I was having a discussion with a friend about light, spacetime, max speeds, etc. I mentioned that light passing through matter travels slower than c. He responded, "Then because there isn't any perfect vacuum [SpaceWeather.com today reports 8 protons/cm^3], light never really travels at c." Thinking about this drove me to Feynmann, Vol I, Chap 31 to read about refractive index.
What I gleaned from a quick reading:
1) All E&M fields (and changes thereto ) propagate at c ... always.
2) Atoms (& electrons attached) oscillate in response to other E&M fields producing their own E&M field.
3) We detect light/E&M radiation which is a sum of all the sources of E&M fields, some of which are strong and others which are ignorable.
4) Light passing through (near??) matter is phase shifted so that it appears to have traveled slower through the matter than it would have without the matter.
Did I understand Feymann properly?
If so, it seems that light passing through any field of matter (even 8 protons/cm^3) would produce other E&M fields which would produce the phase shifted field, giving the appearance of slower than c (even if it's 1 part per 10^15). But the field from any individual source always propogates at c.