Spacetime has a `thickness'

Understand

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.