Can Light Travel Faster Than c in Diamagnetic Materials?

[conquerer7]

The speed of light in vacuum is 1 / sqrt(mu_0 e_0). In matter, this is modified using relative permittivity and permeability, so the speed of light in matter is c / sqrt(k_m k_e).

In diamagnetic materials, isn't k_m < 1? Then we'd have light going faster than 3 * 10^8 m/s in, say, nitrogen gas, which is diamagnetic. What's wrong?

 

[dban]

You're mostly correct, except for the assumption that C would be the variable that changes. C will stay constant, and relative permittivity will go up accordingly.

 

[mfb]

so the speed of light in matter is c / sqrt(k_m k_e).

This is the phase velocity, not the group velocity. And don't forget the k_e-part, which is usually dominant in matter.

There are materials with a phase velocity larger than the speed of light in vacuum and even materials with negative phase velocity. However, the group velocity (the thing which can carry energy and information, neglecting losses here) is always smaller than that.