Exchange interaction independently by physicists Werner Heisenberg and Paul Dirac in 1926. The exchange interaction is sometimes called the "exchange force". However, it is not a true force and should not be confused with the exchange forces produced by the exchange of force carriers, such as the electromagnetic force produced between two electrons by the exchange of a photon, or the strong force between two quarks produced by the exchange of a gluon. Although sometimes erroneously described as a force, the exchange interaction is a purely quantum mechanical "effect" unlike other forces. Quantum mechanical particles are classified as bosons or fermions. The
Gauge fixing well to non-abelian gauge groups such as the SU(3) of QCD. The couplings between physical and unphysical perturbation axes do not entirely disappear under the corresponding change of variables; to obtain correct results, one must account for the non-trivial Jacobian of the embedding of gauge freedom axes within the space of detailed configurations. This leads to the explicit appearance of forward and backward polarized gauge bosons in Feynman diagrams, along with Faddeev–Popov ghosts, which are even more "unphysical" in that they violate the spin-statistics theorem. The relationship between these entities, and the reasons why they do not appear as particles