To clarify what I meant by "past uncertainty" and tie it into this thread
title, "Which universe are we in", it is necessary to adopt a certain
view towards instantiations of consciousness.
If my mind, as a physical or computational system, is instantiated
in multiple places, whether multiple universes, multiple branches of a
many-worlds interpretation (MWI), or even multiple places and times in one
universe, the question is how that appears to me from the first-person
perspective. I am adopting a position that from my point of view, it
is indeterminate which of those instantiations I am now experiencing.
There is no "fact of the matter" as to which one is me, now.
The alternative is to say that although all of these instantiations
are in some sense indistinguishable, nevertheless the instances of
consciousness produced by these systems are all distinct. That is, for
each instance of consciousness, there is a single physical system which
creates that consciousness. All of the physical systems are similar, or
even locally identical, so that the consciousness instances produced are
all structurally the same. But nevertheless we would not say that there
is one consciousness which spans all the implementations; rather, there
are multiple consciousnesses which merely "look alike from the inside".
Anyway, that is the opposite of the view I am taking for the purposes
of this discussion. I am assuming the former position, that my present
consciousness is being instantiated widely throughout the multiverse
and I can with equal justification think that I am experiencing any
of those instantiations. My consciousness, in that sense, spans many
parts of the multiverse, and the question of "which universe am I in"
has no unique answer.
That is how we can reconcile the notion of a cleanly determined "third
person" past with an indeterminate "first person" past. Each universe
has a uniquely defined past history (we will stipulate), but since it
is ambiguous which universe my mentality is currently occupying, there
is also ambiguity about which past is mine.
We might note that in the MWI we also have a somewhat similar effect
with regard to the future. In the MWI, the universe is completely
deterministic. There is no randomness. Yet we are able to reconcile
a deterministic "third person" view with an indeterminate, random,
"first person" perspective. It is quite possible to have objective
determinism while having subjective indeterminism, as long as "identity"
is sufficiently slippery.
And note too that as we move into the future, although we face subjective
indeterminism, we will still have a consistent consensus reality.
The events of the quantum future are undefined for us, but we know that
we will all experience consistent versions of those events. In the same
way, subjective indeterminism in the past still allows for consistent
consensus reality. As we gain information about the past, we will all
learn consistent information.
The way this works, in both the future and the past, is that as we gain
this information, there is a subjective "split" and different versions
of ourselves will gain different sets of information. But each group
of observers which is communicating together will see a consistent view.
The future splitting is that which is described by the QM wave function,
the formation of a relative state. The past splitting is different
and relies on the speculative philosophical interpretation I described
above; as we learn about the past, mind instances which were formerly
indistinguishable become distinct.