strong force

The strong force (or strong interaction) is one of the four known fundamental forces (along with electromagnetism, the weak force, and gravity). It is the force which holds together quarks to form nucleons and that which holds together the latter to form nuclei, and must be overcome for fission to occur. Unlike the electric force or gravity, it is not described by an inverse square law, but is extremely strong (enough to overcome the electrical repulsion of protons and charged quarks) at very short distances, but falls off abruptly by distance. At the small distances in question, the ratio of electric to strong forces is approximately 1/137, the number being the fine structure constant.

Descriptions of the strong force (e.g., between quarks) posit something analogous to electric charge but while electric charge has two polarities termed positive and negative, this different kind of charge has three such possibilities. The terms red, green, and blue (instead of positive and negative) are used for these three charges, and this type of charge is termed a color charge, which is a property of a quark. (These color terms are used analogously: the color charge red has nothing to do with the red that our eyes perceive.) A type of boson associated with these color charges is termed a gluon (somewhat like the photon's association with electric charges). For each of these colors, there is an anticolor, which antiquarks possess. The physical laws describing the behavior of color charge, quarks, and gluons, is termed quantum chromodynamics (QCD).

Electroweak theory treats electromagnetism and the weak force as two aspects of a single force. Proposed theories positing a single force that also includes the strong force as a third aspect are termed Grand Unified Theories or GUTs.