twin paradox
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The twin paradox is a famous thought experiment in Einstein's theory of special relativity where one twin travels away from Earth at extremely high speed and returns younger than the twin who stayed behind, which seems to contradict the idea that motion is relative. It matters because exploring this puzzle helps us understand how time actually works differently for objects moving at different speeds, a counterintuitive prediction of modern physics.
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During the ISS year-long mission, astronaut Scott Kelly (right) aged about 8.6 milliseconds less than his twin brother Mark (left), on Earth, due to relativistic effects.
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