Black holes, the Wheeler-DeWitt equation and the semi-classical   approximation
  The definition of matter states on spacelike hypersurfaces of a 1+1 dimensional black hole spacetime is considered. Because of small quantum fluctuations in the mass of the black hole, the usual approximation of treating the gravitational field as a classical background on which matter is quantized, breaks down near the black hole horizon. On any hypersurface that captures both infalling matter near the horizon and Hawking radiation, a semiclassical calculation is inconsistent. An estimate of the size of correlations between the matter and gravity states shows that they are so strong that a fluctuation in the black hole mass of order exp[-M/M_{Planck}] produces a macroscopic change in the matter state. (Talk given at the 7th Marcel Grossmann Meeting on work in collaboration with E. Keski-Vakkuri, G. Lifschytz and S. Mathur.) 
Miguel E. Ortiz