On 14/11/2017 5:51 pm, smitra wrote:
On 13-11-2017 03:52, Bruce Kellett wrote:On 13/11/2017 12:15 pm, smitra wrote:On 12-11-2017 22:54, Bruce Kellett wrote:On 13/11/2017 7:19 am, smitra wrote:On 12-11-2017 11:21, Bruce Kellett wrote:On 12/11/2017 9:14 pm, smitra wrote:On 12-11-2017 07:57, Bruce Kellett wrote:On 12/11/2017 5:39 pm, Brent Meeker wrote:In Bruno's model the "influence at a distance" is determing which world you're in.If that means anything at all, it is still non-local because Bruno has to rule out the worlds in which angular momentum is not conserved; he has not shown how he can do this. If it is simply that you cannot find yourself in a world in which AM is not conserved, then that isjust an unabashed appeal to magic, since such worlds have not been shown not to exist. BruceThere are two correlated copies of Alice and Bob induced by the correlated spins, there is nothing nonlocal about that in the MWI. There is only a non- locality problem here if you assume a collapse interpretation of QM. In the MWI the correlation arises via an originally local common cause.There is no collapse assumption in anything that I have written about this scenario. What is the local common cause in MWI? Is that a localhidden variable? Such would work in the time-like case, but not in general -- that is Bell's result. But you have still not answered the fundamental question as to whatcauses Bob to necessarily measure spin down when Alice joins him witha spin-up result? What turns Bob's particle from an unpolarized to a polarized state so that the probabilities change from 50/50 to 100% for spin down? BruceBob's particle never changed due to anything Alice did in the MWI.Right.All that happened was that Alice got entangled with her particle (and not just Alice, her entire environment gets entangled with the state of her particle), which in turn was entangled with Bob's particle. So, Bob has the same probabilities for finding spin up or down,Right. In fact, when Alice_up meets Bob, and Alice_down meets Bob in the parallel universe, they can both tell Bob their result, and the direction in which they measured the spin. This makes no difference, since Bob is now entangled with the Alice's that have definite results. None of this makes any difference to the particle Bob measures, because, by the definition of locality, nothing has interacted with Bob's particle, so it must be in the same spin state as when it was produced.except that he can now measure the state of his particle by performing a measurement on Alice, by asking her what she found for her spin.That is not a measurement, that is making a prediction based on the conservation of angular momentum.It's not true that before Bob knows what Alice has found that only one of the two version's of Alice has arrived and that the information of her spin state is then already present in Bob's sector. This is not true in the MWI, decoherence simple means that you can't demonstrate the existence of the two versions of Alice via an interference experiment. But the inability to do so, doesn't by itself imply that only one version really exists.I don't think you have fully understood the scenario I have outlined. There is no collapse, many worlds is assumed throughout. Alice splits according to her measurement result. Both copies of Alice go to meet Bob, carrying the other particle of the original pair. Since they both have now met Bob, the split that Alice occasioned has now spread to entangle Bob as well as the rest of her environment. So there are now two worlds, each of which has a copy of Bob, and an Alice, who has a particular result. Locality says that Bob's particle is unchanged from production, so when he measure its spin, he splits into two copies, according to spin up or spin down. Since Alice is standing beside him, she also becomes entangled with his result. But Alice already has a definite result in each branch, so we now have four branches: with results 'up-up', 'up-down', 'down-up', and 'down-down'. However, only the 'up-down' and 'down-up' branches conserve angular momentum. How do you rule out the other branches? BruceThe splitting as an apparent nonlocal aspect to it, which is due to a common cause effect, the spins were entangled, and that entanglement happened when the spins were created due to a local interaction in the past.Yes, the entangled spin zero state was created in the past.If you then let Alice and Bob measure the space-like separated spins, they'll split up, and that happens in a correlated way, because the spins are correlated. It is just the MWI variant of Bertlmann's socksThat is not correct. The correlation you seems to be relying on is non-local once the particles have separated. As Bell pointed out, Bertlmann's socks are the wrong way to look at it! Besides, my variation was to look at time-like separated measurements so that one could keep explicit track of the splitting into separate worlds -- Bob and Alice, in their various versions, are always in the same world as each other. This eliminates the confusion that seems to being used to escape the fact of non-locality. BruceWhether Bob and Alice are really in the same worlds has to be considered very carefully within a precise model. If Bob is modeled as a robot with it's conscious thoughts defined by some bitstring stored in his electronic brain and similarly in case of Alice, then decoherence does not lead to a splitting in the bitstring sector. While the atoms out of which the logical units are made out of do decohere, the logical states are macroscopic states that can be "0" or "1" , and Bob as defined by its bistring specifying it's subjective state can be simply factored out of the complete quantum state.
You're floundering. Whether Bob and Alice are robots, bitstrings, or flesh and blood human beings, they get split by being entangled with the environment and the experimental result. What they know or don't know is entirely secondary. Once they are split by being entangled with a particular result, that is that -- nothing can change that, and their knowledge is secondary.
Within this model, Bob does not decohere until that time he is told what Alice has found.
That is simply not true. Decoherence is not subject to a particular person's knowledge. When Alice and Bob are next to each other, they are jointly entangled with a particular result.
Now, one may consider modeling Alice and Bob in some other way, such that the information what Alice has found does affect Bob's subjective state before he is told anything, and the probabilities will then change. So, here the splitting happens differently and there is again nothing nonlocal going on.Bob becoming localized in Alice's sector due to decoherence is a local process, I don't think there is disagreement about that.
Decoherence is a local process, of course. But Alice brings, say an 'up' result, to Bob, who is then entangled with the world in which Alice got 'up'. He then measures his part of the entangled pair, and both Alice and Bob together become entangled with the result -- 'up' or 'down' according to which branch one considers. If everything is completely local, there is no way to avoid the cases in which both Bob and Alice report 'up-up' or 'down-down'. Both of these possibilities violate angular momentum conservation. You have not given any account, local or other, that forbids these particular combinations of results from occurring.
But if Bob is completely localized then there is no splitting anymore, as the splitting has already happened. The whole concept of "splitting" is only an effective concept, in reality you only have the states of Bob, Alice, and the rest of the universe evolving in time due to local interactions, and Bob's and Alice's states are going to be correlated.
Sure, splitting is occasioned by entanglement with the results of the experiment (It is real, not simply 'effective'). This is local, and not under our control, but without non-locality for the entangled pair, some of the Alice-Bob combinations see impossible results.
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