Einstein’s famous equation uses the letter c to stand for the speed of light. Since light always moves at 186,000 miles per second, c is equal to 186,000 miles per second.
Einstein’s theory of special relativity has this equation as one of its parts. The equation shows that a thing’s mass (m) is equal to its kinetic energy (E) divided by the square of the speed of light (c). Einstein made this equation to show that energy and mass are the same thing. With this equation, he was able to show that mass and energy are the same thing and that all you need to do to change one into the other is take into account the speed of light.