Below is a copy of a answer given to this Phys.SE question asked previously: Does every material thing just consist in forces?

In short, assume that we have two labs A and B, in each one there is a polarizer, and each one of the photons flies toward such a lab. Assume that in the lab A the experimenter orientates the polarizer in some direction, picked by him arbitrarily. Assume that in the lab B the experimenter orientates the polarizer by chance in the same direction. Then, what happens is that in the lab A the photon passes the polarizer, so does the photon in the lab B. But if in the lab A the photon doesn't pass, neither does the photon in the lab B. And the labs are far from one another?

Which force acts between the two photons so as to correlate their actions? If one photon behaved in a certain way, passed or not the polarizer, how does it influence the other photon to behave the same way? We don't know. And take into account that by the time the two photons leave the source, the experimenters in the two places, may not have even decided in which directions to orientate the polarizers.

My questions:

  1. Is this answer an informal description of non-locality?

  2. And if it is, how do we know that (any) force is involved in non-locality?


We must be careful with what we mean by "locality." There are two relevant related, but distinct, concepts:

  1. Einsteinian non-locality: superluminal communication/transfer of information is possible. Einsteinian locality is closely related to causality - an effect must lie in the future light-cone of a cause.

  2. Quantum non-locality: superluminal correlations are possible. Quantum non-locality is closely related to entanglement.

Beware, though, that even in quantum mechanics, e.g. in discussions of Bell's inequalities and local hidden-variable theories, Einsteinian locality is meant by locality. In other words, in most contexts, without a modifier, you should assume that locality refers to Einsteinian locality.

There are other concepts of locality, including historical notions such as action at a distance.

The important take-home point is that quantum mechanics permits superluminal correlations (it is quantum non-local), but forbids superluminal propagation of information (it is Einsteinian local).

The set-up described in your question involves entanglement (it is quantum non-local), but, like all quantum mechanical phenomena and arguably all physical phenomena we know of, it is Einsteinian local. There is no Einsteinian non-locality in the set-up.

Let us now turn to the "force" or mechanism behind quantum non-locality. In common interpretations of quantum mechanics, there is no "force" or mechanism that explains this phenomena, nor is there anything to explain. This is related to the wave-function collapse and interpretations of quantum mechanics - broad topics that I encourage you to read and think about, but which I can't do justice here.

  • $\begingroup$ Not related to the question. Which forces are involved in non-locality. So, if you understand non-locality (which I don't according to the examination that you did to my knowledge) please explain us the forces. $\endgroup$ – Sofia Mar 14 '15 at 22:47
  • $\begingroup$ @innisfree no idea on the down-voting part..anyway back to non-locality...i appreciate your explanation which was 70% known previously to me but I will research more in future to ensure my terminology is correct regards and thanks $\endgroup$ – user74893 Mar 14 '15 at 22:58
  • $\begingroup$ sincere apologies to all' I removed a comment in error can it be restored? $\endgroup$ – user74893 Mar 14 '15 at 23:05
  • $\begingroup$ @innisfree of course the nature is local. Then how is the correlation at a distance explained? Do the particle influence one another at enormous distances? How? $\endgroup$ – Sofia Mar 14 '15 at 23:49
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    $\begingroup$ I think this answer sets up a good answer to the question but doesn't quite state it. The answer, of course, would be that there is not in fact any (Einsteinian) nonlocality involved in the setup. Assuming I understand correctly, of course. $\endgroup$ – David Z Mar 15 '15 at 10:11

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