OK, picking up from my last post, I think I see a way whereby we can leave this. Advocates of the Many World Interpretation will agree that it does not pretend to say anything about humans and stuff, and that expecting it to do so is as absurd as expecting someone to write down and solve the Schrödinger equation for a football game. They will agree that all those popular (and sometimes technical) books and articles telling us about our alternative quantum selves and Many-Worlds morality and so forth, are just the wilder speculative fringes of the theory that struggle with problems of logical coherence. They agree that statements like DeWitt’s that “every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies” aren’t actually what the theory says at all. They acknowledge a bit more clearly that the Alices and Bobs in their papers are just representations of devices that can make an observation (yes, I know this is all they have ever been intended as anyway.) They agree that when they say “The world is described by a quantum state”, they are using “world” in quite a special sense that makes no particular claims about our place(s) or even our existence(s) in it*. They admit that if one tries to broaden this sense of “world”, some difficult conundrums arise. They admit that the mathematical and ontological status of these “worlds” are not the same thing, and that the difference is not resolved by saying that the “worlds” are “really” there in Hilbert space, waiting to be realized.
Then – then – I’m happy to say, sure, the Many Worlds Interpretation, which yes indeed we might better relabel the Everettian Interpretation (shall we begin now?), is a coherent way to think about quantum theory. Possibly even a default way, though I shall want to seek advice on that.
Is that a deal?
*I submit that most physicists and chemists, if they write down the Schrödinger equation for, say, a molecular orbital, are not thinking that they are actually writing down the equation for a “world” but with some bits omitted. One might respond “Well, they should, unless they are content to be “shut up and calculate” scientists”. But I would submit that they are just being good scientists in recognizing the boundaries of the system their equations describe and are not trying to make claims about things they don’t know about or understand.