# How much is the maximum posible information travel speed in medium?

Light has slower speed in medium. Should we consider this when we use special relativity in atmosferic(or any other type of medium) motions? If we should use vacuum light speed, does it mean photons in medium are no longer the unreachable, fastest particles?

• Probably related reading: en.wikipedia.org/wiki/Cherenkov_radiation. See also some of these posts (e.g., this one) Commented Jan 22, 2016 at 17:54
• One can have particles traveling faster in a medium than the local speed of light (but never faster than the speed of light in vacuum). If the particles are charged they will generate Cherenkov radiation. What "travels" in optical media are not actually photons but quasi-particle states that are made up of strongly coupled photons and the local polarizations of atoms/molecules/the lattice/electrons in the medium. These quasi-particles do not obey Lorentz symmetry. Commented Jan 22, 2016 at 17:54
• The presence of a medium breaks Lorentz invariance, but the laws of physics are still Lorentz and rotationally invariant. This is analogous to the laws of physics being invariant under rotations, this obviously doesn't stop you from driving your car in a particular direction. What it means is that if you rotate everything then the outcome of an experiment follows from the original situation, scalars remain invariant, and vectors transform under the rotation group. In case of the medium, the entire medium will move under a Lorentz transform. Commented Jan 22, 2016 at 17:54