Relative

@questionWeaver 299,792.458 km/s. I would say I want more digits on it though! Nonetheless, suspect in which way?

The speed is derived via calculations that assume the formulas are correct.

Yet ... enough quantum work has occurred that leads all to believe the formulas have unknowns.

Thus, the derived speed of light may itself not be the constant we once accepted it was.

Well we do also meassure the speed to valid date it. It is true that meters and other "variables" have choosen so it gives that number. Maxwell did in fact find the constant c = (epsilon * mu)^(-1/2). Though nothing invalid of that have been shown yet. It is just an appoximation, that gives; 300,000 km/s instead of the more precise value.
Quantum is also pretty reliable that we use that constant. Since CERN would run into problems. By not being able to use Special relativity. So I am not sure which formulas you a referring to?
To my knowledge Quantum have it harsher because not knowing if it is a wave or a particle.

Regards

The speed of light was defined as the limit the formulas approach.

But ... we can never measure the speed of light ... since the tools to measure it are based upon the formulas we use to define it.

Thus, every challenge to the formulas we theorize define nuclear interaction ... also challenge the speed of light.

All we can conclude is the speed of light is not a constant ... but rather a phenom that we do not completely appreciate.

No, Galileo was the first to try to determine the speed of light, which he did never succe on doing. Ole Rømer (Ole Roemer or Olaus Christensen Roemer) was the first to measure the speed of light. Even though it is not as precise as it is today. We then later had Maxwell that found it by using formulars. Which he found odd, since he was studying electromagnetic waves.
Then we had the whole aether problem. Cause if it was a wave, then what would it travel in. Here the Michelson–Morley experiment comes in.
Which is rather famouse today, not that they ever succed. Then later we had Einstein who killed the Aether.
Today we believe it to be a constant. I am not stating that it is, but every experiment seems to point the way.

Here is some good reading of how to measure the speed of light;
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html

We believe so firmly on that the speed of light is a constant and that it is the natural speed limit. So when CERN data showed they found a particle that moved faster. Then they delayed the release to try to figure out the error. Which they could not find out. So they published it, and physicst came to CERN and found it was a error two articles about it here;

https://www.telegraph.co.uk/news/science/8782895/CERN-scientists-break-the-speed-of-light.html
https://www.telegraph.co.uk/news/science/science-news/9100009/Scientists-did-not-break-speed-of-light-it-was-a-faulty-wire.html

Now if you are considering a prisme for instance. That the light "bend" (light does not bend) which it does not, then form a rainbow. Then you have to consider what it meant with light. Is it the light beam or the photon?. We would acctually have most of modern physics breaks down, if it was proven that Theory of relativity was incorrect. We even use quantum mechanics relativistcly which would break down if not the speed of light where a constant.

Notice when I write "c" I do not talk about the speed of light in a medium. However, I talk about the speed of light in vacuum. Where it is in fact a constant. Then it becomes another constant in air for instance. Which is almost the same as the speed in vacuum.

We do experimental test to show that the formulars/theory holds not the other way around.

Regards
Sorry for the long comment (:

No problem

Makes sense

Of course ... for engineering purposes we must treat the speed of light as a constant.

But, in engineering philosophy class, we recognize something derived ... that can not be separately established ... could never be an absolute ... only an assumption.

well it is an assumption. Same goes with Inertial system, and newtons law! Even schrödinger equation. We assume they hold! Lucky for us, for now it seems to do. But it does not make it the "truth" That word is overused and miss used a lot! D: