[Boogieman]

The speed of light in a vacuum is what is universally invariant. The speed of light in a medium does indeed vary depending on the medium. This leads to a peculiar phenomenon, where some particles can exceed the local speed of light when it is traveling through a medium. When that happens, the particles begin emitting "cherenkov radiation", which is kind of an electromagnetic equivalent of a "sonic boom".

[Zeneta]

Interesting.

I have a feeling that both “Dark Matter and Dark Energy” are a lot darker than most people realize.