Albert Einstein’s Theory of Relativity is in two parts.
The first, Special Relativity (1905) defines space as a vacuum, the speed of light as constant and time as a fourth dimension for measuring space. Relativity means two events will appear as simultaneous from one point of reference but will occur separately viewed from another point.
In the General Theory of Relativity (1915) gravity is a force curving space around an object. The equation E=mc2 refers to the enormous amount of energy contained within an object's atomic structure. It led to the development of atomic power.