In general relativity, post-Newtonian expansions (PN expansions) are used for finding an approximate solution of Einstein field equations for the metric tensor. The approximations are expanded in small parameters that express orders of deviations from Newton's law of universal gravitation. This allows approximations to Einstein's equations to be made in the case of weak fields. Higher-order terms can be added to increase accuracy, but for strong fields sometimes it is preferable to solve the complete equations numerically. This method is a common mark of effective field theories. In the limit, when the small parameters are equal to 0, the post-Newtonian expansion reduces to Newton's law of gravity.
The post-Newtonian approximations are expansions in a small parameter, which is the ratio of the velocity of the matter that creates the gravitational field, to the speed of light, which in this case is more precisely called the speed of gravity.[1] In the limit, when the fundamental speed of gravity becomes infinite, the post-Newtonian expansion reduces to Newton's law of gravity. A systematic study of post-Newtonian expansions within hydrodynamic approximations was developed by Subrahmanyan Chandrasekhar and his colleagues in the 1960s.[2] [3] [4] [5] [6]
Another approach is to expand the equations of general relativity in a power series in the deviation of the metric from its value in the absence of gravity.
h\alpha=g\alpha-η\alpha.
h\alphaη\beta
For example, if one goes one step beyond linearized gravity to get the expansion to the second order in h:
g\mu ≈ η\mu-η\muh\alphaη\beta+η\muh\alphaη\betah\gammaη\delta.
\sqrt{-g} ≈ 1+\tfrac12h\alphaη\beta+\tfrac18h\alphaη\betah\gammaη\delta-\tfrac14h\alphaη\betah\gammaη\delta.
0PN | 1PN | 2PN | 3PN | 4PN | 5PN | 6PN | 7PN | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1PM | (1 | + | v2 | + | v4 | + | v6 | + | v8 | + | v10 | + | v12 | + | v14 | + | ...) | G1 | |
2PM | (1 | + | v2 | + | v4 | + | v6 | + | v8 | + | v10 | + | v12 | + | ...) | G2 | |||
3PM | (1 | + | v2 | + | v4 | + | v6 | + | v8 | + | v10 | + | ...) | G3 | |||||
4PM | (1 | + | v2 | + | v4 | + | v6 | + | v8 | + | ...) | G4 | |||||||
5PM | (1 | + | v2 | + | v4 | + | v6 | + | ...) | G5 | |||||||||
6PM | (1 | + | v2 | + | v4 | + | ...) | G6 | |||||||||||
Comparison table of powers used for PN and PM approximations in the case of two non-rotating bodies.0PN corresponds to the case of Newton's theory of gravitation. 0PM (not shown) corresponds to the Minkowski flat space.[7] |
The first use of a PN expansion (to first order) was made by Albert Einstein in calculating the perihelion precession of Mercury's orbit. Today, Einstein's calculation is recognized as a common example of applications of PN expansions, solving the general relativistic two-body problem, which includes the emission of gravitational waves.
See main article: Newtonian gauge. In general, the perturbed metric can be written as[8]
ds2=a2(\tau)\left[(1+2A)d\tau2-2Bidxid\tau-\left(\deltaij+hij\right)dxidxj\right]
A
Bi
hij
hij
hij=2C\deltaij+\partiali\partialjE-
1 | |
3 |
\deltaij\Box2E+\partiali\hat{E}j+\partialj\hat{E}i+2\tilde{E}ij
\Box
E
\hat{E}i
\tilde{E}ij
\Psi\equivA+H(B-E'),+(B+E')', \Phi\equiv-C-H(B-E')+
1 | |
3 |
\BoxE
H
\tau
Taking
B=E=0
\Phi\equiv-C
\Psi\equivA
ds2=a2(\tau)\left[(1+2\Psi)d\tau2-(1-2\Phi)\deltaijdxidxj\right]
Note that in the absence of anisotropic stress,
\Phi=\Psi
A useful non-linear extension of this is provided by the non-relativistic gravitational fields.