Alastair Malcolm writes:
> For my own part, I give strong credibility (>50%) to the existence of many
> worlds in some guise or other, and in particular to the existence of all
> logically possible(*) worlds (alpw). For me the existence of one world
> (ours) so conveniently life-suited - sufficiently spatio-temporally extended
> and quiescent but with particular properties enabling wide diversity in
> chemistry etc - demands a specific explanation, and the only other candidate
> final explanation - a Creator (say a God, or a 'higher' civilisation) -
> suffers (at least) the problem of requiring an explanation for *it*.

That's a great question.  I agree that assuming that this is the only
world is quite problematic.  On the other hand it does not necessarily
follow that all possible or conceivable worlds exist.  From hearing
some physicists speak, I get the impression that they are being "dragged
kicking and screaming" towards many worlds and anthropic ideas, but are
resisting.  They still hope to come up with some kind of mathematical or
philosophical reason to at least restrict the number of possible worlds.

At a minimum they are looking for dependencies among many superficially
independent aspects of the observable universe.  In fact, you could
describe that as the fundamental goal of physics.  They might accept
that certain physical constants have a certain accidental or contingent
aspect, that there is no fundamental reason why they have those values;
but they want to minimize the number of constants for which this is
true, and find ways to show that other constants and properties depend
on these few arbitrary ones.

I also think that AUH (all universe hypothesis) admits too many
alternative formulations which may not all be consistent.  That would seem
to force the metaverse to choose between, say, Schmidhuber and Tegmark.
Yet how can that be?  It doesn't seem to make sense that there are two
inconsisent ways that all universes can exist.  To me that suggests a
weakness in our understanding which further study will improve upon.
But it means that we can't claim to understand the AUH or to really know
what it would mean for all universes to exist.

As far as the MWI of QM, my understanding is that advanced theoretical
physicists believe QM will be shown to be false(!).  It is expected to
be merely an approximation to some deeper theory which will also explain
general relativity.  If all we had was QM, I think the MWI would very
likely be true.  However, given that QM will be replaced by string theory
or loop quantum gravity or some other model, I don't know enough about
those to say whether the MWI will still be the simplest explanation.

All in all I'd say that I see too much confusion and uncertainty to hold
to any position regarding the existence of multiple universes.

Hal Finney

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