Is the cat alive, dead, both or unknown

[rootone]

States which can only exist given that their opposite exists.
There has to be a movie in there somewhere.

 

[Nick666]

And what about the math thing I wrote ?

 

[zonde]

Allright, I see you say (?) that Bell's theorem (and QM) contradicts naive reality, but Bell(and QM) assumes CFD, so there must be some difference between CFD and naive reality, but what is that subtle difference ?

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).
CFD I perceive as more general idea that it is meaningful to ask "what if" type questions. And these questions are meaningful even with contextual variables.

 

[atyy]

Allright, I see you say (?) that Bell's theorem (and QM) contradicts naive reality, but Bell(and QM) assumes CFD, so there must be some difference between CFD and naive reality, but what is that subtle difference ?

I don't quite understand this myself and am not sure what counterfactual definite really means nor if everyone means the same thing by it, but according to https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics MWI is not counterfactual definite. I would certainly count MWI as "naive reality" (assuming it works). So that could be a case in which naive reality and CFD differ. I find it easier to say that one assumption of a Bell inequality is that each experiment has only one outcome, whereas MWI assumes otherwise.

 

[harrylin]

[..] Basically in QFT locality is that uncorrelated experiments that are sufficiently separated in space have unrelated results. Note the keyword - uncorrelated. Entangled systems are correlated.

Standard QM is non-local to begin with because its based on the Galilean transformations.

Also in classical physics, uncorrelated experiments that are sufficiently separated in space have unrelated results... and in any case, Galilean transformations have nothing to do with non-locality!

 

[bhobba]

Allright, I see you say (?) that Bell's theorem (and QM) contradicts naive reality, but Bell(and QM) assumes CFD, so there must be some difference between CFD and naive reality, but what is that subtle difference ?

Its not me saying it:
http://drchinese.com/David/Bell_Theorem_Easy_Math.htm

Dr Chinse spells out the two assumptions. Here the assumption that is being discussed is:
I call this assumption "Bell Reality". And... this assumption is the equivalent of assuming that the moon is there when no one looks.

This is also called naive realism. But the technical assumption is counter-factual definiteness. Its the ability to speak meaningfully of observations that haven't been done.
http://physics.stackexchange.com/qu...lism-in-locality-and-counterfactual-definiten

I personally think they are the same - but philosophy types draw a distinction and logically they are correct - but its a bit nit picky in my view.

Thanks
Bill

 

[bhobba]

II would certainly count MWI as "naive reality" (assuming it works).

So would I. It however is not counter-factual definite because you can't speak meaningfully about future measurements.

Thanks
Bill

 

[Nick666]

But if in the Bell experiment counterfactual measurements can't physically be done , why would the experiment get physical predictions or physical results?

Or at least that's what I understand, that counterfactual measurements can't physically be done.

 

[bhobba]

But if in the Bell experiment counterfactual measurements can't physically be done , why would the experiment get physical predictions or physical results?

Couterfactual definiteness doesn't say they can't be done.

Did you go through Dr Chinese's proof?

Thanks
Bill

 

[zonde]

I think that people tend to mix what is sufficient assumption for Bell inequalities and what is necessary assumption.
Sufficient assumption allows proving Bell inequalities but relaxing sufficient assumption does not necessarily invalidate Bell inequalities.
On the other hand relaxing necessary assumptions would necessarily invalidate Bell inequalities.

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.

 

[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