Hi, I'm new to this forum, and I don't really consider myself as "worfy" to post here... and nor do I try to claim that Einstein was wrong (I would certainly slap myself if I did) Anyway, I have currently finished to read Relativity by Einstein (I heard that is not he best book on that subject, though), and I'm wondering about the time dilation formula. When K' moves with a constant velocity v relatively to K, and when light is travelling along the y'-axis from origo, how does it reach a particular point L on it. I mean doesn't light move perpendiculary to y-axis and not along the x'-axis (so that when I stand on the origo of K' I don't see light reach L) when an observer who stands on the origo of K'? I mean doesn't light travell perpendiculary to the x-axis but not along the y'-axis. I'm sorry but I have wicked imagination. I am thinking from K, don't I?