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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.