In particle physics, the strong interaction (also called the strong force, strong nuclear force, or color force) is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. At atomic scale, it is about 100 times stronger than electromagnetism, which in turn is orders of magnitude stronger than the weak force interaction and gravitation.
The strong interaction is observable in two areas: on a larger scale (about 1 to 3 femtometers (fm)), it is the force that binds protons and neutrons (nucleons) together to form the nucleus of an atom. On the smaller scale (less than about 0.8 fm, the radius of a nucleon), it is also the force (carried by gluons) that holds quarks together to form protons, neutrons and other hadron particles.
In the context of binding protons and neutrons together to form atoms, the strong interaction is called the nuclear force (or residual strong force). In this case, it is the residuum of the strong interaction between the quarks that make up the protons and neutrons. As such, the residual strong interaction obeys a quite different distance-dependent behavior between nucleons, from when it is acting to bind quarks within nucleons. The binding energy related to the residual strong force is used in nuclear power and nuclear weapons.
The strong interaction is thought to be mediated by gluons, acting upon quarks, antiquarks, and other gluons. Gluons, in turn, are thought to interact with quarks and gluons because all carry a type of charge called "color charge." Color charge is analogous to electromagnetic charge, but it comes in three types rather than one, and it results in a different type of force, with different rules of behavior. These rules are detailed in the theory of quantum chromodynamics (QCD), which is the theory of quark-gluon interactions.
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