Albert Einstein developed the theory of Special Relativity to explain the way in which light behaves. He stated that nothing can travel faster than the speed of light. And only massless objects (such as photons of electromagnetic radiation) actually travel *at* the speed of light. Anything that has mass can never reach the speed of light. As a particle with mass approaches the speed of light, it becomes more and more difficult to make it accelerate. We call its resistance to acceleration its **inertia**. A particle with a bigger inertia has a bigger mass. Einstein showed that, in order to prevent the particle reaching the speed of light, it gains mass equal to its total energy divided by the speed of light squared. So: Using equations developed by Lorentz, Einstein showed that the way in which the mass of a particle changes with speed is given by: | | | | | m is the total mass m_{0} is the rest mass - i.e. the mass of the particle when it is stationary v is the velocity of the particle | Notice that as v tends towards c, the denominator tends towards zero and so the mass tends towards infinity. This makes it impossible to accelerate the particle enough to reach the speed of light. |