Sad how there&#x27;s a simple answer to that problem and nobody has been able to answer it correctly so far.<p>It falls directly out of the requirement that physics works the same in boosted reference frames.  With Gallilean relativity if you were traveling at 0.5c then the speed of light in front of you would be 0.5c and behind you would be 1.5c.  This wouldn&#x27;t just affect tabletop physics experiments but would change everything about atomic and molecular physics which is all just applied electrodynamics.<p>That means your biology would fail in odd ways due to chemical reactions in your body changing if you were traveling fast.<p>SR is just Einstein working backwards from wanting to keep the equations of E+M invariant to however fast you were traveling.<p>As a result, the speed of light needs to be constant in all reference frames.  That means if you are traveling at 0.5c you measure the speed of light to still just be c.<p>All of SR basically falls out of that, including implying that c is s &#x27;speed limit&#x27;.  Although I guess to be technical that&#x27;s also built on principles like conservation of energy and momentum, but you can get those from Noether&#x27;s theory and space and time translational symmetries.<p>There might be a deeper more naval-gazing philosophical question here as to &#x27;why&#x27; we find ourselves in a universe that has physical laws that work the same in highly boosted reference frames.  It actually does us a favor as far as space travel and interplanetary travel goes since otherwise we&#x27;d be forever limited to traveling at substantial fractions of the speed of light to other star systems (to keep our biology from failing).<p>The local relativity of physical laws is also critical to extend the theory to General Relativity and express Gravity as the curvature of space-time.  Without SR, then the theoretical basis of GR will necessarily fall apart.<p>There&#x27;s no a priori reason why the Universe could not have had Galilean relativity, though, but gravity couldn&#x27;t work the same way and physics in boosted reference frames would be very weird.  There might be some perverse cosmological result of taking Galilean relativity to its extreme, but I wouldn&#x27;t know what that was.