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?
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4$\begingroup$ No, phonons are quasiparticles related to the vibrations of the crystal lattice (like when making sound) and have nothing to do with light. There are other quasiparticles related to the interaction of light and matter like the polariton, plasmariton (plasmon and photon), exciton-polariton (exciton and photon) and so on. You can even have a quasiparticle of both photons and phonon called a phonon-polariton. $\endgroup$– MauricioCommented Feb 21, 2023 at 22:49
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1$\begingroup$ You are basically correct except these excitations are not called phonons in standard terminology. $\endgroup$– Andrew SteaneCommented Feb 21, 2023 at 23:03
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1$\begingroup$ Please see Wikipedia regarding the meaning of the word “phonon”. Calling it a “quasiparticle of light” is misleading. $\endgroup$– GhosterCommented Feb 22, 2023 at 0:59
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2$\begingroup$ Hi Mikayla :-) You are quite correct. When we have strong interaction between light and a medium we get a quasiparticle called a polariton that does have a mass. In principle the same applies to weaker interactions such as light passing through glass, although the quasiparticle description is not a terribly useful one for weaker interactions. $\endgroup$– John RennieCommented Feb 22, 2023 at 7:30
1 Answer
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
