The postulate of relativity says that the speed of light in a vacuum is the same for all inertial observers. But does it also say that this speed is the greatest achievable speed?

  • $\begingroup$ The fact that it's the greatest speed, as far as I know, is a derived notion and not a postulate in itself. If you look at the Lorentz transformations for example you immediately see they make no sense for $v>c$, at least not without implying some exotic new physics (Tachyons, etc.) which would be a non-standard addition to the theory. $\endgroup$
    – Amit
    Feb 19 at 11:33
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/267852/2451 , physics.stackexchange.com/q/6406/2451 and links therein. $\endgroup$
    – Qmechanic
    Feb 19 at 11:52