I Entanglement: is there 'action at a distance' due to measurement?

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  • #51
DrChinese said:
There is a saying attributed to Sherlock Holmes: “When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.
This assumes that we know all the possibilities to check. We don't.

For example, I've mentioned statistical interpretations in some of these discussions. Statistical interpretations are non-realist in the sense that the quantum state only describes statistical ensembles (or, equivalently, preparation procedures); it does not describe individual quantum systems. Which means such interpretations don't even try to provide the kind of "narrative" you are looking for. To you, that's a bug. To me, it's a feature, because I think we are simply too ignorant at this stage of our development of understanding quantum mechanics to even know what kinds of models we should be looking at. All we've found out so far is what kinds of underlying models don't work. I'm not saying such research shouldn't continue; of course it should. But we should be very, very cautious about making claims about how an underlying model "has to work". We might simply not even be aware yet of the kinds of models that will ultimately end up working. Bear in mind that before the early 1900s, nobody was even aware that the kinds of models that now make up standard QM were even possible.
 
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  • #52
DrChinese said:
they all claim to make the same predictions.
I don't think you've given a single example of an interpretation which made such a claim wrongly. Your basis for criticizing interpretations you don't like, as far as I can tell, has nothing to do with any actual demonstration that they make wrong predictions. You just don't like the underlying models that they use to explain how they arrive at those predictions. Which is fine as a matter of opinion, but not if you're going to make a factual claim about what predictions an interpretation does or does not make.
 
  • #53
DrChinese said:
And I believe there’s now sufficient accepted evidence to rule out both of those. But that’s simply my opinion and assessment.
Evidence ruling out a model should not be a matter of opinion. Either you can cite the specific evidence and the specific prediction it falsifies, or you can't. Either way this should be a factual matter, not a matter of opinion.
 
  • #54
PeterDonis said:
(There are examples of alternate theories to standard QM, such as the GRW stochastic collapse model, but all of them so far have been falsified by experiments; their predictions haven't worked.)

Not all.
 
  • #55
physika said:
Not all.
Which ones have not? Can you give references?
 
  • #56
DrChinese said:
No, they all claim to make the same predictions. Each has assumptions that are fair to challenge. This forum is the place to discuss within the context of a specific idea.

This thread is about AAAD, but believe me I don’t want to derail others expressing their ideas. I am just one voice. I’ve probably said enough for today… :smile:
The pure QM approach is to take the abstract state mechanics as the mechanism. It's not always possible to find physical characteristics that map to all aspects of what the states imply.

A good example might be where an electron rotated by 360 degrees is not in the same state as it was originally, despite having all the same measurable quantities. It's clear from the state formalism why this is and you can exploit this by a suitable experiment involving interference. If you want to be a realist you need, dare I say, an element of physical reality that corresponds to the change of sign of the state.

Similarly, there is no measurable difference between a particle that is effectively in an eigenstate of spin and one that has been measured. The mechanics according to pure QM is not action at a distance- or any other realist mechanism- but the abstract state mechanics.

What QM, Bell and entanglement swapping show is that abstract state mechanics is richer than any realistic mechanics. And, somehow, nature manages to operate under this abstract regime.
 
  • #57
PeterDonis said:
Which ones have not? Can you give references?

https://www.mdpi.com/1099-4300/25/4/645#B30-entropy-25-00645

"The largest particles interfered so far are of mass 10⁴ amu in the Vienna interferometers [24,25]. This experiment is still about two orders of magnitude too small to test the CSL model with the strongest bound according to Adler"

"Future plans for matter-wave interferometers which achieve even larger macroscopicity involve the OTIMA and LUMI interferometers at Vienna [25,29]. OTIMA and LUMI are Talbot–Lau interferometers and should be able to interfere with particles of masses up to 10⁷ amu and beyond."

"Experiments with a particle of 10 nm diameter [l = 100 nm, t = 2 ms] would directly test the CSL model with Adler’s parameter choice [34]"

----------

Testing the foundation of quantum physics in space via Interferometric and non-interferometric experiments with mesoscopic nanoparticles

https://www.nature.com/articles/s42005-021-00656-7


.....
 
  • #58
PeroK said:
What QM, Bell and entanglement swapping show is that abstract state mechanics is richer than any realistic mechanics.
I agree...
PeroK said:
And, somehow, nature manages to operate under this abstract regime.
...but we still seek an understanding of the descriptions in this abstract regime.

And the correlation of separated measurements, still begs for some kind of explanation with some form of causal order, even if abstract. And herein lies also I think a problem of timeless physics, because the causal order is not fundamental, it's more like an arbitrary parameterisation. And something just doesn't seem right with that.

And it seems an explanation in any direction is going to be "controversial" one way or the other. I think action at distance is really weird, but subjective reality is not weird for me. But others refuse to give up objective reality, and pay the price of other pathologies. So which pathological implication are easier to live with?

/Fredrik
 
  • #59
PeterDonis said:
I don't think you've given a single example of an interpretation which made such a claim wrongly. Your basis for criticizing interpretations you don't like, as far as I can tell, has nothing to do with any actual demonstration that they make wrong predictions.

Evidence ruling out a model should not be a matter of opinion. Either you can cite the specific evidence and the specific prediction it falsifies, or you can't. Either way this should be a factual matter, not a matter of opinion.
I don't really think I have been criticizing interpretations; so much as bringing out and citing specific experiments that all interpretations claiming to make the same predictions as QM must be able to explain. For example, it is generally agreed that any interpretation or theory that posits Local Realism must be rejected. I did mention a batch of those (by writer name), true, but surely there is no "opinion" on this point in this subforum. That is generally accepted science per your definition.

For many interpretations, the details of their mechanism to follow Bell (either by rejecting Locality, Realism, or both) are muddy*. I am hoping that one of their adherents - not particularly in this thread or anytime soon - can fill in detail and compare to some of the experiments I cite. Perhaps an Entanglement Swapping experiment, or a Delayed Choice experiment. That would be enlightening for me. Try as I have, searching the literature for rock solid discussion in this area, I have yet to find it. So I hope to read that here. I think that is what many readers/posters in this subforum seek, I don't think I am alone on this.

This thread is based on a fair question: is there 'action at a distance' due to measurement? In my preferred interpretation (I might even call it Orthodox QM), there is. You and anyone else might say "Orthodox QM does not feature AAAD"; but by right, I believe I am entitled to believe it does. However, when posting here, there are fair limits imposed: I would (and should and usually do) label that belief as "not generally accepted" and/or subject to the reader's own opinion or viewpoint.



So you seem to be asking me to name and document an interpretation that wrongly claims (in my view) to follow all existing experiments. That wasn't my objective, so I am only providing the below to fulfill what I think you are asking.

There are forms of MWI that claim Locality and deny AAAD. They also claim a form of Realism that evades Bell. Bell himself had this to say about MWI (per MWI proponent Vaidman*): "The ‘many world interpretation seems to me an extravagant, and above all an extravagantly vague, hypothesis. I could almost dismiss it as silly. And yet... It may have something distinctive to say in connection to ‘Einstein Podolsky Rosen puzzle’ and it would be worthwhile, I think, to formulate some precise version of it to see if it really so." He said it is vague, and needs to be more precise. I would agree with this, in light of modern experiments which are demanding.

Of course, Bell never got to see the likes of experiments from the groundbreaking teams of Zeilinger and others. Many of these seem to be flat out demonstrations of cause and effect that span distances that will not fit into traditional light speed limits. To quote Zeilinger et al from one such experiment (144 km of separation) here: "Here we report a quantum eraser experiment, in which by enforcing Einstein locality no such [signal] communication is possible. ... No naive realistic picture is compatible with our results ... It is therefore suggestive to abandon such pictures altogether."

Certainly this is not a "disproof" of MWI. But it is relevant to this thread regarding AAAD, see also comments * below. I would state my opinion as saying MWI is incompatible with experiment in this regard, but again, it's just one person's view.



Any person who states or advocates a preferred Interpretation, or criticizes any element of any other Interpretation, is implicitly expressing their opinion. It is normal for posters to express this in this Subforum. And it would be no more or no less factual than anyone's else's similar comments. In fact, anyone who denies there is AAAD is expressing their opinion too.

I don't want to derail further discussion on this thread topic, nor do I want to see the thread closed. I promise that I won't turn this thread into a repeat of some of our previous extended threads. I will limit my comments to addressing specific limited ideas. I have already expressed more than my share of ideas sufficiently.

But I believe that I have stayed within the (more lenient) Foundations/Interpretations subforum rules. In this subforum, I am well-known for experimental (and other) citations; I can't believe my supporting citations fall short of any standard. If they do, just let me know and I will try to fill in the gaps.

Cheers,

-DrC


* Vaidman on Locality in MWI (per Stanford Plato here): "We have to add some ingredients to our theory and adding locality, the property of all known physical interactions, seems to be very natural (in fact, it plays a crucial role in all interpretations). ... Since interactions between particles are local in space, this is what is needed for finding causal connections ending at our experience." Also Vaidman (here): "Although there is no action at a distance [AAAD] in the MWI, it still has nonlocality." And also: "Bell inequalities lead us to a hard choice: either we believe that there is some kind of action at a distance, or that there are multiple realities". Clearly it's not so easy to nail down MWI on the particulars. And even Vaidman sees that rejection of MWI entails embracing AAAD.
 
  • #60
DrChinese said:
it is generally agreed that any interpretation or theory that posits Local Realism must be rejected.
Yes, because that's been ruled out by Bell's Theorem and experimental evidence of Bell inequality violations. But while individual writers might be trying to advocate for some form of this, none of the well known QM interpretations are local realistic.

DrChinese said:
For many interpretations, the details of their mechanism to follow Bell (either by rejecting Locality, Realism, or both) are muddy*.
For some interpretations, there is no mechanism at all. I gave the statistical interpretation, as described in, for example, Ballentine, as an example. The statistical interpretation accounts for Bell inequality violations by pointing at the standard math of QM, and the experiments that confirm its predictions, and saying "that's all we can say".

As for "rejecting locality, realism, or both", if you insist on categorizing things that way, the statistical interpretation "rejects" both (in the sense of not even talking about them at all). My personal quick rundown of some other well known interpretations in this regard would be:

Copenhagen -- rejects both.
MWI -- rejects both. (Though, as you comment, whether MWI actually rejects locality is a subject of some debate.)
Bohmian -- rejects locality.

However, I'm not sure how much value there is in this kind of categorization. But that's probably an aspect of the fact that I'm not sure how much value there is in discussions of QM interpretations, period. What we need are some alternative theories that make different predictions from standard QM, so we can get some idea of what experiments we might run to test them. That's how physics is supposed to be done: building models that make correct predictions. Not telling stories about "what's really going on" that can't be tied to any experiment, because other people are telling other, mutually incompatible stories about the same experiment.
 
  • #61
DrChinese said:
In my preferred interpretation (I might even call it Orthodox QM), there is.
Where is this interpretation published in the literature? Where does that literature make the claim you are making here?
 
  • #62
physika said:
Realism is more than that, is not simply about properties.
Which things/entities have that properties ?
This is a fair question. Some interpretations (e.g. Bohmian mechanics in its traditional non-relativistic version) postulates the existence of point particles with well-defined values for their "properties", such as position or velocity. Other interpretations, such as information-based ones claim that there is nothing out there between quantum interactions/events so that, in a sense, the things/entities are those quantum events and nothing else.

Lucas.
 
  • #63
DrChinese said:
Correct, you can also call it objective reality or non-contextual reality.
Yes, I think objective/non-contextual reality is a good description of what is commonly called "realism".

DrChinese said:
We had a long discussion about Mjelva's paper. He seems to be a local realist without saying so.
Do you think Mjelva is a local realist? In my opinion, the results in his paper don't depend on any specific interpretation. However, I think his analysis is particularly useful in explaining entanglement swapping within realistic interpretations.

DrChinese said:
I’m just saying: We’ve been examining Bohmian ideas since the 40’s and Everettian ideas since the 50’s and have so far got zilch from either. And I believe there’s now sufficient accepted evidence to rule out both of those. But that’s simply my opinion and assessment.
I share a similar view. I believe that the experimental results in the last three decades do not rule out these realistic interpretations, but a progressive shift toward more modern interpretations, such as those based on information-theoretic postulates, could be productive in improving our understanding of these experiments.

DrChinese said:
Many of these seem to be flat out demonstrations of cause and effect that span distances that will not fit into traditional light speed limits.
I'd like to add something about that. In my opinion, rather than "locality", the key concept is "local causality" as introduced by Bell. This is what was ruled out experimentally. If we try to explain these experiments through cause-and-effect relationships, action-at-a-distance is unavoidable. This is Bell's theorem. However, we can still deny causation as fundamental, as some modern information-based interpretations do. Our everyday (classical) experience leads us to think (and develop physical theories) in terms of the time evolution of "permanent" entities that exist at all times. However, we must remember that this is a metaphysical assumption with no experimental support.

DrChinese said:
For many interpretations, the details of their mechanism to follow Bell (either by rejecting Locality, Realism, or both) are muddy*. I am hoping that one of their adherents - not particularly in this thread or anytime soon - can fill in detail and compare to some of the experiments I cite. Perhaps an Entanglement Swapping experiment, or a Delayed Choice experiment. That would be enlightening for me.
If you wish, we could discuss how entanglement swapping and its delayed-choice version is addressed by some "realistic" interpretations, such as Bohmian mechanics. Perhaps it's a bit off-topic for this thread, but in short, the Bohmian "explanation" of the DCES employed the forward-in-time time evolution of the wave function as described by Mjelva. I don't like this kind of interpretation, but it is coherent.

Lucas.
 
  • #64
DrChinese said:
For me: Swapping experiments are the nail in the coffin for locality.
In my opinion, any experiment demonstrating the violation of Bell inequalities for a pair of entangled particles is sufficient to rule out local causality (assuming statistical independence).

The different versions of the entanglement swapping experiments disfavor the notion of an absolute (observer-independent) quantum state. We could still claim that some ##\Psi##-ontic interpretation, such as many-worlds or Bohmian mechanics are not yet ruled out (after all, they predict the same experimental results of standard QM), but the fact that a relational view of the wave function is possible seems to me similar to the claim that absolute velocity exists even when we cannot measure it.

Lucas.
 
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  • #65
Morbert said:
The biparticle system is in the state$$\mathrm{tr}_\epsilon \rho = \frac{1}{2}(\ket{00}\bra{00} + \ket{11}\bra{11})$$which is an unentangled state.
Not according to the MWI. According to the MWI, there is no such thing as the state of the biparticle system by itself; there is only the state of the overall system including all degrees of freedom that have ever interacted. The traced out "state" of the biparticle system alone is not a real state according to the MWI; it's a mathematical convenience only and does not correspond to anything real. Only the overall state of the full system is real.
 
  • #66
Sambuco said:
well-defined values for their properties.

Lucas.

Values of properties are not the issue/point, (up or down, for spin, e.g.
or any qualities, magnitudes, values),
for realism, the things/events are the issue/point, not values/properties.

......
 
  • #67
DrChinese said:
Of course, Bell never got to see the likes of experiments from the groundbreaking teams of Zeilinger and others. Many of these seem to be flat out demonstrations of cause and effect that span distances that will not fit into traditional light speed limits. To quote Zeilinger et al from one such experiment (144 km of separation) here: "Here we report a quantum eraser experiment, in which by enforcing Einstein locality no such [signal] communication is possible. ... No naive realistic picture is compatible with our results ... It is therefore suggestive to abandon such pictures altogether."
I think you misunderstand the quoted part. Their conclusion says nothing about locality. It says that the experiments suggest that we should abandon the naive realistic picture.
 
  • #68
So what are the possible alternatives to the naive realistic picture?
What types of explanatory mechanisms are at play in such "non-naive" picture of nature?

As far as I can see this "naive realism" is (if not the same very closely) related to "objective reality" as in some kind of "objective beables" combined with an objective law that defines the flow of time? Which in itself is to me seem closely related to the system dynamics paradigm or newtonian paradigm/schema?

And this is supiciously realted to mechanisms such as apparent action at distance, perhaps beacuse these are possible artifacts of pressing something that does not fit, into the wrong paradigm? Ie the "constraints" of hte paradigm itself will introduce artifacts.

Anyone disagree(I bet) or any other options?

/Fredrik
 
  • #69
Fra said:
So what are the possible alternatives to the naive realistic picture?
Um, the QM interpretations that don't use it?

The problem is not that nobody has come up with alternatives. The problem is that none of the alternatives satisfy everybody; every single one has some showstopper that makes some segment of the physics community simply refuse to buy into it.
 
  • #70
Fra said:
So what are the possible alternatives to the naive realistic picture?
What types of explanatory mechanisms are at play in such "non-naive" picture of nature?
PeterDonis said:
Um, the QM interpretations that don't use it?
Yes, but these don't qualify as "explanatory mechanisms" - they describe but don't explain. There's no reason why any theory should do more than accurately describe, but many people find it exasperating when a theory seems so hostile to any proposed explanatory mechanism.
 
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  • #71
Nugatory said:
There's no reason why any theory should do more than accurately describe
And predict. I think what makes the situation with QM so hard for many to accept is that here we have this theory that makes accurate predictions, but nobody has come up with any "explanatory mechanism" that tells us why the predictions are so accurate.
 
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  • #72
PeterDonis said:
And predict.
Yes, that's an essential property not necessarily implied by "describe"
 
  • #73
Nugatory said:
Yes, but these don't qualify as "explanatory mechanisms" - they describe but don't explain.
...
but many people find it exasperating when a theory seems so hostile to any proposed explanatory mechanism.
I think this something likely everyone can agree with! I think drives many, including me.

Nugatory said:
There's no reason why any theory should do more than accurately describe
This is where I don't agree, I think it also get into the interpretation and context of a theory and it even touches on the philosophy of science I think. And is directly reflected by our preference for paradigm!

In the system dynamics paradigm, description includes the statespace and the dynamical law, which often imples a finetuning. This finetunning is managable as long as we study small subsystems (like the prime substrated for QM) where initial conditions can be precisely controlled, and experiments can be repeated. From all this, we "infer" the time evolution operators and mape out the state space.

But for me theoris is about "learning", the interesting part is not the final fitted fine tuned theory. The interesting part is the context where the theory emerges. And you can view this either trivially in the context of human science, best characterized by Popper. A theory is either corroborated or falsified, and there is nothing more to it. Poppers take on the philosophy of science comes out as the "descriptive analog" to the take of theory of physics here. Is there really nothing more do it than to describe? I think there is, and this is where explanation or understanding comes in for me.

/Fredrik
 
  • #74
PeterDonis said:
And predict.
Yes, agreed.

But it is interesting in this discussion compare the similarity of the notions of "prediction" with "causation" or "betting guides".

What is the context and purpose of a prediction? In the system dynamics paradigm the whole time history follows "logically" from the initial conditions and law. To the law + initial condition "predicts" the future. But what is the deeper purpose of "predicting" and already predetermined future? And the process of inferring the laws, requires that we can repeat the experiment.

In a agent based model, or learning perspective, the role of "prediction" is more like an "expectation" of the future, on which you based your actions, for the benefit of survival or learning. This is a trial and error process and the purpose is to learn and improve, the theory itself, to ultimaltey evolve it.

The first stance also seems to secretly contain an idea that science "unravels" corroborated facts about nature, rather than participating in creating them as subsystems of the same. Also very similary to "naive realism"? At least this is an unvoidable reflection for to make.

Thus the thesis would be - does our underlying take of "philosophy of science" correlate with our view on the context of a theory, and thus choice of paradigm - at least for those that even reflected upon this connection?

/Fredrik
 
  • #75
PeterDonis said:
Not according to the MWI. According to the MWI, there is no such thing as the state of the biparticle system by itself; there is only the state of the overall system including all degrees of freedom that have ever interacted. The traced out "state" of the biparticle system alone is not a real state according to the MWI; it's a mathematical convenience only and does not correspond to anything real. Only the overall state of the full system is real.
MWI vanguards like David Deutsch and David wallace regularly invoke subsystem decomposition and their subsystem states to represent emergent structures instantiated in the full quantum state. Otherwise the quantum state of the universe could never be related to our quasiclassical experiences.

Emergent structures are still real under MWI. What we are not allowed to do is take them as the totality of what exists. E.g. the state of the biparticle subsystem and the state of the environment subsystem together would not describe the entanglement between the two. For that you need a state of the joint biparticle + environment system.
 
  • #76
physika said:
Values of properties are not the issue/point, (up or down, for spin, e.g.
or any qualities, magnitudes, values),
for realism, the things/events are the issue/point, not values/properties.......
I agree, that's why I mentioned that some realistic interpretations, such as Bohmian mechanics, postulate (1) the existence of point particles, and (2) that these particles have well-defined values for certain properties (e.g. position, velocity) at all times.

Lucas.
 
  • #77
Morbert said:
Emergent structures are still real under MWI
Yes, but that's not the same as saying that the biparticle system itself, considered as an emergent structure, has a definite state all by itself that corresponds to the traced out state you wrote down. It doesn't.

It's true that such a decomposition of the full quantum state into emergent structures is necessary for MWI proponents to be able to claim any correspondence at all with experience (and experimental results). But that correspondence can be treated without having to treat the traced out state of a subsystem as real. Indeed it must be so treated, because the traced out state does not describe a subsystem with a single definite state--which is what we experience and what we measure in experiments. It describes a statistical mixture of such states. So any interpretation of the traced out state of the subsystem has to be statistical--meaning the state does not correspond to the actual, physical states that we experience and measure. As I said, it's a mathematical convenience to help us calculate probabilities.
 
  • #78
PeterDonis said:
And predict. I think what makes the situation with QM so hard for many to accept is that here we have this theory that makes accurate predictions, but nobody has come up with any "explanatory mechanism" that tells us why the predictions are so accurate.
What physical theory has an explanatory mechanism? There's no mechanism for how stress-energy creates spacetime curvature. In fact, many lay posters on here don't accept that interpretation: that something without physical substance can have a geometry. The main difference between GR and QM is that there is a natural interpretation of the mathematics of GR that is amenable to human intelligence.

In classical EM it all seems similarly intuitive until you dig a bit deeper. How does an electric charge create an EM field in the first place?

Even if we found a mechanism for entanglement, that mechanism would need a sub-mechanism to explain that mechanism.

I honestly think it's hopeless to expect to find a neat, intuitive mechanism that lies beneath all physical phenomena. Entanglement is perhaps the final nail in the coffin, but the classical patient was already dead!
 
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  • #79
PeroK said:
What physical theory has an explanatory mechanism? There's no mechanism for how stress-energy creates spacetime curvature. In fact, many lay posters on here don't accept that interpretation: that something without physical substance can have a geometry. The main difference between GR and QM is that there is a natural interpretation of the mathematics of GR that is amenable to human intelligence.
...
I honestly think it's hopeless to expect to find a neat, intuitive mechanism that lies beneath all physical phenomena. Entanglement is perhaps the final nail in the coffin, but the classical patient was already dead!
With the major exception of finding someing "neat" I am much more optimistic about finding a better understanding that can be intuitive.

For me the major huge improvement of QM as compare to classical mechanics, is that the concept to "measurement" is taken more seriously, and it (at least in my preferreed interpretation) contains the concept of information. So it is less simplistic than the old classical pictures that measurements could alsways be pictured as a "passive" arbitrarilty neglectable probing of the system, without affecting it. When you think about that for some time, while simple, really does NOT make sense, it is not intutive IMO. So I think QM paradigm is in a sense a huge improvement. I do not want to go back.

But there are more problems to solve, and while QM takes seriously the relation between the external observer, and the "system" and involving the measurement into the model, via choice of measurement basis, and a whole equivalence class of "external observers" via spacetime transformations. It still fails to FULLY incoroporate the "context" into the interactions. QM insiste swe NEED and observer, but it also only is an EXTERNAL one.

There is alot of interesting connection between geometry and non-physical substance via information geomemtry, but what QM IMHO still does not explain, is why "physical basis of information". It does provide and information basis for describing and speaking about "physical substance", but the constrained from physical pre-requirements of the "external observer" is still unsolved.

When we get this, I hink it will will ALOT more intuitive. So while we have no answers yet, I think there are many reasons to be optimistic.

PeroK said:
Even if we found a mechanism for entanglement, that mechanism would need a sub-mechanism to explain that mechanism.
Maybe evolution will reduce the explanatory burden via a tower or turtles until top turtle is just a irreduccble stochastic turtle, that needs no fine tuning.

/Fredrik
 
  • #80
PeterDonis said:
Yes, but that's not the same as saying that the biparticle system itself, considered as an emergent structure, has a definite state all by itself that corresponds to the traced out state you wrote down. It doesn't.

It's true that such a decomposition of the full quantum state into emergent structures is necessary for MWI proponents to be able to claim any correspondence at all with experience (and experimental results). But that correspondence can be treated without having to treat the traced out state of a subsystem as real. Indeed it must be so treated, because the traced out state does not describe a subsystem with a single definite state--which is what we experience and what we measure in experiments. It describes a statistical mixture of such states. So any interpretation of the traced out state of the subsystem has to be statistical--meaning the state does not correspond to the actual, physical states that we experience and measure. As I said, it's a mathematical convenience to help us calculate probabilities.
In MWI, mixed states are not interpreted statistically, but still have an interpretation. If we limit concern to relative states, then a proper mixture can be interpreted as an observer's ignorance of the relative state of the subsystem wrt a particular branch, and hence a mixed state can be a subjective statement of ignorance about a relative state. But more fundamentally, a mixed state is an improper mixture and an objective representation of the subsystem.
David Wallace said:
Say we prepare an EPR pair and throw one element of the pair away: the only quantum state available to represent the other element is a mixed state, and I don’t see anything particularly wrong with saying that that really is the state of the qubit. Then if we let the qubit get decohered — say, if we measure it but don’t look at the result — then it gets entangled with the macroscopic degrees of freedom of its vicinity, but it is still in a mixed state.
https://sites.pitt.edu/~dmw121/papers/Interview_for_website.pdf
 
  • #81
Morbert said:
In MWI, mixed states are not interpreted statistically, but still have an interpretation.
The fact that David Wallace "doesn't see a problem" with this doesn't mean there isn't one. But at this point I think we're in the realm of personal opinion, so I'll leave this issue where it is.
 
  • #82
Maudlin writes about the proponents of the different interpretations of QM (Quantum Non-Locality and Relativity):
They may correctly note that according to every one of their rival theories, God was malicious, and having thus eliminated every other possibility, claim their own theory the victor. The problem is that every partisan can argue in this way since every theory posits some funny business on the part of the Deity.
Peter Donis wrote the equivalent earlier in this thread (#69):
The problem is not that nobody has come up with alternatives. The problem is that none of the alternatives satisfy everybody; every single one has some showstopper that makes some segment of the physics community simply refuse to buy into it.
and it summarizes the quagmire of quantum foundations.

In response, Rovelli wrote (https://arxiv.org/abs/quant-ph/9609002):
[Q]uantum mechanics will cease to look puzzling only when we will be able to derive the formalism of the theory from a set of simple physical assertions ("postulates", "principles") about the world. Therefore, we should not try to append a reasonable interpretation to the quantum mechanics formalism, but rather to derive the formalism from a set of experimentally motivated postulates.

The reasons for exploring such a strategy are illuminated by an obvious historical precedent: special relativity. ... Special relativity is a well understood physical theory, appropriately credited to Einstein’s 1905 celebrated paper. The formal content of special relativity, however, is coded into the Lorentz transformations, written by Lorentz, not by Einstein, and before 1905. So, what was Einstein's contribution? It was to understand the physical meaning of the Lorentz transformations.
In other words, Rovelli challenged the foundations community to render QM a "principle theory" like SR (see quote below), which was spawned from a similar quagmire in the late 19th century.

In that situation, physicists were trying to find a causal mechanism ("constructive efforts" per Einstein, see quote below) like the luminiferous aether responsible for length contraction that would then account for the fact that everyone measures the same value for the speed of light c, regardless of their relative motions (observer-independence of c). Einstein played this game himself before giving up, writing (What is the Theory of Relativity? 1919):
By and by I despaired of the possibility of discovering the true laws by means of constructive efforts based on known facts. The longer and the more despairingly I tried, the more I came to the conviction that only the discovery of a universal formal principle could lead us to assured results.
Instead of a "constructive" (causal) account:

aether --> length contraction --> observer-independence of c​

he formulated a "principle" account:

Relativity principle --> observer-independence of c --> length contraction.​

Here is the distinction between constructive and principle theories per Einstein (What is the Theory of Relativity? 1919):
We can distinguish various kinds of theories in physics. Most of them are constructive. They attempt to build up a picture of the more complex phenomena out of the materials of a relatively simple formal scheme from which they start out. Thus the kinetic theory of gases seeks to reduce mechanical, thermal, and diffusional processes to movements of molecules – i.e., to build them up out of the hypothesis of molecular motion. When we say that we have succeeded in understanding a group of natural processes, we invariably mean that a constructive theory has been found which covers the processes in question.

Along with this most important class of theories there exists a second, which I will call "principle-theories." These employ the analytic, not the synthetic, method. The elements which form their basis and starting point are not hypothetically constructed but empirically discovered ones, general characteristics of natural processes, principles that give rise to mathematically formulated criteria which the separate processes or the theoretical representations of them have to satisfy. Thus the science of thermodynamics seeks by analytical means to deduce necessary conditions, which separate events have to satisfy, from the universally experienced fact that perpetual motion is impossible.
Accordingly, SR is a principle theory since its kinematic formalism (Lorentz transformations) follows from an empirically discovered fact ("experimentally motivated postulate" per Rovelli), i.e., the observer-independence of c (light postulate). Of course, the light postulate follows from the relativity principle and Maxwell's equations, e.g., from Physics for Scientists and Engineers with Modern Physics 5e, R. Knight Pearson, San Francisco p. 1057 (2022):
1752682449661.webp

and from Physics for Scientists and Engineers with Modern Physics, R. Serway and J. Jewett, Cengage, Boston
10th ed., Section 38.3 (2019):
1752682617633.webp

Accordingly, length contraction is not a dynamical effect due to some causal mechanism like the luminiferous aether, it is a kinematic fact following from the relativity principle and Maxwell's equations. Now let's return to QM.

Quantum information theorists have since satisfied Rovelli's challenge and rendered QM a principle theory whose kinematic formalism (Hilbert space) follows from the empirically discovered fact called Information Invariance & Continuity (Brukner & Zeilinger, https://arxiv.org/abs/0905.0653):
The total information of one bit is invariant under a continuous change between different complete sets of mutually complementary measurements.
Essentially, that just says the quantum bit of information (qubit) exhibits quantum superposition. The rest of Hilbert space is built starting with the qubit, so quantum entanglement follows from quantum superposition. Of course, the quantum foundations community already knew that the mystery of entanglement follows from the mystery of superposition, so this did nothing to solve the mystery of entanglement. But, it's easy to show that qubit superposition follows from the observer-independence of Planck's constant h between reference frames related by spatial rotations and translations, which can obviously be justified by the relativity principle just like the light postulate ("Einstein's Entanglement: Bell Inequalities, Relativity, and the Qubit", Oxford UP, 2024; https://iopscience.iop.org/article/10.1088/1742-6596/2948/1/012009).

Consequently, quantum entanglement is not a dynamical effect due to some nonlocal or superdeterministic or retro causal mechanism, it is a kinematic fact following from the the relativity principle and Planck's radiation law (where h was introduced as a constant of Nature).

So, the bottom line is that quantum entanglement does not entail "spooky actions at a distance" or causes from the future with effects in the present or conspiratorial causal control over experimental procedures or infinitely many realities or the violation of intersubjective agreement (a la QBism). Just as physicists long ago accepted length contraction and time dilation as kinematic facts rather than dynamical effects, it is perhaps time to do the same with quantum superposition and entanglement. But, that's up to you :-)
 

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  • #83
RUTA said:
Accordingly, SR is a principle theory since its kinematic formalism (Lorentz transformations) follows from an empirically discovered fact ("experimentally motivated postulate" per Rovelli), i.e., the observer-independence of c (light postulate). Of course, the light postulate follows from the relativity principle and Maxwell's equations, e.g., from Physics for Scientists and Engineers with Modern Physics 5e, R. Knight Pearson, San Francisco p. 1057 (2022):
View attachment 363298
and from Physics for Scientists and Engineers with Modern Physics, R. Serway and J. Jewett, Cengage, Boston
10th ed., Section 38.3 (2019):
View attachment 363300
Accordingly, length contraction is not a dynamical effect due to some causal mechanism like the luminiferous aether, it is a kinematic fact following from the relativity principle and Maxwell's equations.
1. Agreed, as fine a statement of relativity as there is. (The observer independence here is the reference frame of the observer.)

RUTA said:
But, it's easy to show that qubit superposition follows from the observer-independence of Planck's constant h between reference frames related by spatial rotations and translations, which can obviously be justified by the relativity principle just like the light postulate ("Einstein's Entanglement: Bell Inequalities, Relativity, and the Qubit", Oxford UP, 2024; https://iopscience.iop.org/article/10.1088/1742-6596/2948/1/012009).
2. I don't understand what you mean when you relate "observer-independence of Planck's constant h" and "spatial rotations and translations". I don't see any relevant connection.

I am referring specifically to the situation with the usual entangled pairs where any same measurement (say polarization) is being made by distant observers. Their perfectly correlated results (ideal case) don't depend on relativistic considerations. Planck's constant is not a factor either, nor is the gravitational constant. In fact, I would quote the first paragraph of your reference, which says there is no reason to suspect a conflict with SR:

"It is widely believed that the violation of Bell inequalities in spacelike separated quantum measurement events creates tension or outright conflict between special relativity (SR) and quantum mechanics (QM) [Bell, 1986, Popescu and Rohrlich, 1994, Albert and Galchen, 2009, Maudlin, 2011, Mamone-Capria, 2018]. However, since the Bell state distribution of quantum events in spacetime is independent of the spatial separation and temporal order of the two measurement events in each trial of the experiment, the joint probabilities (and hence the correlations) are invariant under Lorentz boosts. Additionally, Schr¨odinger’s equation (dynamics) of QM, which gives the time evolution of the state vector in a fixed-dimensional Hilbert space, is simply the low-energy approximation of the Lorentz-invariant Klein-Gordon equation [Zee, 2003, p. 172]. Thus, unless one invokes an additional (hidden) causal mechanism beyond the formalism of QM in an attempt to account for Bell-inequality-violating correlations via causal mechanisms and/or dynamical processes (constructively), there is no reason to suspect a conflict with SR."

There is no particular element of entanglement for which SR need be considered. Entanglement and SR don't really have a point of overlap.
RUTA said:
Consequently, quantum entanglement is not a dynamical effect due to some nonlocal or superdeterministic or retro causal mechanism, it is a kinematic fact following from the the relativity principle and Planck's radiation law (where h was introduced as a constant of Nature).

So, the bottom line is that quantum entanglement does not entail "spooky actions at a distance" or causes from the future with effects in the present or conspiratorial causal control over experimental procedures or infinitely many realities or the violation of intersubjective agreement (a la QBism). Just as physicists long ago accepted length contraction and time dilation as kinematic facts rather than dynamical effects, it is perhaps time to do the same with quantum superposition and entanglement. But, that's up to you :-)
3. Here you are saying that entanglement is not dynamical, it's kinematic. OK, I am fine with that distinction, and am equally fine with the term "adynamical". So if we wanted to be strict, we might modify the phrase "spooky action at a distance" to remove anything that implies the "action" is dynamic. Fair enough.

But I think the term "spooky" is entirely appropriate. We already know that the ONLY predictive methodology for analyzing entangled experimental outcomes is a complete future quantum context. And that context can be nonlocal, involving distant observers making measurement choices outside of common (relativistic) light cones. On the other hand: If you disregard the arrow of time (past to future), there are always entanglement limits involving c. We might then use the term "retrocausal", but we have just agreed that entanglement is "acausal".

So I guess we might say we have in entanglement: "spooky acausal limited nonlocality".
 
  • #84
Thanks much for the detailed response, DrChinese. Let me attempt to clarify my post.

DrChinese said:
1. Agreed, as fine a statement of relativity as there is. (The observer independence here is the reference frame of the observer.)
Inertial reference frames are related by the Poincare transformations -- boosts, spatial rotations and translations, and time translations. The observer-independence of c under boosts is the light postulate of SR leading to length contraction and time dilation. The observer-independence of h under spatial rotations and translations is the "Planck postulate" of QM leading to quantum superposition and entanglement.

DrChinese said:
2. I don't understand what you mean when you relate "observer-independence of Planck's constant h" and "spatial rotations and translations". I don't see any relevant connection.
This is just qubit superposition, e.g., look at spin measurements in the Stern-Gerlach (SG) experiment. As Weinberg said, you're measuring h when you measure spin in the SG experiment. Now suppose you subject a ##\hat{z}## spin up electron to a subsequent measurement in the ##\hat{b}## direction making an angle ##\theta## with ##\hat{z}##:

1752902516085.webp


According to Newtonian mechanics, an angular momentum of ##+1\hat{z}## (in units of ##\frac{\hbar}{2}##) measured in the ##\hat{b}## direction will give ##+1\cos{\theta}##.

1752907864319.webp


But, you can't measure a fractional value for spin because that means you're measuring a fractional value for h (like moving along a light beam at v relative to its source and getting c - v for the speed of the light beam). Instead you get +1 and -1 outcomes that average to ##+1\cos{\theta}##. That and normalization give you the spin-1/2 qubit probabilities. That's the observer-independence of h under spatial rotations leading to quanta of angular momentum. For the photon polarizer, the observer-independence of h under spatial rotations leads to quanta of energy E = hf (if a fraction of a photon passed the polarizer, you'd be effectively reducing h since f doesn't change).

In the double-slit experiment, your different measurements are made by the detector moving from immediately behind the slits ('which-slit' or 'position' measurement) to far from the slits ('interference' or 'momentum' measurement). There qubit superposition is the result of the observer-independence of h under spatial translations leading to quanta of momentum ##p = \frac{h}{\lambda}##).

I'll have to break this post up, since PF is saying my post is too long.
 
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  • #85
DrChinese said:
I am referring specifically to the situation with the usual entangled pairs where any same measurement (say polarization) is being made by distant observers. Their perfectly correlated results (ideal case) don't depend on relativistic considerations. Planck's constant is not a factor either, nor is the gravitational constant. In fact, I would quote the first paragraph of your reference, which says there is no reason to suspect a conflict with SR: ...

There is no particular element of entanglement for which SR need be considered. Entanglement and SR don't really have a point of overlap.
Don't confuse the relativity principle with being "relativistic," Newtonian mechanics follows from conservation and the relativity principle, for example. Here is how the observer-independence of h gives you the joint probabilities for the Bell spin triplet state in its symmetry plane (where same measurement direction means same outcomes to conserve spin angular momentum).

Alice measures in some direction ##\hat{a}## and Bob measures in ##\hat{b}##. Just like the single-particle case, Alice reasons that Bob would have gotten the same ##\pm 1## outcomes she did if he had measured in ##\hat{a}##, but since he measured in ##\hat{b}##, he should get ##\pm 1 \cos{\theta}##:

1752908458172.webp


Of course, Bob can say the same thing about Alice's outcomes:

1752908412220.webp


These requirements and normalization give the Bell spin triplet state joint probabilities in its symmetry plane. If ##\theta## is 60 deg, here is an idealized table of data partitioned differently per Alice and Bob:

1752908541913.webp

Notice that Alice's partition shows clearly that Bob must average his data to satisfy conservation of spin angular momentum, while Bob's partition shows the exact same thing about Alice's data. This should remind you of length contraction where Alice's partition of spacetime events per her surfaces of simultaneity shows clearly that Bob's meter sticks are short, while Bob's partition shows clearly that Alice's meter sticks are short.

I'll have to stop here and do the last part in yet another post.
 
  • #86
DrChinese said:
3. Here you are saying that entanglement is not dynamical, it's kinematic. OK, I am fine with that distinction, and am equally fine with the term "adynamical". So if we wanted to be strict, we might modify the phrase "spooky action at a distance" to remove anything that implies the "action" is dynamic. Fair enough.

But I think the term "spooky" is entirely appropriate. We already know that the ONLY predictive methodology for analyzing entangled experimental outcomes is a complete future quantum context. And that context can be nonlocal, involving distant observers making measurement choices outside of common (relativistic) light cones. On the other hand: If you disregard the arrow of time (past to future), there are always entanglement limits involving c. We might then use the term "retrocausal", but we have just agreed that entanglement is "acausal".

So I guess we might say we have in entanglement: "spooky acausal limited nonlocality".
You have pointed out something I ignored in showing how SR/QM results from the relativity principle ("no preferred reference frame" NPRF) justifying the observer-independence of c/h (NPRF + c and NPRF + h). That is, to get an ontology when we went from Newtonian mechanics to SR, all we had to do was represent the bodily objects of Newtonian ontology as worldtubes in M4. But, we don't have any such obvious way to generate an ontology having discovered QM. That's why, in Chapter 9 of our book and the last section of our paper referenced above, we introduced an ontology that does not entail "spooky actions at a distance" or violate statistical independence, intersubjective agreement, or the uniqueness of experimental outcomes. It is a form of multiscale contextual emergence we call quantum-classical contextuality. Accordingly, NPRF + c is an adynamical global constraint on the distribution of the worldtubes for bodily objects in spacetime and NPRF + h is an adynamical global constraint on the distribution of the quanta of interaction between those bodily objects in spacetime. Essentially, as with SR, we start with the Newtonian ontology of bodily objects in spacetime to find our new ontology given QM. Accordingly, the QM Hilbert space adds a low-energy kinematics for the interactions between those bodily objects via the exchange of quanta.

Here are a couple of PowerPoint slides from my talk at a school on quantum foundations at the University of Trieste last month that sum up this ontology in picture form:

1752909723901.webp


1752910143549.webp


It's spatiotemporally global, so it satisfies the "complete future quantum context" as you recognize is necessary to understand QM phenomena. The type of "nonlocality" that is violated (something about classical ontology has to be violated) is separability, i.e., it is spatiotemporally holistic. It deflates the measurement problem trivially, but I better stop here.
 

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  • #87
RUTA said:
Consequently, quantum entanglement is not a dynamical effect due to some nonlocal or superdeterministic or retro causal mechanism,
I agree with this, but I don't really see why it needs to be totally one way or the other? Even if it is perfectly true that here and now we have some adynamical constraints, that appears to just hold. This does not excluded that there is a constructive account for them. But this constructive account does not need to take the naive form of regular dynamicas, or substance mediated ether etc.

RUTA said:
it is a kinematic fact following from the the relativity principle and Planck's radiation law (where h was introduced as a constant of Nature).
...
or the violation of intersubjective agreement (a la QBism). Just as physicists long ago accepted length contraction and time dilation as kinematic facts rather than dynamical effects, it is perhaps time to do the same with quantum superposition and entanglement. But, that's up to you :-)
I see plenty of "room" for unifiying constructive and principle accounts, dynamical and non-dynamical accounts, if you take the larger evolutionary perspective:

In evolutionary perspective, there can be dynamics within any given kinematical space. But on a larger scale of time, the constraints themselves may evolve in a way that is not captured by a known fixed dynamical law. So I think in the discussion one should distinguish between "dynamical explanations" and "evolutionary explanations" as the former implies alot more assumptions and limitations following from the system dynamics paradigm.

Just a random input. I like you principle account, but I don't think that even if you are correct, there might exists a future explanation for the principle. So if we can keep two thoughts at a time, how about embracing the principle account, but not thing that it menas there is not meaningul constructive account to be found in the future?

/Fredrik
 
  • #88
Fra said:
I agree with this, but I don't really see why it needs to be totally one way or the other? Even if it is perfectly true that here and now we have some adynamical constraints, that appears to just hold. This does not excluded that there is a constructive account for them. But this constructive account does not need to take the naive form of regular dynamicas, or substance mediated ether etc.


I see plenty of "room" for unifiying constructive and principle accounts, dynamical and non-dynamical accounts, if you take the larger evolutionary perspective:

In evolutionary perspective, there can be dynamics within any given kinematical space. But on a larger scale of time, the constraints themselves may evolve in a way that is not captured by a known fixed dynamical law. So I think in the discussion one should distinguish between "dynamical explanations" and "evolutionary explanations" as the former implies alot more assumptions and limitations following from the system dynamics paradigm.

Just a random input. I like you principle account, but I don't think that even if you are correct, there might exists a future explanation for the principle. So if we can keep two thoughts at a time, how about embracing the principle account, but not thing that it menas there is not meaningul constructive account to be found in the future?

/Fredrik
We have not ignored the empirical fact that dynamical subjective experience is the phenomenological starting (and ending) point of all science (e.g., Einstein's "On the Method of Theoretical Physics," 1934). We don't claim the block universe objective spacetime model of reality that unifies the disparate subjective spacetime models renders dynamical subjective experience an 'illusion'. The title of our first book is "Beyond the Dynamical Universe: Unifying Block Universe Physics and Time as Experienced" (Oxford UP, 2018) and the last third of that book deals exclusively with the co-fundamentality of the dynamical subjective and the adynamical objective (radical empiricism per James). We write extensively about this (but just phenomenologically) via Einstein's essay "Physics and Reality" (1936) in Chapter 1 of "Einstein's Entanglement." But, we should stop here because now we've moved beyond the OP of "action at a distance resulting from quantum measurement" :-)
 
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  • #89
RUTA said:
Just as physicists long ago accepted length contraction and time dilation as kinematic facts rather than dynamical effects, it is perhaps time to do the same with quantum superposition and entanglement. But, that's up to you :-)
As there is at least one more critical constant to worry about if you like Einstein seems to see no way to avoid the spacetime continuum.

Can we expect that the next step in the work - in your spirit - to be something like "NPRF+c+h+G"?

/Fredrik
 
  • #90
A principles-based understanding of a physical theory is desirable, but I don't think it dissolves interpretational disputes. Even in relativity, these disputes persist (see e.g. Substantivalism vs Relationalism). It's simply the case that quantum theories are more protean than classical theories, with a richer landscape of successful interpretations.
 
  • #91
Fra said:
As there is at least one more critical constant to worry about if you like Einstein seems to see no way to avoid the spacetime continuum.

Can we expect that the next step in the work - in your spirit - to be something like "NPRF+c+h+G"?

/Fredrik

“‘Mysteries’ of Modern Physics and the Fundamental Constants c, h, and G,” W.M. Stuckey,
Timothy McDevitt and Michael Silberstein. Honorable Mention in the Gravity Research
Foundation 2021 Awards for Essays on Gravitation.

Quanta 11(1), 5-14 (2022).

https://dankogeorgiev.com/ojs/index.php/quanta/article/view/66
 
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  • #92
Morbert said:
A principles-based understanding of a physical theory is desirable, but I don't think it dissolves interpretational disputes. Even in relativity, these disputes persist (see e.g. Substantivalism vs Relationalism). It's simply the case that quantum theories are more protean than classical theories, with a richer landscape of successful interpretations.
What I have in mind is the intepretation of the principles and how that influence how we view and navigate in theory space.

If we view them as unexplainable fixed facts of nature for us to discover empirically, a typical theory ansatz has plenty fine tuning room. The challenge then is how to enlarge theory space without getting lost in the same.

Or if we see the principles that relate subsystems as emergent as having been tuned by nature itself. Then there may be deeper principles for how persistent principles emerge, that may allow for a more convergent approach to enlarge theory space that maintain stability over evolution. In this case principles likely has an hierarchy, some came about before others.

This is what i meant by principle and constructive principles can complement each other, or be different sides of the same coin.

/Fredrik
 
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