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Time
From A Storehouse of Knowledge
Time is the third component of the velocity and position equation. A photon has a constant velocity of c, it will change its position by about 300,000 kilometers in every second of time for all observers, no matter what their relative velocity is. Einstein's theory of Special relativity holds that space and time itself will actually change to maintain the perceived velocity of light as a constant, and he in fact unites space and time into a kind of elastic sheet that compresses in the direction of motion and forms dents in the presence of gravity.
