  We present a coherent picture of the quantum mechanics of black holes. The
picture does not require the introduction of any drastically new physical
effect beyond what is already known; it arises mostly from synthesizing and
(re)interpreting existing results in appropriate manners. We identify the
Bekenstein-Hawking entropy as the entropy associated with coarse-graining
performed to obtain semiclassical field theory from a fundamental microscopic
theory of quantum gravity. This clarifies the issues around the unitary
evolution, the existence of the interior spacetime, and the thermodynamic
nature in black hole physics--any result in semiclassical field theory is a
statement about the maximally mixed ensemble of microscopic quantum states
consistent with the specified background, within the precision allowed by
quantum mechanics. We present a detailed analysis of information transfer in
Hawking emission and black hole mining processes, clarifying what aspects of
the underlying dynamics are (not) visible in semiclassical field theory. We
also discuss relations between the black hole entropy and the entanglement
entropy across the horizon. We then extend our discussions to more general
contexts in quantum gravity. The subjects include extensions to de Sitter and
Minkowski spaces and implications for complementarity and cosmology, especially
the eternally inflating multiverse.
