Monday, September 24, 2007

Well Versed

I have been delving into David Deutsch’s work on Parallel universes. Go here, to get some insight into Deutsch’s recent papers on the subject. I’m not familiar enough with the formalism of quantum computation to be able to follow Deutsch’s papers without a lot more study. However, some salient points arise. In this paper dated April 2001 and entitled “The Structure of the Multiverse” Deutsch says:
“.. the Hilbert space structure of quantum states provides an infinity of ways of slicing up the multiverse into ‘universes’ each way corresponding to a choice of basis. This is reminiscent of the infinity of ways in which one can slice (‘foliate’) a spacetime into spacelike hypersurfaces in the general theory of relativity. Given such a foliation, the theory partitions physical quantities into those ‘within’ each of the hypersurfaces and those that relate hypersurfaces to each other. In this paper I shall sketch a somewhat analogous theory for a model of the multiverse”
That is, as far as I understand it, Deutsch is following the procedure I mentioned in my last blog entry – he envisages the relationships of the multiple universes similar to the way in which we envisage the relationships of past, present and future.

On the subject of non-locality Deutsch in
this paper on quantum entanglement, states:
“All information in quantum systems is, notwithstanding Bell’s theorem, localized. Measuring or otherwise interacting with a quantum system S has no effect on distant systems from which S is dynamically isolated, even if they are entangled with S. Using the Heisenberg picture to analyse quantum information processing makes this locality explicit, and reveals that under some circumstances (in particular, in Einstein-Podolski-Rosen experiments and in quantum teleportation) quantum information is transmitted through ‘classical’ (i.e. decoherent) channels.”
Deutsch is attacking the non-local interpretation of certain quantum experiments. In
this paper David Wallace defends Deutsch and indicates the controversy surrounding Deutsch’s position and its dependence on the multiverse contention. In the abstract we read:

It is argued that Deutsch’s proof must be understood in the explicit context of the Everett interpretation, and that in this context it essentially succeeds. Some comments are made about the criticism of Deutsch’s proof by Barnum, Caves, Finkelstein, Fuchs and Schack; it is argued that the flaw they point out in the proof does not apply if the Everett interpretation is assumed. "

And Wallace goes on to say:

“…it is rather surprising how little attention his (Deutsch’s) work has received in the foundational community, though one reason may be that it is very unclear from his paper that the Everett interpretation is assumed from the start. If it is tacitly assumed that his work refers instead to some orthodox collapse theory, then it is easy to see that the proof is suspect… Their attack on Deutsch’s paper seems to have been influential in the community; however, it is at best questionable whether or not it is valid when Everettian assumptions are made explicit.”

The Everett interpretation equates to the multiverse view of quantum mechanics. Deutsch’s interpretation of QM is contentious. It seems that theorists are between a rock and a hard place: on the one hand is non-locality and absolute randomness and on the other is an extravagant ontology of a universe bifurcating everywhere and at all times. It is perhaps NOT surprising that Deutsch’s paper received little attention. Theoretical Physics is starting to give theorists that “stick in the gullet” feel and that’s even without mentioning String Theory!

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