[2][3] Ever since Stephen Hawking suggested information is lost in an evaporating black hole once it passes through the event horizon and is inevitably destroyed at the singularity, and that this can turn pure quantum states into mixed states, some physicists have wondered if a complete theory of quantum gravity might be able to conserve information with a unitary time evolution.
Leonard Susskind, Lárus Thorlacius, and John Uglum[1] proposed a radical resolution to this problem by claiming that the information is both reflected at the event horizon and passes through the event horizon and cannot escape, with the catch being no observer can confirm both stories simultaneously.
According to the external observer, infalling information heats up the stretched horizon, which then reradiates it as Hawking radiation, with the entire evolution being unitary.
This isn't to say there are two copies of the information lying about — one at or just outside the horizon, and the other inside the black hole — as that would violate the no-cloning theorem.
It has been suggested that validity of effective field theory near the horizon combined with the monogamy of entanglement implies the existence of an AMPS "firewall",[4] where high energy, short wavelength photons are present in the horizon.