Editing Quantum gravity (section)

{{short description|Description of gravity using discrete values}}
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'''Quantum gravity''' ('''QG''') is a field of [[theoretical physics]] that seeks to describe [[gravity]] according to the principles of [[quantum mechanics]]. It deals with environments in which neither gravitational nor quantum effects can be ignored,<ref name="scholarpedia">{{cite journal | last = Rovelli | first = Carlo | author-link = Carlo Rovelli | year = 2008| title = Quantum gravity | journal = [[Scholarpedia]] | volume = 3 | issue = 5| page = 7117 | doi = 10.4249/scholarpedia.7117 | bibcode = 2008SchpJ...3.7117R | doi-access = free }}</ref> such as in the vicinity of [[black holes]] or similar compact astrophysical objects, as well as in the early stages of the universe moments after the [[Big Bang]].<ref>{{Cite book |last=Kiefer |first=Claus |title=Quantum gravity |date=2012 |publisher=Oxford University Press |isbn=978-0-19-958520-5 |edition=3rd |series=International series of monographs on physics |location=Oxford |pages=1–4 |language=en}}</ref>

Three of the four [[fundamental force]]s of nature are described within the framework of quantum mechanics and [[quantum field theory]]: the [[Electromagnetism|electromagnetic interaction]], the [[Strong interaction|strong force]], and the [[Weak interaction|weak force]]; this leaves gravity as the only interaction that has not been fully accommodated. The current understanding of gravity is based on [[Albert Einstein]]'s [[general theory of relativity]], which incorporates his theory of special relativity and deeply modifies the understanding of concepts like time and space. Although general relativity is highly regarded for its elegance and accuracy, it has limitations: the [[Gravitational singularity|gravitational singularities]] inside black holes, the ad hoc postulation of [[dark matter]], as well as [[dark energy]] and its relation to the [[cosmological constant]] are among the current unsolved mysteries regarding gravity,<ref>{{Cite journal |last=Mannheim |first=Philip |date=2006 |title=Alternatives to dark matter and dark energy |journal=Progress in Particle and Nuclear Physics |language=en |volume=56 |issue=2 |pages=340–445 |doi=10.1016/j.ppnp.2005.08.001|arxiv=astro-ph/0505266 |bibcode=2006PrPNP..56..340M |s2cid=14024934 }}</ref> all of which signal the collapse of the general theory of relativity at different scales and highlight the need for a gravitational theory that goes into the quantum realm. At distances close to the [[Planck length]], like those near the center of a black hole, [[quantum fluctuations]] of spacetime are expected to play an important role.<ref>{{cite web
 | url = https://www.quantamagazine.org/black-hole-singularities-are-as-inescapable-as-expected-20191202/
 | title = Black Hole Singularities Are as Inescapable as Expected
 | last = Nadis
 | first = Steve
 | date = 2 December 2019
 | website = quantamagazine.org
 | publisher = [[Quanta Magazine]]
 | access-date = 22 April 2020
 | archive-date = 14 April 2020
 | archive-url = https://web.archive.org/web/20200414150244/https://www.quantamagazine.org/black-hole-singularities-are-as-inescapable-as-expected-20191202/
 | url-status = live
 }}</ref> Finally, the discrepancies between the predicted value for the [[vacuum energy]] and the observed values (which, depending on considerations, can be of 60 or 120 orders of magnitude)<ref>{{cite journal|last=Bousso |first=Raphael |title=The cosmological constant |journal=General Relativity and Gravitation |volume=40 |year=2008 |issue=2–3 |pages=607–637 |arxiv=0708.4231 |doi=10.1007/s10714-007-0557-5|bibcode=2008GReGr..40..607B }}</ref><ref>{{cite journal|doi=10.1088/2058-7058/34/03/32 |first=Rob |last=Lea |title=A new generation takes on the cosmological constant |journal=Physics World |volume=34 |number=3 |page=42 |year=2021|bibcode=2021PhyW...34c..42L }}</ref> highlight the necessity for a quantum theory of gravity.

The field of quantum gravity is actively developing, and theorists are exploring a variety of approaches to the problem of quantum gravity, the most popular being [[M-theory]] and [[loop quantum gravity]].<ref>{{cite book |last=Penrose |first=Roger |title=The road to reality : a complete guide to the laws of the universe |url=https://archive.org/details/roadtoreality00penr_319 |url-access=limited |date=2007 |publisher=Vintage |oclc=716437154 |page=[https://archive.org/details/roadtoreality00penr_319/page/n1045 1017]|isbn=9780679776314 }}</ref> All of these approaches aim to describe the quantum behavior of the [[gravitational field]], which does not necessarily include [[Theory of everything|unifying all fundamental interactions]] into a single mathematical framework. However, many approaches to quantum gravity, such as [[string theory]], try to develop a framework that describes all fundamental forces. Such a theory is often referred to as a [[theory of everything]]. Some of the approaches, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other forces. Other lesser-known but no less important theories include [[causal dynamical triangulation]], [[Noncommutative geometry#Applications in mathematical physics|noncommutative geometry]], and [[twistor theory]].<ref>{{Cite arXiv |last=Rovelli |first=Carlo |date=2001 |title=Notes for a brief history of quantum gravity |eprint=gr-qc/0006061 }}</ref>

One of the difficulties of formulating a quantum gravity theory is that direct observation of quantum gravitational effects is thought to only appear at length scales near the [[Planck scale]], around 10<sup>−35</sup> meters, a scale far smaller, and hence only accessible with far higher energies, than those currently available in high energy [[particle accelerator]]s. Therefore, physicists lack experimental data which could distinguish between the competing theories which have been proposed.<ref group="n.b.">Quantum effects in the early universe might have an observable effect on the structure of the present universe, for example, or gravity might play a role in the unification of the other forces. Cf. the text by Wald cited above.</ref><ref group="n.b.">On the quantization of the geometry of spacetime, see also in the article [[Planck length]], in the examples</ref>

[[Thought experiment]] approaches have been suggested as a testing tool for quantum gravity theories.<ref>
{{cite journal
|last1 = Lindner
|first1 = Nethanel H.
|last2 = Peres
|first2 = Asher
|title = Testing quantum superpositions of the gravitational field with Bose-Einstein condensates
|date = 2005
|journal = [[Physical Review A]]
|volume = 71
|issue = 2
|pages = 024101
|doi = 10.1103/PhysRevA.71.024101
|arxiv = gr-qc/0410030
|bibcode = 2005PhRvA..71b4101L
}}</ref><ref>
{{cite arXiv
|last1 = Kafri
|first1 = Dvir
|last2 = Taylor
|first2 = Jacob M
|title = A noise inequality for classical forces
|date = 2013
|class = quant-ph
|eprint = 1311.4558
}}</ref> In the field of quantum gravity there are several open questions – e.g., it is not known how spin of elementary particles sources gravity, and thought experiments could provide a pathway to explore possible resolutions to these questions,<ref name="Spin-Spacetime Censorship AdP">{{cite journal|journal=Annalen der Physik|first1=J.|last1=Nemirovsky|first2=E.|last2=Cohen|title=Spin Spacetime Censorship|volume = 534|issue = 1|doi = 10.1002/andp.202100348|date=5 November 2021|last3=Kaminer|first3=I.|arxiv=1812.11450|s2cid=119342861}}</ref> even in the absence of lab experiments or physical observations.

In the early 21st century, new experiment designs and technologies have arisen which suggest that indirect approaches to testing quantum gravity may be feasible over the next few decades.<ref name="nautilus">{{cite web |last1=Hossenfelder |first1=Sabine |title=What Quantum Gravity Needs Is More Experiments |url=http://nautil.us/issue/45/power/what-quantum-gravity-needs-is-more-experiments |website=Nautilus |accessdate=21 September 2020 |date=2 February 2017 |archive-date=28 January 2018 |archive-url=https://web.archive.org/web/20180128021051/http://nautil.us/issue/45/power/what-quantum-gravity-needs-is-more-experiments |url-status=dead }}</ref><ref name="springer1">{{cite book |title=Experimental search for quantum gravity |date=2017 |publisher=Springer |location=Cham |isbn=9783319645360}}</ref><ref name="tabletop">{{cite journal |last1=Carney |first1=Daniel |last2=Stamp |first2=Philip C. E. |last3=Taylor |first3=Jacob M. |title=Tabletop experiments for quantum gravity: a user's manual |journal=Classical and Quantum Gravity |pages=034001 |doi=10.1088/1361-6382/aaf9ca |date=7 February 2019 |volume=36 |issue=3 |arxiv=1807.11494 |bibcode=2019CQGra..36c4001C |s2cid=119073215 }}</ref><ref>{{Cite journal |last1=Danielson |first1=Daine L. |last2=Satishchandran |first2=Gautam |last3=Wald |first3=Robert M. |date=2022-04-05 |title=Gravitationally mediated entanglement: Newtonian field versus gravitons |url=https://link.aps.org/doi/10.1103/PhysRevD.105.086001 |journal=Physical Review D |volume=105 |issue=8 |pages=086001 |arxiv=2112.10798 |doi=10.1103/PhysRevD.105.086001 |bibcode=2022PhRvD.105h6001D |s2cid=245353748 |access-date=2022-12-11 |archive-date=2023-01-22 |archive-url=https://web.archive.org/web/20230122174555/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.105.086001 |url-status=live }}</ref> This field of study is called [[phenomenological quantum gravity]].