On 04-Sep-02, Hal Finney wrote:
> Brent - FYI you sent your comment just to me again. I
> don't know if you intended to send it to the list or not.
> But I will reply just to you based on how you sent it.

Sorry, Hal - my mistake.

> You wrote:
>> 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.

> I would make two comments about this. The first, following
> the lines of my earlier message, is that the same
> consideration applies to the multiverse models we have
> discussed on the everything list. We have all possible
> universes existing, which means that all possible
> alternatives are explored. Whether you think of it as one
> universe "splitting" or multiple formerly-identical
> universes becoming distinct, the process is the same. And
> you have the same problem, that conceptually the smallest
> change anywhere in the universe now makes there be two
> different universes where formerly there were one. So if
> you don't like the MWI, you ought to object to more
> general multiverse models (and maybe you do!).

Yes, I think I understand this similarity between
all-universes-are-computed and the MWI of QM.

> But second, I don't think this objection is as bad as it
> sounds. As is perhaps made more clear in the multiverse
> example, this "splitting" is a somewhat figurative or
> subjective phenomon. It's not so much that the universe
> splits; there is only one universe, in the largest sense.
> We use the word multiverse to describe this grand
> ensemble, but in its traditional meaning of "everything
> there is" it is fair to call it the universe. In the MWI,
> there is only one universe, which evolves
> deterministically by following the Schrodinger equation.
> That's all there is to the physics.

OK, except don't the universes interfere to produce
(approximate) diagonalization of the density matrix?  So
that is why we observe classical physics at the macroscopic
level.

> The splitting comes in when we study the wave function of
> the universe and find that it can be decomposed into
> pieces which add together to form the whole, and where the
> time evolution of each separate piece is essentially
> independent of the others. And then, each of these pieces
> can further be interpreted as being themselves made of
> separate pieces which become causally independent as time
> moves forward. So it's not that the universe splits, it's
> that the global wave function can be interpreted as having
> causally independent parts.

The decomposition though is only an approximation holding
for macroscopic observables that average over many
"universes" or in other words, projects out onto a subspace
of the universes Hilbert space. So the separation and
independence have to do with our macroscopic perspective
and the fact that our knowledge (or all knowledge) must be
encoded at a macroscopic (irreversible) level.

>> 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?

> That's a good question and one I don't really know the
> answer to. It's possible that you're right, it has
> something to do with initial conditions. Or it's possible
> that universes are merging at the micro-scale even as they
> are splitting at the macro-scale, which would also be
> fundamentally due to initial conditions. For example,
> dual-slit diffraction can be seen as worlds where
> particles go through the slits separately merging to show
> the interference.

My concern goes deeper than that. I think that a TOE should
explain time, not assume it.  But MWI universe splitting or
evolution of the wave equation of the universe seems to
assume time.

Brent Meeker
There is a theory which states that if ever anybody
discovers exactly what the Universe is for and why it is
here, it will instantly disappear and be replaced by
something even more bizarre and inexplicable. There is
another theory which states that this has already happened.
            -- Douglas Adams (Hitchhiker's Guide to the
Galaxy)

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