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Re: [Phys-L] speed c

On 2013, Mar 14, , at 13:42, Bill Nettles <> wrote:

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 [ 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.

I'd read about slow light and see F. et aliorum explanation applies.

bc read this in that former artist's text also.