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{{Short description|Physics inside a bounded region is fully captured by physics at the boundary of the region}}
{{Redirect|Holographic universe|the Scar Symmetry album|Holographic Universe (album)|the Epica album|The Holographic Principle}}
{{Use dmy dates|date=November 2020}}The '''holographic principle''' is a property of [[string theory|string theories]] and a supposed property of [[quantum gravity]] that states that the description of a volume of [[space]] can be thought of as encoded on a lower-dimensional [[Boundary (topology)|boundary]] to the region – such as a [[light-like]] boundary like a [[apparent horizon|gravitational horizon]].<ref name="NYT-20221010">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Black Holes May Hide a Mind-Bending Secret About Our Universe – Take gravity, add quantum mechanics, stir. What do you get? Just maybe, a holographic cosmos. |url=https://www.nytimes.com/2022/10/10/science/black-holes-cosmology-hologram.html |date=10 October 2022 |work=[[The New York Times]] |accessdate=10 October 2022 }}</ref><ref name="SA-20230214">{{cite news |last=Ananthaswamy |first=Anil |title=Is Our Universe a Hologram? Physicists Debate Famous Idea on Its 25th Anniversary – The Ads/CFT duality conjecture suggests our universe is a hologram, enabling significant discoveries in the 25 years since it was first proposed |url=https://www.scientificamerican.com/article/is-our-universe-a-hologram-physicists-debate-famous-idea-on-its-25th-anniversary1/ |date=14 February 2023 |work=[[Scientific American]] |accessdate=15 February 2023 }}</ref> First proposed by [[Gerard 't Hooft]], it was given a precise string theoretic interpretation by [[Leonard Susskind]],<ref name="SusskindArXiv">{{cite journal |title=The World as a Hologram |last=Susskind |first=Leonard |doi=10.1063/1.531249 |year=1995 |journal=Journal of Mathematical Physics |volume=36 |issue=11 |pages=6377–6396|arxiv=hep-th/9409089 |bibcode = 1995JMP....36.6377S |s2cid=17316840 }}</ref> who combined his ideas with previous ones of 't&nbsp;Hooft and [[Charles Thorn]].<ref name="SusskindArXiv" /><ref>{{cite conference |first=Charles B. |last=Thorn |title=Reformulating string theory with the 1/N expansion |conference=International A.D. Sakharov Conference on Physics |location=Moscow |date=27–31 May 1991 |isbn=978-1-56072-073-7 |arxiv=hep-th/9405069 |pages=447–54|bibcode=1994hep.th....5069T }}</ref> Susskind said, "The three-dimensional world of ordinary experience—the universe filled with galaxies, stars, planets, houses, boulders, and people—is a hologram, an image of reality coded on a distant two-dimensional surface."<ref name="The Black Hole War">{{cite book |last=Susskind |first=Leonard |url=https://archive.org/details/blackholewarmyba0000suss/page/410/mode/2up |title=The Black Hole War – My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics |date=2008 |publisher=Little, Brown and Company |isbn=9780316016407 |page=410 |url-access=limited}}</ref> As pointed out by [[Raphael Bousso]],<ref>{{cite journal |last=Bousso |first=Raphael |year=2002 |title=The Holographic Principle |journal=[[Reviews of Modern Physics]] |volume=74 |issue=3 |pages=825–874 |doi=10.1103/RevModPhys.74.825 |arxiv=hep-th/0203101 |bibcode=2002RvMP...74..825B|s2cid=55096624 }}</ref> Thorn observed in 1978, that string theory admits a lower-dimensional description in which gravity emerges from it in what would now be called a holographic way. The prime example of holography is the [[AdS/CFT correspondence]].

The holographic principle was inspired by the [[Bekenstein bound]] of [[black hole thermodynamics]], which conjectures that the maximum [[entropy]] in any region scales with the radius {{em|squared}}, rather than cubed as might be expected. In the case of a [[black hole]], the insight was that the [[information content]] of all the objects that have fallen into the hole might be entirely contained in surface fluctuations of the [[event horizon]]. The holographic principle resolves the [[black hole information paradox]] within the framework of string theory.<ref name="The Black Hole War" /> However, there exist classical solutions to the [[Einstein field equations|Einstein equations]] that allow values of the entropy larger than those allowed by an area law (radius squared), hence in principle larger than those of a black hole. These are the so-called "[[John Archibald Wheeler|Wheeler's]] bags of gold". The existence of such solutions conflicts with the holographic interpretation, and their effects in a quantum theory of gravity including the holographic principle are not yet fully understood.<ref name="Marolf01a">{{cite journal |first=Donald |last=Marolf |author-link=Donald Marolf |title=Black Holes, AdS, and CFTs |year=2009 |journal=General Relativity and Gravitation |volume=41 |issue=4 |pages=903–17 |doi=10.1007/s10714-008-0749-7 |bibcode=2009GReGr..41..903M|arxiv = 0810.4886 |s2cid=55210840 }}</ref>

==High-level summary==

The physical universe is widely seen to be composed of "matter" and "energy". In his 2003 article published in [[Scientific American]] magazine, [[Jacob Bekenstein]] speculatively summarized a current trend started by [[John Archibald Wheeler]], which suggests scientists may "regard the physical world as made of [[information]], with energy and matter as incidentals". Bekenstein asks "Could we, as [[William Blake]] memorably penned, 'see a world in a grain of sand', or is that idea no more than '[[poetic license]]'?",<ref>[https://www.scientificamerican.com/article/information-in-the-hologr-2003-08 Information in the Holographic Universe<!-- Bot generated title -->]</ref> referring to the holographic principle.

===Unexpected connection===

Bekenstein's topical overview "A Tale of Two Entropies"<ref>{{Cite web|url=http://ref-sciam.livejournal.com/1190.html|title=Information in the Holographic Universe by Jacob D. Bekenstein &#91;July 14, 2003&#93;}}</ref> describes potentially profound implications of Wheeler's trend, in part by noting a previously unexpected connection between the world of [[information theory]] and classical physics. This connection was first described shortly after the seminal 1948 papers of American applied mathematician [[Claude Shannon]] introduced today's most widely used measure of information content, now known as [[Shannon entropy]]. As an objective measure of the quantity of information, Shannon entropy has been enormously useful, as the design of all modern communications and data storage devices, from cellular phones to [[modems]] to hard disk drives and [[DVD]]s, rely on Shannon entropy.

In [[thermodynamics]] (the branch of physics dealing with heat), entropy is popularly described as a measure of the "[[Order and disorder (physics)|disorder]]" in a physical system of matter and energy. In 1877, Austrian physicist [[Ludwig Boltzmann]] described it more precisely in terms of the number of distinct microscopic states that the particles composing a macroscopic "chunk" of matter could be in, while still "looking" like the same macroscopic "chunk". As an example, for the air in a room, its thermodynamic entropy would equal the logarithm of the count of all the ways that the individual gas molecules could be distributed in the room, and all the ways they could be moving.

===Energy, matter, and information equivalence===

Shannon's efforts to find a way to quantify the information contained in, for example, a telegraph message, led him unexpectedly to a formula with the same form as [[Boltzmann's entropy formula|Boltzmann's]]. In an article in the August 2003 issue of ''Scientific American'' titled "Information in the Holographic Universe", Bekenstein summarizes that "Thermodynamic entropy and Shannon entropy are conceptually equivalent: the number of arrangements that are counted by Boltzmann entropy reflects the amount of Shannon information one would need to implement any particular arrangement" of matter and energy. The only salient difference between the thermodynamic entropy of physics and Shannon's entropy of information is in the units of measure; the former is expressed in units of energy divided by temperature, the latter in essentially dimensionless "bits" of information.

The holographic principle states that the entropy of ordinary mass (not just black holes) is also proportional to surface area and not volume; that volume itself is illusory and the universe is really a [[hologram]] which is [[isomorphism|isomorphic]] to the information "inscribed" on the surface of its boundary.<ref name="sciam2003"/>

==The AdS/CFT correspondence==

{{Main|AdS/CFT correspondence}}
[[File:AdS CFT.png|thumb|upright=0.5|Conjectured relationship of adS/CFT]]
The '''anti-de Sitter/conformal field theory correspondence''', sometimes called '''Maldacena duality''' (after ref.<ref name=":0">{{Cite journal |last=Maldacena |first=Juan |date=March 1998 |title=The large $N$ limit of superconformal field theories and supergravity |url=https://www.intlpress.com/site/pub/pages/journals/items/atmp/content/vols/0002/0002/a001/ |journal=Advances in Theoretical and Mathematical Physics |language=EN |volume=2 |issue=2 |pages=231–252 |doi=10.4310/ATMP.1998.v2.n2.a1 |bibcode=1998AdTMP...2..231M |issn=1095-0753|doi-access=free |arxiv=hep-th/9711200 }}</ref>) or '''gauge/gravity duality''', is a conjectured relationship between two kinds of physical theories. On one side are [[anti-de Sitter space]]s (AdS) which are used in theories of [[quantum gravity]], formulated in terms of [[string theory]] or [[M-theory]]. On the other side of the correspondence are [[conformal field theory|conformal field theories]] (CFT) which are [[quantum field theory|quantum field theories]], including theories similar to the [[Yang–Mills theory|Yang–Mills theories]] that describe elementary particles.

The duality represents a major advance in understanding of string theory and quantum gravity.<ref name="de Haro et al. 2013, p.2">de Haro et al. 2013, p. 2.</ref> This is because it provides a [[non-perturbative]] formulation of string theory with certain [[boundary condition]]s and because it is the most successful realization of the holographic principle.

It also provides a powerful toolkit for studying [[coupling (physics)|strongly coupled]] quantum field theories.<ref>Klebanov and Maldacena. 2009.</ref> Much of the usefulness of the duality results from a strong-weak duality: when the fields of the quantum field theory are strongly interacting, the ones in the gravitational theory are weakly interacting and thus more mathematically tractable. This fact has been used to study many aspects of [[nuclear physics|nuclear]] and [[condensed matter physics]] by translating problems in those subjects into more mathematically tractable problems in string theory.

The AdS/CFT correspondence was first proposed by [[Juan Maldacena]] in late 1997.<ref name=":0" /> Important aspects of the correspondence were elaborated in articles by [[Steven Gubser]], [[Igor Klebanov]], and [[Alexander Markovich Polyakov]], and by [[Edward Witten]]. By 2015, Maldacena's article had over 10,000 citations, becoming the most highly cited article in the field of [[high energy physics]].<ref name="inspire">{{cite web |url= https://inspirehep.net/info/hep/stats/topcites/2014/alltime.html |title=Top Cited Articles of All Time (2014 edition) | publisher= [[INSPIRE-HEP]] |author=<!--Staff writer(s); no by-line.--> | access-date=26 December 2015}}</ref>

==Black hole entropy==
{{Main|Black hole thermodynamics}}
An object with relatively high entropy is microscopically random, like a hot gas. A known configuration of classical fields has zero entropy: there is nothing random about [[Electric field|electric]] and [[magnetic field]]s, or [[gravitational wave]]s. Since black holes are exact solutions of [[Einstein field equations|Einstein's equations]], they were thought not to have any entropy.

But Jacob Bekenstein noted that this leads to a violation of the [[second law of thermodynamics]]. If one throws a hot gas with entropy into a black hole, once it crosses the [[event horizon]], the entropy would disappear. The random properties of the gas would no longer be seen once the black hole had absorbed the gas and settled down. One way of salvaging the second law is if black holes are in fact random objects with an [[entropy]] that increases by an amount greater than the entropy of the consumed gas.

Given a fixed volume, a black hole whose event horizon encompasses that volume should be the object with the highest amount of entropy. Otherwise, imagine something with a larger entropy, then by throwing more mass into that something, we obtain a black hole with less entropy, violating the second law.<ref name="SusskindArXiv" />

[[File:Deriving Emergent gravity from microscopic theories.jpg|thumb|upright=1.2|The illustration demonstrates the way of thinking of the entropic gravity, holographic principle, entropy distribution, and derivation of Einstein General Relativity equations from these considerations. The Einstein equation takes the form of the first law of thermodynamics when Bekenstein and Hawking equations are applied.]]
In a sphere of radius ''R'', the entropy in a relativistic gas increases as the energy increases. The only known limit is [[gravitation]]al; when there is too much energy, the gas collapses into a black hole. Bekenstein used <!-- Redshift~golden number~greenshift  -->this to put an upper bound on the entropy in a region of space, and the bound was proportional to the area of the region. He concluded that the black hole entropy is directly proportional to the area of the event horizon.<ref>{{cite journal |first=Jacob D. |last=Bekenstein |title=Universal upper bound on the entropy-to-energy ratio for bounded systems |journal=Physical Review D |volume=23 |issue=215 |pages=287–298 |date=January 1981 |doi=10.1103/PhysRevD.23.287 |bibcode = 1981PhRvD..23..287B |s2cid=120643289 }}</ref> [[Gravitational time dilation]] causes time, from the perspective of a remote observer, to stop at the event horizon. Due to the natural limit on [[Speed of light|maximum speed of motion]], this prevents falling objects from crossing the event horizon no matter how close they get to it. Since any change in quantum state requires time to flow, all objects and their quantum information state stay imprinted on the event horizon. Bekenstein concluded that from the perspective of any remote observer, the black hole entropy is directly proportional to the area of the event horizon.

[[Stephen Hawking]] had shown earlier that the total horizon area of a collection of black holes always increases with time. The horizon is a boundary defined by light-like [[geodesics]]; it is those light rays that are just barely unable to escape. If neighboring geodesics start moving toward each other they eventually collide, at which point their extension is inside the black hole. So the geodesics are always moving apart, and the number of geodesics which generate the boundary, the area of the horizon, always increases. Hawking's result was called the second law of [[black hole thermodynamics]], by analogy with the [[Second law of thermodynamics|law of entropy increase]].

At first, Hawking did not take the analogy too seriously. He argued that the black hole must have zero temperature, since black holes do not radiate and therefore cannot be in thermal equilibrium with any black body of positive temperature.<ref>{{Cite journal |last1=Bardeen |first1=J. M. |last2=Carter |first2=B. |last3=Hawking |first3=S. W. |date=1973-06-01 |title=The four laws of black hole mechanics |url=https://doi.org/10.1007/BF01645742 |journal=Communications in Mathematical Physics |language=en |volume=31 |issue=2 |pages=161–170 |doi=10.1007/BF01645742 |bibcode=1973CMaPh..31..161B |s2cid=54690354 |issn=1432-0916}}</ref> Then he discovered that black holes do radiate. When heat is added to a thermal system, the change in entropy is the increase in [[mass–energy equivalence|mass–energy]] divided by temperature:
::<math>
{\rm d}S = \frac{{\rm\delta }M \ c^2}{T}.
</math>
(Here the term ''δM&nbsp;c<sup>2</sup>'' is substituted for the thermal energy added to the system, generally by non-integrable random processes, in contrast to d''S'', which is a function of a few "state variables" only, i.e. in conventional thermodynamics only of the [[Kelvin]] temperature ''T'' and a few additional state variables, such as the pressure.)

If black holes have a finite entropy, they should also have a finite temperature. In particular, they would come to equilibrium with a thermal gas of photons. This means that black holes would not only absorb photons, but they would also have to emit them in the right amount to maintain [[detailed balance]].

Time-independent solutions to field equations do not emit radiation, because a time-independent background conserves energy. Based on this principle, Hawking set out to show that black holes do not radiate. But, to his surprise, a careful analysis convinced him that [[Hawking radiation|they do]], and in just the right way to come to equilibrium with a gas at a finite temperature. Hawking's calculation fixed the constant of proportionality at 1/4; the entropy of a black hole is one quarter its horizon area in [[Planck units]].<ref>{{cite journal | first = Parthasarathi | last = Majumdar | title = Black Hole Entropy and Quantum Gravity | arxiv = gr-qc/9807045 | journal =   Indian Journal of Physics B| date = 1998|bibcode = 1999InJPB..73..147M | volume = 73 | issue = 2 |page=147 }}</ref>

The entropy is proportional to the [[logarithm]] of the number of [[microstate (statistical mechanics)|microstates]], the enumerated ways a system can be configured microscopically while leaving the macroscopic description unchanged. Black hole entropy is deeply puzzling – it says that the logarithm of the number of states of a black hole is proportional to the area of the horizon, not the volume in the interior.<ref name="sciam2003">{{cite magazine | first = Jacob D. | last = Bekenstein | author-link = Jacob Bekenstein | url = http://www.sciam.com/article.cfm?articleid=000AF072-4891-1F0A-97AE80A84189EEDF | title = Information in the Holographic Universe – Theoretical results about black holes suggest that the universe could be like a gigantic hologram |magazine= [[Scientific American]] |date=August 2003 |page=59 }}</ref>

Later, [[Raphael Bousso]] came up with a [[Bousso's holographic bound|covariant version of the bound]] based upon null sheets.<ref>{{cite journal
| last         = Bousso
| first        = Raphael
| year         = 1999
| title        = A Covariant Entropy Conjecture
| journal      = Journal of High Energy Physics
| issue        = 7
| doi          = 10.1088/1126-6708/1999/07/004
| arxiv        = hep-th/9905177
|bibcode = 1999JHEP...07..004B
| volume=1999
| page=004| s2cid = 9545752
}}</ref>

==Black hole information paradox==
{{Main|Black hole information paradox}}
Hawking's calculation suggested that the radiation which black holes emit is not related in any way to the matter that they absorb. The outgoing light rays start exactly at the edge of the black hole and spend a long time near the horizon, while the infalling matter only reaches the horizon much later. The infalling and outgoing mass/energy interact only when they cross. It is implausible that the outgoing state would be completely determined by some tiny residual scattering.{{citation needed|date=January 2019}}

Hawking interpreted this to mean that when black holes absorb some photons in a pure state described by a [[wave function]], they re-emit new [[photons]] in a thermal mixed state described by a [[density matrix]]. This would mean that quantum mechanics would have to be modified because, in quantum mechanics, states which are superpositions with probability amplitudes never become states which are probabilistic mixtures of different possibilities.<ref group=note>except in the case of measurements, which the black hole should not be performing</ref>

Troubled by this paradox, Gerard 't Hooft analyzed the emission of [[Hawking radiation]] in more detail.<ref>{{Cite book|url=https://books.google.com/books?id=ZE-yCQAAQBAJ&q=Troubled+by+this+paradox%2C+Gerard+%27t+Hooft+analyzed+the+emission+of+Hawking+radiation+in+more+detai&pg=PA100|title=The Cosmic Compendium: Black Holes|last=Anderson|first=Rupert W.|date=2015-03-31|publisher=Lulu.com|isbn=9781329024588|language=en}}{{self-published source|date=April 2020}}</ref>{{self-published inline|date=February 2020}} He noted that when Hawking radiation escapes, there is a way in which incoming particles can modify the outgoing particles. Their [[gravitational field]] would deform the horizon of the black hole, and the deformed horizon could produce different outgoing particles than the undeformed horizon. When a particle falls into a black hole, it is boosted relative to an outside observer, and its gravitational field assumes a universal form. 't Hooft showed that this field makes a logarithmic tent-pole shaped bump on the horizon of a black hole, and like a shadow, the bump is an alternative description of the particle's location and mass. For a four-dimensional spherical uncharged black hole, the deformation of the horizon is similar to the type of deformation which describes the emission and absorption of particles on a string-theory [[Worldsheet|world sheet]]. Since the deformations on the surface are the only imprint of the incoming particle, and since these deformations would have to completely determine the outgoing particles, 't Hooft believed that the correct description of the black hole would be by some form of string theory.

This idea was made more precise by Leonard Susskind, who had also been developing holography, largely independently. Susskind argued that the oscillation of the horizon of a black hole is a complete description{{refn|"Complete description" means all the ''primary'' qualities.  For example, [[John Locke]] (and before him [[Robert Boyle]]) determined these to be ''size, shape, motion, number, ''and'' solidity''. Such ''secondary quality'' information as ''color, aroma, taste ''and'' sound'',<ref>{{cite book|last=Dennett|first=Daniel|title=Consciousness Explained|date=1991|publisher=Back Bay Books|location=New York|isbn=978-0-316-18066-5|page=[https://archive.org/details/consciousnessexp00denn/page/371 371]|title-link=Consciousness Explained}}</ref> or internal quantum state is not information that is implied to be preserved in the surface fluctuations of the event horizon. (See however "path integral quantization")|group=note}} of both the infalling and outgoing matter, because the world-sheet theory of string theory was just such a holographic description. While short strings have zero entropy, he could identify long highly excited string states with ordinary black holes. This was a deep advance because it revealed that strings have a classical interpretation in terms of black holes.

This work showed that the black hole information paradox is resolved when quantum gravity is described in an unusual string-theoretic way assuming the string-theoretical description is complete, unambiguous and non-redundant.<ref>{{cite journal |last=Susskind |first=Leonard |date=February 2003 |title=The Anthropic landscape of string theory |journal=The Davis Meeting on Cosmic Inflation |page=26 |arxiv=hep-th/0302219 |bibcode=2003dmci.confE..26S}}</ref> The space-time in quantum gravity would emerge as an effective description of the theory of oscillations of a lower-dimensional black-hole horizon, and suggest that any black hole with appropriate properties, not just strings, would serve as a basis for a description of string theory.

In 1995, Susskind, along with collaborators [[Tom Banks (physicist)|Tom Banks]], [[Willy Fischler]], and [[Stephen Shenker]], presented a formulation of the new M-theory using a holographic description in terms of charged point black holes, the D0 [[Membrane (M-theory)|branes]] of [[Type II string theory|type IIA string theory]]. The matrix theory they proposed was first suggested as a description of two branes in [[eleven-dimensional supergravity]] by [[Bernard de Wit]], [[Jens Hoppe]], and [[Hermann Nicolai]]. The later authors reinterpreted the same matrix models as a description of the dynamics of point black holes in particular limits. Holography allowed them to conclude that the dynamics of these black holes give a complete [[non-perturbative]] formulation of [[M-theory]]. In 1997, [[Juan Maldacena]] gave the first holographic descriptions of a higher-dimensional object, the 3+1-dimensional [[Type II string theory|type IIB]] [[Membrane (M-theory)|membrane]], which resolved a long-standing problem of finding a string description which describes a [[gauge theory]]. These developments simultaneously explained how string theory is related to some forms of supersymmetric quantum field theories.

==Limit on information density==
[[File:Bekenstein-Hawking entropy of a black hole.svg|thumb|upright=1.2|The Bekenstein-Hawking entropy of a black hole is proportional to the surface area of the black hole as expressed in Planck units.]]
[[Information content]] is defined as the logarithm of the reciprocal of the probability that a system is in a specific microstate, and the [[information entropy]] of a system is the expected value of the system's information content. This definition of entropy is equivalent to the standard [[Gibbs entropy]] used in classical physics. Applying this definition to a physical system leads to the conclusion that, for a given energy in a given volume, there is an upper limit to the density of information (the Bekenstein bound) about the whereabouts of all the particles which compose matter in that volume. In particular, a given volume has an upper limit of information it can contain, at which it will collapse into a black hole.

This suggests that matter itself cannot be subdivided infinitely many times and there must be an ultimate level of [[elementary particle|fundamental particles]]. As the [[degrees of freedom (physics and chemistry)|degrees of freedom]] of a particle are the product of all the degrees of freedom of its sub-particles, were a particle to have infinite subdivisions into lower-level particles, the degrees of freedom of the original particle would be infinite, violating the maximal limit of entropy density. The holographic principle thus implies that the subdivisions must stop at some level.

The most rigorous realization of the holographic principle is the [[AdS/CFT correspondence]] by Juan Maldacena. However, J. David Brown and [[Marc Henneaux]] had rigorously proved in 1986, that the asymptotic symmetry of 2+1 dimensional gravity gives rise to a [[Virasoro algebra]], whose corresponding quantum theory is a 2-dimensional conformal field theory.<ref>{{Cite journal |first1=J. D. |last1=Brown |name-list-style=amp |first2=M. |last2=Henneaux |date=1986 |title=Central charges in the canonical realization of asymptotic symmetries: an example from three-dimensional gravity |journal=Communications in Mathematical Physics |volume=104 |issue=2 |pages=207–226 |doi=10.1007/BF01211590 |bibcode = 1986CMaPh.104..207B |s2cid=55421933 |url=http://projecteuclid.org/euclid.cmp/1104114999 }}.</ref>

==Experimental tests==
[[File:Fermi National Accelerator Laboratory - Holographic Noise Chart.png|thumb|upright=1.8|This plot shows the sensitivity of various experiments to fluctuations in space and time. Horizontal axis is the log of apparatus size (or duration time the speed of light), in meters; vertical axis is the log of the rms fluctuation amplitude in the same units. The lower left corner represents the Planck length or time. In these units, the size of the observable universe is about 26. Various physical systems and experiments are plotted. The "holographic noise" line represents the rms transverse holographic fluctuation amplitude on a given scale.]]
The [[Fermilab]] physicist [[Craig Hogan]] claims that the holographic principle would imply quantum fluctuations in spatial position<ref>{{Cite journal |last=Hogan |first=Craig J. |year=2008 |title=Measurement of quantum fluctuations in geometry |journal=[[Physical Review D]] |volume=77 |issue=10 |page=104031 |arxiv=0712.3419 |bibcode=2008PhRvD..77j4031H |doi=10.1103/PhysRevD.77.104031 |s2cid=119087922}}</ref> that would lead to apparent background noise or "holographic noise" measurable at gravitational wave detectors, in particular [[GEO 600]].<ref>{{Cite news |last=Chown|first=Marcus|title=Our world may be a giant hologram|newspaper=NewScientist|date=15 January 2009|url=https://www.newscientist.com/article/mg20126911.300|access-date=2010-04-19}}</ref> However these claims have not been widely accepted, or cited, among quantum gravity researchers and appear to be in direct conflict with string theory calculations.<ref>"Consequently, he ends up with inequalities of the type... Except that one may look at the actual equations of Matrix theory and see that none of these commutators is nonzero... The last displayed inequality above obviously can't be a consequence of quantum gravity because it doesn't depend on G at all! However, in the G→0 limit, one must reproduce non-gravitational physics in the flat Euclidean background spacetime. Hogan's rules don't have the right limit so they can't be right." – [[Luboš Motl]], [http://motls.blogspot.com/2012/02/hogans-holographic-noise-doesnt-exist.html Hogan's holographic noise doesn't exist], 7 February 2012.</ref>

Analyses in 2011 of measurements of gamma ray burst [[GRB 041219A]] in 2004 by the [[INTEGRAL]] space observatory launched in 2002 by the [[European Space Agency]], shows that Craig Hogan's noise is absent down to a scale of 10<sup>−48</sup>&nbsp;meters, as opposed to the scale of 10<sup>−35</sup>&nbsp;meters predicted by Hogan, and the scale of 10<sup>−16</sup>&nbsp;meters found in measurements of the [[GEO 600]] instrument.<ref>{{cite web|url=http://www.esa.int/Our_Activities/Space_Science/Integral_challenges_physics_beyond_Einstein|title=Integral challenges physics beyond Einstein|date=30 June 2011|publisher=[[European Space Agency]]|access-date=3 February 2013}}</ref> Research continued at Fermilab under Hogan as of 2013.<ref>{{cite web|url=http://holometer.fnal.gov/faq.html|title=Frequently Asked Questions for the Holometer at Fermilab|date=6 July 2013|access-date=14 February 2014}}</ref>

[[Jacob Bekenstein]] claimed to have found a way to test the holographic principle with a tabletop photon experiment.<ref>{{cite news|url=http://www.nature.com/news/single-photon-could-detect-quantum-scale-black-holes-1.11871|title=Single photon could detect quantum-scale black holes|last=Cowen|first=Ron|date=22 November 2012|work=[[Nature (journal)|Nature]]|access-date=3 February 2013}}</ref>

==See also==
{{cols}}
* [[Bekenstein bound]]
* [[Black hole thermodynamics#Beyond black holes|Beyond black holes]]
* [[Bousso's holographic bound]]
* [[Brane cosmology]]
* [[Digital physics]]
* [[Entropic gravity]]
* [[Implicate and explicate order]]
* [[Quantum speed limit theorems]]
* [[Physical cosmology]]
* [[Quantum foam]]
{{colend}}

==Notes==
{{Reflist|group=note}}

==References==
;Citations
{{Reflist|30em}}

;Sources
* {{cite journal
 | first = Raphael | last = Bousso | title = The holographic principle
 | journal = Reviews of Modern Physics
 | volume = 74
 | year = 2002
 | pages = 825–874
 | arxiv = hep-th/0203101
 | doi = 10.1103/RevModPhys.74.825
 | bibcode=2002RvMP...74..825B
 | issue = 3| s2cid = 55096624 }}
* {{Cite arXiv |last='t Hooft |first=Gerard |date=1993 |title=Dimensional Reduction in Quantum Gravity  |eprint=gr-qc/9310026 }}. 't Hooft's original paper.

==External links==
* Alfonso V. Ramallo: ''Introduction to the AdS/CFT correspondence'', {{arxiv|1310.4319}}, pedagogical lecture. For the holographic principle: see especially Fig. 1.
* [http://www.uctv.tv/shows/The-World-as-a-Hologram-11140 UC Berkeley's Raphael Bousso gives an introductory lecture on the holographic principle – Video.]
* [https://web.archive.org/web/20100511025920/http://community.livejournal.com/ref_sciam/1190.html ''Scientific American'' article on holographic principle by Jacob Bekenstein]
*{{cite web |first=Matt |last=O'Dowd |author-link=Matt O'Dowd (astrophysicist) |title=The Holographic Universe Explained |work=[[PBS Space Time]] |date=10 April 2019 |url=https://www.youtube.com/watch?v=klpDHn8viX8&list=PLsPUh22kYmNCHVpiXDJyAcRJ8gluQtOJR&index=10 | archive-url=https://ghostarchive.org/varchive/youtube/20211211/klpDHn8viX8| archive-date=2021-12-11 | url-status=live|via=[[YouTube]] }}{{cbignore}}

{{String theory topics |state=collapsed}}
{{Black holes}}
{{quantum gravity}}
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{{DEFAULTSORT:Holographic Principle}}
[[Category:Theoretical physics]]
[[Category:Black holes]]
[[Category:Quantum information science]]
[[Category:Holography]]