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