The question is, how exactly it is that light can travel slower when not in vaccum, and how for example Bose-Einstein condensates can be used to slow light a lot, or even stopping it? http://www.osa-opn.org/abstract.cfm?URI=OPN-16-5-30 [Broken] I've read: http://en.wikipedia.org/wiki/Photon#Photons_in_matter http://en.wikipedia.org/wiki/Dispersion_(optics) http://en.wikipedia.org/wiki/Phonon http://en.wikipedia.org/wiki/Exciton http://en.wikipedia.org/wiki/Polariton http://en.wikipedia.org/wiki/Quantum_superposition What I don't understand from the discription given by wikipedia is when it says, syllogistically: In a material, photons couple to the excitations of the medium. "Coupling" means here that photons can transform into these excitations (that is, the photon gets absorbed and medium excited, involving the creation of a quasi-particle) and vice versa (the quasi-particle transforms back into a photon, or the medium relaxes by re-emitting the energy as a photon). However, as these transformations are only possibilities, they are not bound to happen and what actually propagates through the medium is a polariton; That is the quantum-mechanical superposition of the energy-quantum, i.e. the q-m superposition of the photon; being a photon and of it being a quasi-particle matter excitations. That is; the linear combination of the two eingenstate of the photon and the electron. Thus, a polariton is the result of the mixing of a photon with an excitation of a material. The most discussed types of polaritons are [...]; exciton-polaritons, resulting from coupling of visible light with an exciton; The linear combination of two or more eigenstates results in quantum superposition of two or more values of the quantity. An exciton is a bound state of an electron [its eingenstate???] and an imaginary particle called an electron hole in an insulator or semiconductor, and such is a Coulomb correlated electron-hole pair. It is an elementary excitation, or a quasiparticle of a solid. So what does it say? There IS a photon coupling (1), which is photons transforming into polaritons (3) by the superpositioning (4) of the electron- (5) and quantum/photon-eigenstates (2), into the quasi-particle: "exciton-polaritons" (3). What bothers me is that it also states that the photon absorptions/transformations doesn't necessaryly happen (2). The quasi-particle is the polariton (probably named from the electron-polarity from its spin?) Have I understood it correctly? It begs the question - what is it that makes photons/exciton-polaritons speed slower because of the superpositioning? Can you say that the photon enters the medium, interferes with the electron(s) to create a matter excitation/an exciton, whereafter the same photon's eigenstate becomes quantum mechanically superpositioned with the eigenstate of the quasi-particle-exciton, forming the exciton-polariton, which cannot go above c, because it has rest-mass? Has it got something to do with: "The propagation speed is the derivative of the dispersion relation with respect to momentum."? Has it got rest-mass at all, or is it just to be taken metaphorically? Post scripum: I'd love maths, to explain it, as long as I can follow it upwards, from the level of an IB Mathemathics Higher Level education.