Antimatter
For fast particles, it is no longer true that the energy is . Instead, Dirac's new equation worked for particles where the energy was given by . In the new equation for energy, the symbol is 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, and . You can think of the answer with negative energy as being antimatter.