Undergrad EPR and Non-Locality - For and Against

[PeterDonis]

There MUST be some channel of some kind of communication, influence, selection or otherwise between Alice's setting or particle, and Bob's setting or particle.

This is one possible interpretation, but not the only one. As just one other possibility: you mention the "acausal" interpretation. On that interpretation, there is no "channel" at all; there is just a global constraint imposed by conservation laws that leads to correlations that violate the Bell inequalities.

 

[DrChinese]

Of course, they can be individually manipulated, because the valid theory to describe them, QED, is a local (sic!) relativistic QFT and the interaction with the measurement device are local. The photons are indistinguishable always.

I guess your definition of indistinguishable is different than mine. Experimentally, we can have entangled photons that have never interacted in the past, as they are from different sources. They are clearly identifiable, and can have markers (such as their wavelength) that identify them uniquely. They can be passed through filters to confirm that uniqueness.

If QFT is local and realistic, as has been said a hundred times, it's wrong. So tell me exactly where you consider it non-realistic? Non-realistic meaning there is observer dependent reality.

 

[PeterDonis]

your example is half of the equation we want to explain. That is, the perfect correlations.

If perfect correlations were all that needed to be explained, a local hidden variable model would work just fine. The thing that rules out local hidden variable models is the degree of imperfect correlation over the entire range of possible relative angles between Alice's and Bob's measurement axis. The Bell inequality violations occur at such intermediate angles, not at the "perfect correlation" angles.

The antirealist says that there is ONLY reality to those observables that can be simultaneously predicted.

"Antirealist" seems like a strange term to describe this viewpoint, since as you say right here, the viewpoint asserts that there is reality to a certain set of observables. Maybe "limited realist" would be better?

 

[DrChinese]

This is one possible interpretation, but not the only one. As just one other possibility: you mention the "acausal" interpretation. On that interpretation, there is no "channel" at all; there is just a global constraint imposed by conservation laws that leads to correlations that violate the Bell inequalities.

In acausal type interpretations, there is of course a channel. It involves a connection between the different quantum objects/measurements in spacetime. But there is no relevant "now" as we experience it. Those connections don't follow the usual path from past to present to future. But the connection is limited to c.

And yet, to us, it looks exactly as described by the Weinberg quote.

 

[PeterDonis]

In acausal type interpretations, there is of course a channel. It involves a connection between the different quantum objects/measurements in spacetime.

Now it's my turn to ask you for a reference. "Acausal", as I understand it, means there is no need for any such "channel", because the correlations are not being produced by a "mechanism". "Channel" and "mechanism" are causal concepts; an acausal interpretation does not use them.

 

[PeterDonis]

Those connections don't follow the usual path from past to present to future. But the connection is limited to c.

This looks to me like the transactional interpretation, which, while it is certainly different, is not "acausal". It just reinterprets "causal" to include causal connections going along light cones from future to past, as well as from past to future.

 

[DrChinese]

If perfect correlations were all that needed to be explained, a local hidden variable model would work just fine. The thing that rules out local hidden variable models is the degree of imperfect correlation over the entire range of possible relative angles between Alice's and Bob's measurement axis. The Bell inequality violations occur at such intermediate angles, not at the "perfect correlation" angles.

I followed my 1. (perfect correlations) with 2. (Bell) in my post #90 which you referenced. I would have thought you would have picked that up when I started my post with "Agreed, your example is half of the equation we want to explain." The Bell inequalities being the other half. But mine was a long post, so that is understandable.

Your purpose seems to contradict and refute everything I say without addressing the issue at hand, which is: QFT has a problem explaining how Alice and Bob sync up without any nonlocal action occurring. I thought that's what bhobba's thread was about. Every physicist here is aware of the essential experimental difficulty (which you seem to both accept and deny in alternating posts), and which no theory (or interpretation) explains to the satisfaction of the physics community. Certainly QFT does not, or there would be no Interpretations subforum.

 

[DrChinese]

This looks to me like the transactional interpretation, which, while it is certainly different, is not "acausal". It just reinterprets "causal" to include causal connections going along light cones from future to past, as well as from past to future.

I didn't bring up "acausal"; you did in your post #91. So I can't really respond to this. All I said was:

There MUST be some channel of some kind of communication, influence, selection or otherwise between Alice's setting or particle, and Bob's setting or particle. That being the mystery "channel" we seek to solve.

I have no idea what that is or looks like, and don't pretend I do (like some people). But how you can read that as me asserting anything more than the obvious, as embodied by the words of Weinberg (or Zeilinger or a hundred others who can state the mystery better than I) - I don't follow your reasoning. The combination of perfect correlations and violation of Bell Inequalities in hundreds of experiments point to the same mystery: quantum nonlocality exists and remains unexplained*.*Beyond the predictions of QM themselves. I am unaware of any predictions QFT makes on entanglement that go beyond QM, but that probably is a reflection of my ignorance on QFT... which is why I am participating here.

 

[PeterDonis]

QFT has a problem explaining how Alice and Bob sync up without any nonlocal action occurring.

I disagree with this as you state it. I would state it as: Some interpretations of QFT have a problem explaining how Alice and Bob sync up, according to proponents of other interpretations.

QFT predicts the correlations between Alice and Bob just fine. There is no problem in matching theoretical predictions to experimental results. As far as I can tell, everyone agrees on that.

What everyone does not agree on is interpretation--on what kind of story to tell in ordinary language in order to "explain" why the experimental results are what they are. When you say "without any nonlocal action occurring", this "nonlocal action" thing does not come from experimental results. There is no big light that goes on when an experiment is finished that says "Nonlocal action here!" There are just correlations that violate the Bell inequalities. Which, as I have just noted, QFT predicts just fine. And there is no label pasted on any part of the QFT calculations that says "Nonlocal action here!" There are just the predictions of Bell inequality violation. Does that count as "nonlocal"? Depends on whose definition of "nonlocal" you want to use. There is no single definition that everyone agrees on.

Every physicist here is aware of the essential experimental difficulty (which you seem to both accept and deny in alternating posts)

The difficulty is not experimental. It's not even theoretical, since the theory predicts the experimental results just fine. The difficulty is with interpretation.

which no theory (or interpretation) explains to the satisfaction of the physics community

Theories aren't in the business of explaining things by generating interpretations. Theories are in the business of explaining things by making predictions that differ from the predictions of other theories, and then winning out over those other theories when the predictions get tested by experiment.

"Interpretations", IMO, are what we fall back on when we don't have competing theories to test. We don't have competing theories of QM/QFT that make different predictions. (Some have been tried--for example, the GRW stochastic collapse theory--but so far no alternatives have panned out.) We just have a set of predictions that seem counterintuitive, and a bunch of different interpretations that tell different stories about why the predictions come out the way they do, but which can't be tested against each other because they all make the same predictions.

My personal preference in such a situation is not to insist on any particular interpretation, but to accept the fact that, for now, we don't have a single, accepted way to explain why quantum systems behave the way they do. Trying to concoct more interpretations won't help. What will help is figuring out more follow-on theories that extend QM/QFT in different ways, making different testable predictions, and then testing them. That's how progress is made in science.

But if anyone wants to insist on talking about interpretations, at the very least, one has to recognize that, since, as you agree, there is no interpretation that explains things to everyone's satisfaction, all interpretations have to be treated as personal opinions that cannot be tested by experiment. And that means that you cannot talk about features of particular interpretations as if they were requirements that all intepretations had to meet. "Nonlocal action" is a feature of particular interpretations. It is not a requirement that every interpretation has to meet.

 

[DrChinese]

I disagree with this as you state it. I would state it as: Some interpretations of QFT have a problem explaining how Alice and Bob sync up, according to proponents of other interpretations.

I agree with most of the rest of your post, except the above.

The only people I have ever heard talk about QFT as if it is "problem solved" are a few people here. In the hundreds of published papers I read on entanglement and interpretations annually, this has never been said by anyone else. Of course, I haven't read every paper written by a long shot. So maybe I missed it.

A better summary of the position of the physics community is embodied by Weinberg's words, which you so readily dismiss. As he literally wrote the textbook on QFT, I think his words are worthy of repeating:

Weinberg: "There is a troubling weirdness about quantum mechanics. Perhaps its weirdest feature is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas. ... Of course, according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem - it just doesn't change the density matrix."

So no, this is not a case of "he said she said". And no, I am not appealing to authority - just using his words so you will quit semantically disassembling every statement I make (and a few I didn't make). There are no interpretations of QFT that solve Weinberg's "weirdness" [what I call "mystery"]. As best I can tell, it is silent; and further, I don't see where it adds anything past what earlier QM does. The physics community considers this an open problem (to the extent they think about it at all).

I'd love to see more discussion of the relevant points of bhobba's original question - but with at least some acknowledgment that there is an open problem to discuss. And my apologies to @bhobba to the extent I steered the discussion away from the OP. Certainly the opposite of my intent.

@PeterDonis : If you assert that QFT resolves the mystery, then please say so outright; and I will bow out of this discussion. Else acknowledge what the rest of the physics community believes, which is that this is THE open question (mystery, weirdness) by which all candidate theories and interpretations are judged by. That being the quantum nonlocality of entanglement, post Bell.

 

[RUTA]

I didn't bring up "acausal"; you did in your post #91. So I can't really respond to this. All I said was:

There MUST be some channel of some kind of communication, influence, selection or otherwise between Alice's setting or particle, and Bob's setting or particle. That being the mystery "channel" we seek to solve.

I have no idea what that is or looks like, and don't pretend I do (like some people). But how you can read that as me asserting anything more than the obvious, as embodied by the words of Weinberg (or Zeilinger or a hundred others who can state the mystery better than I) - I don't follow your reasoning. The combination of perfect correlations and violation of Bell Inequalities in hundreds of experiments point to the same mystery: quantum nonlocality exists and remains unexplained*.*Beyond the predictions of QM themselves. I am unaware of any predictions QFT makes on entanglement that go beyond QM, but that probably is a reflection of my ignorance on QFT... which is why I am participating here.

You guys post much faster than I can keep up with, but let me jump in here and use this post to make my point again. The mystery is, as DrChinese states, the existence of spacelike separated correlations that violate Bell’s inequality to 8 sigma (that’s the largest I’ve seen anyway) as predicted by QFT. Bell stated famously that ”correlations cry out for explanation.” While QFT predicts these Bell-inequality-violating correlations, QFT is just a mathematical formalism. So, unless you accept QFT’s mathematical description of the correlations as “explanatory,” QFT doesn’t solve the mystery, it only predicts it. Here is a nice quote from Fuchs in 2016:

Compare [quantum mechanics] to one of our other great physical theories, special relativity. One could make the statement of it in terms of some very crisp and clear physical principles: The speed of light is constant in all inertial frames, and the laws of physics are the same in all inertial frames. And it struck me that if we couldn't take the structure of quantum theory and change it from this very overt mathematical speak -- something that didn't look to have much physical content at all, in a way that anyone could identify with some kind of physical principle -- if we couldn't turn that into something like this, then the debate would go on forever and ever. And it seemed like a worthwhile exercise to try to reduce the mathematical structure of quantum mechanics to some crisp physical statements.

And Smolin in 2020:

So, my conclusion is that we need to back off from our models, postpone conjectures about constituents, and begin again by talking about principles.

Both of these quotes evidence the frustration of the foundations community with a debate that shows no sign of ever ending.

 

[PeterDonis]

according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem

If Weinberg were posting here, I'd ask him the same question I have already asked you, and which you have refused to answer: how can the state vector of the other subsystem "change" when it didn't even have a well-defined state vector before the measurement? (And, for extra credit, why should we care about the state vector rather than the density matrix, which everyone agrees does not change?)

This is what I mean about appealing to authority. You just read Weinberg's words and repeat them as though they were unchallengeable. I don't accept anything anyone says as unchallengeable. I apply my own intelligence to what they say to see what I think. And if I have a question about what is said, I ask it. Of course it's up to you whether you answer the question I ask, or even pay attention to it. But if you won't answer it, then I have no reason to change my opinion. (I might not even if you do answer it, but there's only one way to find that out.)

If you assert that QFT resolves the mystery

I have said no such thing. I have not said anything resolves any mystery. Others might have, but I haven't.

I have simply asked what seem to me to be obvious questions about assertions that others have made. The reason I ask is to see if anyone will answer them, and if so, what their answers are.

 

[PeterDonis]

Here is a nice quote from Fuchs in 2016

And Smolin in 2020

Can you give links? (If they were already given earlier in this thread, a pointer or at least a rough idea of the post number would be fine.)

 

[RUTA]

Can you give links? (If they were already given earlier in this thread, a pointer or at least a rough idea of the post number would be fine.)

Sorry, you can find those quotes with citations in this Insight.

 

[PeterDonis]

Sorry, you can find those quotes with citations in this Insight.

Ok, thanks!

 

[vanhees71]

I guess your definition of indistinguishable is different than mine. Experimentally, we can have entangled photons that have never interacted in the past, as they are from different sources. They are clearly identifiable, and can have markers (such as their wavelength) that identify them uniquely. They can be passed through filters to confirm that uniqueness.

If QFT is local and realistic, as has been said a hundred times, it's wrong. So tell me exactly where you consider it non-realistic? Non-realistic meaning there is observer dependent reality.

Photons are indistinguishable bosons. That's a very well defined mathematical property and has nothing to do with any interpretational issues.

I don't know, what "realistic" means. It's an undefined philosophical expression. QFT is by construction a local relativistic QFT. Local means that

(a) the Heisenberg-picture field operators transform under proper orthochronous Poincare transformations as their classical analogues. The transformation property is determined by the unitary representation of this group the corresponding field operators refer to.

(b) the Hamilton density commutes with any operator representing a local observable if the arguments of these are space-like separated

Further one assumes

(c) the Hamilton is bounded from below. By convention the ground-state energy (vacuum energy) is choosen to be 0.

From this the spin-statistics theorem follows, i.e., you have to quantize the field theory either as bosons (if the spin of the field is integer) or fermions (if the spin of the field is half-integer). This implies indistinguishability of the quanta represented by these fields.

There is no observer-dependent reality. States of quantum systems are well-defined, and I don't know, what entanglement swapping, which you seem to refer to once more, has to do with all this. There's no problem to describe entanglement swapping within standard local relativistic QFT.

Once more the claim that the outcome of local measurements of B's photon would instantaneously be changed by the outcome of local measuremnts by A on a photon pair prepared in a Bell state, cannot be made within local relativistic QFTs. What B measures are unpolarized photons. What A measures are unpolarized photons. There's no dependence of this result on the A's or B's choice which observable they measure (e.g., they can measure the polarization of the photons in arbitrary directions). Only if they compare measurement protocols one finds the correlations between A's and B's outcome of measurements as predicted by QFT for the photons prepared in this entangled state. This implies the violation of Bell's inequality and related properties of this kind. There's not tension between Einstein causality and locality in the sense of relativistic local QFTs whatsoever. Inseparability is implied by any kind of quantum theory, including local relativistic QFTs and no contradiction with locality either.

I call this "realistic", because it's in accordance with all known very accurate experiments on this issue. For me there's no other notion of "reality" which makes sense as a scientific rather than a philosophical notion.

 

[vanhees71]

The physics community considers this an open problem (to the extent they think about it at all).

A small very specialized part of the philosophy-of-physics community considers this as an open problem. Practitioners of standard relativistic QFT either don't care about this apparent problem or, if they are interested in it, don't see at as a problem, because relativistic QFT describes very well all observations. Particularly experimentalists in the quantum optics and/or AMO community I know, have a very clear scientific probabilistic-only interpretation of QT. I don't know anybody who claims there is a contradiction between the usual notion of locality of relativistic QFTs (see my previous posting) and experiments.

 

[Sunil]

I take the view correlation does not imply influences.

Yes, this is a message which has to be explained and repeated many times in everyday life and usual applications of statistics that there exist other explanations for correlations. Namely, common causes.

But in the case of the Bell inequalities, those common cause explanations are excluded by the theorem, common causes cannot lead to violations of the Bell inequalities. So, in this case correlation does imply causal influences.

 

[Sunil]

I don't know, what "realistic" means. It's an undefined philosophical expression. QFT is by construction a local relativistic QFT.

The notion of realism is precisely defined in the EPR paper (the EPR criterion of reality) as well as in various presentations of the theorem. It is what is necessary to obtain the formula

$E(AB|a,b) = \int A(a,b,\lambda) B(a,b,\lambda) \rho(a,b,\lambda) d \lambda$

Then you have to exclude superdeterminism to get

$E(AB|a,b) = \int A(a,b,\lambda) B(a,b,\lambda) \rho(\lambda) d \lambda$

and Einstein causality in a stronger form than what holds in QFT to obtain

$E(AB|a,b) = \int A(a,\lambda) B(b,\lambda) \rho(\lambda) d \lambda$

There is Stone's theorem that for every Boolean algebra there exists a space $$\Lambda$$ so that it can be embedded into the Boolean algebra of subsets of this space, so the existence of this space of possible states of reality is a triviality too. But, trivial or not, the requirement that

$E(AB|a,b) = \int A(a,b,\lambda) B(a,b,\lambda) \rho(a,b,\lambda) d \lambda$

has to follow from realism is precise enough.

Once more the claim that the outcome of local measurements of B's photon would instantaneously be changed by the outcome of local measuremnts by A on a photon pair prepared in a Bell state, cannot be made within local relativistic QFTs.

That QFT is not realistic is nothing in favor of QFT. Whatever, realistic interpretations of QFT exist too.

I call this "realistic", because it's in accordance with all known very accurate experiments on this issue. For me there's no other notion of "reality" which makes sense as a scientific rather than a philosophical notion.

You are not free to use an already established scientific notion for something completely different.

 

[PeterDonis]

and Einstein causality in a stronger form than what holds in QFT to obtain

What follows here is the same equation as the one just above it, so I think you have a typo somewhere. Perhaps you meant the equation after this sentence to read as follows?

$E(AB|a,b) = \int A(a,\lambda) B(b,\lambda) \rho(\lambda) d \lambda$

 

[Sunil]

If Weinberg were posting here, I'd ask him the same question I have already asked you, and which you have refused to answer: how can the state vector of the other subsystem "change" when it didn't even have a well-defined state vector before the measurement? (And, for extra credit, why should we care about the state vector rather than the density matrix, which everyone agrees does not change?)

If it didn't even have a state vector before, but has one after, the description of the state of the system has, obviously, changed. From "state without state vector, described by a non-pure density matrix" to "state with state vector". So this is at best a minor quibble about words, which can be used only if someone has sloppily applied "state vector" to that part of an entangled state.

(And, for extra credit, why should we care about the state vector rather than the density matrix, which everyone agrees does not change?)

Because we are interested to find out whatever can be found out. And the density matrix, of course, changes if it takes into account the additional information measured at the place far away. So, it is unchanged only for those who don't have the complete information.

What follows here is the same equation as the one just above it, so I think you have a typo somewhere. Perhaps you meant the equation after this sentence to read as follows?

$E(AB|a,b) = \int A(a,\lambda) B(b,\lambda) \rho(\lambda) d \lambda$

Of course. Thanks, corrected.

 

[Sunil]

Since this is the interpretations forum: if you want strict adherence to Einsteinian locality, why not simply adopt one of those interpretations that feature retrocausal effects, or time symmetry, or similar (including the acausal Relational Blockworld).

Retrocausality would be an exceptional, extraordinary hypothesis. But for an extraordinary claim one would need extraordinary evidence.

There is no such extraordinary evidence, given that what is known allows for a simple and straightforward causal explanation using a hidden preferred frame.

Non-locality of such preferred frame explanations is not a serious objection, given that non-local theories may appear as limits of local theories when the limiting speed goes to infinity, and against local theories with a hidden preferred frame with limiting speed greater than c there is nothing to object in principle. Moreover, we have enough strong hints that GR is no fundamental (it has singularities, it is non-renormalizable, thus, good for an effective QFT but not more), so that there is also no base for assuming that symmetries of GR should be really fundamental, instead of being approximations.

 

[PeterDonis]

the description of the state of the system has, obviously, changed

This is the same problem. If the system didn't even have a well-defined state vector before, then if "description" means "state vector", it didn't have a well-defined "description" before the measurement. "The description has changed" is not a valid description of this situation.

From "state without state vector, described by a non-pure density matrix" to "state with state vector".

No, that's not correct. As has already been pointed out, the density matrix of Bob's particle does not change when Alice makes her measurement. It only changes when Bob makes his measurement.

this is at best a minor quibble about words

No, it isn't. See above.

the density matrix, of course, changes if it takes into account the additional information measured at the place far away

Bob can only take that information into account when he learns it. He doesn't learn it when Alice makes her measurement. He only learns it when the information about Alice's result reaches him, at the speed of light (or slower, depending on how the information is transmitted).

it is unchanged only for those who don't have the complete information.

Which includes Bob. (And if we consider Alice's knowledge about Bob's measurement result, we get the same issue.)

 

[Sunil]

This is the same problem. If the system didn't even have a well-defined state vector before, then if "description" means "state vector", it didn't have a well-defined "description" before the measurement. "The description has changed" is not a valid description of this situation.

If the system doesn't even have a well-defined state vector, the "description" is the density matrix.

No, that's not correct. As has already been pointed out, the density matrix of Bob's particle does not change when Alice makes her measurement. It only changes when Bob makes his measurement.

No. Once Alice has made the measurement, the state of Bob's part of the system has a state vector, which is identified by the measurement made by Alice and its result.

Bob can only take that information into account when he learns it.

So what? The state of a system is not the state of the mind of Bob, but what is known about the preparation in the whole universe.

Which includes Bob. (And if we consider Alice's knowledge about Bob's measurement result, we get the same issue.)

But in QT we consider the whole state, which includes the knowledge about the preparation procedure available at a given moment of time in the whole universe.

 

[Lynch101]

The only people I have ever heard talk about QFT as if it is "problem solved" are a few people here.
...
There are no interpretations of QFT that solve Weinberg's "weirdness" [what I call "mystery"]. As best I can tell, it is silent; and further, I don't see where it adds anything past what earlier QM does. The physics community considers this an open problem (to the extent they think about it at all).
...
If you assert that QFT resolves the mystery, then please say so outright

I've read through the whole thread and I was going to start a separate thread with questions about what I've read here, but I thought that might be redundant. If it would be better to start a new thread that's no problem, I can do that, but I'll post here for the time being. I'll outline my understanding and maybe someone can - if they are so inclined - point out my error.

It reads to me as though you and others are talking past each other somewhat. It reads as though two different questions are being addressed:
@DrChinese: Is nature non-local?
Others: Is QM non-local?

The argument from the others appears to be: it cannot be stated that QM is non-local because QFT is, by construction, a local theory. Alice and Bob both make local measurements and can only share information at or slower than, c. The observed correlations are precisely those predicted by QFT, therefore it cannot be said that QM necessitates non-locality.

@DrChinese, you seem to be speaking to the explanation of those correlations which, as you have said, QFT does not provide.

To me, this reads as though it goes back to the question of completeness. Is QFT a complete description of nature? If the mathematics of QFT are taken to be nothing more than a computational tool, which make predictions about the outcomes of experiments, then it would seem not to be complete. That doesn't mean that it is incomplete in the manner that EPR suggested, but in the more general sense of not describing the system prior to its interaction with a measurement device.

Lee Smolin puts it better than I can.

Edited to shorten quote:

despite the successes of quantum field theory, many physicists, beginning with Einstein, have wanted to go beyond it to a deeper theory that gives a complete description of each individual experiment--which, as we have seen, no quantum theory does. Their searches have consistently found an irreconcilable conflict between quantum physics and special relativity.

As long as we’re just checking the predictions of quantum mechanics at the level of statistics, we don’t have to ask how the correlations were actually established. It is only when we seek to describe how information is transmitted within each entangled pair that we need a notion of instantaneous communication.

 

[bhobba]

But in the case of the Bell inequalities, those common cause explanations are excluded by the theorem, common causes cannot lead to violations of the Bell inequalities. So, in this case correlation does imply causal influences.

It proves classical correlations do not apply unless you have non local influences. But QM is a generalised probability model where the statistics of classical correlations do not hold, nor would you a-priori expect them to. It does not imply there must be non-local influences - simply that it is not standard probability theory, but a generalisation of it that does not always have the same properties.

Thanks
Bill

 

[RUTA]

To me, this reads as though it goes back to the question of completeness. Is QFT a complete description of nature? If the mathematics of QFT are taken to be nothing more than a computational tool, which make predictions about the outcomes of experiments, then it would seem not to be complete. That doesn't mean that it is incomplete in the manner that EPR suggested, but in the more general sense of not describing the system prior to its interaction with a measurement device.

Lee Smolin puts it better than I can.

Edited to shorten quote:
despite the successes of quantum field theory, many physicists, beginning with Einstein, have wanted to go beyond it to a deeper theory that gives a complete description of each individual experiment--which, as we have seen, no quantum theory does. Their searches have consistently found an irreconcilable conflict between quantum physics and special relativity.

As long as we’re just checking the predictions of quantum mechanics at the level of statistics, we don’t have to ask how the correlations were actually established. It is only when we seek to describe how information is transmitted within each entangled pair that we need a notion of instantaneous communication.

It looks like you're referring to two distinct objections to QT based on Bell state entanglement -- locality and completeness -- as in this Mermin article in Physics Today. I argue against both objections in this Insight.

 

[PeterDonis]

Once Alice has made the measurement, the state of Bob's part of the system has a state vector, which is identified by the measurement made by Alice and its result.

Perhaps this is true in your preferred interpretation. But you cannot just help yourself to the claim that it is true, period.

The state of a system is not the state of the mind of Bob, but what is known about the preparation in the whole universe.

Same response as above.

in QT we consider the whole state, which includes the knowledge about the preparation procedure available at a given moment of time in the whole universe.

Same response as above. Also note that this viewpoint can't possibly be valid in a relativistic model, since there is no such thing as a unique "given moment of time in the whole universe" in a relativistic model.

 

[Lynch101]

It looks like you're referring to two distinct objections to QT based on Bell state entanglement -- locality and completeness -- as in this Mermin article in Physics Today. I argue against both objections in this Insight.

Thanks RUTA, I had read your insight article and as much as I understood it, it was very interesting, but I don't quite understand it to the level of seeing how it refutes the challenge of incompleteness.

I'm talking about a specific attribute of "anti-realist" interpretations of the mathematics, which says that the mathematics is simply a tool which makes predictions about the outcomes of experiments. If this is the case, then the mathematics does not describe the system prior to its interaction with the measurement device, which would render it an incomplete description of nature.

 

[Sunil]

Perhaps this is true in your preferred interpretation. But you cannot just help yourself to the claim that it is true, period.

No, this is true in the minimal interpretation, and the minimal interpretation is not my preferred interpretation. What is true in the minimal interpretation is true in QT.

The minimal interpretation does not contain relations between particular measurements and observers localized somewhere. We have measurements, defined by operators, and a measurement leads to a change of the state. There is no measurement process, we have a state before the measurement $|\psi\rangle = \sum_\alpha c_\alpha| \psi_\alpha\rangle$, then a miracle the measurement happens, and after this we have a state after the measurement, which is, with probability $|c_\alpha|^2$, the state $|\psi_\alpha\rangle$. The minimal interpretation does not contain any notion of space, which could be used to define something like a local measurement, or a measurement made only in the environment of Alice but not of Bob.

There are also no different notions of time in the minimal interpretation, there is one time, and the change of the state during a measurement happens immediately, without any delay, and without any specification how it happens.

Also note that this viewpoint can't possibly be valid in a relativistic model, since there is no such thing as a unique "given moment of time in the whole universe" in a relativistic model.

It can be valid in a relativistic model with a hidden preferred frame. So, it is only in conflict with a particular interpretation of relativity, which forbids, for some metaphysical reasons, a hidden preferred frame. Such a conflict with relativistic metaphysics does not change QT in its minimal interpretation.