Do not measure the same lengths of objects. Do not measure the same masses for objects. Universe looks the same to all observers. United by light into Spacetime . Only spacetime has an absolute reality independent of the observer.
All observers measure the same speed of light.
But, objects are accelerated by gravity. Einstein took 8 years to generalize relativity. This effort was to lead to the formulation of a completely new theory of gravity. 1998 February 20 Copyright 1998 Richard W. Pogge, All Rights Reserved.