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If I understand correctly, per special relativity, anything that travels at a speed of $c$ must be massless and conversely, anything massless must travel at precisely $c$ in akl reference frame. We usually say light is massless, and yet, it seems to travel at less than c when moving through water, glass, etc. My understanding is that there are a couple different ways to think about what's happening at a microscopic level, one of which is to say that individual photons always travel at exactly $c$, but that when light interacts with matter all the crazy wave interference results in "collective excitations" that we call them quasiphotons, and these quasiparticles have different properties from photons because they aren't just excitations in the EM field, but in the "combination" (for lack of a better term) of the EM field and the electron field (since most photon interactions would presumably be with electrons). If that's so, it seems these quasiparticles should have at least some small amount of mass, since they're moving at less than $c$, with the specific mass value depending on the refractive index of the medium. Is that correct? If not, where is the flaw in my reasoning?

Qmechanic
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Mikayla Eckel Cifrese
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1 Answers1

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If that's so, it seems these quasiparticles should have at least some small amount of mass, since they're moving at less than c, with the specific mass value depending on the refractive index of the medium. Is that correct? If not, where is the flaw in my reasoning?

Mass of quasiparticles is usually determined by the shape of their dispersion relation near minimum - if it is approximately parabolic, the mass is simply the curvature at the minimum, whereas in case of linear dispersion relation, the particles are massless. As the examples of the latter one could mention acoustic phonons and electrons/holes in graphene (which have conical dispersion relation at the point where the conduction and the valence bands touch each other.)

So quasiparticles are not necessarily massive. On the other hand, quasiparticles corresponding to photons propagating in a media can sometimes be massive, as discussion in the second answer linked below.

References:
Are photons inside the media massive? If yes, why there is no Meissner effect?
Photon effective mass in plasma

Roger V.
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