RE: Sociological approach

From: Brent Meeker <meekerdb.domain.name.hidden>
Date: Mon, 23 May 2005 22:02:49 -0000

>-----Original Message-----
>From: Patrick Leahy [mailto:jpl.domain.name.hidden]
>Sent: Monday, May 23, 2005 6:50 PM
>To: EverythingList
>Subject: Re: Sociological approach
>
>
>
>QM is a well-defined theory. Like any theory it could be proved wrong by
>future experiments. My point is that R. Miller's suggestions would
>definitely constitute a replacement of QM by something different. So would
>aet.radal's (?) suggestion of information tunnelling between macroscopic
>branches. The crucial point, which is not taught in introductory QM
>classes, is the theory of Quantum decoherence, for which see the wikipedia
>article and associated references (e.g. the Zurek quant-ph/0306072).
>
>This shows that according to QM, the decay time for quantum decoherence is
>astonishingly fast if the product ((position shift)^2 * mass *
>temperature) is much bigger than the order of a single atom at room
>temperature. Moreover, the theory has been confirmed experimentally in
>some cases.
>
>Since coherence decays exponentially, after say 100 decay times there is
>essentially no chance of observing interference phenomena, which is the
>*only* way we can demonstrate the existence of other branches. "No chance"
>meaning not once in the history of the universe to date.
>
>No existing animal is small enough or cold enough to participate directly
>in quantum interference effects (i.e. to perceptibly inhabit different
>micro-branches simultaneously), hence my claim that your "behaviour
>system", whatever it is, must be in the fully-decohered regime.
>
>I have to backpedal some though, because by definition an intelligent
>quantum computer would be in this regime (in practice, by being very
>cold). I certainly don't want to imply that this goal is known to be
>impossible.
>
>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.

Brent Meeker
Received on Tue May 24 2005 - 02:00:10 PDT