Perhaps the most famous of the paradoxes of special relativity, is the twin paradox. The scenario is as follows. One of two twins - the sister -- is an astronaut. She sets off in a relativistic spaceship to alpha-centauri. When she gets there, she immediately turns around and comes back.  As seen by her brother on earth, her clocks ran slowly, so although the round trip took some years by earth time, she has aged less than that. So when she steps out of the spaceship, she is younger than her twin brother.  You can see it here.

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