Modelling the onset of decoherence in a subspace as a transition from this subspace http://www.ba.infn.it/~pascazio/publications/sudarshan_seven_quests.pdf (Section 10 is relevant) I am currently reading papers discussing the Zeno Effect. The linked paper discusses modelling a transition out of a wanted subspace as the onset of decoherence. Thus by frequently projecting back onto the subspace prevents decoherence. I don't quite understand this model. Why does remaining in the subspace mean that coherence must be preserved? Surely environmental effects can decohere the system even if it stays in the subspace. I guess that's what I'm having trouble with, why decoherence is modeled as a transition out of the subspace.