  Bringing gravity into a quantum-mechanical framework is likely the most
profound remaining problem in fundamental physics. The "unitarity crisis" for
black hole evolution appears to be a key facet of this problem, whose
resolution will provide important clues. Investigating this raises the
important structural question of how to think about subsystems and localization
of information in quantum gravity. Paralleling field theory, the answer to this
is expected to be an important ingredient in the mathematical structure of the
theory. Perturbative gravity results indicate a structure different from that
of quantum field theory, but suggest an avenue to defining subsystems. If black
holes do behave similarly to familiar subsystems, unitarity demands new
interactions that transfer entanglement from them. Such interactions can be
parameterized in an effective approach, without directly addressing the
question of the fundamental dynamics, whether that is associated with quantum
spacetime, wormholes, or something else. Since such interactions need to extend
outside the horizon, that raises the question of whether they can be
constrained, or might be observed, by new electromagnetic or gravitational wave
observations of strong gravity regions. This note overviews and provides
connections between these developments.
