The twin paradox is a thought experiment that illustrates a consequence of Einstein's theory of special relativity. The theory of special relativity makes two fundamental claims: The speed of light, c, is a constant and never changes; and, the laws of physics must be identical for all observers who are either stationary or in constant velocity motion. If one sits inside a windowless van that rides along at a constant speed, it is impossible to tell if one is moving or standing still. Any physics experiments that are conducted in the van would have the same results whether the van speeds along or sits in a parking lot.

An effect of a fixed speed of light is that measurements of time and space vary depending on the motion of the observer. Since most observers generally move much slower than that of light, the effects of special...