Graviton
Composition  Elementary particle 

Statistics  Bosonic 
Interactions  Gravitation 
Status  Theoretical 
Symbol  G^{[1]} 
Antiparticle  Self 
Theorized 
1930s^{[2]} The name is attributed to Dmitrii Blokhintsev and F. M. Gal'perin in 1934^{[3]} 
Discovered  Hypothetical 
Mass  0 
Mean lifetime  Stable 
Electric charge  0 e 
Spin  2 
In physics, the graviton is a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton is expected to be massless (because the gravitational force appears to have unlimited range) and must be a spin2 boson. The spin follows from the fact that the source of gravitation is the stress–energy tensor, a secondrank tensor (compared to electromagnetism's spin1 photon, the source of which is the fourcurrent, a firstrank tensor). Additionally, it can be shown that any massless spin2 field would give rise to a force indistinguishable from gravitation, because a massless spin2 field must couple to (interact with) the stress–energy tensor in the same way that the gravitational field does.^{[4]} This result suggests that, if a massless spin2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin2 particle.^{[5]}
Contents
Theory
The three other known forces of nature are mediated by elementary particles: electromagnetism by the photon, the strong interaction by the gluons, and the weak interaction by the W and Z bosons. The hypothesis is that the gravitational interaction is likewise mediated by an – as yet undiscovered – elementary particle, dubbed as the graviton. In the classical limit, the theory would reduce to general relativity and conform to Newton's law of gravitation in the weakfield limit.^{[6]}^{[7]}^{[8]}
Gravitons and renormalization
When describing graviton interactions, the classical theory (i.e., the tree diagrams) and semiclassical corrections (oneloop diagrams) behave normally, but Feynman diagrams with two (or more) loops lead to ultraviolet divergences; that is, infinite results that cannot be removed because the quantized general relativity is not renormalizable, unlike quantum electrodynamics. That is, the usual ways physicists calculate the probability that a particle will emit or absorb a graviton give nonsensical answers and the theory loses its predictive power. These problems, together with some conceptual puzzles, led many physicists to believe that a theory more complete than quantized general relativity must describe the behavior near the Planck scale.
Comparison with other forces
Unlike the force carriers of the other forces, gravitation plays a special role in general relativity in defining the spacetime in which events take place. In some descriptions, matter 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.^{[9]} Because the diffeomorphism invariance of the theory does not allow any particular spacetime background to be singled out as the "true" spacetime 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 spacetime.^{[10]} A theory of quantum gravity is needed in order to reconcile these differences.^{[11]} Whether this theory should be background independent is an open question. The answer to this question will determine our understanding of what specific role gravitation plays in the fate of the universe.^{[12]}
Gravitons in speculative theories
String theory predicts the existence of gravitons and their welldefined interactions. A graviton in perturbative string theory is a closed string in a very particular lowenergy 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.
An interesting 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 higherdimensional 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. See brane cosmology.
Experimental observation
Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector.^{[13]} 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.^{[13]}
However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are underway (e.g., LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton.^{[14]} For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass (however, gravitational waves must propagate slower than "c" in a region with nonzero mass density if they are to be detectable).^{[15]} Astronomical observations of the kinematics of galaxies, especially the galaxy rotation problem and modified Newtonian dynamics, might point toward gravitons having nonzero mass.^{[16]}
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 high energies (processes involving energies close to or above the Planck scale) because of infinities arising due to quantum effects (in technical terms, gravitation is nonrenormalizable). 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 believed to be mathematically consistent.^{[17]}
See also
 Gravitomagnetism
 Gravitational wave
 Planck mass
 Gravitation
 Static forces and virtualparticle exchange
 Multiverse
 Gravitino
References
 ^ G is used to avoid confusion with gluons (symbol g)
 ^ Rovelli, C. (2001). "Notes for a brief history of quantum gravity". arXiv:grqc/0006061 [grqc].
 ^ Blokhintsev, D. I.; Gal'perin, F. M. (1934). "Gipoteza neitrino i zakon sokhraneniya energii" [Neutrino hypothesis and conservation of energy].
 ^ Lightman, A. P.; Press, W. H.; Price, R. H.; Teukolsky, S. A. (1975). "Problem 12.16".
 ^ For a comparison of the geometric derivation and the (nongeometric) spin2 field derivation of general relativity, refer to box 18.1 (and also 17.2.5) of
 ^ Feynman, R. P.; Morinigo, F. B.; Wagner, W. G.; Hatfield, B. (1995). Feynman Lectures on Gravitation.
 ^ Zee, A. (2003). Quantum Field Theory in a Nutshell.
 ^ Randall, L. (2005). Warped Passages: Unraveling the Universe's Hidden Dimensions.
 ^ See the other articles on General relativity, Gravitational field, Gravitational wave, etc
 ^ Colosi, D.; et al. (2005). "Background independence in a nutshell: The dynamics of a tetrahedron".
 ^ Witten, E. (1993). "Quantum Background Independence In String Theory". arXiv:hepth/9306122 [hepth].
 ^ Smolin, L. (2005). "The case for background independence". arXiv:hepth/0507235 [hepth].
 ^ ^{a} ^{b} Rothman, T.; Boughn, S. (2006). "Can Gravitons be Detected?".
 ^ Freeman Dyson (8 October 2013). "Is a graviton detectable?".
 ^ Will, C. M. (1998). "Bounding the mass of the graviton using gravitationalwave observations of inspiralling compact binaries".
 ^ S. Trippe (2013), "A Simplified Treatment of Gravitational Interaction on Galactic Scales", J. Kor. Astron. Soc. 46, 41. arXiv:1211.4692
 ^
External links
 Graviton on In Our Time at the BBC. (listen now)


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