1Z wrote:
> Brent Meeker wrote:
>>Stathis Papaioannou wrote:
>>>Peter Jones writes:
>>>>By youur definitions, it's a straight choice between metaphysics and
>>>>I choose metaphsyics.
>>>>We can posit the unobservable to expalint he observable.
>>>Solipsism is a metaphysical position.
>>>>(BTW: it it is wrong to posit an unobserved substrate, why is it
>>>>OK to posit unobserved worlds/branches ?)
>>>It's debatable, but perhaps MWI is a better and simpler explanation of
>>>the facts of quantum mechanics than is CI, for example.
>>Multiple-worlds are a consequence of dropping the collapse of the wave 
>>which was inexplicable and ad hoc.
> It's neither. If anything there is an embarassment of explanations for
> it,
> and number of motivations for positing it.
> A genuine problem with MWI: it starts with the assumption that the
> universe is in a 'pure' state. However, unitary evolution under the SWE
> is unable to fully transform a pure state into a genuine mixture. It
> can generate (by mechanisms similar to environmental decoherence) an
> approximate mixture -- For All Practical Purposes. Since collapse does,
> by stipulation, produce orthogonal states, there is a difference
> between collapse interpretations and MW. The residual interferences
> could be detectable. (It is also believed by many that collapse itself
> is detectable).
> "In fact it turns out that in the general case , there will be a unique
> pair of orthogonal perception states accompanying a pair of orthogonal
> cat states. This is something known as the Schmidt decomposition of an
> entangled state. However this is not much use for resolving the
> measurement paradox (...) because gernerally this mathematically
> preferred pair of cat states (..) would not be the desired |live cat> +
> | dead cat> at all, but some linear superposition of these! [...] Since
> the mathematics alone will not single out the |live cat> and |dead cat>
> states as being in any way 'preferred' we still need a theory of
> perception before we can make sense of [MWI] and such a theory is
> lacking.Moreover the onus on such a theory would be not only to explain
> why superpositions of dead and alive cats (or anything else
> macroscopic) occur in do not occur in the physical world but also why
> the wonderous and extraordinarily precise squared-modulus rule actually
> gives the right answers for probabilities in quantum mechanics!"
> R. Penrose, Road to Reality p809
> Is MWI a complete solution to the paradoxes of QM? Is an Universal Wave
> Function feasible ?
> A genuine problem with MWI: Reasonableness of all-embracing unitary
> evolution. MWI-ers claim that the unitary evolution of the SWE (or some
> variation) is the single all-embracing law of the universe -- the other
> main part of the QM universe, the process of collapse (AKA reduction)
> is not needed. However, QM itself is not an all-encompassing physical
> theory because it does not include gravity and relativity. It might be
> possible to include gravity in an extended WE, but the conventional SWE
> requires a derivative against time, wich is difficult to achieve in a
> way that is compatible with the requirements of relativity. There is
> also a more conceptual argument against large-scale branching; since
> all branches co-exist in the same space-time, and since the disposition
> of matter determines how space bends in general relativity, large-scale
> differences between the branches would leave space not "knowing" which
> way to bend.

I'm well aware of the problems of MWI.  But I think Roger is too pessimistic 
the potential of a theory of einselection.  There's an excellent review paper 
available on arXiv.org:

Decoherence, the measurement problem, and interpretations of quantum mechanics
Authors: Maximilian Schlosshauer
Comments: 41 pages. Final published version
Journal-ref: Rev. Mod. Phys. 76, 1267-1305 (2004)
DOI: 10.1103/RevModPhys.76.1267

     Environment-induced decoherence and superselection have been a subject of 
intensive research over the past two decades, yet their implications for the 
foundational problems of quantum mechanics, most notably the quantum 
problem, have remained a matter of great controversy. This paper is intended to 
clarify key features of the decoherence program, including its more recent 
and to investigate their application and consequences in the context of the 
interpretive approaches of quantum mechanics.

It discusses the sucesses and problems of the decoherence program and their 
to other intepretations.  I think you are wrong in saying the are plenty of 
explanations for collapse of the wave-function.  Penrose's is the only one that 
close to being an explanation, i.e. something with a physical basis that could 
tested.  The others are just hueristic models.  Decoherence is experimentally 
and provides "collapse FAPP"; but the "FAPP" still leaves some problems.

Personally, I like Omnes' viewpoint, which is that QM is a probabilistic 
theory, so 
it predicts probabilities and probability implies that some things happen and 
don't.  But that still leaves a problem in interpreting those (vanishingly 
small) off 
diagonal terms in the density matrix.

Brent Meeker

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