How is speed of light in glass defined in QM/QED?

[LarryS]

How is the "speed of light in glass", or any other non-vacuum classical medium, defined at the quantum level? Between atoms, the photons are traveling exactly at c, right?

As always, thanks in advance.

 

[HallsofIvy]

The "speed of light" in glass is defined in Quantum Physics just as it is in an other field of physics- the distance the light travels through glass divided by the time it takes to travel that distance. In terms of "distance between atoms" that would be an average of the time taken passing "through" an atom and the time taken between atoms. Of course, this varies for different kinds of glass.

 

[atyy]

The speed of light in vacuum is always c in QED. The speed of light in glass is an emergent property in QED, as is glass itself. See ZappaerZ's explanation: https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/ .

Feynman also has an accessible explanation, but let me see if I can find it.

 

[DrDu]

The speed of light is defined via the dispersion relation between k and omega where the propagator of the photons has a pole. In a medium, a photon will mix with excitonic (electron hole pair) excitations which don't move as fast as a free photon, hence the interacting propagator will have poles corresponding to a lower speed than for photons in vacuo.