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The 'twin paradox' shows us what it really means for time to be relative - Space.com
May 17, 2022 1 min, 57 secs

Put simply, special relativity tells us that moving clocks run slowly.

If we want the laws of physics — which also means the experience of physics — to be the same regardless of your reference frame, perspectives have to shift for observers at different speeds.

Special relativity says that a clock attached to a moving object will tick at a slower rate than one standing still.

And what goes for moving clocks goes for everything else; physics, chemistry and biology all operate at a slower pace in moving frames of reference.

When the clocks returned, they were out of sync with a ground-based clock by exactly the amount predicted by special relativity.

This is exactly what the physics of special relativity demands: Different observers in the universe will have different calculations of the flow of time depending on their velocity. .

Nothing feels different for Bob when he's traveling.

Everything just feels totally normal — so normal, in fact, that as long as he's not accelerating, he can't even tell he's moving at all.

This is the crucial thought experiment that helped Einstein develop special relativity.

If you're moving at the exact same speed (no accelerations or decelerations), it's impossible to tell if you're the one who's moving, or if you're staying still and the rest of the universe is moving.

Yes, it sounds improbable, but from the point of view of physics, Bob can't tell the difference.

And he'll be able to tell he's doing it not by looking at the outside universe but by making observations while inside his rocket — for example, the vibrations of the engines and the push of his body against the floor during deceleration.

— Our universe may have a twin that runs backward in time .

The mathematics of special relativity tells us how much Alice and Bob will age in their own frames of reference, and that same math demonstrates that they will end up agreeing on the numbers in the end.

Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars.

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