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ThomasT is offline
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.

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(θ30)≠P(θ) is 25%, or to put it another way the probability that P(θ30)=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.