Einstein's Elevator

CORE DEFINITION

A thought experiment Albert Einstein used on the path to general relativity. Inside a sealed elevator, an observer cannot tell whether the box rests on Earth in a gravitational field or accelerates upward at one g in gravity-free space, because every local measurement gives the same result. This equivalence of gravity and acceleration became the equivalence principle, the foundation on which general relativity treats gravity as the geometry of spacetime rather than a force. The equivalence holds only locally; over a large enough region, tidal effects reveal the difference.