  In canonical quantum gravity, the wave function for a hypersurface inside a
Schwarzschild black hole can be obtained by solving the Wheeler-DeWitt
equation. What is of prime importance is the behavior of the wave function for
the future boundary near the singularity, and the DeWitt boundary condition
implies that it should vanish here. In this paper, we provide several
generalizations, and new interpretations, of the DeWitt boundary condition.
First, we summarize existing works on the wave function inside the black hole
to justify the DeWitt boundary condition. Next, we investigate the wave
function for the collapsing null shell to show that due to the reflection
symmetry in space and time, there exists a destructive interference near the
singularity and hence a vanishing boundary condition can be natural. If we
extend this point of view to the black hole spacetime itself, then the DeWitt
boundary condition is equivalent to saying that there exists a symmetric
anti-black hole contribution, such that eventually these two geometries are
annihilated-to-nothing near the quantum transition surface. This symmetric
model can be realized within black hole models of loop quantum gravity with a
novel interpretation for the arrow(s) of time.
