Albert Einsten published theories of general relativity and special relativity, both of which described space and time as a unified spacetime in Einsten field equations, which formed a set of coupled nonlinear partial differential equations.
General relativity generalizes special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime. The relation is specified by the Einstein field equations, a system of partial differential equations. The Einstein field equations are nonlinear and very difficult to solve. Soon after publishing his work on special relativity, Einstein began working to extend the theory to gravitational fields; he then published a paper on general relativity in 1916, introducing his theory of gravitation. 
