>-----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