>>12392097I used to align with MWI a few years back, when I had realized that COPEnhagen was, as the name implies, pure cope, yet no other options out there seemed any less coping as well. It's nice to believe that all we have is a Hilbert space for the whole universe, some ungodly Hamiltonian that defines what's in it, and simple unitary evolution to make it go. But I think it's founded on the assumption, or perhaps desire, that pure states are "true" quantum states, and mixed states are just part of our effective models of quantum phenomena.
But that's really not true, or at least, it isn't necessarily so. This false belief probably stems from Stinespring dilation -- if we just buy into the Church of the Larger Hilbert Space, all the mixed states we observe in reality can be explained from tracing out various degrees of freedom from the larger pure state. But is that really true? Or is it just another coping mechanism? Stinespring dilation isn't unique, and it can require an arbitrarily large embedding. Are the additional degrees of freedom required to expand to the larger Hilbet space actually real, physical degrees of freedom? Or are they just mathematical conveniences?
I take the latter viewpoint. Imagining that pure states are the "true" states of reality seems like a coping mechanism as well, albeit a more sophisticated one. Do MWIers truly believe that, once you reach the entire Hilbert space of the universe, and not one degree of freedom more, the state of the universe we inhabit and currently occupy will be precisely a pure state? It's a bold claim and entirely unsubstantiated. We have a description of states that don't require projective Hilbert space, and we have a description of their evolution using CPTP maps, not just unitary ones. And they work so marvelously well, so robustly, that we shouldn't relegate them to just being effective descriptors. They ARE quantum mechanics, full stop.
We don't need pure unitarity. Hence we don't need MWI.
