In 1928, the physicist Paul Dirac was looking for an equation that would predict how very fast particles should behave. There was already another equation that could describe slow-moving particles, the Schrödinger equation, but Einstein's theory of special relativity said that fast particles could be very different from slow particles. Dirac knew that particles like electrons were usually moving very fast. He realized the old equation would not make good predictions for fast particles. So he came up with a new equation that could describe particles moving close to the speed of light.