I Entanglement and FTL signaling in professional scientific literature

[Grinkle]

@PeterDonis Referring to post 126, is it possible for a one-way causal connection between the two measurement events to commute, or is it pick one (commutes) or the other (one-way causality)?

 

[vanhees71]

What do you mean by "one-way causal connection"? By the usual definition a cause and effect relation is possible only between time-like or light-like separated events, and the event A can only be the cause and event B if A is temporally before B. In this sense "causal connections" are always "one way", i.e., if A is a cause for B, then B cannot be a cause for A.

 

[PeterDonis]

is it possible for a one-way causal connection between the two measurement events to commute

If the two measurements commute, the relationship between them is symmetric, but as has already been pointed out, a one-way causal connection is not symmetric (that's what "one-way" means).

 

[PeterDonis]

What do you mean by "one-way causal connection"?

A causal connection in which one event is the "cause" and the other is the "effect". That's a one-way relation.

 

[vanhees71]

Of course, that's implied by "cause-and-effect relation", in short "causality", and it also introduces a "causal time ordering". That's why space-like separated events cannot be in a "cause-and-effect relation", because there is no frame-independent temporal ordering for them.

 

[DrChinese]

a. The correlations are due to the preparation of the photon pair as an entangled quantum state... just to the causal connection due to their common creation...

b. It rules out that there are causal connections between space-like separated observations.

a. The Alice/Bob outcomes are ONLY consistent with a sharply defined measurement basis chosen by Alice or Bob. Further, the only viable conclusion is that there is an influence/synchronization between their results based on the choices made by Alice and Bob, which are distant from each other. Let's call the distance between Alice and Bob, as measured at light speed and denoted by absolute value of time duration, as the "separation duration" (SD=separation duration).

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". We have no idea what happens between the beginning and end of Alice and Bob's measurements, while the bubble exists. Let's call the absolute value of that duration of time (for lack of a better term) the "equilibrium duration" (ED=equilibrium duration). We are completely blind during the equilibrium duration (the life of the bubble).

What we do know is that the results are consistent with the two 1 particle systems being at some quantum equilibrium of states upon completion of the equilibrium duration ED. In a classical system, we would expect that SD/ED<=1 so that c is respected. Of course, with entanglement there is no such constraint.b. Causal? No one knows what happens during the "equilibrium duration" ED so there is no way to attach this label meaningfully. Perhaps it is not causal! You'd be 100% correct (by saying nothing causal is happening) and yet completely wrong at the same time, as there is no one - least of all me - saying there IS something causal occurring in the first place. Keep in mind that the individual outcomes Alice and Bob themselves see are completely random. As far as anyone knows, there is nothing happening in the equilibrium duration ED that "causes" the random outcomes (since we can't see into that process).

c. How can two events be causally connected, if the time order of these events are frame dependent?

c. There are many things implied (and wrong) with this "question".

i) No one is saying there is a causal connection - you are the only person using this terminology, and it obscures the discussion.
ii) QFT does not predict there to be a time order dependency of any kind. The ONLY input variables to obtain the quantum predictions are Alice and Bob's measurement choices.
iii) QFT similarly does not predict a frame dependency of any type, regardless of which element of the experiment you make the frame and regardless of what frame the other elements of the experiment are placed in.
iv) The results can be obtain in a frame in which Alice and Bob unambiguously coexist with the entanglement source.

In other words: your insertion of the concepts of "causality" and "frame dependency" into the sentence (and this discussion) are red herrings. They do nothing but distract. QFT does not predict any relationship between these and the actual experimental outcomes in the first place, so why mention? There is also no association with the price of tea in China, we don't need to discuss that either.Cheers and happy Friday!

 

[PeterDonis]

Of course, that's implied by "cause-and-effect relation", in short "causality", and it also introduces a "causal time ordering".

Of course if you define "causal" to mean a one-way relation, then no relation between spacelike separated measurements can be "causal". But that's due to the definition you picked.

 

[agnick5]

I am new here and not a physicist. I apologize in advance if I mis-use any terminology or am not exact. What I do not understand from Vanhees responses here (and everywhere else) is the following:

1) There is no "spooky action at a distance or FTL communication" due to the microcausility provision.
2) The states of each photon (or whatever) are maximally indetermined. Meaning that the outcome of a measurement at a given angle is random.

So this should mean that measuring entangled particles at different angles should give standardized statistical outcomes. Yet they don't. So something is wrong. Either there are "hidden variables", or FTL communication or something else.

So when we measure one particle, the other particle, which cannot have any hidden variables (Due to Bell and subsequent experiments), takes a DETERMINED state. That is, we automatically know the other particles state even though it must, by definition, be undetermined until measured.

Therefore to me and my untrained opinion, the problem is the discrepancy between hidden variables (and undertermined states) and what happens to the other particle when we measure it's entangled partner.

So my question to Vanhees is as follows:

If the states of each particle are undertermined until measured (not when prepared), how is it possible that by measuring one entangled particle that we then know the state of other one? If we are "preparing" it this way then isn't the state then determined? Which is contrary to my understanding. And why doesn't an ensemble of measurements of different angles yield a normal statistical distribution if there is no FTL signalling and no hidden variables and indeterminancy of states?

I'm sure I can be corrected on my terminology and/or description, and I hope that I am. But I also hope that whoever reads this understands the fundamental point I'm trying to understand. Which again is the supposed random outcomes of measurements against the correlations that suggest otherwise.

Thank you in advance.

 

[physicsworks]

I think the best sources about microcausality and its implications are Weinberg's and Haag's QFT books. I'd count them to the "professional scientific literature"...

I would also add Duncan's "The Conceptual Framework of Quantum Field Theory" to this list.

 

[PeroK]

So this should mean that measuring entangled particles at different angles should give standardized statistical outcomes. Yet they don't. So something is wrong. Either there are "hidden variables", or FTL communication or something else.

So when we measure one particle, the other particle, which cannot have any hidden variables (Due to Bell and subsequent experiments), takes a DETERMINED state. That is, we automatically know the other particles state even though it must, by definition, be undetermined until measured.

Therefore to me and my untrained opinion, the problem is the discrepancy between hidden variables (and undertermined states) and what happens to the other particle when we measure it's entangled partner.

So my question to Vanhees is as follows:

If the states of each particle are undertermined until measured (not when prepared), how is it possible that by measuring one entangled particle that we then know the state of other one?

It's possible because nature does it. Non-locality, in that sense, is part of nature. This is what QM tells us.

Why should I trust your human prejudices about how nature must be?

Why add an unnecessary component to QM of which there is no evidence because of philosophical prejudices?

 

[PeroK]

PS and part of the importance of this thread is to highlight that "FTL" doesn't solve the problem. Not least because you lose relativity and then having added something unnecessary to QM to fix its philosophical problems, you additionally topple another pillar of modern physics. And, IMO, you've lost a lot more than you've gained.

 

[Grinkle]

@PeroK I accept your assertion in post 161, and indeed I felt the same before you posted that (for what that's worth, I am not really in a position to make informed judgements on the positions of the experts in this thread), that said I think post 160 is a bit harsh as a response to post 158; it hasn't been completely clear to me at least that the main argument against the position being argued by Dr Chinese is Occam's razor.

Is it not the case that there is an unresolved paradox, though? It seems so to me. Adding complexity does not feel good as an approach, but neither does leaving things at "nature does it" feel good. By paradox I mean, similar to post 158, that something (the unmeasured entangled particle) shouldn't be random and deterministic simultaneously. To me, dissatisfaction with not understanding how that works is a philisophical predjudice that I would hope all who understand the situation would share.

 

[PeroK]

@PeroK I accept your assertion in post 161, and indeed I felt the same before you posted that (for what that's worth, I am not really in a position to make informed judgements on the positions of the experts in this thread), that said I think post 160 is a bit harsh

It sounds harsher than I intended, but I wanted to get @agnick5 to really question why this is a problem. Dissatisfaction with QM is not totally unjustified, but that itself can't just be thrown into the ring like human dissatisfaction is some unimpeachable judgement on QM.

By paradox I mean, similar to post 158, that something (the unmeasured entangled particle) shouldn't be random and deterministic simultaneously. To me, dissatisfaction with not understanding how that works is a philisophical predjudice that I would hope all who understand the situation would share.

There is no "unmeasured" particle. An entangled two particle state was prepared. Not two independent particles. That two-particle system was measured. @Vanadium 50 put in well in a previous post:

Somehow the combination of the ideas that "QM is fundamentally probabilistic" and "you can have a state consisting of multiple particles" confuses and/or bothers people. They try and shoehorn this into the idea that each particle is independent, except for a spooky causal connection that causes a measurement of one to influence the other.

That's exactly what you are doing here.

 

[Grinkle]

That's exactly what you are doing here.

Agreed, thanks for pointing that out and highlighting Vanadium 50's snip.

 

[Vanadium 50]

highlighting Vanadium 50's snip.

People do say I'm snippy, that's for sure.

 

[PeroK]

I am new here and not a physicist. So something is wrong.

I'm sorry if my previous response sounded harsh. But, why is something wrong? It may be counterintuitive, but try to fully justify why it's downright wrong! Wrong is in itself a harsh judgement (on QM)!

 

[vanhees71]

Of course if you define "causal" to mean a one-way relation, then no relation between spacelike separated measurements can be "causal". But that's due to the definition you picked.

It's not the definition I picked but the definition picked for centuries.

 

[WernerQH]

Non-locality, in that sense, is part of nature. This is what QM tells us.

I agree.

Why should I trust your human prejudices about how nature must be?

Almost everybody believes that
(1) It is obvious that correlations must have causal explanations.
(2) It is obvious that something travels from the source to the detectors, carrying information.

But I think it is not obvious that quantum theory must be based on these beliefs. On the contrary, I think that correlations should be thought of as fundamental, as a brute fact that we cannot explain along familiar (classical) lines of thinking. A fact as strange as that the speed of light in vacuo is the same in every reference system.

It has become conventional to speak of "entangled photons", and that quantum theory provides a complete description of such systems. But it is stretching the meaning of "locality" too much to say that it is a "local" description when the "system" of entangled photons extends over meters or even kilometers. I think we shouldn't assume that anything travels from the source to the detectors. It's neither waves nor particles, and its properties are uncertain or even undefined.

Why add an unnecessary component to QM of which there is no evidence because of philosophical prejudices?

Of course there are lots of experiments that are taken to be evidence for the existence of electrons and photons. But it is misleading to think of them as "objects", in my opinion it is more natural to view them as special patterns of events in space-time.

 

[vanhees71]

a. The Alice/Bob outcomes are ONLY consistent with a sharply defined measurement basis chosen by Alice or Bob. Further, the only viable conclusion is that there is an influence/synchronization between their results based on the choices made by Alice and Bob, which are distant from each other. Let's call the distance between Alice and Bob, as measured at light speed and denoted by absolute value of time duration, as the "separation duration" (SD=separation duration).

Of course, any quantum description is about some well-defined measurement. For von Neumann projective measurements that's equivalent to a choice of a basis of eigenvectors of the self-adjoint operators representing the measured observables.

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.

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". We have no idea what happens between the beginning and end of Alice and Bob's measurements, while the bubble exists. Let's call the absolute value of that duration of time (for lack of a better term) the "equilibrium duration" (ED=equilibrium duration). We are completely blind during the equilibrium duration (the life of the bubble).

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

What we do know is that the results are consistent with the two 1 particle systems being at some quantum equilibrium of states upon completion of the equilibrium duration ED. In a classical system, we would expect that SD/ED<=1 so that c is respected. Of course, with entanglement there is no such constraint.

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

b. Causal? No one knows what happens during the "equilibrium duration" ED so there is no way to attach this label meaningfully. Perhaps it is not causal! You'd be 100% correct (by saying nothing causal is happening) and yet completely wrong at the same time, as there is no one - least of all me - saying there IS something causal occurring in the first place. Keep in mind that the individual outcomes Alice and Bob themselves see are completely random. As far as anyone knows, there is nothing happening in the equilibrium duration ED that "causes" the random outcomes (since we can't see into that process).

Within microcausal relativistic QFT there is no other logical interpretation than the conclusion that space-like separated events cannot be causally connected. I'm not aware of any realization of relativistic QT that's not a microcausal QFT. So I can't speculate whether there are consistent realizations where there are causal influcences between space-like separted events. It would need a completely new definition of spacetime structures and the very fundamental notion of causality. I don't see any necessity for introducing such problems given the contemporary state of observations.

c. There are many things implied (and wrong) with this "question".

i) No one is saying there is a causal connection - you are the only person using this terminology, and it obscures the discussion.

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.

ii) QFT does not predict there to be a time order dependency of any kind. The ONLY input variables to obtain the quantum predictions are Alice and Bob's measurement choices.

Exactly, and that's why there cannot be a causal connection between the measurement choices nor between the detection events in the measurement if these are spacelike separted.

iii) QFT similarly does not predict a frame dependency of any type, regardless of which element of the experiment you make the frame and regardless of what frame the other elements of the experiment are placed in.

Exactly, but @RUTA claimed otherwise, and that's why I argued against this claim.

iv) The results can be obtain in a frame in which Alice and Bob unambiguously coexist with the entanglement source.

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.

In other words: your insertion of the concepts of "causality" and "frame dependency" into the sentence (and this discussion) are red herrings. They do nothing but distract. QFT does not predict any relationship between these and the actual experimental outcomes in the first place, so why mention? There is also no association with the price of tea in China, we don't need to discuss that either.

It was not me, who introduced such claims. To the contrary I argued always against them!

Cheers and happy Friday!

Have a nice weekend too :-).

 

[vanhees71]

I am new here and not a physicist. I apologize in advance if I mis-use any terminology or am not exact. What I do not understand from Vanhees responses here (and everywhere else) is the following:

1) There is no "spooky action at a distance or FTL communication" due to the microcausility provision.
2) The states of each photon (or whatever) are maximally indetermined. Meaning that the outcome of a measurement at a given angle is random.

So this should mean that measuring entangled particles at different angles should give standardized statistical outcomes. Yet they don't. So something is wrong. Either there are "hidden variables", or FTL communication or something else.

I don't know what you mean by "standardized statistical outcomes". The statistics is correctly described by the probabilities as predicted from microcausal relativistic QFT. These are incompatible with the probabilities predicted by a class of theories which Bell called "realistic local hidden-variable theories". Note that Bell uses another meaning of the word "local" than what's used in the QFT community.

So when we measure one particle, the other particle, which cannot have any hidden variables (Due to Bell and subsequent experiments), takes a DETERMINED state. That is, we automatically know the other particles state even though it must, by definition, be undetermined until measured.

There is no contradiction in this.

Therefore to me and my untrained opinion, the problem is the discrepancy between hidden variables (and undertermined states) and what happens to the other particle when we measure it's entangled partner.

Nothing happens to the other particle, at least not instantaneously. Within microcausal relativistic QFT a local measurement at A can have a causal influence at B only if these events are time-like or light-like separated. That's the whole point of the discussion.

So my question to Vanhees is as follows:

If the states of each particle are undertermined until measured (not when prepared), how is it possible that by measuring one entangled particle that we then know the state of other one? If we are "preparing" it this way then isn't the state then determined? Which is contrary to my understanding. And why doesn't an ensemble of measurements of different angles yield a normal statistical distribution if there is no FTL signalling and no hidden variables and indeterminancy of states?

The single-particle states are maximally indetermined, i.e., they are described by maximum-entropy statistical operators. The two-particle state is maximally determined, i.e., it's a pure state. This implies the strong correlations between the single-particle measurements, i.e., entanglement. The correlations are already there due to the preparation of the two-particle system in the entangled state although the single-particle properties are maximally uncertain. There's no need for any FTL signalling, because the correlation is not caused by space-like separated measurements but by the preparation of the two particles in an entangled state.

I'm sure I can be corrected on my terminology and/or description, and I hope that I am. But I also hope that whoever reads this understands the fundamental point I'm trying to understand. Which again is the supposed random outcomes of measurements against the correlations that suggest otherwise.

That's indeed what we discuss all the time in this thread :-).

Thank you in advance.

 

[PeroK]

I don't know what you mean by "standardized statistical outcomes".

I took it to mean using classical probabilities relating to hidden variables; as opposed to using complex probability amplitudes as in QM.

 

[agnick5]

Thank you for the responses. @PeroK @vanhees71 @Grinkle (Who was also kind enough to message me and explain certain things). I have read in detail all of the responses, including this entire thread and many, many others.

I am quite literally trying to understand. And I don't know how else to say it. I accept modern physics. I have no agenda. I am not fighting anything, do not want to overthrow anything and I am willing to give up my intuition. These are honest questions, and I'm willing to work to understand the answers provided from those having more knowledge than me. I am also open to any suggestions on professional literature I can read if that is a better and/or additional way for me to understand - and I am willing to study the mathematics involved (which I am comfortable with). I realize that the words and motivations of posters are important, but I do not want to get trapped in this diversion unless it is creating a fundamental misunderstanding. I would rather discuss the substance (unless that is impossible due to too many incorrect words). But please do me the small courtesy of assuming that my questions are honest questions, with no agenda or hidden motivations.

I'll try one more time. If I use the wrong words, then please correct me and forgive me. This is my layman's understanding using plain English and there is no agenda. I also do my best to rid myself of preconceived notions.

We prepare two particles that are maximally entangled. Certain traits about them are indeterminate and not determined until measurement. So we have a 2 particle quantum system and quantum indeterminancy. We then separate these entangled particles by some distance. We still have a 2 particle quantum system and quantum indeterminancy, and yet somehow they are still connected and inseparable, despite the fact that they are physically separated at arbitrarily large distances. So if we measure one particle, and from this measurement "determine" a trait, we are really measuring the entire system, and therefore know the answer to what would happen to the other particle when measured. These are not 2 separate particles, each indeterminate, but rather one "quantum particle system" that follows quantum indeterminancy and some kind of inseparability. We cannot use this information to transfer signals faster than light. Is this GENERALLY correct?

If what I said is generally true (even if I used the wrong words), do we know what exactly allows for such a system to exist? A system that appears connected or inseparable regardless of distance? I believe in causality. Is there a causal mechanism? We cannot have FTL signalling. I accept that. We cannot have hidden variables (predetermined states), as shown by Bell and subsequent experiments. I accept that as well. So what exactly is this connection that allows you to measure and determine a quantum system of two physically separated particles?

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

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? Is that not a contradiction? Either it is pre-determined or indetermined. It cannot be predetermined (per Bell et al) yet it appears so, or something else is going on. I realize the answer is likely "because it is a quantum system", not two separate particles. But I don't see how that actually explains things. Why do we even need to measure both particles then? Which then brings me back to question 1. How can two particles that are physically separated be connected and inseparable? What makes this system inseparable and not individualistic?

3) What exactly and precisely is this inseparability? I guess that's really the "word" I would like a scientific definition of.

Maybe Perok already gave me the answer in his first sentence. It's just what nature does and is, deal with it and don't probe much deeper as there is nothing deeper. I can accept that too, if that's the generally accepted answer. I don't like it one bit (that's my intuition), but I can still accept it and try to better understand it then.

Thank you.

 

[gentzen]

These are not 2 separate particles, each indeterminate, but rather one "quantum particle system" that follows quantum indeterminancy and some kind of inseparability. We cannot use this information to transfer signals faster than light. Is this GENERALLY correct?

Yes, this is generally correct.

If what I said is generally true (even if I used the wrong words), do we know what exactly allows for such a system to exist?

The word "exactly" is unfortunate, but there was a sort of explanation in a related thread which caused the discussion about words in this thread. In special relativity, different groupings of events into being simultaneous can explain how time dilation and length contraction can be consistent. The analog for quantum mechanics is to group different measurement outcomes based on the combined measurement settings, in a process called post-selection.
Here, the phrase "can be consistent" is a specific (mathematical) interpretation of "allows ... to exist".

1) ...
2) ...
3) ...

You seem to have many different detailed questions. Let me suggest to ask them in a separate thread.

This thread in its current state is a fight about words, and you risk not getting the most appropriate answer here. Some answers might weaken certain positions in that fight, so they might get attacked, or not even uttered in the first place.

 

[Grinkle]

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.

 

[vanhees71]

Thank you for the responses. @PeroK @vanhees71 @Grinkle (Who was also kind enough to message me and explain certain things). I have read in detail all of the responses, including this entire thread and many, many others.

I am quite literally trying to understand. And I don't know how else to say it. I accept modern physics. I have no agenda. I am not fighting anything, do not want to overthrow anything and I am willing to give up my intuition. These are honest questions, and I'm willing to work to understand the answers provided from those having more knowledge than me. I am also open to any suggestions on professional literature I can read if that is a better and/or additional way for me to understand - and I am willing to study the mathematics involved (which I am comfortable with). I realize that the words and motivations of posters are important, but I do not want to get trapped in this diversion unless it is creating a fundamental misunderstanding. I would rather discuss the substance (unless that is impossible due to too many incorrect words). But please do me the small courtesy of assuming that my questions are honest questions, with no agenda or hidden motivations.

I'll try one more time. If I use the wrong words, then please correct me and forgive me. This is my layman's understanding using plain English and there is no agenda. I also do my best to rid myself of preconceived notions.

The problem with "plain English" is that you cannot really communicate about these "quantum properties". The only precise way is to use mathematics. Nevertheless, one can of course always try to explain these things in a way that's understandable without this math, but the danger is huge to get it somehow inaccurate or even wrong.

Another problem in this context is that many popular-science books love the "sensation" more than to provide a scientific picture in "plain English". First of all they think their books sell better, if you have some esoteric touch with it.

We prepare two particles that are maximally entangled. Certain traits about them are indeterminate and not determined until measurement. So we have a 2 particle quantum system and quantum indeterminancy. We then separate these entangled particles by some distance. We still have a 2 particle quantum system and quantum indeterminancy, and yet somehow they are still connected and inseparable, despite the fact that they are physically separated at arbitrarily large distances. So if we measure one particle, and from this measurement "determine" a trait, we are really measuring the entire system, and therefore know the answer to what would happen to the other particle when measured. These are not 2 separate particles, each indeterminate, but rather one "quantum particle system" that follows quantum indeterminancy and some kind of inseparability. We cannot use this information to transfer signals faster than light. Is this GENERALLY correct?

That's a very good description. I'd not say the particles are separated at all when they are prepared in such an entangled state. One should also be aware that even a single photon has no well-defined position in the sense of a point particle to begin with. It's impossible to localize a photon as you can localize a massive particle. That's another specialty of massless relativistic particles which can only be described by relativistic quantum-field theory. All we can know about them, given the quantum state they are prepared in, are the probabilities to detect a photon at a given place and time. The two photons in an entangled state have no individuality and they are thus not separated in any sense. All you can calculate is the probability to find two photons at given times and positions of the detector and their polarization (using also some polarization measurement device like a polarization filter or a polarizing beam splitter). So even the separation into two individual photons is only manifest after registration of these photons (with their polarization state when measured) by the corresponding detectors.

If what I said is generally true (even if I used the wrong words), do we know what exactly allows for such a system to exist? A system that appears connected or inseparable regardless of distance? I believe in causality. Is there a causal mechanism? We cannot have FTL signalling. I accept that. We cannot have hidden variables (predetermined states), as shown by Bell and subsequent experiments. I accept that as well. So what exactly is this connection that allows you to measure and determine a quantum system of two physically separated particles?

I hope, I've made this clear above.

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

I don't know, what you expect as "explanation". Physics doesn't explain why we observe phenomena as we do but describes these observations and provides theories to predict what we'll observe given some situation, and that's very well achieved concerning the correlations, which cannot be explained by any "local realistic hidden-variable theory", and that's the math describing quantum states in general, including these special "far-from-classical kind" called "entangled states".

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? Is that not a contradiction? Either it is pre-determined or indetermined. It cannot be predetermined (per Bell et al) yet it appears so, or something else is going on. I realize the answer is likely "because it is a quantum system", not two separate particles. But I don't see how that actually explains things. Why do we even need to measure both particles then? Which then brings me back to question 1. How can two particles that are physically separated be connected and inseparable? What makes this system inseparable and not individualistic?

The most surprising result of the entire quantum business for me indeed is that this is not a contradiction, if you accept the result that nature is indeterministic. You can prepare a particle or photon in a state, where a given observable takes a precisely defined (determined) value. Then any measurement of this observable will give with 100% probability this determined value. However, for some observables you cannot prepare the particle in a state, where all of them take determined values at once. That's the content of the famous Heisenberg uncertainty relations. E.g., if you localize a particle very precisely, i.e., you determine its position very precisely, then necessarily its momentum is very imprecisely determined, i.e., when measuring the position of a such prepared particle you find it with very high probability in a pretty small region, but if you measure instead its momentum, the probability distribution for getting a certain momentum is very broad, i.e., it is very little known, which momentum you'll measure.

3) What exactly and precisely is this inseparability? I guess that's really the "word" I would like a scientific definition of.

I think this was first introduced by Einstein in his famous disputes with Bohr and other proponents of (the Copenhagen interpretation of) quantum theory. It describes the property that when two particles are prepared in an entangled state, it is impossible to consider them as separate individual entities, which manifests itself by these strong correlation when measuring their individual properties, which are very indetermined in such a state, at far distances.

Maybe Perok already gave me the answer in his first sentence. It's just what nature does and is, deal with it and don't probe much deeper as there is nothing deeper. I can accept that too, if that's the generally accepted answer. I don't like it one bit (that's my intuition), but I can still accept it and try to better understand it then.

Thank you.

The problem is that this is a generally accepted answer only in a wide part of the physics community. Philosophers and some philosophy-inclined physicists think otherwise. They still consider QT in some sense incomplete, and that's why there's still this debate about the foundations of quantum theory is going on although from a physics point of view there are no problems, given that QT describes all observations correctly so far.

The one big physics problem left on a foundational level, in my opinion, is that there is no satisfactory description of gravity within quantum (field) theory. To describe gravity on the most fundamental level we use General Relativity, which however is a classical theory, not taking into account quantum effects. The problem to find a quantum description of gravitational effects is that it is very hard to observe possible quantum effects, because the gravitational interaction only becomes relevant for large (astronomical!) objects and is practically unobservable between individual microscopic particles.

 

[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).