Speed of relativistic particles
T= 1/2 mv2, the well-known formula for kinetic energy, is no longer valid for particles traveling at speeds in excess of ten thousand kilometers per second (also known as relativistic speeds). This is the case for those electrons produced by beta decay whose speeds approach (without ever surpassing) the speed of light in vacuo (300,000 km per second). According to classical physics, no such speed limit should exist. The experimental observation that particles never travel faster than light is therefore a good confirmation of the validity of the theory of relativity
IN2P3