In
theoretical physics
Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experim ...
, back-reaction (or backreaction) is often necessary to calculate the self-consistent behaviour of a
particle
In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object which can be described by several physical property, physical or chemical property, chemical ...
or an object in an external field.
Intuitive definition
When a particle is considered to have no mass or to have an
infinitesimal
In mathematics, an infinitesimal number is a quantity that is closer to zero than any standard real number, but that is not zero. The word ''infinitesimal'' comes from a 17th-century Modern Latin coinage ''infinitesimus'', which originally referr ...
charge, this can be described as saying that we deal with a probe and that back-reaction is neglected. However, a real object also carries (in general) a mass and a charge itself. These properties imply that the model of the original environment needs to be modified to reach self-consistency. For example, a particle can be described as helping to curve the space in
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
. Taking into account the constraints implied on the model by the particle's properties – the back-reaction – is one way of reaching a more accurate model than if those constraints are ignored.
Cosmology
In
inhomogeneous cosmology
An inhomogeneous cosmology is a physical cosmological theory (an astronomical model of the physical universe's origin and evolution) which, unlike the currently widely accepted cosmological concordance model, assumes that inhomogeneities in the ...
, in which structure formation is taken into account in a general-relativistic model of the Universe, the term "backreaction" is used for a measure of the non-commutativity of the averaging procedure
:
(which comes from the non-linearity of Einstein field equations) and the dynamical evolution of spatial slices of space-time.
, the role of backreaction in possibly leading to an alternative to
dark energy
In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univer ...
is an open question of debate among cosmologists.
The existence of a homogeneity length scale can be considered to be that at which the calculations with and without backreaction give the same results. , the existence of such a scale needs experimental confirmation.
References
External links
Theoretical physics
{{theoretical-physics-stub