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Manfred comments on On Being Decoherent - Less Wrong
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If the wavefunction collapses upon measurement, and no adequate definition of the term "measurement" has been given, then the theory as it stands is incoherent. (I realize that a Copenhagenist thinks that they can get around this by simply denying that the wavefunction exists, but the price of that move is that they don't have any coherent picture of reality underneath the mathematics.)
OK, but let's note in passing that the MWI believer needs show this a priori, whether from the mathematics of QM or by deconstructing the concept of "experiencing a superposition" or both. I don't think that should be particularly difficult, though.
I know what that means.
The trouble is at some point you need to explain why our measurement records show sequences of random outcomes distributed according to the Born rules. Now it may be the case that branches where measurement records show significant deviation from the Born rules have 'low amplitude', but then you need to explain why we don't experience ourselves in "low amplitude branches". More precisely, you need to explain why we seem to experience ourselves in branches at rates proportional to the norm squared of the amplitudes of the branches (whatever this talk of 'experiencing ourselves' is supposed to mean, and whatever a 'branch' is supposed to be). Why should that be true? Why shouldn't the subjective probabilities simply be a matter of 'counting up' branches irrespective of their weighting? After all, the wavefunction still contains all of the information even about 'lightly weighted' branches.
The MWI believer thinks that, by talking about reduced density matrices obtained by 'tracing out' the environment, you've thereby made good progress towards showing where the Born probabilities come from. But the MWI skeptic thinks that the 'last little bit' that you still have to do (i.e. explaining why we experience ourselves in heavily weighted branches more than lightly weighted ones) is and always was the entire problem.
Sorry, I don't know what you're talking about.
Not even the quantum-information-ey definition of "transfer of information from the measuree to the measurer?"
What I'm taking from this is that you don't know many copenhagenists familiar with Bell's theorem.
Well I suppose I could tell the MWI (or any sort of) skeptic about what density matricies and mixed states are (other readers: to wikipedia!), and how when you see a mixed state it is by definition probabilistic.
Well, even after the next question of "why do density matrices work that way?", you can always ask "why?" one more time. But eventually we, having finite information, will always end with something like "because it works." So how can we judge explanations? Well, one "why" deeper is good enough for me.
Tsk, fine, 0 guesses then: the probabilities you get from tracing out the environment are the Born probabilities. But as I said this doesn't count as deducing them, they're hidden in the properties of density matrices, which were in turn determined using the Born probabilities.
That sounds fine, but there's no objective way of defining what a "measurer" is. So essentially what you have is a 'solipsistic' theory, that predicts "the measurer's" future measurements but refuses to give any determinate picture of the "objective reality" of which the measurer herself is just a part.
I have to concede that many thinkers are prepared to live with this, and scale down their ambitions about the scientific enterprise accordingly, but it seems unsatisfactory to me. Surely there is such a thing as "objective reality", and I think science should try to tell us what it's like.
That may very well be true, but how does pointing out that the Copenhagen Interpretation denies the objective existence of the wavefunction entail it?
Perhaps. My post in the discussion section, and my subsequent comments, try (and fail!) to explain as clearly as I can what troubles me about MWI.
It's true that an MWI non-collapsing wavefunction has 'enough information' to pin down the Born probabilities, and it's also true that you can't get the empirical predictions exactly right unless you simulate the entire wavefunction. But it still seems to me that in some weird sense the wavefunction contains 'too much' information, in the same way that simulating a classically indeterministic universe by modelling all of its branches gives you 'too much information'.
I know what you mean, but as I'm sure you know it's not mere perversity that has led many to accept "modeling all the branches" of the QM universe. In the case of a classically indeterministic universe, you can model just one indeterministic branch, but in the case of the QM universe you can't do that, or you can't do it anywhere near as satisfactorily. The "weirdness" of QM is precisely that aspect of it which (in the eyes of many) forces us to accept the reality of all the branches.