What is the question - would ask you - even a high school student knows now that the speed of light in a vacuum is constant and equals to 299,792,458 m/s!
As a diligent student, I also thought at the same way and unconditionally trusted my teachers of physics. But after a certain moment of my life I came to conclusion that the issue is not so evident as it seems to be and might need a general revision.
I would like to share my deliberations with all skeptical persons.
As it is well known from the experiments and observations of the modern physics that the light is a phenomenon of double nature. On one hand it is a ray, on another it is a wave. Why is it so, is still a topic for a fundamental investigation. For us now the main important is the fact thatthe light is a wave. (I would like to emphasize right away for my would be opponents, that I don't say that it is only a wave!).
Again, from the physical lessons we know that any wave needs some medium for its propagation. If we want to see water waves, we need to have water, if we want to hear sound waves, we need to have air (as the sound media). If we want to talk about the light wave we, apparently, need to admit some luminiferous medium to carry it. Some people, indeed, may say that the light wave is a special electromagnetic one, which doesn't need a medium, since it is a medium for itself, but in this case such a wave, obviously, stops to be a wave as we know this word, and becomes something magic.
Now we need to figure out what is a vacuum. Before any investigation we need to remember that the matter concerns obviously a true vacuum, that is a vacuum as it is according to the meaning of the word. Otherwise, obviously, the word false or so-called would need to be added before the word vacuum. The meaning of the Latin word vacuum is void, emptiness, absence of anything. Since vacuum according to the very meaning of the word is absence of anything, it means that it can be accurately defined as the absence of any media (as a part of anything).
Strictly speaking, the very phrase speed of light in a vacuum is a sort of self-contradiction. Really, to whose mind would come to measure or discuss, say, the speed of speech in a silence or the speed of electricity in a dielectric? To measure or calculate the speed of light in an absence of light is about the same ridiculous. Of course, if physicists-relativists for some reasons want to use the words like vacuum, wave, or field in some special indirect meaning, they, probably, have a right to do it (on their own risk, of course!), even if it leads them to some serious misconceptions. But when they finally present their theories for the general public they, apparently, must discard such a slang and either use the words in their correct direct meaning, or put it in quotes to avoid different misunderstandings.
So, now we can do a very easy thought experiment. Thought experiments were a favorite technique of such an authority in the modern physics as Albert Einstein, so we can use it easily. Let us take some space, remove any media from it and direct a ray-wave of light on this area. Since the wave aspect of light needs some media for the propagation of its waves and there is no media in a true, logical vacuum by definition, this area would stop the light.
There is no light in a true, 100% vacuum! Or, if we would like to express this logical truth by mathematical language, it may be said that the speed of light in a true vacuum is also 0. In this case, 0 wouldn't be considered as a member of numerical series, but rather as a synonym for absence in sense that there is no speed of light in an absence of light, just as there is no change in the speed of light in an absence of light, etc. Zero multiplied upon any number (attribute) gives us the same zero. In this sense, one might say that the speed of light in a true vacuum is constant and equals 0 m/s.
Our thought experiment could be successfully reproduced in the laboratory, if we could have the technology of removing (neutralizing) all kind of media from a certain piece of space. It would be scientifically inaccurate to dismiss out of hand the possibility of such a technology sometime in the future on the base of some doubtful ancient superstition that "nature abhors a vacuum or something like this. It was a time when it seemed impossible to remove the air and gas from some space, but now we can do it with a high degree of success. Of course, someone may reject the very possibility of such a technology, but, apparently, only on the grounds of reasons, which are not empirical themselves.
So, you may ask, if there is no light in a true vacuum, what is 299,792,458 m/s?
There are, seemingly, only two options of how this digit might be obtained: theoretical or experimental.
I have read in some books that the speed of light in a vacuum may be calculated logically without or before any experiments. In this way we may deduct it as, say, Pythagoras' theorem. But it is very hard (if not to say impossible!) to see the way such a theorem about the speed of light in a vacuum could be proved. It would be the same hard as, for instance, to deduct the orbit of the Moon from the basic geometric premises.
There were indeed, a few theoretical attempts made to attach an absolute importance to the speed of light in vacuum. For example, at the 1983 Conference Generale des Poids et Mesures the following SI (Systeme International) definition of the metre was adopted:
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
This, apparently, defines the speed of light in vacuum to be exactly 299,792,458 m/s so it seems to be constant by definition. I recognize, of course, the importance of the accuracy in measurements, but here the respectable Conference undoubtedly had made an obvious logical mistake. To check this let us imagine that, say, the speed of walk of an average adult person is officially admitted a priori to be absolutely invariable and makes up 6 km/h or, accordingly, 1,6666666667 m/s. In this case, we could define meter in the following way:
The metre is the length of the path travelled by an adult person in walk during a time interval of 1/1,666666667 of a second.
Which, of course, sounds funny.
The definition of the Conference presupposes the speed of light in some vacuum to be 299,792,458 m/s and in no way may be used to prove it by definition.
So, 299,792,458 m/s may be obtained experimentally and only experimentally. We can do our experiments in a vacuum chamber with a certain degree of vacuum, which is always less than a complete, true vacuum, unavailable in physical laboratories now. Or, we may try to measure the speed of light in the interstellar space by astronomical observations. Then, some may call these media as a physical vacuum, bringing in a big possibility for confusion. In this way they, obviously, would need to put a special reservation - what is called physical vacuum, namely the medium in a vacuum chamber or outer space. I have looked through a lot of physical (or, it is better to say, metaphysical) web-sites and textbooks and I never met the expression the speed of light in a physical vacuum as a definition for c. It was always the speed of light in a vacuum without any additional clarifications. If it is just negligence, it is definitely an important negligence.
Hence, we may conclude that 299,792,458 m/s is the speed of light wave in the medium of a vacuum chamber or in anouter space. In transparent media such as air, water and glass light is slowed down comparatively to vacuum chamber or outer space. The ratio by which it is slowed is called the refractive index of the medium. This was discovered by Jean Foucault in 1850. There is a possibility, indeed, of the discovering of some media in the future with the refractive index less then in a vacuum chamber with the speed of light in it greater than absolute 299,792,458 m/s.
Thus, there is, apparently, absolutely no reason for a man of good sense to follow the popular physical misconception and declare the rarefied medium of a vacuum chamber or outer space as something special, not to mention postulating the speed of light in these media as something super, independent of the motion of the observer and invariant with time and place. Yes, in some sense it is a record speed we observed so far, but it would definitely require a leap of faith to accept this record as absolute.
Relativists often blame their opponents in anthropocentrism. I will not analyze here, if the anthropocentrism is a correct attitude or not, but it definitely has a lot of practical benefits, and it is much more forgivable to put the humankind in the center of our universe, than the speed of light in a vacuum chamber.
Of course, if we would mean under constancy an immutability of the speed of light in a certain medium, then the speed of light in a vacuum chamber might be called constant. Everything depends upon interpretation of the word constant. But in these case the speeds of light in other media such as air, water or glass are constants also! That is, for a certain medium we would always have a certain speed of a light wave...
From this point of view, famous c seems to be nothing but an abbreviation for 299,792,458 m/s - the speed of light in a vacuum chamber (the letter c here might be interpreted as a first letter of the word chamber). Following this pattern, we could also measure accurately the speed of light in a vacuum tube and abbreviate is as t, or the speed of light in a vacuum bottle and put it under a shortcut b. Then we could create a table of such constants for practical needs without, apparently, a reasonable reservation for any place special in this table.
Along with the above-stated arguments, I would like to mention one more common and less special argument against the absolutism of the speed of light in a vacuum (chamber). We know from our daily experience that these two phenomena - light and sound - often adjoin each other. Like in a storm, for example. So, would it be fair to pick out one phenomenon of the pair and offer its speed as something absolute? Let alone inventing some relativity theory upon this absolute value...
Of course, all the said may sound paradoxically and superficial on the face of it. But it is definitely not a game of idle mind, and I strongly believe that the future development of a true physical science will bring us some experimental data, which would illustrate the logically consistent point of view upon the speed of light for any unprejudiced person.
What is really constant, it is the speed of light in a true vacuum, which equals to 0 m/s. The light fades in an absence of light, or vacuum stops the light. It is logical evidence, as we could see above. Moreover, it can be said for sure that this constant is constant not only for the light, but also for anything else. Speed of everything in a vacuum is 0, since there is not anything moving in a true vacuum!
To summarize all the above we could conclude:
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An assumption about the absolutism of the speed of light in a vacuum requires an essential revision.
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Under in a vacuum we suppose to understand according to the dictionary a true, logical vacuum, until there is a special reservation attached.
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There is no light in a true vacuum, since a) there is nothing in a true vacuum, and b) there is no medium to carry light waves
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299,792,458 m/s, obtained experimentally is not a speed of light in a true vacuum, but the maximal measured so far speed of light in a vacuum chamber or interstellar space.
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With the development of technology there is a probability of experimental discovering of the speed of light more than 299,792,458 m/s in some medium.