English: in 1900, Henri Poincaré noted a paradox in modern physics as it was then understood: When he applied well-known results of Maxwell's equations to the equality of action and reaction, he could describe a cyclic process which would result in creation of a reactionless drive, i.e. a device producing motion without the exhaust of a propellant, in violation of the conservation of momentum. Einstein resolved Poincaré's paradox using by invoking his mass-energy relationship.