Jerry Stuckle wrote in news:m19tmu$snl$1@dont-
email.me:

Well, one thing - the speed of light is not actually a constant. It is
a constant in vacuum, but in other materials it is slower. So if the
friction/viscosity effects of glass were ignored, for instance, you'd
still have a maximum velocity. It would just be rather significantly
less than in a vacuum.


That's true.. refractive index and such. I read recently that a negative
refractive index can exist, but I don't remember how that works. The thing
is, once the light leaves the glass and returns to air or vacuum, normal
'speed' is instantly resumed. Maybe that Bose Einstein condensate (it was
definitely such, though the same name occurs in other things even less
understood by me) had basically just a humungous refractive index, but I
don't think I read of any obvious relation to refraction either in that
slowed-light report, so I tend to have a 'watch-this-space' view in its
general direction. I suspect it will take several reports of new things
before some pattern emerges than I will understand.

I'm wary of thinking of refractive index's effects as friction or viscosity.
I suspect that those notions relate to things with rest mass and electrical
charges and don't model closely to what light is up to.

At the risk of sounding silly, I think Terry Pratchett had a point when he
said that wherever light gets to, the dark is already there, waiting for it.
Personally I think that neither exists without the other, and the only reason
we can posit 'nothing' is because we can posit 'a thing'. Whether such talk
obstructs or helps science I am never entirely sure.