Patrick Leahy writes:
> 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?

First, welcome to the list.

You are right that in the strict MWI, if we define an observer-moment
to be restricted to one branch, then observer moments do not merge.

I might mention that there is some disagreement among aficionados of
the MWI as to what constitutes a branch.  Some reserve the concept of a
unique branch, and branch splitting, to an irreversible measurement-like
interaction, as you are doing.  Others say that even reversible operations
create new branches, in which sense it is OK to say that branches can
merge.  David Deutsch does this, for example, when he says that quantum
computers use the resources of many branches of the MWI (and hence prove
the reality of the MWI!).

However, particularly as we look to larger ensembles than just the MWI,
it becomes attractive to define observers and observer-moments based
solely on their internal information.  If we think of an observer as
being a particular kind of machine, then if we have two identical such
machines with identical states, they represent the same observer-moment.

>From the first-person perspective of that observer-moment, there is no
"fact of the matter" as to which of the infinite number of possible
implementations and instantiations of that observer moment is the real
one.  They are all equally real.  From the inside view, the outside is
a blur of all of the possibilities.

If we apply that concept to the MWI, then we retrieve the concept of an
observer-moment that spans multiple branches.  As long as the information
state of the OM is consistent between the various branches, there is
no fact of the matter as to which branch it is really in.  That is the
sense in which we can say that observers merge and that observer moments
have multiple pasts.

Hal Finney

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