Universal force of attraction that acts between all bodies that have mass .
Though it is the weakest of the four known forces, it shapes the structure and evolution of stars, galaxies, and the entire universe. The laws of gravity determine the trajectories of bodies in the solar system and the motion of objects on earth, where all bodies experience a downward gravitational force exerted by earth's mass, the force experienced as weight. Albert Einstein proposed a whole new concept of gravitation involving the four-dimensional continuum of {{link=space-time">space-time which is curved by the presence of matter. In his general theory of relativity , he showed that a body undergoing uniform acceleration is indistinguishable from one that is stationary in a gravitational field.