Originally Posted by

**trilosohpical**
So in geometry SR is very clear and beautiful.

Yes, and that is all there is to it. SR is a geometric model of flat space-time.

We know light as EM or particle/wave or wavefunction quite well now, but for light as constancy in speed, we still know nothing about it.
In flat space-time, all inertial observers see the same vacuum ground state

, regardless of their states of relative motion; since the speed of light is a function of vacuum permittivity and permeability, it is hence obvious that all observers measure the same

*c*. All that SR does is explore the consequences of this fact - the constancy of the speed of light is a

**postulate** for SR, not a prediction or consequence of it, just like everyone seeing the same laws of physics is a

**postulate** rather than a consequence. If your issue is with

**why** the speed of light is constant, then you need to seek the answer in QFT, not SR.

But in terms of physics I don’t think we know what is going on in SR
The upshot is that you are missing the point. SR only explores the consequences of ( i.e. the relationship of space and time in flat space-times ), but not the reasons for the constancy of

*c*. And in that it is very successful.