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## Graviton
In physics, the Gravitons are postulated because of the great success of the quantum field theory (in particular, the Standard Model) at modeling the behavior of all other forces of nature with similar particles: electromagnetism with the photon, the strong interaction with the gluons, and the weak interaction with the W and Z bosons. In this framework, the gravitational interaction is mediated by gravitons, instead of being described in terms of curved spacetime as in general relativity. In the classical limit, both approaches give identical results, including Newton's law of gravitation. However, attempts to extend the Standard Model with gravitons run into serious theoretical difficulties at high energies (processes with energies close to or above the Planck scale) because of infinities arising due to quantum effects (in technical terms, gravitation is nonrenormalizable.) Some proposed theories of quantum gravity (in particular, string theory) address this issue. In string theory, gravitons (as well as the other particles) are states of strings rather than point particles, and then the infinities do not appear, while the low-energy behavior can still be approximated by a quantum field theory of point particles. In that case, the description in terms of gravitons serves as a low-energy effective theory. ## Additional recommended knowledge
## Gravitons and models of quantum gravityWhen describing graviton interactions, the classical theory (i.e. the tree diagrams) and semiclassical corrections (one-loop 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. In popular terms, the discreteness of quantum theory is not compatible with the smoothness of Einstein's general relativity. These problems, together with some conceptual puzzles, led many physicists to believe that a theory more complete than just general relativity must regulate the behavior near the Planck scale. Superstring theory finally emerged as the most promising solution; it is the only known theory with finite corrections to graviton scattering at all orders. String theory predicts the existence of gravitons and their well-defined interactions which represents one of its most important triumphs. 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. 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 some theorists) this "leakage" of gravitons from the brane into higher-dimensional space could explain why gravity 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 for more details. Some proposed quantum theories of gravity do not predict a graviton. ## Experimental observationUnambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are already underway (e.g. LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than ## Is gravity like the other forces?Some question the analogy which motivates the introduction of the graviton. Unlike the other forces, gravitation plays a special role in general relativity in defining the spacetime in which events take place. Because it does not depend on a particular spacetime background, general relativity is said to be background independent. In contrast, the Standard Model is ## See also- Gravitomagnetism
## References**^**Feynman, R. P.; Morinigo, F. B., Wagner, W. G., & Hatfield, B. (1995).*Feynman lectures on gravitation*. Addison-Wesley.__ISBN 0201627345__.**^**Zee, A. (2003).*Quantum Field Theory in a Nutshell*. Princeton University Press.__ISBN 0-691-01019-6__.**^**Randall, Lisa (2005).*Warped Passages: Unraveling the Universe's Hidden Dimensions*. Ecco.__ISBN 0-06-053108-8__.- ^
^{a}^{b}Rothman, Tony; and Stephen Boughn (November 2006). "Can Gravitons be Detected?".*Foundations of Physics***36**(12): 1801–1825. Retrieved on 2007-07-02. **^**Will, Clifford M. (February 1998). "Bounding the mass of the graviton using gravitational-wave observations of inspiralling compact binaries".*Physical Review D***57**(4): 2061–2068. Retrieved on 2007-07-02.
Categories: Bosons | Hypothetical elementary particles |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Graviton". A list of authors is available in Wikipedia. |