Imagine a hole drilled through the Earth from which all air has been removed thus creating a vacuum. Let a cluster of test particles in the shape of a sphere be dropped into this hole. The volume of the balls should start to decrease. However, in his article "The Meaning of Einstein's Equation" page 18, John Baez and Emory Bunn indicate that the rate of decrease of the volume, per volume, should equal the t-t component of the Ricci tensor. But how can this be if the Ricci tensor is zero in a vacuum? Any ideas would be appreciated.