graviton, postulated quantum that is thought to be the carrier of the gravitational field. It is analogous to the well-established photon of the electromagnetic field. Gravitons, like photons, would be massless, electrically uncharged particles traveling at the speed of light. Since gravitons would apparently be identical to their antiparticles, the notion of antigravity is questionable. Gravitons have not been directly observed; as of 2016, observations of weak lensing of distant galaxies have placed the tightest bound on the graviton’s mass as being less than 6 × 10−32 electron volts.