Thanks for these clarifications Bruce. I find your explanations to be very lucid and helpful - they also confirm my own understanding. IIRC, you weren't a particular fan of MWI when I last conversed with you on this list. I wonder if you'd care to comment on my original argument on this thread - which has of course now been swamped by the usual brawls. Does not a single history + the physical insignificance of the notion of a current moment mean that there is also only a single possible future? And if the future is predetermined in this way, isn't this a serious issue for single universe models of QM? How can the outcome of quantum events be both inevitable and random?
On Wednesday, May 31, 2017 at 12:01:23 PM UTC+10, Bruce 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. Consequently, indispensable phase information is lost *in > principle*, so the recoherence is, in general, impossible. 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. 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. > > > >>> FWIW, you > >>> are expressing my own understanding of the situation: there can be no > >>> superposition of red and green screens or dinosaurs, or dead and > >>> live cats, > >>> because there can be no quantum superposition of macroscopic objects. > >>> Superpositions of wave functions are only possible for systems > >>> isolated from > >>> interaction with their environment, which is why quantum computers > >>> are so > >>> fricking hard to make: keeping aggregates of particles isolated from > >>> interactions with the surrounding environment is exponentially more > >>> difficult as the system grows in size. > >> > >> The main question for me is this: can two branches hold different > >> observer states, if they differ only by things that are not > >> observable? > > > > I would say no, intuitively. I would even say "no" just for the things > > not observed, even when observable. > > I previously answered Telmo's question in the affirmative, viz., two > fully decohered branches will hold different observer states, even if > the differences are not observed or observable. So if some trivial > physical event happens to your body, such as the decay of a K 40 nucleus > in your foot, this would not be noticeable, or even particularly > observable even if you were looking for it. But such an event causes at > least two branches to form every instant -- one in which the decay has > occurred, and one in which it has not. And since this is a beta decay, a > neutrino is lost along the light cone in every case of decay. Perfect > recombination of the branches is, then, according to the above argument, > not possible. You might object that this decay in my toe did not alter > my conscious state -- that is correct, but there are now two copies of > the Moscow man as in step 3, and these can evolve in different > directions while each remains unaware of the existence of the other. > They can never recombine and compare diaries! > > > But this has to be tempered by the fact that any interaction will > > count as an observation, making super-exponentially hard to indeed > > recover a macroscopic superposition in the past, even the very close > > past. Of course, that might change the day we succeed in building a > > fault tolerant (topological perhaps) quantum computer. > > That will not help in the general case. Our future quantum computer > might be able to delay decoherence for some useful finite time, but that > still only retaines the superposition in the said computer, it does not > help with recombination of decohered branches in general. > > > Unfortunately, the T-rex missed them. yet, if a T-rex made a solid > > topological quantum qubit, in the state 0+1, we would have a past with > > 0, and a past with 1, as long as we don't look at it. I read, already > > a long time ago, some experimental evidence of temporal Bell's > > inequality going in this direction, and I think we don't even need to > > test them, as we get them with the usual Bell's inequality violation, > > if we accept special relativity (and some amount of physical realism > > (not the full materialism, to be sure). > > > The temporal version of Bell's inequality simply shows that the > ubiquitous non-locality of quantum entanglement occurs even over time. > And despite your protestations to the contrary, it is now generally > accepted that MWI does not remove the essential non-locality associated > with entangled states. 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. > > Bruce > -- You received this message because you are subscribed to the Google Groups "Everything List" group. 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