  The detection of gravitational waves (GWs) from black hole (BH) mergers
provides an inroad toward probing the interior of astrophysical BHs. The
general-relativistic description of the BH interior is that of empty spacetime
with a (possibly) singular core. Recently, however, the hypothesis that the BH
interior does not exist has been gaining traction, as it provides a means for
resolving the BH information-loss problem. Here, we propose a simple method for
answering the following question: Does the BH interior exist and, if so, does
it contain some distribution of matter or is it mostly empty? Our proposal is
premised on the idea that, similar to the case of relativistic, ultra-compact
stars, any BH-like object whose interior has some matter distribution should
support fluid modes in addition to the conventional spacetime modes. In
particular, the Coriolis-induced Rossby (r-) modes, whose spectrum is mostly
insensitive to the composition of the interior matter, should be a universal
feature of such BH-like objects. In fact, the frequency and damping time of
these modes are determined by only the object's mass and speed of rotation. The
r-modes oscillate at a lower frequency, decay at a slower rate and produce
weaker GWs than do modes of the spacetime class. Hence, they imprint a
model-insensitive signature of a non-empty interior in the GW spectrum
resulting from a BH merger. We find that future GW detectors, such as Advanced
LIGO with its design sensitivity, have the potential of detecting such r-modes
if the amount of GWs leaking out quantum mechanically from the interior of a
BH-like object is sufficiently large.
