Equivalence

Think about what happens when an elevator chord snaps and you, together with the elevator, plummet earthwards. Now imagine you're up in a spacecraft, and the craft is accelerating at 9.8 ms^-1, if you drop a ball what happens? It will drop to the surface of the craft exactly as if you'd dropped it on Earth. The obvious explanation for what happens in the rocket is that the floor comes up to meet the ball. Nothing special there, except that in this simple example Einstein saw the key to General Relativity.

It is important to note that this principle applies only to the local scale, on a large scale gravitational fields will vary, and the motion of two bodies a certain distance apart will vary, something which does not happen in an accelerated co-ordinate system.

The Principle's most important result is that it is impossible to unambiguously separate the effects of gravitational pull from those of the accelerated nature of your chosen non-inertial (accelerating) reference frame. Because we therefore cannot completely determine whether or not we are in a gravitational field, the gravitational field is fundamentally different from all other force fields.