The term graviton was originally coined in 1934 by Soviet physicists Dmitrii Blokhintsev and F.M. 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. All three of these forces appear to be accurately described by the Standard Model of particle physics. The three other known forces of nature are mediated by elementary particles: electromagnetism by the photon, the strong interaction by gluons, and the weak interaction by the W and Z bosons. It is hypothesized that gravitational interactions are mediated by an as yet undiscovered elementary particle, dubbed the graviton. This result suggests that, if a massless spin-2 particle is discovered, it must be the graviton. 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 that gravitational interactions do. 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). 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. In string theory, believed to be a consistent theory of quantum gravity, the graviton is a massless state of a fundamental string. There is no complete quantum field theory of gravitons due to an outstanding mathematical problem with renormalization in general relativity. In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction. The name is attributed to Dmitrii Blokhintsev and F. Hypothetical elementary particle that mediates gravitation Graviton Composition
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