Is the cat alive, dead, both or unknown

[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