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

black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...

encounters high-energy quanta at (or near) the

event horizon In astrophysics, an event horizon is a boundary beyond which events cannot affect an outside 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 c ...

. The "firewall" phenomenon was proposed in 2012 by physicists Ahmed Almheiri, Donald Marolf,

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. (19 ...

, and James Sully as a possible solution to an apparent inconsistency in

black hole complementarity Black hole complementarity is a conjectured solution to the black hole information paradox, proposed by Leonard Susskind, Lárus Thorlacius, John Uglum, and Gerard 't Hooft. Overview Ever since Stephen Hawking suggested information is lost in an ...

. The proposal is sometimes referred to as the AMPS firewall, an

acronym An acronym is a type of abbreviation consisting of a phrase whose only pronounced elements are the initial letters or initial sounds of words inside that phrase. Acronyms are often spelled with the initial Letter (alphabet), letter of each wor ...

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 uses a semi-classical approach; it treats spacetime as a fixed ...

, a single emission of

Hawking radiation Hawking radiation is black-body radiation released outside a black hole's event horizon due to quantum effects according to a model developed by Stephen Hawking in 1974. The radiation was not predicted by previous models which assumed that onc ...

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 Page most commonly refers to: * Page (paper), one side of a leaf of paper, as in a book Page, PAGE, pages, or paging may also refer to: Roles * Page (assistance occupation), a professional occupation * Page (servant), traditionally a young m ...

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 or apparently true premises, leads to a seemingly self-contradictor ...

: 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 The equivalence principle is the hypothesis that the observed equivalence of gravitational and inertial mass is a consequence of nature. The weak form, known for centuries, relates to masses of any composition in free fall taking the same t ...

, unitarity, or existing

quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines Field theory (physics), field theory and the principle of relativity with ideas behind quantum mechanics. QFT is used in particle physics to construct phy ...

.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 Locality may refer to: * Locality, a historical named location or place in Canada * Locality (association), an association of community regeneration organizations in England * Locality (linguistics) * Locality (settlement) * Suburbs and localitie ...

. 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 and

Leonard Susskind Leonard Susskind (; born June 16, 1940)his 60th birth anniversary 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 Americ ...

suggested in the

ER=EPR ER = EPR is a conjecture in physics stating that two entangled particles (a so-called Einstein–Podolsky–Rosen or EPR pair) are connected by a wormhole (or Einstein–Rosen bridge) and is thought by some to be a basis for unifying general rel ...

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 Stephen William Hawking (8January 194214March 2018) was an English theoretical physics, theoretical physicist, cosmologist, and author who was director of research at the Centre for Theoretical Cosmology at the University of Cambridge. Between ...

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 oscillations of the gravitational field that travel through space at the speed of light; they are generated by the relative motion of gravitating masses. They were proposed by Oliver Heaviside in 1893 and then later by ...

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