twin effect

Effect of special relativity, variant of the time dilation effect: A twin that uses a high-powered rocket to travel in space with a speed near that of light before returning ages less than his twin sibling that has remained on Earth.

The question why this is sometimes thought to be a paradox, while it really isn’t, is explored in the spotlight topic The case of the travelling twins; a geometric analogy that is not about time, but about distance, is developed in Twins on the road.