General relativity

Special relativity explains well that speed of light appears same to all observers and describing what happens when things move at speeds close to speed of light.

However, it was inconsistent with Newtonian theory of gravity, which said that objects attracted each other with a force that depended on the distance between them. This meant that if one moved one of the objects, the force on the other one would change instantaneously. Or, in other words, gravitational effects should travel with infinite velocity, instead of at or below the speed of light, as the special theory of relativity required. 

Einstein made unsuccessful attempts between 1908 and 1914 to find a theory of gravity that was consistent with special relativity. In 1915 he proposed the general theory of relativity.

He made the revolutionary suggestion that *gravity is not a force like other forces*, but is a *consequence of the fact that space-time is not flat*, as had been previously assumed: it is curved, or "warped", by the distribution of mass and energy in it.