Hi Patrick,

Let me also welcome you to the list.
I agree with Hal that there are several schools of thoughts regarding many pasts. I believe that a crucial ingredient in accepting the many past concept is the concept of indiscernibles by Leibniz. If two objects are indiscernible then they are one and the same object. Similarly, if two OMs are indiscernible then they are one and the same. Thus if two past OM's lead to two identical present OM's then these two present OM's are one and the same OM. Hence this present OM has two pasts.

Following this reasoning, OM's in the past present and future are a huge network rather than on a huge branching tree.

Note that the whole issue hinges very much on a perception of indiscernibles. The story becomes more complicated if we ask the question whether the concept of indiscernible is first person or third person. I am of the opinion that third person is essentially only an illusion caused by the sharing of almost identical frames of references, and that first person perspective is the only perspective that matters.

Assuming first person, we see that not only we have different pasts and futures but that each one of us has different pasts, presents and futures. In this merging and splitting network, some of us may reach identical OM's. When we do we become the same person for a short "time." Soon after we split again.


Hal Finney wrote:
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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