On Wed, Dec 22, 2021 at 4:54 PM Bruce Kellett <bhkellet...@gmail.com> wrote:
> On Wed, Dec 22, 2021 at 10:12 PM smitra <smi...@zonnet.nl> wrote: > >> On 21-12-2021 22:48, Bruce Kellett wrote: >> > >> > In general, that is not true. When both Alice and Bob set their >> > polarizers randomly while the particles are in flight, the fact that >> > Alice might get |up> tells her nothing about what Bob will get at some >> > randomly different polarizer orientation. You seem to be stuck with >> > thinking in terms of parallel polarizer orientations. >> >> It's not true only when the polarizers are orthogonal. Whenever the >> polarizers are not orthogonal, Alice will gain some amount of >> information about what Bob will find given the result of her >> measurement. For Bob, the probability of finding up or down are always >> 1/2, but after Alice makes her measurement, the conditional probability >> of what Bob will find, given her measurement result will not be equal to >> 1/2 for both outcomes if her polarizer was not orthogonal to that of >> Bob, so Alice will have gained information about Bob's measurement >> result. >> > > The conditional probability you refer to is defined only non-locally. > > >> In the MWI >> >> there is no such mysterious gain of information due to the correlation >> >> being caused by common cause when the entangled pair is created >> > >> > Rubbish. If there were a common cause, then that would have to depend >> > on the final polarizer orientations. And those are not known at the >> > time of creation of the entangled pair. You are, then, back with some >> > non-local influence (or retro-causation). >> >> The setting of the polarizers will be the result of some physical >> process. Whatever you specify for that process should be included in the >> analysis of the problem. But when you do so, it's inevitable that in an >> MWI analysis, there is not going to be any nonlocal effect other than >> trivial common cause effects. >> > > I see. So in desperation you resort to the superdeterminism escape. > I don't think Saibal was referring to superdeterminism? Or are you suggesting the MWI version of locality involves superdeterminism? If so that's wrong, superdeterminism involves some special constraint on initial conditions such that variables associated with the entangled particles (hidden or non-hidden) at the moment they are sent out in opposite directions are correlated in advance with the future choices of detector settings by the experimenters. MWI is not necessary for the understanding of the correlations of entangled > particles, as my simple example shows. In an actual experiment, the > analysis is identical in many-worlds and collapse models. The additional > worlds in MWI add nothing to the explanation. > They allow it to be local without superdeterminism, because the "matching" of local worlds can be done at a point in spacetime that has both experimenter's measurements in its past light cone, I gave you a toy model demonstrating how this can work in my post at https://www.mail-archive.com/everything-list@googlegroups.com/msg91022.html One way to think about local vs. non-local explanations is to imagine running a *simulation* of a Bell type experiment, using three or more separate computers that are each responsible for simulating what's going on in a localized region of space, say the location of experimenter A ('Alice'), the location of experimenter B ('Bob'), and the location of the emitter midway between them. The computer simulating the location of the emitter has to run some algorithm that assigns states to the two emitted particles (the algorithm is allowed to involve something like a random number generator, it need not be deterministic), and then it can transmit some or all of that information to the computers simulating the locations of Alice and Bob. Then once the computer simulating Alice's location receives that information about the state of the simulated particle arriving there, it runs some algorithm to decide what detector setting Alice selects, and what happens in that local region when she measures the particle with that detector setting (again we are allowed to use a random number generator), and the computer simulating Bob's location does the same. If we want to simulate a model of physics that obeys locality, then computers simulating events with a spacelike separation, like Alice performing her measurement and Bob performing his, cannot be in communication at the moment they each compute the local outcome at their own location. And if we want to avoid superdeterminism, the computer simulating the emitter cannot have any way to predict in advance what measurement setting Alice and Bob are going to use at their own locations--over many trials, the states it assigns to the particles on each trial cannot be statistically correlated with the future choices of detector settings by Alice and Bob on that trial. A simulation based on a MWI style toy model could respect both of these conditions, locality (no communication between computers when they are determining the results of events that are supposed to be at a spacelike separation, like Alice's measurement and Bob's measurement) and non superdeterminism (the computer simulating the emitter generates the states to send to Alice and Bob with no advanced knowledge of what detector setting they are going to choose in the future). The twist with the MWI is that the computers simulating Alice and Bob's locations don't generate unique outcomes, but rather a collection of different outcomes for different simulated copies of Alice and Bob. If all the copies then send signals reporting their results back towards the location of the emitter at the midpoint between them, then the "matching" between copies of Alice and copies of Bob can be done in the computer simulating the location of the emitter, when it has had time to receive signals traveling at the speed of light or slower from the Alice-computer and the Bob-computer, thus there is no violation of the locality condition. If for example there is a third observer, Carla, who is at the location of the emitter and waiting to receive signals from Alice and Bob, Carla only gets split into different copies receiving different possible messages from Alice and Bob at point when Alice's measurement and Bob's measurement are both in her past light cone. In contrast, no simulated rules for a *single* world could reproduce Bell inequality violating statistics in a situation like this with 3 distinct computers for each local region, not if we imposed the constraints of locality and non-superdeterminism described above. I think you claimed earlier that your fGRW idea is supposed to be local, but if you tried to formalize it sufficiently to build rules for a simulation subject to these constraints, it wouldn't be able reproduce Bell inequality violating statistics either. > They are, therefore, otiose, and MWI can be discarded. > > 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 view this discussion on the web visit > https://groups.google.com/d/msgid/everything-list/CAFxXSLQ5oroH0Gn9HemqBh_310s6zvDjsz81d%2BSGkt0Teq_biA%40mail.gmail.com > <https://groups.google.com/d/msgid/everything-list/CAFxXSLQ5oroH0Gn9HemqBh_310s6zvDjsz81d%2BSGkt0Teq_biA%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "Everything List" group. 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