Equivalence Principle:

Assume that our space ship, rather than accelerating in deep space, is put down on the surface of a planet. It may be tail-downward, in that situation the force of gravity pulls on the objects within as when the ship were accelerating in a forward direction. It may be nose-downward and hence gravity pulls on the objects within as if the ship were decelerating. It could be oriented in some other manner, and the force of gravity would pull on the objects inside as if the ship were varying course in a sideways direction. The acceleration can consist of a change in direction, and change in speed, or both.

When the windows are kept covered, then the radar is shut off, and the navigational assists are positioned on standby, how can passengers in like a vessel tell whether the force is caused by gravity or by the acceleration? Einstein's answer: They can't say the difference.

From this belief came the correspondence principle. The so-called acceleration force is precisely similar as gravitation. Einstein stated that the two forces act in an identical manner on the whole thing, from people's observations to atoms and from light rays to the fabric of space-time. This is the base of the hypothesis of general relativity.