Einstein has stated that the velocity of light is the maximum possible velocity. Particles with mass can come close to the velocity of light but can never actually reach it. If one approaches light-velocity, the energy that normally is used for the acceleration, is converted into mass. The mass increase is expressed with the Lorenz factor gamma. In the figure the increase in mass is expressed by the growth of the box carried by the man. The Lorenz factor is important for the properties of synchrotron radiation.