In 1676, Olaf Romer concluded from observations of the moons of planet Jupiter that the speed of light must be finite. In the time that it took you to read this sentence, a briskly walking pedestrian has covered 5 meters, a car in the city 40 meters and a flash of light a million kilometers - a distance 25 times the length of the earth's equator. The speed of light, just under 300 000 km/s, is so much larger than any other speed in everyday life that for all practical purposes we can treat it as being infinitely large. Not so in astronomy: the fireball in a gamma burst expands into the interstellar medium at nearly the speed of light. And material in the jets of quasars is ejected at nearly lightspeed into space. When we observe high-speed phenomena of this kind, the finite speed of light dramatically affects what we see.
The speed of light is not merely a high velocity. The special theory of relativity, formulated by Albert Einstein in 1905, expresses its character as a limiting velocity: The speed of light is the cosmic speed limit that no material object can reach or exceed. At velocities close to this speed limit, relativistic effects are important that are unmeasurably small at everyday velocities. How much time passes between two events, e.g., then depends on who measures it: Two observers in relative motion will give different answers. The same holds for the size of an object: Its length in the direction of motion is largest for an observer moving together with the object; all other observers for whom the object is in motion, find a smaller value. A car that is 5 meters long, e.g., and moves at 100 km/h, is 2 x 10-14 meters shorter than when it is at rest. This distance is 10 000 times smaller than a single atom. At 90 percent of the speed of light, though, a 5-meter-vehicle would measure 2.20 meters.
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