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

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SUMMARY

The speed of light in glass is defined in Quantum Physics as the distance light travels through the medium divided by the time taken for that journey. This speed varies depending on the type of glass, as it incorporates both the time light spends passing through atoms and the time between them. In Quantum Electrodynamics (QED), the speed of light in a vacuum remains constant at c, while in glass, it is an emergent property influenced by interactions with excitonic excitations, resulting in a lower effective speed. The dispersion relation between wave vector (k) and angular frequency (omega) plays a crucial role in this definition.

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  • Quantum Electrodynamics (QED) principles
  • Understanding of dispersion relations in physics
  • Knowledge of excitonic excitations in solid-state physics
  • Familiarity with the concept of photon propagation in various media
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  • Study the dispersion relation between k and omega in detail
  • Research excitonic effects in solid-state physics
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  • Investigate the properties of various types of glass and their impact on light speed
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Physicists, materials scientists, and students interested in the quantum behavior of light in different media, particularly those studying Quantum Electrodynamics and solid-state physics.

LarryS
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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.
 
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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.
 
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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.
 
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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.
 

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