. Still, it's measurable. In 1971, physicists Joseph Hafele and Richard Keating put atomic clocks onboard jet aircraft and sent them flying around the world. When the clocks returned, they were out of sync with a ground-based clock by exactly the amount predicted by special relativity.
This is the crucial thought experiment that helped Einstein develop special relativity. If you're moving at the exact same speed , 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. So, from Bob's perspective, he's staying perfectly still, and it's Alice that's racing away from him. Yes, it sounds improbable, but from the point of view of physics, Bob can't tell the difference. In Bob's calculations, his clock is running normally, and it's Alice's that should be running slowly. So, when they meet again, she should be younger than him, because not as much time has passed for her.
When I first read about this paradox I was quite bugged by it, but then I discovered the explanation and my life resumed xD
Time is the result of the speed of light being finite. According to Einstein, a photon is created by an electron and at the same time absorbed by another electron. Time stops and space disappears. Perhaps this is how photons and fast moving particles can become entangled.
;-)