Einstein’s 2^nd ‘postulate’, carved into stone and worshipped by the faithful, states that light must be a constant 186.000 miles per second in a ‘vacuum’.  This constant is the ‘fastest’ light or any particle can travel. By vacuum Einstotle (Einstein + Aristotle) meant a space of ‘nothing’ which has never existed.  A vacuum is currently defined as the ‘lowest energy state in space’ whatever that means. 

Two physicists, Geraint Lewis and Luke Barnes, believe that the ‘speed of light’ and the related measurement of ‘time’, are fundamentally impossible to prove.  They support what Sagnac had discovered in 1913, and which has been replicated hundreds, if not thousands of times since.  The implications of what they propose are ‘game-changing’.  More here