The need for a consistent quantum evolution for black holes has led to
proposals that their semiclassical description is modified not just near the
singularity, but at horizon or larger scales. If such modifications extend
beyond the horizon, they influence regions accessible to distant observeration.
Natural candidates for these modifications behave like metric fluctuations,
with characteristic length and time scales set by the horizon radius. We
investigate the possibility of using the Event Horizon Telescope to observe
these effects, if they have a strength sufficient to make quantum evolution
consistent with unitarity. We find that such quantum fluctuations can introduce
a strong time dependence for the shape and size of the shadow that a black hole
casts on its surrounding emission. For the black hole in the center of the
Milky Way, detecting the rapid time variability of its shadow will require
non-imaging timing techniques. However, for the much larger black hole in the
center of the M87 galaxy, a variable black-hole shadow, if present with these
parameters, would be readily observable in the individual snapshots that will
be obtained by the Event Horizon Telescope.Comment: To appear in Phys. Rev. D. For animations, see
http://xtreme.as.arizona.edu/~dpsaltis/?page_id=275