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Bob and Alice have the same age.

So in special relativity Bob leaves Alice and travels at very high speed and when it returns is younger than Alice. Bob's time is dilated and his space is contracted from Alice frame of reference.

But now, if I take Bob's frame as the frame of reference I can say the same: Alice leaves Bob and travels at very high speed and when it returns is younger than Bob.

So what is the truth: is Bob younger or is Alice younger?

Question is: how do you decide witch of these 2 are younger when they meet again?

Thak you.