In physics, the exchange interaction (with an exchange energy, and exchange term) is a quantum mechanical effect that only occurs between identical particles. Despite sometimes being called an exchange force in analogy to classical force, it is not a true force, as it lacks a force carrier.
The effect is due to the wave function of indistinguishable particles being subject to exchange symmetry, that is, either remaining unchanged (symmetric) or changing its sign (antisymmetric) when two particles are exchanged. Both bosons and fermions can experience the exchange interaction. For fermions, it is sometimes called Pauli repulsion and related to the Pauli exclusion principle. For bosons, the exchange interaction takes the form of an effective attraction that causes identical particles to be found closer together, as in Bose–Einstein condensation.
The exchange interaction alters the expectation value of the distance when the 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 to distinguishable particles). Among other consequences, the exchange interaction is responsible for ferromagnetism and for the volume of matter. It has no classical analogue.
Exchange interaction effects were discovered independently by physicists Werner Heisenberg and Paul Dirac in 1926.
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