On 15 July 2017 at 00:46, Bruce Kellett <[email protected]> wrote:
> > > On 15/07/2017 4:00 am, David Nyman wrote: > > On 13 July 2017 at 12:12, Bruce Kellett < <[email protected]> > [email protected]> wrote: > > >> [Wallace's] 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? > > > This is where it is a pity that Wallace did not actually work through the > entangled singlet state in detail. He gives a general expression for a > state with two entangled particles in (8.14), but says: "In this case, the > amplitudes of the four sets of branches into which C eventually branches > are not determined simply by the separate weights of the branches at A and > B. Not is this to be expected: as I stressed previously, in Everettian > quantum mechanics interactions are local but states are non-local." > > Without further detail, it is impossible to work out how Wallace thinks > the weights of the four branches into which C eventually branches are to be > determined. I think he is just going to use the standard quantum > calculation of these weights, but, as I have said, this calculation is > explicitly non-local. Since Wallace accepts this non-locality, it would not > be out of the question that he would accept the standard calculation at > face value. > > > 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. > > > Well yes, that is what the singlet state means -- there are no physical > circumstances in which the branches that form could violate QM. But that is > a statement of the fact, not an explanation of that fact. > Well, it's at least pointing in the direction of a mode of explanation in terms of a tight constraint on what could count as viably physical. Others have noted that physics (at least a physics adequate to support our kind of observer-hood) is suspiciously replete with such tight constraints. As I said, this would be a mystery if regarded merely as a brute given (i.e. it could simply have been otherwise) and hence it tends to point in the direction of a constraint on observational selection from a more general basis. And if so, it would of course then be the case that only appropriately-constrained phenomena, arising from such states of affairs as were physically capable of supporting observation, would thereby be observable. > > 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. > > > If you want to go down this route you still have to give an account of how > this selection might come about -- why is it that violations of QM are > fatal to consciousness? > They would be fatal to consciousness if such violations precluded the formation of physical states of affairs adequate to comprise the vehicles of that consciousness and the environments on which their consistent experiences depended (as Brent has often noted). > 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 have raised problems with the idea that consciousness "selects" > specifically quantum behaviour in another thread. The basis of my worry > with this is that there does not seem to be anything to rule out a purely > Newtonian classical world exhibiting consciousness as we know it -- that is > implied by substrate independence, strong AI, and the assumed ability to > replace our consciousness with the equivalent computations carried out on a > classical Turing machine. Hence quantum mechanics is not entailed by > consciousness, so it is difficult to see how consciousness could select out > specific facts about the world that were inconsistent with QM. > But surely the counter-argument to that is that a truly Newtonian classical world wouldn't in fact be a viable physical world at all? For example, the Newtonian 'atom' would collapse more or less instantaneously. So what I'm suggesting is that consciousness in practice would have to depend on a physics actually capable of comprising its vehicles and their environments and that evidently such a physics, to be viable, must be quantum in nature. If so, this would likewise imply the observational selection of the necessary physics from a more general background. If CTM is assumed, this background would be computational and the corresponding physics would be comprised by the 1-person-plural appearance, via the FPI, of QM-like phenomena. On that assumption, Bruno attempts to demonstrate (pace the current state of your discussion with him on this) that the necessary quantum-like logic could in fact be derived from classical computation, in the kind of infinite limit entailed by the UDA. Or at least that the possibility of doing so has not yet been persuasively ruled out. 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.
