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 E2=m2c4+p2c2. (Mathematically, if you ignore the p2c2 and take the square root of the equation, you get E=mc2). p2c2 refers to the velocity of a particle multiplied by a very large number. 