  Black and white holes play remarkably contrasting roles in general relativity
versus observational astrophysics. While there is overwhelming observational
evidence for the existence of compact objects that are "cold, dark, and heavy",
which thereby are natural candidates for black holes, the theoretically viable
time-reversed variants -- the "white holes" -- have nowhere near the same level
of observational support. Herein we shall explore the possibility that the
connection between black and white holes is much more intimate than commonly
appreciated. We shall first construct "horizon penetrating" coordinate systems
that differ from the standard curvature coordinates only in a small
near-horizon region, thereby emphasizing that ultimately the distinction
between black and white horizons depends only on near-horizon physics. We shall
then construct an explicit model for a "black-to-white transition" where all of
the nontrivial physics is confined to a compact region of spacetime -- a
finite-duration finite-thickness, (in principle arbitrarily small), region
straddling the naive horizon. Moreover we shall show that it is possible to
arrange the "black-to-white transition" to have zero action -- so that it will
not be subject to destructive interference in the Feynman path integral. This
then raises the very intriguing possibility that astrophysical black holes
might be interpratable in terms of a quantum superposition of black and white
horizons.
