Strong interaction


 * This article is about technical details of the strong force. For an understanding of the interaction that holds protons and neutrons together inside nuclei see nuclear force.
 * The term 'strong interaction' is also used to describe food webs

The strong interaction is one of the four fundamental interactions along with gravitation, electromagnetic force and weak interaction.

The strong interaction or strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics (QCD). This fundamental force is responsible for the cohesion of particles in the atomic nucleus. The strong force is the fundamental force mediated by gluons, acting upon quarks, antiquarks, and the gluons themselves. Of the four fundamental forces -- gravity, electromagnetism, the strong nuclear force and the weak nuclear force -- the strong interaction is the most powerful.

History
Before the 1970s, when protons and neutrons were thought to be fundamental particles, the phrase "strong force" referred to what is today known as the strong nuclear force or the residual strong nuclear force. What were being observed were the "residual" effects of the strong force, which act on hadrons, both baryons and mesons. This force was postulated to overcome the electric repulsion between protons in the nucleus, and for its strength (at short distances) it was dubbed the "strong force". After the discovery of quarks, scientists realized that the force was actually acting upon the quarks and gluons making up the protons, not the protons themselves. For some time after this realization, the older notion was referred to as the residual strong force, and the "new" strong interaction was called color force.

Details
The strong force only acts directly upon elementary particles. However, a residual of the force is observed between hadrons (the best known example being the force that acts between nucleons in atomic nuclei) as the nuclear force. Here the strong force acts indirectly, transmitted as gluons that form part of the virtual pi and rho mesons, which classically transmit the nuclear force (see this topic for more). As has been shown by many failed free quark searches, the elementary particles affected are unobservable directly. This phenomenon is called confinement, a theory that allows only hadrons to be seen.

Quantum chromodynamics, a part of the standard model of particle physics, is a typical non-Abelian gauge theory based on a local (gauge) symmetry group called SU(3). All the particles in this theory interact with each other, through the strong force. The strength of the interaction is parametrized by the strong coupling constant. This strength is, as usual, modified by the gauge color charge of the particle. This really refers to a group theoretical property whose meaning is explained in the article on color charge and has nothing to do with color as such. Quarks and gluons are the only fundamental particles which carry non-vanishing color charge, and hence participate in the strong interactions.