Exchange Interaction

In physics, the exchange interaction is a quantum mechanical effect between identical particles.

It is due to the wave function of indistinguishable particles being subject to exchange symmetry, that is, the wave function describing two particles that cannot be distinguished must be either unchanged (symmetric) or inverted in sign (antisymmetric) if the labels of the two particles are changed.

Both bosons and fermions can experience the exchange interaction. For fermions, it is sometimes called Pauli repulsion; it is related to the Pauli exclusion principle. For bosons, it causes identical particles to be found closer together, as in Bose-Einstein condensation.

The exchange interaction alters the expectation value of the energy when wave functions of two or more indistinguishable particles overlap. It increases (for fermions) or decreases (for bosons) the expectation value of the distance between identical particles (as compared from distinguishable particles).

Among other consequences, the exchange interaction is responsible for ferromagnetism and for the volume of matter. The exchange interaction has no classical analogue.

Exchange interaction effects were discovered independently by physicists Werner Heisenberg and P. A. M. Dirac in 1926.