What is the theory of relativity?
One-line takeaway
BLUF: Relativity refers to Einstein's Special and General theories.
A fundamental explanation of what is the theory of relativity?
The Explanation
Relativity refers to Einstein's Special and General theories. Special Relativity (SR) (1905) unifies space and time into 4D spacetime, with light speed c constant for all observers. It predicts time dilation, length contraction and mass–energy equivalence (E=mc²). General Relativity (GR) (1915) extends this: gravity is not a force but spacetime curvature produced by mass–energy. Massive objects curve spacetime and objects move along those curves (geodesics). Thus SR reshapes our understanding of space and time, and GR explains gravity as geometry. Together they revolutionized physics, replacing Newtonian absolutes with a relativistic framework.