ABSTRACT
This chapter describes the principal ideas of general relativity and their geometric nature. Generalizing the Law of Inertia, which says that free particles follow geodesics in the flat spacetime of special relativity, Einstein hypothesized that free particles must follow geodesics in the curved spacetime of a gravitational field. In 1916 The Foundation of the General Theory of Relativity Einstein extended his special theory to arbitrary frames of reference, and at the same time proposed a completely new theory of gravitation. An important consequence of the Principle of Equivalence is that light rays are "bent" by a gravitational field. All bodies fall with the same acceleration because, according to Newton's Law of Universal Gravitation, the force exerted by gravity on a given object is likewise proportional to the object's mass. The "mass" appearing in the Law of Gravitation is gravitational mass, a measure of an object's response to gravitational attraction.
