Is gravity a force, or the shape of space?

That time, that place

Special relativity handled only motion at constant velocity. Acceleration, and gravity, were left out. Einstein spent ten years on them.

Why this question

It began with a thought experiment.

A man falling from a roof does not feel his own weight. Inside a freely falling elevator, released objects float. Someone inside cannot tell whether he is falling or drifting in gravity-free space.

The reverse holds too. Someone in a rocket accelerating through space feels as though he is standing on a planet.

If gravity and acceleration cannot be told apart, they are the same thing.

What was found

Then gravity may not be a force at all.

A massive body curves the spacetime around it, and objects simply follow the straightest available path through that curvature. The Earth is not being pulled by the Sun; it is rolling along the valley the Sun has made.

The theory made testable predictions: starlight passing near the Sun must bend, and the small anomaly in Mercury's orbit that Newton could not account for falls out on its own.

Terms on this card

General relativityWormholeTime dilationThe equivalence principleGravitational lensingThe self-consistency principle