View source for Physics:Problem of time

{{short description|Conceptual conflict between general relativity and quantum mechanics}}

In [[Physics:Theoretical physics|theoretical physics]], the '''problem of time''' is a conceptual conflict between [[Physics:General relativity|general relativity]] and [[Physics:Quantum mechanics|quantum mechanics]] in that quantum mechanics regards the flow of time as universal and absolute, whereas general relativity regards the flow of time as malleable and relative.<ref>{{Citation|last=Isham|first=C. J.|title=Canonical Quantum Gravity and the Problem of Time|date=1993|url=https://doi.org/10.1007/978-94-011-1980-1_6|work=Integrable Systems, Quantum Groups, and Quantum Field Theories|pages=157–287|editor-last=Ibort|editor-first=L. A.|series=NATO ASI Series|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-011-1980-1_6|isbn=978-94-011-1980-1|access-date=2021-01-04|editor2-last=Rodríguez|editor2-first=M. A.|arxiv=gr-qc/9210011|s2cid=116947742 }}; http://arxiv.org/abs/gr-qc/9210011</ref><ref>{{cite magazine | title=Quantum Gravity's Time Problem | author=Wolchover, Natalie | date=December 1, 2016 | magazine=Quanta Magazine | url=https://www.quantamagazine.org/quantum-gravitys-time-problem-20161201/}}</ref> This problem raises the question of what [[Time|time]] really is in a physical sense and whether it is truly a real, distinct phenomenon. It also involves the related question of why time seems to flow in a single direction, despite the fact that no known physical laws at the microscopic level seem to require a single direction.<ref>{{cite magazine | title=Newsflash: Time May Not Exist | author=Folger, Tim | date=June 12, 2007 | magazine=Discover | url=https://www.discovermagazine.com/the-sciences/newsflash-time-may-not-exist}}</ref>

== Time in quantum mechanics ==
In [[Physics:Classical mechanics|classical mechanics]], a special status is assigned to time in the sense that it is treated as a classical background parameter, external to the system itself. This special role is seen in the standard formulation of quantum mechanics. It is regarded as part of ''a priori'' given classical background with a well defined value. The classical treatment of time is intertwined with the [[Physics:Copenhagen interpretation|Copenhagen interpretation]] of quantum mechanics, and, thus, with the conceptual foundations of quantum theory: all measurements of observables are made at certain instants of time and probabilities are only assigned to such measurements.

== Overturning of absolute time in general relativity ==
Though classically [[Physics:Spacetime|spacetime]] appears to be an absolute background, within general relativity time is no longer a background parameter, but must be considered on equal footing with space, and time and space evolve together as spacetime. Gravity is a manifestation of spacetime geometry, and spacetime is not fixed, but dynamic, as evidenced by phenomena such as [[Astronomy:Gravitational wave|gravitational wave]]s.

== Proposed solutions to the problem of time ==
The quantum concept of time first emerged from early research on [[Physics:Quantum gravity|quantum gravity]], in particular from the work of [[Biography:Bryce DeWitt|Bryce DeWitt]] in the 1960s:,<ref name="Deutsch2011">{{cite book |author=Deutsch |first=David |url=https://books.google.com/books?id=Z7uFxViR19oC&pg=PT240 |title=The Fabric of Reality |date=14 April 2011 |publisher=Penguin Books Limited |isbn=978-0-14-196961-9 |page=240}}</ref> which resulted in the [[Wheeler–DeWitt equation]].

{{Blockquote|text=''"Other times are just special cases of other universes."''}}

In other words, time is an [[Quantum entanglement|entanglement]] phenomenon, which places all equal clock readings (of correctly prepared clocks – or of any objects usable as clocks) into the same history. This was first understood by physicists Don Page and [[Biography:William Wootters|William Wootters]] in 1983.<ref>{{cite book|first = David|last =  Deutsch|title= The Beginning of Infinity: Explanations that Transform The World| page= 299|date = 2011|publisher = Penguin UK|url = https://books.google.com/books?id=ex5xge75SR4C|isbn = 9780141969695}}</ref> They made a proposal to address the problem of time in systems like general relativity called conditional probabilities interpretation.<ref>{{cite journal|last1 = Page|first1 = Don N.|last2 =  Wootters|first2 =  William K.|date = 15 June 1983|journal =  Phys. Rev. D|volume = 27|page =  2885|title = Evolution without evolution: Dynamics described by stationary observables|issue = 12|doi=10.1103/PhysRevD.27.2885|bibcode =1983PhRvD..27.2885P}}</ref> It consists in promoting all variables to quantum operators, one of them as a clock, and asking conditional probability questions with respect to other variables. They arrived at a solution based on the quantum phenomenon of entanglement. Page and Wootters showed how [[Quantum entanglement|quantum entanglement]] can be used to measure time.<ref>{{cite web|last1=Aron|first1=Jacob|title=Entangled toy universe shows time may be an illusion|url=https://www.newscientist.com//article/dn24473-entangled-toy-universe-shows-time-may-be-an-illusion|archive-url=https://web.archive.org/web/20161018065554/http://www.newscientist.com//article/dn24473-entangled-toy-universe-shows-time-may-be-an-illusion|archive-date=2016-10-18|date=October 25, 2013}}</ref>

In 2013, at the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, Italy, Ekaterina Moreva, together with Giorgio Brida, Marco Gramegna, Vittorio Giovannetti, Lorenzo Maccone, and Marco Genovese performed the first experimental test of Page and Wootters' ideas. They confirmed that time is an emergent phenomenon for internal observers but absent for external observers of the universe just as the [[Wheeler–DeWitt equation]] predicts.<ref>{{cite web|title=Quantum Experiment Shows How Time 'Emerges' from Entanglement|url=https://medium.com/the-physics-arxiv-blog/quantum-experiment-shows-how-time-emerges-from-entanglement-d5d3dc850933|website=The Physics arXiv Blog|date=Oct 23, 2013|archive-url=https://web.archive.org/web/20170603063357/https://medium.com/the-physics-arxiv-blog/quantum-experiment-shows-how-time-emerges-from-entanglement-d5d3dc850933|archive-date=2017-06-03}}</ref><ref>{{cite journal|last1=Moreva|first1=Ekaterina|last2=Brida|first2=Giorgio|last3=Gramegna|first3=Marco|last4=Giovannetti|first4=Vittorio|last5=Maccone|first5=Lorenzo|last6=Genovese|first6=Marco|title=Time from quantum entanglement: An experimental illustration|journal=Physical Review A|date=20 May 2014|volume=89|issue=5|page=052122|doi=10.1103/PhysRevA.89.052122|arxiv=1310.4691|bibcode=2014PhRvA..89e2122M|s2cid=118638346 }}</ref><ref>{{cite journal|last1=Moreva|first1=Ekaterina|last2=Gramegna|first2=Marco|last3=Brida|first3=Giorgio|last4=Maccone|first4=Lorenzo|last5=Genovese|first5=Marco|title=Quantum time: Experimental multitime correlations|journal=Physical Review D|date=16 November 2017|volume=96 |issue=5|page=102005|doi=10.1103/PhysRevD.96.102005|arxiv=1710.00707|bibcode=2017PhRvD..96j2005M|s2cid=119431509 }}</ref>

Consistent discretizations approach developed by [[Biography:Jorge Pullin|Jorge Pullin]] and Rodolfo Gambini have no constraints. These are lattice approximation techniques for quantum gravity. In the canonical approach if one discretizes the constraints and equations of motion, the resulting discrete equations are inconsistent: they cannot be solved simultaneously. To address this problem one uses a technique based on discretizing the action of the theory and working with the discrete equations of motion. These are automatically guaranteed to be consistent. Most of the hard conceptual questions of quantum gravity are related to the presence of constraints in the theory. Consistent discretized theories are free of these conceptual problems and can be straightforwardly quantized, providing a solution to the problem of time. It is a bit more subtle than this. Although without constraints and having "general evolution", the latter is only in terms of a discrete parameter that isn't physically accessible. The way out is addressed in a way similar to the Page–Wooters approach. The idea is to pick one of the physical variables to be a clock and asks relational questions. These ideas, where the clock is also quantum mechanical, have actually led to a new interpretation of quantum mechanics — the [[Physics:Minority interpretations of quantum mechanics#Other|Montevideo interpretation]] of quantum mechanics.<ref>{{cite journal|last1=Gambini|first1=Rodolfo|last2=Pullin|first2=Jorge|title=The Montevideo interpretation of quantum mechanics: frequently asked questions|journal=Journal of Physics: Conference Series|date=1 June 2009|volume=174|issue=1|pages=012003|doi=10.1088/1742-6596/174/1/012003|arxiv=0905.4402|bibcode=2009JPhCS.174a2003G|s2cid=250680865 }}</ref><ref>{{cite journal|last1=Gambini|first1=Rodolfo|last2=Garc?a-Pintos|first2=Luis Pedro|last3=Pullin|first3=Jorge|title=An axiomatic formulation of the Montevideo interpretation of quantum mechanics|journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics|date=November 2011|volume=42|issue=4|pages=256–263|doi=10.1016/j.shpsb.2011.10.002|arxiv=1002.4209|url=http://philsci-archive.pitt.edu/9927/|bibcode=2011SHPMP..42..256G|s2cid=993508 }}</ref> This new interpretation solves the problems of the use of environmental decoherence as a solution to the problem of measurement in quantum mechanics by invoking fundamental limitations, due to the quantum mechanical nature of clocks, in the process of measurement in quantum mechanics. These limitations are very natural in the context of generally covariant theories as quantum gravity where the clock must be taken as one of the degrees of freedom of the system itself. They have also put forward this fundamental decoherence as a way to resolve the [[Astronomy:Black hole information paradox|black hole information paradox]].<ref>{{cite journal|last1=Gambini|first1=Rodolfo|last2=Porto|first2=Rafael A.|last3=Pullin|first3=Jorge|title=No Black Hole Information Puzzle In A Relational Universe|journal=International Journal of Modern Physics D|date=December 2004|volume=13|issue=10|pages=2315–2320|doi=10.1142/S0218271804006383|arxiv=hep-th/0405183|bibcode=2004IJMPD..13.2315G|s2cid=119424485 }}</ref><ref>{{cite journal|last1=Gambini|first1=Rodolfo|last2=Porto|first2=Rafael A.|last3=Pullin|first3=Jorge|title=Realistic Clocks, Universal Decoherence, and the Black Hole Information Paradox|journal=Physical Review Letters|date=6 December 2004|volume=93|issue=24|page=240401|doi=10.1103/PhysRevLett.93.240401|pmid=15697783|arxiv=hep-th/0406260|bibcode=2004PhRvL..93x0401G|s2cid=25174047 }}</ref> In certain circumstances, a matter field is used to de-parametrize the theory and introduce a physical Hamiltonian. This generates physical time evolution, not a constraint.

Reduced phase space quantization constraints are solved first then quantized. This approach was considered for some time to be impossible as it seems to require first finding the general solution to Einstein's equations. However, with use of ideas involved in Dittrich's approximation scheme (built on ideas of Rovelli) a way to explicitly implement, at least in principle, a reduced phase space quantization was made viable.<ref>{{cite journal|last1=Thiemann|first1=T|title=Reduced phase space quantization and Dirac observables|journal=Classical and Quantum Gravity|date=21 February 2006|volume=23|issue=4|pages=1163–1180|doi=10.1088/0264-9381/23/4/006|arxiv=gr-qc/0411031|bibcode=2006CQGra..23.1163T|s2cid=17230218}}</ref>

[[Biography:Avshalom Elitzur|Avshalom Elitzur]] and Shahar Dolev argue that quantum mechanical experiments such as the Quantum Liar<ref>Elitzur, A. C., & Dolev, S. (2005). [https://www.researchgate.net/profile/Shahar_Dolev/publication/226976724_Quantum_Phenomena_Within_a_New_Theory_of_Time/links/02bfe5144e115a6917000000.pdf "Quantum phenomena within a new theory of time"]. In Quo vadis quantum mechanics? (pp. 325-349). Springer, Berlin, Heidelberg.</ref> provide evidence of inconsistent histories, and that spacetime itself may therefore be subject to change affecting entire histories.<ref>Elitzur, A. C., & Dolev, S. (2003). [https://arxiv.org/abs/quant-ph/0207029 "Is there more to T?]. In The Nature of Time: Geometry, Physics and Perception (pp. 297-306). Springer, Dordrecht.</ref> Elitzur and Dolev also believe that an objective passage of time and relativity can be reconciled, and that it would resolve many of the issues with the block universe and the conflict between relativity and quantum mechanics.<ref>Elitzur, A. C., & Dolev, S. (2005). [https://www.researchgate.net/profile/Shahar_Dolev/publication/228343669_Becoming_as_a_bridge_between_quantum_mechanics_and_relativity/links/02bfe5144e11584319000000/Becoming-as-a-bridge-between-quantum-mechanics-and-relativity.pdf "Becoming as a bridge between quantum mechanics and relativity"]. In Endophysics, Time, Quantum And The Subjective: (With CD-ROM) (pp. 589-606).</ref>

One solution to the problem of time proposed by [[Biography:Lee Smolin|Lee Smolin]] is that there exists a "thick present" of events, in which two events in the present can be causally related to each other, but in contrast to the block universe view of time in which all time exists [[Philosophy:Eternalism (philosophy of time)|eternally]].<ref>{{cite journal | last1 = Smolin | first1 = L | year = 2015 | title = Temporal naturalism | arxiv = 1805.12468 | journal = Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics | volume = 52 | pages = 86–102  | doi = 10.1016/j.shpsb.2015.03.005 | bibcode = 2015SHPMP..52...86S | s2cid = 8344858 }}</ref> Marina Cortês and Lee Smolin argue that certain classes of discrete dynamical systems demonstrate time asymmetry and irreversibility, which is consistent with an objective passage of time.<ref>{{cite journal | last1 = Cortês | first1 = M. | last2 = Smolin | first2 = L. | year = 2018 | title = Reversing the irreversible: from limit cycles to emergent time symmetry | arxiv=1703.09696 | journal = Physical Review D | volume = 97 | issue = 2| page = 026004 | doi = 10.1103/physrevd.97.026004 | bibcode = 2018PhRvD..97b6004C | s2cid = 119067096 }}</ref>

== Weyl time in scale-invariant quantum gravity ==
Motivated by the [[Astronomy:Immirzi parameter|Immirzi ambiguity]] in [[Physics:Loop quantum gravity|loop quantum gravity]] and the near conformal invariance of the standard model of elementary particles,<ref>{{Cite journal|date=2007-05-10|title=Conformal symmetry and the Standard Model|journal=Physics Letters B|language=en|volume=648|issue=4|pages=312–317|doi=10.1016/j.physletb.2007.03.023| arxiv=hep-th/0612165|issn=0370-2693|doi-access=free|last1=Meissner|first1=Krzysztof A.|last2=Nicolai|first2=Hermann|bibcode=2007PhLB..648..312M }}</ref> Charles Wang and co-workers have argued that the problem of time may be related to an underlying [[Scale invariance|scale invariance]] of gravity-matter systems.<ref name=":0">{{Cite journal|last1=Wang|first1=Charles H.-T.|last2=Stankiewicz|first2=Marcin|date=2020-01-10|title=Quantization of time and the big bang via scale-invariant loop gravity|journal=Physics Letters B|language=en|volume=800|pages=135106|doi=10.1016/j.physletb.2019.135106|arxiv=1910.03300|bibcode=2020PhLB..80035106W |issn=0370-2693|doi-access=free}}</ref><ref name=":1">{{Cite journal|last1=Wang|first1=Charles H.-T.|last2=Rodrigues|first2=Daniel P. F.|date=2018-12-28|title=Closing the gaps in quantum space and time: Conformally augmented gauge structure of gravitation|url=https://link.aps.org/doi/10.1103/PhysRevD.98.124041|journal=Physical Review D|volume=98|issue=12|pages=124041|doi=10.1103/PhysRevD.98.124041|arxiv=1810.01232 |bibcode=2018PhRvD..98l4041W |hdl=2164/11713|s2cid=118961037 |hdl-access=free}}</ref><ref>{{Cite journal|last=Wang|first=Charles H.-T.|date=2005-10-06|title=Unambiguous spin-gauge formulation of canonical general relativity with conformorphism invariance|url=https://link.aps.org/doi/10.1103/PhysRevD.72.087501|journal=Physical Review D|volume=72|issue=8|pages=087501|doi=10.1103/PhysRevD.72.087501|arxiv=gr-qc/0507044|bibcode=2005PhRvD..72h7501W |s2cid=34995566 }}</ref> Scale invariance has also been proposed to resolve the hierarchy problem of fundamental couplings.<ref>{{Cite journal|last1=Shaposhnikov|first1=Mikhail|last2=Shkerin|first2=Andrey|date=2018-10-03|title=Gravity, scale invariance and the hierarchy problem|journal=Journal of High Energy Physics|language=en|volume=2018|issue=10|pages=24|doi=10.1007/JHEP10(2018)024|issn=1029-8479|doi-access=free|arxiv=1804.06376|bibcode=2018JHEP...10..024S }}</ref> As a global continuous symmetry, scale invariance generates a conversed Weyl current<ref name=":0" /><ref name=":1" /> according to Noether’s theorem. In scale-invariant cosmological models, this Weyl current naturally gives rise to a harmonic time.<ref>{{Cite journal|last1=Ferreira|first1=Pedro G.|last2=Hill|first2=Christopher T.|last3=Ross|first3=Graham G.|date=2017-02-08|title=Weyl current, scale-invariant inflation, and Planck scale generation|journal=Physical Review D|volume=95|issue=4|pages=043507|doi=10.1103/PhysRevD.95.043507|doi-access=free|arxiv=1610.09243|bibcode=2017PhRvD..95d3507F }}</ref> In the context of loop quantum gravity, Charles Wang et al. suggest that scale invariance may lead to the existence of a quantized time.<ref name=":0" />

== References ==
{{reflist}}

==Further reading==
* ''The Order of Time'' by [[Biography:Carlo Rovelli|Carlo Rovelli]]
* ''[[Physics:Time Reborn|Time Reborn]]'' by [[Biography:Lee Smolin|Lee Smolin]]
* ''The Singular Universe and the Reality of Time'' by [[Biography:Lee Smolin|Lee Smolin]] and [[Biography:Roberto Mangabeira Unger|Roberto Mangabeira Unger]]

[[Category:Philosophy of physics]]
[[Category:Philosophical problems]]
[[Category:Quantum gravity]]
[[Category:Theoretical physics]]
[[Category:Philosophy of time]]

{{Sourceattribution|Problem of time}}