I've already completed most of the question, it's an add on at the end that has stumped me. I've calculated using time dilation the difference in ages between the two twins, Joe traveled at v = 24/25 (c=1 units) to a planet for 7 years in HIS reference frame and returned at v = 12/25 The question at the end is this: In Ed's frame (Ed stayed at home, whilst his twin Joe ventured through the galaxy) the distance between Earth and the distant planet is greater than 7 light years. So how does Joe explain that the outbound trip took less than seven years in his frame? I guessed this is something to do with length contraction, in Ed's frame the ship would contract. In Joe's frame he's standing still and space is wooshing past him, so would space contract? Doesn't seem to make sense to me, if we use the length contraction equation: L = L' / γ Where γ = (1 - v^2)^-1/2 Applied to Joe's frame, I get 1.96 lyr (using L' = 7 lyr) But then traveling at v = 24/25 he would get the in 2-3 years, not 7?