Theory of general relativity theory of general relativity deals with the case of movements that occur with variable speed and is fundamental postulate the equivalence principle, whereby the effects of a gravitational field equivalent to those produced by the movement accelerated. The revolutionary assumption made by Einstein was caused by the fact that the theory of special relativity, based on the principle of the constancy of the speed of light whatever the motion reference system that measures (such as demonstrated in the experiment of Michelson and Morley), disagreed with the theory of Newtonian gravity: if the force with which two bodies attract depends on the distance between them, a move that would instantly change the force felt by the other, ie, the interaction would have an infinite propagation speed, in violation of the special theory of relativity that states that nothing can exceed the speed of light. After several failed attempts to accommodate the gravitational interaction with relativity, Einstein suggested that gravity is a force like no other, but is a consequence of space-time is warped by the presence of mass (or energy which is the same.) Then, bodies like the earth does not move in closed orbits because there is a force called gravity, but move in as close to a straight line, but in a space-time is warped by the presence of sunlight.