Relativity and Time

What is the twin paradox?
Answered by Science Channel
  • Science Channel

    Science Channel

  1. The twin paradox is a thought experiment first posited by Albert Einstein. (Einstein used clocks for his paradox, while today people tend to use twins, with clocks.) First let's tell the story of the twins. Twin brothers Joe and Frank are very different people. Frank has been a vagabond his whole life and loves to travel. Joe is a homebody; he barely likes crossing into the next county over from his townhouse. Frank (by some miracle of storytelling) has the chance to fly the world's fastest spaceship out into the cosmos, leaving his twin brother behind.

    Frank zooms out into space at close to the speed of light, but after five years traveling at that speed he decides he wants to come back to Earth, which is the only planet in the stars that has good pizza, for which he has an incredible craving. So he turns around and heads back to Earth, still traveling near the speed of light. When Frank gets home, he is astonished to find that while he has barely aged at all his brother Joe has aged considerably. They don't exactly look like twins anymore.

    So where, then, is the "paradox" in the twins paradox? It comes down to a matter of the perspective of each twin. The seeming paradox is that, from each twin's individual perspective, the other twin has a slower-moving clock, so each should be younger and older than the other on Frank's return, which isn't possible. Frank sees the Earth moving and he "sees" (well, in a theoretical sense) his brother's clock moving more slowly on Earth, making Frank think his clock is faster. Joe, watching his brother zoom away into outer space, sees Franks "clock" on the ship moving more slowly, making his, Joe's, clock seem to be the faster of the two. In the end, of course, it's not at all paradoxical. Frank and Joe's perspectives are, in fact, not the same. Frank is moving at near the speed of light, and Joe remains stationary on Earth. So the fact that Frank has barely aged upon return from his journey while Joe has aged considerably is to be expected: Einstein tells us that time passes more slowly the faster one moves through space. Frank, zipping along at the speed of light, had a slower-moving clock on his rocket ship than did his brother in his townhouse on Earth.

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