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