In summary: a refractive index different from 1, should imply an interaction with matter as well as an absorption.
>>A classical refractive index indeed implies interaction (EM) interaction with matter. It does not require absorption, unless I misunderstand your meaning of absorption. How can light travel through glass if it's absorbed? Or sound with respect to an acoustic refractive index?
Knowing the extremely weak interaction of neutrinos with matter, I can't see how this could lead to a refractive index, even 10^-5 close to 1.
>>I'm sure the authors are not talking classical refractive indices (EM), but something analogous.
If the neutrinos have a FTL light, this can in no way be "explained" by a "refractive index".
The presence of rocks should therefore play no role in the FTL velocity of neutrino, and the same result should be expected for neutrino propagating through vacuum.
>>It is precisely analogous to light propagating through a vacuum vs light propagating through glass. The main egregious, and admittedly speculative, element being a negative refractive index.
Therefore this "refractive index" is even not related to the "rocks", which is a strange syntax for a refractive index.
You could of course call v/c a "refractive index", but this would explain nothing.
I could as well say that I am driving my car at a very low refractive index.
>>This part has lost me entirely. Have you read the paper?