“It seems as if the relativity of time poses a paradox—for how can both the passenger on the train and the person on the platform both see each other’s watches slow down? What happens if now these people meet and compare the time; whose watch has really slowed down? To emphasize this paradox (often called the twin paradox), imagine twins who each set their watches before one of them gets on the train. The train speeds up to nearly the speed of light—at which point each twin will see the other’s watch running slower—and then the train slows down and returns to the station. Which twin is older? From the point of view of the twin on the platform, the one on the train has made a round-trip journey, while for the twin on the train, the twin on the platform is the one who has made the round trip.”