why is superdeterminism not the universally accepted explanation of nonlocality?


by jadrian
Tags: accepted, explanation, nonlocality, superdeterminism, universally
zonde
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#253
Mar4-12, 01:16 AM
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Quote Quote by lugita15 View Post
I agree that this is the point of contention, but keep in mind that he thinks a local realist can believe in the nonlinear correlation given by Malus' law, while at the same time also believing that there is perfect correlation at identical settings. I hope you agree that he's wrong on this point.
Malus' law does not describe correlations between two photons but intensity change for single beam of light.
Otherwise yes, I do agree that this doesn't work.

Quote Quote by lugita15 View Post
Out of curiosity, which experimental loophole of Bell tests do you cling onto? Detector efficiency, communication, freedom of choice, or something else?
It's "fair sampling assumption not holding for photons" loophole.
lugita15
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Mar4-12, 09:51 AM
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Quote Quote by zonde View Post
Malus' law does not describe correlations between two photons but intensity change for single beam of light.
Yes, I was just using the term in a generalized sense to refer to sinusoidal θ-dependence.
Otherwise yes, I do agree that this doesn't work.
I hope we can convince ThomasT of that.
It's "fair sampling assumption not holding for photons" loophole.
But surely, as technology improves, it should be fairly easy to send photons one at a time, have every pair be entangled, and have every pair be collected by a photon detector, so no sampling issue will arise. Also, haven't entanglement experiments been done on all kinds of things, including qubits in the context of quantum computing, so don't you need a more general objection to Bell's theorem than just photons?
DrChinese
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Mar4-12, 10:45 PM
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Quote Quote by lugita15 View Post
But surely, as technology improves, it should be fairly easy to send photons one at a time, have every pair be entangled, and have every pair be collected by a photon detector, so no sampling issue will arise. Also, haven't entanglement experiments been done on all kinds of things, including qubits in the context of quantum computing, so don't you need a more general objection to Bell's theorem than just photons?
You are correct, and you probably already know this, but most consider this to be a disproof of the fair sampling assumption:

http://www.nature.com/nature/journal.../409791a0.html

For reasons I do not fully understand, most local realists simply reject this by pointing out that the locality loophole (closed by Weihs et at in 1998) is not closed simultanoeously.
lugita15
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Mar4-12, 11:29 PM
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Quote Quote by DrChinese View Post
You are correct, and you probably already know this, but most consider this to be a disproof of the fair sampling assumption:

http://www.nature.com/nature/journal.../409791a0.html

For reasons I do not fully understand, most local realists simply reject this by pointing out that the locality loophole (closed by Weihs et at in 1998) is not closed simultanoeously.
I brought this up with zonde in another thread, and his response was somewhat strange:
Quote Quote by zonde View Post
Oh, but I said that it does not hold in photon experiments.
Or do you want to argue that we can apply one to one results of ion experiment to photon experiment?
Quote Quote by zonde View Post
Basically you have to assume that Bell inequality violations appear due to the same (unknown) physical mechanism in ion experiments and photon experiments only then it means something. Obviously it is much more preferable to avoid such assumptions.
Apparently, he thinks that different kinds of particles exploit different loopholes to Bell's theorem! He believes that ions exploit the communication loophole, while photons exploit the fair sampling loophole.
Demystifier
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Mar5-12, 03:49 AM
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Quote Quote by lugita15 View Post
Demystifier, can you answer another question about Bohmian mechanics? ... If everything in the universe is interacting nonlocally with everything else through their pilot waves, then why is it that we only observe the nonlocal correlation caused by this nonlocal interaction when we do measurements of entangled particles? Is there something special about entanglement that reveals the nonlocal interactions that are always present?
In Bohmian mechanics, it is not true that everything in the universe is interacting with everything else. Instead, in Bohmian mechanics a particle interacts with another particle through a quantum potential ONLY when there is entanglement.
lugita15
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Mar5-12, 08:24 AM
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Quote Quote by Demystifier View Post
In Bohmian mechanics, it is not true that everything in the universe is interacting with everything else. Instead, in Bohmian mechanics a particle interacts with another particle through a quantum potential ONLY when there is entanglement.
First of all, I thought the nonlocal stuff like entanglement was handled through the pilot wave, not the quantum potential. I thought before the particle gets to any place, the pilot wave has already gone faster than the speed of light to that location, collected information about it, and has given that info to the particle. So if there is a double slit experiment coming up ahead, the pilot wave goes through the double slit, and depending on what detectors the apparatus has it tells the particle what trajectory to travel through. Do I have that roughly right? If so, doesn't this constitute nonlocal interaction between the particles and distant objects like the double slit apparatus which seemingly have nothing to do with it?

Also, what exactly is the Bohmian view of entanglement?
Demystifier
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Mar5-12, 09:12 AM
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Quote Quote by lugita15 View Post
First of all, I thought the nonlocal stuff like entanglement was handled through the pilot wave, not the quantum potential.
The quantum potential is a quantity uniquely determined by the pilot wave. So anything handled by the quantum potential is handled also by the pilot wave.

Quote Quote by lugita15 View Post
I thought before the particle gets to any place, the pilot wave has already gone faster than the speed of light to that location, collected information about it, and has given that info to the particle.
The pilot wave does not travel faster than the speed of light.

Quote Quote by lugita15 View Post
Also, what exactly is the Bohmian view of entanglement?
Just as in standard QM, the wave function (which is the same thing as pilot wave) of many particles is entangled when this wave function cannot be written as a product of wave functions of single particles.
lugita15
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Mar5-12, 10:02 AM
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Demystifer, if pilot waves don't go faster than light, then what explains the nonlocality of entanglement? Does the quantum potential propagate faster than light?

Also, am I wrong in my impression that a particle's trajectory right now is determined in part by the apparatuses it knows, based on nonlocal interaction, that it's going to encounter later?
ThomasT
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Mar5-12, 11:48 AM
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Quote Quote by lugita15 View Post
OK, and I think one thing that leads to misunderstanding is a terminology issue. You're using local determinism to refer to a philosophical stance, while you're using local realism to refer to a particular formal model which tries to implement this philosophical stance.
Yes, I think it's a good idea to keep the technical physics meaning of local realism separate from the philosophical meaning of local determinism.

Quote Quote by lugita15 View Post
I'm using both local realism and local determinism, pretty much interchangably, to refer to the philosophical stance, not to any formal model or formal constraint. So just keep that in mind when reading my posts.
I'll keep that in mind wrt your posts. But I think it would be a good idea to separate the two.

Quote Quote by lugita15 View Post
I am trying to prove that ANY believer in local determinism MUST disagree with some of the predictions of QM.
Ok, it's clear to me now that that's what you're trying to prove.

Quote Quote by lugita15 View Post
The reason that quantum mechanics is able to have both perfect correlation at identical angles and nonlinear correlations as a function of angle is that QM does not say that the decision about whether the photon goes through the polarizer or not is predetermined by a function P(θ).
Bell showed that the view that individual detection is determined by some (LR) function guiding photon behavior is compatible with QM. A LR model of individual detection isn't a problem, and isn't ruled out. It's trying to model coincidental detection in terms of the function that determines individual detection that's a problem, and is ruled out.

The crux of why I think one can be a local determinist while still believing that Bell-type LR models of quantum entanglement are ruled out is because of the assumption that what determines individual detection is not the same underlying parameter as what determines coincidental detection.

The assumption regarding individual detection is that it's determined by the value of some locally produced (eg., via common emission source) property (eg., the electrical vector) of the photon incident on the polarizing filter. It's further assumed that this is varying randomly from entangled pair to entangled pair. So, there is a 50% reduction in detection rate at each of the individual detectors with the polarizers in place (compared to no polarizers), and a random accumulation of detections. (Wrt individual detection, LR and QM predictions are the same).

The assumption regarding coincidental detection is that, wrt each entangled pair, what is being measured by the joint polarizer settings is the locally produced (eg., via common emission source) relationship between the polarizer-incident photons of a pair.

Because A and B always record identical results, (1,1) or (0,0) wrt a given coincidence interval when the polarizers are aligned, and because the rate of coincidental detection varies predictably (as cos2θ in the ideal), then it's assumed that the underlying parameter (the locally produced relationship between the photons of a pair) determining coincidental detection isn't varying from pair to pair. It might be further assumed that the the value of the relevant property is the same for each photon of a given pair (ie., that the separated polarizers are measuring exactly the same value of the same property wrt any given pair). But that value only matters wrt individual detection, not wrt coincidental detection.

And here's the problem. The LR program requires that coincidental detection be modeled in terms of the underlying parameter that determines individual detection. But how can it do that if the underlying parameter that determines coincidental detection is different than the underlying parameter that determines individual detection?

There have been attempts to model entanglement this way (ie., in terms of an unchanging underlying parameter that doesn't vary from entangled pair to entangled pair), but they've rejected as being non-Bell-type LR models.

Regarding your 12 step LR reasoning (reproduced below), the problem begins in trying to understand coincidental detection in terms of step 2.

I hope the above makes it clearer why I think that one can believe that the LR program (regarding the modelling of quantum entanglement) is kaput, while still believing that the best working assumptions are that our universe is evolving locally deterministically. And so, no need for superdeterministic theories of quantum entanglement.

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Quote Quote by lugita15
1. If you have an unpolarized photon, and you put it through a detector, it will have a 50-50 chance of going through, regardless of the angle it's oriented at.

2. A local realist would say that the photon doesn't just randomly go through or not go through the detector oriented at an angle θ; he would say that each unpolarized photon has its own function P(θ) which is guiding it's behavior: it goes through if P(θ)=1 and it doesn't go through it P(θ)=0.

3. Unfortunately, for any given unpolarized photon we can only find out one value of P(θ), because after we send it through a detector and it successfully goes through, it will now be polarized in the direction of the detector and it will "forget" the function P(θ).

4. If you have a pair of entangled photons and you put one of them through a detector, it will have a 50-50 chance of going through, regardless of the angle it's oriented at, just like an unpolarized photon.

5. Just as above, the local realist would say that the photon is acting according to some function P(θ) which tells it what to do.

6. If you have a pair of entangled photons and you put both of them through detectors that are turned to the same angle, then they will either both go through or both not go through.

7. Since the local realist does not believe that the two photons can coordinate their behavior by communicating instantaneously, he concludes the reason they're doing the same thing at the same angle is that they're both using the same function P(θ).

8. He is in a better position than he was before, because now he can find out the values of the function P(θ) at two different angles, by putting one photon through one angle and the other photon through a different angle.

9. If the entangled photons are put through detectors 30° apart, they have 25% chance of not matching.

10. The local realist concludes that for any angle θ, the probability that P(θ±30°)≠P(θ) is 25%, or to put it another way the probability that P(θ±30°)=P(θ) is 75%.

11. So 75% of the time, P(-30)=P(0), and 75% of the time P(0)=P(30), so there's no way that P(-30)≠P(30) 75% of the time.

12. Yet when the entangled photons are put through detector 60°, they have a 75% chance of not matching, so the local realist is very confused.
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zonde
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#262
Mar5-12, 12:27 PM
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Quote Quote by lugita15 View Post
But surely, as technology improves, it should be fairly easy to send photons one at a time, have every pair be entangled, and have every pair be collected by a photon detector, so no sampling issue will arise.
Maybe not easy but certainly feasible. And yet there are no reports about experiments with improved pair detection efficiency.

Quote Quote by lugita15 View Post
Also, haven't entanglement experiments been done on all kinds of things, including qubits in the context of quantum computing, so don't you need a more general objection to Bell's theorem than just photons?
I don't have any objections to Bell's theorem.
Speaking about experiments, different experiments can have different loopholes or different sources of systematic errors.

But photon tests are way ahead of other entanglement experiments in terms of attention they have got, analysis made and different modifications of similar experiments preformed. So I would like to stick to photon experiments.
Joncon
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#263
Mar5-12, 05:31 PM
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Quote Quote by ThomasT View Post
But how can it do that if the underlying parameter that determines coincidental detection is different than the underlying parameter that determines coincidental detection?
ThomasT, please can you clarify what you mean here? I'm guessing you just typed this wrong and that one "coincidental" should have read "individual".

If so, how could a coincidental parameter/function etc. possibly be different from an individual one?
ThomasT
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Mar5-12, 05:58 PM
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Quote Quote by Joncon View Post
ThomasT, please can you clarify what you mean here? I'm guessing you just typed this wrong and that one "coincidental" should have read "individual".
Thanks, I should have proof read what I wrote. I just corrected it.

Quote Quote by Joncon View Post
If so, how could a coincidental parameter/function etc. possibly be different from an individual one?
The underlying parameter or function that determines individual detection is assumed to be, for any given entangled pair, some value of some property. This value is assumed to vary, randomly, from pair to pair, because the rate of individual detection doesn't vary as a function of polarizer setting.

The underlying parameter or function that determines coincidental detection is assumed to be the relationship between those values. This relationship is assumed to not vary from pair to pair, because the rate of coincidental detection varies, predictably, as a function of the angular difference between the joint polarizer settings.
lugita15
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Mar5-12, 06:05 PM
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ThomasT, I just don't understand your point. If you are a (nonsuperdeterministic) local determinist, and you find that entangled photons measured at polarizers oriented at the same angle behave identically, you can have only one possible response: "The photons are not coordinating their behavior through faster-than-light communication. Rather they are each deciding to go through or not go the polarizer based on a common function P(θ), which equals 1 if the photon is supposed to go through and 0 if not." If you do not agree with this response, how can you consider yourself a local determinist?
lugita15
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Mar5-12, 06:20 PM
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Quote Quote by ThomasT View Post
Thanks, I should have proof read what I wrote. I just corrected it.

The underlying parameter or function that determines individual detection is assumed to be, for any given entangled pair, some value of some property. This value is assumed to vary, randomly, from pair to pair, because the rate of individual detection doesn't vary as a function of polarizer setting.

The underlying parameter or function that determines coincidental detection is assumed to be the relationship between those values. This relationship is assumed to not vary from pair to pair, because the rate of coincidental detection varies, predictably, as a function of the angular difference between the joint polarizer settings.
But a "coincidental detection" is not some magical action. It is nothing more than performing "individual detections" on each of the two particles. So definitionally, what determines the result of a coincidental detection is just what determines the results of individual detections.
ThomasT
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Mar5-12, 06:30 PM
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Quote Quote by lugita15 View Post
ThomasT, I just don't understand your point. If you are a (nonsuperdeterministic) local determinist, and you find that entangled photons measured at polarizers oriented at the same angle behave identically, you can have only one possible response: "The photons are not coordinating their behavior through faster-than-light communication.
Right, but that's just one part of why I'm a (nonsuperdeterministic) local determinist who thinks the mainstream LR program was effectively ruled out by Bell almost 50 years ago.

Quote Quote by lugita15 View Post
Rather they are each deciding to go through or not go the polarizer, through based on a common function P(θ), which equals 1 if the photon is supposed to go through and 0 if not."
Or rather, because entangled photons measured at polarizers oriented at the same angle behave identically, and also because rate of joint detection varies as θ varies, then it's assumed that the underlying parameter that's determining joint detection isn't varying from pair to pair. And because individual detection doesn't vary as the polarizer setting varies, then it's assumed that the underlying parameter that's determining individual detection is varying from pair to pair. Hence, the assumption that there is a different underlying parameter or function determining coincidental detection and individual detection. But the LR program requires that coincidental detection be modelled in terms of the same underlying parameter or function that's determining individual detection.
ThomasT
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#268
Mar5-12, 06:34 PM
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Quote Quote by lugita15 View Post
But a "coincidental detection" is not some magical action. It is nothing more than performing "individual detections" on each of the two particles. So definitionally, what determines the result of a coincidental detection is just what determines the results of individual detections.
The rate of individual detection doesn't vary with the measurement parameter, but the rate of coincidental detection does vary with the measurement parameter. So, what would you infer from this?
Joncon
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Mar5-12, 06:49 PM
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Quote Quote by ThomasT View Post
Or rather, because entangled photons measured at polarizers oriented at the same angle behave identically, and also because rate of joint detection varies as θ varies, then it's assumed that the underlying parameter that's determining joint detection isn't varying from pair to pair. And because individual detection doesn't vary as the polarizer setting varies, then it's assumed that the underlying parameter that's determining individual detection is varying from pair to pair. Hence, the assumption that there is a different underlying parameter or function determining coincidental detection and individual detection. But the LR program requires that coincidental detection be modelled in terms of the same underlying parameter or function that's determining individual detection.
The point is that in an LR theory, each photon is using one and only one function. Each photon is acting individually, totally unaware of what is happening with it's entangled partner. So you either accept that the individual and coincidental functions are the same thing, or you make them different. If so, then the photons "choose" which function to use based on how they will be measured - and then you're back to superdeterminism.
ThomasT
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Mar5-12, 07:00 PM
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Quote Quote by Joncon View Post
The point is that in an LR theory, each photon is using one and only one function. Each photon is acting individually, totally unaware of what is happening with it's entangled partner. So you either accept that the individual and coincidental functions are the same thing, or you make them different.
Ok, I believe they're different.

Quote Quote by Joncon View Post
If so, then the photons "choose" which function to use based on how they will be measured - and then you're back to superdeterminism.
If you could phrase this a bit less anthropically, that would be helpful. Photons aren't people.

We're talking about different measurement parameters. Is it unreasonable to suppose that these different measurement parameters are measuring different underlying parameters?


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