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I saw another question on here concerning light speeds and specifically concerning different types of light waves etc. The typical answer dealt with the speed of light in a vacuum but how do we know that space and especially deep space is truly a vacuum? I mean Einstein implies space is able to be contorted or bent which would beg the question is it actually made of something? Space is also full of matter including dark matter so how can we tell these things don't affect light such its speed? For all we know, light could be twice as fast in a true vacuum.

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Space is not a complete vacuum. On average there's about 1 atom per cm3. The density varies with the location. Dark matter is additional to that (about 20 times more mass).

We have checked the speed of light at densities ranging from normal densities around us (of the order of $10^{23}$ atoms per $cm^3$, down to the density of space (1 atom per $cm^3$). Under all those densities we have found that the speed of light varies with the refractive index of the medium. The speed is slightly lower inside matter. Vacuum has an RI = 1, and the speed of light in vacuum is $299792.458 km/s$. Water has RI = 1.33. Hence the speed of light in water is $3/4$ of the speed in vacuum, or $225,000 km/s$. Air has RI = 1.0002772 and the speed of light in air is $299,704 km/s$, very close to vacuum. In general, the lower the density of the material, the more the RI tends towards 0.

Because of the extremely low matter density of space, the true RI of space will be extremely close to 1, and the speed of light is indistinguishable from complete vacuum. As for dark matter, until we know what it is made of, we won't be able to tell how it affects light. So it is possible that dark matter could have an RI smaller than 1, and light would go faster. However, because of its extremely low density, any effect will be minimal.

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The value of the speed of light in vacuum doesn't come from measurements - It comes from the laws of electrodynamics. It is another thing entirely to verify that this is indeed the speed of light in vacuum.

In electromagnetic theory, there are two quantities known as the permittivity of free space $\epsilon_0$ and another one known as the permeability of free space $\mu_0$. These two quantites are what determine the value of the speed of light in vacuum..

Given these two, you obtain the speed of light in vacuum through the equation

$$ c = \frac{1}{\sqrt{\mu_0 \epsilon_0}} $$

where $c$ is then your speed of light. The values of $\mu$ and $\epsilon$ vary with material, and hence the speed of light will also vary with material.

Therefore debates about whether space is truly a vacuum or not are not related to what we think is the true value of the speed of light in a vacuum.

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