where E is the total energy of the particle (i.e. rest energy plus kinetic energy in kinematic sense), p the momentum, γ {\displaystyle \gamma } the Lorentz factor, c the speed of light, and β the speed as a fraction of c. The variable v can either be taken to be the speed of the particle or the group velocity of the corresponding matter wave. Since the particle speed v < c {\displaystyle v<c} for any particle that has mass (according to special relativity), the phase velocity of matter waves always exceeds c, i.e.