The theory of relativity is that physical theory which is based on a consistent physical interpretation of these three concepts. The name “theory of relativity” is connected with the fact that motion from the point of view of possible experience always appears as the relative motion of one object with respect to another.
Two key concepts are based on the theory.
The first is the Special Theory of Relativity, which deals essentially with the question of whether rest and motion are relative or absolute, and the consequences of Einstein’s assumption that they are relative. Einstein recognized that it is practically impossible to tell if the movement is perfectly uniform — and defined this as a fundamental concept of physics.
Second, at a speed of 186,000 miles per second, light travels. No matter how quickly an observer moves or how quickly a light-emitting object moves, a measurement of the speed of light always yields the same result.