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why does the speed of light in vacuum never change? Under any circumstances, The speed of light in vacuum never change, why?

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    $\begingroup$ The question can be made more comprehendible if you can mention whether you are looking for mathematical proof , experimental proofs available or non mathematical logic. $\endgroup$
    – Tony Stark
    Commented Aug 15, 2020 at 11:50

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To add: It is one of the two postulates of Special Relativity. It is 'proved' by almost foolproof and thorough experimentation.

Note: We can almost never become 100% certain in Physics about anything.

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The simplest explanation: Light consists of photons, sub-atomic particles of light. These photons have no mass. Special Relativity tells us that any massless object has to travel at the speed of light. It cannot travel at any other speed whatsoever. That is why the photons travel only at the speed of light (in vacuum), and no other speed.

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    $\begingroup$ Don't you think that this is a circular explanation? Special Relativity is derived from the assumption that there is something (specifically light in a vacuum according to experimental data) moving with a speed that is invariant in each inertial frame of reference. From that assumption all the other conclusions of Special Relativity follow. $\endgroup$
    – Serge Dundich
    Commented Aug 15, 2020 at 9:48
  • $\begingroup$ @Serge Dundich, actually no. SR was derived from a 'postulate' which turned out to be correct. Then the reason for why the postulate was correct was provided by the same theory. $\endgroup$
    – PNS
    Commented Aug 15, 2020 at 9:59
  • $\begingroup$ Couldn't you please back up your assertion by some facts? Both Maxwell's equations (1865) and Michelson-Morley experiment (1887) preceded by many decades and was the basis of Special relativity (1905). So no - it doesn't look like you are factually correct. $\endgroup$
    – Serge Dundich
    Commented Aug 15, 2020 at 13:56
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That is an assumption. Every checkable prediction can be checked right. Then we believe that assumption is right until any phenomenon break the prediction from the assumption. The more fundamental assumption is that electromagnetic laws, Maxwell equations, do not change under any inertial reference system. Max equations can lead to the invariance of light speed.

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In the absence of extremely large concentrations of matter and/or energy in the vicinity, the electromagnetic constants of the vacuum (its permittivity and permeability) give rise to the speed of light that we measure by the equation

(speed of light) = (1/(square root of (permittivity x permeability)))

So saying that the speed of light does not vary from place to place or from time to time is another way of saying that those constants don't. Physics does not furnish us with any mechanism for doing so; you'd have to invent one entirely from scratch yourself using magic.

Another way of looking at this is to notice that in Einstein's equation

(energy) = (mass) x (speed of light)^2

if we let c vary with time or location, then E varies, and energy is no longer conserved. Energy conservation holds for the universe we inhabit, which implies c isn't going to change.

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