Graviton

The graviton is the hypothetical quantum of the gravitational field, the force carrier in quantum theories of gravity. In analogy with the photon for electromagnetism, the graviton would mediate gravitation between masses and energy. It is expected to be a massless, spin-2 boson, propagating at the speed of light and coupling universally to the energy–momentum tensor. In perturbative approaches to general relativity, spacetime metric fluctuations give rise to quantized excitations interpreted as gravitons. A massless spin-2 particle has two helicity states, +2 and −2, corresponding to the two polarization modes of gravitational waves.

Gravitons remain hypothetical; no experimental evidence yet. Gravitational interactions at everyday energies are extremely weak, making direct detection of single gravitons unlikely with current technology. Classical gravity, described by Einstein's field equations, emerges as the low-energy limit; a gravitational wave can be viewed as a coherent state containing many gravitons.

From a theoretical viewpoint, perturbative quantum gravity with gravitons is non-renormalizable, meaning it cannot be a complete quantum theory by itself; it is studied as an effective field theory at energies well below the Planck scale. A full quantum theory of gravity—such as string theory or loop quantum gravity—offers different perspectives on gravitons as low-energy excitations or emergent phenomena.

Experimental status: not observed as individual quanta; gravitational wave observations by LIGO/Virgo confirm classical predictions; any graviton effects would appear as minute deviations at high energies or in high-precision experiments. As such, the graviton remains a central but unverified component of many quantum gravity frameworks.