Eighty years ago, Albert Einstein discovered something he called "the twins paradox": if one of a pair of twin sisters flies in space at a great speed, upon her return to Earth she'll be younger than her sister. Space flight makes people younger! Time goes slower for an astronaut in space than on Earth. There is no universal time measure, all time is relative, as Albert Einstein demonstrated. Time changes with the speed of motion. If the space-faring sister flies into space for 12 years and reaches 96 percent of the speed of light (which is 300,000 kilometres per second) going out and then again on the return leg, she'll find her Earth-bound sister aged by 43 years while for her just 12 years have gone by–they are 31 years apart!
http://www.hq.nasa.gov/osf/qanda3.html#54
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