  The near-horizon field B of an old black hole is maximally entangled with the
early Hawking radiation R, by unitarity of the S-matrix. But B must be
maximally entangled with the black hole interior A, by the equivalence
principle. Causal patch complementarity fails to reconcile these conflicting
requirements. The system B can be probed by a freely falling observer while
there is still time to turn around and remain outside the black hole.
Therefore, the entangled state of the BR system is dictated by unitarity even
in the infalling patch. If, by monogamy of entanglement, B is not entangled
with A, the horizon is replaced by a singularity or "firewall".
  To illustrate the radical nature of the ideas that are needed, I briefly
discuss two approaches for avoiding a firewall: the identification of A with a
subsystem of R; and a combination of patch complementarity with the
Horowitz-Maldacena final-state proposal.
