  Gravitational-wave astronomy has the potential to substantially advance our
knowledge of the cosmos, from the most powerful astrophysical engines to the
initial stages of our universe. Gravitational waves also carry information
about the nature of black holes. Here we investigate the potential of
gravitational-wave detectors to test a proposal by Bekenstein and Mukhanov that
the area of black hole horizons is quantized in units of the Planck area. Our
results indicate that this quantization could have a potentially observable
effect on the classical gravitational wave signals received by detectors. In
particular, we find distorted gravitational-wave "echoes" in the post-merger
waveform describing the inspiral and merger of two black holes. These echoes
have a specific frequency content that is characteristic of black hole horizon
area quantization.
