Talk:Bell's theorem

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What is called correlations[edit]

To Jochen Burghardt and Arthur Rubin: isn't it better to first discuss the matter here rather than edit forth and back? Boris Tsirelson (talk) 07:19, 7 January 2015 (UTC)

Probably you're right - I didn't expect the issue to be that much complicated. Also, I can't help much in answering questions, as I'm not a physicist, but just a mathematician who wants to understand the article. Arthur Rubin is right in that the caption of File:Bell's theorem.svg explicitly mentions "spin-half". On the other hand, section Bell's_theorem#CHSH inequality speaks about "binary (+/-1 valued) outcomes". Maybe, the "outcome" is a normalized (i.e. scaled by 2 in our case) version of the measured "spin"? As another suggestion, the easiest way to obtain consistency might be to stick with the statistical notion of correlation, unless this is absolutely unusual in quantum physics. - Jochen Burghardt (talk) 18:15, 7 January 2015 (UTC)
I do not think it is complicated (and I am a kind of expert in it). Artur Rubin is right if the spin is treated as mechanical (the angular momentum); in this sense it is really neither 1 nor 1/2 but (plus-minus) a half of the Planck constant. But! This mechanics is rather irrelevant. Here the spin is treated informationally, as just a yes-no observable, encoded (for convenience) as plus-minus 1. (And by the way, the Stern–Gerlach experiment gives just this: splits the electron beam in two beams, without indicating "plus-minus how much" is it, really.)
About the statistical notion of correlation: the quantum calculation in the singlet state shows that the average spins (the expectations) are zero, and therefore there is no conflict between the two "correlations". However, when discussing Bell inequalities, it is usual indeed to call "correlations" the expected product in every case, whether or not the expectations are zero. Boris Tsirelson (talk) 19:14, 7 January 2015 (UTC)

Psychological Aspects[edit]

This article makes no mention of the psychology of the observers. Such factors as memory, subjectivity, and interpretation influence the results that each observer perceives and what they can agree on. Even without a many-worlds interpretation, each observer only perceives part of the entire reality. Which part they perceive affects the correlation. — Preceding unsigned comment added by 153.203.90.130 (talk) 20:05, 23 March 2015 (UTC)

I hope it depends strictly on the mathematics of tensor products and projection operators, and not on literary constructions. Otherwise I have no chance of understanding this gadget ! 178.38.79.96 (talk) 07:44, 12 April 2015 (UTC)

Section "Two classes of Bell inequalities" is too technical[edit]

This section departs from the previous sections by suddenly being full of unexplained technical terms such as "fair sampling", "inhomogeneous", "homogeneous", "dark rate", "dead time", "resolving times".

Nobody needed to perform the experiment, because singles rates with all detectors in the 1970s were at least ten times all the coincidence rates.

Does this mean, performing the experiment would have been futile or inconclusive? The sentence uses irony where declaration would be clearer. I would correct it myself, but I can't be sure of my interpretation.

(I also don't understand why this factor of ten even matters or what it means here, but this is only one of many things that are unclear in this section.)

178.38.79.96 (talk) 07:34, 12 April 2015 (UTC)

Unclear on the intuition of some text[edit]

The article reads,

"Suppose the two particles are perfectly anti-correlated—in the sense that whenever both measured in the same direction, one gets identically opposite outcomes, when both measured in opposite directions they always give the same outcome. The only way to imagine how this works is that both particles leave their common source with, somehow, the outcomes they will deliver when measured in any possible direction. (How else could particle 1 know how to deliver the same answer as particle 2 when measured in the same direction? They don't know in advance how they are going to be measured...)."

The above is an unsourced (perhaps OR) intuitive argument the intuition of which dose not jibe with my own understanding.

The particles in question are photons which are observed. Thus, they are photons that have been both emitted and absorbed. When a photon is emitted and absorbed, it has traveled at the speed of light and, from the photon's perspective, the emission and absorption have happened simultaneously, coupling and conserving mass-energy and spin angular momentum from the emitter to the absorber.

Similarly when a pair of entangled photons is emitted and absorbed, the emission event, and the two absorption events happen simultaneously, again from the photons' perspective, and again conserving spin angular momentum. So the photon does not have to "know" what these angular momenta are while in transit, as from the photon's perspective, the entire coupling event has happened in a single instant, is a single event and there was no concept of being "in transit".

The fact that the two distantly separated absorbers are measured as not being co-located, by some observer, who is neither of the photons, is explained because that's how special relativity works. In different frames you measure different distances and times. The coupling must make sense from the frame of the force-coupling carrier(s), but not from anybody else's frame. So there is no FTL communications paradox, at least not in this example involving photons in a vacuum.

Spope3 (talk) 06:03, 7 May 2015 (UTC)

Maybe. But the effect is the same on electrons and even heavy ions, that are far not massless. That "the emission and absorption have happened simultaneously" is itself not a well-established point of view. Moreover, bare photons are massless but do not exist in reality. Dressed photons are not quite massless because electron-positron virtual pairs matter. Boris Tsirelson (talk) 07:16, 7 May 2015 (UTC)

== (Reply to Boris re. "unclear intuition")

I have two points here:

1) Although the effect "is the same" for electrons and heavy ions, the article makes it clear that the most serious experimental results come from studying the behavior of photons. So it is at this point speculative to say that similar results hold for electrons (although I personally believe they do).

2) It is true that all real-world photons behave as dressed photons. But I'm not sure this means undressed photons don't exist, to me it means that a perfect vacuum does not exist, at least in our section of the universe/multiverse. So a photon might scatter off particles other than its Bell-experiment target (including virtual particles, and this effect can be translated into a mass expression for a dressed photon, but if this happens the Bell-experiment outcome is different whether or not one leans towards my point of view or the article's statements. — Preceding unsigned comment added by Spope3 (talkcontribs) 02:02, 8 May 2015 (UTC)