At present, one of the deepest problems in theoretical physics is harmonizing the theory of [[general relativity]], which describes gravitation, and applicationsapplies to large-scale structures ([[star]]s, [[planets]], [[galaxies]]), with [[quantum mechanics]], which describes the other three [[fundamental forces]] acting on the [[atom]]ic scale. This problem must be put in the proper context, however. In particular, contrary to the popular claim that quantum mechanics and general relativity are fundamentally incompatible, one can demonstrate that the structure of general relativity essentially follows inevitably from the quantum mechanics of interacting theoretical spin-2 massless particles (called [[graviton]]s).<ref name=Kraichnan1955>{{cite journal
