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
Elementary charge known elementary particles, including quarks, have charges that are integer multiples of "e". Therefore, one can say that the "quantum of charge" is "e". In this case, one says that the "elementary charge" is three times as large as the "quantum of charge". On the other hand, all "isolatable" particles have charges that are integer multiples of "e". (Quarks cannot be isolated: they only exist in collective states like protons that have total charges that are integer multiples of "e".) Therefore, one can say that the "quantum of charge" is "e", with the proviso that quarks are not to be