Quantum entanglement information

[vanhees71]

No, Bell proved that if you want to reproduce the probabilistic results of QT with a deterministic hidden-variable model it must necessarily be nonlocal. Relativistic QFT is local but not deterministic!

 

[Mathematech]

Read Bell, he makes statements like "quantum correlations are not locally explicable". He was not aware that a local theory could be anything other than a hidden variable model where one can integrate willy nilly.

 

[Simon Phoenix]

Bell quite explicitly pushed the idea that non-locality is required to explain violation of his inequalities

Not really - Bell said that if we wish to describe nature using variables that have certain seemingly a priori reasonable properties then in a certain experimentally achievable set up these rather general properties imply that the measureable correlation functions are bounded (that is, Bell's inequality, or its more complete version the CHSH inequality)..

QM predicts that this bound can be violated. Therefore QM cannot be described, or replaced by a theory based on these variables possessing those seemingly reasonable properties.

It then becomes a matter of experiment whether the QM prediction holds - if it does, if the inequality is violated, then we have unequivocal proof that nature cannot be described by ANY theory built on variables possessing the reasonable properties our intuition might have demanded.

Colloquially, those reasonable properties are 'realism' and 'locality' - but they have more precisely defined meanings.

So we can sacrifice 'realism' or 'locality' (or both perhaps) and construct a theory that agrees with the QM predictions. Bell did like the 'realism' of the Bohm approach to QM, so in this sense he 'pushed' this interpretation being well aware that (a) it is only one interpretation of QM and (b) it is just a smidgeon on the non-local side of things

But he was also well aware that non-locality is ONLY required if we wish to retain a description of nature using 'realistic' hidden variables. QM tootles along quite merrily without hidden variables - they're not in the slightest bit necessary.

while at the same time being unaware that there could be anything wrong with his hand wavy integration over lambda. I have been reading his papers and to me its mind blowing how non-rigorous and flawed his arguments are regarding probability.

I'm quite prepared to accept that to a mathematician Bell's arguments may well look hopelessly non-rigorous and 'hand-wavy'. Thing is, though, they lead to the correct inequality for the correlation functions - which is rather the essential point, don't you think?

 

[Mathematech]

Thing is, though, they lead to the correct inequality for the correlation functions - which is rather the essential point, don't you think?

No they don't! That's the whole point of it. QM leads to the correct inequality, his non-rigorous hand wavy stuff does not. Yet he uses the fact that it does not to claim that non-locality is necessary.

 

[Simon Phoenix]

QM leads to the correct inequality

I think you may be missing the point. You're probably best looking at the various derivations of the CHSH inequality (the original Bell inequality is a special case of this). I'm sure you'll find one that satisfies your requirement for rigour. The key thing is that these are all classical in the sense that absolutely no notion from QM is required in the derivation - the CHSH inequality is a bound on classical correlation functions. If we hadn't discovered QM yet we could still derive the CHSH inequality.

But I'm guessing you're one of those few brave souls who believe the CHSH inequality is mathematically incorrect anyway. Good luck with that

 

[Mathematech]

Proofs of CHSH use both counterfactual definiteness and locality. As with Bell's theorem the implication is thus again that either counterfactual definiteness is wrong, or locality is wrong or both, so what point are you trying to make?

And let me add that as with Bell, you will find people claiming that it proves locality must be wrong. You see this with Eberhard for example where despite making it explicit that he is appealing to counterfactual definiteness, he does not even consider the possibility that it might be wrong and considers CHSH to be a proof of non-locality.

 

[Simon Phoenix]

so what point are you trying to make?

I'm not the one claiming that the Bell/CHSH inequality is wrong

Counterfactual definiteness is a cornerstone of classical physics - in a nutshell it is the claim that objects have properties independent of measurement. Locality is another very reasonable assumption that says that things done 'there' should not affect results 'here' - we can add in a limit on any possible interaction between 'here' and 'there' by appeal to relativity, but the essential principle as far as the experiments are concerned is that the result I obtain in my lab should not depend on the setting chosen in your lab.

These are both eminently reasonable classical assumptions - and from these one can derive the CHSH inequality (with the further assumption that it is possible to make independent random choices for settings). No QM required.

You're the one claiming that there's something mathematically wrong with the derivation of the classical bound on the correlation functions - but so far you haven't explained where the alleged error is. So the point I'm making, if any, is really to try to tease out from you what on Earth you're talking about.

 

[Mathematech]

I'm not the one claiming that the Bell/CHSH inequality is wrong

Experimental evidence is what claims that they are wrong. The experimental data does not obey Bell's inequality. The experimental data does not obey CHSH. Moreover the experimental data is not the suspect here, it is the assumptions in the proofs of BI and CHSH. One possibility is that the assumption of locality is wrong and the point I am making is that this is what gets pushed as being the only possible thing that is wrong by popular science articles and this is a problem going back to papers by Bell, Eberhard etc who do not consider or are not even aware of the other possibilities.

Counterfactual definiteness is not equivalent to simply claiming particles have properties independent of measurement, there is more to it, but the point of particular importance is the idea that combining counterfactual results with factual results gives the same statistics. It is a mathematical fact that when the counterfactual results are possible alternate results that were not obtained due to an incompatible experiment being performed instead, then when more than 2 variables are involved, the statistics need not be the same as a scenario where the counterfactual results were not obtained simply because no one bothered.

Similarly a "hidden variable theory" is not merely a model where there are hidden parameters present that uniquely determine outcomes, the crucial point is that it is one where one can perform the integrations or averaging required in the proofs of BI and CHSH.

 

[Nugatory]

and the point I am making is that this is what gets pushed as being the only possible thing that is wrong by popular science articles

Yes, but, fixing the world so that popular science articles do not misrepresent QM is beyond the charter of Physics Forums. We do not have the power to accomplish such miraculous results, and if we did we would first take on easier problems like world hunger, global peace, a cure for cancer, and the like.

This thread has moved far beyond its original scope, and it's time to close it.