The final rush of matter toward a compact object generates gravitational waves — ripples in spacetime. This 'plunge' sounds different for a black hole and its impostor — a superdense object that mimics a black hole but doesn't swallow everything irretrievably.
If the center holds a real black hole, the plunge signal is dull and featureless. If it's an impostor, a ringing appears: at low frequencies, the spectrum reveals a comb of evenly spaced resonances, like a bell ringing out pure tones. At high frequencies, this melody abruptly cuts off, like a record getting stuck. Surprisingly, from this 'ringing,' we can determine not only the fact of a fake but also the internal structure of the impostor. By stacking recordings of hundreds of plunges, astronomers amplify weak signals, much like a long-exposure photograph.
Black holes are predictable: according to the 'no-hair theorem,' they have only three parameters — mass, charge, and spin. Impostors, however, can have complex internal structures, which produces their distinctive ringing.
In the movie 'Interstellar,' the protagonist plunges into a black hole seeking data. Perhaps in the future, gravitational detectors will warn astronauts in advance whether a black hole is real or a dangerous impostor by analyzing its 'ringing.'