Time dilation and length contraction are expressed by the following equations: L=L'*sqrt(1-(v^2/c^2)) T=T'/sqrt(1-(v^2/c^2)) However, according to the principle of relativity, you can't tell which is the fixed frame of reference and which the moving frame, so you can swap L and L' and T and T' around in the equations, am I right so far? Well, if that is the case, then time is dilated in system K' as seen from K, but also in K as seen from K' (I'm dropping length contraction here, as the time dilation is the interesting part). Now consider the Twin Paradox. Twin One stays on earth, Twin Two flies off, comes back, and turns out to be younger. However, according to the principle of relativity, one could also say that Twin One flew off with earth while Twin Two stayed put in the spaceship, and thus Twin One should now be younger. I must be missing something, or the Twin Paradox wouldn't be so widely used. I'd greatly appreciate it if someone could tell me what exactly that is.