  We derive and critically examine the consequences that follow from the
formation of a regular black or white hole horizon in finite time of a distant
observer. In spherical symmetry, only two distinct classes of solutions to the
semiclassical Einstein equations are self-consistent. Both are required to
describe the formation of physical black holes and violate the null energy
condition in the vicinity of the outer apparent horizon. The near-horizon
geometry differs considerably from that of classical solutions. If
semiclassical physics is valid, accretion into a black hole is no longer
possible after the horizon has formed. In addition, the two principal
generalizations of surface gravity to dynamical spacetimes are irreconcilable,
and neither can describe the emission of nearly-thermal radiation. Comparison
of the required energy and timescales with established semiclassical results
suggests that if the observed astrophysical black holes indeed have horizons,
their formation is associated with new physics.
