Consider the scenario where you measure the time it takes for light to travel to the left 10 meters and to the right 10 meters. Both measurements will take the same time, even though we are moving through space at crazy speeds. This must mean that light is not moving relative to "space" as a whole. What does it move relative to? The light emitter? If so, try shooting two beams of light 10 meters from the wall. The first time the emitter is stationary; the second time it is moving at 100 m/s. Am I mistaken in thinking that it would hit the wall slightly faster? Wouldn't this light be moving faster than the light emitted from the stationary source?
Answer
Am I mistaken in thinking that it would hit the wall slightly faster?
THE insight of relativity is that this is not true. No matter whose clocks and rulers you use, you will always measure the speed of light (in vacuum) to be the same.
This has as a consequence the fact that lengths and times are not as absolute as was once thought. If you are looking at a ruler that is moving relative to you, then that ruler will appear to be shorter (along the direction it's moving); and a moving clock will appear to be slower.
This must mean that light is not moving relative to "space" as a whole.
Relativity does away with the idea of absolute space and instead all velocities are relative to some object or frame of reference (hence relativity).
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