Originally Posted by

**Noa Drake**
But that is not the same thing as realizing why it would be true.

The underlying cause for this is that all observers should experience the same laws of physics, regardless of their states of relative motion. A physics experiment performed in otherwise empty space, and the same experiment performed in an inertial rocket at relativistic speed wrt to the observer should return the same result. This is an empirical observation as much as it is a reasonable assumption.

In mathematical terms, this is possible

** only **if all observers agree on the separation of events in space-time, i.e. the metric must be the same for all inertial observers. So the question becomes - what kind of operations leave the metric unchanged ? It turns out that those operations are linear transformations that form a group, the generalised orthogonal group O(1,3), the elements of which are precisely the Lorentz transformation matrices - this can be explicitly derived.

The Lorentz transformations ( and hence the constancy of the speed of light ) are thus a direct consequence of the fact that the laws of physics are the same for all inertial observers. If

*c* was not an invariant, observers moving at relative velocities would perceive changing laws of physics. In other words - the geometry of space-time is such so as to ensure that all observers see the same laws of physics, and vice versa.

You can of course go and ask

**why** the laws are the same for all observers, but here is where the explanatory power of currently available models fails - this is a postulate supported by observation and experiment.