Decoherence is when the system is entangled with the environment irreversibile... But when it is reversible.. is it still called Decoherence? However, the universe is said to be a closed system. Does the irreversibility in decoherence means it is just our ignorance that we cant track it. For example. In the ocean. It is chaotic, when a big boat surfs it.. the wave disturbance it bestowed to the water is complex and our ignorance can't make us track all the wave interferences of the boat and the ocean. Is this also the case in Decoherence where we can't track all the massive degrees of freedom of the environment hence we call it irreversible? But like the ocean which in principle still has all the wave interferences there and in principle reversible.. is this also the case in environmental decoherence where the universe in principle still can keep tract of all degrees of freedom and hence in principle reversible?