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In physics, the graviton is a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton must be massless (because the gravitational force has unlimited range) and must have a spin of 2. This is because the source of gravitation is the stressenergy tensor, a secondrank tensor, compared to electromagnetism, the source of which is the fourcurrent, a firstrank tensor. Additionally, it can be shown that any massless spin2 field would be indistinguishable from gravitation, because a massless spin2 field must couple to (interact with) the stressenergy tensor in the same way that the gravitational field does. ^{[4]} This result suggests that if a massless spin2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin2 particle.^{[5]}
Gravitons are postulated because of the great success of quantum field theory (in particular, the Standard Model) at modeling the behavior of all other known forces of nature as being mediated by elementary particles: electromagnetism by the photon, the strong interaction by the gluons, and the weak interaction by the W and Z bosons. The hypothesis is that the gravitational interaction is likewise mediated by a – yet undiscovered – elementary particle, dubbed the graviton. In the classical limit, the theory would reduce to general relativity and conform to Newton's law of gravitation in the weakfield limit.^{[6]}^{[7]}^{[8]}
However, attempts to extend the Standard Model with gravitons have run into serious theoretical difficulties at high energies (processes with energies close to or above the Planck scale) because of infinities arising due to quantum effects (in technical terms, gravitation is nonrenormalizable). Since classical general relativity and quantum mechanics are incompatible at such energies, from a theoretical point of view the present situation is not tenable.^{[9]} Some proposed models of quantum gravity^{[10]} attempt to address these issues, but these are speculative theories.
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