Re: Decoherence and MWI
Le 23-mai-05, à 22:13, Patrick Leahy a écrit : There are also those who have thought very carefully about the issue and have come to a hyper-sophisticated philosophical position which allows them to fudge. I'm thinking particularly of the consistent-histories gang, including Murray Gell-Mann. I particularly liked Roland Omnes' version of this: quantum mechanics can account for everything except actual facts. He thinks this is a *good* thing! I don't think it is a good thing to abandon trying to answer questions. It is the don't ask imperative. Actually I do believe Everett (and Finkelstein, Paulette Fevrier, Graham, Hartle, and many others) are on the track of succeeding to explain, well, not the actual fact themselves, but the correct belief in actual-factness. Note that Omnes justifies some of his views (in particular on the uniqueness of the universe, or of the outcome of experiments), by invoking explicitly the abandon of the cartesian program, and accepting some form of irrationalism. Bruno http://iridia.ulb.ac.be/~marchal/
Re: Decoherence and MWI
On Mon, 23 May 2005, Hal Finney wrote: I'd like to take advantage of having a bona fide physicist on the list to ask a question about decoherence and its implications for the MWI. SNIP If this is true, then how can a physicist not accept the MWI? Beats me... Isn't that just a matter of taking this decoherence phenomenon to a (much) larger degree? Either you have to believe that at some point decoherence stops following the rules of QM, or you have to believe that the mathematics describes physical reality. And the mathematical equations predict the theoretical existence of the parallel yet unobservable branches. The physicists who I really respect, but who do not support MWI, do indeed believe (or hope) that the rules of QM break down at some point. E.g. Roger Penrose, or Tony Leggett. They would point out that to date every theory we have has only been an approximation. Penrose would point out additionally that QM is inconsistent with GR, and there is no reason to suppose that only GR has to be modified. I support these people to the extent that I think it is tremendously important to keep doing experiments, especially ones that can test well-formulated alternatives to QM. I'm just not very hopeful that any discrepancy will show up. Of course, given that they are in practice unobservable, a degree of agnosticism is perhaps justifiable for the working physicist. He doesn't have to trouble himself with such difficult questions, in practice. But still, if he believes the theory, and he applies it in his day to day work, shouldn't he believe the implications of the theory? For most physicists the Copenhagen interpretation (in some half-understood way) works perfectly well at the lab bench. There are also those who have thought very carefully about the issue and have come to a hyper-sophisticated philosophical position which allows them to fudge. I'm thinking particularly of the consistent-histories gang, including Murray Gell-Mann. I particularly liked Roland Omnes' version of this: quantum mechanics can account for everything except actual facts. He thinks this is a *good* thing! To me, it almost requires believing a contradiction to expect that decoherence experiments will follow the predictions of QM, without also expecting that the more extreme versions of those predictions will be true as well, which would imply the reality of the MWI. You either have to believe that a sufficiently accurate decoherence experiment would find a violation of QM, or you have to believe in the MWI. Don't you? Yes. Paddy Leahy PS: this is an endorsement of the MWI of QM, not of any everything theory.
Re: Decoherence and MWI
On Mon, May 23, 2005 at 09:13:49PM +0100, Patrick Leahy wrote: For most physicists the Copenhagen interpretation (in some half-understood way) works perfectly well at the lab bench. Having been such a physicist at some point in my past, I would disagree that you average physicist even uses the Copenhagen interpretation. What I used to use in my days as an atomic physics theoretician was the shut up and calculate interpretation. Ie QM is a mathematical theory - you put your parameters in one end, and crank out your expected answers at the other end, which you could compare with experiment. I remember when being asked to think about it that the notion of wave function collapse sounded wrong, one couldn't have a physical object changing at superluminal speeds, which is what this implies. As for Bohm, his writings were about as meaningful to me as the ravings of a lunatic (note this is a reflection on me, not a reflection of Bohm's actual content). I can say that all experiments I worked on had millions of particles per second, and we worked with density matrices, which describe the behaviour of particle ensembles. Such a description is perfectly deterministic - no collapses or any other funny business going on. It was only when I heard about Aspect's experiments that I realised there was more to it. Ultimately, I realised that the MWI was the only interpretation that made sense (to me). Cheers -- *PS: A number of people ask me about the attachment to my email, which is of type application/pgp-signature. Don't worry, it is not a virus. It is an electronic signature, that may be used to verify this email came from me if you have PGP or GPG installed. Otherwise, you may safely ignore this attachment. A/Prof Russell Standish Phone 8308 3119 (mobile) Mathematics0425 253119 () UNSW SYDNEY 2052 [EMAIL PROTECTED] Australiahttp://parallel.hpc.unsw.edu.au/rks International prefix +612, Interstate prefix 02 pgpy0LtXJKPOf.pgp Description: PGP signature
RE: Decoherence and MWI
-Original Message- From: Patrick Leahy [mailto:[EMAIL PROTECTED] Sent: Monday, May 23, 2005 8:14 PM To: Hal Finney Cc: everything-list@eskimo.com Subject: Re: Decoherence and MWI On Mon, 23 May 2005, Hal Finney wrote: I'd like to take advantage of having a bona fide physicist on the list to ask a question about decoherence and its implications for the MWI. SNIP If this is true, then how can a physicist not accept the MWI? Beats me... Isn't that just a matter of taking this decoherence phenomenon to a (much) larger degree? Either you have to believe that at some point decoherence stops following the rules of QM, or you have to believe that the mathematics describes physical reality. And the mathematical equations predict the theoretical existence of the parallel yet unobservable branches. The physicists who I really respect, but who do not support MWI, do indeed believe (or hope) that the rules of QM break down at some point. E.g. Roger Penrose, or Tony Leggett. They would point out that to date every theory we have has only been an approximation. Penrose would point out additionally that QM is inconsistent with GR, and there is no reason to suppose that only GR has to be modified. I support these people to the extent that I think it is tremendously important to keep doing experiments, especially ones that can test well-formulated alternatives to QM. I'm just not very hopeful that any discrepancy will show up. Of course, given that they are in practice unobservable, a degree of agnosticism is perhaps justifiable for the working physicist. He doesn't have to trouble himself with such difficult questions, in practice. But still, if he believes the theory, and he applies it in his day to day work, shouldn't he believe the implications of the theory? For most physicists the Copenhagen interpretation (in some half-understood way) works perfectly well at the lab bench. There are also those who have thought very carefully about the issue and have come to a hyper-sophisticated philosophical position which allows them to fudge. I'm thinking particularly of the consistent-histories gang, including Murray Gell-Mann. I particularly liked Roland Omnes' version of this: quantum mechanics can account for everything except actual facts. He thinks this is a *good* thing! I too like Omnes' point. If QM is a probabilitistic theory, then it must predict probabilities - not actualities. To me, it almost requires believing a contradiction to expect that decoherence experiments will follow the predictions of QM, without also expecting that the more extreme versions of those predictions will be true as well, which would imply the reality of the MWI. You either have to believe that a sufficiently accurate decoherence experiment would find a violation of QM, or you have to believe in the MWI. Don't you? Yes. Paddy Leahy PS: this is an endorsement of the MWI of QM, not of any everything theory. There are a couple of papers on arXiv, gr-qc/0505052 and gr-qc/0505023, by Gambini and Pullin discussing discrete models of spacetime. Most physicist think that, at the Planck scale, the continuum model of spactime is at best inelegant and at worst simply wrong. Gambini and Pullin show that in their discrete spacetime model decoherence is inherent - so something really does happen. This implies a fundamental arrow-of-time. Brent Meeker
