New Scientist has an article on parallel universes:

> David Deutsch at the University of Oxford and colleagues have shown
> that key equations of quantum mechanics arise from the mathematics of
> parallel universes. "This work will go down as one of the most important
> developments in the history of science," says Andy Albrecht, a physicist
> at the University of California at Davis. In one parallel universe,
> at least, it will - whether it does in our one remains to be seen.

It is behind a paywall at
but I found a copy on Google Groups:

It has a great quote from Tegmark: "The critique of many worlds is
shifting from 'it makes no sense and I hate it' to simply 'I hate it'."

The thrust of the article is about recent work to fix the two perceived
problems in the MWI: non-uniqueness of basis (the universe splits in all
different ways) and recovering the Born rule. The basis problem is now
considered (by supporters) to be resolved via improved understanding
of decoherence. This work (which was not particularly focused on the
MWI) generally seems to lead to a unique basis for measurement-like
interactions, hence there is no ambiguity in terms of which way the
universe splits.

As for the Born rule, the article points to the effort begun by Deutsch in
1999 to base things on decision theory. The idea is that we fundamentally
care about probability insofar as it influences the decisions and choices
we make, so if we can recover a sensible decision theory in the MWI, we
have basically explained probability. I've seen a number of critiques of
Deutsch's paper but according to this article, subsequent work by David
Wallace and Simon Saunders has extended it to the point where things
are pretty solid.

Hence the two traditional objections to the MWI are now at least arguably
dealt with, and given its advantage in terms of formal simplicity (fewer
axioms), supporters argue that it should be considered the leading
model for QM. This is where we get claims about it being among the most
important discoveries in the history of mankind, etc.

It's interesting to see the resistance of the physics community to
multiverse concepts. It all comes back to the tradition of experimental
verification I suppose, which is still pretty much impossible. Really
it is more a question of philosophy than of physics as we currently
understand these disciplines.

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. 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.

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. 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.

Hal Finney

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