Quantum nonlocality and "spooky action at a distance"

[Fra]

Keep in mind that the actual outcomes themselves are completely random as far as is known, but in principle there could be elements/variables that determine the individual outcomes that are currently unknown.

Tha would "in principle" mean? The question is, do You think it in principle be known by the experimenter (without actually destroying the entanglement) and thus changing the premise? Then how would such a hidden variable escape bells theorem?

Note: this is not a rethorical question, I am curious to see your response, to understnand your perspective better.

/Fredrik

 

[DrChinese]

1. That would "in principle" mean? The question is, do You think it in principle be known by the experimenter (without actually destroying the entanglement) and thus changing the premise? Then how would such a hidden variable escape bells theorem?

Note: this is not a rethorical question, I am curious to see your response, to understand your perspective better.

/FredrikAsked by @TimmDeeg:
Sorry for a layman question in between:

2. If not "completely random" do we think that the outcomes - in principle - are determined by [a] microscopic states of the detector or by yet unknown "elements/variables" before measurement?Asked by @GarberMoisha:

I didn't get the impression he was rejecting both. In which post did he do so?

1. Sure. We know that local hidden variables are ruled out by Bell. But there could be nonlocal hidden variables. I will say that an amazing number of interpretations seem to reference "updating of knowledge" (or similar terminology) while simultaneously denying "action at a distance". To me, those seem like they are taking an end run to bypass Bell. I would expect any scientific interpretation to clearly deny either locality or realism (or both) clearly and unambiguously.

Since you asked, I will offer up my own perspective, not that it is worth much (just another opinion):
a) I doubt there are hidden variables anywhere. So I deny realism.
b) Observed outcomes are randomly selected from the many possibilities in a manner I cannot explain, and those random outcomes respect the full context in a manner consistent with the independent nonlocal measurement settings (also in a manner I cannot explain). So I deny strict Einsteinian causality - for such quantum outcomes, even while I say there is exists strict Einsteinian causality for signals.2. For @timmdeeg
[a] There are perfect correlations for distant detectors of entangled particles. Therefore, I don't see how there can be anything happening at the detectors contributing any bias to the outcomes. Their states must always cancel out, and therefore would need to operate in tandem. That would require some new kind of nonlocality just for them, which defeats the purpose of attributing the outcomes to the detectors themselves.

(b) See my answer to @Fra in 1. above.3. I didn't, as explained by @Haborix: basic logic dictates that rejection of both locality and realism is consistent with Bell. On the other hand:

See my answer to @Fra in 1. above.

 

[PeterDonis]

I get chastised for a) using terminology that is common is the literature ("QM is quantum nonlocal")

I was objecting because you didn't make it clear what definition of that term you were using. And statements of yours like this...

@DrClaude said a poster should disavow the idea of spooky action at a distance. That was his choice of words, and he said it as if denying "spooky action at a distance" is generally accepted physics. He may as well have said we should deny "quantum locality".

...made it seem like you were equating "quantum nonlocality" with "spooky action at a distance" (since you objected to @DrClaude denying the latter because it was tantamount to denying the former--see in particular the bolded part of the quote above), which, as you have now agreed, is not correct.

someone else can say b) there is no "action at a distance" with impunity

Based on what we have discussed, such a statement would be interpretation dependent. So it is correct to point out that it belongs in a separate discussion in the interpretations forum. But it is not correct to say it is denying generally accepted physics.

 

[PeterDonis]

there could be nonlocal hidden variables

The Bohmian interpretation is an example of this; particle positions are the hidden variables, and they can be influenced instantaneously by changes in the wave function anywhere in the universe.

 

[Fra]

1. Sure. We know that local hidden variables are ruled out by Bell. But there could be nonlocal hidden variables.

Can you loosely line out how could an experimenter could "in principle" infer these, in realistic time and space scales, as you call them "nonlocal HV", from the preparation procedure, without changing the initial state and thus the whole premise of what we base the expectations on?

I will say that an amazing number of interpretations seem to reference "updating of knowledge" (or similar terminology) while simultaneously denying "action at a distance". To me, those seem like they are taking an end run to bypass Bell. I would expect any scientific interpretation to clearly deny either locality or realism (or both) clearly and unambiguously.

I might see the problem here you mention. If I get you right, IMHO it has to do with that in standard QM/QFT paradigm, information is just encoded in the classical observer part of the cut. One is ignorant about the physical location of where the knowledge is encoded about how it's inferred, because it's effectively encoded in the whole "macroscopic" environment. This is why a consistent treatment of this tend to force the macroscopic environent out at the infinity, and you get just a scattering matrix. This works in the particle lab, but not on scales larger than the experimenter, or on cosmological scales. Then all observers become a fiction as no preparations or tomographies etc are even remotely possible.

Since you asked, I will offer up my own perspective, not that it is worth much (just another opinion):
a) I doubt there are hidden variables anywhere. So I deny realism.

I have no problem with hidden variables, just like I have no problem with that all observers simply don't hold the same information, or even the same "amount" of information. If there is no communication channels can copy the data reliably, then it's "effectively hidden", and observers simply can't come to an agreement. For me this is not so strange to imagine.

b) Observed outcomes are randomly selected from the many possibilities in a manner I cannot explain, and those random outcomes respect the full context in a manner consistent with the independent nonlocal measurement settings (also in a manner I cannot explain).

I agree, but seeking a viable explanation is for me the key question in all these debates.

/Fredrik

 

[DrChinese]

Can you loosely line out how could an experimenter could "in principle" infer these, in realistic time and space scales, as you call them "nonlocal HV", from the preparation procedure, without changing the initial state and thus the whole premise of what we base the expectations on?

@PeterDonis covered this in post #39 above. I don’t think one could infer this at all from any experiment I am aware of. That wouldn’t mean it’s not possible, although I don’t see that it really explains quantum nonlocality. I am not a Bohmian and therefore cannot defend effectively their position, or certain others.

 

[Fra]

@PeterDonis covered this in post #39 above. I don’t think one could infer this at all from any experiment I am aware of. That wouldn’t mean it’s not possible, although I don’t see that it really explains quantum nonlocality. I am not a Bohmian and therefore cannot defend effectively their position, or certain others.

I am not a bohmian either, but as far as understand it - from the perspective where I can connect and symphatise a bit - is via the solipsist HV that Demystifier lined out - I am tempted to say that it is not even inferrable in principle. The value of those hidden variables, are as a sort of thinking tool that (maybe?) can help explain certain things, but as I understand it I do not see that it can ever recover the unpredcitability. I think the randomness is intrinsic to the nature of interactions. This is also the reason why I think the set of alla HV are not inferrable even in principle, by an actual observer or experimenter. This is why I don't understand how "non local HV" can solve that problem. So your perspective is not clear to me except the issues in post#40 you see that I also see and acknowledge.

/Fredrik

 

[martinbn]

... I would expect any scientific interpretation to clearly deny either locality or realism (or both) clearly and unambiguously.
...

This seems inconsistent with what you said earlier. You said that non-locality is interpretation independent fact, backed up by experiment and thousands of papers. So why would you expect locality to be denied or not! There is no locality, period. The only thing they can do is deny or not realism.

 

[vanhees71]

As I'm at an exciting conference at a lovely beach in Sicily right now, I cannot read all this discussion in this thread, and it's again about the already over-discussed issue of locality.

Just a summary: The problem is that different communities means different things by locality. In the quantum-foundations community, which is unfortunately highly infected by the philosophical-noise disise, it simply means that the facorization assumption for the probabilities for common outcomes of measurements at far distant places doesn't hold.

This has nothing to do with what physicists working with relativistic QFT understand as "locality". Using their notion of "locality" settles all these issues, which are non-issues in fact since indeed relativsitic QFT in its working form (i.e., the one which describes real-world phenomena) by construction excludes any "spooky actions at a distance" or "faster-than light signals" or "acauslities". The trick is the microcausality condition, according to which local observable-operators must commute at space-like separated arguments. This particularly includes the Hamilton density, which ensures that there's no faster-than light causal connections for the outcome of measurements on such local observables.

All the Bell tests with photons, e.g., are described by standard QED, which obeys the microcausality principle. The violation of Bell's inequality is due to the the correlations between local observables on far-distant places on an entangled (i.e., inseparable) quantum system like polarization measurements on two entangled photons at far distant places. The correlations are not due to faster-than-light influences of the measurment events but rather due to the correlations inherent in the state, i.e., the initial preparation of the two photons in a "Bell state". This is an interpretation-independent statement since it simply relies only on the mathematical properties of the theory.

The times, where the quantum foundations were a field of philosophical gibberish (a la Bohr et al) should really be over in the 21st century. Science has gone on and resolved all these puzzles from both the experimental as well as from the theoretical point of view. For long it's a purely scientific topic (since the 1980ies at least), and it has lead to last year's physics Nobel prize for Aspect, Clauser, and Zeilinger.

That's why this field now even develops into a branch of engineering, leading to all kinds of promising new technology like quantum cryptography (which is to a large extent already realized today) and quantum computers (which are under vigorous development).

 

[PeterDonis]

don’t think one could infer this at all from any experiment I am aware of.

Unless and until we have some follow-on theory that makes different experimental predictions from QM, it is obviously impossible to infer anything from experiments except whether QM itself is valid or not. All QM interpretations make the same experimental predictions, so there is no way to distinguish between them by experiments.

 

[vanhees71]

There is no contradiction between standard Q(F)T and experiments. So there's no need for new theories (yet). There's also no need for more "quantum interpretations". The one we have so far is sufficient for all scientific purposes. I'd say in practice all these speculations about the "right interpretation" plays no role at all. If you look at what physicists, experimental and theoretical, "really do" concerning physics, i.e., the observation and description of real-world/lab phenomena, what comes to application is simply the minimal statistical interpretation.

 

[Fra]

There is no contradiction between standard Q(F)T and experiments. So there's no need for new theories (yet). There's also no need for more "quantum interpretations". The one we have so far is sufficient for all scientific purposes.

You seem to focus exclusively on "describing nature", and having the relaxed attitude that any description that is correct, is as good as any. This is all fine to me.

But some also wants and "explanation" and not just a description.

The first reaction to this is probably that science just describes nature according to some "model" or "theory" it doesn't "explain" or answer why questions. But for me the difference between description and explanation is in a way also the extent of fine tuning. And the descriptive model that has less free parameters, even if making no better predictions than one with more parameters, can be said to have more explanatory power, and thus be preferred.

So in that light, I think it still makes sense to seek new theories, even if the currently doable experiments are not driving it?

/Fredrik

 

[GarberMoisha]

A 'virtual' world would be manifestly local in the practical sense, FAPP and fail the realism assumption. Newtonian concepts would hold approximately and scientists would mostly stay away from 'how' and 'why' philosophical questions but merely contend with describing observations and having correct predictions. They would not have a single coherent model of the world and there would be dozens of competing interpretations how the world is.
This is a hypothetical scenario, not necessarily pertaining to the current situation. We as a species may not have the brainpower to grasp how the world works.

 

[vanhees71]

You seem to focus exclusively on "describing nature", and having the relaxed attitude that any description that is correct, is as good as any. This is all fine to me.

But some also wants and "explanation" and not just a description.

The first reaction to this is probably that science just describes nature according to some "model" or "theory" it doesn't "explain" or answer why questions. But for me the difference between description and explanation is in a way also the extent of fine tuning. And the descriptive model that has less free parameters, even if making no better predictions than one with more parameters, can be said to have more explanatory power, and thus be preferred.

So in that light, I think it still makes sense to seek new theories, even if the currently doable experiments are not driving it?

/Fredrik

Of course physics doesn't answer the question, why nature is as it is. I also don't see, how you can expect this from any science.

Also, there's no way to find new theories without sufficient empirical input. At least even the greatest physicists like Einstein or Schrödinger who in their last years of their life tried to find a unified classical field theory of everything, made no progress at all. If you read about the well-studied historical development of the hitherto "successful theories" (Newtonian classical mechanics, Maxwellian electrodynamics, SRT, GRT, QM, relativistic QFT) they all were solidly founded and their development guided by experimental evidence of the phenomena they aim at describing (not explaining!).

 

[PeterDonis]

A 'virtual' world

What are you referring to here? Do you have a reference?

 

[GarberMoisha]

What are you referring to here? Do you have a reference?

Max Tegmark

Journal reference:

Found.Phys.38:101-150,2008

I explore physics implications of the External Reality Hypothesis (ERH) that there exists an external physical reality completely independent of us humans. I argue that with a sufficiently broad definition of mathematics, it implies the Mathematical Universe Hypothesis (MUH) that our physical world is an abstract mathematical structure. I discuss various implications of the ERH and MUH, ranging from standard physics topics like symmetries, irreducible representations, units, free parameters, randomness and initial conditions to broader issues like consciousness, parallel universes and Godel incompleteness. I hypothesize that only computable and decidable (in Godel's sense) structures exist, which alleviates the cosmological measure problem and help explain why our physical laws appear so simple. I also comment on the intimate relation between mathematical structures, computations, simulations and physical systems.

https://arxiv.org/abs/0704.0646

 

[PeterDonis]

Max Tegmark

This is off topic in this thread (and probably this forum since the hypothesis has nothing specifically to do with quantum physics). Please start a new thread in the appropriate forum if you want to discuss it.

 

[Fra]

At least even the greatest physicists like Einstein or Schrödinger who in their last years of their life tried to find a unified classical field theory of everything, made no progress at all.

I think that the task is just very complicated apparently even for the greatest minds, especially if the solution maybe requires a bit of a paradigm change. But I don't see why failures of the past, should not discourage us from at least trying to make progress.

This progress is I think not in any conflict with experimental research. Both are needed I think. I'm not suggesting we should all just sit in our closests and think.

Also, there's no way to find new theories without sufficient empirical input.

I see it's similar to trying to find new compression algortims to compress data. You do not need new data, the "empirical input" is to note if the new models requires more compact representation, the same predictions can be made with less memory, computational requirements and less free parameters. This is because the new theories aren't selected as making different predictions (under current data set) but as having a more clever or flexible computational properties. And such things are I think related to problems we see, with renormalizability etc.

It's in this sense i still one can seek an "explanation" to try to understandin the "mechanisms" that allows for strange phenomena such as entanglement. We know it happens, this is not the problem. But it keeps beeing hard, not to accept, but to understand?

/Fredrik

 

[Lord Jestocost]

....... one can seek an "explanation" to try to understandin the "mechanisms" that allows for strange phenomena such as entanglement.

I am skeptical about this.

One should keep in mind a remark by Richard Conn Henry: "The real scandal of quantum mechanics is that so many physicists still insist on thinking about quantum phenomena with classical ideas.” (R. Conn Henry, “The real scandal of quantum mechanics,” Am. J. Phys., 77 (10), pp. 869-870 (2009))

 

[Fra]

One should keep in mind a remark by Richard Conn Henry: "The real scandal of quantum mechanics is that so many physicists still insist on thinking about quantum phenomena with classical ideas.” (R. Conn Henry, “The real scandal of quantum mechanics,” Am. J. Phys., 77 (10), pp. 869-870 (2009))

Fully agree.

But by now, I think it should have be very obvious to residents on here what I refer to has nothing to do with "classical ideas". On the contrary. But the notion of "mechanism" is not reserved for classical mechanics!

/Fredrik

 

[vanhees71]

The true "scandal of quantum mechanics" is very well caught in this letter to the editors of AJP by van Kampen (one of the great no-nonsense physicists when it comes to interpretations of QT and, more importantly, thermodynamics within relativistic physics):

https://doi.org/10.1119/1.2967702

Note the author of the article van Kampen is referring too :-)

https://arxiv.org/abs/physics/0702069v2

 

[Fra]

van Kampen writes, just as someone else often does....

"Actually quantum mechanics provides a complete and adequate description of the observed physical phenomena on the atomic scale. What else can one wish? (It is true that the connection with gravity is still a problem, but that is outside this discussion.)"

Why so categorically deny that the foundations of quantum mechanics have anything todo with the theory of spacetime; ie. the one spacetime which QM requires for it's foundation? Can someone define quantum mechanics, without making use of a background spacetime, and claim it describes atomic physics experiments?

Why simplify things to the point that we think the ground we stand on is not important because we take it for granted?

/Fredrik

 

[vanhees71]

It's true that gravity is not adequately described within QT. That's an open scientific research topic and has nothing to do with some philosophical interpretational problems. For everything else QT in its minimal interpretation is simply the science content of this theory. There are no scientific problems with it. The foundational questions around the infamous paper by EPR is settled completely by all the Bell tests in various forms in favor of Q(F)T.

 

[gentzen]

The true "scandal of quantum mechanics" is very well caught in this letter to the editors of AJP by van Kampen (one of the great no-nonsense physicists when it comes to interpretations of QT and, more importantly, thermodynamics within relativistic physics):

Never heard of van Kampen before, and certainly not as "one of the great no-nonsense physicists when it comes to interpretations of QT". In fact
https://en.wikipedia.org/wiki/Seifert–Van_Kampen_theorem
is not even from the same person.
And the letter itself didn't seem to contain any great explanations or insights either. Or perhaps I already forgot them, or maybe I should have read the letter more carefully. Or perhaps van Kampen was preaching to some choir, and I simply don't know enough about that choir, or about previous work of van Kampen related to that choir, which would enable me to get the message of that letter.

The foundational questions around the infamous paper by EPR is settled completely by all the Bell tests in various forms in favor of Q(F)T.

In fact, the content of the letter feels similarly to this statement to me. The grammar feels wrong: "... questions ... is settled ...". It is stated as some fact: "foundational questions ... EPR ... settled ... by all the Bell tests ..." which seems to mix or replace theoretical analysis by experimental confirmation. The way your statement is made, and the way that letter is written doesn't even seem to try to explain anything. It feels more like "here is my opinion, you should know what to do with it"!

 

[martinbn]

Never heard of van Kampen before, and certainly not as "one of the great no-nonsense physicists when it comes to interpretations of QT". In fact
https://en.wikipedia.org/wiki/Seifert–Van_Kampen_theorem
is not even from the same person.
And the letter itself didn't seem to contain any great explanations or insights either. Or perhaps I already forgot them, or maybe I should have read the letter more carefully. Or perhaps van Kampen was preaching to some choir, and I simply don't know enough about that choir, or about previous work of van Kampen related to that choir, which would enable me to get the message of that letter.In fact, the content of the letter feels similarly to this statement to me. The grammar feels wrong: "... questions ... is settled ...". It is stated as some fact: "foundational questions ... EPR ... settled ... by all the Bell tests ..." which seems to mix or replace theoretical analysis by experimental confirmation. The way your statement is made, and the way that letter is written doesn't even seem to try to explain anything. It feels more like "here is my opinion, you should know what to do with it"!

About EPR, I thought that Bohr's reply was crystal clear.

 

[gentzen]

About EPR, I thought that Bohr's reply was crystal clear.

In fact, I actually like Bohr's reply. He certainly tries to explain something, namely the contextuality of QM, and how it applies to the EPR scenario.

 

[martinbn]

In fact, I actually like Bohr's reply. He certainly tries to explain something, namely the contextuality of QM, and how it applies to the EPR scenario.

I wrote it to sound like a joke, but I was serious. People may complain about Bohr, but he is the only one in the philosphy of QM that makes sense to me.

Off topic, by the way how does one formulate EPR within the statistical interpretation?

 

[Lord Jestocost]

Never heard of van Kampen before, and certainly not as "one of the great no-nonsense physicists when it comes to interpretations of QT".

Use Google Scholar, keyword “Ten theorems about quantum mechanical measurements” (a paper by N.G. van Kampen).

You find a lot in “Quantum Measurement Theory as simply explained by N G van Kampen” by M. G. Burt
https://arxiv.org/abs/2206.03219

 

[vanhees71]

I wrote it to sound like a joke, but I was serious. People may complain about Bohr, but he is the only one in the philosphy of QM that makes sense to me.

Off topic, by the way how does one formulate EPR within the statistical interpretation?

Bohr is the main culprit for obscuring QT as a physical theory. I never understood, why he was so famous at his time.

 

[Fra]

Bohr is the main culprit for obscuring QT as a physical theory. I never understood, why he was so famous at his time.

I agree with Martinbn, I find Bohr to express things in honest and clear ways, in particular how QM is defined relative to the classical/macroscopic side of the cut. This to me, both clearly explains that QM can not be defined without a "classical/macroscopic observer". Yet, Bohr to my knowledge never claimed that there is a classical reality that is more fundamental, nor that there is a quantum reality.

Taken together, this both states clearly what QM is AND it's limitations. While others may pretend there is only a quantum reality, but without realizing that without the "macro background" we can not DEFINE QM. I think Bohr never made any exaggerated claims here, he expressed what we know, not more, not less, this is why he has my respect.

/Fredrik

 

[martinbn]

My impression is that if your view and understanding of QM is similar to Bohr's you find him clear. If they are different you find him completely incomprehensible.

 

[vanhees71]

I agree with Martinbn, I find Bohr to express things in honest and clear ways, in particular how QM is defined relative to the classical/macroscopic side of the cut. This to me, both clearly explains that QM can not be defined without a "classical/macroscopic observer". Yet, Bohr to my knowledge never claimed that there is a classical reality that is more fundamental, nor that there is a quantum reality.

This is an unfounded claim. Nothing in contemporary experiments hints at a quantum-classical cut, which can be objectively defined. I've no clue, what "reality" means, because this word is made useless by all the philosophical undertones attached to it.

Taken together, this both states clearly what QM is AND it's limitations. While others may pretend there is only a quantum reality, but without realizing that without the "macro background" we can not DEFINE QM. I think Bohr never made any exaggerated claims here, he expressed what we know, not more, not less, this is why he has my respect.

/Fredrik

The only limitation of QT (QM is of course a non-relativistic approximation, with clear limits of applicability) is its lack to describe satisfactorily the gravitational interaction. This has nothing to do with philosophical interpretational issues though.

 

[Structure seeker]

I'd like to take a post in another thread "number systems in science":

The question is whether we could ever confirm by measurement that the ratio of a circle's circumference to its diameter is π.

The thread has an analogy with this thread: if only what we measure is interesting, there's no need for all the complicated number systems like the reals or complex numbers as just the most common examples. Just the rationals suffice, with error bars.
What @PeroK calls the question in that thread is an example of knowledge that gives great guidance in what is interesting to measure, but it cannot be concluded from measurement. I am sure we will encounter similar results in quantum physics. We would not be able to prove which rational number sequences go to π, since infinity isn't measurable.

 

[Fra]

This is an unfounded claim. Nothing in contemporary experiments hints at a quantum-classical cut

I'm not talking about uniqueness of the cut, it's clearly not, just like there are many possible measurement devices. They aren't only one, but that doesn't mean it isn't important.

But our mesaurement apparatouses, and all the data we get from observations are stored in reliable records, that - modulo special relativity - all observers agree upon. Ie. there is no non-commutativity or no-cloning issues with information sharing of measurement results. This is the classical cut. Without this cut, how can you imagine reliably preparing and repeating experiments, that all observers agree upon?

/Fredrik

 

[vanhees71]

Yes, and this has nothing to do with a quantum-classical cut either. The "appearance of a classical world" out of QT is much better understood today than at Bohr's times!