A hypothetical quantum of gravitation whose role is the mediation of gravitational force between masses; mass 0, spin 2, charge 0. Gravitons are the quanta of gravitational waves, as photons are the quanta of electromagnetic waves; but, unlike photons, gravitons will interact with one another, one reason why quantum gravity is hard to formulate.
For other uses, see Graviton (disambiguation).| Graviton | |
| Composition: | Elementary particle |
|---|---|
| Interaction: | Gravity |
| Status: | Hypothetical |
| Mass: | 0 |
| Mean lifetime: | Stable |
| Electric charge: | 0 |
| Spin: | 2 |
In physics, the graviton is a hypothetical elementary particle that mediates the force of gravity in the framework of quantum field theory.
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.
However, attempts to extend the Standard Model with gravitons run into serious theoretical difficulties at high energies (processes with energies close 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.
Gravitons and models of quantum gravity
When describing graviton interactions, the classical theory (i.e. it is the only known theory in which the quantum corrections of any order to graviton scattering are finite.
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.
Some proposed quantum theories of gravity do not predict a graviton.
Gravitons and experiments
Detecting a graviton, if it exists, would prove rather problematic. The answer to this question will determine whether gravity plays a "special role" in this underlying theory similar to its role in general relativity.
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