It would be useful to know the mechanism by which light is "slowed down" when traveling through a medium. It's been some years since I've studied this stuff and I seem dimly to recall that the light is absorbed and retransmitted by the atoms of the medium, thus causing the delay. I suspect the accuracy of this recollection, as well as the accuracy of the original "factoid". But it leaves me wondering as to the actual mechanism.
The photon or wavefront is passing through a three dimensional array of atoms, consisting mostly of empty space--the central nucleus, and the surrounding electron particle/wave-ness--and the passing light interacts somehow with the resident matter such that the light emerges unaffected save for a delay. Howzat?
With the Bose-Einstein condensate the delay is so long that, if absorbtion/reemission is the mechanism, then it seems like the photon would get sucked up and then held onto for a goodly piece, and then spit out again without any evidence of its sequestration. I don't know. Time to send this theory back in for an upgrade.
I've been reading about quantum stuff lately: EPR, Bell, Aspect, nonlocality, etc. But the more I read the more confused I get. It's supposed to be the other way round. But that's the nature of QM, it turns logic on its head. Hmmmmm.
Best, Jeff Davis
"Everything's hard till you know how to do it."