View source for Graviton

{{Short description|Hypothetical elementary particle that mediates gravity}}
{{About|the hypothetical particle}}
{{pp-move|small=yes}}
{{Infobox Particle
| name = Graviton
| num_types =
| composition = [[Elementary particle]]
| statistics = [[Bose–Einstein statistics]]
| group = spin-2 boson
| generation =
| interaction = [[Gravitation]]
| particle =

| status = [[Hypothetical]]
| theorized = 1930s<ref>
  {{cite arXiv
   |last = Rovelli |first=C.
   |date = 2001
   |title = Notes for a brief history of quantum gravity
   |eprint = gr-qc/0006061
  }}</ref><br />The name is attributed to Dmitrii Blokhintsev and F. M. Gal'perin in 1934<ref name=Blokhintsev>
  {{cite journal
   |last1 = Blokhintsev |first1=D. I.
   |last2 = Gal'perin |first2=F. M.
   |date = 1934
   |title = Гипотеза нейтрино и закон сохранения энергии|url=https://books.google.com/books?id=V2ktDAAAQBAJ&pg=PA664
   |trans-title=Neutrino hypothesis and conservation of energy
   |journal = Pod Znamenem Marxisma
   |volume = 6 |pages=147–157
   |language=ru|isbn=978-5-04-008956-7
  }}</ref>
| discovered = 
| symbol = G<ref>G is used to avoid confusion with [[gluon]]s (symbol g)</ref>
| mass = 0
<br />{{nowrap|&lt; {{val|6|e=-32|ul=eV/c2}} }}<ref name="Particle_table_2020">{{cite journal  |last=Zyla |first=P.  |display-authors=etal |journal=Progress of Theoretical and Experimental Physics |collaboration=[[Particle Data Group]]  |year=2020  |url=https://pdg.lbl.gov/2020/tables/rpp2020-sum-gauge-higgs-bosons.pdf |archive-url=https://web.archive.org/web/20200930163307/https://pdg.lbl.gov/2020/tables/rpp2020-sum-gauge-higgs-bosons.pdf |archive-date=2020-09-30 |url-status=live  |title=Review of Particle Physics: Gauge and Higgs bosons }}</ref>
| mean_lifetime = stable
| decay_particle =
| electric_charge = 0&nbsp;[[elementary charge|''e'']]
| color_charge = No
| spin = 2&nbsp;[[reduced Planck constant|''ħ'']]
}}
In theories of [[quantum gravity]], the '''graviton''' is the hypothetical [[elementary particle]] that mediates the force of gravitational interaction. There is no complete [[quantum field theory]] of gravitons due to an outstanding mathematical problem with [[renormalization]] in [[general relativity]]. In [[string theory]], believed by some to be a consistent theory of quantum gravity, the graviton is a [[Massless particle|massless]] state of a fundamental string.

If it exists, the graviton is expected to be [[Mass in special relativity|massless]] because the gravitational force has a very long range, and appears to propagate at the speed of light. The graviton must be a [[Spin (physics)|spin]]-2 [[boson]] because the source of gravitation is the [[stress–energy tensor]], a second-order [[tensor]] (compared with [[electromagnetism]]'s spin-1 [[photon]], the source of which is the [[four-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.<ref>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) of {{cite book
 | last1=Misner | first1=C. W. | author-link=Charles W. Misner
 | last2=Thorne | first2=K. S. | author2-link=Kip Thorne
 | last3=Wheeler | first3=J. A. | author3-link=John A. Wheeler
 | date=1973
 | title=Gravitation
 | publisher=[[W. H. Freeman]]
 | isbn=0-7167-0344-0
}}</ref>

== Theory ==
It is hypothesized that gravitational interactions are mediated by an as yet undiscovered elementary particle, dubbed the ''graviton''. The three other known [[Fundamental interaction|forces]] of nature are mediated by elementary particles: [[electromagnetism]] by the [[photon]], the [[strong interaction]] by [[gluon]]s, and the [[weak interaction]] by the [[W and Z bosons]]. All three of these forces appear to be accurately described by the [[Standard Model]] of particle physics. In the [[classical limit]], a successful theory of gravitons would reduce to [[general relativity]], which itself reduces to [[Newton's law of gravitation]] in the weak-field limit.<ref>
{{cite book
 |last1=Feynman
 |first1=R. P.
 |last2=Morinigo
 |first2=F. B.
 |last3=Wagner
 |first3=W. G.
 |last4=Hatfield
 |first4=B.
 |date=1995
 |title=Feynman Lectures on Gravitation
 |publisher=[[Addison-Wesley]]
 |isbn=0-201-62734-5
 |url-access=registration
 |url=https://archive.org/details/feynmanlectureso0000feyn_g4q1
}}</ref><ref>
{{cite book |last1=Zee |first1=Anthony |title=Quantum Field Theory in a Nutshell |date=2003 |publisher=[[Princeton University Press]] |isbn=0-691-01019-6 |location=Princeton, New Jersey |language=en-us}}</ref><ref>
{{cite book
 |last=Randall
 |first=L.
 |date=2005
 |title=Warped Passages: Unraveling the Universe's Hidden Dimensions
 |publisher=[[Ecco Press]]
 |isbn=0-06-053108-8
 |url-access=registration
 |url=https://archive.org/details/warpedpassagesun00rand_1
}}</ref>

== History ==
[[Albert Einstein]] discussed quantized gravitational radiation in 1916, the year following his publication of [[general relativity]].<ref name=Stachel1999/>{{rp|525}}
The term ''graviton'' was coined in 1934 by Soviet physicists [[Dmitry Blokhintsev]] and {{ill|Fyodor Galperin|ru|Гальперин, Фёдор Матвеевич}}.<ref name=Blokhintsev/><ref name=Stachel1999>{{cite book| chapter=The Early History of Quantum Gravity (1916–1940)| title=Black Holes, Gravitational Radiation and the Universe| date=1999| last1=Stachel| first1=John| pages=525–534| isbn=978-90-481-5121-9| series=Fundamental Theories of Physics |volume=100| doi=10.1007/978-94-017-0934-7_31}}</ref> [[Paul Dirac]] reintroduced the term in a number of lectures in 1959, noting that the energy of the gravitational field should come in quanta.<ref>{{Cite book |last=Farmelo |first=Graham |author-link=Graham Farmelo |title=The Strangest Man : The Hidden Life of Paul Dirac, Quantum Genius |publisher=Faber and Faber |year=2009 |isbn=978-0-571-22278-0 |pages=367–368 |language=en}}</ref><ref name="Debnath">{{Cite journal |last=Debnath |first=Lokenath |author-link=Lokenath Debnath |date=2013 |title=A short biography of Paul A. M. Dirac and historical development of Dirac delta function |url=http://www.tandfonline.com/doi/abs/10.1080/0020739X.2013.770091 |journal=International Journal of Mathematical Education in Science and Technology |language=en |volume=44 |issue=8 |pages=1201–1223 |doi=10.1080/0020739X.2013.770091 |bibcode=2013IJMES..44.1201D |issn=0020-739X}}</ref> A mediation of the gravitational interaction by particles was anticipated by [[Pierre-Simon Laplace]].<ref name="Zee_Gravity">{{Cite book |url=https://press.princeton.edu/books/hardcover/9780691174389/on-gravity |title=On Gravity: A Brief Tour of a Weighty Subject |last=Zee |first=Anthony |date=2018-04-24 |publisher=Princeton University Press |isbn=978-0-691-17438-9 |location=Princeton, New Jersey |language=en-us}}</ref> Just like [[Light#Particle theory|Newton's anticipation of photons]], Laplace's anticipated "gravitons" had a greater speed than the speed of light in vacuum <math>c</math>, the speed of gravitons expected in modern theories, and were not connected to [[quantum mechanics]] or [[special relativity]], since these theories didn't yet exist during Laplace's lifetime.

=== Gravitons and renormalization ===
When describing graviton interactions, the [[classical theory]] of [[Feynman diagram]]s and semiclassical corrections such as [[one-loop diagram]]s behave normally. However, Feynman diagrams with at least two loops lead to [[ultraviolet divergence]]s.<ref>{{Cite journal |last1=Bern |first1=Zvi |last2=Chi |first2=Huan-Hang |last3=Dixon |first3=Lance |last4=Edison |first4=Alex |date=2017-02-22 |title=Two-loop renormalization of quantum gravity simplified |url=https://www.slac.stanford.edu/pubs/slacpubs/16750/slac-pub-16905.pdf |journal=Physical Review D |language=en |volume=95 |issue=4 |page=046013 |arxiv=1701.02422 |doi=10.1103/PhysRevD.95.046013 |bibcode=2017PhRvD..95d6013B |issn=2470-0010}}</ref> These infinite results cannot be removed because quantized [[general relativity]] is not [[Perturbation theory (quantum mechanics)|perturbatively]] [[renormalizable]], unlike [[quantum electrodynamics]] and models such as the [[Yang–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 the [[Planck scale]].

=== Comparison with other forces ===
Like the [[force carrier]]s of the [[fundamental interaction|other forces]] (see [[photon]], [[gluon]], [[W and Z bosons]]), the graviton plays a role in [[general relativity]], in defining the [[spacetime]] in which events take place. [[General relativity#Definition and basic applications|In some descriptions]] energy modifies the "shape" of [[spacetime]] itself, 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.<ref>See the other Wikipedia articles on [[general relativity]], [[gravitational field]], [[gravitational wave]], etc.</ref> Because the [[diffeomorphism]] invariance 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 be [[background-independent]]. In contrast, the Standard Model is ''not'' background-independent, with [[Minkowski space]] enjoying a special status as the fixed background space-time.<ref>
{{cite journal
 |last1= Colosi|first1=D.
 |display-authors=etal
 |date=2005
 |title=Background independence in a nutshell: The dynamics of a tetrahedron
 |journal=[[Classical and Quantum Gravity]]
 |volume=22 |issue=14 |pages=2971–2989
 |arxiv=gr-qc/0408079
 |bibcode=2005CQGra..22.2971C
 |doi=10.1088/0264-9381/22/14/008
 |s2cid=17317614
}}</ref> A theory of quantum gravity is needed in order to reconcile these differences.<ref>
{{cite arXiv
 |last1=Witten |first1=E.
 |date=1993
 |title=Quantum Background Independence In String Theory
 |eprint=hep-th/9306122
}}</ref> 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.<ref>
{{cite arXiv
 |last1=Smolin |first1=L.
 |date=2005
 |title=The case for background independence
 |eprint=hep-th/0507235
}}</ref>

== Energy and wavelength ==
While gravitons are presumed to be [[massless particle|massless]], they would still carry [[energy]], as does any other quantum particle. [[Photon energy]] and [[gluon energy]] are also carried by massless particles. It is unclear which variables might determine graviton energy, the amount of energy carried by a single graviton.

Alternatively, [[massive gravity|if gravitons are massive at all]], the analysis of gravitational waves yielded a new upper bound on the [[mass]] of gravitons. The graviton's [[Compton wavelength]] is at least {{val|1.6|e=16|u=[[metre|m]]}}, or about 1.6 [[light-year]]s, corresponding to a graviton mass of no more than {{val|7.7|e=-23|u=[[electronvolt|eV]]/[[speed of light|c]]<sup>2</sup>}}.<ref name="Abbott2017">{{cite journal |last=Abbott |first=B. P. |display-authors=etal |date=1 June 2017 |title=GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2 |journal=[[Physical Review Letters]] |volume=118 |issue=22 |page=221101 |arxiv=1706.01812 |bibcode=2017PhRvL.118v1101A |doi=10.1103/PhysRevLett.118.221101 |pmid=28621973 |s2cid=206291714 |collaboration=[[LIGO Scientific Collaboration]] and [[Virgo interferometer|Virgo Collaboration]]}}</ref> This relation between wavelength and mass-energy is calculated with the [[Planck–Einstein relation]], the same formula that relates electromagnetic [[wavelength]] to [[photon energy]]. 

== Experimental observation ==
Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, has been thought to be impossible with any physically reasonable detector.<ref name="Rothman">
{{cite journal
 |last1=Rothman |first1=T.
 |last2=Boughn |first2=S.
 |date=2006
 |title=Can Gravitons be Detected?
 |journal=[[Foundations of Physics]]
 |volume=36 |issue=12 |pages=1801–1825
 |arxiv=gr-qc/0601043
 |bibcode=2006FoPh...36.1801R
 |doi=10.1007/s10701-006-9081-9
|s2cid=14008778
 }}</ref> The reason is the extremely low [[cross section (physics)|cross section]] for the interaction of gravitons with matter. For example, a detector with the mass of [[Jupiter]] and 100% efficiency, placed in close orbit around a [[neutron 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 of [[neutrino]]s, since the dimensions of the required neutrino shield would ensure collapse into a [[black hole]].<ref name="Rothman" /> It has been proposed that detecting single gravitons would be possible by quantum sensing.<ref name="Tobar">{{Cite journal |last=Tobar |first=Germain |display-authors=etal |date=22 August 2024|title=Detecting single gravitons with quantum sensing|journal=Nat Commun|volume=15 |issue=1 |page=7229 |language=en |arxiv=2308.15440|doi=10.1038/s41467-024-51420-8 |pmid=39174544 |pmc=11341900 }}</ref> Even quantum events may not indicate quantization of gravitational radiation.<ref>{{Cite journal |last1=Carney |first1=Daniel |last2=Domcke |first2=Valerie |last3=Rodd |first3=Nicholas L. |date=2024-02-05 |title=Graviton detection and the quantization of gravity |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.109.044009 |journal=Physical Review D |volume=109 |issue=4 |pages=044009 |doi=10.1103/PhysRevD.109.044009|arxiv=2308.12988 }}</ref>

[[LIGO]] and [[Virgo interferometer|Virgo]] collaborations' observations have [[First observation of gravitational waves|directly detected]] gravitational waves.<ref name="Abbot">{{Cite journal |last=Abbott |first=B. P. |display-authors=etal |date=2016-02-11 |year=2016 |others=LIGO Scientific Collaboration and Virgo Collaboration |title=Observation of Gravitational Waves from a Binary Black Hole Merger |url=https://link.aps.org/doi/10.1103/PhysRevLett.116.061102 |journal=Physical Review Letters |language=en |volume=116 |issue=6 |page=061102 |arxiv=1602.03837 |bibcode=2016PhRvL.116f1102A |doi=10.1103/PhysRevLett.116.061102 |issn=0031-9007 |pmid=26918975 |s2cid=124959784}}</ref><ref name="Discovery 2016">{{cite journal |title=Einstein's gravitational waves found at last |journal=Nature News|date=February 11, 2016 |last1=Castelvecchi |first1=Davide |last2=Witze |first2=Witze |doi=10.1038/nature.2016.19361 |s2cid=182916902}}</ref><ref name="NSF">{{cite web |title=Gravitational waves detected 100 years after Einstein's prediction |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=137628 |access-date=2016-02-11 |website=NSF – National Science Foundation}}</ref> Others have postulated that graviton scattering yields gravitational waves as particle interactions yield [[coherent state]]s.<ref>{{cite journal | last1 = Senatore | first1 = L. | last2 = Silverstein | first2 = E. | last3 = Zaldarriaga | first3 = M. | year = 2014 | title = New sources of gravitational waves during inflation | journal = Journal of Cosmology and Astroparticle Physics | volume = 2014 | issue = 8| page = 016 | doi=10.1088/1475-7516/2014/08/016| arxiv = 1109.0542 | bibcode = 2014JCAP...08..016S | s2cid = 118619414 }}</ref> Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton.<ref name="detecting graviton">{{cite journal|first=Freeman |last= Dyson|date=8 October 2013|journal=[[International Journal of Modern Physics A]]|volume=28|issue=25|pages=1330041–1–1330035–14|title=Is a Graviton Detectable?|doi=10.1142/S0217751X1330041X|bibcode = 2013IJMPA..2830041D }}</ref> For example, if gravitational waves were observed to propagate slower than ''c'' (the [[speed of light]] in vacuum), that would imply that the graviton has mass (however, gravitational waves must propagate slower than ''c'' in a region with non-zero mass density if they are to be detectable).<ref>
{{cite journal
 |last=Will |first=C. M.
 |date=1998
 |title=Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries
 |journal=[[Physical Review D]]
 |volume=57 |issue=4 |pages=2061–2068
 |arxiv=gr-qc/9709011
 |bibcode=1998PhRvD..57.2061W
 |doi=10.1103/PhysRevD.57.2061
 |s2cid=41690760
 |url=https://cds.cern.ch/record/333219/files/9709011.pdf
 |archive-url=https://web.archive.org/web/20180724135835/https://cds.cern.ch/record/333219/files/9709011.pdf
 |archive-date=2018-07-24
 |url-status=live
}}</ref> Observations of gravitational waves put an upper bound of {{val|1.76|e=-23|u=eV/c2}} on the graviton's mass.<ref name="Abbot2">{{cite journal|doi=10.1103/PhysRevD.103.122002|title= Tests of General Relativity with Binary Black Holes from the second LIGO-Virgo Gravitational-Wave Transient Catalog|journal= [[Physical Review Letters]]|date= 15 June 2021|author=R Abbot|display-authors=etal|volume=103|issue=12|pages=122022|arxiv=2010.14529|bibcode= 2021PhRvD.103l2002A}}</ref> Solar system planetary trajectory measurements by space missions such as [[Cassini–Huygens|Cassini]] and [[MESSENGER]] give a comparable upper bound of {{val|3.16|e=-23|u=eV/c2}}.<ref name="Bernus2020">{{cite journal|doi=10.1103/PhysRevD.102.021501|title= Constraint on the Yukawa suppression of the Newtonian potential from the planetary ephemeris INPOP19a|journal= [[Physical Review Letters]]|date= 15 July 2020|author=L. Bernus|display-authors=etal|volume=102|issue=2|pages=021501(R)|arxiv=2006.12304|bibcode= 2020PhRvD.102b1501B}}</ref> The gravitational wave and planetary ephemeris need not agree: they test different aspects of a potential graviton-based theory.<ref>{{Cite journal |last1=Fienga |first1=Agnès |last2=Minazzoli |first2=Olivier |date=2024-01-29 |title=Testing theories of gravity with planetary ephemerides |journal=Living Reviews in Relativity |language=en |volume=27 |issue=1 |pages=1 |doi=10.1007/s41114-023-00047-0 |issn=1433-8351|doi-access=free |arxiv=2303.01821 }}</ref>{{rp|71}}

Astronomical observations of the kinematics of galaxies, especially the [[galaxy rotation curve|galaxy rotation problem]] and [[modified Newtonian dynamics]], might point toward gravitons having non-zero mass.<ref>{{Cite journal|arxiv = 1211.4692|doi = 10.5303/JKAS.2013.46.1.41|last1 = Trippe|first1 = Sascha|title = A Simplified Treatment of Gravitational Interaction on Galactic Scales|year = 2012|journal=Journal of the Korean Astronomical Society |volume=46 |issue=1 |pages=41–47 |bibcode=2013JKAS...46...41T}}</ref><ref>{{cite journal |title=Long range effects in gravity theories with Vainshtein screening |year=2018 |last1=Platscher |first1=Moritz |last2=Smirnov |first2=Juri |last3=Meyer |first3=Sven |last4=Bartelmann |first4=Matthias|journal=Journal of Cosmology and Astroparticle Physics |volume=2018 |issue=12 |page=009 |doi=10.1088/1475-7516/2018/12/009|arxiv=1809.05318 |bibcode=2018JCAP...12..009P|s2cid=86859475}}</ref>

== Difficulties and outstanding issues ==
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 the [[Planck scale]]. This is because of infinities arising due to quantum effects; technically, gravitation is not [[renormalizable]]. Since classical general relativity and [[quantum mechanics]] seem to be incompatible at such energies, from a theoretical point of view, this situation is not tenable. One possible solution is to replace particles with [[String (physics)|strings]]. 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.<ref>
{{cite news
 |last=Sokal |first=A.
 |author-link=Alan Sokal
 |title=Don't Pull the String Yet on Superstring Theory
 |url=https://query.nytimes.com/gst/fullpage.html?res=9D0DE7DB1639F931A15754C0A960958260
 |date=July 22, 1996
 |work=[[The New York Times]]
 |access-date=March 26, 2010
}}</ref>

== See also ==
{{cols|colwidth=21em}}
* [[Gravitino]]
* [[Dual graviton]]
* [[Gravitoelectromagnetism]]
* [[Planck mass]]
* [[Static forces and virtual-particle exchange]]
* [[Soft graviton theorem]]
* [[Polarizable vacuum]]
{{colend}}

== References ==
{{reflist|30em}}

== External links ==
{{Wikiversity|Graviton}}
* {{In Our Time|Graviton|p003k9ks|Graviton}}

{{Particles}}
{{quantum gravity}}
{{Theories of gravitation}}
{{String theory topics|state=collapsed}}

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[[Category:Bosons]]
[[Category:Gauge bosons]]
[[Category:Quantum gravity]]
[[Category:String theory]]
[[Category:Hypothetical elementary particles]]
[[Category:Force carriers]]