From: braham@physics.ubc.ca (Stephen Braham)
Subject: Re: Quantum Cosmological Boundaries and Determinisim?
Keywords: Quantum cosmology
Organization: University of British Columbia, Vancouver, B.C., Canada
Date: Wed, 22 Jul 1992 01:56:41 GMT

jtwamley@physics.adelaide.edu.au (Jason Twamley) writes:

Greetings Jase :),

>Question:
>Assume the the while universe can be described by a quantum theory.ie. there
>exists a wave function for the universe. There have been a number of proposals
>for the quantum boundary conditions for this wavefunction at the begining of
>the universe.

My first remark would be that modern quantum cosmology doesn't use
a wavefunction, or even a density matrix. The alternative decohered
histories approach does not have an intermediate state in general (at
best a density matrix, which has many superselection sectors, and
thus describes, roughly, an ensemble of states). A single quantum
state, at best, has old-fashioned decoherence (loss of diagonal
terms in the Schroedinger picture density picture rather than the
Heisenberg picture decoherence functional) and thus reality is
a tad fuzzy.

If one of these boundary conditions succeed in specifiying
>a unique wavefunction what does this say concerning the uncertainty in the
>evolution of the everyday classical world and free will? Frequently the 
>wavefunction of the universe is called on to calculate the characteristics
>of large structures ie. inflation, density perturbations etc. However,
>in principle, how far can the information content of this wavefunction reach?
>Can it (is it suposed to?) describe the milk mixing in my tea? The standard
>probabalistic interpretation of QM is not clear here as we are attempting to
>describe the one universe by a single wavefunction and attempting to make
>predictions. Or is the Many-world description more appropriate?

Again, in generalized quantum mechanics, this has little meaning.
ADH, of course, supersedes many-worlds, but many worlds comes close
to the paradigm (As Jim Hartle likes to say, though, it's really
a many _histories_ interpretation). A single state probably lacks
the information for a realistic quantum cosmology.

>To put my
>question another way.. do the correlations contained in the quantum 
>cosmological boundary conditions contain all the information necessary to
>describe the complete future of the `classical' world deterministically?

<grin>, that's a hard one. That's the entire quasiclassical domain
problem, of course. You basically have to come up with a definition
of _classical_world_ first to answer that one. Do that, and we
can finally make generalized quantum mechanics into a real internal
QM interpretation scheme. In general, you _expect_ many quasiclassical
domains and thus many different histories for the universe. If we
had a SINGLE domain for our (generalized) quantum state, you could
almost say that we have a unique classical world. It's a little more
complicated that that, though, because further finegraining would
give you incompatible histories, and thus there's still a certain
quantum fuzziness in the system.

>Jason Twamley

				Steve Braham
--
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