In the realm of physics, a graviton is a hypothetical elementary particle that mediates the force of gravitation in quantum field theory. According to quantum mechanics, forces between particles are usually transmitted by other particles known as "force carriers" or "gauge bosons." For example, photons are the force carriers of electromagnetic force, while W and Z bosons mediate weak nuclear force.
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A graviton is the spin-two quantum appearing in a perturbative quantization of gravitational waves about a suitable background. In the massless theory, physical flat-space modes have two helicities. This identifies a theoretical quantum excitation, rather than an experimentally established single-particle detection.
A classical gravitational wave corresponds to collective field behavior and can be measured without resolving individual gravitons. Quantizing weak fluctuations is useful as a low-energy effective description, but does not by itself settle the high-energy completion of quantum gravity. In closed string theory, the massless spectrum includes a symmetric transverse tensor state with the graviton's properties. Its presence motivates a gravitational interpretation of that sector, with the complete background and consistency conditions still specified.
