c is the speed of light in a vacuum. It is impossible to send information faster than c. x-rays are also just light so everything I said before also applies to x-rays.

I’m not very familiar with how light interacts with materials since it wasn’t covered in the electromagnetism class that I took, so the next part might be wrong. The speed of light is calculated by looking at Maxwell’s laws of electromagnetism and recognizing that it is the equation for a wave. The speed is then calculated as c = 1/sqrt(epsilon_0 * mu_0) where epsilon_0 and mu_0 are the vacuum permittivity and the vacuum permeability. Inside a material, I think the permittivity and permeability are different and thus the speed of light as determined by Maxwell’s equations would be different. I know its not possible for the speed (group velocity) of light in a material to be faster than the speed of light in a vacuum because otherwise, you could send information faster than c, which is impossible.

If you had a material with no light passing through it, then you shined a light through it, it would only move at the group velocity. The whole “faster than light” thing refers to the phase velocity, but you can’t have a phase velocity if the light hasn’t gotten there yet.

Saying that gamma rays (or any light in general) moves faster than light is technically true but very misleading and relies heavily on when you define “speed” as.

3Blue1Brown has a good video on why the phase velocity is different from the group velocity if you’re interested in why. The faster than light phase velocity happens when the “phase kicks” are so delayed that the phase appears to move forward.

Its because “speed” can refer to two different things in this context. Group velocity is the speed that your light waves can send a message it. This cannot move faster than light. Phase velocity on the other hand is the speed of the peaks of the wave. This can move faster but its not possible to send a signal through this.