Entanglement and FTL signaling in professional scientific literature

[PeroK]

Referencing posts 163 & 164 ...

The reminder that we are talking about a single system composed of more than one particle took me some time to internalize, and was very helpful. At least I hope it was - as often as not when I post about directions my understanding is going, I am course corrected!

Take 2 entangled particles, A and B, and specify a specific axis of measurement. When I specify a time, I mean the elapsed time post-preparation. Measure particle A at t=1s, say the result is "up". If I can choose to believe the untestable hypothesis that the "up" result depended only on my choice of axis, and not at all on my choice of when to measure (if I did the measurement at t=10s or t=0.01s I'd still have gotten "up"), then this for me resolves all of the mystery, as it implies that particle B will measure "down" no matter when I measure it and with no need to have any post-preparation interactions with particle A.

Does such a hypothesis break anything? Is it already demonstrably false by experiment despite my thinking that this is basically not testable? I think its consistent with relativity, since there is no unique privileged duration between the preparation event and the measurement event, so its hard to see how one can get a different result just by choosing to take the measurement at a different time.

To try to answer what I take to be at the heart of your question.

All QT gives you is the probability of measuring a certain value at a certain time. In this case we could have a time-independent equal probability of up or down. But, if a particle was measured as "up" at time ##t##, there is no sense you can conclude that you would have got up at any other time. And, in fact, to assume otherwise may lead to the requirement for hidden variables.

This is generally the case across QM. All you can say about measurement outcomes is about the outcomes you actually measure. In general, you cannot infer the results of measurements you didn't make.

 

[Grinkle]

there is no sense you can conclude that you would have got up at any other time

Ok, I understand. Regarding my comment on relativity, thinking a little more I guess the measurement is a unique event, and a different measurement would be a different unique event, and all observers would agree they are unique events, which is what matters, even though both events can be rotated around on a space-time co-ordinate system, and one cannot hypothesize that both events would have given the same result. So all one can say about relativity is that the specific result of a specific measurement is frame independent.

 

[PeroK]

Ok, I understand. Regarding my comment on relativity, thinking a little more I guess the measurement is a unique event, and a different measurement would be a different unique event, and all observers would agree they are unique events, which is what matters, even though both events can be rotated around on a space-time co-ordinate system, and one cannot hypothesize that both events would have given the same result. So all one can say about relativity is that the specific result of a specific measurement is frame independent.

A given event is identifiable independent of the coordinate system. You cannot map event A to event B by changing coordinates.

 

[Grinkle]

A given event is identifiable independent of the coordinate system. You cannot map event A to event B by changing coordinates.

Yes, I agree- I am saying my statement that relativity implies two measurements taken at two different times should give the same result is (was) wrong, they remain two different measurements, even if one rotates them around on a co-ordinate system. All one can say is that a single measurement can be rotated around and the result of that one measurement won't change.

 

[agnick5]

Thank you, sincerely, for the detailed response ( @vanhees71 @gentzen). I have a better understanding now and appreciate you taking the time. Your words are not lost on me, and I will continue to consider them in detail. I feel I now have a little bit better understanding and will commence additional studies. I understand that I actually cannot understand in fully this subject right now, based on limited prior training and education in this area (in particular mathematics), but your efforts are not in vain. And the "final answers" (if there are any) cannot probably be fully understood by someone like me without the appropriate back-round and mathematical expertise. As explanations are continually "dumbed down", then something important is likely lost. Therefore, like most things important in life, to understand something beyond a superficial analysis requires study and time.

On a side note, there must be so much hidden and valuable knowledge buried within countless detailed posts on this forum. It's hard for me to fathom these efforts or their purpose, but I do appreciate them very, very much.

Last question and then I go back to lurking: Is this a good textbook to begin or help to further my admittedly limited understanding? If so, I will purchase it immediately.

Introduction to Quantum Mechanics (3rd Edition) by David J. Griffiths

Thank you again.

 

[PeroK]

Thank you, sincerely, for the detailed response ( @vanhees71 @gentzen). I have a better understanding now and appreciate you taking the time. Your words are not lost on me, and I will continue to consider them in detail. I feel I now have a little bit better understanding and will commence additional studies. I understand that I actually cannot understand in fully this subject right now, based on limited prior training and education in this area (in particular mathematics), but your efforts are not in vain. And the "final answers" (if there are any) cannot probably be fully understood by someone like me without the appropriate back-round and mathematical expertise. As explanations are continually "dumbed down", then something important is likely lost. Therefore, like most things important in life, to understand something beyond a superficial analysis requires study and time.

On a side note, there must be so much hidden and valuable knowledge buried within countless detailed posts on this forum. It's hard for me to fathom these efforts or their purpose, but I do appreciate them very, very much.

Last question and then I go back to lurking: Is this a good textbook to begin or help to further my admittedly limited understanding? If so, I will purchase it immediately.

Introduction to Quantum Mechanics (3rd Edition) by David J. Griffiths

Thank you again.

I like Griffiths. As an alternative, check out James Cresser's notes from his webpage at McQuarrie University. That's at University level and focuses more on justifying why QM is the way it is. It's extremely insightful in my opinion.

 

[DrChinese]

a. Some experiments make this choice locally and randomly at A's and B's places such that these choices are space-like separated. According to the microcausality principle thus the choices cannot be in causal connection when interpreted within standard microcausal relativistic QFT.

b. There is no medium. The correlations are due to the preparation of the photon pair in an entangled state.

c. I don't know, what you mean by "quantum equilibrium". The entangled two-photon state is not an equilibrium state.

d. You are the one repeatedly claiming a causal connection between spacelike separated events, not I. I try to explain, why within the standard microcausal relativistic QFTs this cannot be right.

e. If A and B coexist with the entanglement source in one frame they coexist with it in any frame. The physics doesn't change under Poincare transformations, because microcausal QFT is Poincare-covariant and physical observables are Poincare invariant.

a. This is the question we seek to resolve. The problem is your use of the word "causal". There is an influence, but it does not meet the definition of "causal". See d. below.b. The "medium" I referred to is the entangled 2 particle quantum state. I said: "The medium for the influence is a spatiotemporally extended (i.e. across space and/or time) quantum system of 2 particles, which then becomes 2 quantum systems of 1 particle - once both Alice and Bob make their measurements. It's a bubble, if you will, which eventually "pops".

In other words: i) we start with a single 2 particle quantum system with spatial extent. Surely there is no controversy about this. ii) We later end up with 2 distant systems of 1 particle. Surely there is no controversy about this either. We don't know the particulars about what happens in between i) and ii). Surely there is no controversy about this either. c. After we have ii) above, the 2 separated particles are in what I would call a quantum equilibrium. The measurement outcomes are consistent with either of 2 measurement bases: either that of Alice, or that of Bob (or both if they are the same). By consistent I mean: they agree with the quantum expectation, which is ONLY dependent on both Alice and Bob's choices from an infinite set of measurement bases. Let me be more specific by way of an example.

We start with a polarization entangled 2 photon state such that both photons are parallel (this is your "correlations are due to the preparation"). We'll call this the "E2" state. Alice measures a photon (we'll call that A1) at 0 degrees, and Bob measures a photon (we'll call that B1) at 5 degrees offset from Alice. According to QM, the correlation will be 99.24%, which tells us that the A1/B1 outcomes are sharply defined by Alice and Bob's measurement bases. The equilibrium I refer to is a result consistent with Alice's single particle being polarized at 0 degrees, and Bob's single particle is the same; or Bob's single particle being polarized at 5 degrees, and Alice's single particle is the same. Since A1 and B1 are distant, there is no way for this equilibrium (it might also be called "symmetry") to have evolved from the earlier 2 particle system E2 unless there is some mutual influence between the measurements of Alice and Bob, regardless of their distance. [If you prefer a different terminology: you could also describe E2, A1, and B1 as part of a common context, noting that the context of A1 and B1 relative to each other is distant and that the E2 system expanded to form a spacetime bubble (or spacetime volume, if you prefer).]

That influence need not meet the criteria of a "causal" influence or connection for 2 reasons: a) we don't know the time direction of the influence; there is no evidence for it moving from past to present, or from present to past; b) the outcome as 0 or 1 at the measured angles are random, even if A1 and B1 are the same 99.24% of the time. If those results are either 0 & 0 or 1 & 1 equally, totaling 99.24%; and we have no clue as to what "causes" them to be both 0 or both 1, then we should reject there being a root cause. Certainly, it was not predetermined from the E2 state (as shown by Bell).

Note however, we would get the exact same statistical results if Alice measured at 45 degrees and Bob measured at 50 degrees!

d. Hopefully you agree now that I am not claiming the existence of a causal influence. I don't know what is influencing what, or by what mechanism. And I don't know what to call this quantum influence. I just know that it does not respect classical limits (c). The professional community calls this "quantum nonlocality", and as mentioned previously there are literally thousands of references in the literature to the same just in titles of recent papers. It doesn't matter that Gell-Mann did not like the word, or that you deny the existence of quantum nonlocality. I have described it as best as I can, and I don't think there is any factual element of the description that you will disagree with OTHER than the conclusion. e. We agree! So there is no need for you to mention frame, as there is no difference in results regardless of any choice of relativistic frame. It's a red herring.

---------------------

Hoping you are enjoying the weekend. If I am not mistaken, you are perhaps in Germany or thereabouts? I am in the US - it's been a hot summer here in Texas. Highs around 37 or 38 C.

 

[Nugatory]

1) Is there any explanation as to how a quantum system of entangled particles that is separated by an arbitrarily large distance is connected and inseparable?

No. Or at least if there is such an explanation no one has found it yet and if one is found Bell’s Theorem tells us that it will be just as offensive to our classical intuition as the simple “No”.

If there is not, what exactly about my intuition must I give up?

There are many different ways of describing the insult to our intuition here. See below.

2) And if whatever we are measuring is indeterminate until measured, how is it that we can know the answer to other particle's measurement, which itself isn't determined until measured?

We don’t. We know what the answer would be if the measurement is made, but that is not the same thing as knowing what the answer is. Our classical intuition that these must be the same thing may be what you have to give up.

 

[Fra]

1) Is there any explanation as to how a quantum system of entangled particles that is separated by an arbitrarily large distance is connected and inseparable?

The inseparable concept makes more sense once you notice to that it's not like the two entagled parts are just sent off in different direction into the unknown environment without control and expect them to be treated as one system whatever happens.

Instead one must protect the isolate the entangled parts from interacting with the environment(prevent decoherence). This is not a trivial thing to accomplish experimentally from the perspective of engineering. So when one sees that the parts of the entangled system needs to be isolated from the environment, it makes a lot more sense to see it as "one system", simply because it's isolated since created! (no matter how far separated) (The difficulty in actually KEEPING them isolated is a different discussion, but it's also easier to understand entanglement once when one understands that is difficult)

If there is not, what exactly about my intuition must I give up?

You will get different proposals from different people I fear.

This is food for the interpretation section, for example there is a thread already. https://www.physicsforums.com/threa...ntum-entanglement.1017194/page-7#post-6790053

/Fredrik

 

[vanhees71]

Last question and then I go back to lurking: Is this a good textbook to begin or help to further my admittedly limited understanding? If so, I will purchase it immediately.

Introduction to Quantum Mechanics (3rd Edition) by David J. Griffiths

Thank you again.

Given the many posters, who are confused by this book, I cannot recommend it. My favorite as a first book at the university level is Sakurai, Modern Quantum Mechanics (Revised edition or 2nd edition).

 

[Fra]

I've never seen a good book on QM ;) One of the best moments was when i pressed a lecturer who admitted that most of the explanations in the book as ad hoc stuff (while others tried to stay behind smoke) justified by experimental verification. So if you read and don't get the explanations, you are probably getting it just right and if you think you get it, you probably missed something!

/Fredrik

 

[Fra]

To add to the post above, I think the thing that you CAN understand, is to understand how and why QM was constructed they way it is within science, based on the history of physics and experiments back then etc. This is why I think the "best" books are those that are faithful to the history and how ideas and even their philsophies developed by it's founders who "came from" classical physics, without hiding the wrong turns.

Then can at least achieve this "understanding"

1. Why science came up with QM - as a farily rational guess based on the history
2. Learn and understand that this corroborates well

Any "understanding" beyond this, which many seek, I have never found in a textbook. Some books plays our supposedly plausible arguments though, but rarely succedd. The better ones are the historically honest arguments, ie. to explain how the founders reasoned.

/Fredrik

 

[PeroK]

Any "understanding" beyond this, which many seek, I have never found in a textbook. Some books plays our supposedly plausible arguments though, but rarely succedd. The better ones are the historically honest arguments, ie. to explain how the founders reasoned.

I personally have found nothing like this in the textbooks I have. Griffiths, Sakurai and Lancaster & Blundell.

It may be your personal opinion that there are no good books on QM and that only the historical development contains anything of value, but that is an eccentric, idiosyncratic and contrary position, which is hardly likely to help a modern student.

 

[vanhees71]

I think the historical approach has both its merits and its flaws, and as usual the dose makes the poison. For sure one should not start with a historical approach but with a clear exposition of the theory as it is understood since its development in the late 1920ies, i.e., using the adequate math (rigged Hilbert spaces) and the minimal statistical interpretation (which is a flavor of Copenhagen but without cut nor a collapse). In developing this, it is helpful to give some overview over the historical development, but excluding "old quantum mechanics" entirely, i.e., there should be no "wave-particle duality" nor "Bohr-Sommerfeld model of atoms" and also no point-particle picture for photons, since these often discussed issues are misleading and contradicting the findings of modern quantum mechanics rather than helping to understand them.

A good approach is that by Feynman, discussing the double-slit experiment with massive (non-relativistic) particles or using polarization states of the em. field for a bra-ket-first approach instead of a wave-mechanics-first approach. Last but not least there must be a good balance between sloppiness and mathematical rigor concerning the mathematical foundations. The two-particle-system approach has the advantage that you don't start with the somewhat complicated issue of self-adjoint operators with a continuous spectrum (or, in mathematical terms, unbound operators). So you can develop for quite a while all the physically challenging subjects, including the probabilistic interpretation, entanglement, etc. without being disturbed with mathematical subtleties like domains and co-domains of self-adjoint operators, proper and generalized "eigenvalues" and "eigenvectors", but also this must of course be covered pretty soon, and here some rigor is necessary, and this is where Griffiths's book lacks. I don't know, how it could become so popular. My favorite for a first introduction is Sakurai, Modern Quantum Mechanics. A very good supplement for additional reading is Ballentine, Quantum Mechanics since it gently introduces rigged Hilbert spaces and has a very thorough discussion of the representation theory of the (quantum) Galilei group. Last but not least it makes you somewhat immune against all kinds of "interpretational quantum esoterics" by sticking stricty to the minimal statistical interpretation.

 

[Grinkle]

ii) We later end up with 2 distant systems of 1 particle. Surely there is no controversy about this

Can you help me understand what you mean by this? To my understanding, what makes the 2 particles a system is the preparation that causes entanglement. In other words, we call the two particles a system soley because they are entangled. This entanglement is not affected by distance. In the sense the multiple particles were a system post-preparation, they remain so until one of them is 'measured', or interacts with another particle, or collapses, or whatever (really trying not to inject vague and distracting words here ironically by using a whole string of them!) irrespective of distance between them, which distance anyway is frame-dependent. What am I missing here?

 

[vanhees71]

Of course we have a two-photon system, not a single-photon system, be they prepared in an entangled state or not.

 

[Fra]

there must be a good balance between sloppiness and mathematical rigor concerning the mathematical foundations.

Beeing clear on what is a deduction, what are educated guesses that are corroborated, and what are definitions and what are assumptions might help some. At the time of the first QM courses, my recent classes was mathematics and some classical and stat mech, any everythign was quite clear up until that point, it was a nice harmony between deductive systems, axiomatic approaches and determinism that got together well.

But QM, really challenged a lot of this, and while it was not a problem to understand the model of linear functional spaces, and the axioms of QM, the correspondence to nature was not explained with the same rigour and clarity that you had been used to up until that point. And this is what I felt was the main topic, to just solve an computational task withing QM, is essentially mathematics. But I think most wish to get and understanding as well.

My approach has been to read several books, as they have pros and cons. I had Sakurai, Itzykson Zuber and Branschen, and some other books as official books. But some teacher just justed their own notes and rarely referred to the books.

When I read some books and wrtings (not textbooks) from Hiesenberg, Dirac and Neumann you could from combining there perspectives understand the relation between the quantum world, from the thikning they were coming from. Sometimes that I did not quite grasp from the official textbooks, and lecturers mainly offered the usual physics style "inferences" which are admittedly not stringent, yet in the end, it's used as it's facts.

/Fredrik

 

[DrChinese]

Can you help me understand what you mean by this? To my understanding, what makes the 2 particles a system is the preparation that causes entanglement. In other words, we call the two particles a system soley because they are entangled. This entanglement is not affected by distance.

In the sense the multiple particles were a system post-preparation, they remain so until one of them is 'measured', or interacts with another particle, or collapses, or whatever (really trying not to inject vague and distracting words here ironically by using a whole string of them!) irrespective of distance between them, which distance anyway is frame-dependent. What am I missing here?

DrChinese earlier said:
i) we start with a single [entangled] 2 particle quantum system with spatial extent.
ii) We later end up with 2 distant systems of 1 particle.
iii) We don't know the particulars about what happens in between i) and ii).

As you say: Bell tests start with a system prepared per i).

You then mention them remaining in this state until one of the two is measured. Actually, we cannot be entirely sure of this particular point - as there is no test which can determine if entanglement continues after the "first" measurement. What you can say confidently is that the entanglement ends* after BOTH particles have been measured. That is my ii).

Although you could say distance is frame dependent, distance is not a factor in observed outcomes - and neither is the frame. As a result: if anything propagates/collapses/influences anything else in the process of the system evolving from i) to ii), then c is not respected in that evolution.

------------------------

*In principle: entangled pairs are often entangled in more than one basis (or degree of freedom). For example, they can be both spin entangled, and position-momentum entangled. In such case, it is possible to observe spin entanglement (by spin measurements) without disturbing the remaining entanglement (and vice versa). This is called hyperentanglement. This experiment demonstrates hyperentanglement and discusses its theoretical nature in detail (although it is not an exact demonstration of entanglement persisting in the manner I mention):

https://arxiv.org/abs/2207.09990
We experimentally investigate the properties of hyperentangled states displaying simultaneous entanglement in multiple degrees of freedom...

And as a snarky aside (please forgive me this indulgence!): 2 quotes from the above paper showing the acceptance of the descriptive terminology which is generally accepted by the physics community (since this is a 2022 paper from a team led by Paul Kwiat): "...the original purpose of Bell tests, providing a measurable criteria for separating local and nonlocal theories, has been largely fulfilled..." and "In this paper, we investigate nonlocality tests on hyperentangled quantum states." Just sayin'... "nonlocality" it is. Of course, they mean "quantum nonlocality". As they are simply endorsing the usual view within the community, and not endorsing an explicitly nonlocal theory such as Bohmian Mechanics.

 

[Morbert]

Gell-Mann dismisses Bell's essential point by claiming that it is explained by the "decoherent [sometimes consistent] histories" interpretation. So I guess in that respect, your point about my description being "interpretation dependent and therefore not obvious" has some merit. I just don't see how Alice's selection of a measurement basis *here* - which casts distant Bob's outcome into a precisely synchronized result *there* - should not be described as a (quantum) nonlocal influence. It doesn't happen by coincidence, as Bell showed us...

It's the bit in bold that needs to be reified. Are you presenting it as an interpretation-independent description? A consistent historian would not say Alice's selection of a measurement basis casts Bob's outcome into a synchronised result. They would instead say if we want to use quantum mechanics to successfully predict the correlations Alice and Bob can reproduce in their experiment, we must use the appropriate basis spanning the photon pair Hilbert space (not just the space of Alice's photon). I.e. the basis selection is not done in the real world by, say, the measurements Alice and Bob decide to do. It's done in the notebook of the physicist who wants to make contact between QM and experiment.

 

[DrChinese]

Are you presenting it as an interpretation-independent description? A consistent historian would not say Alice's selection of a measurement basis casts Bob's outcome into a synchronised result. They would instead say if we want to use quantum mechanics to successfully predict the correlations Alice and Bob can reproduce in their experiment, we must use the appropriate basis spanning the photon pair Hilbert space (not just the space of Alice's photon). I.e. the basis selection is not done in the real world by, say, the measurements Alice and Bob decide to do. It's done in the notebook of the physicist who wants to make contact between QM and experiment.

In my mind, the description I present is an accepted fact; and it matters not whether Alice's measurement occurs before Bob's. Of course, it is equally true from Bob's perspective vis a vis Alice. Only Alice and Bob's measurement choices contribute to the observed outcomes. If Alice and Bob choose the same measurement basis (say 0 degrees for a spin measurement): the results are perfectly correlated (or anti-correlated as the case may be). This is true for any angle (0 degrees, 1 degree, 2 degrees, etc). Yet the outcomes cannot be predetermined (since the predetermination would by definition need to be independent of both Alice's and Bob's choice, since Bell showed us there are no possible states consistent with quantum predictions). Either Alice's measurement casts Bob's particle into a state synchronized with Alice, or Bob's measurement casts Alice's particle into a state synchronized with Bob. How is there any other conclusion? Either are best described as a quantum nonlocal influence (or collapse, or context or whatever you want to label it).

Pretty much every interpretation brings about the same conclusion: MWI (worlds split), Bohmian (action at a distance), RBW (acausal context), physical collapse interpretations (nonlocal collapse). Of course, a few give us crickets (silence) but that obviously begs the question.

 

[PeroK]

Either Alice's measurement casts Bob's particle into a state synchronized with Alice, or Bob's measurement casts Alice's particle into a state synchronized with Bob. How is there any other conclusion?

There's the conclusion "neither". At the moment that seems the most robust answer, since there is no experimental evidence of any influence or which way the influence goes. Moreover, any attempt to find an influence between the particles casts doubt on the theory of relativity.

I would say that if there is an influence, then it should be experimentally detectable. How is there any other conclusion?

 

[DrChinese]

There's the conclusion "neither". At the moment that seems the most robust answer, since there is no experimental evidence of any influence or which way the influence goes. Moreover, any attempt to find an influence between the particles casts doubt on the theory of relativity.

I would say that if there is an influence, then it should be experimentally detectable. How is there any other conclusion?

It IS experimentally detected! The influence doesn't come from anywhere OTHER than Alice or Bob. Only Alice and Bob's choices matter, and the results are consistent with nothing else. As I mentioned, "crickets" is an interpretation as well, but it avoids this obvious point.

 

[PeroK]

It IS experimentally detected!

It's not in the standard model. Where is the mathematical description of that interaction?

All that is experimentally detected is a correlation. Not a communication, not an interaction, not an influence. Only a correlation.

 

[CoolMint]

Only Alice and Bob's choices matter, and the results are consistent with nothing else.

There are no particles as such and your conditions are too stringent. There are only fields.

Nature allows certain events at the quantum scales that defy the Newtonian worldview.

 

[agnick5]

In my mind, the description I present is an accepted fact; and it matters not whether Alice's measurement occurs before Bob's. Of course, it is equally true from Bob's perspective vis a vis Alice. Only Alice and Bob's measurement choices contribute to the observed outcomes. If Alice and Bob choose the same measurement basis (say 0 degrees for a spin measurement): the results are perfectly correlated (or anti-correlated as the case may be). This is true for any angle (0 degrees, 1 degree, 2 degrees, etc). Yet the outcomes cannot be predetermined (since the predetermination would by definition need to be independent of both Alice's and Bob's choice, since Bell showed us there are no possible states consistent with quantum predictions). Either Alice's measurement casts Bob's particle into a state synchronized with Alice, or Bob's measurement casts Alice's particle into a state synchronized with Bob. How is there any other conclusion? Either are best described as a quantum nonlocal influence (or collapse, or context or whatever you want to label it).

Pretty much every interpretation brings about the same conclusion: MWI (worlds split), Bohmian (action at a distance), RBW (acausal context), physical collapse interpretations (nonlocal collapse). Of course, a few give us crickets (silence) but that obviously begs the question.

I probably should have stopped reading this thread after @vanhees71 and @Nugatory explained many things to me. But the bolded part above remains the basis for my previous questions. I realize fully that it is not me who has come to this realization and that this "problem" (not a problem for some) is well understood and already studied (simply new to me). I do not argue this point then, and accept the answers given. But it appears (to me) then that either there is no answer that can reconcile with my intuition (just the way it is, deal with it and start calculating), or I must move my study to the various interpretations for a deeper analysis if not satisfied, and open myself up to philosophizing. (after first understanding fully the minimal interpretation, including the appropriate math - textbook ordered).

 

[DrChinese]

It's not in the standard model. Where is the mathematical description of that interaction?

All that is experimentally detected is a correlation. Not a communication, not an interaction, not an influence. Only a correlation.

Haha, well I think you boxed yourself in on this one. The standard model predicts the results depend on Alice and Bob's choices, and there are no other input variables to the formulae like cos^2(Alice- Bob). The outcome of Alice's choice allows me to exactly predict the result that distant Bob sees. Cause, meet effect!

Further, a critique complaining of it being "only" a correlation fails, as all scientific experiments depend on correlations where certain inputs are held constant. The issue is that with entanglement, there is no earlier predecessor variable which you can identify as a candidate "common cause", which you would certainly like to have. But that is not a flaw in my description. In other words: for the results to represent "only" a correlation - as you assert above: there would need to exist a such a prior common "root" cause. But according to Bell: there can be no prior common cause explaining your hypothetical "correlation"!

My explanation is as accurate as it gets, and it really shouldn't be controversial in the least if you read it without attempting to tear apart things word by word. My point is not that the mystery disappears with this explanation, it doesn't. My point is that there are facts and logical deductions we can agree upon, and we should start there. Obviously, it is not an accident or coincidence that Alice (or Bob) cast the resulting 2 individual particles (coming from a 2 particle entangled system) into a state consistent with Alice's measurement choice. That's true in every Bell test.

And finally: as I have quoted extensively, the existence of quantum nonlocality is generally accepted within the leaders of the physics community. Which is what we should be referencing in this Quantum Physics thread, as this is the PF standard. Further discussion that debates generally accepted physics should rightfully be discussed in a different forum. This is but one example of generally accepted physics, demonstrating that quantum nonlocality has been demonstrated in Bell tests:

Paul Kwiat et al (2022): ...the original purpose of Bell tests, providing a measurable criteria for separating local and nonlocal theories, has been largely fulfilled..."

 

[PeterDonis]

Either Alice's measurement casts Bob's particle into a state synchronized with Alice, or Bob's measurement casts Alice's particle into a state synchronized with Bob.

But, as has already been pointed out, neither of these can be right, because they are both asymmetric, but the situation is symmetric between Alice and Bob. So whatever is going on "behind the scenes" would need to be symmetric between them as well.

How is there any other conclusion?

Nobody really knows at present, but that's not at all the same as any other conclusion being impossible. This is simply something we don't fully understand. But part of what we do understand is that the situation is symmetric between Alice and Bob: we know the measurements commute. So any resolution, it seems to me, would have to be consistent with that.

 

[Morbert]

If Alice and Bob choose the same measurement basis (say 0 degrees for a spin measurement): the results are perfectly correlated (or anti-correlated as the case may be). This is true for any angle (0 degrees, 1 degree, 2 degrees, etc). Yet the outcomes cannot be predetermined (since the predetermination would by definition need to be independent of both Alice's and Bob's choice, since Bell showed us there are no possible states consistent with quantum predictions).

Say Alice measures the spin of her photon at some angle and observes "up". Is the following statement true:

"If bob performs the same measurement on his photon, there is a 100% chance he will observe 'up', but if Alice had not measured her particle, there would be less than 100% chance that Bob will observe 'up'"

 

[Grinkle]

"If bob performs the same measurement on his photon, there is a 100% chance he will observe 'up', but if Alice had not measured her particle, there would be less than 100% chance that Bob will observe 'up'"

FWIW, I think that if what I posited in post 162 is not a palatable line of thinking, then the answer should be yes, there is a less than 100% chance that Bob will observe 'up'. If the answer is otherwise, then there is no weirdness going on, right?

 

[Fra]

all scientific experiments depend on correlations where certain inputs are held constant. The issue is that with entanglement, there is no earlier predecessor variable which you can identify as a candidate "common cause", which you would certainly like to have.

I agree.

But according to Bell: there can be no prior common cause explaining your hypothetical "correlation"!

I disagree. Bell only excludes the kind of common cause that follows his ansatz; which is a hidden correlation of "ignorance type". Why would this be exhaustive??

I don't see the problem with a "hidden variable" that explains the correlation only, but where the interaction at either detector does NOT depend on the hidden variable, but just on the preparation (as QM describes, but not necessarily "explains"). (conceptual arguements for this is interpretation dependent so i omit them)

/Fredrik

 

[PeterDonis]

there is no earlier predecessor variable which you can identify as a candidate "common cause"

Sure there is: the entangled state that was prepared. The issue some people have is that that isn't the kind of "variable" they are looking for as a common cause.

 

[vanhees71]

In my mind, the description I present is an accepted fact; and it matters not whether Alice's measurement occurs before Bob's. Of course, it is equally true from Bob's perspective vis a vis Alice. Only Alice and Bob's measurement choices contribute to the observed outcomes. If Alice and Bob choose the same measurement basis (say 0 degrees for a spin measurement): the results are perfectly correlated (or anti-correlated as the case may be). This is true for any angle (0 degrees, 1 degree, 2 degrees, etc). Yet the outcomes cannot be predetermined (since the predetermination would by definition need to be independent of both Alice's and Bob's choice, since Bell showed us there are no possible states consistent with quantum predictions). Either Alice's measurement casts Bob's particle into a state synchronized with Alice, or Bob's measurement casts Alice's particle into a state synchronized with Bob. How is there any other conclusion? Either are best described as a quantum nonlocal influence (or collapse, or context or whatever you want to label it).

The "other conclusion" is that this point of view violates the mathematical properties of QED (or any other microcausal realtivistic QFT): There cannot be any causal influence between space-like separated events. Thus the much less spectacular but consistent conclusion is that the correlations are due to the entanglement of the state prepared before any of the two measurements have been performed. This property of entangled states, i.e., maximal indeterminism of the measured single-photon properties with strong correlations between the outcome of their space-like separated measurements is what distinguishes QT (including microcausal relativistic QFTs) from classical ("local realistic") theories, and all experimental findings are in favor of QT.

This is in so far a "interpretation independent statement", because it just uses the mathematical formulation of the theory and the empirical findings applying it to the corresponding experiments within a "minimal interpretation", i.e., the assumption that the "kinematics and dynamics" of the theory is complete, including the "probabilistic and only probabilistic" meaning of quantum states.

Pretty much every interpretation brings about the same conclusion: MWI (worlds split), Bohmian (action at a distance), RBW (acausal context), physical collapse interpretations (nonlocal collapse). Of course, a few give us crickets (silence) but that obviously begs the question.

You can add as many "interpretation" as you like, but you must stay consistent with the mathematical formulation of microcausal QFT. Otherwise you propose new theories. Particularly "non-local collapse" is very difficult to establish in accordance with the special-relativistic causality structure of Minkowski space.

 

[CoolMint]

The presumed nonlocality of entangled systems would have been a mystery if it happened in a fundamentally classical reality.
The issue, if such exists at all, is that nature is fundamentally non-classical. And we are not particularly good at dealing with emergent properties. At least not yet.

 

[Morbert]

Is the following statement true: "If bob performs the same measurement on his photon, there is a 100% chance he will observe 'up', but if Alice had not measured her particle, there would be less than 100% chance that Bob will observe 'up'"

FWIW, I think that if what I posited in post 162 is not a palatable line of thinking, then the answer should be yes, there is a less than 100% chance that Bob will observe 'up'. If the answer is otherwise, then there is no weirdness going on, right?

If the answer is yes then there is absolutely a nonlocal influence in the common sense of the words. But the answer is interpretation-dependent. Consistent histories let's us readily evaluate counterfactual claims like this, and would in fact imply the statement is false: even if Alice had not carried out her measurement, there would still be a 100% chance that Bob will measure 'up'. Other interpretations might be more conservative and deny any evaluation of counterfactuals, and under any interpretation where the statement is not considered true, it seems hard to conclude nonlocal influence.

 

[gentzen]

Pretty much every interpretation brings about the same conclusion: MWI (worlds split), Bohmian (action at a distance), RBW (acausal context), physical collapse interpretations (nonlocal collapse). Of course, a few give us crickets (silence) but that obviously begs the question.

You can add as many "interpretation" as you like, but you must stay consistent with the mathematical formulation of microcausal QFT. Otherwise you propose new theories. Particularly "non-local collapse" is very difficult to establish in accordance with the special-relativistic causality structure of Minkowski space.

I had to look-up what "give us crickets" means: It is such a perfect silence that you can hear the crickets! And I have to agree, you are giving us exactly this sort of silence. Notice that "the mathematical formulation of microcausal QFT" is not an interpretation, and mumbling "minimal interpretation" without any clarifying explanations doesn't turn it into an interpretation either. When I tried to go a bit into the details (how the perfect locality that "microcausal QFT let's us expect" translates back to an interpretation)

The minimal statistical interpretation is just fine, what is problematic is ...

you just shrugged it off

Yes: Alice meets Bob for a cup of tea bringing her laptop with the measurement protocol to compare it with Bob's on his laptop ;-)).

But it was a mistake anyway for me to try to do your work. My own position is that QM is nonlocal, and that it is the randomness which is nonlocal. This is a widely shared position, Nicolas Gisin's book Quantum Chance convincingly argues for it, as do many of his articles and papers. And it is much easier to explain than performing the interpretative dance required for getting rid of this type of nonlocality in some specific interpretation.

The "other conclusion" is that this point of view violates the mathematical properties of QED (or any other microcausal realtivistic QFT): There cannot be any causal influence between space-like separated events. Thus the much less spectacular but consistent conclusion is that the correlations are due to the entanglement of the state prepared before any of the two measurements have been performed.

This is fine, if you accept the remaining nonlocal randomness. If not, then just looking at the preparation can be insufficient.

It IS experimentally detected! The influence doesn't come from anywhere OTHER than Alice or Bob. Only Alice and Bob's choices matter, and the results are consistent with nothing else. As I mentioned, "crickets" is an interpretation as well, but it avoids this obvious point.

Indeed, the default interpretation of the experimental results is "nonlocality". Even if you don't like it, silence is not a convincing argument against that interpretation.