selfAdjoint said:
If there were a clarification of "relative state" that avoided "MWI" I would jump at it. But as it is, it leads sensible people like vanesch to bring back the "mind of the observer" and that gets us back again to "does the Moon exist when nobody is looking at it" and "How do nuclear processes go on in the heart of the Sun when there is no mind there to observe them?
Well, the moon DOES exist even if "no mind" is looking at it. It even exists in several entangled states. You just have to pick out one, that's all there is to it. The same with nuclear processes. Sooner or later, you WILL observe something that is somehow related to it, which will make you choose one or another "interaction history" completely compatible with an "objective reaction".
In fact, your remark is more to the point concerning "parts of the universe outside of the visible universe", say, at 500 billion lightyears from here. Does that part of the unobservable universe "exist" or not ?
I will remain agnostic about measurement and hope for a successful completion of the decoherence program. Next year in Stockholm!
I'm convinced the decoherence program IS already "completed". It did what it could": it indicated that interactions with the environment lead to Schmidt decompositions which turn out to be "classical" states.
But there's something that Decoherence, nor ANYTHING can solve, within strict unitary QM: that is: at the end of the day, you end up with a sum of terms, and you have to pick one of them. Because of the linearity of the time evolution operator, you cannot do anything to do so.
What you can hope to do is that there is some *natural* way of assigning probabilities to each of the terms which turns out to be equivalent to the Born rule (I'm convinced you cannot even do that because I know of OTHER ways to "naturally assign" probabilities, but that's something else).
But THE VERY FACT that you have to pick out one term, and not all together (even if there is a "natural way" to pick it out), means that this is *an extra postulate* which is NOT included in quantum theory itself. After all, nothing stops me from being entangled with something else, and _experience_ that entanglement !
I agree that my "mind" stuff is only a shortcut to "the observing something that must somehow pick out ONE term to be observed". But given CURRENT STATE of quantum theory, embracing strict linearity, I don't see another way out.
My real hope resides infact in gravity. Gravity might somehow induce a genuine probabilistic (or even deterministic ?) collapse in an objective way, and then all that mind stuff can go in the dust bin.
But that would then imply true non-local interactions (EPR...), GR and SR would fall on their faces in one way or another etc...
If the superstring approach to quantizing gravity holds, then I think we're stuck with strictly linear quantum theory. And then I don't see any other way out, honestly.
cheers,
Patrick.