gravitons

Gravitons are hypothetical elementary particles that mediate the gravitational force in quantum theories of gravity. In quantum field theory, gravity would be carried by a massless, spin-2 boson called the graviton. If gravitons exist, they would be quanta of the quantized perturbations of the spacetime metric, arising when one linearizes general relativity around flat Minkowski space and quantizes the small fluctuations hμν.

Properties: massless, spin-2; two helicity states of ±2; neutral; no known charge; extremely weak coupling to matter, with the coupling set by Newton's constant G_N, making interactions far weaker than other forces. The gravitational interaction would be described by the exchange of gravitons, with the coupling strength proportional to energy and momentum, via the energy-momentum tensor Tμν.

Theoretical role: Graviton is the gauge boson of diffeomorphism invariance in perturbative quantum gravity. In effective field theory, general relativity treated as a low-energy limit yields a predictive framework at energies well below the Planck scale, where graviton exchanges can be described perturbatively. The concept is central in approaches such as string theory, where the graviton arises as a massless excitation of a closed string, and in loop quantum gravity.

Experimental status: no direct detection of gravitons. Classical gravitational phenomena are well explained by general relativity; gravitational waves observed by LIGO/Virgo are classical manifestations. A single-graviton detection would require energies and detectors far beyond current capabilities.

Planck-scale issues: a consistent quantum theory of gravity remains unresolved; gravity is nonrenormalizable in perturbation theory, motivating nonperturbative or alternative frameworks. At energies near the Planck scale (~1.22×10^19 GeV), quantum gravity effects would become significant and perturbative descriptions may fail.