Mass increases with speed

Setting the speed of light as a universal speed limit led Einstein to his remarkable discovery that mass and energy are equivalent. He arrived at this by considering what would happen if an object travelling at relativistic speed continued to pour energy into acceleration. The energy could be provided internally, for instance on a rocket powered by an ion engine, or externally, for example in a particle accelerator.

Einstein realised that in either case the object’s kinetic energy would have to increase even if its velocity could not. Since kinetic energy is reliant on both mass and velocity, this indicated that adding energy to an object moving at near-light speed tends to increase its mass. In fact, Einstein found that the energy content of an object is equal to its mass times the speed of light, or c, squared.