Undergrad 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.