SPECIAL RELATIVITY No possible experiment can distinguish between two observers in constant relative motion. The first relativity postulate can be taken as stating that no possible experiment can distinguish between two observers in constant relative motion. Einstein's third article in 1905 on the special theory of relativity forever changed the way we think about time and space. In it he said space is what we measure with a measuring rod and time is what we measure with a clock. Einstein asked how the measurement of space and time changes between two observers moving at a constant velocity relative to each other. He showed that time is measured differently for persons moving relative to one another. Einstein infers from this statement regarding space and time, that measuring the same thing and receiving different results, could somehow have an effect upon the reality of their existence. He felt two clocks having different times could mean that time itself slowed down at one of the two clock locations. He felt that if a difference could be shown in the measurements using a rod in space, that with different results, he could conclude that the length of something was variable. It was dependent upon the observer. Whereas it is true that different results from the measurements will be obtained, I cannot agree with the conclusions that time is altered or space is altered through any observation or observations.