FALLING AND FLOATING Imagine waking up in a box, completely weightless. Are you falling toward Earth or floating in outer space?

GRAVITY AND ACCELERATION Now imagine waking up on the floor of the box. Are you resting on Earth or accelerating in a spaceship?

LIGHT IS BENT BY SPACE AND TIME In your accelerating spaceship, a beam of light would bend slightly. The sun’s mass also bends starlight.

Einstein realized that falling freely in gravity feels the same as floating in space with no gravity. Gravity seems to be absent in free fall, at least in the small area around you.

Since floating in space and falling in gravity feel the same, Einstein reasoned there must be no “force” of gravity at work in either state.

Einstein also realized that resting in gravity feels the same as accelerating at 1 g. Since acceleration and gravity feel the same, he argued that they must have the same cause.

Einstein proposed that the massive objects like the Earth curve space and time, and that what we call gravity arises from this curvature. Acceleration feels the same as resting in a region of curved space and time.

A beam of light across the ship seems to bend slightly, because the ship moves forward as the light travels. Einstein predicted that light should also be bent by gravity, since acceleration and gravity are equivalent.