The ultrarelativistic limit is the maximum velocity that a particle can move at. Albert Einstein found the equation for energy: E=mc². This means that energy is equal to a certain amount of mass. However, this is a simplified version of his equation. The longer version of his equation is E=mc+pc. (Mathematically, if you ignore the pc and take the square root of the equation, you get E=mc). pc refers to the velocity of a particle multiplied by a very large number. 