Strong interaction
The strong interaction, or strong force, is one of the four fundamental forces. Its two main jobs are to bind quarks together into protons, neutrons, and other particles (hadrons), and to hold protons and neutrons together inside atomic nuclei (the nuclear force).
Most of the mass of protons and neutrons comes from the energy of the strong interaction, not from the mass of the quarks themselves. The force is incredibly strong at very short distances: about 100 times stronger than electromagnetism, a million times stronger than the weak force, and vastly stronger than gravity at around 1 femtometer (10^-15 meters).
Inside protons and neutrons, quarks are held together by gluons, the force carriers. Quarks carry color charge, and gluons themselves carry color charge too. This theory is called quantum chromodynamics (QCD). A key feature is color confinement: isolated quarks or gluons cannot be observed. If you try to pull them apart, the energy produces new quark–antiquark pairs, so you always end up with new hadrons rather than free quarks.
Between nucleons in a nucleus, there is a weaker residual force acting at slightly larger distances (up to about 3 femtometers). This nuclear force is transmitted by mesons (such as pions) and keeps nuclei bound. It fades with distance, which is why larger nuclei can be unstable.
Nuclear fusion (joining nuclei) powers the Sun and other stars, while nuclear fission (splitting nuclei) is used in reactors and some weapons. In both cases energy comes from the strong interaction, shown by the mass defect: nuclei are slightly lighter than the sum of their parts because binding releases energy.
The strong force is described by QCD, a part of the Standard Model. It has a special property called asymptotic freedom: the force becomes weaker at very high energies.
This page was last edited on 2 February 2026, at 01:44 (CET).