On 13 July 2017 at 12:12, Bruce Kellett <[email protected]> wrote:
I have recently acquired Wallace's book and read his section on > non-locality, so I am re-opening this thread (under a different header) to > discuss Wallace's views in more detail. More below...... > > > On 7/06/2017 9:24 am, David Nyman wrote: > > On 6 June 2017 at 01:46, Bruce Kellett < <[email protected]> > [email protected]> wrote: > >> On 6/06/2017 10:21 am, David Nyman wrote: >> >> >> Bruce, I'm reading The Emergent Multiverse by David Wallace at the >> moment. He's well known as a prominent theorist of MWI. I don't know >> whether he falls under your definition of competence in this area, but as >> far as I've understood him, he fully accepts that MWI must be consistent >> with QM in all respects, including of course nonlocality. The distinction >> he makes is between nonlocality and the question of whether this requires >> us to think in terms of instantaneous transfer of information at >> greater-than-light speed, or "action at a distance". I can't say I've been >> able to get my head around his full exposition of this yet, but I'm pretty >> sure he doesn't go along with your exposition of Price's seemingly faulty >> version of this. >> >> >> It is interesting that Wallace has come to this view. He, with Deutsch, >> was one of those who attempted to argue that MWI restored full locality. >> They also tried to derive the Born Rule from within MWI, and failed in that >> too. >> >> I do not know the book you refer to, but if Wallace now accepts that QM >> and Bell implies non-locality, then I fully agree. I have always argued, on >> this list and elsewhere, that non-locality does not mean the instantaneous >> transfer of physical information -- if you think about it, that would, in a >> sense, be a local, albeit FTL, effect. The core of the quantum singlet >> state is that it does not involve the physical positions of the particles. >> It is expressed in configuration space, and the difficulties appear to >> arise from interpreting configuration space as though it were the same as >> ordinary 3-space. What has been said is that the singlet state is always >> local in configuration space, which translates to non-locality in 3-space. >> And this without some FTL information transfer. If there were FTL >> information transfer, then that could be manipulated to give FTL >> signalling, and there are all sorts of theorems in QM that show that FTL >> signalling is not possible. >> >> But it seems as though Wallace is coming to see these things as do the >> majority of other physicists -- non-locality is intrinsic to quantum >> entanglement. >> > > Wallace uses the term non-separability. He makes an analogy, to a > certain extent, with the ontology of field theories such as > electromagnetism, about which he says "The structural complexity of a given > electromagnetic field is represented not in the properties of very small > spacetime regions (indeed in the limit as these regions become point sized, > the field's structure becomes almost trivial) but in the way in which those > properties vary across spacetime. Furthermore, this general model is > characteristic of pretty much any classical field theory, except that > vector fields seem mathematically tame compared to the sorts of > mathematical objects used to represent the field values of many classical > field theories.". He gives a number of examples of these latter objects > including the affine connections of General Relativity. He then goes on > from this analogy to propose an ontology for quantum field theory which he > calls Spacetime State Realism. I can't really attempt to elaborate on this > here. > > Moving on this basis to the question "Does Everettian quantum mechanics > display action at a distance?" he answers in the negative. He justifies > this by elaborating on the observation that "In a quantum field theory, the > quantum state of any region depends on the quantum state of some cross > section of the past light cone of that region. Disturbances cannot > propagate into that light cone." To the question "Does Everettian quantum > mechanics display non-separability?" he answers in the positive. He > justifies this by elaborating on the observation that "Because of > entanglement, knowing the density operators of regions A and B does not > suffice to fix the density operator of (the union of) A and B. Some of the > properties of (the union of) A and B are genuinely non-local: they have > local physical manifestations only if we arrange appropriate dynamics.". > > > That is a good extract from the heart of his exposition. In a way, it is > more a matter of words than of substance -- his description of > 'nonseparability' is essentially what I have been calling 'non-locality', > and Wallace himself actually lapses into this usage from time to time. His > summary is: > "The overall story about locality in Everettian quantum physics, then, is > this: the dynamics of the theory are local: there is no action at a > distance, and no clash with relativistic covariance. But quantum > entanglement means that a great deal of the information contained within > the quantum state is non-local, associated with large spatial regions but > not with any given subregion of those regions. As David Deutsch once put > it, quantum theory is a theory of local interactions and non-local states." > > I cannot find anything in that summary to which I could object. > > Wallace than goes on to discuss some examples. His first example is of a > single particle system, such as Schrödinger's cat. He describes the initial > local interaction, and the spreading of the branching via decoherence. If > the cat is system A, and the environment with which it becomes entangled a > set of systems B_i, (i = 1,2,...), then the spread of the branching results > in a situation in which the individual states of the systems A and B_i > become mixed, but the combined state of (A ⋃⋃ B) remains pure. "The state > A itself does not change at all in the process; what changes are the > non-local states of successively larger regions including A." The spreading > of entanglement by decoherence is thus a process that introduces a degree > of non-locality. Wallace illustrates this in his Figure 8.1 on page 307. > > Then we get to measurements on two independent particles, and, finally, > two entangled particles. Here Wallace really wimps out, and does not give > any systematic analysis. He does not consider the entangled singlet state > explicitly at all. All he says is that the entanglement between the > particle at A and the particle at B is a non-local property of A ⋃⋃ B. > "That property propagates outwards, becoming a non-local property of the > forward light cone of A and that of B. Only in their interactions can it > have locally determinable effects--and it does, giving rise to the branch > weights which, in turn, give rise to the sorts of statistical result > recorded in Aspect's experiments and their successors: statistical results > which violate Bell's inequality." > > That is just the standard quantum account, since it is always accepted > that the correlations only become apparent when the results of measurements > by A and B are combined at some later time, when their light cones overlap. > > Wallace seems to find this relatively uninteresting. Bell's result entails > non-separability (non-locality), but not action at a distance -- but then, > no one said it did involve action at a distance. He then claims that Bell's > theorem does not apply to the Everett interpretation anyway, because is > assumes that experiments have unique, definite outcomes. That is the usual > MWI claim against Bell, but Bell's results are not specifically quantum -- > the inequalities obtain for any theory in which the measurements at A and B > are independent, so this passing swipe at Bell is rather unnecessary. > > The upshot, it seems to me, is that Wallace acknowledges non-locality as I > have used the term, only he prefers to call it nonseparability. The change > in terminology does not change the physics, so Wallace accepts that the > Everettian interpretation of QM does not eliminate non-locality, despite > the claims of many MWI supporters. > The question then is what are we to make of such non-separability (I'll stick to this usage, for what it's worth) if it is not to imply a corresponding action at a distance? How can it be that the Alice who has observed, say, spin-up doesn't get to encounter the Bob who has also observed spin-up? In what does this so-called entanglement consist? ISTM, on reflection, that my tentative suggestion that the alternative cases are in some deeper sense 'unphysical' have some sort of explanatory force. It's circular of course, but not I think in a vicious way. IOW it would be the case that there just were no well-formed physical circumstances in which such encounters could occur. ISTM however that any such notion implies a deeper level from which QM as we observe it gets 'selected' on what is essentially an anthropic (hate that term) basis. This in turn would imply that an entangled QM-physics is a prerequisite for (at least our species of) observation itself. Of course, if we hypothesise, on the basis of a computational theory of mind, that QM is indeed observationally selected from a deeper arithmetical level, then a 'canonical' physics of precisely this kind is indeed implied. In this scenario, computational continuations implying non-entangled encounters simply don't correspond to any of the observable 'physical worlds'. I realise in advance that you are unsympathetic to this kind of thing Bruce, but I would still value your opinion on whether there is anything contradictory in such an idea. You commented before that it was conspiratorial and anti-scientific, but I'm not sure why you have this particular view of what might constrain a properly scientific enquiry. David > > Bruce > > -- > 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 [email protected]. > To post to this group, send email to [email protected]. > Visit this group at https://groups.google.com/group/everything-list. > For more options, visit https://groups.google.com/d/optout. > -- 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 [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
