  We show that acoustic crystalline wave gives rise to an effect similar to
that of a gravitational wave to an electron gas. Applying this idea to a
two-dimensional electron gas in the fractional quantum Hall regime, this allows
for experimental study of its dynamical gravitational response. To study such
response we generalize Haldane's geometrical description of fractional quantum
Hall states to situations where the external metric is time-dependent. We show
that such time-dependent metric (generated by acoustic or effective
gravitational wave) couples to collective modes of the system, including a
quadrapolar mode similar to graviton at long wave length, and magneto-roton at
finite wave length. Energies of these modes can be revealed in spectroscopic
measurements. We argue that such gravitational probe provides a potentially
highly useful alternative probe of quantum Hall liquids, in addition to the
usual electromagnetic response.
