How do you understand Non-Locality ?

[Varon]

How do you understand "Non-Locality"?

Some people state that "non-locality" is a wrong word to describe it because there is nothing to be non-local about.. meaning properties don't exist before measurement... that is, the particles don't exist before measurement.. so how can you say the particles are in non-local communication. Is this argument valid?

 

[phinds]

Some people state that "non-locality" is a wrong word to describe it because there is nothing to be non-local about.. meaning properties don't exist before measurement... that is, the particles don't exist before measurement.. so how can you say the particles are in non-local communication. Is this argument valid?

From what I've read, the term non-locality seems to perfectly fit the phenomenon being described. Your statement that the particles don't exist before measurement is non-nonsensical to me. The particles EXIST, it's just that some of their characteristics are only in a probabilistic state prior to measurement. That is, the characteristics also EXIST, but their value, or state, is unknown until measured. That's a perfectly normal circumstance in QM.

What gets weird about non-locality is that measuring the state of a particular characteristic of one of a pair of entangled particles dictates the value of that characteristic in the other particle of the entangled pair, and this happens instantaneously and regardless of distance. This is utterly bizarre (well, I think "impossible" is not too strong a term) from the point of view of classical mechanics, but it is not theory, it is experimentally observed fact.

SO ... "non-locality" means that a local event (measurement) has non-local effects.

On the other hand, saying that entangled particles are "in communication" gets into a semantic discussion. Since no useful information is transferred, this is not communication as it is sometimes understood. You can't send any kind of message using entangled pairs, so "communication" doesn't seem to quite fit.

I'm not an expert on this, so I'm open to hearing about it if I have any of this wrong.

 

[Varon]

When I first learned about the word Non-locality, someone said the word is not valid, due to the so called Kochen-Speaker theorem. Is anyone familiar with this?

 

[Hurkyl]

Some people state that "non-locality" is a wrong word to describe it because there is nothing to be non-local about.. meaning properties don't exist before measurement... that is, the particles don't exist before measurement.. so how can you say the particles are in non-local communication. Is this argument valid?

Newtonian gravity, electostatics and magnetostatics were all non-local. Forces didn't depend on what was "nearby", but instead acted at a distance. (of course, electrostatics and magnetostatics were unified into electromagnetism which does act locally)

With the advent of Special Relativity, non-locality became more serious, because the causal structure of space-time gave us reason to very strongly desire that the conditions in a region of space-time affect only what lies within its future and past light-cones.

(General Relativity subsequently turned gravity into a local theory)



With quantum mechanics, a simple treatment of an entangled photon experiment is

• Before Alice does her measurement, the result Bob will get is not determined.
• After Alice does her measurement, the result Bob gets is determined.
• Therefore, the time evolution of Bob's photon depends on what happens in Alice's laboratory

From which non-locality is concluded.

Of course, we now know a variety of ways to treat the situation.

 

[Varon]

From what I've read, the term non-locality seems to perfectly fit the phenomenon being described. Your statement that the particles don't exist before measurement is non-nonsensical to me. The particles EXIST, it's just that some of their characteristics are only in a probabilistic state prior to measurement. That is, the characteristics also EXIST, but their value, or state, is unknown until measured. That's a perfectly normal circumstance in QM.

No. Bohr stateD that in the absence of measurement to determine it's position, the particle has no position nor other attributes.. so one can for all intent and purposes say the particles don't exist before measurement. It is only in the ad hoc Bohmian mechanics and silly Many Worlds that particles have positions at all times. But in the best Bohr tradition, let's accept that particles properties don't exist before measurement... which includes property like position.

But something bugs me. To othe posters. In the quarks. Isn't it they are in superposition, or not? But their dynamics create mass which we can measure.. so I guess the quarks are still there even before measurement? Or do the quarks measure one another collapsing the wave function making mass exist?

What gets weird about non-locality is that measuring the state of a particular characteristic of one of a pair of entangled particles dictates the value of that characteristic in the other particle of the entangled pair, and this happens instantaneously and regardless of distance. This is utterly bizarre (well, I think "impossible" is not too strong a term) from the point of view of classical mechanics, but it is not theory, it is experimentally observed fact.

SO ... "non-locality" means that a local event (measurement) has non-local effects.

On the other hand, saying that entangled particles are "in communication" gets into a semantic discussion. Since no useful information is transferred, this is not communication as it is sometimes understood. You can't send any kind of message using entangled pairs, so "communication" doesn't seem to quite fit.

I'm not an expert on this, so I'm open to hearing about it if I have any of this wrong.

 

[Math Is Hard]

Note: moved from Philosophy. This seems a better fit. -MIH

 

[ThomasT]

Some people state that "non-locality" is a wrong word to describe it because there is nothing to be non-local about.. meaning properties don't exist before measurement... that is, the particles don't exist before measurement.. so how can you say the particles are in non-local communication. Is this argument valid?

Nonlocality is something of a misnomer when applied to entanglement. There's no physical evidence for ftl communication, instantaneous action at a distance is, by definition, noncommunicative, and there's no particularly compelling reasoned approach which requires anything in entanglement preparations to be communicating ftl. The correlations that are observed are precisely the correlations that one would expect assuming the validity of certain standard optical principles.

 

[Varon]

Nonlocality is something of a misnomer when applied to entanglement. There's no physical evidence for ftl communication, instantaneous action at a distance is, by definition, noncommunicative, and there's no particularly compelling reasoned approach which requires anything in entanglement preparations to be communicating ftl. The correlations that are observed are precisely the correlations that one would expect assuming the validity of certain standard optical principles.

Yes, there seems to be many different ways to look at non-locality. In some context like yours.. if there is no ftl and no signalling even if the instantaneous correlations has distance of 1 billion light years, it can't be called "non-local". Then what's a proper term to call this "non-local correlations"??

 

[ThomasT]

Yes, there seems to be many different ways to look at non-locality. In some context like yours.. if there is no ftl and no signalling even if the instantaneous correlations has distance of 1 billion light years, it can't be called "non-local". Then what's a proper term to call this "non-local correlations"??

Correlations. Period. For example, optical Bell tests generate a correlation between rate of coincidental detection and the angular difference between the analyzers. The observed correlations are in line with what would be expected given the validity of certain standard optical principles which are part of the standard qm treatment. The local realist correlations differ from this, and the point of departure is the requirement of formally explicating realism and locality. The now conventional way of doing this, a la Bell, is necessarily incompatible with the standard qm treatment and established optical principles which are part of that treatment. So Bell inequalities are violated, and it's neither mysterious nor deep why they're violated. Nevertheless, insofar as it can be considered the general LR form, then the LR program has been effectively refuted. But this doesn't inform us about the reality underlying the instrumental results. And, given that nonlocality hasn't been empirically or reasonably demonstrated, then, apparently, LR theories of entanglement are impossible even in an exclusively locally evolving universe.

The ONLY reason that the idea of nonlocality gained a foothold is because it provides an ad hoc answer to how the observed correlations might be accounted for in a realistic, formally separable model of entanglement.

One key point here is that the formal nonseparability of standard qm doesn't imply or refer to nonlocality. So, continuing to use the term nonlocality in this way just perpetuates the confusion.

 

[Vanadium 50]

In physics, locality means something very specific - it means that we can write down the Hamiltonian using a finite number of space derivatives. For example, a charge can respond to the electric field, or the gradient of the electric field, or the 8th derivative of the electric field.

 

[Antiphon]

My problem with non-locality like the situation with Bob and Alice above is that there is always a reference frame in which the measurements are simultaneous. I know I'll probably catch hell for this, but to me that's a preferred natural frame for eliminating the apparent paradoxes. Its reminiscent of the relativistic car in the short garage paradox.

 

[phinds]

... one can for all intent and purposes say the particles don't exist before measurement.

The information you gave about Bohr was interesting. Thanks.

I do question the quoted statement, however, and stand by my contention that the particle does exist. My rationale, and I'd be happy to hear your point of view on this, is that if you are talking about a particle that has mass, say an electron in an entangled pair, then to say that the particle doesn't exist before it is measured is, to me, exactly the same as saying that the mass of the particle suddenly popped into being in order that some characteristic of the electron could be measured. THAT is what I find to be nonsensical. This is not like virtual particles popping in and out of existence, this is an electron that "exists".

Your thoughts?

 

[IllyaKuryakin]

The simplist explanation is the Universe is a quantum computer running the program we call reality. Then, the entanglement is simply the math running on the computer. If the math says the corrolations are valid at any distance, then it is. It's all in the math.

 

[Varon]

Correlations. Period. For example, optical Bell tests generate a correlation between rate of coincidental detection and the angular difference between the analyzers. The observed correlations are in line with what would be expected given the validity of certain standard optical principles which are part of the standard qm treatment. The local realist correlations differ from this, and the point of departure is the requirement of formally explicating realism and locality. The now conventional way of doing this, a la Bell, is necessarily incompatible with the standard qm treatment and established optical principles which are part of that treatment. So Bell inequalities are violated, and it's neither mysterious nor deep why they're violated. Nevertheless, insofar as it can be considered the general LR form, then the LR program has been effectively refuted. But this doesn't inform us about the reality underlying the instrumental results. And, given that nonlocality hasn't been empirically or reasonably demonstrated, then, apparently, LR theories of entanglement are impossible even in an exclusively locally evolving universe.

The ONLY reason that the idea of nonlocality gained a foothold is because it provides an ad hoc answer to how the observed correlations might be accounted for in a realistic, formally separable model of entanglement.

One key point here is that the formal nonseparability of standard qm doesn't imply or refer to nonlocality. So, continuing to use the term nonlocality in this way just perpetuates the confusion.

You admit there are correlations between the entangled pair one billion light years away and instantaneously and you still want to simply refer to it as "correlations". No. We must make more emphasis. Maybe call it "Instantaneous universe wide correlations". Unless you are trying to say above that Bell's Theorem is not really violated and the correlation is just some kind of local hidden variable? But Aspect, etc. experiments are meant to show that Bell's Theorem is violated and universe wide instantaneous correlations exist. Do you deny this?

 

[ThomasT]

You admit there are correlations between the entangled pair one billion light years away and instantaneously and you still want to simply refer to it as "correlations". No. We must make more emphasis. Maybe call it "Instantaneous universe wide correlations".

That would be a mistake, and only contributes to your confusion on this. The correlations are acausal. Any emphasis beyond the usual meaning of correlation is superfluous.

Unless you are trying to say above that Bell's Theorem is not really violated and the correlation is just some kind of local hidden variable?

This sentence doesn't make any sense.

But Aspect, etc. experiments are meant to show that Bell's Theorem is violated and universe wide instantaneous correlations exist. Do you deny this?

It's a fact that Bell inequalities are experimentally violated. In the Aspect experiment(s) the correlations are due to paired detection attributes being associated with photons emitted by the same atom during the same atomic transition and thus being related via conservation of angular momentum. The distance between the analyzers and/or the detectors in an ideal experimental setup is irrelevant.

It isn't clear to me what you're trying to say, or what your problem is with correlations. Is it that you want the spatially separated events of the experimental wings to be in some way causing each other? Well, there's no reason to think that they do. The correlations (both between the separated individual photon fluxes at certain settings, and between coincidental photon flux and the angular difference between the analyzers at any settings) are fairly precisely accounted for via the standard optics and the conservation law(s) incorporated in the standard qm treatment. The entire business about nonlocality is just a contrived pseudo-problem. There isn't any nonlocality to be explained.

 

[Varon]

That would be a mistake, and only contributes to your confusion on this. The correlations are acausal. Any emphasis beyond the usual meaning of correlation is superfluous.

This sentence doesn't make any sense.

It's a fact that Bell inequalities are experimentally violated. In the Aspect experiment(s) the correlations are due to paired detection attributes being associated with photons emitted by the same atom during the same atomic transition and thus being related via conservation of angular momentum. The distance between the analyzers and/or the detectors in an ideal experimental setup is irrelevant.

It isn't clear to me what you're trying to say, or what your problem is with correlations. Is it that you want the spatially separated events of the experimental wings to be in some way causing each other? Well, there's no reason to think that they do. The correlations (both between the separated individual photon fluxes at certain settings, and between coincidental photon flux and the angular difference between the analyzers at any settings) are fairly precisely accounted for via the standard optics and the conservation law(s) incorporated in the standard qm treatment. The entire business about nonlocality is just a contrived pseudo-problem. There isn't any nonlocality to be explained.

No. You misunderstood the experiments. It was not simply due to "the paired detection attributes being associated with photons emitted by the same atom during the same atomic transition and thus being related via conservation of angular momentum". It was more than that. They can change the setting of the polarizer angles after the particles left their source. And the correlations is due to the particles somehow being in connected even if they 500 billion light years away. Try to see messages of JesseM and DrChinese on Bell's Theorem to see where you misunderstood it. They are experts and I would leave you to them to settle your misunderstanding.

 

[Hurkyl]

We must make more emphasis. Maybe call it "Instantaneous universe wide correlations".

If I randomly select a color and prepare two boxes each containing a ball of that color, give the boxes to Alice and Bob, and send them to opposite corners of the universe where they open their boxes, there is an instantaneous universe-wide correlation between what color they each observe.

But it's certainly nothing deserving of emphasis.

Unless you are trying to say above that Bell's Theorem is not really violated and the correlation is just some kind of local hidden variable? But Aspect, etc. experiments are meant to show that Bell's Theorem is violated and universe wide instantaneous correlations exist. Do you deny this?

I think you completely misunderstand Bell's Theorem. It's point isn't that correlations exist, it's that those correlations cannot be produced by the kind of model I described above.

 

[Hurkyl]

instantaneous action at a distance is, by definition, noncommunicative

What could you possibly mean by that?

In Newtonian gravity, for example, you can actually send signals instantaneously over arbitrary distances by moving a massive object and letting gravity transmit the signal

The correlations that are observed are precisely the correlations that one would expect assuming the validity of certain standard optical principles.

Care to elaborate? I'm pretty sure any variety of classical optical principles satisfies the hypotheses of Bell's theorem, and thus cannot reproduce the results of experiment.

and there's no particularly compelling reasoned approach which requires anything in entanglement preparations to be communicating ftl.

Except, of course, that's exactly what's required by collapse-based interpretations of QM.

 

[Varon]

If I randomly select a color and prepare two boxes each containing a ball of that color, give the boxes to Alice and Bob, and send them to opposite corners of the universe where they open their boxes, there is an instantaneous universe-wide correlation between what color they each observe.

But it's certainly nothing deserving of emphasis.



I think you completely misunderstand Bell's Theorem. It's point isn't that correlations exist, it's that those correlations cannot be produced by the kind of model I described above.

I understand perfectly well Bell's Theorem having taken a crash course on it. Maybe let's change
"instantaneous universe-wide correlation" to "on the fly real time instantaneous universe-wide correlation" for accuracy.

 

[ThomasT]

instantaneous action at a distance is, by definition, noncommunicative

What could you possibly mean by that?

First ask what physical meaning "instantaneous action at a distance" could possibly have. I take it to refer to two or more events happening simultaneously. The relationship between the events is acausal. There's no signal and no propagation involved. Communication, on the other hand, is causal. For A to communicate with B a disturbance has to propagate between them.

In Newtonian gravity, for example, you can actually send signals instantaneously over arbitrary distances by moving a massive object and letting gravity transmit the signal

'Instantaneous transmission' is simply a contradiction in terms. Moving massive objects entails simultaneously moving the fields surrounding them. No communication or 'instantaneous transmission' is necessary.

The correlations that are observed are precisely the correlations that one would expect assuming the validity of certain standard optical principles.

Care to elaborate? I'm pretty sure any variety of classical optical principles satisfies the hypotheses of Bell's theorem, and thus cannot reproduce the results of experiment.

I was referring to the standard optical principles, such as Malus' Law, that are preserved in the standard qm treatment.

and there's no particularly compelling reasoned approach which requires anything in entanglement preparations to be communicating ftl.

Except, of course, that's exactly what's required by collapse-based interpretations of QM.

I don't find collapse based interpretations of qm to be particularly compelling. If anything they've simply added to the confusion.

Bottom line is that there's no extant causal physical nonlocality to understand.

 

[ThomasT]

I understand perfectly well Bell's Theorem having taken a crash course on it. Maybe let's change "instantaneous universe-wide correlation" to "on the fly real time instantaneous universe-wide correlation" for accuracy.

The term 'instantaneous' is superfluous in the context of correlations. There's no essential difference in correlating events separated by 5 meters or 5 light years or 5 billion light years. Correlations are acausal. There's no nonlocality involved.

It still isn't clear what you're trying to say.

 

[Varon]

The term 'instantaneous' is superfluous in the context of correlations. There's no essential difference in correlating events separated by 5 meters or 5 light years or 5 billion light years. Correlations are acausal. There's no nonlocality involved.

It still isn't clear what you're trying to say.

You have a severe misunderstanding of Bell's Theorem and the experiments. You think it is about the momentum being conserved hence if one side has up, the other side has down and this is already determined from initial position? No. in QM, They are interactive.. for example.. if blue and red colors are used. If Alice used red, Bob will always have red in addition to the opposite momentum. This is why the correlations have on the fly real time effect on one another even 500 billion light years away. Even if you don't want to call it non-local. Something superluminal is occurring behind the scene. Hurkyl can better explain this to you with more precision language.

 

[ThomasT]

No. You misunderstood the experiments. It was not simply due to "the paired detection attributes being associated with photons emitted by the same atom during the same atomic transition and thus being related via conservation of angular momentum". It was more than that. They can change the setting of the polarizer angles after the particles left their source. And the correlations is due to the particles somehow being in connected even if they 500 billion light years away. Try to see messages of JesseM and DrChinese on Bell's Theorem to see where you misunderstood it. They are experts and I would leave you to them to settle your misunderstanding.

Changing the polarizer settings in mid-flight doesn't change the fact that there's one and only one polarizer setting associated with each detection attribute. The paired emitted disturbances don't need to be physically connected or communicating with each other. The experiments are measuring a relationship between them, a relationship that's produced via the emission process.

 

[ThomasT]

You have a severe misunderstanding of Bell's Theorem and the experiments. You think it is about the momentum being conserved hence if one side has up, the other side has down and this is already determined from initial position? No. in QM, They are interactive.. for example.. if blue and red colors are used. If Alice used red, Bob will always have red in addition to the opposite momentum. This is why the correlations have on the fly real time effect on one another even 500 billion light years away. Even if you don't want to call it non-local. Something superluminal is occurring behind the scene. Hurkyl can better explain this to you with more precision language.

Varon, Hurkyl has already indicated that he thinks you're clueless on this. I'm fairly certain that DrC and JesseM will tell you the same thing. What you're writing here is fairly common among ill-informed victims of popularizations. I'm curious as to what exactly it is that makes you think that the entangled disturbances are 'communicating' with each other. What you've written so far is just nonsense.

 

[Hurkyl]

First ask what physical meaning "instantaneous action at a distance" could possibly have.

In Newton gravity, the magnitude of gravitational force I experience here on Earth depends on the position of objects in the Andromeda galaxy right now^*. If I had a friend in Andromeda speaking in sign language and a gravitometer sensitive enough to distinguish his motions (and a computer capable of rendering the data into an image), I could read what he's saying. If he had a similar device, we could carry on a conversation in real time.

But you say there is no signal, no cause-and-effect, and no communication. I don't think those words mean what you think they mean.

*: Newton gravity, of course, uses Newtonian space-time, so "right now" makes sense without further qualification.

I was referring to the standard optical principles, such as Malus' Law, that are preserved in the standard qm treatment.

Malus' law doesn't tell you what you will see from an Aspect experiment. It is only part of the "standard qm treatment".

 

[ThomasT]

In Newton gravity, the magnitude of gravitational force I experience here on Earth depends on the position of objects in the Andromeda galaxy right now*. If I had a friend in Andromeda speaking in sign language and a gravitometer sensitive enough to distinguish his motions (and a computer capable of rendering the data into an image), I could read what he's saying. If he had a similar device, we could carry on a conversation in real time.

But you say there is no signal, no cause-and-effect, and no communication. I don't think those words mean what you think they mean.

*: Newton gravity, of course, uses Newtonian space-time, so "right now" makes sense without further qualification.

What gravity is and how it works are open questions. Are there any isolated gravitational fields? We can view the universe as overlapping, interacting gravitational fields or wave complexes with different centers defined by ponderable objects. If one of several interacting fields is moved (say, by moving the massive ponderable object at its center), then this will ultimately affect the other overlapping, interacting fields and the behavior of the objects at their centers. But I don't see any reason to believe that this happens instantaneously wrt the encompassing field complex if only a part of the encompassing field complex is initially moved.

What I would expect to happen instantaneously, more or less, is the movement of the field attached to the ponderable object that was moved, because the ponderable object and its associated wave complex or gravitational field comprise a singular entity. The field simply moves with its center, and there's no signal or transmission necessary to do this as they're the same thing. Much like when you move a chair you're moving the arms, legs, seat, and back in unison. There's no 'communication' between the contiguous parts of the chair. They're simply part of the same object. On the other hand, if I slide the chair into another chair, then causal transmissions are involved.

Or maybe gravity doesn't work that way. I don't know.

Anyway, aside from the obvious semantic problems with 'instantaneous transmission', why do you believe that you and your friend in the Andromeda galaxy would be communicating instantaneously, or at least via ftl transmissions?

Malus' law doesn't tell you what you will see from an Aspect experiment. It is only part of the "standard qm treatment".

That's what I said, although I didn't stress that it's "only part" of the standard qm treatment of, say, an Aspect experiment. But it's an important part. And it's fairly obvious from the experimental setups that the correlations will deviate from the linear via some function at least approximating a Malus' Law dependency.

 

[skippy1729]

Some people state that "non-locality" is a wrong word to describe it because there is nothing to be non-local about.. meaning properties don't exist before measurement... that is, the particles don't exist before measurement.. so how can you say the particles are in non-local communication. Is this argument valid?

Alice and Bob prepare a pair of entangled particles and Bob takes one with him on his trip to never-never-land. Alice performs a measurement on her particle and immediately can assign a new state to Bob's particle. No experiment can detect a physical change in Bob's remote lab. Until Bob performs a measurement or receives a message from Alice (along a timelike curve) his state vector is unchanged. At this point all that can be said is that Alice has better information about Bob's particle than Bob has. Wither Spukhafte Fernwirkungen?

 

[skippy1729]

When I first learned about the word Non-locality, someone said the word is not valid, due to the so called Kochen-Speaker theorem. Is anyone familiar with this?

It is the Kochen-Specker theorem. There is a vast literature and Wikipedia has a decent article on it.

For example it states that there is no possible assignment of the squared spin of a spin-1 particle along all possible orientations of three orthogonal axes (which always will be a permutation 0f 101) that do not lead to a contradiction. The conclusion is that the spin of the particle cannot be a pre-existing value waiting to be measured. The particle and the measurement perform an "act of creation" which results in the spin value.

 

[Hurkyl]

But I don't see any reason to believe that this happens instantaneously wrt the encompassing field complex if only a part of the encompassing field complex is initially moved.

$$F = G m_1 m_2 / r^2$$​

Much like when you move a chair you're moving the arms, legs, seat, and back in unison.

Rigid objects are an even more blatant example of non-locality -- and can be used in a far more obvious fashion to communicate over arbitrary distances.

Or maybe gravity doesn't work that way. I don't know.

I find it difficult to believe you can feel competent enough to discuss in threads like this without knowing Newton's law of universal gravitations.

why do you believe that you and your friend in the Andromeda galaxy would be communicating instantaneously, or at least via ftl transmissions?

$$F = G m_1 m_2 / r^2$$​

 

[Varon]

Varon, Hurkyl has already indicated that he thinks you're clueless on this. I'm fairly certain that DrC and JesseM will tell you the same thing. What you're writing here is fairly common among ill-informed victims of popularizations. I'm curious as to what exactly it is that makes you think that the entangled disturbances are 'communicating' with each other. What you've written so far is just nonsense.

Yes, they are communicating non-locally but you can't use this to transfer information because of randomness. But then since particles properties don't exist before measurement, it can't be called superluminal. Somehow it is the whole measurement setup that is non-locally connected 300 billion light years distance. So we call say that the "the entangled disturbances are 'communicating' with each other" but not directy between the particles which don't even have positions before measurement. Hope others can elaborate what I'm describing. ThomasT was saying there was absolutely no non-local correlations, that the correlations is because the entangled particles already have fixed values after emissions. Perhaps ThomasT doesn't understand in the first place what it meant to violate Bell's Theorem. Hope others who knew his arguments before can state his position so as not to waste precious time arguing with him.