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I have heard of the thought experiment where someone fly off in spaceships and come back younger than their twin on earth. But a fundamental idea behind general relativity was that motion can only be described as relative to others. Therefore, the twins would have the same relative acceleration to each other. If we think about them as floating in space, there should be no way to tell who is doing the accelerating. So how does physics determine which one ends up younger?

Arkleseisure
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    Accelleration isn't relative, it is absolute. And the frame changes that the acceleration causes can be used to figure who is older and why. – zeta-band Dec 03 '18 at 17:44
  • The time experienced by both twins is calculable, but the calculations use different expressions because one twin is accelerating and one is not. For a look at how to calculate the time elapsed by the twin on the rocket, taking finite acceleration into account, see https://en.wikipedia.org/wiki/Twin_paradox#Difference_in_elapsed_time_as_a_result_of_differences_in_twins%27_spacetime_paths – probably_someone Dec 03 '18 at 18:11

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Therefore, the twins would have the same relative acceleration to each other. If we think about them as floating in space, there should be no way to tell who is doing the accelerating. So how does physics determine which one ends up younger?

This post explains it in depth, but very simply the two setups are not identical. During the time the rocket turns around and accelerates back to Earth, they see something very different than the person on Earth. Basically, the "rocket person" sees the "Earth person" suddenly age a whole lot, and they retain that difference through the rest of the journey.

Maury Markowitz
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