Einstein’s 1905 theory claims that light moves through a vacuum at a constant speed relative to any observer, no matter what the observer’s motion.

Relativity and time:

A moving clock runs slower than a stationary one from the perspective of a stationary observer. For example, a man riding a moving train is timing a light beam that travels from the ceiling to the floor and back again. From his point of view, the light moves straight down and straight up. The observer riding the train thinks the light bulb and mirror are standing still.

From trackside, Einstein sees man, bulb and mirror moving sideways: the light traces a diagonal path. From Einstein’s viewpoint, the light goes farther. But since lightspeed is always the same, the event must take more time by his clock. The distance that the light pulse travels as seen by Einstein is farther.

The observer watching the train thinks the light bulb and mirror are moving.

Relativity and length:

A moving object appears to shrink in the direction of motion, as seen by a stationary observer. For example, the man now observes a light beam that travels the length of the train car. Knowing the speed of light and the travel time of the light beam, he can calculate the length of the train. The observer on the train sees only the motion of the light beam.

Einstein is not moving, so the rear of the train is moving forward from his point of view to meet the beam of light: for him the beam travels a shorter distance. Because the speed of light is always the same, he will calculate the train’s length as shorter – even after he allows for his faster ticking clock. As the train approaches the speed of light, its length shrinks to nearly zero. Someone watching from outside sees the light beam moving but with the motion of the train added.


  • Time Magazine, Dec 31, 1999


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