Geometry is at the heart of Einstein's picture of gravity. The best place to see how gravity as curvature works is in the Solar System, where the predictions must be very close to the description given by Newton. In this familiar arena, we can compare the old and new ways of looking at gravity. In this arena, too, general relativity meets and passes its first two crucial tests: explaining the anomalous advance in the perihelion of the planet Mercury (which we puzzled over in Chapter 5), and predicting that light should be deflected as it passes the Sun by twice the amount that would be calculated from Newtonian gravity (see Chapter 4).

We saw in the last chapter that the equivalence principle tells us that it is not possible to represent gravity just by the curvature of space; the curvature of spacetime must include the curvature of time as well.