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This equation relates to the curvature of space and time, telling that stress–energy is what causes the disturbance of spacetime, thus gravitation. Einstein used Newton's law of universal gravitation in his field equations, and the constant of κ is directly proportional to Newton's gravitational constant G:
The above is for the stress-energy tensor in units of mass density (i.e., mass per volume). Writing Einstein's constant depends on how the stress–energy tensor is defined, so an alternative choice for T with units of energy density (i.e., energy per volume) yields
(see § About the two possible forms below for details).
In the following, the value of Einstein's constant will be calculated. To do so, at the beginning a field equation where the cosmological constant Λ is equal to zero is taken, with a steady state hypothesis. Then we use the Newtonian approximation with hypothesis of a weak field and low velocities with respect to the speed of light.
The Newton law will arise and its corollary Poisson's equation.
In this approximation, Poisson's equation appears as the approached form of the field equation (or the field equation appears as a generalization of Poisson's equation). The identification gives the expression of Einstein's constant related to quantities G and c.
The Einstein field equations in non-empty space
We have to obtain a suitable tensor to describe the geometry of space in the presence of an energy field. Einstein proposed this equation in 1917, written as:
(const) is what will become Einstein's constant. We will take the cosmological constant Λ equal to zero (one of the requirements of the properties of the gravitational equations is that they reduce to the free-space field equations when the density of energy in space Tαγ is zero, therefore that the cosmological constant Λ appearing in this equation is zero) so the field equation becomes:
where Rαγ is the Ricci tensor, gαγ is the metric tensor, R the scalar curvature and κ is Einstein's constant we will calculate in the next section..
This equation can be written in another form, contracting indices:
where T is the scalar Tαα which we shall refer to as the Laue scalar.
Using this result we can write the field equation as:
Classical limit of the gravitational equations
It will be shown that the field equations are a generalization of Poisson's classical field equation. The reduction to the classical limit, besides being a validity check on the field equations, gives as a byproduct the value of the constant κ.
|i and |i|j respectively indicate ∂/ and ∂2/. Thus, |i|i means ∂2/.
Consider a field of matter with low proper density ρ, moving at low velocity v. The stress–energy tensor can be written:
If the terms of order (v/)2
and ρ(v/) are neglected, it becomes:
One assumes the flow to be stationary and therefore expects the metric to be time-independent. We use the coordinates of special relativity ct, x, y, z that we write as x0, x1, x2, x3. The first coordinate is time, and the three others are the space coordinates.
The second term corresponds to the small perturbation (due to the presence of a gravitating body) and is also time-independent:
Thus we write the metric:
Clarifying the length element:
If we neglect terms of order ερ0, the Laue scalar Tμ
And the right side of the field equations is to first order in all the small quantities ρ0, v/ and εγμν is written:
Neglecting second-order terms in εγμν gives the following approximate form for the contracted Riemann tensor:
Thus the approximate field equations may be expressed as:
First consider the case μ = ν = 0. As the metric is time-independent, the first term of the equation above is zero. What remains is:
The Christoffel symbol of the first kind is defined by:
Since the Lorentz metric is constant in space and time, this simplifies to:
Moreover, γμν is time-independent, so [00,0] is zero. Neglecting second-order terms in the perturbation term εγμν, we get:
which is zero for β = 0 (which then corresponds to the derivative with respect to time). Substituting into (1) we obtain the following approximate field equation for :
or, by virtue of time independence:
This notation is just a writing convention. The equation can be written:
which can be identified with Poisson's equation if we write:
Therefore, it is established that the classical theory (Poisson's equation) is the limiting case (weak field, low velocities with respect to the speed of light) of a relativistic theory where the metric is time-independent.
To be complete, gravity has to be demonstrated as a metric phenomenon. In the following, without detailing all calculation, the simplistic description of the complete calculation is given. Again, at first start from a perturbed Lorentz metric:
Suppose the velocity v to be low with respect to the speed of light c, with a small parameter β = v/. We have:
We can write:
Limiting to the first degree in β and ε gives:
The approximate form of the Christoffel symbol is:
Introducing this result into the geodesic equation (2) gives:
This is a vector equation. Since the metric is time-independent, only space variables are concerned. Therefore, the second member of the equation is a gradient.
Coding the position-vector by the letter X and the gradient by the vector ∇, one can write:
This is no more than Newton's law of universal gravitation in classical theory, deriving from the gravitational potential φ if one makes the identification:
Conversely, if we set a gravitational potential φ, the movement of a particle will follow a spacetime geodesic if the first term of the metric tensor is like:
That step is important. Newton's law appears as a particular aspect of the general relativity with the double approximation:
- weak gravitational field
- low velocity with respect to the speed of light
With the calculation above, we have made the following statements:
- A metric g, solution of the Einstein field equation (with a cosmological constant Λ equal to zero).
- This metric would be a weak perturbation in relation to a Lorentz metric η (relativistic and flat space).
- The perturbation term would not depend on time. Since the Lorentz metric does not depend on time either, that metric g is also time-independent.
- The expansion into a series gives a linearization of the Einstein field equations.
- This linearized form is found to identify to Poisson's equation because a field is a curvature, linking the perturbation term to the metric and to the gravitational potential thanks to the relation:
And this rewards the value of the constant κ, called "Einstein's constant" (which is not the cosmological constant Λ or the speed of light c):
One can then write the Einstein field equation:
Dimensionally, each term on the left hand side of the equation has dimension 1/length2. If the stress-energy tensor T on the right has units of energy/volume or mass/[length*time2], then the Einstein's constant must have units time2/[length*mass] to mediate between the two sides of the Einstein field equation. But see below for other unit choices in the tensor T.
About the possible forms for stress-energy tensor of each dimension
We have seen, neglecting the terms of order (v/)2
and ρ(v/), that the Laue scalar could be written:
which gives the corresponding Einstein's constant for a tensor with dimension of mass per volume:
But another valid choice for writing the form of the stress–energy tensor, is in units of energy per volume:
Neglecting the same term orders, the corresponding Laue scalar is:
which contains an additional term c2 to convert units of mass density to units of energy density, so the corresponding Einstein's constant in the field equations is then:
This is just a question of choice, since for any chosen value unit, the Einstein field equations are the same.
The Einstein field equation has zero divergence. The zero divergence of the stress–energy tensor is the geometrical expression of the conservation law. So it appears constants in the Einstein equation cannot vary, otherwise this postulate would be violated.
However, since Einstein's constant had been evaluated by a calculation based on a time-independent metric, this by no mean requires that G and c must be unvarying constants themselves, the only postulate derived from conservation of energy is that the ratio G/ must be constant. Depending on the choice of natural units, this ratio can be set to a defined constant value; subject to measurement is the dimensionless gravitational coupling constant, variation in which would not necessarily amount to violation of the conservation of four-momentum.
- Heller, Michael (1992). Theoretical Foundations of Cosmology: Introduction to the Global Structure of Space-time. World Scientific. p. 63. ISBN 981-02-0756-5.