Yes, you are quite right, and I put that badly. I was thinking of Omnes'
version of the consistent histories interpretation in which the histories
have purely probabilistic status (i.e. one is realized and others are not).
However, I'm in the process of reading a paper by Dowker and Kent,
gr-qc/9412067, which claims to show that these theories that purport to
derive the classical world from the quantum are all unsatisfactory.
One particular form of non-linearity would arise if there were a lower
bound on non-zero values of terms in the density matrix. In that case
cross terms would become strictly zero and the theory could be given a
simple probabilistic interpretation. I've only seen one paper discussing
this possibility and it had to assume a closed universe.
Brent
Jan Harms wrote:
> Hello Brent,
>
> perhaps one remark concerning your understanding of "einselection".
> Please correct me if I am wrong here, but einselection - which is as you
> say related to decoherence - is not selecting one state out of a
> superposition of states, but selecting a certain basis in a Hilbert
> space. The corresponding basis states are usually what we experience as
> classical states. So it serves as an explanation why the world appears
> as it is. However, this does stay in conflict with Everett's
> relative-state multiverse theory. Instead, einselection is something
> heavily required to make multiverse theories a serious candidate of QM
> foundation. The only thing which may eventually undermine the beautiful
> QM multiverse theory is, that someone discovers a nonlinear effect
> inherent to the Schrödinger equation, which hasn't been discovered yet,
> because it acts on an incredible small timescale.
>
> Jan
>
> Brent Meeker schrieb:
>> Ronald Held wrote:
>>
>>> I am giving a talk on the Multiverse to Star Trek fans in several
>>> weeks. I would appreciate any advice and suggestions, since as of now,
>>> I have an outline based on Tegmark's four levels.
>>>
>>>
>> One thing I would avoid is presenting the multiverse (of any level) as the
>> latest "gee-whiz, science has discovered that..." It is interesting
>> speculative metaphysics. Good fodder for SciFi fans but not yet science. I
>> cringe when Scientific American or the L.A. Times or some other popular
>> publication takes scientific speculation and hypes it as though it were a
>> new revelation of science. It happens most often in medical stories, but
>> also in physics and astronomy pieces. In the end I think it debases
>> science in the popular mind as just more advertising hype and spin noise.
>>
>> ISTM there were several independent threads that lead to different ideas of
>> the multiverse:
>>
>> 1) The apparent inherent randomness of QM inconsistent with "hidden
>> variables" which made the "collapse of the wave function" mysterious and ad
>> hoc. The led Everett to argue for a relative-state interpretation which
>> implied the QM-multiverse. However, theories of einselection based on
>> decoherence and perhaps weak anthropic selection may undermine the
>> QM-multiverse. In this theory there is really only one universe but each
>> of us consists of multiple branches on which different values are projected.
>>
>> 2) Completely independent of (1), development of theories of cosmogony
>> based on quasi-classical quantum gravity showed the universe could arise
>> from "nothing". These theories naturally have the consequence that
>> arbitrarily many other universes could also have arisen. Any natural
>> process is repeatable. This theory of multiverses allows that universes
>> exist with different values of those physical constants that seem arbitrary
>> in our current theories (a set that could change). These universes all
>> exist in the same sense this universe exists, they may even have common
>> points (e.g. in singularities). Again some anthropic selection principle
>> must be invoke to explain *this* universe.
>>
>> 3) The extremely abstract nature of physics has led naturally to the
>> speculation that only the mathematics matters. We seem to know nothing
>> about elementary particles and the interior of black holes beyond the
>> mathematics they satisfy. So perhaps it is only the relational properties
>> of information coded into the mathematics that is the ur-stuff of the
>> world. In that case all possible information structures may be considered
>> equally "existent". This also fits with the idea that reality can be
>> simulated. If reality consists in the relation of information, it doesn't
>> matter how that information is embodied or maybe that it is embodied at
>> all. Therefore reality may *be* a simulation - as on the holodeck of the
>> Enterprise.
>>
>> 4) Finally, what seems to have motivated Tegmark and also perhaps Bruno and
>> others is Wheeler's question, "What makes them fly?" Why is one set
>> equations instantiated in the world and others are not? Why is there some
>> special property of "existing" that some possibilities have and others
>> don't. Tegmark sidesteps the question by answering that they are all
>> instantiated; so "to exist" is simply to be the subject of propositions
>> that form a non-self contradictory set. Starting from this he then must
>> try to recover some explanatory power by limiting what we experience by
>> appealing to some anthropic principle.
>>
>> Brent Meeker
>>
>>
>
>
> >
>
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Received on Mon Jul 07 2008 - 20:24:11 PDT