Editing Holographic principle (section)

{{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>