# Graviton: Wikis

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# Encyclopedia

 Graviton Composition: Elementary particle Statistical behavior: Boson Group: Gauge boson Interaction: Gravity Status: theoretical Symbol(s): g, G[1] Antiparticle: Self Theorized: 1930s[2] The name is attributed to Dmitrii Blokhintsev and F.M. Gal'perin in 1934[3] Discovered: currently 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 gravity in the framework of quantum field theory. If it exists, the graviton must be massless (because the gravitational force has unlimited range) and must have a spin of 2 (because the source of gravity is the stress-energy tensor, which is a second-rank tensor, compared to electromagnetism, the source of which is the four-current, which is a first-rank tensor). To prove the existence of the graviton, physicists must be able to link the particle to the curvature of the space-time continuum and calculate the gravitational force exerted.

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, which are required to conform to Newton's law of gravitation.[4][5][6]

However, attempts to extend the Standard Model with gravitons have 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 models of quantum gravity[7] attempt to address this issue, but these are highly speculative theories.

## Gravitons and renormalization

When 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 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 just general relativity must regulate the behavior near the Planck scale.

## Experimental observation

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector.[8] The reason is simply 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.[8]

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 c (the speed of light in a vacuum), that would imply that the graviton has mass.[9]

## 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. Because it does not depend on a particular space-time background, general relativity is said to be background independent. In contrast, the Standard Model is not background independent.[10] A theory of quantum gravity is needed in order to reconcile these differences.[11] Whether this theory should be background independent or not is an open question. The answer to this question will determine if gravity plays a special role in the universe.[12]

## Gravitons in speculative theories

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.

## References

1. ^ G is often used to avoid confusion with gluons (symbol g)
2. ^ Rovelli, C. (July 2000). "Notes for a brief history of quantum gravity". 9th Marcel Grossmann Meeting in Roma. p. p.5. arΧiv:gr-qc/0006061v3.
3. ^ Blokhintsev, D.I.; Gal'perin, F.M. (1934). "Gipoteza neitrino i zakon sokhraneniya energii (Neutrino hypothesis and conservation of energy)" (in Russian). Pod Znamenem Marxisma 6: pp.147–157.
4. ^ Feynman, R. P.; Morinigo, F. B., Wagner, W. G., & Hatfield, B. (1995). Feynman lectures on gravitation. Addison-Wesley. ISBN 0201627345.
5. ^ Zee, A. (2003). Quantum Field Theory in a Nutshell. Princeton University Press. ISBN 0-691-01019-6.
6. ^ Randall, Lisa (2005). Warped Passages: Unraveling the Universe's Hidden Dimensions. Ecco. ISBN 0-06-053108-8.
7. ^ Roger Penrose(1975)"The Non-Linear Graviton"
8. ^ a b Rothman, Tony; and Stephen Boughn (November 2006). "Can Gravitons be Detected?". Foundations of Physics 36 (12): 1801–1825. doi:10.1007/s10701-006-9081-9. Retrieved 2007-07-02.
9. ^ 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. doi:10.1103/PhysRevD.57.2061. Retrieved 2007-07-02.
10. ^ C. Rovelli et al., Background independence in a nutshell, Class.Quant.Grav. 22 (2005) 2971-2990, gr-qc/0408079
11. ^ Edward Witten, Quantum Background Independence In String Theory, hep-th/9306122
12. ^ L. Smolin, The case for background independence, hep-th/0507235

# Wiktionary

Up to date as of January 15, 2010

## German

### Noun

Graviton n. (plural: Gravitonen)

# Simple English

A graviton is a very small thing (called a particle) that some scientists think is responsible for gravity. At this time it is just an idea because scientists have not yet found or seen a graviton.[1]

Because the graviton is a particle that has force (gravity), it is classed as a gauge boson. Other gauge bosons include the photon, the gluon, and the W and Z particles.

If someone found one, it could lead to a theory which would join together the four main physical forces: electromagnetism, gravity, strong force and weak force.

Particles in Physics
Elementary: Fermions: Quarks: up - down - strange - charm - bottom - top
Leptons: electron - muon - tau - neutrinos
Bosons: Gauge bosons: photon - W and Z bosons - gluons
Composite: Hadrons: Baryons: proton - neutron - hyperon
Mesons: pion - kaon - J/ψ
Atomic nuclei - Atoms - Molecules
Hypothetical: Higgs boson - Graviton - Tachyon

## References

1. Wei, Lisa (2003). "What is a graviton?". Curious about Astronomy. Retrieved 2009-11-01.