Okay, I'm asking a question similar to this one here: Time Dilation – what happens when you bring the observers back together?. Specifically, I am curious about a specific angle on the second part of his question, regarding when two moving frames of reference (FoR) are brought back together, and how "it" knows which one should be still young.
The accepted answer on that question says that it is whichever FoR experienced the forces of acceleration/deceleration. But, isn't that the whole point of relativity, is that it's all … well, relative; that it is not possible to say with a certainty that it was the traveler in the spaceship who was accelerating/decelerating?
Isn't it the case that it is just as legitimate to say that the universe and people on the planet accelerated/decelerated and the traveler in the spaceship was stationary? This would therefore then lead to that the universe and planet-side people should remain young and the spaceship occupant should be old, no?
Does dilation (temporal-spatial) just generally apply to the smaller of the two FoRs, or is there some other system or rule which "decides" which FoR gets dilated?
Best Answer
"Relativity" is actually a misleading word that Einstein didn't like. It doesn't mean "every vantage point is equivalent and it's all relative". It really means only inertial, non-accelerating vantage points are equivalent. You could think of it as, prior to relativity, people believed that there was an absolute position/speed to the universe. Special Relativity shows that there is not, but rather, there is an absolute acceleration to the universe.
This is illustrated by the famous rotating bucket thought experiment. You put a bucket out in the middle of empty space and spin it, and the water in it starts flowing towards the edges. But if all vantage points were the same, couldn't you also think of it as the universe spinning and the bucket stationary? But one vantage point is obviously more correct than the other, because only one involves the water flowing towards the edges. Thus there is a "universal" state of zero acceleration that is unambiguous.
It is interesting to note that Einstein's original idea for his theory of Special Relativity was a theory of Invariance (of the speed of light)