  Here, we develop a comprehensive quantum theory for the phenomenon of quantum
friction. Based on a theory of macroscopic quantum electrodynamics for unstable
systems, we calculate the quantum expectation of the friction force, and link
the friction effect to the emergence of system instabilities related to the
Cherenkov effect. These instabilities may occur due to the hybridization of
particular guided modes supported by the individual moving bodies, and
selection rules for the interacting modes are derived. It is proven that the
quantum friction effect can take place even when the interacting bodies are
lossless and made of nondispersive dielectrics.
