>-----Original Message-----
>From: Patrick Leahy [mailto:[EMAIL PROTECTED]
>Sent: Tuesday, May 24, 2005 9:46 AM
>To: Brent Meeker
>Cc: Everything-List
>Subject: RE: Sociological approach
>
>
>
>On Mon, 23 May 2005, Brent Meeker wrote:
>
>>> -----Original Message-----
>>> From: Patrick Leahy [mailto:[EMAIL PROTECTED]
>
><SNIP>
>>> NB: I'm in some terminological difficulty because I personally *define*
>>> different branches of the wave function by the property of being fully
>>> decoherent. Hence reference to "micro-branches" or "micro-histories" for
>>> cases where you *can* get interference.
>>>
>>> Paddy Leahy
>>
>> But in QM different branches are never "fully decoherent".  The off
>axis terms
>> of the density matrix go asymptotically to zero - but they're never exactly
>> zero.  At least that's standard QM.  However, I wonder if there isn't some
>> cutoff of probabilities such that below some value they are necessarily,
>> exactly zero.  This might be related to the Bekenstein bound and the
>> holographic principle which at least limits the *accessible* information in
>> some systems.
>
>I'm talking about standard QM. You are right that my definition of
>macroscopic branches is therefore slightly fuzzy. But then the definition
>of any macroscopic object is slightly fuzzy. I don't see any need for a
>cutoff probability... the probabilities get so low that they are zero FAPP
>(for all practical purposes) pretty fast, where, to repeat, you can take
>FAPP zero as meaning an expectation of less than once per age of the
>universe.

There's no difference FAPP, but it seems to me there's a philosophical
difference in intepretation.  If there's a probability cutoff then QM can be
regarded as a theory that just predicts the probability of what actually
happens (per Omnes).  Without a cutoff nothing ever actually a happens, i.e.
whatever seems to happen could be quantum erased, and we have the MWI.

Brent Meeker

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