© Blayne Heckel.

Identical clocks are synchronized (top pair). One of the clocks is then taken on a voyage, returning to its original position at a later time (lower pair). Upon its return, the moving clock finds that it has undergone fewer ticks than its stationary partner. Replacing the clocks by twins, one of whom takes a round-trip voyage to a distant planet, is called the "twin paradox"; the twin who travels returns younger than the twin who remained at home. The apparent paradox arises because the theory of special relativity asserts that observers moving at constant velocity relative to one another both see the other's clock running slower than their own. In the example above, the moving clock cannot remain at a constant velocity relative to the stationary one; it must undergo an acceleration to turn around to return home. By the equivalence principle, the acceleration is equivalent to a uniform gravitational field, and clocks tick slower the deeper they are in the "gravitational potential energy well." The "twin paradox" can be explained using special relativity alone, but the equivalence principle provides a simple way to understand the effects of the acceleration. (Unit: 3)