  We show that vacuum fluctuations of the stress-energy tensor in
two-dimensional dilaton gravity lead to a sharp focusing of light cones near
the Planck scale, effectively breaking space up into a large number of causally
disconnected regions. This phenomenon, called "asymptotic silence" when it
occurs in cosmology, might help explain several puzzling features of quantum
gravity, including evidence of spontaneous dimensional reduction at short
distances. While our analysis focuses on a simplified two-dimensional model, we
argue that the qualitative features should still be present in four dimensions.
