8
$\begingroup$

I understand that this is a much debated issue, so I will try to be precise in order to narrow the question. Bell inequality violation rules out Local Realism. From this, I understand that by giving up just one of the two conditions (Locality or Realism) one could generate a viable model. My questions:

  1. Could we provide a local and non realistic model that reproduces the observed correlations? (in particular I do not see how a local theory can reproduce correlations outside of the light cone)
  2. Could we provide a non local and realistic model that reproduces the observed correlations?
  3. What about QM? I this (non) local and/or (non) realistic? Is this just a matter of interpretation?

Claims on these points in the literature are often unclear and (to me) confusing (see for example http://www.physics.drexel.edu/~bob/Entanglement/aspect_nature446.pdf) so I would like to see explicit model for my points 1. and 2.

Improve this question
$\endgroup$
8
  • $\begingroup$ What exactly do you mean by "local" and "non-local"? Note that it is possible for a theory to be non-local (in the sense that it produces correlations between spacelike-separated events that break the CHSH bound) while also non-signalling (i.e. no action of Alice can affect local measurements of Bob at a spacelike-separated event). This category includes quantum theory, but also stranger beasts like Popescu-Rohrlich boxes. Theories with signalling are not that interesting (as they break causality if you allow relativistic motion) but there's a long way from non-signalling to local. $\endgroup$
    – Emilio Pisanty
    Commented Apr 10, 2015 at 13:50
  • $\begingroup$ thx emilio, trying to translate it to my own language you are saying that: QM is non-local (i.e. out of light cone correlated) and non-realistic (i.e. no "element of reality" thx to Bell inequality violation) what I try to understand is whether we could interpret QM by breaking just one of the conditions. $\endgroup$
    – Arnaldo Maccarone
    Commented Apr 10, 2015 at 14:03
  • 1
    $\begingroup$ The short answer is yes: Bell violations imply that if a model reproduces QM it cannot be both realistic and local; in other words, you can get away with only breaking one of them. The detailed answer, however, depends on what you mean by "(non-)local" and "realistic". Specifically: do you consider a non-signalling realistic theory which violates CHSH to be nonlocal? $\endgroup$
    – Emilio Pisanty
    Commented Apr 10, 2015 at 14:57
  • $\begingroup$ Let's try with these definition: 1. Locality: different measures at space-like separations are uncorrelated 2. Reality: (the einstein one) if the outcome of a measure can be predicted with 100% confidence than it is a real property of the system It seems that the answer to your last question in my definitions is yes, isn't it. $\endgroup$
    – Arnaldo Maccarone
    Commented Apr 10, 2015 at 15:26
  • $\begingroup$ That definition of locality is much too strict - you can easily beat it within classical physics. Say Charlie takes a pair of gloves, puts them in different boxes, sends you and me the boxes at random, and we open them simultaneously. If I have a right-hand glove, I can immediately conclude that yours is a left-hand one - there's a perfect (anti)correlation between our spacelike-separated measurements. There's no mystery there! The problem is if our measurements of the boxes start to beat CHSH bounds on their correlations, which the sock model cannot do. $\endgroup$
    – Emilio Pisanty
    Commented Apr 10, 2015 at 15:38

2 Answers 2

Reset to default
1
$\begingroup$

1.) You have to give up not only Einstein-locality (locality itself would be unproblematic), but also causality, because there are variants of the proof which rely on causality only, with Reichenbach's principle of common cause as the base. I do not think theories which reject causality and realism make sense for a scientist, who wants to study the real world and find causal explanations for unexplained correlations. But, in principle, one could use the minimal interpretation of QT for this.

2.)+3.) There are realistic interpretations of QT, namely the de Broglie-Bohm interpretation.

Improve this answer
$\endgroup$
1
$\begingroup$

There are a number of local theories that violate Bell's inequality. This is partially because there are lots of hidden assumptions in Bell's theory which are well described by Wiseman.

The best example I know of is a Event-based simulation of quantum physics experiments where they show that a simple classical event-based simulation can reproduce Bell-CHSH inequalities. It can also be done with negative probabilities. Neither of these are a complete theory, however.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.