Al and Bob are twin brothers on Earth. One day, Bob decides to visit a nearby galaxy, so he hops on a spacecraft and goes off at nearly the speed of light. He comes back a few days later, according to his reckoning, and reunites with Al, who has aged 70 years in the meantime.
This is the essence of the "twin paradox," which has been a matter of controversy among philosophers and also the general public – but not among physicists, who know about time dilation of the Special Theory of Relativity.
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