hypothetical carrier of gravity, if gravity is quantised
massless (infinite range), spin 2 (couples to energy and momentum, always attractive)
never observed
theory breaks down near the Planck energy (~10¹⁹ GeV) → the problem of quantum gravity
single-graviton detection: long thought impossible (Dyson), recent resonator proposals (Tobar et al., 2024), though a classical wave could give the same signal (Carney et al., 2024)