According to
quantum field theory in curved spacetime, a
single emission of
Hawking radiation involves two mutually
entangled particles. The outgoing particle escapes and is emitted as a quantum of Hawking radiation; the infalling particle is swallowed by the black hole. Assume that a black hole formed a finite time in the past and will fully evaporate away in some finite time in the future. Then, it will only emit a finite amount of information encoded within its Hawking radiation. For an old black hole that has crossed the half-way point of evaporation, general arguments from quantum-information theory by
Page and
Lubkin suggest that the new Hawking radiation must be entangled with the old Hawking radiation. However, since the new Hawking radiation must also be entangled with degrees of freedom behind the horizon, this creates a
paradox: a principle called "
monogamy of entanglement" requires that, like any quantum system, the outgoing particle cannot be fully entangled with two independent systems at the same time; yet here the outgoing particle appears to be entangled with both the infalling particle and, independently, with past Hawking radiation. However, it is now accepted that an additional tacit assumption in the monogamy paradox was that of
locality. A common view is that theories of quantum gravity do not obey exact locality, which leads to a resolution of the paradox.
The "firewall" resolution to the paradox Some scientists suggest that the entanglement must somehow get immediately broken between the infalling particle and the outgoing particle. Breaking this entanglement would release large amounts of energy, thus creating a searing "black hole firewall" at the black hole event horizon. This resolution requires a violation of Einstein's equivalence principle, which states that free-falling is indistinguishable from floating in empty space. This violation has been characterized as "outrageous"; theoretical physicist
Raphael Bousso has complained that "a firewall simply can't appear in empty space, any more than a brick wall can suddenly appear in an empty field and smack you in the face." posited that the interior of the black hole was described by the same degrees of freedom as the Hawking radiation. This resolves the monogamy paradox by identifying the two systems that the late Hawking radiation is entangled with. Since, in this proposal, these systems are the same, there is no contradiction with the monogamy of entanglement. Along similar lines,
Juan Maldacena and
Leonard Susskind suggested in the
ER=EPR proposal that the outgoing and infalling particles are somehow connected by wormholes, and therefore are not independent systems. The
fuzzball picture resolves the dilemma by replacing the '
no-hair' vacuum with a stringy quantum state, thus explicitly coupling any outgoing Hawking radiation with the formation history of the black hole.
Stephen Hawking received widespread mainstream media coverage in January 2014 with an informal proposal to replace the
event horizon of a black hole with an "
apparent horizon" where infalling matter is suspended and then released; however, some scientists have expressed confusion about what precisely is being proposed and how the proposal would solve the paradox. ==Characteristics and detection==