# Firewall (physics)

A black hole firewall is a hypothetical phenomenon where an observer falling into a black hole encounters high-energy quanta at (or near) the event horizon. The "firewall" phenomenon was proposed in 2012 by Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully[1] as a possible solution to an apparent inconsistency in black hole complementarity. The proposal is sometimes referred to as the AMPS firewall,[2] an acronym for the names of the authors of the 2012 paper. The use of a firewall to resolve this inconsistency remains controversial, with high-energy physicists divided as to the solution to the paradox.[3] 2016 LIGO observations provided tentative evidence of a firewall, or of some other phenomenon violating general relativity.[4]

## The motivating paradox

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 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. Assume that at time ${\displaystyle t}$, more than half of the information had already been emitted. According to widely accepted research by physicists like Don Page[5][6] and Leonard Susskind, an outgoing particle emitted at time ${\displaystyle t}$ must be entangled with all the Hawking radiation the black hole has previously emitted. 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.[3]

In order to resolve the paradox, physicists may eventually be forced to give up one of three time-tested theories: Einstein's equivalence principle, unitarity, or existing quantum field theory.[7]

### 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 inconceivable 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"; one theorist 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."[3]

### 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.[3]

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.[3]

Juan Maldacena and Leonard Susskind have suggested in ER=EPR that the outgoing and infalling particles are somehow connected by wormholes, and therefore are not independent systems; however, as of 2013, this hypothesis is still a "work in progress".[8][9]

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.[10][11]

Stephen Hawking received widespread mainstream media coverage in January 2014 with an informal proposal[12] 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.[13]

## 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.[3]

In a merger of two black holes, the characteristics of a firewall (if any) may leave a mark on the outgoing gravitational radiation 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 found tentative signs of some such echo in the data from the first black hole merger detected by LIGO; Afshordi calculated that, if random noise is behind the patterns, then the chance of seeing such echoes is only about 1 in 270. Over the next two years, the null "random noise" hypothesis should be more solidly confirmed or rejected, as additional data is accumulated by the Laser Interferometer Gravitational-Wave Observatory (LIGO). If confirmed, these echoes would be strong evidence in favor of a firewall, a fuzzball, or some other novel breakdown of classical general relativity at the event horizon.[4]

## References

1. ^ Almheiri, Ahmed; Marolf, Donald; Polchinski, Joseph; Sully, James (11 February 2013). "Black holes: complementarity or firewalls?". Journal of High Energy Physics. 2013 (2). arXiv:. Bibcode:2013JHEP...02..062A. doi:10.1007/JHEP02(2013)062.
2. ^ Borun D. Chowdhury, Andrea Puhm, "Decoherence and the fate of an infalling wave packet: Is Alice burning or fuzzing?", Phys. Rev. D 88, 063509 (2013)
3. Astrophysics: Fire in the hole!
4. ^ a b Mera;o, Zeeya (2016). "LIGO black hole echoes hint at general-relativity breakdown". Nature. 540. doi:10.1038/nature.2016.21135.
5. ^ Page, Don N. (1993). "Information in black hole radiation". Phys. Rev. Lett. 71: 3743. arXiv:. Bibcode:1993PhRvL..71.3743P. doi:10.1103/PhysRevLett.71.3743.
6. ^ Page, Don N. (1993). "Average entropy of a subsystem". Phys. Rev. Lett. 71: 1291. arXiv:. Bibcode:1993PhRvL..71.1291P. doi:10.1103/PhysRevLett.71.1291.
7. ^ Ouellette, Jennifer (21 December 2012). "Black Hole Firewalls Confound Theoretical Physicists". Scientific American. Retrieved 29 October 2013. Originally published in Quanta, December 21, 2012.
8. ^ Overbye, Dennis (12 August 2013). "A Black Hole Mystery Wrapped in a Firewall Paradox". New York Times. Retrieved 29 October 2013.
9. ^ "The Firewall Paradox". New York Times. 12 August 2013. Retrieved 29 October 2013.
10. ^ S. Mathur (2009). "The information paradox: A pedagogical introduction," Class. Quantum Grav., Vol. 26 No. 22 (2009)
11. ^ Steven G. Avery, Borun D. Chowdhury, Andrea Puhm, "Unitarity and fuzzball complementarity: 'Alice fuzzes but may not even know it!'", JHEP 09 (2013) 012
12. ^ Hawking, Stephen (22 Jan 2014). "Information Preservation and Weather Forecasting for Black Holes". arXiv:.
13. ^ "Why some physicists aren't buying Hawking's new black hole theory". Christian Science Monitor. 29 January 2014. Retrieved 15 March 2014.