I What exactly does quantum entanglement imply?

[Daniel K]

I've been having issues understanding quantum entanglement and non-locality recently, and certain explanations that I have been told has just made the matter more confusing. The first portion of this thread will be explaining what I know and the second part will contain the questions.

First Part: What I understand from entanglement is that a particle becomes connected with another particle, and this generates a phenomenon in which neither particle can any longer be considered independent, but rather intrinsically intertwined to the corresponding entangled particle. Any affects that one particle experiences will affect the other particle instantaneously.

To explain quantum entanglement, physicists postulated the idea of non-locality. Non-locality states that an object can affect other objects that are not in its immediate surroundings.Second Part: My first question lies within non-locality. It seems to me that non-locality was postulated because physicists could just not explain why can object is affected by another object at a distance where faster than light speeds would have to be necessary. Is this true?

Second question lies within a certain entanglement explanation that I've been told. This explanation goes as follows: "Quantum entanglement does not imply faster than light speeds because nothing is being communicated, rather it's just merely correlation between the particles. If one particle is measured to contain spin up, then the other entangled particle can be sure to contain spin down. Nothing is being communicated; we just know what spin the other particle is going to be." This explanation would make sense to me, although the particles do seem to communicate something. Communication can be seen observed by the other particle always containing an opposite spin. The only to explain this using this explanation is to say that particles always have to be opposite spin to entangle together, although isn't this just the hidden variable theory?

Any feedback would be appreciated!

 

[Simon Bridge]

1. Yes. It's the name for the effect; see:
https://en.wikipedia.org/wiki/Bell's_theorem

2. I've been having trouble finding a decent lay description ... when I had the same issue you did I basically had to do the math. The statistics are different.
You can imagine a situation where there are two boxes, a blue ball in one box and a red one in the other - you don't know which is where but looking in one box tells you right away what is in the other one. That is hidden variables - where it does not match up with the entangled case is in the statistics: in a nutshell, the example is too simple.
This looks promising: https://cs.uwaterloo.ca/~watrous/CPSC519/LectureNotes/20.pdf

 

[Salvador]

also my 2 cents if I may, just because once you see one particle reveals the other doesn't mean communication , because for communication the other end (whoever may be there) needs to also know what you saw at your end of the line.

you could say , well okay make a code sheet and then give it to each of the observers at each end so they don't have to communicate back to each other to confirm but as they see their result they can compare it wih what has been written for that exact result but this would work if one could determine the outcomes of the entangled particles spin states etc, but we can't determine those , we only know the other particles state once the first one is revealed, so it's more of a lottery.

 

[DrChinese]

you could say , well okay make a code sheet and then give it to each of the observers at each end so they don't have to communicate back to each other to confirm but as they see their result they can compare it wih what has been written for that exact result but this would work if one could determine the outcomes of the entangled particles spin states etc,

Just to be clear: there are no such code sheets possible which would agree with the predictions of QM.

 

[entropy1]

My two cents: the measured value by Bob depends on the measured value by Alice, or vice-versa, which is equivalent. So, Alice's measurement basis of her free choice influences the correlation (between Alice and Bob), and similarly for Bob. This is non-locality. The problem is that you can't tell who influences who, the correlation just is! So in this case, local hidden variables are ruled out, though non-local hidden variables are not.

 

[gjonesy]

A good lay description I've heard is: imagine walking into a shoe store taking a box off the shelf and inside you have a LEFT white size 9 nike with blue trim, immediately you know that somewhere regardless of distance space or time, that there is a RIGHT white size 9 nike with blue trim, we know this simply because they are a correlated pair. As for this spooky "action" at a distance that's a bit harder to explain, entangled pairs act as one.

 

[Nugatory]

A good lay description I've heard is: imagine walking into a shoe store taking a box off the shelf and inside you have a LEFT white size 9 nike with blue trim, immediately you know that somewhere regardless of distance space or time, that there is a RIGHT white size 9 nike with blue trim, we know this simply because they are a correlated pair. As for this spooky "action" at a distance that's a bit harder to explain, entangled pairs act as one.

Unfortunately, this is the classic example of local variables at work - we know that a shoe is created with a size, a color, and a handedness footedness and it has these properties even if we don't measure them. Entangled quantum particles don't behave like that..

Suppose you and I somewhere else in the universe are looking at the pairs of shoes as they come by, but we're each only allowed to measure at random one of the three properties: color (white or red), size (9 or 10), left-foot or right-foot. If, for a given pair, you measure the size and get nine and I measure the color and get red, we might conclude when we compare notes that you had a white size nine shoe of unknown footedness while I had a red size nine shoe of unknown footedness; similar logic works for all the other possible pairs of measurements. (If we both randomly measure the same property, the other two will be unknown even after we compare notes).

However, suppose that when we compare notes we discovered that the number of white size nine shoes (using the logic above, where we combine your measurement and mine to infer something about my shoe) that I saw is more than the sum of the number of white left shoes that passed me plus the the number of size nine right shoes? That's the equivalent quantum mechanical prediction; it has been confirmed experimentally and it tells us pretty clearly that the two particles in the entangled pair were not created with definite values of all three attributes.

 

[Daniel K]

And so something is being communicated between the entangled particles, right? From my research I know that we cannot tell what is being communicated or who's doing the communication, but just to clarify, we know that something is being communicated?

 

[Simon Bridge]

That would be the classical interpretation of the results... otherwise, how does the distant particle know which set of statistics to follow?

Technically, what we are saying is that any "hidden variables" style communication would have to be ftl... this potentially allows for causality violations[1] so we don't think this is what happens. Instead we have to face the idea that reality can be non-local. ie action at a distance is real. In this case, two objects can be part of the same quantum system even at very large distances.

[1] Summary of (2012) Nature paper, link to paper at bottom.
http://arstechnica.com/science/2012/10/quantum-entanglement-shows-that-reality-cant-be-local/

 

[bhobba]

Before discussing entanglement you need to understand what it is.

Simply its an extension of the principle of superposition to different systems. Suppose two systems can be in state |a> and |b>. If system 1 is in state |a> and system 2 is in state |b> that is written as |a>|b>. If system 1 is in state |b> and system 2 is in state |a> that is written as |b>|a>. But we now apply the principle of superposition so that c1*|a>|b> + c2*|b>|a> is a possible state, The systems are entangled - neither system 1 or system 2 are in a definite state - its in a peculiar non-classical state the combined systems are in.

If you observe system 1 and get state |a> then you know system 2 is in state |b>, and similarly if you observe system 1 and get |b> you know system 2 is in state |a>. That's all entanglement is - a correlation. That's it, that's all. Let that sink in devoid of the stuff you read about it in the pop sci literature.

Imagine you have two slips of paper a red and a green one and put them in envelopes. Send one to the other side of universe and keep the other. Open the envelope and you see red - you immediately know the other is green, and conversely. Nothing weird or mysterious here. That's all that's going on with entanglement with a twist I will explain.

Now for the QM twist. It turns out the paper analogy is not quite the same as QM. The correlation is a bit different - its still just a correlation - but has statistical properties different to the paper example. Why the difference? The difference is in QM things do not have properties until observed to have them, whereas the slips of paper remain red or green at all times. But what if we insist it's like the slips of paper - then it turns out you need some kind of non local superluminal communication. That's really weird. But there is nothing compelling anyone to insist its like the slips of paper - simply accept QM allows a different kind of correlation and things are no longer mysterious.

Without going into the details there is good reason to not apply the concept of locality to correlated systems (its to do with QFT an the so called cluster decompostion property). You can define it to be applicable but it doesn't sit well with other things. If you do that then you don't run into issues in the first place. Only by allowing it to apply can you say its like the slips of paper. If you say its not an applicable concept then the answer is simple - it can never be like the slips of paper. QM correlations are different from classical ones - big deal.

Thanks
Bill

 

[zonde]

And so something is being communicated between the entangled particles, right? From my research I know that we cannot tell what is being communicated or who's doing the communication, but just to clarify, we know that something is being communicated?

Right, as long as you remember that all knowledge in science is tentative.
Or the longer answer would be that we know there can't be any light speed limit compatible scientific explanation of our loophole free Bell test experiments as far as we can trust that experiments are loophole free.

 

[Daniel K]

If you observe system 1 and get state |a> then you know system 2 is in state |b>, and similarly if you observe system 1 and get |b> you know system 2 is in state |a>. That's all entanglement is - a correlation. That's it, that's all. Let that sink in devoid of the stuff you read about it in the pop sci literature.

The difference is in QM things do not have properties until observed to have them, whereas the slips of paper remain red or green at all times.

That is the explanation in which I am having difficulty understanding. How would system 1 know that system 2 is measured if there is no communication but merely correlation? You said yourself that "QM things do not have properties until you have observed them.." and so how would a certain system know that it has to be opposite to the opposing system, if it has the superposition of both systems prior to measurement?

 

[entropy1]

Imagine you have two slips of paper a red and a green one and put them in envelopes. Send one to the other side of universe and keep the other. Open the envelope and you see red - you immediately know the other is grenn, and conversely. Nothing weird or mysterious here. That's all that's going on with entanglement with a twist

I would rephrase that: suppose you have two envelopes and in envelope A you put a green/red striped sheet of paper, and in envelope B a red/green striped sheet of paper. If you send one to Alice and the other to Bob, and Alice finds out that, after putting the envelope through some process, she has a green sheet, then she know Bob has a red one. But you can view this the other way round: suppose Bob puts his envelope to some process and after opening it finds it contains a green sheet! The he knows Alice must have the red one! Since who opens the envelope first depends on the (relativistic) frame of reference, you can't tell who should be the communicator. You can only establish that there is a correlation. Am I making sense?

 

[bhobba]

Am I making sense?

Sure. Its just a correlation.

Thanks
Bill

 

[Daniel K]

Sure. Its just a correlation.

Yes, it's correlation. Through the correlation one can infer that there is some kind of communication being transferred between the two entangled states. You stated yourself that within the confines of quantum mechanics two entangled states are in a superposition of being both state A and B when not measured. So my question is simple: if it is merely correlation and there is no communication, then how does an entangled particle know to be opposite to the other particle?

Let me try to be my clear and give an example in what I am asking.Let's say we have an entangled pair of particles that are separated by an arbitrarily long distance. We'll call the particles system 1 and system 2.
Both systems are in a superposition of being in state A and B.
If then system 1 is measured to contain state A, then we can infer that system 2 contains state B. However how does system 2 itself know that it has to be in state B? Prior to measurement, it should be in a superposition of both A and B. And so then how can a measurement that is occurring in some far off galaxy affect the of state of it?

 

[zonde]

Since who opens the envelope first depends on the (relativistic) frame of reference, you can't tell who should be the communicator. You can only establish that there is a correlation. Am I making sense?

Not quite. It does not make sense to incorporate into single model symmetries of relativity and FTL communication. Of course you get contradiction because symmetries of relativity are simply incompatible with any FTL communication or causality.

 

[entropy1]

However how does system 2 itself know that it has to be in state B?

In my view, it isn't in state B. The correlation measured leaves room for the interpretation it may have been in state B, but the measurements and their correlations is all we have!

 

[bhobba]

Yes, it's correlation. Through the correlation one can infer that there is some kind of communication being transferred between the two entangled states.

That's incorrect. It knows it the same way the colored papers knows it ie you have arranged it so by the entanlement. The only difference is in QM properties do not exist until measured.

To be specific because |a>|b> and |b>|a> are in superposition you have arranged things so that the only outcomes of an observation are |a>|b> or |b>|a>. That is the exact analogue of the coloured papers where the only outcomes are red and green or green and red.

If you still don't see it you need to explain exactly why there is no communication between the slips of paper but there must be communication between entangled particles.

Thanks
Bill

 

[entropy1]

Not quite. It does not make sense to incorporate into single model symmetries of relativity and FTL communication. Of course you get contradiction because symmetries of relativity are simply incompatible with any FTL communication or causality.

But they both apply to the observable world, don't they? I am just combining the two in this context!

By the way, I am on the contrary not talking about FTL communication. I think that decoherence is an important contributor in this case. Hoewever, I have little knowledge of it right now.

 

[zonde]

By the way, I am rather thinking about non-local hidden variables or the deBroglie-Bohm model

For deBroglie-Bohm model you have to use preferred frame and incorporate relativity using so called Lorentz relativity interpretation.

 

[Daniel K]

If you still don't see it need to explain exactly why there is no communication between the slips of paper but there must be communication between entangled particles?

Within the confines of the paper analogy, there are definite values.
However for the quantum mechanical one, there are not. Once measurement occurs in one entangled particle, you are destroying the superposition of the other entangled particle that could be light years apart instantly.

 

[bhobba]

It seems to me that your explanation is putting inherit values within the particles.

I am not.

All that has happened is the entanglement has ensured the only outcomes are |a>|b> or |b>|a>. It says nothing about what properties it has prior to observation.

I know with the pop-sci half truths that surrounds this its difficult to shake the confusion they engender. But really its quite simple.

But don't take my word for it - you can read Bell's original paper:
https://cds.cern.ch/record/142461/files/198009299.pdf

The only difference is he uses Bertlmann's socks.

Thanks
Bill

 

[Daniel K]

But don't take my word for it - you can read Bell's original paper:
https://cds.cern.ch/record/142461/files/198009299.pdf

Thanks I'll take a look at it.
Also, if you are correct and there is no communication between the particles, is non-locality not necessary then?

 

[bhobba]

Within the confines of the paper analogy, there are definite values.
However for the quantum mechanical one, there are not. Once measurement occurs in one entangled particle, you are destroying the superposition of the other entangled particle that could be light years apart instantly.

So? Entanglement is not some kind of mystical connection between objects - its simply a superposition.

Thanks
Bill

 

[bhobba]

Thanks I'll take a look at it.
Also, if you are correct and there is no communication between the particles, is non-locality not necessary then?

It's exactly as I explained. You only need locality broken if you want it to be like the slips of paper ie have the properties irrespective of observation.

Thanks
Bill

 

[entropy1]

Thanks I'll take a look at it.
Also, if you are correct and there is no communication between the particles, is non-locality not necessary then?

It is non-local in the sense that there is a correlation between two measurement values that could not have communicated.

 

[jerromyjon]

But they both apply to the observable world, don't they?

Tell me how to observe FTL communication... I doubt you can because it still hasn't. And I don't mean group velocities or phase velocities (or whatever it is I mean and can't think of a better term for) Information can never be transferred from A to B instantly. I choose to think of entanglement as existing in complex tension between two states so that an action affects that tension which is separate from the "observable" reality of spacetime.

 

[zonde]

It is non-local in the sense that there is a correlation between two measurement values that could not have communicated.

You must be using very uncommon definition of non-locality.

 

[entropy1]

You must be using very uncommon definition of non-locality.

Tell me yours! Well, perhaps it goes a little limp formulated this way...

 

[zonde]

Tell me yours!

I try to avoid it. Basically there are two possible meanings:
first, FTL causality or communication;
second, distance is meaningless.

For the first meaning it is less ambiguous to call it FTL whatever.
About the second meaning, it is novel philosophical concept that does not really have much to do with science.

 

[Daniel K]

So? Entanglement is not some kind of mystical connection between objects - its simply a superposition.

Could the crash of a superposition between two objects be instantaneous?

 

[DrChinese]

Entanglement is sometimes referred to as "Quantum Nonlocal" in order to distinguish it amongst the many definitions. The underlying physical process whereby entangled particles become entangled and/or cease to be such are not understood, even though the formalism itself is relatively well understood. You can entangle particles that do not even exist at the same time, for example. That too is "quantum nonlocal". So if you want to describe entanglement in a manner outside the theoretical formalism, there really is nothing that works better as a definition. Because there is no further physical description possible except by adopting an interpretation of QM.

Please note that even individual particles have nonlocal attributes. A particle's position can be [1/2 here] and [1/2 there], and then collapse to a state of [all here]. That collapse is also quantum nonlocal.

 

[Daniel K]

You only need locality broken if you want it to be like the slips of paper ie have the properties irrespective of observation.

DrChinese, do you then believe that particles have properties irrespective of observation?

 

[bhobba]

Could the crash of a superposition between two objects be instantaneous?

There is no could about it - it is.

But it doesn't mean anything.

Thanks
Bill

 

[Daniel K]

There is no could about it - it is.

Alright, so just to be clear of your interpretation on quantum entanglement-

You believe what occurs is that two particles become entangled, and upon measurement of one particle, the superposition of both systems instantly crashes and they become defined states.

However the instant superposition crash of both particles does not imply faster than light communication/non-locality; rather just correlation.

Is this correct?

 

[.Scott]

Alright, so just to be clear of your interpretation on quantum entanglement-

You believe what occurs is that two particles become entangled, and upon measurement of one particle, the superposition of both systems instantly crashes and they become defined states.

However the instant superposition crash of both particles does not imply faster than light communication/non-locality; rather just correlation.

Is this correct?

Let me chime in here.
First, I think that the word "communication" has too much baggage to be used in describing what happens with entangled particles. It suggest that information is moving from one to the other. In fact the experimental results are statistics that do not indicate any directionality. Also, the quantum states we are working with seem to have a life of their own - independent of other quantum states of the particle - see quantum Cheshire cat http://phys.org/news/2015-06-quantum-cheshire-cat-effect-standard.html. So the common way of "explaining" the observed correlations is to describe the state shared by the entangled particles without presuming that the state exists "at" either particle.

Regarding "upon measurement of one particle, the superposition of both systems instantly crashes".
I don't like the term "instantly" - it not as bad as "simultaneously" which would be outrightly wrong, but it's almost that bad.
The problem is that there is no instant in time that is independent of the reference-frame when the collapse happens. It's not that its wrong, it just doesn't fit with the common notion of "instant".

For example, if both particles are measured at about the same time - so that from a reference frame shared by both detectors they are measure within a picosecond of each other and the detectors are a few meters apart, we would call this a space-like separation of the events. Depending on your reference frame, you could say that either measurement A precedes B or measurement B precedes A. And so what is the "instant" when the collapse occurs? And which particle collapsed first? It is meaningless to say.

Now, you might ask, what if the measurements are time-like separated - so that all reference frames agree that A was measured before B? Well, we get exactly the same statistics from time-like separation as we do from space-like separation, so do we really want to say something different happens with one then the other? So if I have one particle stored in a box and it's entangled twin was measure last year, do I want to say that the one in the box collapsed last year? Probably not. We probably don't want to talk about "when" the collapsed occurred because it didn't happen in a way that corresponds to the common experience of "when".

 

[DrChinese]

DrChinese, do you then believe that particles have properties irrespective of observation?

Not in the classical sense, no, and not in my view. For example: If you knew property p precisely, then non-commuting property q is completely indeterminate and cannot be said to have *any* value.

If you choose to call that indeterminate property to be "existing" then it is, else it isn't, but either way it would be a matter of your semantics and nothing else. It still (in my viewpoint) has no value.

 

[gjonesy]

Unfortunately, this is the classic example of local variables at work - we know that a shoe is created with a size, a color, and a handedness footedness and it has these properties even if we don't measure them. Entangled quantum particles don't behave like that..

Suppose you and I somewhere else in the universe are looking at the pairs of shoes as they come by, but we're each only allowed to measure at random one of the three properties: color (white or red), size (9 or 10), left-foot or right-foot. If, for a given pair, you measure the size and get nine and I measure the color and get red, we might conclude when we compare notes that you had a white size nine shoe of unknown footedness while I had a red size nine shoe of unknown footedness; similar logic works for all the other possible pairs of measurements. (If we both randomly measure the same property, the other two will be unknown even after we compare notes).

However, suppose that when we compare notes we discovered that the number of white size nine shoes (using the logic above, where we combine your measurement and mine to infer something about my shoe) that I saw is more than the sum of the number of white left shoes that passed me plus the the number of size nine right shoes? That's the equivalent quantum mechanical prediction; it has been confirmed experimentally and it tells us pretty clearly that the two particles in the entangled pair were not created with definite values of all three attributes.

The shoe description is an example of how simple English descriptions become very complicated, its just an illustration of why correlated pairs do not need to "communicate" faster than the speed of light. like the Alice and Bob description featured here.

http://www.livescience.com/50262-spooky-action-is-real.html

There are many more "bad" simple English descriptions that unless you really study and research the material its fairly hard to paint an accurate picture of what entanglement really is.

 

[Jeff Rosenbury]

Thanks I'll take a look at it.
Also, if you are correct and there is no communication between the particles, is non-locality not necessary then?

I don't think he claimed no communication exists. He claimed it might not exist or to put it another way, it is not needed. There are other possible solutions.

The math is the math. "Why" is a human question and says more about us than the underlying physics I think.

In any case, simple hidden variables are disallowed by Bell. Simple FTL communication is disallowed by Einstein. That doesn't mean a more complex hidden variable theory might not work, or a more complex FTL communications theory might work, or both, or something else entirely. But any such theory would simply be a way to translate our human, macroscopic understanding to math which already works.

Or I could be wrong.

 

[StevieTNZ]

You may find this interview helpful:
Four minutes 55 seconds long.
@1:44 he talks about quantum correlations (entanglement).

 

[bhobba]

You believe what occurs is that two particles become entangled, and upon measurement of one particle, the superposition of both systems instantly crashes and they become defined states.

Its the collapse of the wave-function issue in another guise. The observation doesn't occur instantaneously - but when its completed the state has changed so we say entanglement has been broken - and for simplicity its taken as instantaneously. Its like when you flip a coin - it doesn't become heads or tails instantaneously but when it does we say that the probability of a head or tail as 1/2 instantly changed to one being zero and the other one. Its of no concern because probabilities do not exist in a real sense but as a theoretical concept. The same with a superposition and quantum states in general. Note that's the formalism - interpretations can and sometimes do have a different take.

Although the math is advanced the following post may help in understanding the role of states in the QM formalism (see post 137):
https://www.physicsforums.com/threads/the-born-rule-in-many-worlds.763139/page-7

Basically they are, like probabilities, just a useful concept and do not exist in a real sense like say the results of observations that are very real.

Thanks
Bill

 

[Daniel K]

You may find this interview helpful:
Four minutes 55 seconds long.
@1:44 he talks about quantum correlations (entanglement).

I think the reason why myself and so many other individuals find entanglement confusing is because of stuff like this. This video is directly contradicting with many peoples' statements within this thread, and so it can difficult to perceive which description is the correct one.

In the video, the person being interviewed says that quantum non-locality is the single solution to the correlations found in quantum entanglement. However, within this thread, they are many people who have advocated that quantum non-locality isn't even real.

 

[bhobba]

In the video, the person being interviewed says that quantum non-locality is the single solution to the correlations found in quantum entanglement.

That's the reason we have acceptable sources around here. Videos like that generally are not acceptable. I haven't seen it, but can assure you, locality is only violated if you want realism (ie like the slips of paper) - technically its called counterfactual definiteness:
http://www.johnboccio.com/research/quantum/notes/paper.pdf

Here you get the real deal from people who have studied it in detail.

Everything I have said is standard textbook stuff.

Generally videos like you posted are opinions - but don't make clear its an opinion.

Thanks
Bill

 

[Daniel K]

That's the reason we have acceptable sources around here. Videos like that generally are not acceptable. I haven't seen it, but can assure you, locality is only violated if you want realism (ie like the slips of paper) - technically its called counterfactual definiteness:
http://www.johnboccio.com/research/quantum/notes/paper.pdf

Thanks for the paper - I'll read it in a bit!

Also before I start reading it, can you tell me if it will only deal with the bell inequality or will it also explain why quantum entanglement does not imply faster than light communication?

 

[bhobba]

Also before I start reading it, can you tell me if it will only deal with the bell inequality or will it also explain why quantum entanglement does not imply faster than light communication?

It deals with Bells Theorem that states QM does not allow both locality and realism - by realism it means QM properties are like the slips of paper - they are red or green regardless of if you observe them or not - as I have been saying in this thread. If you reject realism you can have locality ie no FTL.

Thanks
Bill

 

[stevendaryl]

I think that there is a sense of "locality" whereby QM with definite outcomes (as opposed to MWI) is nonlocal. If Bob performs a measurement confined to some region of space R_{bob}, then we can say that the outcome is {local}_{daryl} (to distinguish it from a dozen other subtly different notions of "local") if it depends only on conditions in region R_{bob}, and not on conditions in other, distant regions. Conversely, we can say that the outcome is {nonlocal}_{daryl} if facts about a second region R_{alice} can tell us more about Bob's outcome than facts about R_{bob} alone can.

Quantum mechanical correlations are certainly nonlocal in this sense. In an EPR-type experiment with anti-correlated fermions, if Alice measures spin-up for her particle along an axis \vec{\alpha}, then (assuming she performs her measurement slightly before Bob performs his), she knows that Bob will measure spin-down along that axis. But there is nothing about conditions local to Bob's measurement that could be used to predict this outcome ahead of time. So Bob's outcome is {nonlocal}_{daryl}.

Note: The classical analog of the EPR experiment uses two slips of paper, one red and one green, placed into envelopes. One envelope is sent to Alice, and the other is sent to Bob. When Alice opens her envelope and sees a red piece of paper, she knows that Bob's result will be green. But that's not {nonlocal}_{daryl}, because Bob's outcome is completely determined by the state of his envelope, and that state is a fact local to Bob.

 

[.Scott]

That's the reason we have acceptable sources around here. Videos like that generally are not acceptable. I haven't seen it, but can assure you, locality is only violated if you want realism (ie like the slips of paper) - technically its called counterfactual definiteness:
http://www.johnboccio.com/research/quantum/notes/paper.pdf

Here you get the real deal from people who have studied it in detail.

Everything I have said is standard textbook stuff.

Generally videos like you posted are opinions - but don't make clear its an opinion.

Thanks
Bill

One problem here is the definition of "locality" - and whether it, by definition, presumes realism. You are taking "locality" to simply exclude FTL information transfer. Others (including Wikipedia), tie it to any FTL-like influence.
Rereading the original 1979 Scientific American article (https://www.scientificamerican.com/media/pdf/197911_0158.pdf), it is difficult to resolve the definition. According to the author, Bernard d'Espagnat, QM is either unrealistic or non-local. But he defines realism as follows:

One is realism, the doctrine that regularities in observed phenomena are caused by some physical reality whose existence is independent of human observers.

By that definition, most physicists would assert that QM conforms to realism. That leaves locality, which he defines as follows:

The third premise is called Einstein separability or Einstein locality, and it states that no influence of any kind can propagate faster than the speed of light.

These seems to match the issue with QM much better than "realism". The terms "separability" and "locality" are used synonymously throughout the article. So, by those terms, QM would be a non-local realistic theory. In the article, QM is described repeatedly by combining the terms - as not a "local realistic theory".

 

[zonde]

If you reject realism you can have locality ie no FTL.

No, you can't (if I am correctly "translating" realism as counterfactual definiteness).
While it's true that Bell theorem relies on assumption of counterfactual definiteness (so that you might hope to construct valid local model of entanglement by relaxing this assumption) there are other proofs of Bell type inequalities. Say Eberhard in his derivation (http://dx.doi.org/10.1103/PhysRevA.47.R747) assumes factual definiteness and locality and still gets Bell type inequality.

 

[Nugatory]

No, you can't (if I am correctly "translating" realism as counterfactual definiteness).
While it's true that Bell theorem relies on assumption of counterfactual definiteness (so that you might hope to construct valid local model of entanglement by relaxing this assumption) there are other proofs of Bell type inequalities. Say Eberhard in his derivation (http://dx.doi.org/10.1103/PhysRevA.47.R747) assumes factual definiteness and locality and still gets Bell type inequality.

Is there a copy not behind paywall? The abstract is less than helpful

 

[stevendaryl]

No, you can't (if I am correctly "translating" realism as counterfactual definiteness).
While it's true that Bell theorem relies on assumption of counterfactual definiteness (so that you might hope to construct valid local model of entanglement by relaxing this assumption) there are other proofs of Bell type inequalities. Say Eberhard in his derivation (http://dx.doi.org/10.1103/PhysRevA.47.R747) assumes factual definiteness and locality and still gets Bell type inequality.

I have said before that I don't believe that Bell's theorem assumes counterfactual definiteness. As somebody else put it recently, in an EPR-type experiment with two experimenters Alice and Bob, the assumption needed to get Bell's theorem is this:

P(B| \lambda, \beta, \alpha, A) = P(B|\lambda, \beta)

where:

• B is the result of some yes/no measurement performed by Bob,
  
• \beta represents variables describing conditions at Bob's measuring device,
  
• \alpha represents variables describing conditions at Alice's measurement device,
  
• A is the result of some yes/no measurement performed by Alice,
• \lambda represents variables describing conditions in the intersection of the backward lightcones of Alice's and Bob's measurements.

In other words, Bell is assuming that the only way for Alice's measurement result to reveal any information about Bob's measurement result is if both results are affected by their common past, described by \lambda. That isn't counter-factual definiteness, although you can derive counter-factual definiteness from that assumption, together with the fact of perfect correlation (or anti-correlation) between Alice's and Bob's results when they choose specific settings.