  We model vacuum fluctuations in quantum gravity with a scalar field,
characterized by a high occupation number, coupled to the metric. The
occupation number of the scalar is given by a thermal density matrix, whose
form is motivated by fluctuations in the vacuum energy, which have been shown
to be conformal near a light-sheet horizon. For the experimental measurement of
interest in an interferometer, the size of the energy fluctuations is fixed by
the area of a surface bounding the volume of spacetime being interrogated by an
interferometer. We compute the interferometer response to these "geontropic"
scalar-metric fluctuations, and apply our results to current and future
interferometer measurements, such as LIGO and the proposed GQuEST experiment.
