Let's suppose that I am the Captain at a space dock sitting in interstellar space. My crew decides to go on a trip in our ship, so they board and set off at .9c. t = t0 / sqrt( 1 - v^2/c^2 ), t0 = 1, v = .9c, therefore t = 1 / sqrt( .19 ) = 2.3 (approximately) So as they speed off, 1 second of the time that I experience corresponds to 2.3 seconds of the time that they experience, which suggests that if I log that they travel (assuming they say go in a big circle back to our dock, not stopping/altering their speed) a month they come back having experienced/aged 2.3 months. I believe this is all correct. Now here is my problem: since motion is relative, could the crew in the ship not have said that me, sitting in our dock, is instead in motion and that they were stationary? From their respective, shouldn't have I (and really the rest of the universe) have aged 2.3 seconds for their every second? What makes one perspective more correct than the other or how is this apparent contradiction otherwise solved?