a central idea in general relativity is the "principle of equivalence." an example is that two people, one in an elevator sitting on the surface of the earth, and the other in an elevator in outer space but accelerating upwards at the speed of 9.8 m/s2 (every second the object picks up speed of 9.8 m/s), will each observe the same behavior of an object they drop from their hands. the object will accelerate to the floor at 9.8 m/s2 in either case, making it impossible for either to distinguish whether or not they are at rest in a gravitational field or accelerating upward creating the same effect as gravity would on earth. other versions of this type of "thought experiment" were used to show that light would curve in an accelerating frame of reference (perspective). there are several forms of the equivalence principle. these include: newton's equivalence principle, the weak equivalence principle, the gravitational weak equivalence principle, einstein's equivalence principle and the strong equivalence principle.