Physics:Quantum gravity - Revision history

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	* [http://abyss.uoregon.edu/~js/cosmo/lectures/lec20.html "Planck Era" and "Planck Time"] {{Webarchive\|url=https://web.archive.org/web/20181128045313/http://abyss.uoregon.edu/~js/cosmo/lectures/lec20.html \|date=2018-11-28 }} (up to 10<sup>−43</sup> seconds after birth of Universe) (University of Oregon).		* [http://abyss.uoregon.edu/~js/cosmo/lectures/lec20.html "Planck Era" and "Planck Time"] {{Webarchive\|url=https://web.archive.org/web/20181128045313/http://abyss.uoregon.edu/~js/cosmo/lectures/lec20.html \|date=2018-11-28 }} (up to 10<sup>−43</sup> seconds after birth of Universe) (University of Oregon).
	* [http://www.bbc.co.uk/programmes/p00547c4 "Quantum Gravity"], BBC Radio 4 discussion with John Gribbin, Lee Smolin and Janna Levin (''In Our Time'', February 22, 2001)		* [http://www.bbc.co.uk/programmes/p00547c4 "Quantum Gravity"], BBC Radio 4 discussion with John Gribbin, Lee Smolin and Janna Levin (''In Our Time'', February 22, 2001)

	~~{{Quantum gravity}}~~
	~~{{Theories of gravitation}}~~
	{{Relativity}}		{{Relativity}}
	{{Standard model of physics}}		{{Standard model of physics}}
	~~[[Category:Quantum gravity\| ]]~~
	~~[[Category:General relativity]]~~
	~~[[Category:Physics beyond the Standard Model]]~~
	{{Author\|Harold Foppele}}		{{Author\|Harold Foppele}}

	{{Sourceattribution\|Physics:Quantum gravity\|1}}		{{Sourceattribution\|Physics:Quantum gravity\|1}}

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	{{Relativity}}		{{Relativity}}
	{{Standard model of physics}}		{{Standard model of physics}}
	~~{{Quantum mechanics topics}}~~

	[[Category:Quantum gravity\| ]]		[[Category:Quantum gravity\| ]]
	[[Category:General relativity]]		[[Category:General relativity]]

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	{{~~short~~ description\|Description of gravity using discrete values}}		{{Short description\|Description of gravity using discrete values}}
	{{Quantum book backlink\|Advanced and frontier topics}}		{{Quantum book backlink\|Advanced and frontier topics}}
	{{Quantum article nav\|previous=Physics:Quantum Holographic principle\|previous label=Holographic principle\|next=Physics:Quantum De Sitter invariant special relativity\|next label=De Sitter invariant special relativity}}		{{Quantum article nav\|previous=Physics:Quantum Holographic principle\|previous label=Holographic principle\|next=Physics:Quantum De Sitter invariant special relativity\|next label=De Sitter invariant special relativity}}

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	{{Quantum book backlink\|Advanced and frontier topics}}		{{Quantum book backlink\|Advanced and frontier topics}}
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			{{Quantum article nav\|previous=Physics:Quantum Holographic principle\|previous label=Holographic principle\|next=Physics:Quantum De Sitter invariant special relativity\|next label=De Sitter invariant special relativity}}

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	{{Quantum book backlink\|Advanced and frontier topics}}		{{Quantum book backlink\|Advanced and frontier topics}}
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	<div style="flex:1; line-height:1.45; color:#006b45; column-count:2; column-gap:32px; column-rule:1px solid #b8d8c8;">		<div style="flex:1; line-height:1.45; color:#006b45; column-count:2; column-gap:32px; column-rule:1px solid #b8d8c8;">
	Although general relativity is highly regarded for its elegance and accuracy it has limitations: the gravitational singularities inside of 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 ~~[[Wikipedia:~~Planck length~~\|Planck length]]~~, like those near the center of the black hole, quantum fluctuations of spacetime are expected to play an important role.<ref>{{cite web		Although general relativity is highly regarded for its elegance and accuracy it has limitations: the gravitational singularities inside of 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 the 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/		\| url = https://www.quantamagazine.org/black-hole-singularities-are-as-inescapable-as-expected-20191202/
	\| title = Black Hole Singularities Are as Inescapable as Expected		\| title = Black Hole Singularities Are as Inescapable as Expected
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	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 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, 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>		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 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, 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 accelerators. 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>		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 accelerators. 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		Thought experiment approaches have been suggested as a testing tool for quantum gravity theories.<ref>{{cite journal
	\|last = Bose		\|last = Bose
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	=== Graviton ===		=== Graviton ===
	~~{{Main\|Physics~~:Graviton}}		''Related topic:'' Graviton
	The observation that all fundamental forces except gravity have one or more known messenger particles leads researchers to believe that at least one must exist for gravity. This hypothetical particle is known as the ''graviton''. These particles act as a force particle similar to the photon of the electromagnetic interaction. Under mild assumptions, the structure of general relativity requires them to follow the quantum mechanical description of interacting theoretical spin-2 massless particles.<ref name=Kraichnan1955>{{cite journal		The observation that all fundamental forces except gravity have one or more known messenger particles leads researchers to believe that at least one must exist for gravity. This hypothetical particle is known as the ''graviton''. These particles act as a force particle similar to the photon of the electromagnetic interaction. Under mild assumptions, the structure of general relativity requires them to follow the quantum mechanical description of interacting theoretical spin-2 massless particles.<ref name=Kraichnan1955>{{cite journal
	\|last = Kraichnan		\|last = Kraichnan
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	=== Nonrenormalizability of gravity ===		=== Nonrenormalizability of gravity ===
	~~{{Further\|Physics~~:Renormalization~~\|Physics:~~Asymptotic safety in quantum gravity}}		''Related topic:'' Renormalization, Asymptotic safety in quantum gravity
	General relativity, like electromagnetism, is a classical field theory. One might expect that, as with electromagnetism, the gravitational force should also have a corresponding [[Physics:Quantum field theory\|quantum field theory]].		General relativity, like electromagnetism, is a classical field theory. One might expect that, as with electromagnetism, the gravitational force should also have a corresponding [[Physics:Quantum field theory\|quantum field theory]].

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	=== Quantum gravity as an effective field theory ===		=== Quantum gravity as an effective field theory ===
	~~{{Main\|Physics~~:Effective field theory}}		''Related topic:'' Effective field theory
	In an effective field theory, all but the first few of the infinite set of parameters in a nonrenormalizable theory are suppressed by huge energy scales and hence can be neglected when computing low-energy effects. Thus, at least in the low-energy regime, the model is a predictive quantum field theory.<ref name=":0">{{cite book		In an effective field theory, all but the first few of the infinite set of parameters in a nonrenormalizable theory are suppressed by huge energy scales and hence can be neglected when computing low-energy effects. Thus, at least in the low-energy regime, the model is a predictive quantum field theory.<ref name=":0">{{cite book
	\|last = Donoghue		\|last = Donoghue
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	=== Spacetime background dependence ===		=== Spacetime background dependence ===
	~~{{Main\|Physics~~:Background independence}}		''Related topic:'' Background independence
	A fundamental lesson of general relativity is that there is no fixed spacetime background, as found in Newtonian mechanics and special relativity; the spacetime geometry is dynamic. While simple to grasp in principle, this is a complex idea to understand about general relativity, and its consequences are profound and not fully explored, even at the classical level. To a certain extent, general relativity can be seen to be a relational theory,<ref>{{cite book		A fundamental lesson of general relativity is that there is no fixed spacetime background, as found in Newtonian mechanics and special relativity; the spacetime geometry is dynamic. While simple to grasp in principle, this is a complex idea to understand about general relativity, and its consequences are profound and not fully explored, even at the classical level. To a certain extent, general relativity can be seen to be a relational theory,<ref>{{cite book
	\|last = Smolin		\|last = Smolin
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	=== Semi-classical quantum gravity ===		=== Semi-classical quantum gravity ===
	{{Main article\|Physics:Quantum field theory in curved spacetime~~\|Physics:Semiclassical gravity~~}}		{{Main article\|Physics:Quantum field theory in curved spacetime}}
	Quantum field theory on curved (non-Minkowskian) backgrounds, while not a full quantum theory of gravity, has shown many promising early results. In an analogous way to the development of quantum electrodynamics in the early part of the 20th century (when physicists considered quantum mechanics in classical electromagnetic fields), the consideration of quantum field theory on a curved background has led to predictions such as black hole radiation.		Quantum field theory on curved (non-Minkowskian) backgrounds, while not a full quantum theory of gravity, has shown many promising early results. In an analogous way to the development of quantum electrodynamics in the early part of the 20th century (when physicists considered quantum mechanics in classical electromagnetic fields), the consideration of quantum field theory on a curved background has led to predictions such as black hole radiation.

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	=== Problem of time ===		=== Problem of time ===
	~~{{Main\|Physics~~:Problem of time}}		''Related topic:'' Problem of time
	A conceptual difficulty in combining quantum mechanics with general relativity arises from the contrasting role of time within these two frameworks. In quantum theories, time acts as an independent background through which states evolve, with the Hamiltonian operator acting as the generator of infinitesimal translations of quantum states through time.<ref>{{Cite book\|title=Modern Quantum Mechanics\|last1=Sakurai\|first1=J. J.\|last2=Napolitano\|first2=Jim J.\|date=2010-07-14\|publisher=Pearson\|isbn=978-0-8053-8291-4\|edition=2\|page=68\|language=en}}</ref> In contrast, general relativity treats time as a dynamical variable which relates directly with matter and moreover requires the Hamiltonian constraint to vanish.<ref>{{Cite book\|url=https://books.google.com/books?id=vDWvUBiNgNkC\|title=Cosmology and Gravitation: Xth Brazilian School of Cosmology and Gravitation; 25th Anniversary (1977–2002), Mangaratiba, Rio de Janeiro, Brazil\|last1=Novello\|first1=Mario\|last2=Bergliaffa\|first2=Santiago E.\|date=2003-06-11\|publisher=Springer Science & Business Media\|isbn=978-0-7354-0131-0\|page=95\|language=en}}</ref> Because this variability of time has been observed macroscopically, it removes any possibility of employing a fixed notion of time, similar to the conception of time in quantum theory, at the macroscopic level.		A conceptual difficulty in combining quantum mechanics with general relativity arises from the contrasting role of time within these two frameworks. In quantum theories, time acts as an independent background through which states evolve, with the Hamiltonian operator acting as the generator of infinitesimal translations of quantum states through time.<ref>{{Cite book\|title=Modern Quantum Mechanics\|last1=Sakurai\|first1=J. J.\|last2=Napolitano\|first2=Jim J.\|date=2010-07-14\|publisher=Pearson\|isbn=978-0-8053-8291-4\|edition=2\|page=68\|language=en}}</ref> In contrast, general relativity treats time as a dynamical variable which relates directly with matter and moreover requires the Hamiltonian constraint to vanish.<ref>{{Cite book\|url=https://books.google.com/books?id=vDWvUBiNgNkC\|title=Cosmology and Gravitation: Xth Brazilian School of Cosmology and Gravitation; 25th Anniversary (1977–2002), Mangaratiba, Rio de Janeiro, Brazil\|last1=Novello\|first1=Mario\|last2=Bergliaffa\|first2=Santiago E.\|date=2003-06-11\|publisher=Springer Science & Business Media\|isbn=978-0-7354-0131-0\|page=95\|language=en}}</ref> Because this variability of time has been observed macroscopically, it removes any possibility of employing a fixed notion of time, similar to the conception of time in quantum theory, at the macroscopic level.

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	=== String theory ===		=== String theory ===
	~~{{Main\|Physics~~:String theory}}		''Related topic:'' String theory
	[[File:Calabi-Yau.png\|thumb\|Projection of a Calabi–Yau manifold, one of the ways of compactifying the extra dimensions posited by string theory]]		[[File:Calabi-Yau.png\|thumb\|Projection of a Calabi–Yau manifold, one of the ways of compactifying the extra dimensions posited by string theory]]

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	=== Loop quantum gravity ===		=== Loop quantum gravity ===
	~~{{Main\|Physics~~:Loop quantum gravity}}		''Related topic:'' Loop quantum gravity
	[[File:Spin network.svg\|right\|thumb\|200px\|Simple spin network of the type used in loop quantum gravity]]		[[File:Spin network.svg\|right\|thumb\|200px\|Simple spin network of the type used in loop quantum gravity]]
	Loop quantum gravity seriously considers general relativity's insight that spacetime is a dynamical field and is therefore a quantum object. Its second idea is that the quantum discreteness that determines the particle-like behavior of other field theories (for instance, the photons of the electromagnetic field) also affects the structure of space.		Loop quantum gravity seriously considers general relativity's insight that spacetime is a dynamical field and is therefore a quantum object. Its second idea is that the quantum discreteness that determines the particle-like behavior of other field theories (for instance, the photons of the electromagnetic field) also affects the structure of space.

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Clean Book I red links, intro, and image slots

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	{{Quantum book backlink\|Advanced and frontier topics}}		{{Quantum book backlink\|Advanced and frontier topics}}
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	== Overview ==		== Overview ==
	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 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>		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 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, 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 accelerators. 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>		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 accelerators. 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>
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	=== Problem of time ===		=== Problem of time ===
	{{Main\|Physics:Problem of time}}		{{Main\|Physics:Problem of time}}
	A conceptual difficulty in combining quantum mechanics with general relativity arises from the contrasting role of time within these two frameworks. In quantum theories, time acts as an independent background through which states evolve, with the Hamiltonian operator acting as the generator of infinitesimal translations of quantum states through time.<ref>{{Cite book\|title=Modern Quantum Mechanics\|last1=Sakurai\|first1=J. J.\|last2=Napolitano\|first2=Jim J.\|date=2010-07-14\|publisher=Pearson\|isbn=978-0-8053-8291-4\|edition=2\|page=68\|language=en}}</ref> In contrast, general relativity treats time as a dynamical variable which relates directly with matter and moreover requires the Hamiltonian constraint to vanish.<ref>{{Cite book\|url=https://books.google.com/books?id=vDWvUBiNgNkC\|title=Cosmology and Gravitation: Xth Brazilian School of Cosmology and Gravitation; 25th Anniversary (1977–2002), Mangaratiba, Rio de Janeiro, Brazil\|last1=Novello\|first1=Mario\|last2=Bergliaffa\|first2=Santiago E.\|date=2003-06-11\|publisher=Springer Science & Business Media\|isbn=978-0-7354-0131-0\|page=95\|language=en}}</ref> Because this variability of time has been ~~[[Physics:Gravitational time dilation#Experimental confirmation\|~~observed macroscopically]], it removes any possibility of employing a fixed notion of time, similar to the conception of time in quantum theory, at the macroscopic level.		A conceptual difficulty in combining quantum mechanics with general relativity arises from the contrasting role of time within these two frameworks. In quantum theories, time acts as an independent background through which states evolve, with the Hamiltonian operator acting as the generator of infinitesimal translations of quantum states through time.<ref>{{Cite book\|title=Modern Quantum Mechanics\|last1=Sakurai\|first1=J. J.\|last2=Napolitano\|first2=Jim J.\|date=2010-07-14\|publisher=Pearson\|isbn=978-0-8053-8291-4\|edition=2\|page=68\|language=en}}</ref> In contrast, general relativity treats time as a dynamical variable which relates directly with matter and moreover requires the Hamiltonian constraint to vanish.<ref>{{Cite book\|url=https://books.google.com/books?id=vDWvUBiNgNkC\|title=Cosmology and Gravitation: Xth Brazilian School of Cosmology and Gravitation; 25th Anniversary (1977–2002), Mangaratiba, Rio de Janeiro, Brazil\|last1=Novello\|first1=Mario\|last2=Bergliaffa\|first2=Santiago E.\|date=2003-06-11\|publisher=Springer Science & Business Media\|isbn=978-0-7354-0131-0\|page=95\|language=en}}</ref> Because this variability of time has been observed macroscopically, it removes any possibility of employing a fixed notion of time, similar to the conception of time in quantum theory, at the macroscopic level.

	== Candidate theories ==		== Candidate theories ==
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	}}</ref> The theory is successful in that one mode will always correspond to a graviton, the messenger particle of gravity; however, the price of this success is unusual features such as six extra dimensions of space in addition to the usual three for space and one for time.<ref>For the graviton as part of the string spectrum, e.g. {{Harvnb\|Green\|Schwarz\|Witten\|1987\|loc=sec. 2.3 and 5.3}}; for the extra dimensions, ibid sec. 4.2.</ref>		}}</ref> The theory is successful in that one mode will always correspond to a graviton, the messenger particle of gravity; however, the price of this success is unusual features such as six extra dimensions of space in addition to the usual three for space and one for time.<ref>For the graviton as part of the string spectrum, e.g. {{Harvnb\|Green\|Schwarz\|Witten\|1987\|loc=sec. 2.3 and 5.3}}; for the extra dimensions, ibid sec. 4.2.</ref>

	In what is called the ~~[[Physics:History of string theory#1984–1989: first superstring revolution\|~~second superstring revolution]], it was conjectured that both string theory and a unification of general relativity and supersymmetry known as supergravity<ref>{{Cite book		In what is called the second superstring revolution, it was conjectured that both string theory and a unification of general relativity and supersymmetry known as supergravity<ref>{{Cite book
	\|last=Weinberg		\|last=Weinberg
	\|first=Steven		\|first=Steven
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	* Causal Set Theory		* Causal Set Theory
	* Covariant Feynman path integral approach		* Covariant Feynman path integral approach
	* ~~[[Physics:Dilaton#The dilaton in quantum gravity\|~~Dilatonic quantum gravity]]		* Dilatonic quantum gravity
	*Double copy theory		*Double copy theory
	* Group field theory		* Group field theory

Maintenance script: Clean Book I red links, intro, and image slots

2026-05-20T08:18:42Z

Clean Book I red links, intro, and image slots

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Maintenance script: Normalize Quantum book page structure and short text

2026-05-19T23:06:44Z

Normalize Quantum book page structure and short text

← Older revision		Revision as of 23:06, 19 May 2026
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	{{Author\|Harold Foppele}}		{{Author\|Harold Foppele}}

	{{Sourceattribution\|Quantum gravity\|1}}		{{Sourceattribution\|Physics:Quantum gravity\|1}}

← Older revision		Revision as of 11:12, 22 May 2026
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	{{short description\|Description of gravity using discrete values}}		{{short description\|Description of gravity using discrete values}}

	{{Quantum book backlink\|Advanced and frontier topics}}		{{Quantum book backlink\|Advanced and frontier topics}}
	{{Quantum article nav\|previous=Physics:Quantum Holographic principle\|previous label=Holographic principle\|next=Physics:Quantum De Sitter invariant special relativity\|next label=De Sitter invariant special relativity}}		{{Quantum article nav\|previous=Physics:Quantum Holographic principle\|previous label=Holographic principle\|next=Physics:Quantum De Sitter invariant special relativity\|next label=De Sitter invariant special relativity}}