A black hole firewall is a hypothetical phenomenon where an observer falling into a

black hole A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can defo ...

encounters high-energy

quanta Quanta is the plural of quantum. Quanta may also refer to: Organisations * Quanta Computer, a Taiwan-based manufacturer of electronic and computer equipment * Quanta Display Inc., a Taiwanese TFT-LCD panel manufacturer acquired by AU Optronic ...

at (or near) the

event horizon In astrophysics, an event horizon is a boundary beyond which events cannot affect an observer. Wolfgang Rindler coined the term in the 1950s. In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive compact ob ...

. The "firewall" phenomenon was proposed in 2012 by physicists

Ahmed Almheiri Ahmad ( ar, أحمد, ʾAḥmad) is an Arabic male given name common in most parts of the Muslim world. Other spellings of the name include Ahmed and Ahmet. Etymology The word derives from the root (ḥ-m-d), from the Arabic (), from the ve ...

Donald Marolf Donald Marolf is a theoretical physicist, a Professor of Physics, and former head of the physics department at the University of California, Santa Barbara. Biography Marolf gained his Ph.D. from University of Texas at Austin in 1992, under Bryce ...

Joseph Polchinski Joseph Gerard Polchinski Jr. (; May 16, 1954 – February 2, 2018) was an American theoretical physicist and string theorist. Biography Polchinski was born in White Plains, New York, the elder of two children to Joseph Gerard Polchinski Sr. (1929 ...

, and James Sully as a possible solution to an apparent inconsistency in black hole complementarity. The proposal is sometimes referred to as the AMPS firewall, an initialism for the names of the authors of the 2012 paper. The potential inconsistency pointed out by AMPS had been pointed out earlier by Samir Mathur who used the argument in favour of the fuzzball proposal. The use of a firewall to resolve this inconsistency remains controversial, with physicists divided as to the solution to the paradox.

The motivating paradox

According to

quantum field theory in curved spacetime In theoretical physics, quantum field theory in curved spacetime (QFTCS) is an extension of quantum field theory from Minkowski spacetime to a general curved spacetime. This theory treats spacetime as a fixed, classical background, while givin ...

, a single emission of

Hawking radiation Hawking radiation is theoretical black body radiation that is theorized to be released outside a black hole's event horizon because of relativistic quantum effects. It is named after the physicist Stephen Hawking, who developed a theoretical ar ...

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 paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically u ...

: 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. AMPS initially argued that to resolve the paradox physicists may eventually be forced to give up one of three time-tested principles: Einstein's equivalence principle, unitarity, or existing

quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles a ...

.Originally published
in Quanta, December 21, 2012. 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. On the other hand, some physicists argue that such violations of locality cannot resolve 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."

Non-firewall resolutions to the paradox

Some scientists suggest that there is in fact no entanglement between the emitted particle and previous Hawking radiation. This resolution would require black hole information loss, a controversial violation of unitarity. Others, such as Steve Giddings, suggest modifying quantum field theory so that entanglement would be gradually lost as the outgoing and infalling particles separate, resulting in a more gradual release of energy inside the black hole, and consequently no firewall. The Papadodimas–Raju proposal 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 Juan Martín Maldacena (born September 10, 1968) is an Argentine theoretical physicist and the Carl P. Feinberg Professor in the School of Natural Sciences at the Institute for Advanced Study, Princeton. He has made significant contributions to t ...

and

Leonard Susskind Leonard Susskind (; born June 16, 1940)his 60th birthday was celebrated with a special symposium at Stanford University.in Geoffrey West's introduction, he gives Suskind's current age as 74 and says his birthday was recent. is an American physicis ...

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 Fuzzball may refer to: * Fuzzball (sport), a variation of baseball similar to stickball * Fuzzball (string theory), an alternative quantum description of black holes * Fuzzball router, the first modern routers on the Internet * ''Fuzzball'', a car ...

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

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

The firewall would exist at the black hole's event horizon, and would be invisible to observers outside the event horizon. Matter passing through the event horizon into the black hole would immediately be "burned to a crisp" by an arbitrarily hot "seething maelstrom of particles" at the firewall. In a merger of two black holes, the characteristics of a firewall (if any) may leave a mark on the outgoing

gravitational radiation Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...

as "echoes" when waves bounce in the vicinity of the fuzzy event horizon. The expected quantity of such echoes is theoretically unclear, as physicists don't currently have a good physical model of firewalls. In 2016, cosmologist Niayesh Afshordi and others argued there were tentative signs of some such echo in the data from the first black hole merger detected by LIGO; more recent work has argued there is no statistically significant evidence for such echoes in the data.

References

{{quantum gravity Black holes Quantum gravity Quantum mechanical entropy Theorems in general relativity

The motivating paradox

The "firewall" resolution to the paradox

Non-firewall resolutions to the paradox

Characteristics and detection

See also

References