  The observation of electromagnetic radiation emitted or absorbed by matter
was instrumental in revealing the quantum properties of atoms and molecules in
the early XX century, and constituted a turning-point in the development of the
quantum theory. Quantum mechanics changes dramatically the way radiation and
matter interact, making the probability of emission and absorption of light
strongly frequency dependent, as clearly manifested in atomic spectra. In this
essay, we advocate that gravitational radiation can play, for the quantum
aspects of black holes, a similar role as electromagnetic radiation did for
atoms, and that the advent of gravitational-wave astronomy can bring this
fascinating possibility to the realm of observations.
