OOPS! I sent this reply only to Hal - instead of the list.
So here's the fwd to the list.

Brent Meeker

On 04-Sep-02, Hal Finney wrote:
> I think on this list we should be willing to seriously
> consider the many-worlds interpretation (MWI) of quantum
> mechanics as the ontology for our universe. In particular,
> we should not assume that wave function collapse is
> anything more than an illusion caused by decoherence of
> formerly interacting components of the universal wave
> function. Almost all of the supposed paradoxes of QM go
> away if you eliminate wave function collapse.

> There are a few objections which I am aware of which have
> been raised against the MWI. The first is its lack of
> parsimony in terms of creating a vast number of universes.
> We gain some simplification in the QM formalism but at
> this seemingly huge expense. The second is its
> untestability, although some people have claimed
> otherwise. And the third is that it retains what we might
> call the problem of measure, that is, explaining why we
> seem to occupy branches with a high measure or amplitude,
> without just adding that as an extra assumption.

I have always had two problems with the MWI. Initially it
was measurements that caused the splitting into different
universes, and that's apparently still the view of people
who propose tests like Plaga, but later it was realized
that there was no prinicpled way to distinguish
measurements from other interactions.  But then it seems
that the universe must split everytime a photon is emitted
by an atom anywhere in the visible universe.  Since that
atom could have interacted with some atom near us - all the
visible universe was in interaction before inflation - then
that split implies a split here & now.  But to what effect?
 MWI seems to commit us to an essentially infinite rate of
splitting; yet nobody uses it except in a QM measurement
analysis.  To take it seriously I would need to see why all
this infinite splitting can be ignored and whay I need only
pay attention to certain cases.  This seems very much like
Bohr's division into classical/quantum realms.

The second problem has to do with time and casuality. At a
microscopic level QM is time symmetric.  If we say there is
no real collapse of the wave function - all evolution is
unitary (and therefore reversible) - then it seems we
should from each datum or measurement result, compute the
past as well as the future.  All that can be known of the
past, what happened within our past light cone, is
determined by the present.  So does the universe split as
we go back in time?  If not, why not?  Are we stepping
outside physics and assuming that the experimenter, as an
agent, sets up initial, but not final, conditions and so
defines the arrow-of-time by his causal action as an agent
outside of physics?

Brent Meeker

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