Many Pasts? Not according to QM...

From: Patrick Leahy <jpl.domain.name.hidden>
Date: Wed, 18 May 2005 16:57:37 +0100 (BST)

I've recently been reading the archive of this group with great interest
and noted a lot of interesting ideas. I'd like to kick off my contribution
to the group with a response to a comment made in numerous posts that a
single observer-moment can have multiple pasts, including macroscopically
distinct pasts, e.g. in one memorable example, pasts which differ only
according to whether a single speck of dust was or was not on a
confederate soldier's boot in 1863.

Does anybody believe that this is consistent with the many-worlds
interpretation of QM? If so, please think again! Even such an apparently
minor change is sufficient to split the universal wave function into two
distinct branches (i.e. branches peaking in vastly-separated regions of
configuration space), which can recombine with probability effectively
zero. The reason for this is "decoherence" in the technical sense used by
Zurek and others.

To counter one obvious rejoinder, I'm not denying that micro-histories can
recombine, as in the two-slit experiment. Rather, decoherence ensures that
states with macroscopic (or even mesoscopic) entropy spread their
information so effectively that it is practically impossible to erase it
("practically" in the sense that even the entire resources of the universe
would be insufficient, as emphasised by Omnes).

Of course, many of you (maybe all) may be defining pasts from an
information-theoretic point of view, i.e. by identifying all
observer-moments in the multiverse which are equivalent as perceived by
the observer; in which case the above point is quite irrelevant. (But you
still have to distinguish the different branches to find the total measure
for each OM).

======================================================
Dr J. P. Leahy, University of Manchester,
Jodrell Bank Observatory, School of Physics & Astronomy,
Macclesfield, Cheshire SK11 9DL, UK
Tel - +44 1477 572636, Fax - +44 1477 571618
Received on Wed May 18 2005 - 12:25:21 PDT