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?