I've never heard of "Polaritons" or "Excitons"?
One of those theoretical ideas that have no practical realisation?
N.
Well, they do not exist in the condensed matter model to explain certain interactions. Room-temperature Polaritons had not been observed in 2016, after they had not been detected in plasma before.
Confused?
Just note, that those are quasi-particles. They actually don't exist except for making complex math easier. A quasi-particle is a number of particles (one or more) behaving like a different single particle. For example an electron moving through dense matter has a quasi-particle brother, the quasi-electron with a slightly higher mass than the actual electron.
Polaritons are interactions between photons and other quasi-particles.
Excitons are quasi-particles in semiconductors created by binding negative electrons and positive holes, explaining how energy can be transported through them without motion of a charge, as the excitons are electrically neutral - they are for example used for explaining how photosynthesis works on the protein level.
While a quasi-particle acts like a real particle in the theory and can be observed like a real particle, in reality they are not existing - the particles and interactions that create them do.
