Question

Time Dilation - what happens when you bring the observers back together?

Answer 1

"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)