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The strong interaction or strong nuclear force is one of the four fundamental forces in physics.
The other fundamental forces are electromagnetism, the weak interaction, and gravitation. They are called fundamental because there is no simpler way for physicists to understand what the forces do or how they do it.
The strong nuclear force is what holds most ordinary matter together. It does this in two ways: it holds subatomic particles, like neutrons and protons, together, and then it holds the atomic nucleus together.
It is the strongest fundamental force—many times stronger than gravity (1038 times stronger: that's 1 followed by 38 zeros). But it works only over very short distances of a few femtometres (fm). A femtometre is 10−15 (0.00000 00000 00001) metres.
Scientists often think about the two ways the strong interaction works as separate forces: the color force and the nuclear force. At distances of 0.8 fm and less, the color force holds subatomic particles like protons and neutrons together. At distances of 1 to 3 fm, the residual (leftover) strong force is what keeps protons and neutrons together in the atomic nucleus, so it is called the nuclear force. (This is like thinking of electricity and magnetism as separate forces, when the fundamental force is electromagnetism.)
The strong interaction is often thought to be the action of gluons, which 'glue' quarks together. Gluons can be exchanged (moved) between quarks, antiquarks and other gluons. All of those particles are said to carry a color charge, something that some elementary particles have which is like electric charge. Particles with color charge exchange gluons, like particles with electric charge exchange photons.
The theory of quantum chromodynamics (QCD) says that the strong force acts between quarks and gluons. Quantum chromodynamics is the theory that explains different colors [source?]. The strong force is the basic force controlled by gluons: affecting quarks, antiquarks, and the gluons themselves.
The strong force affects only quarks directly (as the color force). Between hadrons (like protons and neutrons), made up of quarks, the effect of the strong force is known as the nuclear force (which is not fundamental).
The strength of the strong force is the reason why we cannot detect free quarks (that is, quarks that are by themselves). The theory is that so much energy would be needed (to separate a quark) that new hadrons would be created instead. This is called color confinement and it is seen to happen in particle accelerators.