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?
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