General relativity is the modern theory of the gravitational ?eld. It is a deep subject that couples ?uid dynamics to the geometry of spacetime through the Einstein equations. The subject has seen a resurgence of interest recently, partlybecauseofthespectacularsatellitedatathatcontinuestoshednewlight on the nature of the universe. . . Einstein's theory of gravity is still the basic theorywehavetodescribetheexpandinguniverseofgalaxies. ButtheEinstein equations are of great physical, mathematical and intellectual interest in their own right. They are the granddaddy of all modern ?eld equations,...