THE EQUIVALENCE PRINCIPLE

Imagine that our space ship, instead of accelerating in deep space, is set down on the surface of a planet. It might be tail-downward, in which case the force of gravity pulls on the objects inside as if the ship were accelerating in a forward direction. It might be nose-downward so that gravity pulls on the objects inside as if the ship were decelerating. It could be oriented some other way, and the force of gravity would pull on the objects inside as if the ship were changing course in a lateral direction. Acceleration can consist of a change in speed, a change in direction, or both.
If the windows are kept covered, the radar is shut off, and the navigational aids are placed on standby, how can passengers in such a vessel tell whether the force is caused by gravity or by acceleration? Einstein’s answer: They cannot tell the difference.

From this notion came the equivalence principle. The so-called acceleration force is exactly the same as gravitation. Einstein reasoned that the two forces act in an identical way on everything, from people’s perceptions to atoms and from light rays to the fabric of space-time. This is the basis of the theory of general relativity.