An abstract long-distance correlation experiment

[stevendaryl]

Under these conditions I want to discuss what the human Alice
knows about Bob's results after she has completed her experiments.

My claim is that she knows nothing definite at all.

For the results Bob gets depend on what he is doing, and she is not
informed about the latter. At best she can draw conditional inferences
''If Bob's pointer position was set to ... then his results were ...''.

Okay, but consider the case in which Alice and Bob agree ahead of time what their detector settings will be. For example, they decide to measure spins (or polarizations--I can't remember which one) along the same axis. In that case, Alice's measurement tells her exactly (modulo detection loopholes) what Bob's measurement result will be.

So then we're in the situation where, it seems to me, there are two possibilities:

1. Either Bob's measurement result was fixed before Alice did her measurement (that is, her measurement just informed her about a pre-existing situation), or
2. Alice's measurement affected Bob; it made his situation go from some superposition or mixed state of possibilities to a definite, single possibility.

I think both possibilities are weird and implausible, given everything else that we know about QM and relativity. You seem to be claiming that quantum field theory alone allows us to say that future measurement results are determined now, by the detailed state of the entire universe, and that the probabilities only reflect our lack of knowledge about these details. That seems wildly improbable to me.

If I have a single electron that is in the state "spin-up in the z-direction", then does quantum mechanics have a definite answer to the question "Will it be spin-up or spin-down in the x-direction 10 seconds from now?" It definitely does not. It only gives probabilistic answers. I don't see a difference in principle if you let the system become more complex, to include measuring devices and human scientists, and you let the question change from "Will the electron be spin-up in the x-direction?" to "Will the macroscopic system be such that there is a record of measuring spin-up in the x-direction?"

I understand that classically, systems with a huge number of degrees of freedom can be in metastable state, and that small perturbations can push it over into a discrete number of more stable "pointer states". But I don't think it is at all appropriate to borrow results from classical mechanics here. There is a huge difference between the classical and the quantum state in that superpositions don't exist, classically. So if I delicately balance a coin on its edge, and I perturb it, it will either land on "heads" or it will land on "tails". There is no state corresponding to "a superposition of heads and tails". In quantum mechanics, there is such a state. So the argument that the metastable system will end up in one or the other state just doesn't go through, quantum mechanically.

So I think it's wildly improbable that QM can be made deterministic through the use of metastable states.

 

[ddd123]

Or suppose that Alice and Bob are machines set up by the experimenter. At that point there's no talk about assuming unseen phenomena, the experimenter looks at the correlator results later on and finds them weird.

 

[A. Neumaier]

but consider the case in which Alice and Bob agree ahead of time what their detector settings will be. For example, they decide to measure spins (or polarizations--I can't remember which one) along the same axis. In that case, Alice's measurement tells her exactly (modulo detection loopholes) what Bob's measurement result will be.

... if he keeps the agreement and the detectors work properly. Alice cannot know whether this will be the case. Thus her knowledge is still conditional. But causality is only about what actually happens, not about what happens if...

Or suppose that Alice and Bob are machines set up by the experimenter. At that point there's no talk about assuming unseen phenomena, the experimenter looks at the correlator results later on and finds them weird.

The experimenter doesn't even have to look at the correlated results.

By the same assumptions that allow Alice to know what will happen on Bob's side, Norbert knows already all future correlations - against all causality understood in a naive way.

But suppose that we grant that there is no causal barrier for Norbert to know the correlations that the results of Alice and Bob will have. In this case, what is good for Norbert will even more be good for Alice, who is in the future cone of Norbert. Therefore, in this case there is no causal barrier for her to know of Bob's results. What remains of the weirdness?

The conclusion is that anything seemingly acausal in the class of experiments considered is not due to the material aspects of Nature but to the intelligence of an observer.

But the nonlocal nature of intelligence is familiar from ordinary experience: The use of models and their predictions do not respect causality. We can model and predict what happens in the interior of a black hole although no information is supposed to escape from there. We can model and predict the interior of the sun at any time although we'll never receive direct signals from there. We can model and predict collision or noncollision of comets with the Earth in the far future, although it is not in our past light cone. We can predict the correct local clock time of our twin light years away in his accelerated relativistic journey.

 

[ddd123]

But suppose that we grant that there is no causal barrier for Norbert to know the correlations that the results of Alice and Bob will have. In this case, what is good for Norbert will even more be good for Alice, who is in the future cone of Norbert. Therefore, in this case there is no causal barrier for her to know of Bob's results. What remains of the weirdness?

Keeping the optical illusion analogy, the point is that there's no illusion, it's really happening. That's like saying that Norbert walks up the stairs in Escher's ladder:

He then gets back where he started. But since he's done that already, he knew that was going to happen. So he should find that non-weird.

I don't think so...

 

[A. Neumaier]

it seems to me, there are two possibilities:

1. Either Bob's measurement result was fixed before Alice did her measurement (that is, her measurement just informed her about a pre-existing situation), or
2. Alice's measurement affected Bob; it made his situation go from some superposition or mixed state of possibilities to a definite, single possibility.

In a deterministic universe, the first is the case. The conditions of the density matrix of the universe at any given time determines everything at any later time. Nobody found anything weird in this at the time of Laplace, where in place of the density matrix we had the positions and momenta of all atoms in the universe. Now we have a far more realistic theory, but whatever let Laplace conclude that a deterministic universe is rational and not weird is applicable in the same way to the density matrix of the universe, which, according to orthodox (shut up and calculate) quantum mechanics, evolves in a deterministic way.

 

[stevendaryl]

... if he keeps the agreement and the detectors work properly. Alice cannot know whether this will be the case. Thus her knowledge is still conditional. But causality is only about what actually happens, not about what happens if...

You can certainly reason about the case where Alice and Bob are not humans, but are machines, programmed to perform particular measurements at a particular time. You are saying in that case that Alice's and Bob's results are determined ahead of time. That does not seem plausible to me. It also doesn't seem to really address the issue of quantum weirdness. As I said, in the case of a single electron in the state of being spin-up in the z-direction, QM does not in any way support the idea that it has a definite (but unknown) spin in the x-direction. It says the opposite. As the systems under consideration become more and more complex, it becomes more and more difficult to see them as superpositions of possibilities, and so, for practical matters, we go to density matrix descriptions. These descriptions can be interpreted via classical probability, that the system is actually in this state or that one, but we just don't know which, and the density matrix reflects our lack of knowledge. But looking back, you can see that you introduced density matrices as a practical matter of dealing with very large, complex systems. There is nothing essentially different between the case of a single electron and the case of a detector, other than complexity. So an interpretation that treats the two as fundamentally different (in the electron case, certain questions have no definite answers--it can be in a superposition of possibilities, in the detector case, we say that all macroscopic questions have definite answers--we just don't know what they are ahead of time) seems like cheating to me.

I would say that this effort to show that QM is not weird really amounts to the fact that we have ad hoc rules for dealing with it, and they pretty much work. It doesn't actually make the weirdness go away, or explain it. It just says you don't have to worry about it.

 

[stevendaryl]

In a deterministic universe, the first is the case. The conditions of the density matrix of the universe at any given time determines everything at any later time. Nobody found anything weird in this at the time of Laplace,

That's because in classical mechanics, dynamical variables have definite values at all times, and we can understand statistical phenomena as being due to our lack of knowledge about the precise state of the universe. That interpretation doesn't work in quantum mechanics. If an electron is in the state of being spin-up in the z-direction, then it's not the case that it has a definite (but unknown) spin in the x-direction.

So the comparison with non-weird classical mechanics just heightens how weird QM is. It doesn't lessen it.

 

[A. Neumaier]

That interpretation doesn't work in quantum mechanics.

But it works in quantum field theory. I'll start a new thread about it; please continue any discussion of a deterministic universe there. It has no direct relations with what we are discussing here.

In this thread, the topic is solely the experiment depicted in the initial post, and a discussion of why, or the extend to which, it is weird (independent of any particular explanation of the quantum result).

 

[stevendaryl]

But it works in quantum field theory.

I don't believe it. Quantum field theory certainly can be used to describe a single electron in a state with a definite spin-up in the z-direction, can't it? In that case, is it, or is it not true that the spin in the x-direction is indeterminate?

 

[A. Neumaier]

Quantum field theory certainly can be used to describe a single electron in a state with a definite spin-up in the z-direction, can't it?

Please continue this part of the discussion (relating to quantum field theory) not here but in the following thread:
https://www.physicsforums.com/threads/particles-in-quantum-field-theory.855770/

 

[A. Neumaier]

OK, since no more comments on Stage 2 or 3 are coming in, I declare their discussion closed. My conclusion of the two stages is that the weirdness in the present experimental setting has two sources:

• inappropriate use of relativistic thinking in an otherwise nonrelativistic context (simultaneity) - see post #119
• contradictory assumptions in the theoretical inference of knowledge (which uses quantum mechanical reasoning) and of weirdness (which uses classical reasoning) - see post #173.

The main concern that creates the weirdness seems to be the apparent conflict with causality. This is the topic of the final Stage 4, which begins with the next post #187.

 

[A. Neumaier]

What I did in the leading post #2 to stevendaryl's blueprint is the following:

• I made the language fully precise.
• I removed from the basic setting any model-dependent features.
• I separated all anthropomorphic language from the physics.

The result is a fully local description of how Nature appears to any particular non-intelligent observer in the sense that everything visible to an observer (Norbert, Alice, Bob, Yvonne, or you) was caused in the past light cone of this observer.

Indeed, Alice knows her results (which follow causally from Norbert's signals) and has uncheckable beliefs about Bob's results. Symmetrically, Bob knows his results (which follow causally from Norbert's signals) and has uncheckable beliefs about Alice's results. Yvonne has access in her past light cone to a bigger quantum system (consisting of both Alice's and Bob's results) and hence finds that her correlation analysis satisfy causality, too.

The conclusion is that anything nonlocal in this class of experiments is not due to the material aspects of Nature but to the intelligence of an observer - which generates beliefs about unseen results far away.

But the nonlocal nature of intelligence is familiar from ordinary experience: The use of models and their predictions do not respect causality. We can model and predict what happens in the interior of a black hole although no information is supposed to escape from there. We can model and predict the interior of the sun at any time although we'll never receive direct signals from there. We can model and predict collision or noncollision of comets with the Earth in the far future, although it is not in our past light cone. We can predict the correct local clock time of our twin light years away in his accelerated relativistic journey.

Closer to our everyday experience, we can know the time our bus goes tomorrow morning, although this is an event not in today's past light cone. Of course, we cannot be 100% sure, since the bus might be delayed due to an accident, say. But by the same token, the intelligent Alice behind the dumb robot Alice - cannot know Bobs's measurement for sure since perhaps he is unable to measure anything due to a power outage, a defect transistor, or the limited efficiency of his detector.

If we look closer of what kind of knowledge Alice can infer we find no true knowledge but only conditional knowledge of the form ''if the detector was working properly and Bob did this or that then his results are this or that''. But for lack of knowledge of whether the hypothesis holds she knows nothing about the actual observations - the color of Bob's light (if any).

On the other hand, even Norbert has conditional knowledge about the future. He knows that if Alice and Bob choose the same settings there will (given the particular signals Norbert is sending) be coincident lights of opposite color. Again, he knows nothing definite since Norbert knows neither the color nor whether or not Alice and Bob will (or can) really choose the same setting.

Given that Norbert's action is known, an intelligent Alice at spacetime position $x$ can infer conditional knowledge about what Bob observes at spacetime position $y$ under the assumption that Bob's preparation satisfies a property $p$ only when she has a theory that predicts Bob's observation from information in Alice's past light cone together with property $p$. This is the proper form causality takes for the potential local knowledge at any space-time position $x$, and it is valid for each agent in this experiment, if assumed intelligent.

If this theory is quantum mechanics Alice gets exactly the quantum mechanical (and in practice observed) predictions, and hence true conditional knowledge. If Alice uses instead a classical theory with local hidden variables she gets predictions (and hence ''apparent knowledge'') that contradict Bob's observation - as you as analyzer find out after the completed experiment that includes the comparison of the results of Alice and Bob. There is no way to distinguish inferred true knowledge from inferred apparent knowledge except by

• either waiting till material causality allows one to compare the data,
• or inconclusive plausibility reasoning that leads to endless debates.

Taken together there is nothing intrinsically strange or acausal about the results of Bell-type experiments. For every local observer, the correlations are nonlocal in the sense of relativity theory only as long as they are inferred by intelligent reasoning rather than known by measurement. The extent to which this intelligent reasoning produces true knowledge depends on the extent to which the underlying theory on which the reasoning is based reflects the true properties of Nature.

Therefore the weirdness perceived in certain interpretations of quantum mechanical experiments is fully explained by the futility to assess the weirdness by classical mechanics although it is already well-known that one needs quantum mechanics to be consistent with experiment. Indeed, quantum mechanical experience is already silently assumed in the traditional interpretations of Bell-type experiments, since without it Alice cannot infer anything conditionally about Bob's experiments (except perhaps fake knowledge obtained from local hidden variable theories.)

 

[stevendaryl]

Indeed, Alice knows her results (which follow causally from Norbert's signals) and has uncheckable beliefs about Bob's results.

I don't find this very satisfying. Instead of human Bob, and human Alice, we could program a robot Bob and a robot Alice to choose a particular detector setting at a particular time. Then if the programs are such that robot Alice chooses the same detector setting as robot Bob, she will know exactly what result robot-Bob will get. What kind of knowledge is that? Is that knowledge about the value of a predetermined result? Maybe. But that seems pretty weird. What if their instructions were encrypted, so that it takes up until the moment right before the measurements to figure out what setting to choose?

I don't find this attempt to show why QM is not weird to be at all effective. It seems to me that it amounts to: Let's ignore some of the details that make QM different from non-weird theories. Then it doesn't look so weird, does it? That's right. It's the details that make it weird, that make non-weird ways of understanding what's going on implausible (if not impossible).

 

[A. Neumaier]

she will know exactly what result robot-Bob will get

She is a robot, how can she know anything?

You can program her of course in any way you like and pretend that certain facts represented in the program represent knowledge. But she has no way to check whether her knowledge conforms to reality. This can only be checked by Yvonne.

Checking whether conjectured knowledge is correct is possible only in a causal way. A rational understanding of Nature only needs this much. Nature has facts but no conjectured knowledge, hence need not be causal about the latter. It is only (human or artificial) intelligence that can form well-informed conjectures about things that happen at causally unreachable locations in spacetime.

 

[A. Neumaier]

I don't find this attempt to show why QM is not weird to be at all effective.

At this stage I am not trying to show why QM isn't weird, just why it isn't acausal.

I had already explained the weirdness in Stage 3 - it is due to measuring with double standards. This produces true weirdness, not only in the quantum domain but also in the classical domain whenever in a complex situation the double standards are not clearly recognized and eliminated.

 

[stevendaryl]

She is a robot, how can she know anything?

Well, what does it mean to "know" something, and can machines be said to know things, and are humans a kind of machine, or not? Those are philosophical issues that I would hope do not need to be resolved in order to understand quantum mechanics.

 

[A. Neumaier]

Well, what does it mean to "know" something, and can machines be said to know things, and are humans a kind of machine, or not? Those are philosophical issues that I would hope do not need to be resolved in order to understand quantum mechanics.

Well, if you use these terms in an argument about robots, you better clarify the terms. I deliberately avoided the need to do that by making sure my experimental setting only involved dumb robots without any trace of artificial intelligence. My arguments only used the informal concept of knowledge every educated human is acquainted with.

 

[stevendaryl]

Well, if you use these terms in an argument about robots, you better clarify the terms.

You were using such terms:

Indeed, Alice knows her results (which follow causally from Norbert's signals) and has uncheckable beliefs about Bob's results.

I was questioning why you considered her information about Bob to be "uncheckable beliefs", rather than "knowledge". Is that an important distinction? I brought up robot-Bob only because in that case, Bob's detector settings are predetermined, so are as "knowable" as anything else. Is it important that Alice's beliefs about Bob are uncheckable?

 

[stevendaryl]

I know that this thread was prompted partly by me, but I am not finding it at all enlightening. It seems that the questions that I most want answers to are being dismissed and/or ignored, rather than answered. That's fine--if they don't have good answers that are easily explainable, then so be it. But then, what, exactly is the point of the thread? I thought it was to address the various lingering qualms about the foundations of quantum mechanics.

I am dropping out of this discussion.

 

[A. Neumaier]

I brought up robot-Bob only because in that case, Bob's detector settings are predetermined, so are as "knowable" as anything else.

They are indirectly knowable (by an intelligent Alice behind the dumb robot Alice, as I had mentioned when I started to talk about knowledge) by inference, not by observation. They are not truly knowable: If a power outage causes Bob's preprogrammed detector not to respond during the whole experiment, Alice ''knows'' nonexistent results.

I am dropping out of this discussion.

Maybe you can look at just one more response (to be written; it takes a bit more preparation) where I relate everything to Lorentz invariance (points 1 and 2 in maline's summary of Bell's reasoning). I wouldn't have invested a lot of time in this discussion without having something definite to contribute that I learned during the discussion, and that makes a real difference (in my opinion). After that I'll have said what can be said from my point of view, and the thread can be abandoned or closed.

 

[A. Neumaier]

Bell locality is intended as a stronger assumption than "relativity holds". It is justified (for me) by:
1.The intuition that causation occurs from past to present to future, in an objective sense. Since relativity does not define regions outside the light-cone as "past" or "future", causation should be confined to this cone.
2.FTL signalling would imply a possibility of sending messages to the past, and I see no fundamental reason why signals should differ from other forms of influence.
Therefore, to me, the violation of locality is weird.

Too strong assumptions in a no-go theorem considerably weaken the relevance of the conclusion. Once upon a time, von Neumann had disproved the existence of all hidden variable theories to the satisfaction of everyone. His seemingly innocent assumptions were proved to be ridiculously narrow only when Bohm discovered his hidden variable theory.

If you say that the assumptions are deliberately too strong, it means that there might be sensible causally impeccable hidden variable theories deliberately excluded by this too strong assumptions.

 

[A. Neumaier]

To be able to discuss why I find the assumptions of Bell far too strong, let me distinguish two kinds of causality: extended causality and separable causality. Both kinds of causality are manifestly local Lorentz invariant and imply a signal speed bounded by the speed of light. Here a signal is defined as a dependence of measured results at one spacetime point caused by a preparation at another spacetime point.

Separable causality is what is assumed in Bell-type theorems, and is thereby excluded by the standard experiments (assuming that all other conditions used in the derivation of such theorems hold in Nature). On the other hand, extended causality is far less demanding, and therefore is not excluded by the standard arguments.

To define these two kinds of causality I use the following terminology. A point object has, at any given time in any observer's frame, properties only at a single point, namely the point in the intersection of its world line and the spacelike hyperplane orthogonal to the observer's 4-momentum at the time (in the observer frame) under discussion. An extended object has properties that, in some observer frames at some time depend on more than one space-time position. A joint property is a property that explicitly depends on more than one space-time location within the space-time region swept out by the extended object in the course of time.

Both kinds of causality agree on the causality properties of point objects (''point causality'') but differ on the causality properties of extended objects. Extended causality takes into account what was known almost from the outset of modern quantum mechanics - that quantum objects are intrinsically extended and must be treated as whole. This is explicitly expressed in Bohr's writing (N. Bohr, On the notions of causality and complementarity, Dialectica 2 (1948), 312. Reprinted in Science, New Ser. 111 (1950), 51-54.):

Phrases often found in the physical literature as 'disturbance of phenomena by observation' or 'creation of physical attributes of objects by measurements' represent a use of words like 'phenomena' and 'observation' as well as 'attribute' and 'measurement' which is hardly compatible with common usage and practical definition and, therefore, is apt to cause confusion. As a more appropriate way of expression, one may strongly advocate limitation of the use of the word phenomenon to refer exclusively to observations obtained under specified circumstances, including an account of the whole experiment.

(Thanks to Danu for locating this quote!)

Here are the definitions:

• Point causality: Properties of a point object depend only on its closed past cones, and can influence only its closed future cones.
• Extended causality: Joint properties of an extended object depend only on the union of the closed past cones of their constituent parts, and can influence only the union of the closed future cones of their constituent parts.
• Separable causality: Joint properties of an extended object consist of the combination of properties of their constituent points.

I believe that only extended causality is realized in Nature. It can probably be derived from relativistic quantum field theory. If this is true, there is nothing acausal in Nature. In any case, causality in this weaker, much more natural form is not ruled out by current experiments.

 

[ddd123]

Suppose that extended causality is correct. This seems to be related with your abandoning the particle picture. Then we treat these quantum objects as fields: but the fields we know propagate at the speed of light in vacuum, at most. It is so in QFT. So to account for Bell pairs phenomenology with the fields-only picture, you need to hypotesize an instantaneous field: is it an extra field, kind of like the Higgs field? Or should we extend the behavior of the fields we currently use to somehow instantaneously jump in certain cases?

 

[A. Neumaier]

the fields we know propagate at the speed of light in vacuum, at most. It is so in QFT.

Do you really know QFT well enough to be able to claim this?

Quantum field theory is the theory underlying QED, so it agrees with the quantum mechanical predictions about photons and electrons, if the latter are derived consistent with QED (which they should, to be reliable). Hence the QFT predictions violate separable causality (because of the Bell experiments). One desn't need additional fields for this - just standard QED, together with a thorough understanding of quantum correlation functions.

Relativistic classical fields propagate at the speed of light in vacuum, yes, but quantum fields are different.

If you want to discuss this further, please do it, citing this post, in the corresponding thread on QFT - after you have read and understood which measurable information is obtainable from QFT.

 

[Mentz114]

To be able to discuss why I find the assumptions of Bell far too strong, let me distinguish two kinds of causality: extended causality and separable causality.
..
..
I believe that only extended causality is realized in Nature. It can probably be derived from relativistic quantum field theory. If this is true, there is nothing acausal in Nature. In any case, causality in this weaker, much more natural form is not ruled out by current experiments.

I find this argument very believable.

I venture that extended causality does not require changing of properties at a distance but only changing probability (ie interference) 'at a distance'.

Given that phase is unobservable but a phase change can change probability, why can't the change propagate at phase velocity ?

 

[A. Neumaier]

why can't the change propagate at phase velocity ?

I don't have any arguments.

But I have the intuition that extended (rather than separable) causality should be related to the fact that $N$-particle wave functions propagate in an $6N$-dimensional phase space (rather than in 6 dimensions). Together with knowing that extended causality is a mathematically natural concept and cannot be ruled out by Bell theorems, this is evidence strong enough to motivate me to research the subject.

 

[ddd123]

Given that phase is unobservable but a phase change can change probability, why can't the change propagate at phase velocity ?

That's very interesting, are there explicit constructions?

 

[Mentz114]

That's very interesting, are there explicit constructions?

I don't know but I am exploring some ideas.

 

[Mentz114]

I don't have any arguments.

But I have the intuition that extended (rather than separable) causality should be related to the fact that $N$-particle wave functions propagate in an $6N$-dimensional phase space (rather than in 6 dimensions). Together with knowing that extended causality is a mathematically natural concept and cannot be ruled out by Bell theorems, this is evidence strong enough to motivate me to research the subject.

Indeed.

I also believe that Bell's theorems are classical and cannot be applied to QT. If they were quantum theoretical there would be an inconsitency because it is clear that QT ignores them.

 

[stevendaryl]

Indeed.

I also believe that Bell's theorems are classical and cannot be applied to QT. If they were quantum theoretical there would be an inconsitency because it is clear that QT ignores them.

Yes, Bell's theorems are about local realistic theories, which quantum mechanics clearly is not. What some people (Einstein, for example) hoped was that quantum mechanics could somehow be obtained from a local realistic theory (in the same sort of way that statistical mechanics can be understood as Newtonian physics plus ignorance of the details of the current state of the system), but Bell showed that it couldn't.

 

[wle]

What some people (Einstein, for example)

...and Bell. I don't know why this is so often left out, but Bell was clearly dissatisfied with the standard formulation of quantum physics. (Personally, I suspect that this has a lot to do with how "realism", "contextuality", "counterfactual definiteness", etc. somehow entered the discussion: people had their own stories about what type of theory quantum physics is and how it is different from the "classical" theories Bell was considering instead of noticing that, for Bell, quantum physics just did not qualify as a well-defined theory in the first place. Given this, it also makes a bit more sense that Bell could be worrying about a potential conflict between quantum physics and relativity after modern "relativistic" QFTs had been developed.)

 

[Lord Crc]

Yvonne has access in her past light cone to a bigger quantum system (consisting of both Alice's and Bob's results) and hence finds that her correlation analysis satisfy causality, too.

The conclusion is that anything nonlocal in this class of experiments is not due to the material aspects of Nature but to the intelligence of an observer - which generates beliefs about unseen results far away.

I'm not following this conclusion. In which way does Yvonnes find that her correlation analysis satisfies causality?

 

[A. Neumaier]

I'm not following this conclusion. In which way does Yvonnes find that her correlation analysis satisfies causality?

Both Alice's and Bob's decisions lie in her past light cone. Hence no faster than light information transfer was necessary to produce her observation of the joint correlations.

 

[Lord Crc]

Both Alice's and Bob's decisions lie in her past light cone. Hence no faster than light information transfer was necessary to produce her observation of the joint correlations.

Surely you don't suggest the values that Alice and Bob measure change between the time they're measured and sent, and received by Yvonne? If not, how can Yvonne draw the conclusion you claim she will? That's the part I'm struggling to see.

 

[A. Neumaier]

Surely you don't suggest the values that Alice and Bob measure change between the time they're measured and sent, and received by Yvonne? If not, how can Yvonne draw the conclusion you claim she will? That's the part I'm struggling to see.

She draws the conclusion based on the recordings transmitted to her, at a time when everything observed lies in her past light cone. The joint recordings were done on an extended object (since they involve observations in two far away space-time points), hence the rules of extended causality in the sense of post #197 apply, not those of separable causality.

This leaves a lot of leeway for the required nonlocal influences consistent with causality and relativity. To limit the speed of information transfer by the speed of light, Nature only has to make sure that the correct quantum mechanical correlations exist whenever they can be compared some time in the future. I know that Nature achieves this (and we can understand this) by means of quantum mechanics, without any violation of (the just newly born concept of) extended causality, while our classical intuition is currently trained only to think in terms of (the now 110 years old concept of) separable causality in the sense of that post, and hence has difficulties to grasp what really happens.

Inferred but unchecked knowledge does not have to respect causality constraints. I had demonstrated this by reminding us that we can infer what happens inside a black hole where causality forbids that any information leaks out.

The kind of nonlocal correlations permitted by extended causality is not acausal. In quantum mechanics we work all the time with extended coherent objects, and their extendedness implies these nonlocal correlations. For example, in theory, we work a lot with plane waves. They have perfect correlations (at integal wavelength) and anticorrelations (at halfintegal wavelength) at distances of the size of the diameter of the visible universe. So it shouldn't be a surprise that some of the states we can actually prepare inherit from this infinitely extended nonlocality some observable residue. We cannot prepare objects in a perfect plane wave state, but we can prepare (with sufficiently hard work that requires a lot of experimental ingenuity) less perfect nonlocal states extended over very long distances which show in experiments not the perfect (anti)correlations but at least good approximations of it - sufficiently good to prove that separable causality is not realized in nature.

But extended causality seems to be realized in Nature. In any case, extended causality is enough to guarantee that signal speed is limited by the speed of light. It is Lorentz invariant and respects everything we should reasonably expect from a consistent theory of relativity.

Note that I do not deny nonlocal correlations but only the appropriateness of the overstrict causality assumption in Bell-type reasoning! Bell's no-go theorem (like earlier von Neumann's no-go theorem) is a valid mathematical theorem but because of its overly strong assumptions it tells nothing about the compatibility of quantum mechanics with relativity.