  Suppose we assume that (a) information about a black hole is encoded in its
Hawking radiation and (b) causality is not violated to leading order in gently
curved spacetime. Then we argue that spacetime cannot just be described as a
manifold with a shape; it must be given an additional attribute which we call
`thickness'. This thickness characterizes the spread of the quantum gravity
wavefunctional in superspace -- the space of all 3-geometries. Low energy
particles travel on spacetime without noticing the thickness parameter, so they
just see an effective manifold. Objects with energy large enough to create a
horizon do notice the finite thickness; this modifies the semiclassical
evolution in such a way that we avoid horizon formation and the consequent
violation of causality.
