In theories of quantum gravity, the graviton is the hypothetical elementary particle that mediates the force of gravitational interaction. There is no complete quantum field theory of gravitons due to an outstanding mathematical problem with renormalization in general relativity. In string theory, believed by some to be a consistent theory of quantum gravity, the graviton is a massless state of a fundamental string.
If it exists, the graviton is expected to be massless because the gravitational force has a very long range and appears to propagate at the speed of light. The graviton must be a spin-2 boson because the source of gravitation is the stress–energy tensor, a second-order tensor (compared with electromagnetism's spin-1 photon, the source of which is the four-current, a first-order tensor). Additionally, it can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field would couple to the stress–energy tensor in the same way gravitational interactions do. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton.[For a comparison of the geometric derivation and the (non-geometric) spin-2 field derivation of general relativity, refer to box 18.1 (and also 17.2.5) of ]
Theory
It is hypothesized that gravitational interactions are mediated by an as yet undiscovered elementary particle, dubbed the
graviton. The three other known forces of nature are mediated by elementary particles:
electromagnetism by the
photon, the strong interaction by
, and the
weak interaction by the W and Z bosons. All three of these forces appear to be accurately described by the
Standard Model of particle physics. In the
classical limit, a successful theory of gravitons would reduce to general relativity, which itself reduces to Newton's law of gravitation in the weak-field limit.
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History
Albert Einstein discussed quantized gravitational radiation in 1916, the year following his publication of general relativity.
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The term graviton was coined in 1934 by Soviet physicists Dmitry Blokhintsev and .]
Gravitons and renormalization
When describing graviton interactions, the classical theory of and semiclassical corrections such as behave normally. However, Feynman diagrams with at least two loops lead to ultraviolet divergences.
Energy and wavelength
While gravitons are presumed to be massless, they would still carry energy, as does any other quantum particle.
Alternatively, massive gravity, the analysis of gravitational waves yielded a new upper bound on the mass of gravitons. The graviton's Compton wavelength is at least , or about 1.6 , corresponding to a graviton mass of no more than .
Experimental observation
Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, has been thought to be impossible with any physically reasonable detector.[
] 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 , since the dimensions of the required neutrino shield would ensure collapse into a black hole.
LIGO and Virgo collaborations' observations have directly detected gravitational waves.[
] Observations of gravitational waves put an upper bound of on the graviton's mass.
Astronomical observations of the kinematics of galaxies, especially the galaxy rotation problem and modified Newtonian dynamics, might point toward gravitons having non-zero mass.
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 energies close to or above the Planck scale. This is because of infinities arising due to quantum effects; technically, gravitation is not renormalizable. 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 thought to be mathematically consistent.[
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See also
External links
