nLab backreaction

Idea

In physics, the backreaction of an object or field configuration is its effect on other objects/fields by their mutual interaction.

Typically one speaks of backreaction only in situations where it has (previously) been neglected, in that all other fields have been regarded as a fixed background field for the given object/field. Given such a situation of a field in a background field one may then ask what the backreaction of the former on the latter is or would have been.

Examples

Particle mechanics

The orbit of an electron around a nucleus or of a planet around the sun is to good approximation computed by taking into account the force that the nucleus/sun exerts on the electron/planet, but neglecting the (much smaller) force exerted by the electron/planet on the nucleus/sun. The latter force is the backreaction; and one will speak about taking it into account or not.

Cosmic inhomogeneities

Not all backreaction considered is between small and large point masses.

For example, in inhomogeneous cosmology one speaks about the backreaction (or not) of inhomogeneities in the matter density filling the observable universe on the gravitational field filling the universe.

Branes

In string theory one speaks of probe branes when the backreaction of a small number of branes on some large number of background black branes is neglected.

References

See also

• Wikipedia, Backreaction