We know what is twin paradox is, and also why it is not a paradox, but my question is, "if we can not define absolute motion, how can we decide that the twin which went to space will age less, and not the home one?" That is to say that, when peter at home sees paul's clock moving slow (in space ship), similarly, paul should see peter's clock to be slowed down. So for paul, peter should be the younger! How is it that when paul comes, he is the one who is younger? for example, let's say (arbitrarily) that, the absolute speed of earth (which is of course not measurable) is 70,000 km/s. the speed of paul's spaceship is 30,000 km/s. and they are moving in opposite direction. So, if we talk about the absolute speed, peter should be younger, and if we talk about relative speed, both should be younger for each other. What is it that lets us decide, that each time, paul will be the one who aged less?