Decoherence is easiest to see in a MW setting. The basic observed fact is that at some point you need an observer that makes a clear, consistent observation. D

Decoherence is easiest to see in a MW setting. The basic observed fact is that at some point you need an observer that makes a clear, consistent observation. Decoherence is a concept about an isolated state (that can be multi-valued, like dead cat / alive cat) that leaks out into the environment by entanglement with more and more environment states, and proceeds to "pull in" versions of you (as the observer) one by one as the versions of you are also entangled with the environment. You can recurse this analysis and eventually you reach a point where the "leaf" quantum state only involves one outcome and is entangled with a single version of you, and at that point you don't need any collapse anymore. Also this process is completely deterministic. The Copenhagen interpretation is a shortcut that basically says let's ignore all those intermediate interactions between the original "weird" state and the observers, in a very real sense you "collapse" the chain of entanglements. You also ignore that you have multiple observer versions of yourself and instead explain it as a probabilistic distribution from the point of view of the "pre-split" observer. This has turned out to be a good enough strategy in practical physics since the 1920s! Note that none of this explains the quantitative aspect of it, the Born rule, or what it actually means that something's more probable than not. It just discusses how you can start with superpositions and end up with observers who don't see the superpositions.