Is the cat alive, dead, both or unknown

[bhobba]

Say assumption that hidden variables are non-contextual is sufficient assumption of Bell inequalities but it is not necessary assumption because contextual hidden variables can't violate Bell inequlities either.

Where exactly in Dr Chinese's proof is there non-contextuality?

Its got nothing to do with it and I have zero idea why you want to bring it up. Bringing up irrelevancies really makes things hard to discuss.

Thanks
Bill

 

[zonde]

Where exactly in Dr Chinese's proof is there non-contextuality?

Well acctually this sounds exactly as non-contextuality:
"we are simply saying that the answers to the 3 questions "What is the polarization of a photon at: 0, 120 and 240 degrees?" exist independently of actually seeing them."

But my remark actually was not meant exatly at your last post.

Its got nothing to do with it and I have zero idea why you want to bring it up. Bringing up irrelevancies really makes things hard to discuss.

This discussion goes around assumptions of Bell theorem. It seems relevant to distinguish which ones are sufficient to speak about Bell inequalities being satisfied and which ones are such that relaxing them is suficient to violate Bell inequalities.
For example, you said that relaxing "naive realism" is alternative to "action at a distance". This would be true if you could demonstrate that "naive realism" is necessary condition for Bell inequalities.

 

[bhobba]

Well acctually this sounds exactly as non-contextuality:

Before going any further, not with links, but in your own words, can you please describe what non-contextuality is?

Thanks
Bill

 

[bhobba]

This discussion goes around assumptions of Bell theorem. It seems relevant to distinguish which ones are sufficient to speak about Bell inequalities being satisfied.

It is well known what they are:
http://www.johnboccio.com/research/quantum/notes/paper.pdf

Let us define a “local” theory as a one where the outcomes of an experiment on a system are independent of the actions performed on a different system which has no causal connection with the first. For example, the temperature of this room is independent on whether I choose to wear purple socks today. Einstein’s relativity provides a stringent condition for causal connections: if two events are outside their respective light cones, there cannot be any causal connection among them.

Let us define a “counterfactual” theory as one whose experiments uncover properties that are pre-existing. In other words, in a counterfactual theory it is meaningful to assign a property to a system (e.g. the position of an electron) independently of whether the measurement of such property is carried out. Sometime this counterfactual definiteness property is also called “realism”, but it is best to avoid such philosophically laden term to avoid misconceptions

Bell’s theorem can be phrased as “quantum mechanics cannot be both local and counterfactual”. A logically equivalent way of stating it is “quantum mechanics is either non-local or non-counterfactual”.

Now you can keep locality if you give up realism, you can keep realism if you give up locality, or you can give up both.

Another issue is if locality is meaningful for correlated systems. The cluster decomposition principle that defines locality in QFT specifically precludes it. So another out is to say locality isn't meaningful for entangled systems.

Thanks
Bill

 

[zonde]

Before going any further, not with links, but in your own words, can you please describe what non-contextuality is?

In post #93 i described non-contextuality using example:

Maybe bhobba associates "naive realism" with non-contextual (hidden) variables. Say we believe that photon has objective property "polarization" and it can be determined by polarizer regardless of the state of polarizer (idependently from any hidden variables polarizer might have).

 

[bhobba]

In post #93 i described non-contextuality using example:

Tha'ts not non contextuality.

Its that observations are basis independent as per the assumption of Gleason's theorem:
http://arxiv.org/pdf/quant-ph/0507182v2.pdf
'It was tacitly assumed that measurement of an observable must yield the same value independently of what other measurements may be made simultaneously' ie if I have the basis defined by an observable and I keep some of the basis but replace the others to form another basis, hence another observable, the probabilities of the outcomes of the elements I kept are the same.

Its another aspect of the hidden variable issue - but nothing to do with Bell.

Thanks
Bill

 

[my2cts]

It's called removing system B from control by a partial trace.

The bible on this is Schlosshauer - Decoherence - And The Quantum To Classical Transition. See section 2.4.6 on the reduced density matrix.

Of course it is entangled with system B - I am not denying that - in fact I specifically said it was. However if you just observe system A then its in a mixed state. There is no attempt to hoodwink anyone, tell an incomplete story etc etc. Its simply if you just observe system A you are not observing system A+B. In fact often, like Schroedingers cat, you don't even have access to system B.

Thanks
Bill

The system consisting of the cat and the stochastic killing device is in a superposition of two coherent eigenstates.
In the one the cat is dead, in the other it is alive. An argumentum ad absurdum, not even a thought experiment.
It is going too far to address partial coherence, the level of coherence of a cat (dead or alive)
or the non-linear aspects of dying by cyanide.

 

[zonde]

Bell’s theorem can be phrased as “quantum mechanics cannot be both local and counterfactual”. A logically equivalent way of stating it is “quantum mechanics is either non-local or non-counterfactual”.

Bell theorem proves that any local counterfactual model for paired measurements can not violate Bell inequalities. It proves nothing about QM.
What you say is interpretation of Bell proof in cojungtion with predictions of QM. And it is rather common interpretation but it's wrong. That's because Bell theorem does not say what it takes to violate Bell inequalities (it gives sufficient conditions but does not give necessary conditions).
And indeed if we relax assumption of counterfactual definiteness while keeping enough definiteness that we can still talk about paired measurement events we can't model violation of Bell inequalities just the same. You can check this using that simple model from my link.

 

[Haelfix]

There is nothing in Bells setup that precludes local, non deterministic physics. Rather it is as Bhobba states, that Bells setup merely forces you into a choice. Something like consistent histories is an example of an interpretation that is the former. The link you give does not preclude locality either, it just precludes locality and classical statements like (either A or B) which don't allow for interference.

The modern point of view in teaching this tends to be very information theoretical, which is just the usual circuit diagram of quantum gates, however everything remains manifestly local in the operational definition of the dynamical laws.

 

[atyy]

There is nothing in Bells setup that precludes local, non deterministic physics. Rather it is as Bhobba states, that Bells setup merely forces you into a choice. Something like consistent histories is an example of an interpretation that is the former. The link you give does not preclude locality either, it just precludes locality and classical statements like (either A or B) which don't allow for interference.

The modern point of view in teaching this tends to be very information theoretical, which is just the usual circuit diagram of quantum gates, however everything remains manifestly local in the operational definition of the dynamical laws.

But that is the important point - it is operationally local.

The confusion is that there are claims that quantum field theory remains local beyond operational definitions. For example, vanhess71 has argued many times in this forum that the collapse of the wave function is not physical, because it would violate locality. This notion goes beyond an operational definition of locality, because as far as I understand, no predictions of the theory are changed, and whether the collapse of the wave function is physical or not does not affect operational locality.

Also, vanhees71's confusion shows that there is an important sort of locality that is ruled out by Bell's inequality - that is the causality of classical relativistic spacetime. When he says that a physical collapse of the wave function violates locality, this is the sort of locality he is referring to.

 

[Haelfix]

Yes, so I don't really want to put words in other people's mouths, but I think the statement refers to the notion that in some interpretations of quantum mechanics, like the Bohmian point of view where the wavefunction is a physical classical object (the pilot wave). Therefore in order to stay consistent with the violation of Bell's inequalities you must therefore abandon exact statements about the speed of light. The pilot wave itself is allowed to propagate nonlocally, or something of that nature.

I can't say too much about this, b/c I don't understand it and I don't know if it has ever been succesfully merged with relativity. I mean there is no lagrangian that you can write down to describe such an object is there?

Another thing I wanted to mention is there is another clarification about locality that I thought was a little ambiguous in the other thread. Namely that object A and object B cannot become entangled when they are spacelike separated (where we only consider objects A and B in the whole world for precision). This is NOT merely a statement about the propagation of information. For instance, imagine that you measure a particle in some galaxy. It would be damn odd if you then discovered that it was entangled with another particle in another galaxy that could never have been in causal contact. Indeed this is exactly what happens in astrophysics with the horizon problem. The conclusion is not that quantum mechanics can evade this constraint (it can't) but rather that the assumption is wrong and that the particles were, contrary to what you might think, in causal contact. (here the setup would involve measuring particle A and then allowing its partner particle to reenter your Hubble horizon, and making a measurement on that one. Note that information has not been transferred here, you haven't signaled any change, but you have verified something that seems like it might naively clash with locality)

 

[atyy]

Yes, so I don't really want to put words in other people's mouths, but I think the statement refers to the notion that in some interpretations of quantum mechanics, like the Bohmian point of view where the wavefunction is a physical classical object (the pilot wave). Therefore in order to stay consistent with the violation of Bell's inequalities you must therefore abandon exact statements about the speed of light. The pilot wave itself is allowed to propagate nonlocally, or something of that nature.

I can't say too much about this, b/c I don't understand it and I don't know if it has ever been succesfully merged with relativity. I mean there is no lagrangian that you can write down to describe such an object is there?

Whether pilot wave theory can handle exact relativity is still being researched. There are some proposals like Demystifier's, but I don't think there is consensus at the moment on the status of these proposals.

An easier way to see that the pilot wave theory can handle some relativistic phenomena is to assume that relativity is not exact, so we take say QED to be lattice QED with a fine but finite spacing. Then QED will be just a non-relativistic theory.

Another thing I wanted to mention is there is another clarification about locality that I thought was a little ambiguous in the other thread. Namely that object A and object B cannot become entangled when they are spacelike separated (where we only consider objects A and B in the whole world for precision). This is NOT merely a statement about the propagation of information. For instance, imagine that you measure a particle in some galaxy. It would be damn odd if you then discovered that it was entangled with another particle in another galaxy that could never have been in causal contact. Indeed this is exactly what happens in astrophysics with the horizon problem. The conclusion is not that quantum mechanics can evade this constraint (it can't) but rather that the assumption is wrong and that the particles were, contrary to what you might think, in causal contact. (here the setup would involve measuring particle A and then allowing its partner particle to reenter your Hubble horizon, and making a measurement on that one. Note that information has not been transferred here, you haven't signaled any change, but you have verified something that seems like it might naively clash with locality)

This is quite different from my intuition, which is that the only thing that matters quantum mechanically is that there is no superluminal communication. So for example, if we count the anti-symmetrization requirement for identical fermions as a kind of entanglement, then that should be allowed, no matter how far apart the particles are. But maybe you don't consider the symmetrization requirement to be entanglement?

 

[Haelfix]

This is quite different from my intuition, which is that the only thing that matters quantum mechanically is that there is no superluminal communication. So for example, if we count the anti-symmetrization requirement for identical fermions as a kind of entanglement, then that should be allowed, no matter how far apart the particles are. But maybe you don't consider the symmetrization requirement to be entanglement?

So I wouldn't exactly call that entanglement, although I concede there is a subtle point there. I would instead say that it is a rather interesting statement about the form certain types of entanglement can take. Also, the anti-symmetrization of the wavefunction is really a consequence of the spin-statistics theorem, which crucially relies on the existence of local relativistic field theory.

Now I want to emphasize that this is not merely intuition, but rather the history of a long line of failed attempts. So, if someone thinks that they can write down a local theory that can 'create' entanglement at spacelike separation out of thin air, without using a local, and causal third party (like a messenger particle in the case of entanglement swapping), then write down that theory. The problem will become obvious the second you attempt to do that, as you will find that you need to write down an interaction Hamiltonian that will either involve fields that are evaluated at different spacetime points, or that will require higher derivatives. I think this is where the Bohmians run into issues. They have to be able to allow ftl communication between say EPR pairs, but not for anything else, which then requires imposition of extra rules that adds theoretical baggage. As I said, I don't really know how successful they are with that.

 

[Nick666]

Indeed this is exactly what happens in astrophysics with the horizon problem. The conclusion is not that quantum mechanics can evade this constraint (it can't) but rather that the assumption is wrong and that the particles were, contrary to what you might think, in causal contact. (here the setup would involve measuring particle A and then allowing its partner particle to reenter your Hubble horizon, and making a measurement on that one. Note that information has not been transferred here, you haven't signaled any change, but you have verified something that seems like it might naively clash with locality)

So it could be that what we see as entanglement are in fact photons that were in causal contact at the big bang ?

 

[bhobba]

So it could be that what we see as entanglement are in fact photons that were in causal contact at the big bang ?

That makes no sense at all.

Here is what entanglement is. Suppose we have two systems A and B that can only be in state |a> and state |b>. If system A is in state |a> and system B in state |b> that is written as |a>|b>. Similarly if system A is in state |b> and system B in state |a> that is written as |b>|a>. But from the principle of superposition any superposition of |a>|b> and |b>|a> is also a state eg 1√2|a>|b> + 1√2|b>|a>. Such systems are called entangled. Neither system is in a definite pure state. Now let's say you observe system A, then since its the only two states it can be in you will get |a> or |b>. But because of the superposition if system A is in state |a> the total system A+B is in state |a>|b> ie you have immediately determined and know the state of system B. This is the spooky action at a distance that is talked about.

Note there is nothing in what I said about influences going between system A and B. All we have done is observe system A. It may simply be that its just a correlation like the green and red slips mentioned before. It is to investigate this Bell came up with his theorem. He showed if it was like the green and red slips then it would obey a certain inequality - but it turns out QM doesn't obey that inequality. Its a different kind of correlation. That's all this is about - coming to terms with a different kind of correlation than you have classically. Don't be fooled by all the mystique around this about locality being violated, naive reality overthrown, and all the other stuff bandied about - at rock bottom its not really that hard.

Bell didn't use green and red slips - he used Berlemann's socks:
https://en.wikipedia.org/wiki/Reinhold_Bertlmann
http://cds.cern.ch/record/142461/files/198009299.pdf

Thanks
Bill

 

[StevieTNZ]

So it could be that what we see as entanglement are in fact photons that were in causal contact at the big bang ?

All systems of the same type are entangled - Asher Peres made this clear in a book which I don't have the title to hand. I will do some searching and come back.

 

[Nick666]

He showed if it was like the green and red slips then it would obey a certain inequality - but it turns out QM doesn't obey that inequality.

Yeah, but the anti-Bell camp says that QM does obey the inequality.

From what I understood, and from that Dr.Chinese website you gave me, the anti-Bell camp just doesn't believe that measuring the same photon at the same time but in different places (different angles) , is not equivalent to a measurement of entangled particles, in other words (not mine) "Quantum mechanics takes it for granted that the times are the same because both (entangled) particles are described by the same wavefunction" .

 

[bhobba]

"Quantum mechanics takes it for granted that the times are the same because both (entangled) particles are described by the same wavefunction" .

I have zero idea what you are talking about.

Can you give a précis of the argument, not a link, I have had a lot of trouble with links that are supposed to show this or that, only to find it does no such thing; but post the argument in a nutshell. If you can't follow it, that's OK - just say that and I will see what I can glean out of it - but please, if such is the case, can you post something like I can't follow it but they seem to be claiming something at odds with accepted physics.

The above quote for example is just a tautological statement about wave-functions.

Thanks
Bill

 

[Nick666]

As Dr.Chinese says

We can test (angles) A, B or C one at a time (for a photon), but there is no way to test for all 3 simultaneously.

d. Bell anticipated that this result sounded good in theory, but needed more to make sense - because the above conclusion could not be tested. And in his next step he once again drew from EPR. He was aware that it was theoretically possible to have entangled particles that had identical but unknown spin attributes. Using these entangled particles, it would be possible to measure 2 of the 3 settings mentioned above simultaneously,

I think the anti-Bell camp has a problem with the "simultaneous" in the first sentence being equivalent to the "simultaneous" in the second paragraph.

I will give you a link in private message.

 

[bhobba]

We can test (angles) A, B or C one at a time (for a photon), but there is no way to test for all 3 simultaneously.

Of course you can't. So?

Bell anticipated that this result sounded good in theory, but needed more to make sense - because the above conclusion could not be tested. And in his next step he once again drew from EPR. He was aware that it was theoretically possible to have entangled particles that had identical but unknown spin attributes. Using these entangled particles, it would be possible to measure 2 of the 3 settings mentioned above simultaneously,

Please, please, can you explain, in your own words what the issue is - because the above makes no sense due to lack of context.

I think the anti-Bell camp has a problem with the "simultaneous" in the first sentence being equivalent to the "simultaneous" in the second paragraph.

What first sentence? And again please please explain it in your own words.

I will give you a link in private message.

I would rather discuss it here.

Thanks
Bill

 

[Nick666]

Just please look at the link, you'll see why I can't post the link here.

 

[bhobba]

Just please look at the link, you'll see why I can't post the link here.

There is no reason you can't post it here. Simply do what I said - give a precis of the argument. If you can't follow it simply say so and ask what others think. I have had a quick glance and as far as I can see its crank rot - claiming Bell was sloppy - and in such a way that hardly anyone else spotted it - I mean - really - is that creditable? Its not impossible and has happened before - in fact Bell picked up that exact issue with Von-Neumann - but its very very unlikely. However if you have concerns state them clearly. And if you can't follow it just say so - myself and/or others will get to the bottom of it.

Thanks
Bill

 

[Nick666]

In this website that you gave me http://drchinese.com/David/Bell_Theorem_Easy_Math.htm

Dr Chinese says that you can't measure angle A and angle B at the same time for a photon.

But then at the end, Dr.Chinese says that you can do the measurement with entangled particles.

But anti-Bell folks say that the measurement is only possible if somehow you would do the measurement for the entangled particles at precisely precisely precisely the exact same time for entangled particle a and for entangled particle b, cause if its done at a later time for one of the entangled particles the result is meaningless because obviously the measurements were done at different times. They think that the Bell experiment is useless because the measurements can't be practically made at the same time on the two entangled particles.I don't want to start with the math they use cause its too advanced for me.

 

[bhobba]

Dr Chinese says that you can't measure angle A and angle B at the same time for a photon.

Of course you cant.

But then at the end, Dr.Chinese says that you can do the measurement with entangled particles.

I can't find that. Like I said I have had problems with people claiming such and such shows this or that when in fact it does nothing of the sort. Can you please post the bit you think says that.

Added Later:
Is it the following:
'do A, B and C correspond to SIMULTANEOUS elements of reality?'

That's not saying it can be measured simultaneously.

Thanks
Bill

 

[Nick666]

 

[Nick666]

Just please don't think of me as anti-Bell. I really don't care about Bell or anti-Bell, I care about the discussions because they seem to be interesting, they are interesting at least to me.

 

[bhobba]

They are not measuring them simultaneously - Dr Chinese specifically states that's not it. What he is saying is by measuring a second particle with the same attributes you can infer it. But that has a flaw - well you can read the flaw.

Again so?

Thanks
Bill

 

[bhobba]

Just please don't think of me as anti-Bell. I really don't care about Bell or anti-Bell, I care about the discussions because they seem to be interesting, they are interesting at least to me.

That's Ok. That's what this forum is about. But to discuss it we need to be clear what is being discussed.

Thanks
Bill

 

[Nick666]

Before going further, I want to get this outta my way.

That makes no sense at all.

But why can't I assume that at the beginning of the universe, certain universal properties were created that still linger to these day? Like entanglement . What if entanglement its a property of spacetime/matter/whatever that goes somehow all they way back to the big bang ?

 

[bhobba]

But why can't I assume that at the beginning of the universe, certain universal properties were created that still linger to these day? Like entanglement . What if entanglement its a property of spacetime/matter/whatever that goes somehow all they way back to the big bang ?

I explained what entanglement was. It has nothing to do with anything you suggest. Its like saying what if the cause of nuclear fusion is that fire engines are red.

Please read my explanation of what entanglement is and you should see its got nothing to do with what you wrote.

If you think otherwise explain, in full detail, how that systems can be entangled, which follows from the principle of superposition, has anything to do with photons from the big bang?

Thanks
Bill

 

[Feeble Wonk]

All systems of the same type are entangled - Asher Peres made this clear in a book which I don't have the title to hand. I will do some searching and come back.

It's been a very interesting thread. But I've been hoping that Steve would come back with his source, and further expand on this point.

What does "All systems of the same type" refer to, and in what way are they entangled?

 

[StevieTNZ]

It's been a very interesting thread. But I've been hoping that Steve would come back with his source, and further expand on this point.

What does "All systems of the same type" refer to, and in what way are they entangled?

I'm here -- just woke up. Haven't had a chance to look for the copies I made from his book I got out of my uni library, but all systems of the same type I mean photons, electrons etc. I have two other references about all systems being entangled -- I also need to look those up from my books. Please bare with me.

 

[Feeble Wonk]

Thanks for effort. But, for now, do understand you to be saying that ALL particles (bosons and fermions) of any specific type are entangled on a cosmological scale?

 

[Feeble Wonk]

*...do "I" understand you to be saying...?

 

[StevieTNZ]

Some references:
"Quantum Mechanics: A New Introduction"(https://www.amazon.com/dp/0199560277/?tag=pfamazon01-20) pgs 512-513, section 18.4 Factorisation versus Entanglement

Entanglement is a very general feature of quantum mechanics, as all sub-systems in the universe do interact, or have interacted with each other in the past, to various degrees.

"Sneaking a Look at God's Cards" (https://www.amazon.com/dp/069113037X/?tag=pfamazon01-20) pgs 339-343
"Entangled World" (https://www.amazon.com/dp/3527404708/?tag=pfamazon01-20) Chapter 10

I have yet to find the Asher Peres photocopy.

 

[BiGyElLoWhAt]

...Its like saying what if the cause of nuclear fusion is that fire engines are red.Thanks
Bill

I really don't think that's what nick is saying. Correct me if I'm wrong here nick, but I'm going to try to elaborate on what I think you're trying to say.

At the big bang, it is reasonable to view all particles as being created from a common source, i.e. the big bang. So by "watching" 2 photons evolve throughout time from some "god frame" that was there before the big bang (not saying that it's reasonable to have an observer before the big bang, but bear with me), you can sum the quantum states and statistically determine the state of particle b by measuring the state of particle a. This can be increased in statistical accuracy up to 100% by assuming a 2 particle universe.

 

[StevieTNZ]

I have my Asher Peres photocopy, which I believe came from this book: https://www.amazon.com/dp/0792336321/?tag=pfamazon01-20 (pages 126-131):

An immediate consequence of Eqs (5.37) and (5.38) [given on page 127] is that two particles of the same type are always entangled, even if they are prepared independently, far away from each other, in different laboratories. We must now convince ourselves that this entanglement is not a matter of concern: No quantum prediction, referring to an atom located in our laboratory, is affected by the mere presence of similar atoms in remote parts of the universe.

 

[atyy]

I have my Asher Peres photocopy, which I believe came from this book: [URL='https://www.amazon.com/dp/0792336321/?tag=pfamazon01-20
Concepts-Fundamental-Theories/dp/0792336321[/URL] (pages 126-131):

An immediate consequence of Eqs (5.37) and (5.38) [given on page 127] is that two particles of the same type are always entangled, even if they are prepared independently, far away from each other, in different laboratories. We must now convince ourselves that this entanglement is not a matter of concern: No quantum prediction, referring to an atom located in our laboratory, is affected by the mere presence of similar atoms in remote parts of the universe.

I think Peres is referring to the symmetrization/anti-symmetrization of the wave function for identical particles that Haelfix and I discussed in posts #112-113 about whether a local interaction is needed for two particles to become entangled.

 

[StevieTNZ]

This thread has become so long I probably missed those posts. But yes, it is about idential particles.

 

[bhobba]

So by "watching" 2 photons evolve throughout time from some "god frame" that was there before the big bang (not saying that it's reasonable to have an observer before the big bang, but bear with me), you can sum the quantum states and statistically determine the state of particle b by measuring the state of particle a. This can be increased in statistical accuracy up to 100% by assuming a 2 particle universe.

Before the big bang? Its the birth of space-time - there is no before. And watching photons - you can't watch photons - that makes no sense at all - photons are what you use to watch with and they interact very very weakly - beams of light pass through each other.

Entanglement has nothing to do with anything like that - its simply applying the principle of superposition to systems. I gave a very careful explanation before - its really all there is to it. Nothing weird in the sense of being mystical etc etc is going on - it simply leads to a different type of correlation than occurs classically. The difference is classically you know it has properties all the time ie the green and red slips of paper are always green and red. In QM its more subtle as Bells theorem shows - but it's still just a correlation - its not some phenomena that needs further explanation. We know its explanation - systems can be in superposition and hence are correlated in a way different to classical correlations.

Thanks
Bill

 

[zonde]

Rather it is as Bhobba states, that Bells setup merely forces you into a choice.

This seems to be popular misconception so I will reiterate my argument but I hope more clearly.

Bell’s theorem can be phrased as “quantum mechanics cannot be both local and counterfactual”.

Yes, this is basically correct. We can check this by converting Bell's theorem into logical statement:
L and CD => BI (1)
transposition is valid rule of replacement, so we get:
not BI => not (L and CD) (2)
and if we add that QM can violate Bell inequality we get original statement.

A logically equivalent way of stating it is “quantum mechanics is either non-local or non-counterfactual”.

Yes, we get this by rewriting consequent in statement (2):
not BI => not L or not CD (3)

Now you can keep locality if you give up realism, you can keep realism if you give up locality, or you can give up both.

Let's rephrase this statement to make it more clear:
Model that gives up counterfactual definiteness or locality can violate Bell inequality (I suppose that realism in this context was meant as counterfactual definiteness).

Now it is clear that this statement is converse of (3):
not L or not CD => not BI (4)
and the truth of converse does not follow from truth of original statement i.e. it's possible that it's false.

 

[bhobba]

The logic is simple and doesn't require formal logic. You can give up locality or counter-factual definiteness or both. There is also another out not generally talked about and my personal view. Locality in QFT does not apply to correlated systems so locality may not even be a valid concept in this case.

Thanks
Bill

 

[zonde]

There is also another out not generally talked about and my personal view. Locality in QFT does not apply to correlated systems so locality may not even be a valid concept in this case.

Please specify in what sense you use "locality" here? Because it seems that your references for this statement might have used "locality" in different sense than any of the two used in discussions about Bell theorem.

 

[bhobba]

Please specify in what sense you use "locality" here? Because it seems that your references for this statement might have used "locality" in different sense than any of the two used in discussions about Bell theorem.

I have mentioned it a number of times. Its the cluster decomposition property which is the statement of locality in QFT:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

It does not apply to correlated systems and entangled systems are correlated. Hence the concept of locality is not relevant. And inherent in the discussion of Bell is the idea locality is a relevant concept - but that's precisely what I don't agree with. If you think it's relevant then Bell is airtight. But if its of no relevance then its of no concern at all.

Note also, as I have mentioned before, standard QM is based on the Galilean transformations so is inherently non-local at it very foundations.

This leads to the view this EPR stuff is not an issue - its just some interesting correlations.

Its not a common view - but some hold to it eg Brian Green:


Thanks
Bill

 

[zonde]

I have mentioned it a number of times. Its the cluster decomposition property which is the statement of locality in QFT:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

In your link I found this statement:
Here Weinberg introduces the concept of Cluster Decomposition: “It is one of the fundamental principles of physics (indeed, of all science) that experiments that are sufficiently separated in space have unrelated results…”

So it seems that "locality" here means that distant experiments have unrelated results. Is this right?

 

[bhobba]

In your link I found this statement:
Here Weinberg introduces the concept of Cluster Decomposition: “It is one of the fundamental principles of physics (indeed, of all science) that experiments that are sufficiently separated in space have unrelated results…” So it seems that "locality" here means that distant experiments have unrelated results. Is this right?

I will be more concise:
Uncorrelated experiments that are sufficiently separated in space have unrelated results.

There is a bit more to it at a technical level but basically that's the statement of locality in QM. And in that form its easy to see why you need the caveat uncorrelated.

EPR type experiments, and entanglement in general, are correlated so are not part of this definition of locality. Hence its meaningless, under this definition, to speak about locality in entangled systems. But of course you can have other definitions, and if you do that, then what Bell says comes into play - namely you can have counter-factual definiteness if some kind of instantaneous influence travels between entangled systems.

Thanks
Bill

 

[BiGyElLoWhAt]

I really don't think that's what nick is saying. Correct me if I'm wrong here nick, but I'm going to try to elaborate on what I think you're trying to say.

At the big bang, it is reasonable to view all particles as being created from a common source, i.e. the big bang. So by "watching" 2 photons evolve throughout time from some "god frame" that was there before the big bang (not saying that it's reasonable to have an observer before the big bang, but bear with me), you can sum the quantum states and statistically determine the state of particle b by measuring the state of particle a. This can be increased in statistical accuracy up to 100% by assuming a 2 particle universe.

Sigh... alright let me be more careful with my wording.
[Mentor's note: An extended speculation based on a misunderstanding of the Big Bang has been removed from this post]

I read through this thread, by the way, I have also worked somewhat with entanglement, if this is not how it works, then would you please bear with me and give me a dumbed down version of what entanglement actually means in a modern context? Because I was apparently unable to grasp it from what you've said...

If this is, in fact, how it works, then why are not ALL particles from the time of the big bang entangled, as nick666 has said?

 

[Nugatory]

If this is, in fact, how it works, then why are not ALL particles from the time of the big bang entangled, as nick666 has said?

The big bang is indeed in the causal past of everything, but any residual entanglement between two particles that have not interacted since then is near as no never mind zero. That is, the wave function of the two-particle system is for all practical purposes completely factorizable and the two particles can be treated separately.

(This might, however, be a good time to suggest googling for "quantum superdeterminism", as long as everyone promises not to prolong this thread based on what they find).

 

[Feeble Wonk]

I think Peres is referring to the symmetrization/anti-symmetrization of the wave function for identical particles that Haelfix and I discussed in posts #112-113 about whether a local interaction is needed for two particles to become entangled.

Can you elaborate more about this type of entanglement in conceptual language rather than mathematical? And how would this type of entanglement contribute, if at all, to decoherence effects?

 

[atyy]

Can you elaborate more about this type of entanglement in conceptual language rather than mathematical? And how would this type of entanglement contribute, if at all, to decoherence effects?

Good question! Actually, I don't know whether everyone counts this as "true" entanglement.

In the old sense, this is for electrons and protons, simply the Pauli exclusion principle, which is key to chemistry and the solids and liquids you see in everyday life.

I think the debate as to whether this constitutes "true" entanglement continues. There are recent papers trying to address the issue, eg.:

http://arxiv.org/abs/1312.4311
Phys. Rev. Lett. 112, 150501 (2014)
Extracting entanglement from identical particles
N. Killoran, M. Cramer, M. B. Plenio
(Submitted on 16 Dec 2013 (v1), last revised 22 Apr 2014 (this version, v2))
Identical particles and entanglement are both fundamental components of quantum mechanics. However, when identical particles are condensed in a single spatial mode, the standard notions of entanglement, based on clearly identifiable subsystems, break down. This has led many to conclude that such systems have limited value for quantum information tasks, compared to distinguishable particle systems. To the contrary, we show that any entanglement formally appearing amongst the identical particles, including entanglement due purely to symmetrization, can be extracted into an entangled state of independent modes, which can then be applied to any task. In fact, the entanglement of the mode system is in one-to-one correspondence with the entanglement between the inaccessible identical particles. This settles the long-standing debate about the resource capabilities of such states, in particular spin-squeezed states of Bose-Einstein condensates, while also revealing a new perspective on how and when entanglement is generated in passive optical networks. Our results thus reveal new fundamental connections between entanglement, squeezing, and indistinguishability.