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In summary, as long as the particles are not disturbed by any external influence, the entanglement in all these cases persists due to conservation laws!

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Physics news on Phys.org

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So, as I suggested, they are common in time?

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What does that mean?

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Suppaman said:An alternate way to say this is that they are not communicating at a distance faster than light but rather they communicate back to/from the instant in time they were entangled.

They don't communicate at all, nor do they need to - entanglement is just a statistical correlation.

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Yes, they are connected from the very beginning by being prepared in the entangled state. To be more precise you have to say, in which observables they are entangled.Suppaman said:

The most famous example is the EPR example going back to a famous debate between Einstein and Bohr. Einstein, to his later regret with his coauthors Podolsky and Rosen, wrote a paper, asking the question, whether quantum mechanics can be considered complete, and Bohr answered with another article with the same title.

There the example was a particle, initially at rest, decaying into two other particles running with opposite momenta away. Then these particles are entangled with respect to their momenta. As often here the entanglement is due to a conservation law (here conservation of momentum).

Another example is the decay of a scalar particle into two particles of spin 1/2. Due to angular-momentum conservation the two particles' spins are entangled.

The most accurate Bell experiments are made with photons, because nowadays it's very easy to prepare two-photon states whose polarizations are entangled by a mechanism called "parametric down conversion". You shoot with a laser at a certain type of birefringent crystal and then a photon, picked up out of the laser field, becomes split into two photons that have opposite polarization ("horizontal" and "vertical" with respect to any arbitrarily chosen direction).

In all these examples the entanglement between the observables is due to the "preparation" of the corresponding quantum state. Another important point is that, as long as the so prepared quantum systems are not disturbed by any external influence, the entanglement in all these cases persists due to conservation laws! This means the particles or photons in all these examples can get very far apart of each other if you wait long enough, but the entanglement still persists, and thus, although being very far apart, the particles/photons still are one system.

Now, if Alice observes the entangled observable on one of the particles/photons, nothing happens instantaneously to the other particle according to our present understanding of this question, since all interactions of the particles with A's measurement apparatus obey themselves the rules of local relativistic quantum field theory, and this theory has built into its foundations the principle of microcausality and locality, i.e., there are only local interactions, and observables (like the energy density of the electromagnetic field, which describes the probability to find a photon in a detector put at a given place) which are separated by a space-like distance commute, i.e., the measurement at A's place cannot do anything at Bob's (perhaps far distant place).

Nevertheless, although the polarizations of each of the photons in the example with the two-photon polarization state, are maximally unknown, i.e., when Alice and Bob measure very many so prepared photons, they cannot predict in any way what they will find. Both A and B just have a stream of unpolarized photons. Nevertheless, if they keep the time of their measurement events carefully enough, so that after the measurement they can check on correlations between the polarizations of photons belonging always to a polarization-entangled two-photon state, they will find 100% correlation, i.e., when A has found a H-polarized photon B will have found a V-polarized one and vice versa.

Now taken these two statements, which both are fully consistent with relativistic local quantum field theory (here particularly Quandum Electrodynamics, describing the electromagnetic field and its interactions with matter), together can only lead to the conclusion that the 100% correlation must be due to the preparation in the entangled state, and that it is not caused on B's photon by A's measurement (or vice versa). This has also been verified with many very accurate measurements, where the choice of what was measured at A's and B's place was decided so short before the photon's registration that there cannot be any influence from the measurements at the other place.

This shows that local relativistic quantum field theory combines both the principles of locality of interactions and microcausality with the possibility of 100% correlations between separate parts of quantum systems, which are described by entanglement. As Einstein made clear later with another paper, his main concern was this inseparability of far-distant parts of quantum systems, not so much the "spooky action at a distance", which is an issue only if one assumes the socalled "collapse of the quantum state" due to measurement processes, but that's an unnecessary additional assumption on top of the quantum theoretical formalism. As the above example shows, you do not need to assume it anywhere to fully describe what's measured on such entangled systems! The demonstration of entanglement, however, shows that Nature is very different from a naive worldview based on classical (i.e., non-quantum) physics in the sense that the quantum states describe very strong correlations between far-distant parts of a quantum system, which cannot be explained with local deterministic hidden-variable models, i.e., they cannot be described with classical statistics within a deterministic local classical theory.

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vanhees71 said:As often here the entanglement is due to a conservation law (here conservation of momentum).

I thought it was always due to a conservation law, not just often, are there exceptions?

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NIST team proves 'spooky action at a distance' is really real"

So, reading vanhees71 and the above article I am convinced it is both real and not real. Did someone let a cat into the discussion?

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I'm not sure. I've no counterexample in mind.ddd123 said:I thought it was always due to a conservation law, not just often, are there exceptions?

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Suppaman said:

NIST team proves 'spooky action at a distance' is really real"

So, reading vanhees71 and the above article I am convinced it is both real and not real. Did someone let a cat into the discussion?

It's a language issue. "Spooky action at a distance" in the article (and I think in the OP as well) is a violation of the Bell inequality. Vanhee didn't say that this is not real.

vanhees71 said:The demonstration of entanglement, however, shows that Nature is very different from a naive worldview based on classical (i.e., non-quantum) physics in the sense that the quantum states describe very strong correlations between far-distant parts of a quantum system, which cannot be explained with local deterministic hidden-variable models, i.e., they cannot be described with classical statistics within a deterministic local classical theory.

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Suppaman said:"Because I see there is some doubt about the concept of time that perhaps when the particles are entangled that they are linked to a specific instant in time and when separated and tested they are still connected to that point in time. An alternate way to say this is that they are not communicating at a distance faster than light but rather they communicate back to/from the instant in time they were entangled."

There are interpretations of quantum theory that incorporates backward-in-time influences such as John Cramer's transactional interpretation and the two-state vector formalism by Yakir Aharonov and others. I'm not familiar with them so I can't say more about them. Perhaps others in this forum can. But IMO these "explanations" aren't any more palatable than having faster-than-light communication between particles.

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https://quantumfrontiers.com/2013/06/07/entanglement-wormholes/

But from when I attended Susskind's talk a year ago, it seems like you need each of the particles to be a black hole itself (to create a wormhole).

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Markus Hanke said:They don't communicate at all, nor do they need to - entanglement is just a statistical correlation.

Indeed. Nothing mysterious at all. Its just different to classical correlations.

Thanks

Bill

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Suppaman said:NIST team proves 'spooky action at a distance' is really real"

These and similar claims are simply misunderstandings of so called weak measurents as has been discussed here many times.

Thanks

Bill

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Suppaman said:

See this paper on conservation laws and entanglement : http://arxiv.org/pdf/quant-ph/0407041.pdf

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I like to think of the analogy that particles are "too big" in information sense. Our spacetime doesn't have enough information capacity to hold the full state of the particle, so we have to encode part of it in the space occupied by other particles. The consequence is that not all multi-particle states are allowed or (direct equivalence from probability theory) the states of individual particles are correlated.

If you like Matrix-like interpretations, it may mean that our universe is compressed, i.e. the computer that simulates us uses some compression algorithm.

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Of course, the violation of the Bell inequality is one of the best checked features of QT. I only deny the necessity of a collapse and I deny that one has to assume something else than Born's Rule to give meaning to the quantum mechanical state (statistical operator, ray in Hilbert space). This means that there's no necessity to assume any "spooky action at a distance" at all, which is only a necessary conclusion if you introduce the collapse hypothesis, which thus is violating causality in the relativsitic context. This view of the meaning of the state (minimal statistical interpretation) implies that the observed correlations, responsible for the violation of the Bell inequality, are due to the state preparation and not due to the measurement of A causing something relevant for the measurement at B (and vice versa).Truecrimson said:It's a language issue. "Spooky action at a distance" in the article (and I think in the OP as well) is a violation of the Bell inequality. Vanhee didn't say that this is not real.

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morrobay said:See this paper on conservation laws and entanglement : http://arxiv.org/pdf/quant-ph/0407041.pdf

The same author has published this: https://arxiv.org/abs/1102.1187 , from the text "We have discovered local variables that are quantum compatible that allow coding the shared information without violating Einstein locality or the requirements arising from quantum superposition". Isn't this a controversial claim? I'd like to have some opinions.

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A statistical correlation which one cannot explain with a common cause in the past. This is Bell's theorem.Markus Hanke said:They don't communicate at all, nor do they need to - entanglement is just a statistical correlation.

So you have the choice: Or to accept that one measurement has a direct causal influence on the other one, which would violate Einstein causality.

Or to give up the very idea that it is the job of science to find causal explanations of observed correlations. This choice would be preferred by astrologs and the tobacco industry, for obvious reasons. But why scientists prefer this choice is beyond me.

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Ilja said:...to accept that one measurement has a direct causal influence on the other one, which would violate Einstein causality...

But why violate Einstein? Before those two measurements, there is the quantum superposition of potential spacetime worlds - in each world,

(That's all actually explained by Heisenberg in his philosophic writings.)

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Do you have a reference where Einstein expresses his regret?vanhees71 said:The most famous example is the EPR example going back to a famous debate between Einstein and Bohr. Einstein, to his later regret with his coauthors Podolsky and Rosen, wrote a paper, asking the question, whether quantum mechanics can be considered complete, and Bohr answered with another article with the same title.

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Indeed there is nothing mysterious in how to calculate the correlations for an entangled pair, QM gives a clear recipe. Nonetheless, I find that nature is in accord with those correlations extremely mysterious and I am in good company. Don't you find that masses experience gravitational attraction mysterious in spite of knowing how to calculate the force.bhobba said:

Indeed. Nothing mysterious at all. Its just different to classical correlations.

Thanks

Bill

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However, you do give the false impression that the 100% anti-correlation results (e.g. H vs V) are sufficient to undermine local hidden variables.vanhees71 said:The demonstration of entanglement, however, shows that Nature is very different from a naive worldview based on classical (i.e., non-quantum) physics in the sense that the quantum states describe very strong correlations between far-distant parts of a quantum system, which cannot be explained with local deterministic hidden-variable models, i.e., they cannot be described with classical statistics within a deterministic local classical theory.

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Zafa Pi said:Indeed there is nothing mysterious in how to calculate the correlations for an entangled pair, QM gives a clear recipe. Nonetheless, I find that nature is in accord with those correlations extremely mysterious and I am in good company. Don't you find that masses experience gravitational attraction mysterious in spite of knowing how to calculate the force.

I think before making statements like that a bit of thought needs to be put into the nature of explanation. An explanation assumes some things to explain others. Every explanation, every single one, has that 'mysterious' aspect to it. Its how you react to it that determines your attitude - its very personal and not science.

Regarding gravity - GR explains that attraction as the result of space-time curvature which in modern times is known to be more or less implied by the very intuitive principle of no prior geometry - why should nature single out one geometry over another? Still its an assumption and how you react to it determines if its mysterious or not - personally for me its not mysterious - but that's just me - although I suspect the vast majority would feel that way as well.

I post this a lot because I think its very important (those that have seen before just ignore it - its purely to make a point):

https://arxiv.org/pdf/quant-ph/0101012.pdf

QM can be presented in such a way, like the principle of no prior geometry, so its not 'mysterious'. From that the different kinds of statistical correlations follow. In particular as the above paper shows its the requirement of continuous transformations between pure states that takes the place of no prior geometry. It turns out that is equivalent to having entanglement:

https://arxiv.org/abs/0911.0695

Its entirely how you view and react to it - 'mysterious' is a human reaction - nature doesn't care a hoot and certainly science doesn't.

Thanks

Bill

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Because else you can prove Bell's inequality. All we need for this is the EPR criterion of reality: "If, without in any way disturbing a system, we can predict with certainty the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity. " And Einstein causality to show that the measurement by Alice does not in any way disturb Bobs part of the system.AlexCaledin said:But why violate Einstein?

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AlexCaledin said:But why violate Einstein? Before those two measurements, there is the quantum superposition of potential spacetime worlds - in each world,allthe observed/observable events are in the most perfect agreement with Einstein causality. The two measurements together make the choice of the one actual world, one measurement just reducing the choice for another.

Virtually all of that is interpretive supposition. The QM formalisn says nothing about potential space time worlds etc etc.

Take on board the writings of the early pioneers with caution - things have moved on a lot since then.

Before forming any views on QM study a good modern text like Ballentine.

Thanks

Bill

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How does mass pull off the warping of space?bhobba said:I think before making statements like that a bit of thought needs to be put into the nature of explanation. An explanation assumes some things to explain others. Every explanation, every single one, has that 'mysterious' aspect to it. Its how you react to it that determines your attitude - its very personal and not science.

Regarding gravity - GR explains that attraction as the result of space-time curvature which in modern times is known to be more or less implied by the very intuitive principle of no prior geometry - why should nature single out one geometry over another? Still its an assumption and how you react to it determines if its mysterious or not - personally for me its not mysterious - but that's just me - although I suspect the vast majority would feel that way as well.

I post this a lot because I think its very important (those that have seen before just ignore it - its purely to make a point):

https://arxiv.org/pdf/quant-ph/0101012.pdf

QM can be presented in such a way, like the principle of no prior geometry, so its not 'mysterious'. From that the different kinds of statistical correlations follow. In particular as the above paper shows its the requirement of continuous transformations between pure states that takes the place of no prior geometry. It turns out that is equivalent to having entanglement:

https://arxiv.org/abs/0911.0695

Its entirely how you view and react to it - 'mysterious' is a human reaction - nature doesn't care a hoot and certainly science doesn't.

Thanks

Bill

The Hardy "axioms" to mathematicians are incomprehensible fluff, at least all the ones I've showed them to.

As far as we know nature doesn't give a hoot, nor does a specific theory/ model. But science as a human endeavor does care.

Is there any scientific result you find mysterious?

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Zafa Pi said:How does mass pull off the warping of space?

If you examine a simple model of dust and assume a pseudo-riemannian geometry (that's the no prior geometry idea) then the equations of GR pretty much follow. Most books on GR explains it, at least the ones I have read, but here is not the place to discuss it - the relativity subsection is.

Zafa Pi said:The Hardy "axioms" to mathematicians are incomprehensible fluff, at least all the ones I've showed them to.

Well my background is math as well and that's not my view. But obviously its a matter of opinion.

Zafa Pi said:As far as we know nature doesn't give a hoot, nor does a specific theory/ model. But science as a human endeavor does care. Is there any scientific result you find mysterious?

Of course eg the Feynman sum over history approach explains the principle of least action in classical physics, but why can QFT also be put into that form. That's just one example of course.

Thanks

Bill

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