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Dear Physics Stack Exchange community,

I am intrigued by the concept of quantum entanglement and its implications for our understanding of the fundamental nature of reality. I have come across various explanations regarding the phenomenon of non-locality in quantum systems, where entangled particles appear to instantaneously influence each other's properties regardless of the distance between them. However, I am seeking further clarification on whether this non-local behavior is an inherent aspect of quantum mechanics or if it poses a violation of causality.

I would greatly appreciate if someone could shed light on the following questions:

1. Does quantum entanglement necessarily imply non-locality, or are there alternative interpretations that can explain entangled particle correlations without invoking non-local influences?

2. Can non-locality in quantum systems be reconciled with the principle of causality? Are there any theoretical frameworks or experimental evidence suggesting that non-locality does not violate causality?

3. What are the prevailing theories or models that attempt to explain the mechanism behind non-locality in quantum systems? Are there any ongoing research efforts aimed at further understanding this phenomenon?

I have a solid background in quantum mechanics and would welcome any insights, references to relevant research papers, or further reading materials on this topic. Thank you in advance for your expertise and guidance.

Raihan Sarker
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It's hard to answer your first two questions without knowing precisely what definition of "locality" and "causality" you're assuming. But even without this detail, I think your key question is 3, which might be paraphrased as: 'something weird is going on; what are the main ways to explain it?'

Here are the main categories:

  1. Bohmian-style explanations: Physical things exist in space and time, and can all interact with the same non-local wavefunction which does not exist in ordinary space. Information can be mediated from one thing to another through this wavefunction, and there must be some special reference frame where that mediation is instantaneous.

  2. ER=EPR style explanations: Physical things exist in space and time, but space and time is littered with wormholes connecting different regions, wherever entangled particles exist. Information can be mediated through these wormholes.

  3. Retrocausal explanations: Physical things exist in space and time, and are connected through their worldlines, making a physical network in spacetime. Information can be mediated from one event to another through their past and/or future worldlines, following the paths where they intersect. This viewpoint is hard to make sense of without both a block-universe view of spacetime and a more subjective/traditional view of information (so that the information isn't literally "moving" through spacetime).

And finally, there's dodging the need for an explanation via either:

  1. No Reality / Many-Worlds Viewpoint: There's nothing actually existing anywhere in space-time, everything real resides in a Hilbert or Fock space. Worrying about something "over here" affecting something "over there" is therefore not a worry; there's no conventional sense of "here" or "there" in a huge-dimensional Hilbert space. (So called "Copenhagen" accounts tend to shift back and forth between this and something closer to #1.)

or

  1. Superdeterminism: The universe was set up such that what we see locally and causality follows from that setup; this explains everything except for the setup itself. There is no need to in turn explain how it was set up in such a special way; that's just how it was.
Ken Wharton
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To understand the concept of entanglement, it may be helpful to look at the history of the development of the concept. J.S. Bell used two simple assumptions to derive what is now called Bell's inequality. One of his assumptions is locality. The other one is that there is only one unique reality. Later, it was shown by using entangled states that nature violates Bell's inequality, which shows that at least one of his assumptions must be wrong.

Which one? Well, special relativity tells us that his assumption about locality must be correct. So the violation of the Bell's inequality, which leads to the concept of entanglement does not violate locality, or causality for that matter. Therefore, the assumption that must be wrong is the assumption that there is a unique reality. It is on this basis that QM is formulated.

You can see this notion of multiple realities in the expressions that are used for entangled states. They always consist of superpositions of multiple terms. Each term represents a reality. When an observation produces a result that is consistent with one of these terms then any other observations will produce result from the same term.

Now, one must be care not to confuse the notion of such multiple realities with the idea of a multiverse. That is not the case, because one can always change the state with a unitary transformation that would combine some the terms in the state to produce a new representation of the state. The entanglement would remain the same but the nature of the observations associated with each term would be different.

flippiefanus
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