[pgrmdave]

I know that we are able to slow down light by moving it through different mediums, but does it remain the maximum speed through that medium? As in, are we able to move matter through that medium faster than the light can move, or does light move as fast as possible through any given medum? Could we use this to see things in the past in any way?

[zadojla]

I know subatomic particles, such as those given off by radioactive decay can move faster than the speed of light in a medium, like glass. However, they give off energy, called Cherenkov radiation, until they are slowed to the speed of light.

[Qfwfq]

What zadojla says is quite correct, pgrmdave, I fully agree.

The point about causality is essentially a matter of the (local) geometry of space-time. This is what the velocity of light in the vacuum has an essential role in. The effect of a material medium on the propagation of EM waves (or whatever) is really not relevant to the topic, it doesn't change the geometry of space-time.