The Fabric of the Cosmos: Space, Time, and the Texture of Reality

by Brian Greene

fit well a maxim often attributed to Einstein: “Make everything as simple as possible, but no simpler.”

Special relativity declares a similar law for all motion: the combined speed of any object’s motion through space and its motion through time is always precisely equal to the speed of light. At

complementary. When the parked car you were looking at speeds away, what really happens is that some of its light-speed motion is diverted from motion through time into motion through space, keeping their combined total unchanged. Such diversion unassailably means that the car’s motion through time slows down.

Absolute space does not exist. Absolute time does not exist. But according to special relativity, absolute spacetime does exist.

All this led Einstein to conclude that the force one feels from gravity and the force one feels from acceleration are the same. They are equivalent. Einstein called this the principle of equivalence.

Since gravity and acceleration are equivalent, if you feel gravity’s influence, you must be accelerating.

Clearly, this is a radically different way of thinking about motion. But it’s anchored in the simple recognition that you feel gravity’s influence only when you resist it. By contrast, when you fully give in to gravity you don’t feel it.

Through special relativity, Einstein showed that every observer cuts up spacetime into parallel slices that he or she considers to be all of space at successive instants of time, with the unexpected twist that observers moving relative to one another at constant velocity will cut through spacetime at different angles.