SPACE AND TIME ARE NOT ABSOLUTE. TOGETHER, SPACETIME IS FLEXIBLE AND SUBJECT TO THE CONDITIONS UNDER WHICH IT IS BEING MEASURED. THESE CONDITIONS ARE MOST EASILY SEEN WHEN OBJECTS MOVE VERY FAST OR WHEN MEASUREMENTS ARE MADE NEAR LARGE SOURCES OF GRAVITATION.

Nearly all of the discussion up until this point in General Physics has been based on two assumptions: (1) that all observers will agree on the size of a meter and a second, regardless of their conditions, and (2) that all events take place in a constant and unchanging universe. While common sense seems to uphold these, it turns out that neither of these assumptions is correct. This startling realization, first made in full by Albert Einstein, is based on the notion that the light is always observed to travel at the speed of light, regardless of the conditions of the observer.