View source for Graviton

{{Short description|Hypothetical elementary particle that mediates gravity}}
{{About|the hypothetical particle}}
{{pp-move|small=yes}}
{{Infobox Particle
| name = Graviton
| composition = [[Elementary particle]]
| statistics = [[Bose–Einstein statistics]]
| group = [[Boson]]ic
| interaction = [[Gravitation]]
| 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 [[Dmitry Blokhintsev]] and F. M. Gal'perin in 1934<ref name=Blokhintsev>
  {{cite journal
   |last1 = Blokhintsev |first1=D. I.
   |author-link1=Dmitry Blokhintsev
   |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>
| 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}}(limit)}}<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
| electric_charge = 0&nbsp;[[elementary charge|''e'']]
| color_charge = No
| spin = 2&nbsp;[[reduced Planck constant|''ħ'']]
| num_spin_states = +2&nbsp;''ħ'',&nbsp; −2&nbsp;''ħ''
| parity = +1
| c_parity = +1
}}

In theories of [[quantum gravity]], the '''graviton''' is the hypothetical [[elementary particle]] that mediates the force of gravitational interaction. It is a quantum of [[gravitational wave]] energy.<ref name="Tobar"/> There is no complete [[quantum field theory]] of gravitons due to the unsolved mathematical problem of [[renormalization]] in [[general relativity]]. This problem is avoided in [[string theory]], which has the graviton as a [[Massless particle|massless]] state of a fundamental string, but that theory has not made sufficient progress.

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 ==
[[File:Do Gravitons Really Exist - Finding the Particles of Gravity.webm|thumb|How we can potentially discover the existence of Gravitons with current technology such as what is found at LIGO and some potential pitfalls that these methods have.]]
It is hypothesized that an undiscovered elementary particle mediates gravitational interactions, 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 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|author-link=John Stachel| 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|url-access=subscription }}</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]], as he predated these theories by a century.

=== 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 |article-number=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]].

== Energy and wavelength ==
While gravitons are presumed to be [[massless particle|massless]], they would still carry [[energy]], as does any other quantum particle.<ref>{{Cite web |last=O'Keefe |first=Madeleine |date=2019-07-23 |title=Massless particles can't be stopped {{!}} symmetry magazine |url=https://www.symmetrymagazine.org/article/massless-particles-cant-be-stopped?language_content_entity=und |access-date=2025-07-13 |website=www.symmetrymagazine.org |language=en}}</ref> [[Photon energy]] and [[gluon energy]] are also carried by massless particles. 

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|ul=m}}, or about 1.6 [[light-year]]s, corresponding to a graviton mass of no more than {{val|7.7|e=-23|ul=eV/c2}}.<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 |article-number=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 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 quantum sensing would make detecting single gravitons possible.<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 |article-number=7229 |language=en |arxiv=2308.15440|doi=10.1038/s41467-024-51420-8 |pmid=39174544 |pmc=11341900 |bibcode=2024NatCo..15.7229T }}</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 |article-number=044009 |doi=10.1103/PhysRevD.109.044009|arxiv=2308.12988 |bibcode=2024PhRvD.109d4009C }}</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 |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 |article-number=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|doi-access=free }}</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>  Although these experiments cannot detect individual gravitons,<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> they might provide information about certain properties of the graviton. 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> Furthermore, in the presence of a gravitational wave, it should be possible to observe (via interference or beating effects after a delay line) signatures of stimulated emission or absorption of gravitons with present-day technology.<ref>{{Cite journal| title = Stimulated Emission or Absorption of Gravitons by Light | last1 = Schützhold | first1= Ralf | journal = Phys. Rev. Lett. | volume = 135 | issue = 17 | page = 171501 | year = 2025 | publisher = American Physical Society | doi = 10.1103/xd97-c6d7 | url = https://link.aps.org/doi/10.1103/xd97-c6d7| doi-access = free }}</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|page=122022|article-number= 122002|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)|article-number= 021501|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 |article-number=1 |doi=10.1007/s41114-023-00047-0 |issn=1433-8351|doi-access=free |arxiv=2303.01821 |bibcode=2024LRR....27....1F }}</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 severe theoretical difficulties at energies close to or above the [[Planck scale]]. Infinities arise due to quantum effects; technically, gravitation cannot be [[renormalized]]. Since classical general relativity and [[quantum mechanics]] seem incompatible at such energies, this situation is not tenable from a theoretical point of view. 

One possible solution is to replace particles with [[String (physics)|strings]]. Strings are one-dimensional loops that avoid divergences by smearing out the gravitational interactions. A particle identified with the graviton appears in string theory with long-distant interactions described by general relativity. Unfortunately models based on strings have only been worked out for a few weakly interacting strings.<ref>{{Cite journal |last=Greene |first=Brian R. |last2=Morrison |first2=David R. |last3=Polchinski |first3=Joseph |date=September 15, 1998 |title=String theory |url=https://pnas.org/doi/full/10.1073/pnas.95.19.11039 |journal=Proceedings of the National Academy of Sciences |language=en |volume=95 |issue=19 |pages=11039–11040 |doi=10.1073/pnas.95.19.11039 |issn=0027-8424 |pmc=33894 |pmid=9736684}}</ref>

== See also ==
* {{Annotated link|Dual graviton}}
* {{Annotated link|Gravitino}}
* {{Annotated link|Gravitoelectromagnetism}}
* {{Annotated link|Planck units}}
* {{Annotated link|Polarizable vacuum}}
* {{Annotated link|Soft graviton theorem}}
* {{Annotated link|Static forces and virtual-particle exchange}}

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

== External links ==
{{Wikiversity|Graviton}}
{{commons category}}
* {{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]]
[[Category:Gravitational waves]]