Editing Quantum gravity (section)

== Experimental tests ==
As was emphasized above, quantum gravitational effects are extremely weak and therefore difficult to test. For this reason, the possibility of experimentally testing quantum gravity had not received much attention prior to the late 1990s. However, since the 2000s, physicists have realized that evidence for quantum gravitational effects can guide the development of the theory. Since theoretical development has been slow, the field of [[phenomenological quantum gravity]], which studies the possibility of experimental tests, has obtained increased attention.<ref>{{cite book |last=Hossenfelder |first=Sabine |title=Classical and Quantum Gravity: Theory, Analysis and Applications |date=2011 |publisher=Nova Publishers |isbn=978-1-61122-957-8 |editor=Frignanni |editor-first=V. R. |chapter=Experimental Search for Quantum Gravity |access-date=2012-04-01 |chapter-url=https://www.novapublishers.com/catalog/product_info.php?products_id=15903 |archive-url=https://web.archive.org/web/20170701040647/https://www.novapublishers.com/catalog/product_info.php?products_id=15903 |archive-date=2017-07-01 |url-status=dead}}</ref>

The most widely pursued possibilities for quantum gravity phenomenology include gravitationally mediated entanglement,<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>
violations of [[Lorentz covariance|Lorentz invariance]], imprints of quantum gravitational effects in the [[cosmic microwave background]] (in particular its polarization), and decoherence induced by fluctuations<ref>{{Cite journal|last1=Oniga|first1=Teodora|last2=Wang|first2=Charles H.-T.|date=2016-02-09|title=Quantum gravitational decoherence of light and matter|url=https://link.aps.org/doi/10.1103/PhysRevD.93.044027|journal=Physical Review D|volume=93|issue=4|pages=044027|doi=10.1103/PhysRevD.93.044027|arxiv=1511.06678|bibcode=2016PhRvD..93d4027O|hdl=2164/5830|s2cid=119210226|hdl-access=free|access-date=2021-01-01|archive-date=2023-01-22|archive-url=https://web.archive.org/web/20230122174605/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.93.044027|url-status=live}}</ref><ref>{{Cite journal|last1=Oniga|first1=Teodora|last2=Wang|first2=Charles H.-T.|date=2017-10-05|title=Quantum coherence, radiance, and resistance of gravitational systems|url=https://link.aps.org/doi/10.1103/PhysRevD.96.084014|journal=Physical Review D|volume=96|issue=8|pages=084014|doi=10.1103/PhysRevD.96.084014|arxiv=1701.04122|bibcode=2017PhRvD..96h4014O|hdl=2164/9320|s2cid=54777871|hdl-access=free|access-date=2021-01-01|archive-date=2023-01-22|archive-url=https://web.archive.org/web/20230122174600/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.96.084014|url-status=live}}</ref><ref>{{Cite journal|last1=Quiñones|first1=D. A.|last2=Oniga|first2=T.|last3=Varcoe|first3=B. T. H.|last4=Wang|first4=C. H.-T.|date=2017-08-15|title=Quantum principle of sensing gravitational waves: From the zero-point fluctuations to the cosmological stochastic background of spacetime|url=https://link.aps.org/doi/10.1103/PhysRevD.96.044018|journal=Physical Review D|volume=96|issue=4|pages=044018|doi=10.1103/PhysRevD.96.044018|arxiv=1702.03905|bibcode=2017PhRvD..96d4018Q|hdl=2164/9150|s2cid=55056264|hdl-access=free|access-date=2021-01-02|archive-date=2023-01-22|archive-url=https://web.archive.org/web/20230122174603/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.96.044018|url-status=live}}</ref> in the [[space-time foam]].<ref>{{Cite journal|last1=Oniga|first1=Teodora|last2=Wang|first2=Charles H.-T.|date=2016-09-19|title=Spacetime foam induced collective bundling of intense fields|url=https://link.aps.org/doi/10.1103/PhysRevD.94.061501|journal=Physical Review D|volume=94|issue=6|pages=061501|doi=10.1103/PhysRevD.94.061501|arxiv=1603.09193|bibcode=2016PhRvD..94f1501O|hdl=2164/7434|s2cid=54872718|hdl-access=free|access-date=2021-01-02|archive-date=2023-01-22|archive-url=https://web.archive.org/web/20230122174605/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.94.061501|url-status=live}}</ref> The latter scenario has been searched for in light from [[gamma-ray burst]]s and both astrophysical and atmospheric [[neutrino]]s, placing limits on phenomenological quantum gravity parameters.<ref>{{Cite journal |last1=Vasileiou |first1=Vlasios |last2=Granot |first2=Jonathan |last3=Piran |first3=Tsvi |last4=Amelino-Camelia |first4=Giovanni |date=2015-03-16 |title=A Planck-scale limit on spacetime fuzziness and stochastic Lorentz invariance violation |journal=Nature Physics |volume=11 |issue=4 |pages=344–346 |doi=10.1038/nphys3270 |bibcode=2015NatPh..11..344V |s2cid=54727053 |issn=1745-2473|doi-access=free }}</ref><ref>{{Cite journal |last1=The IceCube Collaboration |last2=Abbasi |first2=R. |last3=Ackermann |first3=M. |last4=Adams |first4=J. |last5=Aguilar |first5=J. A. |last6=Ahlers |first6=M. |last7=Ahrens |first7=M. |last8=Alameddine |first8=J. M. |last9=Alispach |first9=C. |last10=Alves Jr |first10=A. A. |last11=Amin |first11=N. M. |last12=Andeen |first12=K. |last13=Anderson |first13=T. |last14=Anton |first14=G. |last15=Argüelles |first15=C. |date=2022-11-01 |title=Search for quantum gravity using astrophysical neutrino flavour with IceCube |url=https://www.nature.com/articles/s41567-022-01762-1 |journal=Nature Physics |language=en |volume=18 |issue=11 |pages=1287–1292 |doi=10.1038/s41567-022-01762-1 |bibcode=2022NatPh..18.1287I |s2cid=243848123 |issn=1745-2473|arxiv=2111.04654 }}</ref><ref>{{arxiv|hep-ex/2308.00105}}</ref>

[[ESA]]'s [[INTEGRAL]] satellite measured polarization of photons of different wavelengths and was able to place a limit in the granularity of space that is less than 10<sup>−48</sup>&nbsp;m, or 13 orders of magnitude below the Planck scale.<ref>{{Cite web|date=2011-06-30|title=Integral challenges physics beyond Einstein|url=https://www.esa.int/Science_Exploration/Space_Science/Integral_challenges_physics_beyond_Einstein|url-status=live|access-date=2021-11-06|website=European Space Agency|archive-date=2021-11-13|archive-url=https://web.archive.org/web/20211113230038/https://www.esa.int/Science_Exploration/Space_Science/Integral_challenges_physics_beyond_Einstein}}</ref><ref>{{Cite journal|last1=Laurent|first1=P.|last2=Götz|first2=D.|last3=Binétruy|first3=P.|last4=Covino|first4=S.|last5=Fernandez-Soto|first5=A.|date=2011-06-28|title=Constraints on Lorentz Invariance Violation using integral/IBIS observations of GRB041219A|url=https://link.aps.org/doi/10.1103/PhysRevD.83.121301|journal=Physical Review D|language=en|volume=83|issue=12|pages=121301|doi=10.1103/PhysRevD.83.121301|arxiv=1106.1068|bibcode=2011PhRvD..83l1301L|s2cid=53603505|issn=1550-7998|access-date=2021-11-06|archive-date=2023-01-22|archive-url=https://web.archive.org/web/20230122175047/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.83.121301|url-status=live}}</ref>{{better source needed|date=September 2024}}

The [[BICEP and Keck Array|BICEP2 experiment]] detected what was initially thought to be primordial [[B-modes|B-mode polarization]] caused by [[gravitational wave]]s in the early universe. Had the signal in fact been primordial in origin, it could have been an indication of quantum gravitational effects, but it soon transpired that the polarization was due to [[cosmic dust|interstellar dust]] interference.<ref name="nature-20150130">
{{cite journal
 |last=Cowen |first=Ron
 |date=30 January 2015
 |title=Gravitational waves discovery now officially dead
 |journal=[[Nature (journal)|Nature]]
 |doi=10.1038/nature.2015.16830
|s2cid=124938210
 }}</ref>