Einstein's Box

This thought experiment proposed by Albert Einstein in 1906 considers a box of mass and length in which a quantity of radiant energy is emitted from one end toward the other. The box moves a distance (exaggerated in the picture) in a time with a velocity in the opposite direction to the photons, which carry the equivalent of a mass . The momentum carried by the radiation is given by . Assuming conservation of momentum and energy (for the case ) leads to Einstein's celebrated equation , applicable, at least, to photons.