[glossybrown]

Here’s something I’ve never understood; perhaps a more knowledgeable commenter can explain it to me.

If I get in a spaceship and accelerate up to about 0.9c, then cruise for a while, then flip around and come home at the same speed, I will have experienced much less time than the people on Earth. But from my frame of reference, they were the ones going really fast, and I was sitting still. If all motion is relative, what makes me the one to experience less time?

[jiggawatts]

This is a well known paradox: https://en.wikipedia.org/wiki/Twin_paradox

TL;DR: The two scenarios are not symmetric, the person in the spaceship undergoes acceleration whereas the observer staying behind does not.