Time and the Twin Paradox; Scientific American Time; Special Editions; by Ronald C. Lasky; 4 Page(s)

As maxims go, “time is relative” may not be quite as famous as “time is money.” But the notion that time speeds up or slows down depending on how fast one object is traveling relative to another surely ranks as one of Albert Einstein’s most inspired insights.

The term “time dilation” was coined to describe the slowing of time caused by motion. To illustrate the effect of time dilation, Einstein proposed an example—the twin paradox—that is arguably the most famous thought experiment in relativity theory. In this supposed paradox, one of two twins travels at near the speed of light to a distant star and returns to Earth. Relativity dictates that when he comes back, he is younger than his identical twin [see “How to Build a Time Machine,” by Paul Davies, on page 24].