>-----Original Message-----
>From: Patrick Leahy [mailto:[EMAIL PROTECTED]
>Sent: Monday, May 23, 2005 8:14 PM
>To: Hal Finney
>Cc: everything-list@eskimo.com
>Subject: Re: Decoherence and MWI
>On Mon, 23 May 2005, Hal Finney wrote:
>> I'd like to take advantage of having a bona fide physicist on the list to
>> ask a question about decoherence and its implications for the MWI.
>> If this is true, then how can a physicist not accept the MWI?
>Beats me...
>> Isn't that just a matter of taking this decoherence phenomenon to a
>> (much) larger degree?  Either you have to believe that at some point
>> decoherence stops following the rules of QM, or you have to believe that
>> the mathematics describes physical reality.  And the mathematical
>> equations predict the theoretical existence of the parallel yet
>> unobservable branches.
>The physicists who I really respect, but who do not support MWI, do indeed
>believe (or hope) that the rules of QM break down at some point. E.g.
>Roger Penrose, or Tony Leggett. They would point out that to date every
>theory we have has only been an approximation. Penrose would point out
>additionally that QM is inconsistent with GR, and there is no reason to
>suppose that only GR has to be modified.  I support these people to the
>extent that I think it is tremendously important to keep doing
>experiments, especially ones that can test well-formulated alternatives to
>QM.  I'm just not very hopeful that any discrepancy will show up.
>> Of course, given that they are in practice unobservable, a degree of
>> agnosticism is perhaps justifiable for the working physicist.  He doesn't
>> have to trouble himself with such difficult questions, in practice.
>> But still, if he believes the theory, and he applies it in his day to
>> day work, shouldn't he believe the implications of the theory?
>For most physicists the Copenhagen interpretation (in some half-understood
>way) works perfectly well at the lab bench.
>There are also those who have thought very carefully about the issue and
>have come to a hyper-sophisticated philosophical position which allows
>them to fudge. I'm thinking particularly of the consistent-histories gang,
>including Murray Gell-Mann. I particularly liked Roland Omnes' version of
>this: "quantum mechanics can account for everything except actual facts".
>He thinks this is a *good* thing!

I too like Omnes' point.  If QM is a probabilitistic theory, then it must
predict probabilities - not actualities.

>> To me, it almost requires believing a contradiction to expect that
>> decoherence experiments will follow the predictions of QM, without also
>> expecting that the more extreme versions of those predictions will be
>> true as well, which would imply the reality of the MWI.  You either have
>> to believe that a sufficiently accurate decoherence experiment would
>> find a violation of QM, or you have to believe in the MWI.
>> Don't you?
>Paddy Leahy
>PS: this is an endorsement of the MWI of QM, not of any "everything"

There are a couple of papers on arXiv, gr-qc/0505052 and gr-qc/0505023, by
Gambini and Pullin discussing discrete models of spacetime.  Most physicist
think that, at the Planck scale, the continuum model of spactime is at best
inelegant and at worst simply wrong.  Gambini and Pullin show that in their
discrete spacetime model decoherence is inherent - so something really does
happen.  This implies a fundamental arrow-of-time.

Brent Meeker

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