A Professor of astro physics once told me that the speed of light is actually higher when light passes through cesium as its medium as opposed to vacuum or worse mediums.
I dont really find a consensus that this is true, apparently there where some experiments in around 2000, but it doesnt seem widely accepted – OR it doesnt matter.
The implications of FTL even in such a restricted way, especially with regards to causality should be incredible.
But apparently that is not the case.
How is light moving in a medium like cesium, at > c, not the same as without cesium and not a big deal?
Best Answer
I think you may be confusing group velocity and phase velocity. The thing that we usually call the "speed of light" refers to the group velocity - how fast light can carry information from A to B.
When you look closely at the waves in a medium other than vacuum (which is "no medium"), a packet of finite duration can be thought of as having a mixture of frequencies present. If the medium is dispersive, not all those frequencies travel at the same speed. When you put them all back together, it looks as though there is a wave traveling faster than $c$. However, when you look at the envelope, it always travels at a speed $\le c$.
More on this at this link