On 4 Jun 2017 1:05 p.m., "Bruno Marchal" <marc...@ulb.ac.be> wrote:
On 02 Jun 2017, at 03:01, Bruce Kellett wrote: On 1/06/2017 10:19 pm, Bruno Marchal wrote: > >> On 01 Jun 2017, at 02:26, Bruce Kellett wrote: >> On 1/06/2017 4:43 am, Bruno Marchal wrote: >> >>> On 31 May 2017, at 04:01, Bruce Kellett wrote: >>>> >>>>> On 30/05/2017 9:35 pm, Bruno Marchal wrote: >>>>> >>>>>> On 30 May 2017, at 11:28, Telmo Menezes wrote: >>>>>> >>>>>> I get your point with decoherence. >>>>>>> Again, I would say that it all depends on theories of mind. What does >>>>>>> mind supervene on? Perhaps it is true that every single coupling with >>>>>>> the environment prevents the current observer state to become >>>>>>> compatible with other branches. But can we be sure? I feel that such >>>>>>> certainties come from a strong belief in emergentism (which I cannot >>>>>>> disprove, but find problematic). >>>>>>> >>>>>> >>>>>> It is impossible to recohere the past, FAPP. >>>>>> >>>>>> But only FAPP. To make the blue T-rex interfereing with the >>>>>> red-T-rex, we must erase the trace of particle interaction between the >>>>>> T-rex in its whole light-cone, and this without forgetting the particles >>>>>> "swallowed" by the black-holes, etc. It is just completely impossible, >>>>>> but >>>>>> to derive from that the unicity of the past, is, it seems to me (and you >>>>>> if >>>>>> I understood well) is invalid. >>>>>> >>>>> >>>>> I think the recoherence of paths that have completely decohered is >>>>> more than just FAPP impossible, I think it is impossible in principle. One >>>>> major problem with recoherence in general is that information leaks from >>>>> the paths at the speed of light (as well as less slowly for other >>>>> interactions). Since this vital information goes out along the light cone, >>>>> it can never be recaptured and returned to the original interaction. >>>>> >>>> >>>> In QM + special relativity OK. (note that to me QM + special relativity >>>> => no collapse (and even the Many dreams, but we have agreed to disagree on >>>> this if I remember well). >>>> >>> >>> Reasoning in QM without SR is not very profitable. Besides, QM + SR does >>> not particularly imply MWI -- it is perfectly possible to have a consistent >>> collapse model of QM+SR. I know you don't agree because of the non-locality >>> implied by EPR, but this non-locality is not removed in MWI, regardless of >>> what you might say. >>> >> >> I don't see it, and the mast time we discuss this, we conclude on some >> vocabulary problem. When there is no collapse, measurement tells to people >> light separated in which branch theiy belongs, but to exploits that >> information, they need to come into contact. >> > > Last time we discussed this, no resolution was achieved. > That is not what I remembered, but I will not insist. In QM, with or without collapse, decoherence and the transition from the > pure state to a mixture gives a definite measurement result. > In particular branches only. When looking at the whole wave including the observers, decoherence explain why it *looks*, to all observers in the different branches, that mixed states have been obtained, but that is not the case in the global description. Without collapse, different branches get different results, but once > obtained, these results are fixed, and are not affected by whether Alice > and Bob exchange information or not. > I agree. That is used in the fact that in EPR like situation, when Alice and Bob are space-time separated, what we have is "only" an infinity of Alices and Bobs, all with their spin correlated, and when Alice makes her measurement, at any angle, she will know Bob's possible result, without needing any action at a distance. She just localize herself, and her corresponding Bob, in which branch they belong. There is no influence at a distance, although we would need it to talk of token unique Alice and Bob in case there would be only one universe. > Consequently, indispensable phase information is lost *in principle*, so >>>>> the recoherence is, in general, impossible. >>>>> >>>> >>>> OK. (I was reasoning in naive classical QM) >>>> >>>> Of course, with carefully constructed systems, where the loss of >>>>> information along the light cone is prevented, recoherence is possible in >>>>> special circumstances, but not in general. >>>>> >>>>> From this, the uniqueness of the past of any decoherent history is >>>>> assured. So deriving the unicity (if I understand this use of the word) is >>>>> by no means invalid -- it is proved. >>>>> >>>> >>>> In QM + SR. OK. >>>> >>>> Even if one encounters one of those rare situations in which >>>>> recoherence is achieved, that still does not invalidate the uniqueness of >>>>> the past history -- recoherence, if it occurs, simply means that no new >>>>> branches are formed at that point, so the decoherent history remains >>>>> unique. >>>>> >>>> >>>> OK. That might suggest that we identify our indistinguishible past in >>>> arithmetic, if we assume mechanism. I use the Y = II principle, or the >>>> "quantum" linearity of the tensor product "@": we have that a @ (b + c) = >>>> (a @ b) + (a @ c). >>>> >>>> That makes sense. >>>> >>> >>> Linearity is the heart of QM. It is linearity that allows >>> superpositions, and leads to all the "quantum weirdness". >>> >> >> I agree. Both "linearities" (the quantum evolution, and the tensor >> products). >> >> >> ....... >> >>> But those state difference are accessible to the observers, and indeed, >>>> only this makes the analogy with step 3 working. >>>> >>> >>> In MWI, the differences are not observable by anyone. Any observer has >>> access to only one branch, so only one copy. They can say nothing about the >>> other branch. >>> >> >> The difference are not observable, but are very gross, like seeing a cat >> dead, or alive. Linearity prevent any direct view of that difference, but >> it exists, when we assume QM (and non collapse). >> > > It is not linearity that prevents macro-superpositions > That is what I just said. -- it is decoherence and the reduction to a mixed state. > In each branch, not in the universal wave. Superposition remains superposition. Only the collapse of a wave would destroy a superposition, and indeed, in a non-local way. Without collapse we get only local appearance of mixed states. D'Espagnat called them "improper" to distinguish them from a proper mixed state that we could get with a collapse. The difference between the measurement outcomes exists whatever > interpretation of QM you impose. > That is what I try to explain to John Clark in the mechanist context of the WM-duplication. The measurement of self-localization is a precise outcome, but that does not make the other "branch" disappearing. > ....... > >> Of course, it assures them in all branches, where indeed Aspect like >>>> experiences can be made. It seems to me that we did agree on this: that >>>> non-locality does not entail any physical influence in the past. That does >>>> happen in the unique universe view though; even if there is no possible >>>> communication of information is done. >>>> >>> >>> Non-locality means that there is no physical transfer of information, >>> but that there is non-physical(non-local) transfer of information. But this >>> information transfer cannot be used for signalling. Signalling is possible >>> only with actual; physical transfer, a consequence of SR and the fact that >>> 'information' is physical. >>> >> >> So we agree. Yet, with unicity of outcomes assumed for measurement, we >> still cannot signal, but must assume some "reduction of a wave packet at a >> distance", or super-deterministic conspiracies. >> >> This non-locality is even more evident in the more recent delayed choice >>>>> experiments that use entangled photons to manipulate photon polarization >>>>> states non-locally. >>>>> >>>> >>>> I have no problem with one-branch observable, apparent, non-locality. >>>> I have a problem only with the action at a distance that you need in >>>> case you assume one contextually well defined physical reality. >>>> >>> >>> Non-locality is not removed in MWI as you appear to believe. >>> >> >> In the sense above, you are right. I was juste arguing against John >> Clarks idea that the Bell's inequality violation introduce physical action >> at a distance, even with the MWI. >> >> There are two things: >> >> 1) no influence at a distance, >> 2) no signaling at a distance. >> >> With QM-without collapse, there is no influence at a distance. The >> no-locality above is explained without them. >> With QM+ collapse, there is no signaling at a distance, but there is an >> influence at a distance. >> > > There is no non-local signalling whatever interpretation you adopt. > Yes. But there is non-locality -- non-local influence -- in all interpretations > since it is inherent in the quantum formalism. > I don't see any non-locality in the MWI. EPR, Bell, assumes always one Alice and Bob, and as Everett shows, decoherence explains the manitenance of coherent first person plural description, and the absence of collapse prevent any non-local influence. Non-locality is not explained away in MWI. > Indeed, the MWI explains well the appearance of non-locality, and this without needing a physical influence at speed light. > > > For me the abandon of the collapse is the solution of the EPR "paradox", >>>> and Aspect experience is somehow the confirmation of our belonging to >>>> macrosuperposition. >>>> >>> >>> The non-local (paradoxical) nature of EPR remains even without collapse. >>> >> >> I know some claims this, but I have never sen a proof. All proofs relies >> on unicity of the outcome of some experiences. >> > > You cannot get away by reversing the onus of proof. Bell's theorem is > independent of whether or not a collapse is assumed, > To interpret the experimental violation, you need to identify the Alice and Bob you talk about. But EPR and Bell talk of Alice and Bob like if they were in a definite universe all along the experience, when that is never the case. They do assume implicitly one physical universe. so if you want to argue that MWI removes the non-locality proved by Bell, > then the onus of proof is very much on you: you have to demonstrate how > this can be possible. > It is a trivial consequence of the linear differential shroedinger equation. Or of the fact that the evolution is a rotation (unitary) in Hilbert space. > You say that Bell's theorem relies on the unicity of outcomes. By this, I > presume you mean that Bell assumes counterfactual definiteness (in the > usual terminology). If by counterfactual definiteness you mean that a > measurement gives a definite (though unknown in advance) result, even if > that measurement is not performed. > This does not make sense in the MWI. If I measure a alive+dead cat, it is only a "first person illusion, 1p" that such an experience gives a definite result. In the 3p picture, both results must be said to obtained. Then I accept that counterfactual definiteness is assumed in quantum > mechanics. Without such an assumption, the whole notion of an expectation > value would collapse. > Not in the 1-views. It continues to make sense, and the contagion of superposition (the linearity of the tensor product) even prolongate the 1-views into 1-plural views, that gives the siplitting/differentiation/decoherence, which needs only to propagate at the speed of light or below. So if you abandon counterfactual definiteness, you have a different theory > -- you have abandoned standard QM, and you then have to explain how you can > get and use expectation values. > We got them in the memories of the person's involved. Not from looking at the whole universal wave. It is again like with computationalism. > Your claim appears to be that Bell's theorem is not valid in MWI. > Bell's theorem is valid. His inequality does not even assume QM, but just locality. It is violate when we do the experience, like Aspect, and this shows non-locality in our branch, but when looking at the big picture, we see that this non-locality has a local origin. It would need an action at a distance to destroy the alternante branches alwailable to Bob, but without collapse, non-locality is a local, branch-owned, phenomenon. I take Bells theorem + Aspect as a quasi definite proof that if there is one universe, then there are many universes. This is nonsense. Bell's theorem is a theorem of quantum mechanics, and it > is therefore valid in all interpretations of that theory. > Yes, in all interpretation of quantum mechanics, the relevant branches violate the inequality, but they do that without involving an action at a distance when we look at the entire wave. It is phenomenological. Suppose one were to enquire what makes those branches "relevant". One answer is that other pairings would be in conflict both with the predictions of QM and with observation, but that is circular. What then? Perhaps one might speculate that other pairings would somehow be fundamentally inconsistent with any physics that would permit its own coherent (or for that matter decoherent) observation. This might ultimately be related to the speculation that the appearance of spacetime itself may emerge as a consequence of entanglement. David If it is not valid in MWI, then Many-Worlds is a different theory, and not > just an interpretation of standard QM. > It is valid in the MWI, but interpreted differently than in a mono-universe interpretation which requires non local action at a distance to get the same non-locality. (with or without hidden variables). > > > As on the previous occasion we discussed this, you were unable to >>> demonstrate where the notion of 'collapse' is used in Bell's theorem - all >>> Bell requires is that measurements give results, and that is what the whole >>> of physics is based on: in MWI as well as in any other interpretation. >>> >> >> We did eventually agree. May be reread those post. Bell's supposed that >> when the two measurement are done, Alice and Bob get a precise answer, >> which makes no sense without-collapse. >> > > That is what making a measurement means. It is what happens in all > interpretations. It makes no sense to deny counterfactual definiteness -- > that is not QM. > It is QM without collapse, and using the simple mechanist FPI. > Alice and Bob get *all* (always correlated) answers, but when >> light-separated, it make no sense to compare them. They can only make >> comparison with the person accessible in their light cone, where the >> contagious superposition spread out. >> > > I presume you mean "space-like separated". Alice and Bob do their > measurements; > The infinities of Alices and Bobs do their measurements. they get their results and write them in their lab books. They meet years > later and compare lab books. Are you trying to suggest that they do not > have definite answers in their lab books before then? > The infinities of Alices and Bobs get their infinities of definite results. In MWI (with two-outcome experiments), there is a copy of Alice that writes > '+' in her lab book, and a copy who writes '-' (for a given orientation > theta). Similarly for Bob. There are, therefore, only four possibilities > when they meet: '++', '+-', '--', and '-+'. The non-locality is necessary > to set the probabilities for each of these four possible combinations of > results. If you want to eliminate the non-locality, you have to give a > non-magical way of establishing the necessary probabilities. You have never > been able to do this. > QM does that, and without collapse, I don't see how any influence leaking at the speed of light need to be introduced. > Remember that in a sequence of such experiments, the probabilities for '+' > and '-' are 50/50 for both Alice and Bob. > OK. The joint probabilities, or correlations, depend on the relative > orientations of their polarizers. > Right. It is information about this relative orientation that must be conveyed > non-locally for the correlations to come out correctly when they meet. > Why? That would be the case if you think that it is the same Bob and Alice all along the experiences, but that cannot be the case. It is not sufficient for them simply to exchange this information later, > because their results at particular orientations are already fixed when > they meet. > I don't see this. If the angle is some theta different from 0° or 90° they will both split/differentiate, and whoever they will meet later will be the correspond partner with the correct correlation, obtained by the decoherence local to their respective branch. In this case, it is clear that it does not make sense to attribute to "Alice and Bob" the same identity than the initial one. Bruno > 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 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. > http://iridia.ulb.ac.be/~marchal/ -- You received this message because you are subscribed to the Google Groups "Everything List" group. 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