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 http://space.newscientist.com/article/mg19526223.700-parallel-universes-make-quantum-sense.html but I found a copy on Google Groups: http://groups.google.com/group/alt.kq.p/browse_thread/thread/9631b2e37ba5e7a2/fb3202c9c5b71228?lnk=st&q=%22new+scientist%22+deutsch+albrecht&rnum=1#fb3202c9c5b71228 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 --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to [EMAIL PROTECTED] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/everything-list?hl=en -~----------~----~----~----~------~----~------~--~---