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?
> In physics, the twin paradox is a thought experiment in special relativity involving identical twins, one of whom makes a journey into space in a high-speed rocket and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect[1][2] and naive[3][4] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less. However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey, and so there is no symmetry between the spacetime paths of the twins. Therefore, the twin paradox is not a paradox in the sense of a logical contradiction.
There's multiple explanations included to resolve the "paradox" from different lines of argument; I particularly like this one: https://en.wikipedia.org/wiki/Twin_paradox#A_non_space-time_...