The notion of locality in (Quantum) Physics should be clearly defined

[PeterDonis]

TL;DR Summary: The notion of locality in (quantum) physics should be clearly defined

It has not been clarified what "locality" means.

I don't understand why you continue to harp on this issue when I have repeatedly pointed out the two different definitions of "locality" that are used in the literature.

(1) "Locality" means spacelike separated measurements commute. (This is the definition you use.)

(2) "Locality" means the Bell inequalities are not violated. (This is the definition many papers in the literature on Bell's theorem use.)

The solution to this "problem" is not to keep complaining about "locality" not being clearly defined, but to stop using the term "locality" whenever there is a question about what meaning is intended. Just use the more technical terminology that says exactly what you mean. Why is this a problem? I don't understand.

 

[PeterDonis]

As far as I know that's also the meaning Bell gives always to this notion in his work on Bell inequalities

No, Bell actually uses "locality" in a third sense which also appears elsewhere in the literature, though not as often:

(3) "Locality" means the joint probability distribution for results of spacelike separated measurements factorizes, so that the result of measurement A is only dependent on the setting of measuring device A, and the result of measurement B is only dependent on the setting of measuring device B.

What confuses many people about Bell-type experiments is that those experiments violate "locality" in the sense of #3 above, and of my #2 in my previous post just now, but do not violate locality in the sense of #1, i.e., the standard QFT sense that you always use the term. That is indeed an issue that gets a lot of discussion, but not having a clearly defined meaning for the term "locality" is not the issue that needs discussion.

 

[Demystifier]

What is the measurement postulate in QT? Do you mean Born's rule?

Yes. The Born rule says: When an observable is measured, then the probability is such and such. In classical physics there is no postulate beginning with "When an observable is measured ...".

 

[Demystifier]

The solution to this "problem" is not to keep complaining about "locality" not being clearly defined, but to stop using the term "locality" whenever there is a question about what meaning is intended. Just use the more technical terminology that says exactly what you mean. Why is this a problem? I don't understand.

This is a big problem for @vanhees71 because it would imply that there is some notion of locality which is violated, which would contradict his philosophical prejudices, for which he cannot accept that they are philosophical prejudices, because it would ruin his self-picture of a pure scientist who despises philosophy. To preserve his self-picture of a pure scientist, he is forced to use philosophy to argue that it's the others, not himself, who use philosophy. Vicious circle.

 

[Fra]

but not having a clearly defined meaning for the term "locality" is not the issue that needs discussion.

I agree. So what is it that needs discussion? I fear we can't even agree on that?

I have a feeling some of us here, thinks there is really nothing to be discussed. While others agree that currenty theory describes experiments but does not offer a satisfactory causal explanation of the mechanisms.

Bells "hidden variables" would have been an "explanation", but as it does't work at least in a sensible manner, the quest is still open. But Vanhees as far as I recall, don't see it as a scientific task to "explain" beyond "describe" and consider it a philosophical question?

/Fredrik

 

[PeterDonis]

So what is it that needs discussion?

Off topic for this thread since this thread is about locality.

We have had plenty of other threads about violations of the Bell inequalities.

 

[Demystifier]

Off topic for this thread since this thread is about locality.

The central assumption of the thread starter is that there is only one notion of locality that can be clearly defined. But it seems that most of us agree that there is more than one notion of locality. The need to distinguish them is particularly important in order to understand how relativistic QFT can be compatible with the Bell theorem, i.e. how can QFT be both "local" and "nonlocal" at the same time. This idea deeply disturbs the thread starter, but apparently not the rest of us.

 

[vanhees71]

This is a big problem for @vanhees71 because it would imply that there is some notion of locality which is violated, which would contradict his philosophical prejudices, for which he cannot accept that they are philosophical prejudices, because it would ruin his self-picture of a pure scientist who despises philosophy. To preserve his self-picture of a pure scientist, he is forced to use philosophy to argue that it's the others, not himself, who use philosophy. Vicious circle.

No, it's about a clear terminology, not prejudices. It's just a fact that local QFTs are local in the sense that there are no causal connections between space-like separated events. That's mathematics, not philosophy.

To understand the issues about entanglement and Bell tests (which are for sure not off-topic in this thread, because they are at the heart of these questions) one must clearly distinguish between "non-locality" (i.e., theories, which violate the causality constraint of relativistic space-times and allow for some "action at a distance" or at least "faster-than-light signal propagation) and the possibility inseparability, i.e., strong correlations between outcomes of measurements on far-distant parts of a quantum system. All presumed problems go away, as soon as one sticks to the clear probabilistic meaning of the minimal statistical interpretation.

It's really unfortunate that discussion about these interesting issues inevitably end up in a morast of philosophical confusion. I should have known this and not started this thread in the first place :-(.

 

[Demystifier]

inseparability, i.e., strong correlations between outcomes of measurements on far-distant parts of a quantum system. All presumed problems go away

You can call it inseparability, but it doesn't make the problem go away because the question is what causes the correlation. I know that your answer is that local interactions cause the correlation, but there is no purely scientific paper, without any philosophy, which confirms your thesis that local interactions are enough to cause the correlation. If you claim there is, cite it!

as soon as one sticks to the clear probabilistic meaning of the minimal statistical interpretation.

Does Ballentine stick to clear probabilistic meaning of the minimal statistical interpretation? He concludes that quantum theory is nonlocal.

 

[Fra]

the possibility inseparability, i.e., strong correlations between outcomes of measurements on far-distant parts of a quantum system.

This is what I was thinking about, as the possible question we should be discussing(perhaps not in this THREAD, but as an answer to Peters suggestion that the topic here isn't worth discussing), ie is how can we understand this inseparability?

I wonder if those using different definitions of locality, can even agree to ask this question in principle?

/Fredrik

 

[Demystifier]

It's really unfortunate that discussion about these interesting issues inevitably end up in a morast of philosophical confusion. I should have known this and not started this thread in the first place

Agreed.

 

[AndreasC]

No, Bell's "realism" is a clear scientific postulate

I will never understand why there has to be this arbitrary dichotomy between what concept is "philosophy" and what concept is "physics". Physics is in the business of understanding the world. Philosophy is in the business of understanding the world. Obviously there is overlap. Introducing distinctions based on what is "clear" and what is "vague" or "confusing" says more about the aptitude of the one making the distinction towards these concepts than about the concepts themselves. I say this with all due respect and not specifically directed at you, since from your posts you are clearly one of the most knowledgeable and helpful people on this forum. However this attitude in my humble opinion is not only unhelpful and unjustified, but also disrespectful to both the people working at the intersection of physics and philosophy (which includes Bell) as well as philosophers in general.

 

[vanhees71]

Physics in the business of describing the world as it is objectively observed and not in the business to say, how the world should behave, according to some vague philosophical assumptions (as the EPR idea about, how a physical theory should look like).

The great achievement by Bell was to find a clear scientific definition of what a "realistic local hidden-variable theory" is, which can be obejctively tested by experiments. The clear result of all the Bell tests is that local (!) Q(F)T is the correct description. For me the case is closed. What philosophers make out of this result, I can't say, because it's again a mess of vague statements. As the history of science and philosophy shows, the best philosophy can do is to use the results of science to a posteriori analyze the consequences for our worldview which goes beyond the objectively observable of the sciences.

The history of science also shows that the greatest breakthrough was the deliberation of science from philosophy in the Renaissance, i.e., a clear distinction between objective (theoretical and experimental) natural sciences and subjective philosophical (and religious!) opinion, is key to that success.

 

[WernerQH]

If there were a pair of "magic dice" that show the same face whenever and wherever they are thrown, would you call this an "inseparable" but still "local" system? Would the mystery go away if you explained that the correlations arise because they came from the same factory?

@vanhees71 : At the heart of your local explanation of EPR/B experiments seem to be "magic dice". Don't you find anything mysterious about such dice? Isn't Born's rule relevant in QFT? Have I misunderstood your position?

 

[vanhees71]

There are no "magic dice". There's just relativistic QFT. As in any QT Born's rule, i.e., the probabilistic meaning of the quantum states, is one of the basic postulates and part of the minimal statistical interpretation, which just accepts one of the great objective findings about Nature, i.e., that it is inherently random. There's no magic involved, it's just a fact about Nature, which contradicts the outdated worldview of deterministic classical physics, and that's why many philosophers even today cannot accept it and build up all kinds of presumed paradoxes against this inevitable conclusion.

 

[AndreasC]

Now you are playing with words, like a philosopher

But since when is that what philosophy is?

 

[vanhees71]

Since when isn't this precisely what philosophy is ;-))?

 

[martinbn]

You can call it inseparability, but it doesn't make the problem go away because the question is what causes the correlation. I know that your answer is that local interactions cause the correlation, but there is no purely scientific paper, without any philosophy, which confirms your thesis that local interactions are enough to cause the correlation. If you claim there is, cite it!

What other interactions? The fundametal ones are: gravitational, electromagnetic weak and strong. All of them are local. Can you cite a paper that discribes those nonlocal one that you suggest are there?

ps Why do you assume that correlation implies causation?

 

[vanhees71]

You can call it inseparability, but it doesn't make the problem go away because the question is what causes the correlation. I know that your answer is that local interactions cause the correlation, but there is no purely scientific paper, without any philosophy, which confirms your thesis that local interactions are enough to cause the correlation. If you claim there is, cite it!

The correlation is caused on the preparation procedure. E.g., you produce an entangled photon pair by using parametric down conversion and selecting the corresponding photon pairs. It's all due to local interactions between the electromagnetic field (laser) and the particles building up the BBO crystal.

Does Ballentine stick to clear probabilistic meaning of the minimal statistical interpretation? He concludes that quantum theory is nonlocal.

Where? Ballentine's book is about non-relativistic QM, and there's not even a notion of locality. I'm pretty sure that what he calls "non-local" is just what's described by entanglement, i.e., correlations between far distant parts of a quantum system prepared in an entangled state. As I say, it's unfortunate that physicists are not more strict in the notion of locality, which leads to a great confusion in the literature.

 

[WernerQH]

There are no "magic dice".

Good. At least we agree on something! But I'm still puzzling about how you explain the EPR/B correlations without invoking magic dice. Your account of QED is markedly different from how I think about QED. Perhaps you should explain in a little more detail how QED achieves its "magic", instead of merely pronouncing "There's just relativistic QFT". Or the glib remark "The correlation is caused on the preparation procedure."

In a typical EPR/B experiment there are two amplitudes for the two photons to travel from the source to the detectors. (I just had a look at Feynman's discussion of the decay of positronium in his Lectures on Physics.) And following Born's rule you take the squared modulus of the amplitudes to get the probability. Now the amplitude for a photon to go from A to B is just the complex conjugate of the amplitude for going from B to A, so you can read the squared modulus as the product of the amplitudes for a photon travelling from A to B, and another (anti-) photon going from B to A. That's how I understand QED's "magic": the propagators reach also into the backward light cones. It's a local description, but it requires also waves travelling backwards in time. (This is the core of the transactional interpretation.) From a previous discussion I remember that you disagree strongly with that. But how else can you uphold your cherished locality?

 

[gentzen]

The great achievement by Bell was to find a clear scientific definition of what a "realistic local hidden-variable theory" is, which can be obejctively tested by experiments. The clear result of all the Bell tests is that local (!) Q(F)T is the correct description. For me the case is closed. What philosophers make out of this result, I can't say, because it's again a mess of vague statements. As the history of science and philosophy shows, the best philosophy can do is to use the results of science to a posteriori analyze the consequences for our worldview which goes beyond the objectively observable of the sciences.

As a step towards clarifying Bell's own opinion on the relation of his great achievement to QFT, I carefully checked which articles in "Speakable and Unspeakable in Quantum Mechanics" (second edition, 2004) discuss QFT:
7 The theory of local beables (1975)
19 Beatles for quantum field theory (1984)
23 Against 'measurement' (1989)
24 La nouvelle cuisine (1990)

 

[gentzen]

To understand the issues about entanglement and Bell tests (which are for sure not off-topic in this thread, because they are at the heart of these questions) one must clearly distinguish between "non-locality" (i.e., theories, which violate the causality constraint of relativistic space-times and allow for some "action at a distance" or at least "faster-than-light signal propagation) and the possibility inseparability, i.e., strong correlations between outcomes of measurements on far-distant parts of a quantum system. All presumed problems go away, as soon as one sticks to the clear probabilistic meaning of the minimal statistical interpretation.

Also the "minimal statistical interpretation" is an interpretation, with limitations and paradoxical conclusions of its own. As an example for a limitation, I see no way how to apply it without a cut or something similar. As an example of a paradoxical conclusion, my "envisioned proof" that a qubit "in context" contains at most two classical bits of information relies on the minimal statistical interpretation:

I've no clue, how you think you can describe a qubit with two classical bits at all. A qubit can be in a continuity of states, for two classical bits you have only 4 states. So how can there be a one-to-one connection between them?

Also in this case, what I have to "check" to confirm that two classical bits can describe a single qubit "in context" are statistics of measurement results. ... In the end, this means that I work in the minimal statistical interpretation, and use it as my criterion for "checking" correctness of proposed "protocols".

 

[AndreasC]

Physics in the business of describing the world as it is objectively observed

You can't define what "objective" means without philosophy. You also can't possibly come up with a theory that does that.

On the other hand, what physics does do is... Model how the world SHOULD behave, according to various theories. Which is what you assert philosophy does, with the only difference that you call it "vague". Essentially you define philosophy as "vague". But that says more about your preferences than philosophy itself.

The great success of Bell was that he effectively demolished this supposed "purist" point of view, by pursuing a field that was previously supposedly "philosophy" and "not physics", and extracting results which no one could deny were physics, while also giving more food for thought to philosophy, thus showcasing yet again that there is no arbitrary distinction at all. It also goes completely contrary to your account of the history of science and philosophy, ie that philosophy at best comes after the physics. It didn't come after the physics at the very inception of physics, it didn't come after for Einstein's relativity, and it didn't come after for Bell's investigations. It always came before.

It's also wrong that philosophy and physics separated during the Renaissance. It happened much, much later, during the 19th century. Late/post renaissance "physicists" were, according to themselves, "natural philosophers". Who do you think of when you think of the "great names" of that era for physical science? Galileo, Newton, Leibniz and Descartes. Is it an accident that Leibniz and Descartes were also two of the biggest names in philosophy? What is the full name of Newton's Principia?

 

[AndreasC]

The clear result of all the Bell tests is that local (!) Q(F)T is the correct description. For me the case is closed.

I also find this statement bizarre. The Bell tests rule out some theories. They can't possibly show "local QFT" is "the correct description", unless you believe they somehow demonstrated local QFT is the final theory of physics. If not you must recognize that the case is not closed, and there are multiple possible descriptions.

 

[Demystifier]

Can you cite a paper that discribes those nonlocal one that you suggest are there?

Bohm 1952. Do you need the full reference?

ps Why do you assume that correlation implies causation?

The Reichenbach common cause principle.

 

[Demystifier]

As a step towards clarifying Bell's own opinion on the relation of his great achievement to QFT, I carefully checked which articles in "Speakable and Unspeakable in Quantum Mechanics" (second edition, 2004) discuss QFT:
7 The theory of local beables (1975)
19 Beatles for quantum field theory (1984)
23 Against 'measurement' (1989)
24 La nouvelle cuisine (1990)

In this context, I think 24 is the most important one, because it addresses precisely the point that QFT notion of locality is not the adequate one in the context of Bell theorem.

 

[Demystifier]

Where? Ballentine's book is about non-relativistic QM, and there's not even a notion of locality. I'm pretty sure that what he calls "non-local" is just what's described by entanglement, i.e., correlations between far distant parts of a quantum system prepared in an entangled state.

The relevant quotes from Ballentine are here.
https://www.physicsforums.com/threa...be-a-waste-of-time.910190/page-3#post-5734603

 

[martinbn]

Bohm 1952. Do you need the full reference?

Are you saying that he discovered a new interaction, one that is not any of the four known ones?

The Reichenbach common cause principle.

How exactly does this apply to the Bell test scenario?

 

[Morbert]

Indeed QFT is largely a red herring. Bell specifies a concern here when he says:

What is held sacred is the principle of "local causality" - or "no action at a distance." Of course, mere correlation between distant events does not by itself imply action at a distanct, but only correlation between the signals reaching the two places.[...]Einstein had no difficulty accepting that affairs in different places could be correlated. What he could not accept was that an intervention at one place could influence, immediately, affairs at the other.

QFT will predict the same correlations in EPRB experiments that QM will predict. The question is can we consistently treat these correlations the same way we would correlations in a classical theory: without positing a causal influence between spacelike separated events.

 

[Demystifier]

Are you saying that he discovered a new interaction, one that is not any of the four known ones?

You should know by now that I am too smart to be tricked by such a cheap play with words. "Discovered" in an experimental sense does not mean the same as "discovered" in theoretical physics or math. Yes, he made a theoretical discovery, he discovered a mechanism that can describe a "force" which can explain quantum correlations.

How exactly does this apply to the Bell test scenario?

https://arxiv.org/abs/1311.6852