Editing Quantum gravity (section)

=== Graviton ===
{{Main|Graviton}}
The observation that all [[fundamental forces]] except gravity have one or more known [[messenger particles]] leads researchers to believe that at least one must exist for gravity. This hypothetical particle is known as the ''graviton''. These particles act as a [[force particle]] similar to the [[photon]] of the electromagnetic interaction. Under mild assumptions, the structure of general relativity requires them to follow the quantum mechanical description of interacting theoretical spin-2 massless particles.<ref name=Kraichnan1955>{{cite journal
|last = Kraichnan
|first = R. H.
|author-link = Robert Kraichnan
|title = Special-Relativistic Derivation of Generally Covariant Gravitation Theory
|date = 1955
|journal = [[Physical Review]]
|volume = 98
|issue = 4
|pages = 1118–1122
|doi = 10.1103/PhysRev.98.1118
|bibcode = 1955PhRv...98.1118K }}</ref><ref name=Gupta1954>{{cite journal
|last = Gupta
|first = S. N.
|author-link = Suraj N. Gupta
|title = Gravitation and Electromagnetism
|date = 1954
|journal = [[Physical Review]]
|volume = 96
|issue = 6
|pages = 1683–1685
|doi = 10.1103/PhysRev.96.1683
|bibcode = 1954PhRv...96.1683G }}</ref><ref name=Gupta1957>{{cite journal
|last = Gupta
|first = S. N.
|author-link = Suraj N. Gupta
|title = Einstein's and Other Theories of Gravitation
|date = 1957
|journal = [[Reviews of Modern Physics]]
|volume = 29
|issue = 3
|pages = 334–336
|doi = 10.1103/RevModPhys.29.334
|bibcode=1957RvMP...29..334G
}}</ref><ref name=Gupta1962>{{cite book
|first = S. N.
|last = Gupta
|author-link = Suraj N. Gupta
|title = Recent Developments in General Relativity
|contribution = Quantum Theory of Gravitation
|date = 1962
|pages = 251–258
|publisher = Pergamon Press
}}</ref><ref name=Deser1970>{{cite journal
|last = Deser
|first = S.
|author-link = Stanley Deser
|title = Self-Interaction and Gauge Invariance
|date = 1970
|journal = [[General Relativity and Gravitation]]
|volume = 1
|issue = 1
|pages = 9–18
|doi = 10.1007/BF00759198
|bibcode=1970GReGr...1....9D
|arxiv = gr-qc/0411023 |s2cid = 14295121
}}</ref> Many of the accepted notions of a unified theory of physics since the 1970s assume, and to some degree depend upon, the existence of the graviton. The [[Weinberg–Witten theorem]] places some constraints on theories in which [[composite gravity|the graviton is a composite particle]].<ref>{{cite journal|first1=Steven|last1=Weinberg|first2=Edward|last2=Witten|author-link1=Steven Weinberg|author-link2=Edward Witten|title=Limits on massless particles|journal=[[Physics Letters B]]|volume=96|issue=1–2|year=1980|pages=59&ndash;62|doi=10.1016/0370-2693(80)90212-9|bibcode=1980PhLB...96...59W}}</ref><ref>{{cite book|first1=Gary T.|last1=Horowitz|first2=Joseph|last2=Polchinski|author-link2=Joseph Polchinski|chapter=Gauge/gravity duality|title=Approaches to Quantum Gravity|publisher=[[Cambridge University Press]]|editor-last=Oriti|editor-first=Daniele|isbn=9780511575549|oclc=873715753|arxiv=gr-qc/0602037|bibcode=2006gr.qc.....2037H|year=2006}}</ref> While gravitons are an important theoretical step in a quantum mechanical description of gravity, they are generally believed to be undetectable because they interact too weakly.<ref>{{cite journal
 |last1        = Rothman
 |first1       = Tony
 |last2        = Boughn
 |first2       = Stephen
 |date         = 2006
 |title        = Can Gravitons be Detected?
 |url          = https://link.springer.com/article/10.1007/s10701-006-9081-9
 |journal      = Foundations of Physics
 |volume       = 36
 |issue        = 12
 |pages        = 1801–1825
 |doi          = 10.1007/s10701-006-9081-9
 |arxiv        = gr-qc/0601043
 |bibcode      = 2006FoPh...36.1801R
 |s2cid        = 14008778
 |access-date  = 2020-05-15
 |archive-date = 2020-08-06
 |archive-url  = https://web.archive.org/web/20200806234929/https://link.springer.com/article/10.1007/s10701-006-9081-9
 |url-status   = live
}}</ref>