On 3/06/2017 4:07 pm, smitra wrote:
On 03-06-2017 05:42, Bruce Kellett wrote:On 3/06/2017 11:38 am, smitra wrote:On 03-06-2017 02:10, Bruce Kellett wrote:On 3/06/2017 9:16 am, smitra wrote:In a single universe theory, this implies non-locality, because of the absence of local hidden variables. If local hidden variable were to exist then you could say that Alice and Bob where to find whatever they found anyway only due to their local interactions with the spins and polarizers. But with that ruled out, whatever Alice will find is information that just popped into existence when Bob made his measurement.What is ruled out by Bell's theorem is that local hidden variables can account for all possible correlations between the observations, Bell's theorem does not rule out the possibility of a local hidden variable explanation in special cases, like that of polarizers set at the same angle.Either there exists a local hidden variable theory from which QM can be derived or such a theory doesn't exist. If we assume that a local hidden variable theory underlies QM, then we find that regardless of the details, it cannot reproduce QM in certain cases. Bell inequalities can be derived for such theories that QM violates. Then, with QM confirmed and in particular the violations of the Bell inequalities conformed, we can then discard any local hidden variables theory.This means that even in cases that do not involve violations of Bell inequalities, we can still say that there are no local hidden variables that can explain the results in those experiments, because we've verified that there is no hidden variable theory that can reproduce QM.Reproduce all QM results. It does not rule out cases where local hidden variable, or common cause, explanations are possible.Then it suffices to consider a simple case of entanglement between two spins where in a single universe interpretation there is non-local behavior, e.g. Alice and Bob measuring the z-components of a system of two spin 1/2 particles in the singlet state. You may consider more complex cases where Alice randomly chooses another direction, but I remember from a previous discussion that this led to a disagreement about how to treat the source of this randomness.Not that I remember. There might have been disagreement about how the orientation of one polarizer became known at the other polarizer, but randmoness was never an issue.Ultimately all randomness has a quantum mechanical origin as pointed out in this article:https://arxiv.org/abs/1212.0953 There simply exists no known way to get to purely classical randomness.In the case where both polarizers have the same setting, the fact that Bob knows what Alice will find poses a problem for locality because local hidden variables have been ruled out.No it doesn't. There is a simple common cause explanation available for this case.Which doesn't eliminate non-local effects, see EPR.So, what Alice will find is random, new information will appear at her place after she measures the spin that didn't previously exist locally. But the fact that Bob could predict her result means that this information did exist at Bob's place. This demonstrates the non-locality aspect of single universe theories.No, it simply demonstrates that when there is a common cause explanation, there is no problem. Remember Bertlmann's socks.No, see EPR argument.Then in the MWI, Alice is identical in the two branches, so her measurement result is not predetermined as she is not yet located in either of Bob's branches. Her measurement result will do that.Not so. When the polarizers are aligned, Bob's measurement has a common cause with Alice's, so the agreement between results is determined from the start.Agreement via common cause does not eliminate non-locality problem in single universe theories, as EPR have shown.
The common cause I was referring to was the possibility that Alice getting '+' and Bob getting '-' might have been because the electrons left the common source in that state (not singlet). Not a good explanation, I admit, but akin to Bertlmann's socks.
If we change the set-up by letting Alice choose different setting of her polarizer to bring in the additional baggage of having to rule out hidden variables within the same experiment so that non-locality arguments have to be re-argued based on that, then that's not going to add anything but confusion.It might confuse you, but until you can do this you have not achieved a local explanation of the general case in which the polarizers are at arbitrary angles to each other. It seems that you are making valiant attempts to avoid the case where the shit hits the fan. Good luck with that, but I am not fooled. BruceIt's totally irrelevant, but I'll write out the GHZ state case with 3 entangled spins and 3 observers that measure the x and y components of the spins to demonstrate step by step that there is no problem in the MWI, while in the CI there is a non-locality problem later when I have time.
No need to go to GHZ. Doing it for the EPR singlet state and arbitrary polarizer directions would be enough.
But in general, the EPR thought experiment proves you wrong, the common cause there does not save you from non-local effects in single universe collapse theories. Then Bell proved EPR wrong in the sense that, there is no local hidden variable way out of non-locality without violating QM.Then, if we assume that QM is correct then there is not even the need to actually go about verifying the violation of Bell's inequalities. All that this verification does is that it rules out local hidden variables theories, regardless of whether or not QM is actually true or not (suppose it get's falsified, then you would still have falsified all local hidden variables theories).
OK. So your argument is that since experiment confirms the violation of local hidden variables, and theory of QM that give the standard results cannot have local hidden variables. Since MWI gives the standard QM results, it cannot have local hidden variables. I agree with this. But this does not prove that the theory is local, and that is what the argument is about. You are claiming that MWI gives a local explanation for the violation of the Bell inequalities. I dispute this, claiming that MWI is just as non-local as any other interpretation of QM.
I looked back at the previous discussion of non-locality in MWI on the list in 2016. Rather than rehashing a lot of the same ground, I have put together some of the comments and arguments I made last year; I think many of these are still relevant. I attach a pdf of these extracts from last year's discussion.
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everything2016.pdf
Description: Adobe PDF document
