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I get why, in this model, light isn't really "made of" photons, because photons, by definition have zero mass and travel at $c$, whereas these quasiparticles, if I understand correctly, do have mass and can travel at less than $c$. They're essentially the superposition of photons and the other particles in the medium the photons are interacting with, yes? So clearly they're different from photons. But why does that make them "quasiparticles" instead of just a different type of particle?

Qmechanic
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Mikayla Eckel Cifrese
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1 Answers1

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They're essentially the superposition of photons and the other particles in the medium the photons are interacting with, yes? So clearly they're different from photons.

This is true. In a medium, photons can couple with the excitations (quasiparticles) of that medium. For example, a polariton is a superposition of a photon with a quasiparticle such as an exciton.

But why does that make them "quasiparticles" instead of just a different type of particle?

Quasiparticles are collective excitations that exist in a many body system (like a solid). They are an emergent phenomenon that, unlike fundamental particles like the photon, cannot exist outside of multiparticle systems. See also this PSE post.

joseph h
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  • "Quasiparticles are collective excitations that exist in a many body system (like a solid). They are an emergent phenomenon that, unlike fundamental particles like the photon, cannot exist outside of multiparticle systems." But isn't that also true of, e.g. protons and neutrons? They aren't fundamental particles, but rather arise from the interactions of other particles, yes? But I've never heard them called quasiparticles instead of particles. – Mikayla Eckel Cifrese Jan 16 '23 at 05:38
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    @MikaylaEckelCifrese You may find helpful Roger Vadim's answer to this question: https://physics.stackexchange.com/questions/604029/is-differentiating-particle-and-quasiparticle-meaningless – GiorgioP-DoomsdayClockIsAt-90 Jan 16 '23 at 05:49