On Fri, Apr 22, 2016 at 2:35 AM, Bruce Kellett <bhkell...@optusnet.com.au> wrote:
> On 22/04/2016 2:46 pm, Jesse Mazer wrote: > > On Thu, Apr 21, 2016 at 11:25 PM, Bruce Kellett <bhkell...@optusnet.com.au > > wrote: > >> On 22/04/2016 12:53 pm, Jesse Mazer wrote: >> >> On Thu, Apr 21, 2016 at 9:49 PM, Bruce Kellett < >> <bhkell...@optusnet.com.au>bhkell...@optusnet.com.au> wrote: >> >>> >>> >>> The point here is that some combinations of results are forbidden. How >>> can this happen? >>> >> >> By the appropriate matching rules for locally-generated copies in >> different locations, as in my toy model. There's no reason you can't have >> something similar in a more general model, which I think is exactly what >> people like Rubin are presenting. >> >> >> The best I can make of this is that you have some theory that is not >> quantum mechanics. Quantum mechanics does not give any such "matching rules" >> > > It's important to distinguish between theories of physics and the > mathematical models used to express them--a physical theory is defined > entirely by the predictions about observable outcomes, not any elements of > the model that are not directly measurable even in principle. For example, > curved spacetime is not essential to general relativity as a theory, though > it is a feature of the most commonly-used mathematical model (there is an > alternate formulation that only uses flat spacetime, but has a field > defined on this spacetime which varies the length of rulers and the ticking > rate of clocks at different points in the spacetime, and physicists would > still call this 'general relativity'). Likewise, a state vector in Hilbert > space is not essential to quantum mechanics as a theory. And if one *could* > come up with a model involving "matching rules" that would be equivalent in > its predictions about observable measurement results as the existing > mathematical models, this would merely be a new mathematical model for the > same physical theory. > > > It would seem that you are not a physicist! What you claim here about > physics is actually quite contentious. It seems to constitute an extreme > form of instrumentalism. > I don't think that's the case, I'm basically just talking about how physicists *define* the physical content of a theory. But it would help if you would define what you mean by "instrumentalism". For example, some articles I found googling the term seem to say that it suggests we should not assign any "reality" to elements of the theory beyond the predictions about empirical measurements; I would say any talk of "reality" beyond measurements is more of a philosophical issue than a scientific one, but I don't see anything wrong with having opinions on such philosophical issues. In particular, if there is an element that seems to show up in *all* our mathematical models, like the notion of an "electron" which isn't absent from any formulation of quantum electrodynamics, it certainly makes sense to me to call it "real". Likewise, although we can't ever get evidence that space and matter continue beyond the boundary of the observable universe, it would require a very contrived model to avoid it (one which treated us as being at the exact center of real space, for example), so disbelieving it would to me seem like a ridiculous philosophical view, akin to solipsism (speaking of which, I could also potentially come up with a solipsistic interpretation of quantum physics in which I and only I am capable of collapsing the wavefunction with my observations, but this would seem equally ridiculous despite the fact that I can't come up with any experiment that would falsify it for me). Also, it seems from my googling that many instrumentalists would define the validity of scientific theories solely in terms of what we humans can actually verify in principle, giving up the notion of any objective truth about nature independent of what humans know. If so, I am not taking this position either. I'm saying the physical content of a theory is defined in terms of the complete set of predictions about things that could *in principle* be measured by some arbitrarily advanced physical being at the right place and time (so the fact that we may have no way of verifying most of the predictions of string theory at any time in the forseeable theory does not disqualify it as a scientific theory, for example), and I personally believe there is some objective truth about what mathematical relationship describes the complete set of in-principle-measurable facts about our physical universe. The basic point I was making with my point about physics theories vs. mathematical models is that I'm pretty sure the vast majority of the modern physics community would define a physics theory in terms of its predictions about in-principle-measurable facts, and if two mathematical models can be proved to be identical in *all* their predictions about such facts, then they are defined to be different models of the "same theory", not different theories. Do you disagree with this narrow claim? I can point to plenty of examples of physicists who do treat different models with identical predictions as the same theory. For example, earlier I mentioned the alternate formulation of general relativity which gets rid of any notion of curved spacetime, replacing it with a field that shrinks rulers and slows down clocks differently in different regions of spacetime. I learned about this formulation from Kip Thorne's book "Black Holes and Time Warps" where after discussing this alternate form he writes on p. 400: "What is the real, genuine truth? Is spacetime really flat, as the above paragraphs suggest, or is it really curved? To a physicist like me this is an uninteresting question because it has no physical consequences. Both viewpoints, curved spacetime and flat, give precisely the same predictions for any measurements performed with perfect rulers and clocks, and also (it turns out) the same predictions for any measurements performed with any kind of physical apparatus whatsoever. ... They disagree as to whether that measured distance is the 'real' distance, but such a disagreement is a matter of philosophy, not physics. Since the two viewpoints agree on the results of all experiments, they are physically equivalent. ... Moreover, physicists can and do use the two viewpoints interchangeably when trying to deduce the predictions of general relativity." I think that pretty unambiguously supports the point of view I described, don't you? And Kip Thorne is one of the leading researchers in general relativity (see https://cosmolearning.org/courses/overview-of-gravitational-wave-science-400/about-the-professor/ for an overview of his contributions), so I think he has pretty good authority to speak for how the physics community in general thinks about these matters. But if you're not convinced by that, let me know if you want me to look for more quotes, I'm sure I could find similar comments in the context of other theories--for example, are you aware that quantum physics has multiple mathematical models for generating predictions, like the "Schroedinger picture" (which deals with a quantum state vector that changes with time while measurement operators stay constant), the "Heisenberg picture" (which deals with a quantum state vector that remains fixed forever, while the measurement operators change with time), and the Feynman path integral (based on summing amplitudes to all possible paths a particle could take to get to a given location, to find the total amplitude of finding it there)? But physicists definitely view them all as formulations of the same theory, "quantum mechanics", not different theories. Likewise, when discussing different interpretations of QM like the MWI and Bohmian mechanics, physicists will regularly point out that these are not really distinct physical theories, which is the whole reason they use the alternate term "interpretation". If you do disagree with this point of view on what constitutes a theory of physics, can you point to *any* modern (mainstream) physicists who clearly say something different, pointing to examples of models that they acknowledge make identical predictions but which they still argue should be considered distinct theories of physics? > If a physical theory is determined only by the predictions it gives for > the results of experiments, I am puzzled by why you should have such a > strong reaction against the notion of non-locality. The non-local > calculations of standard QM give a completely straightforward mathematical > model for calculating probabilities; a model that is, in the terms of of > other physical theories, phenomenally successful. If your only concern is > to get an instrument to predict experimental results (probabilities), why > should you worry whether the theory is non-local or not? According to you, > the mathematical model has nothing to do with physical reality (whatever > that is). The anti-realist would have no worries about such trivia. > Again, I never said I was an anti-realist, it's just that I distinguish between questions of philosophy and questions of physics, as Kip Thorne did in the comment of his I quoted. It's only as a philosophical matter that I find the many-worlds picture more plausible, but I certainly don't think there are any knockdown arguments against the idea that the reality underlying the observations involves some genuine non-locality, it's a totally plausible perspective. I'm curious though, what gave you the impression I have a "strong reaction" against non-locality? I have been trying to argue one point and one point only, that there is no knockdown argument *against* the possibility of a local model which reproduces all the predictions of existing models of QM, akin to Bell's theorem which does provide a rigorous proof that you can't have a "local realist" model (involving unique measurement) that reproduces those predictions. *You* seemed to be arguing that there was such a definitive argument against a local version of QM, and that was basically the only reason I decided jumped into the discussion--did I misunderstand you on this point, or was it just a matter of your defining "QM" to include aspects of existing models which go beyond the predictions about in-principle-measurable facts? If you are in fact arguing that there is good reason to be 100% sure no local model can even reproduce the empirical predictions of QM, then please give your argument, since I already showed that Bell's argument isn't sufficient to establish this if you allow for a multiple-local-copies-with-matching model. > > But you are clearly deeply worried about non-locality, which says to me > that you are not a thorough-going instrumentalist after all. So that you > say above about mathematical models being the only concern is all all just > so much hogwash -- you are actually concerned that your physical theories > conform to your own particular set of philosophical prejudices. > > That is your concern, but you cannot expect me to share it. I take an > instrumental (or epistemic) view of the wave function of QM. That is to > say, I view it as a mathematical object that can be used to claculate the > probabilities for experimental outcomes, but it is not a real physical > thing in the same sense as chairs and tables, the earth and the moon, are > real physical things. If the wave function is not physical, then there is > no physical collapse when a measurement is made, there is only a change in > our knowledge, and the wave function changes instantaneously to reflect > this change. In just the same way, our probability function for the outcome > of a horse race, or of a lottery, changes instantaneously once we learn the > actual outcome. From this perspective, MWI is just a ridiculously baroque > construction designed to preserve some persons' realist prejudices that the > wave function is a real physical object, like a chair or a table. > I don't assume the wave function is a real physical object--both Deutsch/Hayden and Rubin seem to say that to formulate the MWI in a local way you have to use some version of the Heisenberg picture, where all the dynamics are described by changes in the local operators, not a changing global wavefunction (a static wavefunction is still used, but this seems more like a calculational tool than anything else, and perhaps it would be possible to modify things so that the same static wavefunction was used for all systems, and one simply adjusted the initial states of the operators to get the right predictions for initial measurements on the system). My philosophical preference for the MWI basically just stems from the fact that it gives an objective description of the physical state of every region of spacetime (not just the ones we humans observe), and a single set of equations for the time-evolution, without any hidden variables that break some of the nice symmetries seen in the basic equations (Bohmian mechanics seems to require breaking Lorentz-symmetry, and also seems to require putting the position operator on a special footing). As for what is "real", my opinion is that if the MWI is true, all the "copies" of a system that arise in the model should be treated as having equal physical reality, in particular I think other copies of human beings or other intelligent beings that would arise in the formalism should be assumed to be just as conscious as the ones we see (this is similar to the belief that if the universe continues on beyond the observable region we can measure, any intelligent life which arises outside our observable bubble should be assumed to be just as conscious as ourselves). Beyond this I don't see a need to assume other more abstract aspects of the model, like operators or a wavefunction, would be "real". But these are just my personal philosophical opinions, I am not trying to convince you that you should adopt them. Again, the only reason I jumped in on this discussion was because you seemed to be making the argument that there was some definitive proof that a many-worlds-type model cannot possibly reproduce all the predicted observations of QM in a local way (and in any arbitrary patch of spacetime, as I clarified later), so please tell me whether you in fact believe this is impossible, or if you're just skeptical but can't say for sure that it can't be done. > > Of course, modern developments in black hole theory and cosmology render > the whole debate about locality otiose. The currently popular theory of > holography is necessarily completely non-local; and in a way that makes EPR > correlations look tame. > If you have an argument as to why holography is absolutely incompatible with any local model involving multiple copies of the physical facts at each point in spacetime, please present it. String theory in its current form is formulated in the framework of quantum field theory, although things like dualities hint at an (unknown) background-independent formulation (some string theorists do think that things like holography indicate the background-independent form will be more explicitly nonlocal, but you can find plenty of other string theorists who disagree, it's a speculative question at present). And if you leave out any notion of "collapse" from quantum field theory it is said to be a theory that does respect locality, I don't understand the details but apparently it has to do with the fact that measurement operators with a spacelike separation always commute, see http://www.scholarpedia.org/article/Local_operator for example. > So I don't think I will waste more time trying to convince you that the > standard non-local quantum theory is perfectly adequate for the explanation > of all observed phenomena in its domain. > > Of course I agree with that statement 100%, but it has nothing to do with the seeming claims of yours that I've been disputing. Jesse -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
