  It is commonly believed that light cannot couple to the collective
excitations of the fractional quantum Hall effect (FQHE). This assumption
relies on Kohn's theorem that states that electron-electron interactions
decouple from homogeneous electromagnetic fields due to Galilean invariance.
Here, we demonstrate that in finite systems light-matter coupling beyond the
dipole approximation breaks Kohn's theorem, and enables the coupling of cavity
photons to the plasmonic edge modes of the FQHE. We derive the coupling using
the FQHE bulk-boundary correspondence and predict the formation of
experimentally detectable plasmon polaritons. In conjunction with recent
experiments, we find that a single cavity mode leaves the system's topological
protection intact. Interestingly, however, a multimode cavity mediates plasmon
backscattering and effectively transforms the edges of the 2D FQHE into a 1D
wire. Such cavity-meditated nonlocal backscattering bodes the breakdown of the
topological protection in the regime of ultra-strong photon-plasmon coupling.
Our analytical framework and findings pave the way for investigating the
topological order of the FQHE via optical spectroscopic probes as well as
provide new opportunities to control FQHE edge excitations using light.
