exchange interaction

The exchange interaction is an effective influence between identical particles that arises purely from the symmetry of their wavefunction, with no new force of nature involved. Because identical fermions must be described by an antisymmetric state, they tend to avoid being in the same place; because identical bosons live in symmetric states, they tend to bunch together. This change in how likely the particles are to be near one another shifts the system's energy, and that shift behaves as if a force were at work.

It is worth being precise about what it is and is not. There is no extra exchange field pushing the particles; the ordinary forces, such as the electric repulsion between electrons, are unchanged. What the exchange symmetry alters is the spatial pattern of where the particles are likely to be, and since the energy depends on those distances, the average energy comes out different. The result is often called an exchange force only as a vivid shorthand for this statistical effect.

Though it is a bookkeeping consequence of symmetry, the exchange interaction has enormous physical reach. It is the dominant reason that the iron in a magnet, or the needle of a compass, is magnetic at all: electron spins line up not because magnets pull on them directly but because the exchange interaction makes aligned spins energetically favourable. The same effect governs the bonding in molecules and the magnetic ordering of many materials.