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.

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A. Einstein, Ann. Phys., 20, 1906 pp. 627–633.
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