Dear PF Forum, In Einstein elevator, Einstein states that we can't tell between gravity and acceleration. I have incoming questions concerning gravity, that's why I create a different thread. Can we use this formula for gravity? Case A: A rest observer watches an accelerating object and that object keeps sending signal containing its clock. The rest observer compares its clock to the object's clock with the above formula, is this true? Case B: What if there is a rest observer, "outside" the Earth gravitational field. And supposed the Earth is floating in space, no near planet no sun nearby by millions of years. The observer and the earth distance is always the same (I know, the observer has to orbit the earth no matter how really slow it moves. The gravitational fields extend infinitely before it is disrupted by other celestial object, just supposed the speed of the object revolves the earth is very small). Does the observer watches the clock on the earth by that formula above as the observer watches an accelerated object 9.8m/sec2? Thanks.