Composition Elementary particle Bose–Einstein statistics spin-2 boson Gravitation Hypothetical G[1] Self 1930s[2]The name is attributed to Dmitrii Blokhintsev and F. M. Gal'perin in 1934[3] 0 < 6×10−32 eV/c2 [4] stable 0 e 2 ħ

In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an 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 state of a fundamental string.

If it exists, the graviton is expected to be 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-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.[5]

## Theory

It is hypothesized that gravitational interactions are mediated by an as yet undiscovered elementary particle, dubbed the graviton. The three other known forces of nature are mediated by elementary particles: electromagnetism by the photon, the strong interaction by gluons, 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.[6][7][8]

## History

The term graviton was originally coined in 1934 by Soviet physicists Dmitrii Blokhintsev [ru; de] and F. M. Gal'perin.[3] Paul Dirac reintroduced the term in a number of lectures in 1959, noting that the energy of the gravitational field should come in quanta, which Dirac referred to as “gravitons”, in a reintroduction terminology manner.[9][10] A mediation of the gravitational interaction by particles was anticipated by Pierre-Simon Laplace.[11] Just like Newton's anticipation of photons, Laplace's anticipated "gravitons" had a greater speed than c (the speed of light), 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 diagrams and semiclassical corrections such as one-loop diagrams behave normally. However, Feynman diagrams with at least two loops lead to ultraviolet divergences.[12] These infinite results cannot be removed because quantized general relativity is not 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 carriers of the 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. 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.[13] 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.[14] A theory of quantum gravity is needed in order to reconcile these differences.[15] 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.[16]

### Gravitons in speculative theories

String theory predicts the existence of gravitons and their well-defined interactions. A graviton in perturbative string theory is a closed string in a very particular low-energy vibrational state. The scattering of gravitons in string theory can also be computed from the correlation functions in conformal field theory, as dictated by the AdS/CFT correspondence, or from matrix theory.[citation needed]

A feature of gravitons in string theory is that, as closed strings without endpoints, they would not be bound to branes and could move freely between them. If we live on a brane (as hypothesized by brane theories), this "leakage" of gravitons from the brane into higher-dimensional space could explain why gravitation is such a weak force, and gravitons from other branes adjacent to our own could provide a potential explanation for dark matter. However, if gravitons were to move completely freely between branes, this would dilute gravity too much, causing a violation of Newton's inverse-square law. To combat this, Lisa Randall found that a three-brane (such as ours) would have a gravitational pull of its own, preventing gravitons from drifting freely, possibly resulting in the diluted gravity we observe, while roughly maintaining Newton's inverse square law.[17] See brane cosmology.

A theory by Ahmed Farag Ali and Saurya Das adds quantum mechanical corrections (using Bohm trajectories) to general relativistic geodesics. If gravitons are given a small but non-zero mass, it could explain the cosmological constant without need for dark energy and solve the smallness problem.[18] The theory received an Honorable Mention in the 2014 Essay Competition of the Gravity Research Foundation for explaining the smallness of the cosmological constant.[19] Also the theory received an Honorable Mention in the 2015 Essay Competition of the Gravity Research Foundation for naturally explaining the observed large-scale homogeneity and isotropy of the universe due to the proposed quantum corrections.[20]

Matthew R. Edwards suggest that the gravito-optical medium is composed of gravitons and may in turn connect with the polarizable vacuum approach.[21]

## Energy and wavelength

While gravitons are presumed to be 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, 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 1.6×1016 m, or about 1.6 light-years, corresponding to a graviton mass of no more than 7.7×10−23 eV/c2.[22] 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, is impossible with any physically reasonable detector.[23] The reason is the extremely low 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 neutrinos, since the dimensions of the required neutrino shield would ensure collapse into a black hole.[23]

LIGO and Virgo collaborations' observations have directly detected gravitational waves.[24][25][26] Others have postulated that graviton scattering yields gravitational waves as particle interactions yield coherent states.[27] Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton.[28] 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).[29] Recent observations of gravitational waves have put an upper bound of 1.2×10−22 eV/c2 on the graviton's mass.[24] Astronomical observations of the kinematics of galaxies, especially the galaxy rotation problem and modified Newtonian dynamics, might point toward gravitons having non-zero mass.[30][31]

## 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 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.[32]

## References

1. ^ G is used to avoid confusion with gluons (symbol g)
2. ^ Rovelli, C. (2001). "Notes for a brief history of quantum gravity". arXiv:gr-qc/0006061.
3. ^ a b 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. ISBN 978-5-04-008956-7.
4. ^ Zyla, P.; et al. (Particle Data Group) (2020). "Review of Particle Physics: Gauge and Higgs bosons" (PDF). Archived (PDF) from the original on 2020-09-30. `((cite journal))`: Cite journal requires `|journal=` (help)
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) of Misner, C. W.; Thorne, K. S.; Wheeler, J. A. (1973). Gravitation. W. H. Freeman. ISBN 0-7167-0344-0.
6. ^ Feynman, R. P.; Morinigo, F. B.; Wagner, W. G.; Hatfield, B. (1995). Feynman Lectures on Gravitation. Addison-Wesley. ISBN 0-201-62734-5.
7. ^ Zee, Anthony (2003). Quantum Field Theory in a Nutshell. Princeton, New Jersey: Princeton University Press. ISBN 0-691-01019-6.
8. ^ Randall, L. (2005). Warped Passages: Unraveling the Universe's Hidden Dimensions. Ecco Press. ISBN 0-06-053108-8.
9. ^ Farmelo, Graham (2009). The Strangest Man : The Hidden Life of Paul Dirac, Quantum Genius. Faber and Faber. pp. 367–368. ISBN 978-0-571-22278-0.
10. ^ 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. doi:10.1080/0020739X.2013.770091. ISSN 0020-739X.
11. ^ Zee, Anthony (2018-04-24). On Gravity: A Brief Tour of a Weighty Subject. Princeton, New Jersey: Princeton University Press. ISBN 978-0-691-17438-9.
12. ^ Zvi Berna; Huan-Hang Chib; Lance Dixonb; Alex Edisona. "Two-Loop Renormalization of Quantum Gravity Simplified" (PDF). www.slac.stanford.edu. Bhaumik Institute for Theoretical Physics – Department of Physics and Astronomy.
13. ^ See the other Wikipedia articles on general relativity, gravitational field, gravitational wave, etc.
14. ^ 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. S2CID 17317614.
15. ^ Witten, E. (1993). "Quantum Background Independence In String Theory". arXiv:hep-th/9306122.
16. ^ Smolin, L. (2005). "The case for background independence". arXiv:hep-th/0507235.
17. ^ Kaku, Michio (2006) Parallel Worlds – The science of alternative universes and our future in the Cosmos. Doubleday. pp. 218–221. ISBN 978-0385509862.
18. ^ Ali, Ahmed Farag (2014). "Cosmology from quantum potential". Physics Letters B. 741: 276–279. arXiv:1404.3093. Bibcode:2015PhLB..741..276F. doi:10.1016/j.physletb.2014.12.057. S2CID 55463396.
19. ^ Das, Saurya (2014). "Cosmic coincidence or graviton mass?". International Journal of Modern Physics D. 23 (12): 1442017. arXiv:1405.4011. Bibcode:2014IJMPD..2342017D. doi:10.1142/S0218271814420176. S2CID 54013915.
20. ^ Das, Saurya (2015). "Bose–Einstein condensation as an alternative to inflation". International Journal of Modern Physics D. 24 (12): 1544001–219. arXiv:1509.02658. Bibcode:2015IJMPD..2444001D. doi:10.1142/S0218271815440010. S2CID 119210816.
21. ^ Matthew R. Edwards (2014). "Gravity from refraction of CMB photons using the optical-mechanical analogy in general relativity". Astrophysics and Space Science. 351 (2): 401–406. doi:10.1007/s10509-014-1864-4.
22. ^ Abbott, B. P.; et al. (LIGO Scientific Collaboration and Virgo 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. PMID 28621973. S2CID 206291714.
23. ^ a b 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. S2CID 14008778.
24. ^ a b Abbott, B. P. et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2016). "Observation of Gravitational Waves from a Binary Black Hole Merger". Physical Review Letters. 116 (6): 061102. arXiv:1602.03837. Bibcode:2016PhRvL.116f1102A. doi:10.1103/PhysRevLett.116.061102. PMID 26918975. S2CID 124959784.
25. ^ Castelvecchi, Davide; Witze, Witze (February 11, 2016). "Einstein's gravitational waves found at last". Nature News. doi:10.1038/nature.2016.19361. S2CID 182916902.
26. ^ "Gravitational waves detected 100 years after Einstein's prediction | NSF – National Science Foundation". www.nsf.gov. Retrieved 2016-02-11.
27. ^ 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. S2CID 118619414.
28. ^ 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.
29. ^ 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. S2CID 41690760. Archived (PDF) from the original on 2018-07-24.
30. ^ 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.
31. ^ 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. S2CID 86859475.
32. ^ Sokal, A. (July 22, 1996). "Don't Pull the String Yet on Superstring Theory". The New York Times. Retrieved March 26, 2010.