# User Contributed Dictionary

see Graviton

### Noun

- A hypothetical gauge boson that regulates the gravitational force. It would have a spin of 2 and zero rest mass.

# Extensive Definition

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 gravity is a second-rank tensor
field).

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.

## Gravitons and models of quantum gravity

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 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 observation

Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is simply the extremely low cross section for the interaction of gravitons with matter. For example, a detector designed to measure the mass of Jupiter with 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, and it would be impossible to shield the neutrinos without the shielding material collapsing into a black hole.## 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 not background independent. In other words, general relativity and the standard model are incompatible. A theory of quantum gravity is needed in order to reconcile these differences. Whether this theory should itself be background independent, or whether the background independence of general relativity arises as an emergent property is an open question. The answer to this question will determine whether gravity plays a "special role" in this underlying theory similar to its role in general relativity.## See also

## References

graviton in Bosnian: Graviton

graviton in Bulgarian: Гравитон

graviton in Catalan: Gravitó

graviton in Czech: Graviton

graviton in Danish: Graviton

graviton in German: Graviton

graviton in Modern Greek (1453-):
Βαρυτόνιο

graviton in Spanish: Gravitón

graviton in Persian: گراویتون

graviton in French: Graviton

graviton in Galician: Gravitón

graviton in Korean: 중력자

graviton in Croatian: Graviton

graviton in Italian: Gravitone

graviton in Hebrew: גרביטון

graviton in Latin: Graviton

graviton in Latvian: Gravitons

graviton in Lithuanian: Gravitonas

graviton in Hungarian: Graviton

graviton in Dutch: Graviton

graviton in Japanese: 重力子

graviton in Norwegian: Graviton

graviton in Low German: Graviton

graviton in Polish: Grawiton

graviton in Portuguese: Gráviton

graviton in Romanian: Graviton

graviton in Russian: Гравитон

graviton in Simple English: Graviton

graviton in Slovak: Gravitón

graviton in Slovenian: Graviton

graviton in Finnish: Gravitoni

graviton in Swedish: Graviton

graviton in Turkish: Graviton

graviton in Ukrainian: Гравітон

graviton in Chinese: 引力子