In particle physics, the strong interaction, or strong force, or color force, holds quarks and gluons together to form protons and neutrons. The strong interaction is one of the four fundamental interactions, along with gravitation, the electromagnetic force and the weak interaction. The word strong is used since the strong interaction is the most powerful of the four fundamental forces; its typical field strength is 100 times the strength of the electromagnetic force, some 1013 times as great as that of the weak force, and about 1038 times that of gravitation.
The strong force is thought to be mediated by gluons, acting upon quarks, antiquarks, and the gluons themselves. This is detailed in the theory of quantum chromodynamics (QCD).
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Before the 1970s, protons and neutrons were thought to be indivisible fundamental particles. It was known that protons carried a positive electrical charge, electric repulsion made same-charge particles repel each other, and multiple protons were bound together in the atomic nucleus. However, it was unknown what force held the like-charged protons together (with neutrons) in the nucleus.
Another, stronger, attractive force was postulated to explain how protons and neutrons were held together in the atomic nucleus, overcoming electromagnetic repulsion. For its high strength at short distances, it was dubbed the "strong" force. It was thought, at that time, this strong force was a fundamental force acting directly on the nuclear particles. Experiments suggested that the force acted equally between any nucleons, whether protons or neutrons.
It was later discovered this phenomenon was only a residual side-effect of another, truly fundamental, force acting directly on particles inside protons and neutrons, called quarks and gluons. This newly-discovered force was initially called the "color force." The name was chosen for convenience, and has no relation to visible color. [1]
Today, the term "strong force" is used for that strong nuclear force that acts directly on quarks and gluons. The original strong force that acts between nucleons (and also between particles made of quarks, or hadrons) is today called the nuclear force, or the "residual strong nuclear force."
The contemporary strong force is described by quantum chromodynamics (QCD), a part of the standard model of particle physics. Mathematically, QCD is a non-Abelian gauge theory based on a local (gauge) symmetry group called SU(3).
Quarks and gluons are the only fundamental particles which carry non-vanishing color charge, and hence participate in strong interactions. The strong force itself acts directly only upon elementary quark and gluon particles.
All quarks and gluons in QCD interact with each other through the strong force. The strength of interaction is parametrized by the strong coupling constant. This strength is modified by the gauge color charge of the particle, a group theoretical property which has nothing to do with ordinary visual color.
The strong force acting between quarks, unlike other forces, does not diminish in strength with increasing distance, after a limit (about the size of a hadron) has been reached. It remains at a strength of about 10 newtons, no matter how far away from each other the particles are, after this limiting distance has been reached. In QCD, this phenomenon is called color confinement, implying that only hadrons can be observed; this is because the amount of work done against a force of 10 newtons is enough to create particle-antiparticle pairs within a very short distance of an interaction. Evidence for this effect is seen in many failed free quark searches.
The elementary quark and gluon particles affected are unobservable directly, but instead emerge as jets of newly created hadrons, whenever energy is deposited into a quark-quark bond, as when a quark in a proton is struck by a very fast quark (in an impacting proton) during an accelerator experiment. However, quark-gluon plasmas have been observed.
A residual effect of the strong force is called the nuclear force. The nuclear force acts between hadrons, such as nucleons in atomic nuclei. This "residual strong force," acting indirectly, transmits gluons that form part of the virtual pi and rho mesons, which, in turn, transmit the nuclear force between nucleons.
The residual strong force is thus a minor residuum of the strong force which binds quarks together into protons and neutrons. This same force is much weaker between neutrons and protons, because it is mostly neutalized within them, in the same way that electromagnetic forces between neutral atoms (van der Waals forces) are much weaker than the electromagnetic forces that hold the atoms internally together.
Unlike the strong force itself, the nuclear force, or residual strong force, does diminish in strength, strongly with distance. The decrease is approximately as an negative exponential power of distance, though there is no simple expression known for this; see Yukawa potential. This fact, together with the less-rapid decrease of the disruptive electromagnetic force between protons with distance, causes the instability of larger atomic nuclei, such as all those with atomic numbers larger than 82.
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