twins paradox

The twins paradox is a thought experiment in which one twin (A) travels into space at nearly the speed of light and returns to find he has aged less than his twin (B) who has remained on Earth. This result is paradoxical because of the seemingly symmetric roles played by A and B in the story. In fact, A returns younger than B because only A travels in a non-inertial (accelerating) reference frame. From B's point of view, A experiences time dilation, but from A's point of view the distance traveled is shortened because of length contraction.