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.