In the formulation of Special Relativity (SRT) Einstein assumed that the atmosphere of the earth extended only to a certain altitude, and that beyond this space was a total vacuum.
“Half a century ago, (i.e. 1950′s) most people visualised our planet as a solitary sphere travelling in a cold, dark vacuum of space around the Sun.”1
Accordingly he firstly further assumed (out of sheer necessity) that light needed no medium for transmission, and secondly that, rather than it accelerating to infinity within this vacuum as some suggested, it would travel in it at a velocity as indicated by experiments using the moons of Jupiter as a yardstick, and in SRT accordingly used this as a basis for the suggestion that light speed is invariable throughout the universe.
As kinetic theory then stated that the gases at sea level were volumetrically 99.9% a vacuum, he also assumed that the velocity of light, in passing from the supposed vacuum of space, would not be inhibited by the meagre distribution of atomic matter within the earth’s atmosphere, and so the velocity here would also be c.
It is now known that a vacuum is ‘a philosophical concept with no basis in reality’2, and that in space throughout the universe there is a consistent distribution of matter at varying densities.
“Absolute vacua cannot be created or found. — for example interplanetary space is not a vacuum, but has density ρ = 10−29g.cm.−3 or 10−5 protons cm.−3. The Universe itself is not a vacuum but has various densities associated with itself -” 3
Thus it is now known that space is not a perfect vacuum, and the cosmic microwave background shows that there is a consistent distribution of matter throughout the vast regions of inter-galactic and inter-stellar spaces in the universe.