General relativity  
Einstein field equations  
Introduction Mathematical formulation Resources


The Einstein field equations (EFE) or Einstein's equations are a set of ten equations in Einstein's theory of general relativity which describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy.^{[1]} First published by Albert Einstein in 1915^{[2]} as a tensor equation, the EFE equate spacetime curvature (expressed by the Einstein tensor) with the energy and momentum within that spacetime (expressed by the stressenergy tensor).
Similar to the way that electromagnetic fields are determined using charges and currents via Maxwell's equations, the EFE are used to determine the spacetime geometry resulting from the presence of massenergy and linear momentum, that is, they determine the metric tensor of spacetime for a given arrangement of stressenergy in the spacetime. The relationship between the metric tensor and the Einstein tensor allows the EFE to be written as a set of nonlinear partial differential equations when used in this way. The solutions of the EFE are the components of the metric tensor. The inertial trajectories of particles and radiation (geodesics) in the resulting geometry are then calculated using the geodesic equation.
As well as obeying local energymomentum conservation, the EFE reduce to Newton's law of gravitation where the gravitational field is weak.
Solution techniques for the EFE include simplifying assumptions such as symmetry. Special classes of exact solutions are most often studied as they model many gravitational phenomena, such as rotating black holes and the expanding universe. Further simplification is achieved in approximating the actual spacetime as flat spacetime with a small deviation, leading to the linearised EFE. These equations are used to study phenomena such as gravitational waves.
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The Einstein field equations (EFE) may be written in the form:^{[1]}
where is the Ricci curvature tensor, the scalar curvature, the metric tensor, is the cosmological constant, is Newton's gravitational constant, the speed of light, and the stressenergy tensor.
The EFE is a tensor equation relating a set of symmetric 4 x 4 tensors. It is written here using the abstract index notation. Each tensor has 10 independent components. Given the freedom of choice of the four spacetime coordinates, the independent equations reduce to 6 in number.
Although the Einstein field equations were initially formulated in the context of a fourdimensional theory, some theorists have explored their consequences in n dimensions. The equations in contexts outside of general relativity are still referred to as the Einstein field equations. The vacuum field equations define Einstein manifolds.
Despite the simple appearance of the equations they are, in fact, quite complicated. Given a specified distribution of matter and energy in the form of a stressenergy tensor, the EFE are understood to be equations for the metric tensor g_{μν}, as both the Ricci tensor and scalar curvature depend on the metric in a complicated nonlinear manner. In fact, when fully written out, the EFE are a system of 10 coupled, nonlinear, hyperbolicelliptic partial differential equations.
One can write the EFE in a more compact form by defining the Einstein tensor
which is a symmetric secondrank tensor that is a function of the metric. The EFE can then be written as
where the cosmological term has been absorbed into the stressenergy tensor as dark energy.
Using geometrized units where G = c = 1, this can be rewritten as
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 interpreted as a 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.
The above form of the EFE is the standard established by Misner, Thorne, and Wheeler. The authors analyzed all conventions that exist and classified according to the following three signs (S1, S2, S3):
The third sign above is related to the choice of convention for the Ricci tensor:
With these definitions Misner, Thorne, and Wheeler classify themselves as , whereas Weinberg (1972) is , Peebles (1980) and Efstathiou (1990) are while Peacock (1994), Rindler (1977), Atwater (1974), Collins Martin & Squires (1989) are .
Authors including Einstein have used a different sign in their definition for the Ricci tensor which results in the sign of the constant on the right side being negative
The sign of the (very small) cosmological term would change in both these versions, if the + metric sign convention is used rather than the MTW −+++ metric sign convention adopted here.
Einstein's field equations can be rewritten in the following equivalent "tracereversed" form
which may be more convenient in some cases (for example, when one is interested in weakfield limit and can replace g_{μν} in the expression on the right with the Minkowski tensor without significant loss of accuracy).
Einstein modified his original field equations to include a cosmological term proportional to the metric
The constant Λ is the cosmological constant. Since Λ is constant, the energy conservation law is unaffected.
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 Λ was abandoned, with Einstein calling it the "biggest blunder [he] ever made".^{[3]} For many years the cosmological constant was almost universally considered to be 0.
Despite Einstein's misguided motivation for introducing the cosmological constant term, there is nothing inconsistent with the presence of such a term in the equations. Indeed, recent improved astronomical techniques have found that a positive value of Λ is needed to explain some observations.^{[4]}^{[5]}
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:
The vacuum energy is constant and given by
The existence of a cosmological constant is thus equivalent to the existence of a nonzero vacuum energy. The terms are now used interchangeably in general relativity.
General relativity is consistent with the local conservation of energy and momentum expressed as
Contracting the differential Bianchi identity
with g^{ac} gives, using the fact that the metric tensor is covariantly constant, i.e. g^{ab}_{;c} = 0,
The antisymmetry of the Riemann tensor allows the second term in the above expression to be rewritten:
which is equivalent to
using the defintion of the Ricci tensor.
Next, contract again with the metric
to get
The definitions of the Riemann tensor and Ricci scalar then show that
which can be rewritten as
A final contraction with g^{ed} gives
which by the symmetry of the bracketed term and the defintion of the Einstein tensor, gives, after relabelling the indices,
Using the EFE, this immediately gives,
which expresses the local conservation of stressenergy. This conservation law is a physical requirement. With his field equations Einstein ensured that general relativity is consistent with this conservation condition.
The nonlinearity of the EFE distinguishes general relativity from many other fundamental physical theories. For example, Maxwell's equations of electromagnetism are linear in the electric and magnetic fields, and charge and current distributions (i.e. the sum of two solutions is also a solution); another example is Schrödinger's equation of quantum mechanics which is linear in the wavefunction.
The EFE reduce to Newton's law of gravity by using both the weakfield approximation and the slowmotion approximation. In fact, the constant appearing in the EFE is determined by making these two approximations.
Newtonian gravitation can be written as the theory of a scalar field, , which is the gravitational potential in joules per kilogram
where is the mass density. The orbit of a freefalling particle satisfies
In tensor notation, these become
In general relativity, these equations are replaced by the Einstein field equations in the tracereversed form
for some constant, K, and the geodesic equation
To see how the latter reduce to the former, we assume that the test particle's velocity is approximately zero
and thus
and that the metric and its derivatives are approximately static and that the squares of deviations from the Minkowski metric are negligible. Applying these simplifying assumptions to the spatial components of the geodesic equation gives
where two factors of have been divided out. This will reduce to its Newtonian counterpart, provided
Our assumptions force α=i and the time (0) derivatives to be zero. So this simplifies to
which is satisfied by letting
Turning to the Einstein equations, we only need the timetime component
the low speed and static field assumptions imply that
So
and thus
From the definition of the Ricci tensor
Our simplifying assumptions make the squares of Γ disappear together with the time derivatives
Combining the above equations together
which reduces to the Newtonian field equation provided
which will occur if
If the energymomentum tensor T_{μν} is zero in the region under consideration, then the field equations are also referred to as the vacuum field equations. By setting T_{μν} = 0 in the full field equations, the vacuum equations can be written as
Taking the trace of this (contracting with g^{μν}) and using the fact that g^{μν}g_{μν} = 4, we get
and thus
Substituting back, we get an equivalent form of the vacuum field equations
In the case of nonzero cosmological constant, the equations are
which gives
yielding the equivalent form
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.
Manifolds with a vanishing Ricci tensor, R_{μν} = 0, are referred to as Ricciflat manifolds and manifolds with a Ricci tensor proportional to the metric as Einstein manifolds.
If the energymomentum tensor T_{μν} is that of an electromagnetic field in free space, i.e. if the electromagnetic stressenergy tensor
is used, then the Einstein field equations are called the Einstein–Maxwell equations (with cosmological constant Λ, taken to be zero in conventional relativity theory):
Additionally, the covariant Maxwell Equations are also applicable in free space:
where the semicolon represents a covariant derivative, and the brackets denote antisymmetrization. The first equation asserts that the 4divergence of the twoform F is zero, and the second that its exterior derivative is zero. From the latter, it follows by the Poincaré lemma that in a coordinate chart it is possible to introduce an electromagnetic field potential A_{α} such that
in which the comma denotes a partial derivative. This is often taken as equivalent to the covariant Maxwell equation from which it is derived.^{[6]} However, there are global solutions of the equation which may lack a globally defined potential.^{[7]}
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 postNewtonian approximations. Even so, there are numerous cases where the field equations have been solved completely, and those are called exact solutions.^{[8]}
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.
Main articles: Linearised Einstein field equations, Linearized gravity
The nonlinearity of the EFE makes finding exact solutions difficult. One way of solving the field equations is to make an approximation, namely, that far from the source(s) of gravitating matter, the gravitational field is very weak and the spacetime approximates that of Minkowski space. The metric is then written as the sum of the Minkowski metric and a term representing the deviation of the true metric from the Minkowski metric. This linearisation procedure can be used to discuss the phenomena of gravitational radiation.
See General relativity resources.
