Hi. So the whole premise of special relativity seems to me to be hinged on the immutability of the speed of light, the fact that it is the same for every inertial frame of reference, and the fact that information and energy cannot travel faster than this. What really puzzles me is this whole traveling forward in time thing. While I can appreciate the use of the twin paradox as a pedagogical device, would a moving frame of reference at a comparable to light speed actually affect how fast a biological system in that frame ages? Has this ever been observed ? Would the living system's biological clock actually slow down relative to an observer on the earth ? Would his sense of time perception alter to make it feel as if he's spending say 2 years on the shuttlecraft, while his twin back home feels like he's spent 50 waiting for him ? Special relativity made perfect sense to me until this came along. From the derivation of the time dilation equation that my textbook shows, the crucial argument seems to me to hinge on the fact that the light ray follows different paths when viewed from one frames of reference or another, and because of the fact that it's speed remains the same, the time measurements must be different. So, if you used a signal other than light, would you even have time dilation ? Why must all clocks be based on light ? A biological clock surely doesn't use light to keep time, does it ?