When you see the title of this thread, I'm sure you will either be bored or excited because this is one of those famous 'paradoxes' in SR. So I read the book by Schutz on the part of explanation behind the twin paradox. There he gave an example of two sisters Diana and Artemis involved in the experiment. In particular Diana leaves her sister on earth with a rocket travelling with close-to-light velocity. Then at certain moment Diana turns around and go back to the earth. The question is, as is very obvious already, which one is older upon the reunion. The author explains it using the spacetime diagram of Artemis, the sister which stays on earth. I can understand the explanation that the reason Diana is still much younger than Artemis is that because they move with very high speed with each other and Diana undergoes abrupt change of direction. But then one can argue why we hadn't used the spacetime diagram of Diana in the rocket? This could have resulted in the opposite conclusion. Up to now I'm still unsure why we should use Artemis's spacetime as the reference, but I think it's because Artemis is inertial while Diana is not, is that the true reason? I am not sure with my own conclusion because it seems that the author doesn't mention that. He only puts a stress on the explanation why there can be such a large difference in the ages, not why we hadn't use the other sister's spacetime to draw the diagram. I think the reason why this gedanken has become a 'paradox' is that because there can be an ambiguity in the final conclusion of who's older when different references are considered, not because there is a large age difference which the author emphasized. Even when the rocket moves with everyday velocity, Artemis is still the older one at the reunion.