Le 24-sept.-07, à 18:39, Hal Finney wrote (in part)

> We see the same thing happening all over again in string theory. I
> don't know if you guys are following this at all. String theory is
> going through a crisis as it has turned out in the past few years that
> it does not predict a single universe, rather a multiverse where there
> is a "landscape" of possible sets of parameters, each of which would
> correspond to a universe.

I think you are confusing two levels. String theory is based on quantum 
mechanics and presuppose a multiverse.
The landscape concerns the "10^500" different string theories with 
possible different set of parameters; but each theory corresponds 
already to a multiverse.
But I would no deduce from this that string theory leads us toward a 
multi-multiverse, given there are no reasons (as far as I know) why 
anything like interference between the multiverses would occur.



> The big problem is that there is no natural
> or accepted measure (unlike with QM where everyone knew all along that
> the measure had to be the Born rule and it was just a matter of how
> many hoops you had to jump through to pull it out of your model).  As a
> result it looks like it might be impossible to get even probabilistic
> predictions out of the string theory landscape.

In each of them, there is. The problem is more that we don't know which 
string theory is correct, and given that there are many of them, this 
can be seen as a big critics. Another problem is that the theory is 
background dependent, and the geometry seems to fall from the sky.


>
> AFAIK no one within the community has followed our path and looked
> at algorithmic complexity as a source of measure (i.e. the Universal
> Distribution, which says that the simplest theories have higher 
> measure).
> Granted, even if that direction were pursued it would probably be
> computationally intractable so they still would not be able to pull 
> much
> out in the way of predictions.

Yes. But not if you postulate comp explicitly (unlike taking comp just 
to borrow kolmogorov complexity theory). Once you take comp, it looks 
worst, at first sight, because although simplest program keep a big 
role, we cannot avoid the role of big programs. But then we can still 
use the intrisically computationalist constraints to derive at least 
the "logic of observable "certainty"" (the case: probability one), and 
this can be compared to emirical data. In particular, with comp, we 
know that classical physics is false. QM, up to now, confirms comp.


> Neverthless physicists are skilled at the
> use of approximation and assumptions to get plausible predictions out 
> of
> even rather opaque mathematics so it's possible they might get 
> somewhere.
>
> But at this point it looks like the resistance is too strong. Rather
> than string theory making the multiverse respectable as we might hope,
> it seems likely that the multiverse will kill string theory.


Well, Witten himself is aware that the "wave" aspect of superstring 
theory is in need of conceptual foundations. But String theories accept 
multiverse (most of the time implicitly by using QM without collapse). 
I think the problem in String Theory are more internal to string 
theory, than in its relation with the interpretation of QM.

Bruno



http://iridia.ulb.ac.be/~marchal/


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