Impact of a plasma on the relaxation of black holes

Our Universe is permeated with interstellar plasma, which prevents propagation of low-frequency electromagnetic waves. Here, we show that two dramatic consequences arise out of such suppression; (i) if plasma permeates the light ring of a black hole, electromagnetic modes are screened entirely from the gravitational-wave signal, changing the black hole spectroscopy paradigm; (ii) if a near vacuum cavity is formed close to a charged black hole, as expected for near equal-mass mergers, ringdown ``echoes'' are excited. The amplitude of such echoes decays slowly and could thus serve as a silver bullet for plasmas near charged black holes.