a gauge theory called quantum chromodynamics.