According to: https://en.wikipedia.org/wiki/Cosmic_microwave_background the CMB (Cosmic Microwave Background) "is faint cosmic background radiation filling all space"
Also, https://en.wikipedia.org/wiki/Vacuum#Outer_space says "no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter"
And https://physics.stackexchange.com/tags/vacuum/info: "This rather theoretical requirement is never achieved in practice, because even if space does not contain any atoms / electrons / nucleons, it does contain a lot of photons and neutrinos. But we still call it a vacuum, as an approximation of the theoretical vacuum."
Then on https://en.wikipedia.org/wiki/Speed_of_light we have "The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 299,792,458 metres per second (approximately 300,000 km/s (186,000 mi/s)). It is exact because by international agreement a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second"
Given that, at least according to Wikipedia, in practice vacuum does not exist, and it's filled with radiation, is it actually possible to measure the speed of light in vacuum? Also, has anyone even observed light in vacuum, ever?
If not, what types of vacuums has light been measured in? Are there any records about this?
Additionally, if true vacuum doesn't even seem to exist and it's filled with radiation (CMB), can we really assume that light doesn't need a medium to propagate? Wouldn't it effectively be propagating through whatever is filling up the space? Would we ever be able to tell the difference?
Best Answer
Science is full of ideals in its wordings. This is one of them.
SI has fixed the speed of light in a vacuum to be 299,792,458 m/s. If there was indeed light propagating through a perfect vacuum, that would be its speed ... because we define it to be.
For practical purposes, however, we need to be able to design experiments with which to measure distances using this definition. We have done these sorts of experiment regularly in high vacuum, on par with or more extreme than the vacuum of interstellar space. When we look at the effect matter has in slowing the speed of light, we find that the difference between its speed in a perfect vacuum and an achievable vacuum is smaller than the measurement error on our experimental devices. Before we had fixed the speed of light to be a constant, we had measured it to within 1 m/s.
How much of an effect does it have? I'm having trouble finding sources to give a definitive answer, but based on the refractive index of hydrogen as a function of pressure, I would expect interstellar levels of hydrogen to slow light by a factor on the order of µm/s. It's very difficult to measure physical things to 8 or 9 digits, and µm/s is 15 digits away from the speed of light, so our measurements in a high vacuum are as usable as if they were in a perfect vacuum.
If, at some point in the future, we discover that this approach is flawed, we will amend it, as has been done several times before—the most recent amendment being fixing the kilogram as a function of several fundamental constants.