Among other problems. It's just bollocks on the level of "but you didn't say it couldn't be this, nanananana."
Genuinely curious, what do you mean by "but you didn't say it couldn't be this, nanananana...?" The Everett interpretation isn't some fantastical notion spun up by a scifi writer...it is simply the consequence of removing collapse of the wavefunction as an objective event from the picture. And if it turns out our observations wouldn't be changed by removing this feature, then perhaps it was an extraneous feature in the first place!
There's also nothing special about observing. Our consciousness isn't super-natural, so the idea is in desperate need of some other underpinning.
And probabilities: if this is one of many, many worlds, the distribution of events as we can observe them is heavily skewed. The next observations should follow a radically different pattern, unless you also assume that each split influences the probabilities of future events.
"Many worlds" is a misnomer. MWI is just wavefunction realism + unitary evolution. There's only one world, and really only one dynamical object: the wavefunction. It evolves according to some unitary operator, and that's the whole story. No splitting, no collapse, no objective classical transition, just quantum mechanics taken at face value.
> There's also nothing special about observing.
Yes, that's the MWI position.
Another MWI proponent here is talking about how the "look classical" thing is "simply an artifact of being a conscious being that can only observe one value".
Is there something special about "large thermalised systems" (and/or humans)? How large have they to be to allow for "our observations"? Where is the boundary between the thing being observed and the observing thing?
MWI seems to still face most of the difficult questions - it not all.
The inner life of rocks is somewhat beyond our reach, I'm afraid. But they'll induce decoherence in the same way as a human, yes.
> Is there something special about "large thermalised systems" (and/or humans)?
Aside from being large and thermalized? No.
> How large have they to be to allow for "our observations"?
It's a continuum. The more internal degrees of freedom you have, and the more thoroughly they're mixed, the faster you'll decohere things.
> Where is the boundary between the thing being observed and the observing thing?
At the level of fundamental physics, there isn't one. "Observation" is an approximate and thermodynamic notion.
From the perspective (?) of the (non-human) thermal bath, will that decoherence result in a single (diagonal, mixture) state or in a particular state of those N separate states that would "look classical"?
Decoherence doesn't make things "look classical" by itself - at least until you define what "looking" is.
> Decoherence doesn't make things "look classical" by itself - at least until you define what "looking" is.
Of course; but that's true of every scientific theory. Decoherence solves the preferred basis problem, not the hard problem of consciousness.
Ok, I guess I misunderstood the scope of "There's also nothing special about observing."
It's not clear to me if you (MWI) would say that rocks had defined positions when nobody was observing them - or whether the question of things having definite positions (and the very existence of those things) wouldn't even make sense in the absence of consciousness.
There's nothing special about the physical processes constituting a scientific experiment. They're unitary evolution like everything else. Whether there's anything special about conscious experience (for my money: obviously yes) is outside the scope of physics.
> or whether the question of things having definite positions (and the very existence of those things) wouldn't even make sense in the absence of consciousness.
If you want to be perfectly precise, the MWI does not contain discrete things at all, any more than the Earth objectively has discrete continents and seas. But the most accurate map is not always the most useful one.
As for observation, I didn't suggest there was anything special about observing. In fact, this is part of what makes Everett among the most parsimonious interpretations: While some (not all) other interpretations are tasked with explaining the nature of observation (what counts as observation, how quickly does collapse propagate, etc), under Everett, there is no notion of observation at all. My point about consciousness is that, being conscious, we are forced to have a point of view which depends on where we exist in/on the wave function. I wasn't suggesting that consciousness has any actual effect whatsoever on the wavefunction.
I admit that the nature of probability is among the most difficult parts of Everett to wrestle with, though I'm not sure I understand why subsequent observations should follow a radically different pattern? It's still the same wavefunction with the same distributions as before...Schrodinger's equation is the same wherever you are on the wavefunction.
> What we observe as the collapse of a function is simply an artifact of being a conscious being that can only observe one value of the of the wavefunction at any particular moment.
If we simply assume that our observations are as if a wave function happened we can indeed have our Schrödinger cake and eat it too.