  Quantum gravity theory is untested experimentally. Could it be tested with
tabletop experiments? While the common feeling is pessimistic, a detailed
inquiry shows it possible to sidestep the onerous requirement of localization
of a probe on Planck length scale. I suggest a tabletop experiment which, given
state of the art ultrahigh vacuum and cryogenic technology, could already be
sensitive enough to detect Planck scale signals. The experiment combines a
single photon's degree of freedom with one of a macroscopic probe to test
Wheeler's conception of "quantum foam", the assertion that on length scales of
the order Planck's, spacetime is no longer a smooth manifold. The scheme makes
few assumptions beyond energy and momentum conservations, and is not based on a
specific quantum gravity scheme.
