The main ideas in the general theory of relativity are that space and time are knit together in a four-dimensional fabric called spacetime, and that spacetime can be bent by mass. Massive objects cause spacetime to “dimple” toward the object (think of the way that a bowling ball set on a trampoline causes the trampoline to dimple).

In the four-dimensional spacetime of the universe, if a less massive object approaches a more massive object (for example, a planet approaches a star), the less massive object will follow the lines of curved space and be drawn toward the more massive one. Thinking of the bowling ball on the trampoline, if a marble rolls past the bowling ball and into the dimpled part of the trampoline, then the marble will fall in toward the bowling ball. According to the general theory of relativity, this is how gravity works. Newton’s Law of Universal Gravitation, according to Einstein, is almost completely correct in describing how gravity works, but it was not quite complete in explaining why it works.