I'm sure there's an easy answer to this, but it's bothered me for a while. It's about the Twin Paradox.
So Allen stays on earth while twin brother Bob roars off in a space ship to some distant star at a significant fraction of the speed of light. Bob reached his destination and completes his business, then roars back to earth again at again, some near-light speed.
When the brothers finally meet back up on earth again, Bob is significantly younger than Allen. Bob's near-c velocity has slowed time for him compared to what Allen experienced.
That's how I understand the Twin Paradox.
But-- All things being relative, couldn't we just as easily look at this as Bob's space ship sitting stationary while the earth, Allen, and the whole universe rushes past Bob at near-light speeds? In that case, isn't it Allen who's traveling at near-c speeds, and Allen who would come back younger than Bob?
Obviously, Bob can't be both older and younger than Allen. Where did I go wrong?