  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.
