Hi. If the twin paradox, etc, is not really a paradox because one twin experiences acceleration, then is the idea of recipricol time dilation etc due to symetry across inertial frames purely theoretical? What I mean is that could we ever find two inertial frames moving relatively to each other in reality? At least one of them must have experienced some acceleration in order to produce the relative motion in the first place, right? Edit* I know i just explained that incorrectly. Of course we can have to inertial frames. What I mean is that because one of them must have experienced acceleration, then we will always have a "stationary" frame, and a "moving frame".