Wikipedia

Graviton

This article is about the hypothetical particle. For other uses, seeGraviton (disambiguation).

In theories ofquantum gravity,thegravitonis the hypotheticalelementary particlethat mediates the force of gravitational interaction. There is no completequantum field theoryof gravitons due to an outstanding mathematical problem withrenormalizationingeneral relativity.Instring theory,believed by some to be a consistent theory of quantum gravity, the graviton is amasslessstate of a fundamental string.

Graviton
CompositionElementary particle
StatisticsBose–Einstein statistics
Familyspin-2 boson
InteractionsGravitation
StatusHypothetical
SymbolG[1]
Theorized1930s[2]
The name is attributed toDmitry Blokhintsevand F. M. Gal'perin in 1934[3]
Mass0
<6×10−32eV/c2[4]
Mean lifetimestable
Electric charge0e
Color chargeNo
Spin2ħ

If it exists, the graviton is expected to bemasslessbecause the gravitational force has a very long range, and appears to propagate at the speed of light. The graviton must be aspin-2bosonbecause the source of gravitation is thestress–energy tensor,a second-ordertensor(compared withelectromagnetism's spin-1photon,the source of which is thefour-current,a first-order tensor). Additionally, it can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton.[5]

Theory

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It is hypothesized that gravitational interactions are mediated by an as yet undiscovered elementary particle, dubbed thegraviton.The three other knownforcesof nature are mediated by elementary particles:electromagnetismby thephoton,thestrong interactionbygluons,and theweak interactionby theW and Z bosons.All three of these forces appear to be accurately described by theStandard Modelof particle physics. In theclassical limit,a successful theory of gravitons would reduce togeneral relativity,which itself reduces toNewton's law of gravitationin the weak-field limit.[6][7][8]

History

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Albert Einsteindiscussed quantized gravitational radiation in 1916, the year following his publication ofgeneral relativity.[9]: 525  The termgravitonwas coined in 1934 by Soviet physicistsDmitry BlokhintsevandFyodor Galperin[ru].[3][9]Paul Diracreintroduced the term in a number of lectures in 1959, noting that the energy of the gravitational field should come in quanta.[10][11]A mediation of the gravitational interaction by particles was anticipated byPierre-Simon Laplace.[12]Just likeNewton's anticipation of photons,Laplace's anticipated "gravitons" had a greater speed than the speed of light in vacuum,the speed of gravitons expected in modern theories, and were not connected toquantum mechanicsorspecial relativity,since these theories didn't yet exist during Laplace's lifetime.

Gravitons and renormalization

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When describing graviton interactions, theclassical theoryofFeynman diagramsand semiclassical corrections such asone-loop diagramsbehave normally. However, Feynman diagrams with at least two loops lead toultraviolet divergences.[13]These infinite results cannot be removed because quantizedgeneral relativityis notperturbativelyrenormalizable,unlikequantum electrodynamicsand models such as theYang–Mills theory.Therefore, incalculable answers are found from the perturbation method by which physicists calculate the probability of a particle to emit or absorb gravitons, and the theory loses predictive veracity. Those problems and the complementary approximation framework are grounds to show that a theory more unified than quantized general relativity is required to describe the behavior near thePlanck scale.

Comparison with other forces

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Like theforce carriersof theother forces(seephoton,gluon,W and Z bosons), the graviton plays a role ingeneral relativity,in defining thespacetimein which events take place.In some descriptionsenergy modifies the "shape" ofspacetimeitself, and gravity is a result of this shape, an idea which at first glance may appear hard to match with the idea of a force acting between particles.[14]Because thediffeomorphisminvariance of the theory does not allow any particular space-time background to be singled out as the "true" space-time background, general relativity is said to bebackground-independent.In contrast, the Standard Model isnotbackground-independent, withMinkowski spaceenjoying a special status as the fixed background space-time.[15]A theory of quantum gravity is needed in order to reconcile these differences.[16]Whether this theory should be background-independent is an open question. The answer to this question will determine the understanding of what specific role gravitation plays in the fate of the universe.[17]

Energy and wavelength

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While gravitons are presumed to bemassless,they would still carryenergy,as does any other quantum particle.Photon energyandgluon energyare also carried by massless particles. It is unclear which variables might determine graviton energy, the amount of energy carried by a single graviton.

Alternatively,if gravitons are massive at all,the analysis of gravitational waves yielded a new upper bound on themassof gravitons. The graviton'sCompton wavelengthis at least1.6×1016m,or about 1.6light-years,corresponding to a graviton mass of no more than7.7×10−23eV/c2.[18]This relation between wavelength and mass-energy is calculated with thePlanck–Einstein relation,the same formula that relates electromagneticwavelengthtophoton energy.

Experimental observation

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Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, has been thought to be impossible with any physically reasonable detector.[19]The reason is the extremely lowcross sectionfor the interaction of gravitons with matter. For example, a detector with the mass ofJupiterand 100% efficiency, placed in close orbit around aneutron star,would only be expected to observe one graviton every 10 years, even under the most favorable conditions. It would be impossible to discriminate these events from the background ofneutrinos,since the dimensions of the required neutrino shield would ensure collapse into ablack hole.[19]It has been proposed that detecting single gravitons would be possible by quantum sensing.[20]Even quantum events may not indicate quantization of gravitational radiation.[21]

LIGOandVirgocollaborations' observations havedirectly detectedgravitational waves.[22][23][24]Others have postulated that graviton scattering yields gravitational waves as particle interactions yieldcoherent states.[25]Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton.[26]For example, if gravitational waves were observed to propagate slower thanc(thespeed of lightin vacuum), that would imply that the graviton has mass (however, gravitational waves must propagate slower thancin a region with non-zero mass density if they are to be detectable).[27]Observations of gravitational waves put an upper bound of1.76×10−23eV/c2on the graviton's mass.[28]Solar system planetary trajectory measurements by space missions such asCassiniandMESSENGERgive a comparable upper bound of3.16×10−23eV/c2.[29]The gravitational wave and planetary ephemeris need not agree: they test different aspects of a potential graviton-based theory.[30]: 71 

Astronomical observations of the kinematics of galaxies, especially thegalaxy rotation problemandmodified Newtonian dynamics,might point toward gravitons having non-zero mass.[31][32]

Difficulties and outstanding issues

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Most theories containing gravitons suffer from severe problems. Attempts to extend the Standard Model or other quantum field theories by adding gravitons run into serious theoretical difficulties at energies close to or above thePlanck scale.This is because of infinities arising due to quantum effects; technically, gravitation is notrenormalizable.Since classical general relativity andquantum mechanicsseem to be incompatible at such energies, from a theoretical point of view, this situation is not tenable. One possible solution is to replace particles withstrings.String theories are quantum theories of gravity in the sense that they reduce to classical general relativity plus field theory at low energies, but are fully quantum mechanical, contain a graviton, and are thought to be mathematically consistent.[33]

See also

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References

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  1. ^G is used to avoid confusion withgluons(symbol g)
  2. ^ Rovelli, C. (2001). "Notes for a brief history of quantum gravity".arXiv:gr-qc/0006061.
  3. ^ab Blokhintsev, D. I.;Gal'perin, F. M. (1934)."Гипотеза нейтрино и закон сохранения энергии"[Neutrino hypothesis and conservation of energy].Pod Znamenem Marxisma(in Russian).6:147–157.ISBN978-5-04-008956-7.
  4. ^Zyla, P.; et al. (Particle Data Group) (2020)."Review of Particle Physics: Gauge and Higgs bosons"(PDF).Progress of Theoretical and Experimental Physics.Archived(PDF)from the original on 2020-09-30.
  5. ^For a comparison of the geometric derivation and the (non-geometric) spin-2 field derivation of general relativity, refer to box 18.1 (and also 17.2.5) ofMisner, C. W.;Thorne, K. S.;Wheeler, J. A.(1973).Gravitation.W. H. Freeman.ISBN0-7167-0344-0.
  6. ^ Feynman, R. P.; Morinigo, F. B.; Wagner, W. G.; Hatfield, B. (1995).Feynman Lectures on Gravitation.Addison-Wesley.ISBN0-201-62734-5.
  7. ^ Zee, Anthony (2003).Quantum Field Theory in a Nutshell.Princeton, New Jersey:Princeton University Press.ISBN0-691-01019-6.
  8. ^ Randall, L. (2005).Warped Passages: Unraveling the Universe's Hidden Dimensions.Ecco Press.ISBN0-06-053108-8.
  9. ^abStachel, John (1999). "The Early History of Quantum Gravity (1916–1940)".Black Holes, Gravitational Radiation and the Universe.Fundamental Theories of Physics. Vol. 100. pp.525–534.doi:10.1007/978-94-017-0934-7_31.ISBN978-90-481-5121-9.
  10. ^Farmelo, Graham(2009).The Strangest Man: The Hidden Life of Paul Dirac, Quantum Genius.Faber and Faber. pp.367–368.ISBN978-0-571-22278-0.
  11. ^Debnath, Lokenath(2013)."A short biography of Paul A. M. Dirac and historical development of Dirac delta function".International Journal of Mathematical Education in Science and Technology.44(8):1201–1223.Bibcode:2013IJMES..44.1201D.doi:10.1080/0020739X.2013.770091.ISSN0020-739X.
  12. ^Zee, Anthony (2018-04-24).On Gravity: A Brief Tour of a Weighty Subject.Princeton, New Jersey: Princeton University Press.ISBN978-0-691-17438-9.
  13. ^Bern, Zvi; Chi, Huan-Hang; Dixon, Lance; Edison, Alex (2017-02-22)."Two-loop renormalization of quantum gravity simplified"(PDF).Physical Review D.95(4): 046013.arXiv:1701.02422.Bibcode:2017PhRvD..95d6013B.doi:10.1103/PhysRevD.95.046013.ISSN2470-0010.
  14. ^See the other Wikipedia articles ongeneral relativity,gravitational field,gravitational wave,etc.
  15. ^ Colosi, D.; et al. (2005). "Background independence in a nutshell: The dynamics of a tetrahedron".Classical and Quantum Gravity.22(14):2971–2989.arXiv:gr-qc/0408079.Bibcode:2005CQGra..22.2971C.doi:10.1088/0264-9381/22/14/008.S2CID17317614.
  16. ^ Witten, E. (1993). "Quantum Background Independence In String Theory".arXiv:hep-th/9306122.
  17. ^ Smolin, L. (2005). "The case for background independence".arXiv:hep-th/0507235.
  18. ^Abbott, B. P.; et al. (LIGO Scientific CollaborationandVirgo Collaboration) (1 June 2017). "GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2".Physical Review Letters.118(22): 221101.arXiv:1706.01812.Bibcode:2017PhRvL.118v1101A.doi:10.1103/PhysRevLett.118.221101.PMID28621973.S2CID206291714.
  19. ^ab Rothman, T.; Boughn, S. (2006). "Can Gravitons be Detected?".Foundations of Physics.36(12):1801–1825.arXiv:gr-qc/0601043.Bibcode:2006FoPh...36.1801R.doi:10.1007/s10701-006-9081-9.S2CID14008778.
  20. ^Tobar, Germain; et al. (22 August 2024)."Detecting single gravitons with quantum sensing".Nat Commun.15(1): 7229.arXiv:2308.15440.Bibcode:2024NatCo..15.7229T.doi:10.1038/s41467-024-51420-8.PMC11341900.PMID39174544.
  21. ^Carney, Daniel; Domcke, Valerie; Rodd, Nicholas L. (2024-02-05)."Graviton detection and the quantization of gravity".Physical Review D.109(4): 044009.arXiv:2308.12988.Bibcode:2024PhRvD.109d4009C.doi:10.1103/PhysRevD.109.044009.
  22. ^Abbott, B. P.; et al. (2016-02-11)."Observation of Gravitational Waves from a Binary Black Hole Merger".Physical Review Letters.116(6). LIGO Scientific Collaboration and Virgo Collaboration: 061102.arXiv:1602.03837.Bibcode:2016PhRvL.116f1102A.doi:10.1103/PhysRevLett.116.061102.ISSN0031-9007.PMID26918975.S2CID124959784.{{cite journal}}:CS1 maint: date and year (link)
  23. ^Castelvecchi, Davide; Witze, Witze (February 11, 2016). "Einstein's gravitational waves found at last".Nature News.doi:10.1038/nature.2016.19361.S2CID182916902.
  24. ^"Gravitational waves detected 100 years after Einstein's prediction".NSF – National Science Foundation.Retrieved2016-02-11.
  25. ^Senatore, L.; Silverstein, E.; Zaldarriaga, M. (2014). "New sources of gravitational waves during inflation".Journal of Cosmology and Astroparticle Physics.2014(8): 016.arXiv:1109.0542.Bibcode:2014JCAP...08..016S.doi:10.1088/1475-7516/2014/08/016.S2CID118619414.
  26. ^Dyson, Freeman (8 October 2013). "Is a Graviton Detectable?".International Journal of Modern Physics A.28(25): 1330041–1–1330035–14.Bibcode:2013IJMPA..2830041D.doi:10.1142/S0217751X1330041X.
  27. ^ Will, C. M. (1998)."Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries"(PDF).Physical Review D.57(4):2061–2068.arXiv:gr-qc/9709011.Bibcode:1998PhRvD..57.2061W.doi:10.1103/PhysRevD.57.2061.S2CID41690760.Archived(PDF)from the original on 2018-07-24.
  28. ^R Abbot; et al. (15 June 2021). "Tests of General Relativity with Binary Black Holes from the second LIGO-Virgo Gravitational-Wave Transient Catalog".Physical Review Letters.103(12): 122022.arXiv:2010.14529.Bibcode:2021PhRvD.103l2002A.doi:10.1103/PhysRevD.103.122002.
  29. ^L. Bernus; et al. (15 July 2020). "Constraint on the Yukawa suppression of the Newtonian potential from the planetary ephemeris INPOP19a".Physical Review Letters.102(2): 021501(R).arXiv:2006.12304.Bibcode:2020PhRvD.102b1501B.doi:10.1103/PhysRevD.102.021501.
  30. ^Fienga, Agnès; Minazzoli, Olivier (2024-01-29)."Testing theories of gravity with planetary ephemerides".Living Reviews in Relativity.27(1): 1.arXiv:2303.01821.Bibcode:2024LRR....27....1F.doi:10.1007/s41114-023-00047-0.ISSN1433-8351.
  31. ^Trippe, Sascha (2012). "A Simplified Treatment of Gravitational Interaction on Galactic Scales".Journal of the Korean Astronomical Society.46(1):41–47.arXiv:1211.4692.Bibcode:2013JKAS...46...41T.doi:10.5303/JKAS.2013.46.1.41.
  32. ^Platscher, Moritz; Smirnov, Juri; Meyer, Sven; Bartelmann, Matthias (2018). "Long range effects in gravity theories with Vainshtein screening".Journal of Cosmology and Astroparticle Physics.2018(12): 009.arXiv:1809.05318.Bibcode:2018JCAP...12..009P.doi:10.1088/1475-7516/2018/12/009.S2CID86859475.
  33. ^ Sokal, A.(July 22, 1996)."Don't Pull the String Yet on Superstring Theory".The New York Times.RetrievedMarch 26,2010.
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