Is it possible to observe anything at all in MWI? If yes, what does it mean to "observe something" in MWI? If not, is there any physical content in MWI?
Observations in MWI are just ordinary physical interactions. Specifically they're perturbative-regime interactions between the thing being observed and large thermalized systems (e.g. humans). From the perspective of the thermal bath, you get exponential suppression of everything but the eigenstates of the interaction Hamiltonian, which is why our observations "look classical".
Will other large thermalised system (e.g. rocks) also experience observations that "look classical"?
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.
> But they'll induce decoherence in the same way as a human, yes.
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.
Depends on whether you're talking about the "perspective" of the whole joint |Rock>|Interaction Eigenstate 1> + |Rock>|Interaction Eigenstate 2> ... system or just one of its components.
> 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.
> 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.
> Ok, I guess I misunderstood the scope of "There's also nothing special about observing."
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.
