  A first-principles approach to the unitarity problem for black holes is
systematically explored, based on the postulates of 1) quantum mechanics 2) the
ability to approximately locally divide quantum gravitational systems into
subsystems 3) correspondence with quantum field theory predictions for
appropriate observers and (optionally) 4) universality of new gravitational
effects. Unitarity requires interactions between the internal state of a black
hole and its surroundings that have not been identified in the field theory
description; correspondence with field theory indicates that these are soft. A
conjectured information-theoretic result for information transfer between
subsystems, partly motivated by a perturbative argument, then constrains the
minimum coupling size of these interactions of the quantum atmosphere of a
black hole. While large couplings are potentially astronomically observable,
given this conjecture one finds that the new couplings can be exponentially
small in the black hole entropy, yet achieve the information transfer rate
needed for unitarization, due to the large number of black hole internal
states. This provides a new possible alternative to arguments for large effects
near the horizon. If universality is assumed, these couplings can be described
as small, soft, state-dependent fluctuations of the metric near the black hole.
Open questions include that of the more fundamental basis for such an effective
picture.
