  If spacetime undergoes quantum fluctuations, an electromagnetic wavefront
will acquire uncertainties in direction as well as phase as it propagates
through spacetime. These uncertainties can show up in interferometric
observations of distant quasars as a decreased fringe visibility. The Very
Large Telescope and Keck interferometers may be on the verge of probing
spacetime fluctuations which, we also argue, have repercussions for cosmology,
requiring the existence of dark energy/matter, the critical cosmic energy
density, and accelerating cosmic expansion in the present era.
