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 and would be emitted only by highly accelerating, extremely massive objects such as stars. Since gravitons would apparently be identical to their antiparticles, the notion of antigravity is questionable.