What is the ‘twin paradox’? © Getty Images

What is the ‘twin paradox’?

Einstein’s theory of Special Relativity states that time isn’t the same for everyone - not even for twins.

Asked by: Anonymous

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According to Einstein’s theory of Special Relativity, time isn’t the same for everyone. For example, if we observe a clock moving at a constant speed relative to us, it appears to mark time slower than a clock remaining with us. The time difference is only significant when you approach the speed of light.
Imagine a pair of twins, one of whom is an astronaut that travels to a distant star. The stay-at-home twin will see his brother age more slowly than him. You’d expect the space-faring twin to see the same happen to his Earth-bound counterpart since they’re moving at the same speed relative to one another. But if the twins are re-united, Einstein said that the space-faring twin will have aged less than the one on Earth, which is odd given that they’ve both performed identical journeys relative to each other.
Einstein’s prediction has been confirmed in experiments with atomic clocks, so what resolves this paradox? The answer lies in the fact that the twins don’t undertake identical journeys. To get back to Earth, the travelling twin experiences a force in order to slow down and reverse direction. The stay-at-home twin doesn’t, making their journeys fundamentally different. Not surprisingly, so are the relative travel times of the twins, thus one of them ages more.


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