Editing Quantum entanglement (section)

=== In quantum gravity ===
There is a fundamental conflict, referred to as the [[problem of time]], between the way the concept of ''time'' is used in quantum mechanics, and the role it plays in [[general relativity]]. In standard quantum theories time acts as an independent background through which states evolve, while  general relativity treats time as a dynamical variable which relates directly with matter. Part of the effort to reconcile these approaches to time results in the [[Wheeler–DeWitt equation]], which predicts the state of the universe is timeless or static, contrary to ordinary experience.<ref name=Moreva2014>{{cite journal|title= Time from quantum entanglement: an experimental illustration|arxiv=1310.4691|bibcode = 2014PhRvA..89e2122M |doi = 10.1103/PhysRevA.89.052122|volume=89|issue= 5|pages=052122|journal=Physical Review A|year=2014 | last1 = Moreva | first1 = Ekaterina|s2cid=118638346}}</ref> 
Work started by [[Don Page (physicist)|Don Page]] and [[William Wootters]]<ref>{{cite journal |last1=Page |first1=Don N. |last2=Wootters |first2=William K. |date=15 June 1983 |title=Evolution without evolution: Dynamics described by stationary observables |url=https://link.aps.org/doi/10.1103/PhysRevD.27.2885 |journal=Physical Review D |volume=27 |issue=12 |pages=2885–2892 |doi=10.1103/PhysRevD.27.2885|bibcode=1983PhRvD..27.2885P }}</ref><ref>{{cite journal |last=Rovelli |first=Carlo |date=15 October 1990 |title=Quantum mechanics without time: A model |url=https://link.aps.org/doi/10.1103/PhysRevD.42.2638 |journal=Physical Review D |volume=42 |issue=8 |pages=2638–2646 |doi=10.1103/PhysRevD.42.2638|pmid=10013133 |bibcode=1990PhRvD..42.2638R }}</ref><ref>{{cite journal |last1=Giovannetti |first1=Vittorio |last2=Lloyd |first2=Seth |last3=Maccone |first3=Lorenzo |date=26 August 2015 |title=Quantum time |url=https://link.aps.org/doi/10.1103/PhysRevD.92.045033 |journal=Physical Review D |volume=92 |issue=4 |pages=045033 |doi=10.1103/PhysRevD.92.045033|arxiv=1504.04215 |bibcode=2015PhRvD..92d5033G |hdl=1721.1/98287 |s2cid=85537706 }}</ref> suggests that the universe appears to evolve for observers on the inside because of energy entanglement between an evolving system and a clock system, both within the universe.<ref name=Moreva2014/> In this way the overall system can remain timeless while parts experience time via entanglement. The issue remains an open question closely related to attempts at theories of [[quantum gravity]].<ref>{{cite journal |last1=Altaie |first1=M. Basil |last2=Hodgson |first2=Daniel |last3=Beige |first3=Almut |date=3 June 2022 |title=Time and Quantum Clocks: A Review of Recent Developments |journal=Frontiers in Physics |language=English |volume=10 |doi=10.3389/fphy.2022.897305 |doi-access=free |arxiv=2203.12564 |bibcode=2022FrP....10.7305A |issn=2296-424X}}</ref><ref>{{cite book |last=Isham |first=C. J. |url=https://link.springer.com/chapter/10.1007/978-94-011-1980-1_6 |title=Integrable Systems, Quantum Groups, and Quantum Field Theories |date=1993 |publisher=Springer Netherlands |isbn=978-94-011-1980-1 |editor-last=Ibort |editor-first=L. A. |location=Dordrecht |pages=157–287 |language=en |doi=10.1007/978-94-011-1980-1_6 |editor-last2=Rodríguez |editor-first2=M. A.}}</ref>

In general relativity, gravity arises from the curvature of spacetime and that curvature derives from the distribution of matter. However, matter is governed by quantum mechanics. Integration of these two theories faces many problems. In an (unrealistic) model space called the [[anti-de Sitter space]], the [[AdS/CFT correspondence]] allows a quantum gravitational system to be related to a quantum field theory without gravity.<ref name=Swingle2018>{{cite journal |last=Swingle |first=Brian |date=10 March 2018 |title=Spacetime from Entanglement |url=https://www.annualreviews.org/doi/10.1146/annurev-conmatphys-033117-054219 |journal=Annual Review of Condensed Matter Physics |language=en |volume=9 |issue=1 |pages=345–358 |doi=10.1146/annurev-conmatphys-033117-054219 |bibcode=2018ARCMP...9..345S |issn=1947-5454}}</ref> Using this correspondence, [[Mark Van Raamsdonk]] suggested that [[spacetime]] arises as an emergent phenomenon of the quantum degrees of freedom that are entangled and live in the boundary of the spacetime.<ref>{{cite journal |last=Van Raamsdonk |first=Mark |date=2010 |title=Building up spacetime with quantum entanglement |url=https://www.worldscientific.com/doi/abs/10.1142/S0218271810018529 |journal=International Journal of Modern Physics D |language=en |volume=19 |issue=14 |pages=2429–2435 |doi=10.1142/S0218271810018529 |bibcode=2010IJMPD..19.2429V |issn=0218-2718|arxiv=1005.3035 }}</ref>