© M. Poessel/Einstein Online/Max Planck Institute for Gravitational Physics.
The apparent equivalence between gravity and acceleration was a key insight for Albert Einstein when he was developing the theory of general relativity. A person inside a rocket ship far from any stars or planets will float freely when the engines are turned off. When the engines are turned on, the rocket accelerates, and the person inside feels a force pushing up against his or her feet. Einstein realized that the person cannot tell the difference between a uniformly accelerating rocket and a rocket sitting on the ground in a uniform gravitational field. (Unit: 3)