To see why equation (2) is equivalent to the usual formulation of Einstein's equation, we need a bit of tensor calculus. In particular, we need to understand the Riemann curvature tensor and the geodesic deviation equation. For a detailed explanation of these, the reader must turn to some of the texts in the bibliography. Here we briefly sketch the main ideas.

When spacetime is curved, the result of parallel transport depends on the path taken. To quantify this notion, pick two vectors and at a point in spacetime. In the limit where , we can approximately speak of a 'parallelogram' with sides and . Take another vector at and parallel transport it first along and then along to the opposite corner of this parallelogram. The result is some vector . Alternatively, parallel transport first along and then along . The result is a slightly different vector, :

The limit

where as usual we sum over repeated indices. The quantity is called the 'Riemann curvature tensor'.

We can use this tensor to compute the relative acceleration of nearby particles in free fall if they are initially at rest relative to one another. Consider two freely falling particles at nearby points and . Let be the velocity of the particle at , and let be the vector from to . Since the two particles start out at rest relative to one other, the velocity of the particle at is obtained by parallel transporting along .

Now let us wait a short while. Both particles trace out geodesics as time passes, and at time they will be at new points, say and . The point is displaced from by an amount , so we get a little parallelogram, exactly as in the definition of the Riemann curvature:

Next let us compute the new relative velocity of the two particles. To compare vectors we must carry one to another using parallel transport. Let be the vector we get by taking the velocity vector of the particle at and parallel transporting it to along the top edge of our parallelogram. Let be the velocity of the particle at . The difference is the new relative velocity. Here is a picture of the whole situation:

The vector is depicted as shorter than for purely artistic reasons.

It follows that over this passage of time, the average
relative acceleration of the two particles is
. By equation (5),

so

This is called the 'geodesic deviation equation'. From the definition of the Riemann curvature it is easy to see that , so we can also write this equation as

Using this equation we can work out the second time derivative
of the volume of a small ball of test particles that start
out at rest relative to each other. For this we must let
range over an orthonormal basis of tangent vectors, and sum the
'outwards' component of acceleration for each one of these.
By equation (6)
this gives

In terms of the 'Ricci tensor'

we may write this as:

In local inertial coordinates where the ball starts out at rest we have , so

In short, the Ricci tensor says how our ball of freely falling test particles starts changing in volume. The Ricci tensor only captures some of the information in the Riemann curvature tensor. The rest is captured by something called the 'Weyl tensor', which says how any such ball starts changing in shape. The Weyl tensor describes tidal forces, gravitational waves and the like.

Now, Einstein's equation in its usual form says

Here the right side is the stress-energy tensor, while the left side, the 'Einstein tensor', is just an abbreviation for a quantity constructed from the Ricci tensor:

Thus Einstein's equation really says

This implies

but , so

Plugging this in equation (8), we get

This is equivalent version of Einstein's equation, but with the roles of and switched! The good thing about this version is that it gives a formula for the Ricci tensor, which has a simple geometrical meaning.

Equation (9) will be true if any one component holds in
all local inertial coordinate systems. This is a bit like the
observation that all of Maxwell's equations are contained in Gauss's law
and
. Of course, this is only true if we
know how the fields transform under change of coordinates. Here we
assume that the transformation laws are known. Given this, Einstein's
equation is equivalent to the fact that

so and

Equation (10) thus says that

By equation (7), this is equivalent to

as desired.

* © 2006 John Baez and Emory Bunn*