If it exists, the graviton is expected to bemassless because the gravitational force has a very long range and appears to propagate at the speed of light. The graviton must be aspin-2boson because the source of gravitation is thestress–energy tensor, a second-ordertensor (compared withelectromagnetism's spin-1photon, the source of which is thefour-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.[5]
Albert Einstein discussed quantized gravitational radiation in 1916, the year following his publication ofgeneral relativity.[9]: 525 The termgraviton was coined in 1934 by Soviet physicistsDmitry Blokhintsev andFyodor Galperin [ru].[3][9]Paul Dirac reintroduced the term in a number of lectures in 1959, noting that the energy of the gravitational field should come in quanta.[10][11] A mediation of the gravitational interaction by particles was anticipated byPierre-Simon Laplace.[12] Just likeNewton's anticipation of photons, Laplace's anticipated "gravitons" had a greater speed than the speed of light in vacuum, the speed of gravitons expected in modern theories, and were not connected toquantum mechanics orspecial relativity, since these theories didn't yet exist during Laplace's lifetime.
When describing graviton interactions, theclassical theory ofFeynman diagrams and semiclassical corrections such asone-loop diagrams behave normally. However, Feynman diagrams with at least two loops lead toultraviolet divergences.[13] These infinite results cannot be removed because quantizedgeneral relativity is notperturbativelyrenormalizable, unlikequantum electrodynamics and models such as theYang–Mills theory. Therefore, incalculable answers are found from the perturbation method by which physicists calculate the probability of a particle to emit or absorb gravitons, and the theory loses predictive veracity. Those problems and the complementary approximation framework are grounds to show that a theory more unified than quantized general relativity is required to describe the behavior near thePlanck scale.
While gravitons are presumed to bemassless, they would still carryenergy, as does any other quantum particle.[14]Photon energy andgluon energy are also carried by massless particles.
Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, has been thought to be impossible with any physically reasonable detector.[16] The reason is the extremely lowcross section for the interaction of gravitons with matter. For example, a detector with the mass ofJupiter and 100% efficiency, placed in close orbit around aneutron 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 ofneutrinos, since the dimensions of the required neutrino shield would ensure collapse into ablack hole.[16] It has been proposed that detecting single gravitons would be possible by quantum sensing.[17] Even quantum events may not indicate quantization of gravitational radiation.[18]
LIGO andVirgo collaborations' observations havedirectly detected gravitational waves.[19][20][21] Others have postulated that graviton scattering yields gravitational waves as particle interactions yieldcoherent states.[22] Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton.[23] For example, if gravitational waves were observed to propagate slower thanc (thespeed of light in vacuum), that would imply that the graviton has mass (however, gravitational waves must propagate slower thanc in a region with non-zero mass density if they are to be detectable).[24] Observations of gravitational waves put an upper bound of1.76×10−23 eV/c2 on the graviton's mass.[25] Solar system planetary trajectory measurements by space missions such asCassini andMESSENGER give a comparable upper bound of3.16×10−23 eV/c2.[26] The gravitational wave and planetary ephemeris need not agree: they test different aspects of a potential graviton-based theory.[27]: 71
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 thePlanck scale. This is because of infinities arising due to quantum effects; technically, gravitation is notrenormalizable. Since classical general relativity andquantum 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 withstrings. 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.[30]
^abStachel, John (1999). "The Early History of Quantum Gravity (1916–1940)".Black Holes, Gravitational Radiation and the Universe. Fundamental Theories of Physics. Vol. 100. pp. 525–534.doi:10.1007/978-94-017-0934-7_31.ISBN978-90-481-5121-9.
