  Given the insight steming from string theory, the origin of the black-hole
(BH) information puzzle is traced back to the assumption that it is physically
meaningful to trace out the density matrix over negative-frequency Hawking
particles. Instead, treating them as virtual particles necessarily absorbed by
the BH in a manner consistent with the laws of BH thermodynamics, and tracing
out the density matrix only over physical BH states, the complete evaporation
becomes compatible with unitarity.
