  To ask a question about a black hole in quantum gravity, one must restrict
initial or boundary data to ensure that a black hole is actually present. For
two-dimensional dilaton gravity, and probably a much wider class of theories as
well, the imposition of a "stretched horizon" constraint alters the algebra of
symmetries at the horizon, introducing a central term. Standard conformal field
theory techniques can then then be used to obtain the asymptotic density of
states, reproducing the Bekenstein-Hawking entropy. The microscopic states
responsible for black hole entropy can thus be viewed as "would-be pure gauge"
states that become physical because the symmetry is altered by the requirement
that a horizon exist.
