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The Copenhagen interpretation of QM ultimately amounts to asserting that non-local correlations occur without a cause since that cause would involve propagation of information faster than the speed of light.

I have two questions in regards to this.

A) Why can’t we simply be wrong in thinking that information can’t be propagated faster than the speed of light?

B) Why can’t there be a common cause to these correlations that don’t involve information transfer between these particles in the first place?

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  • $\begingroup$ A) Of course we can be wrong about this, but as far as experiments and experience tell us, this is not the case. B) I don't know what exactly you mean, but it seems you are asking something which is answered in Bell's theorem/Bell's inequalities and related stuff. $\endgroup$
    – Tobias Fünke
    Commented Mar 31 at 18:59
  • $\begingroup$ If you accept Schroedinger's equation, then you must accept that the states of entangled particles evolve in such a way as to remain correlated. Period! No propagation of information required. $\endgroup$
    – D. Halsey
    Commented Apr 1 at 12:57

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There are many ways to think of this type of phenomena. What we know is the following. You can draw your own conclusions from it.

  1. Theoretical fact. If we try to explain these correlations in terms of a common cause described within a hidden variable theory, it cannot be both realistic (all variables are always defined) and local (only subluminal interactions exist). That is true independently of the validity of QM.
  2. Experimental fact. Non local correlations predicted by QM have been experimentally observed.
  3. Theoretical fact. There is no way to transmit information through these correlations either using a single entangled couple, or an ensemble. This was proved assuming QM (and some further few specific hypotheses depending on the type of quantum system).
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  • $\begingroup$ Regarding 3.: Is this really "proved"? For example, in (relativistic) QFT, you assume micro-causality and related things, to get something like no-communication out of it... So well, yes, it is proven, but you have to assume something additionally, not included in the "standard" axioms of QM... and the reason we want to have this and build the theory like that is, well, to "get along" with special relativity. C. Beck's book on local quantum measurements is nice, but also some papers of Asher Peres (I don't remember the exact names, but if needed I can search). $\endgroup$
    – Tobias Fünke
    Commented Mar 31 at 19:01
  • $\begingroup$ In QM (not QFT) it is sufficient to assume that composite systems are described via tensor product. There is a direct proof. Regarding QFT the situation is much more complicated. I fo not know Becks 'book, but I have his phd thesis. I will have a look at it $\endgroup$
    – Valter Moretti
    Commented Mar 31 at 19:33
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    $\begingroup$ Well, yes and no. You also have to assume a special structure of the time-evolution, i.e. that the time-evolution operator factors... See also arxiv.org/abs/quant-ph/9906036 and arxiv.org/pdf/2309.07715.pdf $\endgroup$
    – Tobias Fünke
    Commented Mar 31 at 19:35
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    $\begingroup$ Thanks Tobias, I will read it. $\endgroup$
    – Valter Moretti
    Commented Mar 31 at 19:36
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A) Why can’t we simply be wrong in thinking that information can’t be propagated faster than the speed of light?

Bacause in special relativity, faster-than-light in one reference frame is backwards-in-time in another frame moving relative to it. In one frame, the first measurement occurs first and causes the outcome of the second. In the other frame, causality is reversed. We wind up with circular causation, each 'causing' the other. That sort of thing makes a nonsense of most of our reasoning about cause and effect.

B) Why can’t there be a common cause to these correlations that don’t involve information transfer between these particles in the first place?

It depends what you mean by "can be" - with or without the Copenhagen interpretation?

Wavefunction collapse in the Copenhagen interpretation is not explained - there is no mechanism described, we don't know how it propagates through space, we can't perform any experiment to detect if or when it happens. We can only speculate.

We have to abandon linearity, realism, locality, determinism to maintain our belief in wavefunction collapse - if we're going to take backwards-in-time causation seriously, then why not causation-without-communication? If you set no limits on possible explanations, no basic principles of "reasonable" behaviour you want your theory to respect, (or abandon the need for explanations entirely,) then we can't rule anything in or out.

But there are other interpretations (e.g. Everett interpretation) that explain the correlations without wavefunction collapse, or faster-than-light, backwards-in-time propagation, that are linear, deterministic, local, and realist. So yes there can be a cause to the correlations that doesn't involve long-range information transfer, but you asked your question specifically in the context of the Copenhagen interpretation, so maybe those explanations are ruled out of scope?

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    $\begingroup$ But the problems with FTL communication do only hold if we accept SR to start with, no? $\endgroup$
    – Tobias Fünke
    Commented Apr 1 at 7:31
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Just to add to the already good answers:

Entanglement is always created locally, at one point. From there the entangled particles can then propagate to different locations. So in that sense, there is always a common cause for entanglement, but it does not involve faster-than-light communication.

The notion of faster-than-light communication is a bigger issue that has been addressed within a broader context in physics. The current consensus is that faster-than-light doesn't exist, which is consistent with everything we observe including measurements on entangled particles.

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