Consider the simplest case of 1D motion on the x axis. Let the photon be traveling in the + x direction and the atom in the − x direction. In each absorption event, the atom loses a momentum equal to the momentum of the photon. The atom, which is now in the excited state, emits a photon spontaneously but randomly along + x or − x. Momentum is returned to the atom. If the photon was emitted along + x then there is no net change; however, if the photon was emitted along − x, then the atom is moving more slowly in either − x or + x.