From: Lee Corbin (lcorbin@tsoft.com)
Date: Sat May 10 2003 - 20:46:53 MDT
Mitchell wrote
> From: Behalf Of Mitchell Porter
> Sent: Monday, May 05, 2003 5:41 AM
> Subject: many worlds (was RE: Einstein)
>
> I think MWI is rather contrived, but you need to know a few
> technicalities to appreciate why. Naively, it might sound
> like this:
>
> 1) Quantum mechanics says that some things happen completely
> at random: one out of a set of possibilities occurs, for no
> reason.
>
> 2) The many-worlds interpretation says that every possibility
> is realized: whenever such a choice-point is reached, the
> universe 'splits', with one offspring for each possibility.
>
> This is MWI according to DeWitt,
This is also MWI as understood by Deutsch, and probably by
Tegmark, who wrote "When she is asked a question, makes a
snap decision and answers, quantum effects at the neuron
level in her brain lead to multiple outcomes, and from the
bird perspective, her single past branches into multiple
futures."
> and it has some problems which are widely recognized:
>
> a) Complementary observables. A quantum particle can't have
> definite position and definite momentum at the same time.
> When the split occurs, does the particle start out in each
> branch with a definite position, or with a definite momentum,
> or in some other state?
I don't know, but I suggest that you post your question to
the Fabric of Reality list, where Deutsch himself often answers
queries.
> b) Relativity. Is the whole universe copied at each split?
> If so, what reference frame are we using? Alternatively,
> one could suppose that the split propagates outwards at
> lightspeed, with the parent universe dynamically cleaving
> in superspace, but no-one has expressed this mathematically.
Yes, the split *does* propagate outward at the speed of
light. The visual image I use is that of a thin sheet
of plastic being pulled apart to form two thinner but
similar sheets.
I also believe that this is in accord with what David
Deutsch says. Frankly, when I was studying this intently
in 2000, I reread his book, "The Fabric of Reality", and
was most impressed. On my initial reading in 1997, there
were still a number of places in the book where I could
not agree. But now, I disagree with virtually nothing in
it. It consists of about 1000 carefully written paragraphs,
and I've studied that book harder than I have any other book
that does not have equations in it.
> c) Interference effects. Quantum effects such as the
> interference fringes which show up in the double-slit
> experiment require *recombination* of separated wavepackets
> (in that example, the recombination of wavepackets that
> have passed through the separate slits). If the universe
> splits before recombination is to occur, recombination would
> seem to be impossible, because the two wavepackets are now
> in separate universes. So either universes can recombine,
> or splits don't occur between measurements, both of which
> pose further problems.
>
> All this is well-known amongst MWI advocates, and so most
> of them prefer MWI according to Everett, which is a more
> subtle approach. Instead of splitting universes, Everett
> talks about "relative states" of subsystems of the universe.
It's a matter of terminology IMO. The idea of calling these
"separate universes" offends the positivistic sensibilities
of many people. And as Tegmark observed, the reluctance of
many people to admit larger universes into their ontology
has been apparent since 1610.
The remainder of your points are well taken, and I don't know
how lucid Deutsch and Tegmark would be in addressing them.
Lee
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