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Relativity and the Twin Paradox

Went off on a tangent last night, sitting through a few episodes of The Inexplicable Universe: Unsolved Mysteries. This is probably the best explanation of Einstein’s twin paradox I’ve ever seen.

In this video lecture, Neil deGrasse Tyson, America’s most noted astrophysicist, describes the Twins Paradox, a hypothetical scenario in which high-speed travel slows down the aging of one twin, while the other twin ages at a normal rate.

This is an excerpt of The Inexplicable Universe: Unsolved Mysteries, a series of online courses presented by Dr. Tyson in Hayden Planetarium, American Museum of Natural History.

One of the great achievements of the human mind and our understanding of the universe is Einstein’s theory of relativity. It has two simple tenets: (1) The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light. (2) The laws of physics are the same for all observers in uniform motion relative to one another. Extraordinary phenomena derive from those tenets: As you travel faster, or if you find yourself in the vicinity of a higher source of gravity, time ticks more slowly for you than it does for other people. This leads us to the famous twins paradox: If we have a pair of twins, and one is sent off at 90% the speed of light, and the other is kept on Earth, how does their aging rates compare?

BONUS NOT INCLUDED IN THE EXCERPT: Neil moves on to ponder what would actually happen if the second twin traveled at 100% the speed of light. This will bake your noodle. The answer: Nothing. Time will stand completely still for him.

It so follows that photons — the fundamental particles of light — only exist in a single instant. No time passes at all for them. So once they’re emitted (from a star, for instance), they are immediately absorbed at their destination, with no time having passed according to their point of view. That’s right. A particle of light can travel across space for 400 billion years as observed to the rest of the universe, but for the particle of light itself, no time has passed.

This is all, of course, based on the notion that nothing in the universe is faster than the speed of light.

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