  Consistency between quantum mechanical and general relativistic views of the
world is a longstanding problem, which becomes particularly prominent in black
hole physics. We develop a coherent picture addressing this issue by studying
the quantum mechanics of an evolving black hole. After interpreting the
Bekenstein-Hawking entropy as the entropy representing the degrees of freedom
that are coarse-grained to obtain a semiclassical description from the
microscopic theory of quantum gravity, we discuss the properties these degrees
of freedom exhibit when viewed from the semiclassical standpoint. We are led to
the conclusion that they show features which we call extreme relativeness and
spacetime-matter duality---a nontrivial reference frame dependence of their
spacetime distribution and the dual roles they play as the "constituents" of
spacetime and as thermal radiation. We describe black hole formation and
evaporation processes in distant and infalling reference frames, showing that
these two properties allow us to avoid the arguments for firewalls and to make
the existence of the black hole interior consistent with unitary evolution in
the sense of complementarity. Our analysis provides a concrete answer to how
information can be preserved at the quantum level throughout the evolution of a
black hole, and gives a basic picture of how general coordinate transformations
may work at the level of full quantum gravity beyond the approximation of
semiclassical theory.
