[ron123456 (OP)]

Is it vacuum ( no ether )? ...... or is it Electromagnetic Field ?.....Why ?.....Why is it it that light's speed has to be constant all the time no matter what frame of reference it is observed in..with respect to the observant's frame of reference?.....thx once again.....

[Kryptid]

The vacuum is referenced because that is where the speed of light is the highest. Light is slower through water or glass. Relativity is still true outside a vacuum, though. As for WHY the local speed of light appears the same in all reference frames... I'm not sure. Perhaps Halc can shed light on it.

[Halc]

Why does vacuum affect frames of reference?

'Why questions' only beget more why questions.
A frame of reference is an abstract choice of coordinate system. Vacuum has little effect on that, except that in a vacuum, a human is quickly going to be rendered incapable of making such an abstract choice.
That's probably not an answer to what you had in mind, but it answers the question actually asked in the title.

Why is it it that light's speed has to be constant all the time no matter what frame of reference it is observed in?

It doesn't have to be, it's just that observation seem to suggest it.

SR postulates that locally (as Kryptid correctly points out), the speed of light in a vacuum is frame independent, and it postulates this because such an assumption is consistent with all experimentation. That experimentation also shows it not being invariant for a medium other than a vacuum.

But it is just a postulate, not something ever actually proven, and it only works for inertial coordinates in Minkowskian spacetime, and spacetime is Minkowskian (has Lorentz symmetry) only locally, so light in a vacuum moves at other speeds depending on your coordinate system of choice and the curvature of the spacetime being considered.

There are alternate theories that deny the postulates of SR, and according to those theories, light speed is not frame independent, and the physics of one frame is not the same as another.

For a comprehensive experimental basis for the postulates of special relativity, see here:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

[varsigma]

Apparently there is a "little problem" with us equating Euclidean space with R³.

. . .this seemingly innocuous act of convenience comes with a heavy price: It is largely responsible for the illusion of quantum nonlocality

.

Understanding what Christian means here, means understanding certain algebras, including Clifford algebras.

There isn't a bumper sticker version, you need a PhD almost. You might be asking, what does quantum nonlocality have to do with inertial frames of reference?

Me too.

[alancalverd]

Since a vacuum contains nothing (by definition) it has no reference points and hence no preferred direction. Therefore it is isotropic and whatever happens in a vacuum will happen in exactly the same way in all directions.

Same would apply to a bullet in vacuo, in the absence of gravity.

So the answer is that vacuum isn't a frame of reference, but anything else is!

[ron123456 (OP)]

thx......"Einstein felt that Maxwell's equations should be the same for all observers in inertial motion. From Maxwell's equations, one can deduce a single speed of light, and there is nothing in this computation that depends on an observer's speed"....just Einstein's feelings developed special relativity until you elaborated....thx once again.....

[varsigma]

Because somehow I can't stop myself doing it, I can point out that modern communications (actually since the telegraph) depend on the constant speed of light.

Imagine how hard it could be getting a working internet with all those optical fibres, if the speed of signal propagation varied around the globe. This is a rough kind of proof that c is constant.

footnote--it does vary because of changing gravitational potential, but this has a negligible effect, and besides, modern communication has robust error-detection/correction.