"[M]y suggestion that the resonator might be able to exert a one-sided, i. e. irreversible, action on the energy of the surrounding radiation field called forth the emphatic protest of Ludwig Boltzmann... showing that according to... classical dynamics... the processes I was considering could take place in... the opposite sense. Thus a spherical wave emitted from a resonator when reversed shrinks... continually decreasing... on to the resonator, is absorbed by it, and so permits the resonator to send out again into space the energy formerly absorbed in the direction from which it came."