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Forget about anti-ageing creams and hair treatments. If you want to stay young, get a fast spaceship. That is what Einstein's Theory of Relativity predicted a century ago, and it is commonly known as "twin paradox".

Imagine two twins. They are identical, except for one thing, one of them has a very advanced and expensive spaceship. The first twin remains on Earth, while the other travels to a distant star and back at velocities close to the speed of light.

When they meet again, the Earth-dwelling twin has aged a lot more than the travelling twin. This is because of what Einstein called time dilation. He predicted that clocks experiencing different accelerations measure time differently. Puzzling as it may seem, these time-dilation effects have been tested in the laboratory many times, and are routinely taken into account by the Global Positioning System (GPS).

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