Subject: 'Vacuum' itself constraining 'actual' Light velocity?

I've searched the archives but can't find an 
alternate explanation for why the velocity of 
Light & the velocity of its 
source_aren't_additive.I'm resubmitting my 
original ques.#1167507035.Ph

Light's velocity seems to be "constant" & not 
additive. Regardless of motion of its source, 
Lt's velocity measures 300,000k/s in 
a "vacuum". While the notion of "The Ether" has 
been discarded, certainly Lt velocity is 
measured in an "Environment" ( "vacuum" etc.) 
that is NOT quantumlly empty. Is it remotely 
possible that this "Quantum Environment" itself 
is the limiting factor-ie nothing can propagate 
within "IT" ( be it "Vacuum","Fabric of 
Space","Field" etc ) more rapidly than 
300,000k/s ?
Is it possible that Lt's max_native_velocity is 
really, say..600,000k/s BUT our 
Universe's "Quantum Vacuum Environment" 
can't "transmit" it faster than 300,000k/s max ?
Maybe Lt speed_is_additive but we can't 
practically discern it. Perhaps a fighter plane 
traveling at 1k/s projects Lt beam ahead that 
theoretically_would_measure at 600,001k/s ( to 
an observer ) OR projects it behind so 
it_would_measure 599,999k/s, EXCEPT that,in 
each case,current "Environment" just 
can't "transmit" energy faster than 300,000k/s, 
so the observed velocities*appear*constant & 
measure at 300,000k/s.??
Please comment on the possibility of 
the "Environment" as the limiting factor in Lt 
velocity.Thank You

Re: 'Vacuum' itself constraining 'actual' Light velocity?