Einstein’s theory of special relativity describes what happens as things near the speed of light. Here are some important special-relativity equations that deal with time dilation, length contraction, and more.
Angle of x' axis in Minkowski spacetime | Special relativity | Physics | Khan Academy - YouTube
Equation for the General Theory of Relativity. General relativity generalises special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime.