In theories of quantum gravity, the graviton is the hypothetical elementary particle that mediates the force of gravitational interaction. It is a quantum of gravitational wave energy.
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 an undiscovered elementary particle mediates gravitational interactions, 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 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 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.[
] Furthermore, in the presence of a gravitational wave, it should be possible to observe (via interference or beating effects after a delay line) signatures of stimulated emission or absorption of gravitons with present-day technology.
Solar system planetary trajectory measurements by space missions such as Cassini and MESSENGER give a comparable upper bound of .
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 severe theoretical difficulties at energies close to or above the Planck scale. Infinities arise due to quantum effects; technically, gravitation cannot be renormalized. Since classical general relativity and quantum mechanics seem incompatible at such energies, this situation is not tenable from a theoretical point of view.
One possible solution is to replace particles with strings. String are one-dimensional loops that avoid divergences by smearing out the gravitational interactions. A particle identified with the graviton appears in string theory with long-distant interactions described by general relativity. Unfortunately models based on strings have only been worked out for a few weakly interacting strings.
See also
External links
