Time dilation with regards to the "no absolute rest frame" in physics.
The famous twin thought experiment where one gets in a spaceship, accelerates away from the planet, turns around, and comes back.
The twin that stayed on earth is old and the traveling twin is young still.
On one hand, I know that time will "pass differently" for each twin....but why is it the twin in the spaceship that ages less? Why isn't it true that the entire universe accelerated away from the spaceship and then returned, leaving the entire earth young?