Einstein's field equation
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In physics, the Einstein field equation or Einstein equation is a equation in Einstein's theory of general relativity. It is a dynamical equation which describes how matter and energy changes the geometry of spacetime, this curved geometry being interpreted as the gravitational field of the matter source. The motion of objects (with a mass much smaller than the matter source) in this gravitational field is described very accurately by the geodesic equation.
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Mathematical form of Einstein's field equation
The Einstein field equation (EFE) is usually written in the form
 <math>R_{ab}  {1 \over 2}R\,g_{ab} = {8 \pi G \over c^4} T_{ab}.<math>
Here <math>R_{ab}<math> is the Ricci tensor, <math>R<math> is the Ricci scalar, <math>g_{ab}<math> is the metric tensor, <math>T_{ab}<math> is the stressenergy tensor, and the constant is given in terms of <math>\pi<math> (pi), <math>c<math> (the speed of light) and <math>G<math> (the gravitational constant). The EFE equation is a tensor equation relating a set of symmetric 4 x 4 tensors. It is written here in terms of components. Each tensor has 10 independent components. Given the freedom of choice of the four spacetime coordinates, the independent equations reduce to 6 in number.
The EFE is understood to be an equation for the metric tensor <math>g_{ab}<math> (given a specified distribution of matter and energy in the form of a stressenergy tensor). Despite the simple appearence of the equation it is, in fact, quite complicated. This is because both the Ricci tensor and Ricci scalar depend of the metric in a complicated nonlinear manner.
One can write the EFE in a more compact form by defining the Einstein tensor
 <math>G_{ab} = R_{ab}  {1 \over 2}R g_{ab}<math>
which is a symmetric secondrank tensor that is a function of the metric. Working in geometrized units where G = c = 1, the EFE can then be written as
 <math>G_{ab} = 8\pi T_{ab}\,<math>
The expression on the left represents the curvature of spacetime as determined by the metric and the expression on the right represents the matter/energy content of spacetime. The EFE can then be interpretted as set of equations dictating how the curvature of spacetime is related to the matter/energy content of the universe.
These equations, together with the geodesic equation, form the core of the mathematical formulation of general relativity.
Properties of Einstein's equation
Conservation of energy and momentum
An important consequence of the EFE is the local conservation of energy and momentum; this result arises by using the differential Bianchi identity to obtain
 <math>\nabla_b G^{ab}=0<math>
which, by using the EFE, results in
 <math>\nabla_b T^{ab}=0<math>
which expresses the local conservation law referred to above.
Nonlinearity of the field equations
The EFE are a set of 10 coupled elliptichyperbolic nonlinear partial differential equations for the metric components. This nonlinear feature of the dynamical equations distinguishes general relativity from other physical theories. For example, Maxwell's equations of electromagnetism are linear in the electric and magnetic fields (i.e. the sum of two solutions is also a solution); another example is Schrodinger's equation of quantum mechanics where the equation is linear in the wavefunction.
The cosmological constant
One can modify Einstein's field equations by introducing a term proportion to the metric:
 <math>R_{ab}  {1 \over 2}R g_{ab} + \Lambda g_{ab} = {8 \pi} T_{ab}<math>
The constant <math>\Lambda<math> is called the cosmological constant.
The cosmological constant term was originally introduced by Einstein to allow for a static universe (i.e., one that is not expanding or contracting). This effort was unsuccessful for two reasons: the static universe described by this theory was unstable, and observations of distant galaxies by Hubble a decade later confirmed that our universe is in fact not static but expanding. So <math>\Lambda<math> was abandoned, with Einstein calling it the "biggest blunder he ever made".
Despite Einstein's misguided motivation for introducting the cosmological constant term, there is nothing wrong (i.e. inconsistent) with the presence of such a term in the equations. Indeed, quite recently, improved astronomical techniques have found that a nonzero value of <math>\Lambda<math> is needed to explain some observations.
Einstein thought of the cosmological constant as an independent parameter, but its term in the field equation can also be moved algebraically to the other side, written as part of the stressenergy tensor:
 <math>T_{ab}^{\mathrm{(vac)}} = \frac{\Lambda}{8\pi}g_{ab}.<math>
The constant
 <math>\rho_{\mathrm{vac}} = \frac{\Lambda}{8\pi}<math>
is called the vacuum energy. The existence of a cosmological constant is equivalent to the existence of a nonzero vacuum energy. The terms are now used interchangeably in general relativity.
Solutions of the field equations
The solutions of the Einstein field equations are metrics of spacetime. The solutions are hence often called 'metrics'. These metrics describe the structure of the spacetime including the inertial motion of objects in the spacetime. As the field equations are nonlinear, they cannot always be completely solved (i.e. without making approximations). For example, there is no known complete solution for a spacetime with two massive bodies in it (which is a theoretical model of a binary star system, for example). However, approximations are usually made in these cases. These are commonly referred to as post Newtonian approximations. Even so, there are numerous cases where the field equations have been solved completely, and those are called exact solutions.
The study of exact solutions of Einstein's field equations is one of the activities of cosmology. It leads to the prediction of black holes and to different models of evolution of the universe.
Tetrad formalism
There is a useful way to study exact solutions by using the tetrad formulation. This involves using four reference vector fields, called a vierbein or tetrad. The four vector fields are denoted by e_{a}, a = 1, 2, 3, 4 and satisfy g(e_{a}, e_{b}) = η_{ab} where η_{ab} is the flat Minkowski metric. One thing to note is that we can perform an independent proper, orthochronous Lorentz transformation at each point (subject to smoothness, of course) and still get a valid tetrad. So, the tetrad formulation of GR is a gauge theory, but with a noncompact gauge group SO(3,1). It is also invariant under diffeomorphisms.
See vierbein and Palatini action for more details.
Vacuum field equations
If the energymomentum tensor <math>T_{ab}<math> is zero in the region under consideration, then the field equations are also referred to as the vacuum field equations, which can be written as:
 <math>R_{ab} = 0\,<math>
The solutions to the vacuum field equations are called vacuum solutions. Flat Minkowski space is the simplest example of a vacuum solution. Nontrivial examples include the Schwarzschild solution and the Kerr solution.
The above vacuum equation assumes that the cosmological constant is zero. If it is taken to be nonzero then the vacuum equation becomes:
 <math>R_{ab} = \Lambda g_{ab}\,<math>
Mathematicians usually refer to manifolds with a vanishing Ricci tensor as Ricciflat manifolds and manifolds with a Ricci tensor is proportional to the metric as Einstein manifolds.
The correspondence principle
Einstein's equation reduces to Newton's law of gravity by using both the weakfield approximation and the slowmotion approximation. In fact, the constant appearing in the EFE's is determined by making these two approximations.
See also
References
 Weinberg, S. Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity (1972) ISBN 0471925675
 Stephani, H., Kramer, D., MacCallum, M., Hoenselaers C. and Herlt, E. Exact Solutions of Einstein's Field Equations (2nd edn.) (2003) CUP ISBN 0521461367de:Einsteinsche Feldgleichungen
es:Ecuación del campo de Einstein ja:アインシュタイン方程式 zhcn:爱因斯坦场方程