Motivation

Time and again I hear people saying that the field equations of GR are "way too difficult", "incomprehensible" and "way over my head". In this thread I wish to show that solving these equations is - although mathematically extremely tedious - fairly straightforward on a conceptual level. Also I have been asked more than once about how to do it by different people, so I would like to show the general procedure in this thread.

Definitions

Recall the definitions of the basic entities used in the equations.

Einstein Field Equations ( without cosmological constant )

(1)

Ricci tensor :

(2)

Christoffel symbols :

(3)

Contracted Christoffel symbols :

(4)

Ansatz

Every solution of the field equations requires an ansatz; in this thread we will look at the simplest possible solution of the equations, which is the vacuum solution of a spherically symmetric gravitational field for a static mass. The solution is called the. The spherical symmetry and the condition that mass and resulting field are static leads to the following simple ansatz :Schwarzschild Metric

(5)

with two as yet unspecified functions A(r) and B(r). Our task will be to find these two functions from the field equations.

Field Equations

In a vacuum ( ) the Einstein Field Equations (1) reduce to

(6)

which is a set of partial differential equations for the unknown functions A(r) and B(r).

Calculating the Christoffel Symbols

The elements of the Christoffel symbols which do not vanish are

Calculating the Ricci Tensor

The non-vanishing elements of the Ricci tensor are thus

Solving the Equations

From the above we obtain the system of equations

We now write

and, doing some algebra, we obtain from this

We also know that the gravitational field vanishes at infinity, i.e for we obtain

and therefore

Now we can insert this into the remaining equations :

One can easily verify that these two differential equations are solved by

with an integration constant a. This constant is determined by the condition that the solution of the field equation must reduce the usual Newton's law at infinity; therefore

Putting all this back into the ansatz (5) gives us the solution of the Einstein field equation we were looking for :

This is called the, and its form is the simplest possible vacuum solution to the original field equations without cosmological constant.Exterior Schwarzschild Metric

Conclusion

As has been shown, solving these equations is conceptually straightforward as one only needs to follow the prescribed steps; however, the actual algebra and analysis is tedious and time consuming, and increasingly so once one abandons some of the presumed symmetries, thereby complicating the ansatz.

The next step up from here would be to perform the same calculation for non-vanishing stress-energy-momentum tensors, i.e. for theof a mass. Finally, one can allow a cosmological constant into the field equations. Each of these steps will rapidly complicate the resulting differential equations, which is why in many cases the solutions are possible only numerically, but not in closed algebraic form.interior

References

Fliessbach, Prof Torsten :AllgemeineRelativitätstheorie, Mannheim/Wien/Zuerich : BI-Wiss.-Verl. 1990

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