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July 18, 2015 at 9:04 am #2859bricmontMember
The de BroglieBohm theory is not an alternative to ordinary quantum mechanics or an interpretation of it but a rational version of quantum mechanics: while the latter provides an accurate algorithm for predicting results of measurements, the de BroglieBohm theory is a real theory about what is going on in the world from which the quantum algorithm can be derived.
We want to emphasize here two of the main merits of the de BroglieBohm theory. The first one is that it exists. The second one is its perfect clarity. For more details see the attched text.
July 18, 2015 at 10:28 am #2863Reinhard WernerMemberDear Jean,
so BM is “a real theory”? As opposed to what? I also find your use of the attribute “rational” pretty strange. I suppose you would describe yourself as a rationalist, and I do like your book with Sokal on his hoax. But I think you are just mistaken that rationality is tied to some kind of ontology (as in that silly slogan that quantum mechanics must be about “something”). Rational thought is as much about relations as it is about things. So a scientific theory like quantum mechanics strives to build relations between contingent facts, and to understand special instances from overarching principles. None of this needs hypothetical mass points to be “realistic” or “rational”.
In contrast, Bohmian mechanics contains a lot of stuff which is highly arbitrary, because unrelatable to experience. No rational argument about these aspects of the theory seems possible, and surely these features cannot be called realistic (in the broad sense) either.
I was also puzzled by your description of BM as “deterministic”. You have that massive source of randomness stuck into the initial conditions by the Maker of the Unverse. That is a construction you can make in any probabilistic theory so in that way every probabilistic theory is deterministic. So even if you can argue that our universe just sits on one trajectory and still displays quantum randomness almost surely, the “almost” is with respect to God’s measure. So your randomness assumption can be rephrased as a “typicality” condition. Again you can do this in any probabilistic theory. So a more accurate statement would be that in BM randomness is shifted to \infty.
The “perfect clarity” is another puzzle to me. Every concrete question I ever followed up on in this theory ended in a heap of evasions. So I suppose you mean the sort of clarity you derive from scripture, in this case maybe the gospel “Unspeakables” by St. John, from which you offer readings in your long text.
With best my regards,
ReinhardJuly 19, 2015 at 4:16 pm #2901Aurelien DrezetParticipantDear Jean, I like what you wrote. For me BM is indeed a rational theory. But we can reverse the problem. Are we sure that the so called orthodox interpretation is a rational theory? I have strong reasons to doubt about that since for Bohr everything should be necessarily described in term of macroscopic languages while at the same time detectors are also made of atoms which should be describd by the theory. It looks like the problem of the chicken of the egg but here touching the ontology of the things themselves. It is only with a clear ontology that problems vanish and this is the main advantage of Bohm over Bohr.
July 19, 2015 at 8:45 pm #2904Robert GriffithsParticipantDear Jean,
My critique of Bohmian mechanics (BM) was posted in the Why Bohmian Tbeory
thread begun by Shelly, and you might want to take a look at it. A key problem
is that Bohmian trajectories can give the wrong answer, at least from the
perspective of an experimentalist who doesn’t like to be told that his detector
was triggered by a particle passing a meter away.Nonlocality in the sense of instantaneous nonlocal dynamical influences (to be
distinguished from nonlocal correlations, which are also present in classical
physics) is a serious problem for BM. There is no experimental evidence for
such influences, as shown by the fact that even those who believe in them agree
that they cannot carry information. The supposed indirect evidence based on
violations of Bell inequalities can be countered by noting the derivation of
such inequalities employs hidden variables which are treated classically (they
commute). See my “Quantum Locality,” Found. Phys. 41 (2011) 705;
arXiv:0908.2914. Nonlocality is also a serious problem when it comes to
constructing a satisfactory relativistic version of BM–a problem which Bell
(in his ‘Against Measurement’) regarded as an important unfinished task. So
far as I know, it remains unfinished.I will grant you the clarity of Bohmian mechanics. But by itself that does not
make it a good theory of the quantum world.Bob Griffiths
July 22, 2015 at 2:47 pm #2947bricmontMemberI am sorry; I was away for a few days and could not respond. I think at some point in the discussion, one has to agree to disagree and let others judge the merits of our arguments. That is why I have written a book (to be published by Springer) “Making sense of QM”, where I explain in detail de BroglieBohm’s theory and reply to the usual objections. I also discussed “alternatives” such as the consistent histories approach (I do not think Shelley’s criticisms in Physics today is full of mistakes, and I have read all the letters).
For Reinhard, we have to agree to disagree on what the goal of a physical theory is and on the meaning of determinism.
For Robert, we have to agree to disagree on the meaning of Bell’s result and the no hidden variable theorems more generally. For me, Bell does prove the existence of non local actions, not just correlations, in the world. And that is a real problem for any “relativistic” quantum theory, not just Bohm. Usual field theories do not deal with the collapse, which can be a non local operation for instance.
For Aurelien, we agree on your comment of course.
Best wishes
Jean

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