For fast particles, it is no longer true that the energy is formula_1. Instead, Dirac's new equation worked for particles where the energy was given by formula_3. In the new equation for energy, the symbol formula_4is called the momentum, and it measures how fast the particle is going and how hard it is to stop. This equation says that very fast particles have more energy, so they are different from slow particles. You can take the square root of each side of this equation, since both sides are equal. However, any real square root has two answers, formula_5and formula_6. You can think of the answer with negative energy as being antimatter.