Do photons slow down in matter?
''A common explanation that has been provided is that a photon moving
through the material still moves at the speed of c, but when it
encounters the atom of the material, it is absorbed by the atom via an
atomic transition. After a very slight delay, a photon is then
re-emitted. This explanation is incorrect and inconsistent with
This is a quote from '' Do Photons Move Slower in a Solid Medium?''. There then follows a long explanation, why. See also: Warum bewegt sich Licht im Medium langsamer als im Vakuum? (in German)
Wikipedia correctly says in its article Quasiparticle: ''In materials, a photon quasiparticle is a photon as affected by its interactions with the material. In particular, the photon quasiparticle has a modified relation between wavelength and energy (dispersion relation), as described by the material's index of refraction.''
But note that the notions ''quasiparticle'' and ''elementary collective
excitation'' are synonymous. The attempted distinction in the article
between these is spurious.
Both phonons and holes (although classified differently in the article are elementary excitations of the quantum field, hence are collective excitations that cannot be decomposed into smaller excitations. Both are described by creation and annihilation operators, hence behave like elementary particles and are therefore called quasiparticles.
On the other hand, the notion of dressed particles is more specialized. A dressed particle is a quasiparticle that would persist as an elementary particle when the environment were switched off. Thus neither holes nor phonons are dressed particles, while electrons and photons in material are.
Arnold Neumaier (Arnold.Neumaier@univie.ac.at) A theoretical physics FAQ