On Wednesday, September 4, 2002, at 10:08  AM, 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.

I remain agnostic on the MWI or EWG interpretation. While I don't 
strongly believe that the MWI is "reality" (cough cough), I agree with 
Hal that it's a plausible ontology. Further, I take more seriously than 
many the "plurality of worlds" ontology of the late philosopher David 
Lewis. (The guy who argues that we should not give special linguistic 
treatment to "our" world and should give equal standing to "the world 
in which World War II was won by Germany," for example. Lewis is 
sometimes caricaturized by capsule summaries of the sort "David Lewis 
believes unicorns really do exist," but what Lewis is claiming is fully 
consistent with modal logic and possible worlds semantics.)

> 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.

The latter is the more important. If, for example, the plurality of 
worlds are out of communication with each other, forever and always, 
then it means nothing to assert that they "actually" exist.

In weaker forms of the MWI, where it's the early state of the Big Bang 
(for example) which are splitting off into N universes, De Witt and 
others have speculated (as early as around 1970) that we may _possibly_ 
see some evidence consistent with the EWG interpretation but NOT 
consistent with other interpretations.

> 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.

What's the problem here? I find it utterly plausible that we would be 
in a universe where matter exists, where stars exist, where entropy 
gradients exist, etc., and NOT in a universe where the physical 
constants or structure of the universe makes life more difficult or 
impossible (or where the densities and entropy gradients mean that 
evolution of complex structures might take 100 billion years, or more, 
instead of the billion or so years it apparently took).

> The point is, all of these objections apply equally to the more
> ambitious multiverse models we consider here.  Our multiverse is even
> more profligate than the MWI; it is if anything less observable; and
> the problem of measure is at least as acute.

I certainly agree with this! Tegmark's and Schmidhuber's and Egan's 
"all mathematics, all programs" models form supersets of the 
conventional MWI.

>  By the metrics we typically use for
> universe complexity, basically the number of axioms or the size of a
> program to specify the universe, the MWI is in fact simpler and 
> therefore
> more probable than the traditional interpretation.

This I'm not convinced of at all. I don't find the Copenhagen (aka 
"Shut up and calculate") Interpretation requires any more axioms. So 
long as we don't try to understand what is "really" happening, it's a 
very simple system.

> Quantum randomness does not exist in the MWI.  It is an illusion 
> caused by
> the same effect which Bruno Marchal describes in his thought 
> experiments,
> where an observer who is about to enter a duplication device has 
> multiple
> possible futures, which he treats as random.  If Schmidhuber would 
> adopt
> this model for the physics of our universe it would improve the quality
> of his predictions.

And, putting in a plug for modal/topos logic, the essence of nearly 
every interpretation, whether MWI or Copenhagen or even Newtonian, is 
that observers at time t are faced with unknowable and branching 
futures. (In classical systems, these arise from limited amounts of 
information available to observers and, importantly, in limited 
positional information. Even a perfectly classical billiard ball 
example is unpredictable beyond a few seconds or maybe tens of seconds, 
because the positions and sets of forces (turbulence in the air 
currents around the balls, even gravitational and static electricity 
effects, etc.) are only known to, say, 20 decimal places (if that, of 
course). Because the "actual" positions, masses, sphericities, static 
charges, etc.  are perhaps defined by 40-digit or even 200-digit 
numbers, the Laplacian dream of a suffiicently powerful mind being able 
to know the future is dashed.

Unpredictability, or randomness, arises even in a fully classical real 

--Tim May
"As my father told me long ago, the objective is not to convince someone
  with your arguments but to provide the arguments with which he later
  convinces himself." -- David Friedman

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