  Black hole complementarity requires that the interior of a black hole be
represented by the same degrees of freedom that describe its exterior.
Entanglement plays a crucial role in the reconstruction of the interior degrees
of freedom. This connection is manifest in "two-sided" eternal black holes. But
for real black holes which are formed from collapse there are no second sides.
The sense in which horizon entropy is entanglement entropy is much more subtle
for one-sided black holes. It involves entanglement between different parts of
the near-horizon system. As a one-sided black hole evaporates the entanglement
that accounts for its interior degrees of freedom disappears, and is gradually
replaced by entanglement with the outgoing Hawking radiation. A principle of
"transfer of entanglement" can be formulated. According to the argument of
Almheiri, Marolf, Polchinski and Sully, it is when the transfer of entanglement
is completed at the Page time, that a firewall replaces the horizon.
Alternatives to firewalls may suffer contradictions which are similar to those
of time travel. The firewall hypothesis would be similar to Hawking's
chronology protection conjecture.
