General Relativity

Abstract

While gravitational and electric fields both have physical “sources” —masses on the one hand and electric charges on the other—there is a crucial distinction between them. In an electric field, objects accelerate differently, depending on their charge; in a gravitational field, all objects experience identical acceleration. So it is possible to “fall” along with those objects—in an orbiting spacecraft, for example—and if we do, the gravitational accelerations will seem to disappear. Conversely, in a spaceship far from other matter, where there is no perceptible gravitational field, we can create one in the spaceship simply by making it accelerate. In other words, with a coordinate change we can make a gravitational field appear or disappear—at least in a small region of space and time.