Brent Meeker wrote:
> Stathis Papaioannou wrote:
> > Peter Jones writes:
> >
> >
> >>By youur definitions, it's a straight choice between metaphysics and
> >>solipsism.
> >>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 
> function,
> which was inexplicable and ad hoc.

It's neither. If anything there is an embarassment of explanations for
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.

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