Is the cat alive, dead, both or unknown

[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.

 

[BiGyElLoWhAt]

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).

I wasn't saying it was realistic to be able to extract any useful information nowadays, just that it didn't "make no sense" to view them as entangled from the beginning, as I think this was a confusing and unnecessary complication to the argument. Thanks for chiming in.

 

[atyy]

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.

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).

@Haelfix, my intuition is closer to what Nugatory is saying - it can be present, but in general it should be very diluted (because of monogamy of entanglement). Also, originally, you mentioned it as analogous to the horizon problem. So would it be right to understand it not so much as "impossible", but more a question of fine tuning - ie. "dman odd" as you put it?

 

[BiGyElLoWhAt]

Monogomy of entanglement? haven't they entangled as many as 5 particles intentionally? That's what I thought until google turned up th world record being 3000, but all the articles seemed to be pop-sci. Here's one on multiple entanglement.
http://m.phys.org/_news63037231.html

 

[atyy]

Monogomy of entanglement? haven't they entangled as many as 5 particles intentionally? That's what I thought until google turned up th world record being 3000, but all the articles seemed to be pop-sci. Here's one on multiple entanglement.
http://m.phys.org/_news63037231.html

It refers to maximal entanglement.

 

[BiGyElLoWhAt]

Hmmm... I'll have to look into that, would you care to elaborate a little bit?

 

[Haelfix]

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

Indeed, and that is the point. People get very tripped up about spooky action at a distance, or how quantum mechanics can seemingly feel measurements that are very far apart and adjust accordingly etc. But at the end of the day, as long as you give up realism (counterfactual definitiveness to use the philosophical lingo) and simply accept that we don't have bits, but instead we have qubits, there is absolutely nothing bizarre about Bells inequalities being violated. We don't have to give up locality, relativity or anything like that. The correlations found in quantum mechanics are different than classical mechanics, but we already knew this 40 years before Bell formulated his thought experiment.

 

[Haelfix]

Sorry, I don't think my point has come across. I am not saying anything controversial here. The point is you can only create entanglement locally. Forget about the horizon problem, or anything like that, I regret bringing that up b/c it seems like it has caused confusion.

So how about this. Take the usual EPR setup with system A and B prepared in a maximally entangled pure state and allow another system C to be nearby. Take B off to Alpha Centauri and after some time make a measurement on every box and write down the results. We have now broken the entanglement between A and B, and can allow those systems to develop new entanglements, by say allowing them to interact with the environment.

Now at some fixed and agreed upon later time after those initial measurements but before the light travel time between Earth and Alpha Centauri, make a measurement on box B and C. Write the results down, and have your partner fly back to compare notes.

Here is the important thing. There will never be a correlation between the results in box B and C. Repeat the experiment however many times you want, you will always find the same result. The conclusion is obvious. B/c A and B were in a maximally entangled state, by monogamy of entanglement they could not be entangled with C. Further, once C was spacelike seperated, it could never create entanglement with B, even after the original entanglement was broken. This is a physical statement about the locality of the laws of physics and is not just about the transfer of information.

 

[atyy]

Sorry, I don't think my point has come across. I am not saying anything controversial here. The point is you can only create entanglement locally. Forget about the horizon problem, or anything like that, I regret bringing that up b/c it seems like it has caused confusion.

So how about this. Take the usual EPR setup with system A and B prepared in a maximally entangled pure state and allow another system C to be nearby. Take B off to Alpha Centauri and after some time make a measurement on every box and write down the results. We have now broken the entanglement between A and B, and can allow those systems to develop new entanglements, by say allowing them to interact with the environment.

Now at some fixed and agreed upon later time after those initial measurements but before the light travel time between Earth and Alpha Centauri, make a measurement on box B and C. Write the results down, and have your partner fly back to compare notes.

Here is the important thing. There will never be a correlation between the results in box B and C. Repeat the experiment however many times you want, you will always find the same result. The conclusion is obvious. B/c A and B were in a maximally entangled state, by monogamy of entanglement they could not be entangled with C. Further, once C was spacelike seperated, it could never create entanglement with B, even after the original entanglement was broken. This is a physical statement about the locality of the laws of physics and is not just about the transfer of information.

Is that the same thing as eg. a limitation on the states that say LOCC can prepare? Or more than that?

Edit: Or is it the same as eg. if B and C are in a product state, then local unitaries can never cause B and C to be entangled?

 

[atyy]

Hmmm... I'll have to look into that, would you care to elaborate a little bit?

Monogamy of entanglement is misleading but standard jargon. It certainly doesn't mean that one particle can only be entangled with another - that's considered so "obvious" that the misleading jargon is considered not that misleading. http://quantiki.org/wiki/Monogamy_of_entanglement

 

[rootone]

Excuse my naivety please. I want to understand entanglement.
Two objects exist, one is red, the other is blue.
These objects are then separated. they can be a few meters separated or maybe light years;
When we look at either object we instantaniously know what the state of the other object must be,
and information does not have to travel any distance at all for this to be so.
It is not spooky at all..

 

[bhobba]

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...

See post 115. I will repeat it in summery - entanglement is the principle of superposition applied not just to the same system but composite systems.

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?

Thats assuming they were all entangled to begin with - something those that know more cosmology than I do question:
http://physics.stackexchange.com/questions/55026/particles-entangled-after-the-big-bang

Thanks
Bill

 

[bhobba]

and information does not have to travel any distance at all for this to be so. It is not spooky at all..

No it inst.

Its only spooky if you want to hold to counter-factual definiteness. That's the root cause of all this angst. And even if you still want to hold to it then pick an interpretation like Bohmian Mechanics and see what happens there. In that interpretation you can see exactly why you have locality broken:
http://arxiv.org/pdf/quant-ph/0611032v1.pdf

Also let's look at the definition of locality in the paper on Bell I linked to before:
'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.'

That is a slightly less clearly stated version of the cluster decomposition property - it doesn't state a necessary caveat - namely - it can't apply to correlated systems. Entangled systems are correlated so - it isn't even an applicable definition. Only if you want counter-factual definiteness is it even a worry. If you do then you have to allow locality to apply to correlated systems.

Thanks
Bill

 

[atyy]

Also let's look at the definition of locality in the paper on Bell I linked to before:
'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.'

That is a slightly less clearly stated version of the cluster decomposition property - it doesn't state a necessary caveat - namely - it can't apply to correlated systems. Entangled systems are correlated so - it isn't even an applicable definition. Only if you want counter-factual definiteness is it even a worry. If you do then you have to allow locality to apply to correlated systems.

But aren't Bertlmann's socks correlated systems?

 

[bhobba]

But aren't Bertlmann's socks correlated systems?

Of course. But its a classically correlated system and obeys Bell's inequalities. Quantum correlated systems do not and there is the rub. But if you remove correlated systems from the definition of locality, and you are pretty well forced to in the cluster decomposition property for it to be reasonable, then its of no concern as far as locality goes. If however you want counter-factual definiteness then it must be included and you get something like: 'experiments that are sufficiently separated in space have unrelated results'. It immediately jumps out - what if they are correlated to begin with. So you say - of course - that needs to be removed and you get 'Uncorrelated experiments that are sufficiently separated in space have unrelated results'. But if you do that you can't have counter-factual definiteness because in order for Bells inequality to be broken correlated systems need to break locality. In a sense you are led to something quite unnatural.

Thanks
Bill

 

[Nick666]

There are some who think entanglement is something that could exist at macroscopic scale, cause of a topological property of spacetime . They proposed some sort of macroscopic experiment based on an arvix paper, and that arvix paper was apparently reviewed by these guys . If anyone's interested I can post the paper.

 

[bhobba]

There are some who think entanglement is something that could exist at macroscopic scale,

Everything is quantum. Of course it exists at the macroscopic scale. The very reason for classical properties is classical objects are all the time being observed by its environment and entangled with it.

I am unaware of any relation to the topology of space-time. But it sounds highly speculative and probably best in its own thread.

Thanks
Bill

 

[Nick666]

Yeah the proposer of the experiment says the macroscopic things will also be corelated just like the entangled photons, so there would be nothing special about the quantum realm when it comes to entanglement, the correlations being consequences of the topological properties of space. And the experiment being macroscopic it would manifestly be local and realistic.

 

[atyy]

Of course. But its a classically correlated system and obeys Bell's inequalities. Quantum correlated systems do not and there is the rub. But if you remove correlated systems from the definition of locality, and you are pretty well forced to in the cluster decomposition property for it to be reasonable, then its of no concern as far as locality goes. If however you want counter-factual definiteness then it must be included and you get something like: 'experiments that are sufficiently separated in space have unrelated results'. It immediately jumps out - what if they are correlated to begin with. So you say - of course - that needs to be removed and you get 'Uncorrelated experiments that are sufficiently separated in space have unrelated results'. But if you do that you can't have counter-factual definiteness because in order for Bells inequality to be broken correlated systems need to break locality. In a sense you are led to something quite unnatural.

I don't know if your definition of cluster decomposition is the same as Wikipedia's https://en.wikipedia.org/wiki/Cluster_decomposition_theorem, which makes use of the vacuum state. I also don't think that Wikipedia's definition of cluster decomposition is the same as that used in Bell's theorem, because Bell's theorem is independent of QM, whereas cluster decomposition is defined with respect to QM, since it uses the concept of a vacuum state.

 

[StevieTNZ]

There are some who think entanglement is something that could exist at macroscopic scale

I am reminded of this experiment - two macroscopic diamonds entangled: http://www.nature.com/news/entangled-diamonds-vibrate-together-1.9532

I would also be interested in reading the paper.

 

[bhobba]

I don't know if your definition of cluster decomposition is the same as Wikipedia's

I gave a link earlier that addressed that:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

Even Weinberg is a bit loose in his bible textbook. The key point is it doesn't apply to correlated systems in QFT.

The definition of locality I gave from my linked paper on Bell is very similar to the cluster decomposition property.

Thanks
Bill

 

[zonde]

But if you remove correlated systems from the definition of locality, and you are pretty well forced to in the cluster decomposition property for it to be reasonable, then its of no concern as far as locality goes. If however you want counter-factual definiteness then it must be included and you get something like: 'experiments that are sufficiently separated in space have unrelated results'. It immediately jumps out - what if they are correlated to begin with. So you say - of course - that needs to be removed and you get 'Uncorrelated experiments that are sufficiently separated in space have unrelated results'. But if you do that you can't have counter-factual definiteness because in order for Bells inequality to be broken correlated systems need to break locality. In a sense you are led to something quite unnatural.

You are fooling with definitions. You are bringing in irrelevant definition of "locality" while there is common and widely used relevant definition of "locality".
Your irrelevant definition - Uncorrelated experiments that are sufficiently separated in space have unrelated results
Relevant definition - There can't be any influence between spacelike separated events.

 

[atyy]

@bhobba, here is a different explanation of cluster decomposition - the underlying principle would then be no superluminal signalling of classical information.

http://relativity.livingreviews.org/Articles/lrr-2005-5/ (section 2.7.1)

Yes, I do think Weinberg's explanation is wrong.

 

[Nick666]

I am reminded of this experiment - two macroscopic diamonds entangled: http://www.nature.com/news/entangled-diamonds-vibrate-together-1.9532

I would also be interested in reading the paper.

http://arxiv.org/pdf/1211.0784v4.pdf

 

[bhobba]

You are fooling with definitions. You are bringing in irrelevant definition of "locality" while there is common and widely used relevant definition of "locality".

I gave the common and widely used one as found in my linked paper on Bell.

Thanks
Bill

 

[atyy]

Also let's look at the definition of locality in the paper on Bell I linked to before:
'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.'

That is a slightly less clearly stated version of the cluster decomposition property - it doesn't state a necessary caveat - namely - it can't apply to correlated systems. Entangled systems are correlated so - it isn't even an applicable definition. Only if you want counter-factual definiteness is it even a worry. If you do then you have to allow locality to apply to correlated systems.

I think the definition of "local" is ok without the caveat that about correlated systems. It means that if two systems are spacelike separated, then an action on one cannot affect the probabilities of local observables on the other. This prevents superluminal signalling, and that is the basis of the cluster decomposition.

Weinberg's informal statement is just so wrong, I don't think it is worth rescuing by adding caveats. If we use "no superluminal signalling", we get cluster decomposition, and an acceptable definition of locality used in some derivations of Bell's theorem.