>> Speed of light relative to empty space? What is it in empty space that
>> the speed of light is measured relative to? Emptiness? Hyrdrogen
>> atoms? What?
>
> You need to LEARN WHAT SR ACTUALLY SAYS, rather than going on
> like this about nonsense. In SR, and indeed in all current theories of 
> physics, speed is measured relative to a COORDINATE SYSTEM, not
> "empty space". The implications of this are legion, and to understand
> them you need to STUDY.

Measured of not, light cares not.  It's speed is finite and not
constant, and is dependent upon its surrounding.  Measurements
be damned.