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  • $\begingroup$ "The fact that nothing can move with a speed greater than the speed of light is not an "assumption" that ensures causality" - No. This is exactly the basic postulate behind the SR, from which the Lorentz transformations are derived. $\endgroup$ Commented Apr 24, 2011 at 13:29
  • $\begingroup$ @Anixx - I was under the impression that the two postulates said the following: (1) The laws of physics are the same for all inertial observers and (2) the speed of light as measured by all observers is always $c$, irrespective of their state of motion, or the state of motion of the emitter. That nothing can move faster than $c$ is then a consequence, isn't it? Please clarify. $\endgroup$ Commented Apr 24, 2011 at 13:47
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    $\begingroup$ You are absolutely right regarding the above comment. $\endgroup$ Commented Apr 24, 2011 at 14:36
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    $\begingroup$ SR is only geometry and mathematics. The only what You need for SR is as follows: (1) The laws of physics are the same for all inertial observers, (2) coordinates of different inertial observers transform in linear (affine) way. From that two postulates You will give Lorenz group of symmetry, with one constant - velocity of unknown value which is "largest possible velocity inertial observer may notice". Then You may turn on some physics. You may identify this velocity value with the speed of light by comparison with Maxwell equations which obeys Lorentzian symmetry. $\endgroup$ Commented Apr 25, 2011 at 19:19
  • $\begingroup$ So please remember - the maximality of seed of light is kind of experimental results ( from Maxwell theory of electrodynamics) whilst existence of maximal speed as a constant of nature is a effects of Galilean principle ( all inertial observers are equivalent) and certain mathematical assumption about symmetry ( linearity or affinity of coordinate transformations among them) $\endgroup$ Commented Apr 25, 2011 at 19:21