What is the consensus among physicists on whether quantum mechanics has non-locality?

Question

According to this article here by the SEP,

Following Bell's work, a broad consensus has it that the quantum realm involves some type of non-locality (for examples, see Clauser and Horne 1974, Jarrett 1984,1989, Shimony 1984, Redhead 1987, Butterfield 1989, 1992a,b, 1994, Howard 1989, Healey 1991, 1992, 1994, Teller 1989, Clifton, Butterfield and Redhead 1990, Clifton 1991, Maudlin 1994, Berkovitz 1995a,b, 1998a,b, and references therein).

The article in general talks about how if two entangled particles separated at a huge distance have a wave function whose probabilities are joint and thus not factorizable, there is no way to escape non locality in quantum mechanics (even without superluminal signalling it seems) unless you question some plausible assumptions.

Some of the answers on here seem to imply the opposite so I am now wondering as to what the consensus really is.

NOTE: This is not meant to be an opinionated question. I am looking for surveys of some sort or some other data indicating current viewpoints. Asking about whether an opinion has consensus is not a matter of opinion.

Answer 1

I don't think there is a consensus among physicists. I think the reason for the lack of consensus is that this issue is ultimately an issue of interpretation of quantum mechanics, and does not have a direct experimental test that can decide the question one way or the other.

Everyone agrees on the experimental facts. In a Bell-type experiment, you can separate two entangled particles so they are at a spacelike separation. Then measurements of the spin of one particle along some axis will be correlated with measurements of the spin of the other particle along some axis (could be the same or different depending on the whims of the experimenter). It also turns out that this correlation cannot be used to send information from one experimenter to the other, and can only be discovered after the fact when the experimenters re-enter each others light cones and compare their experimental results.

The question is, "how do these particles at spacelike distances know to be correlated with each other in the ways prescribed by quantum mechanics?" Bell's inequalities rule out the possibility that a local hidden variable theory can explain these correlations, assuming the measurements are statistically independent. In other words, it is not possible to explain the observed correlations -- correctly predicted by quantum mechanics -- by assuming the particles really do have a definite spin before we measure them (determined by hidden variables we don't know about) and that they only interacted at the point they were created (unless one appeals to superdeterminism).

At this point, different people have different opinions, depending on their interpretation of quantum mechanics. I think probably the "default" view of physicists who don't work directly with interpretations of quantum mechanics would be to take the Copenhagen interpretation, which would imply some nonlocality when one particle is measured, causing the wavefunction of the other particle to instantly collapse. However, other physicists may have other interpretations where nonlocality may or may not be present. For example, according to superdeterminism, the initial conditions of the Universe are such that the experimenters are not free to choose the axes along which they measure spin; the particles really do have spins and the choices the experimenters make are effectively pre-determined in advance to agree with the predictions of quantum mechanics. In this interpretation, there is no non-local process.

For what it is worth, personally I believe there is some non-locality in this process. I suspect that deep down our notions of space and time are emergent from some more fundamental quantum mechanical degrees of freedom, and entanglement is a kind of clue about that. Entanglement is perfectly natural mathematically if you think about Hilbert space; it's only when we add on our notion of how space and time "should" behave that we run into problems of interpretation. However, that's just my personal speculation and if you challenged me I'm not even sure I could define precisely what I mean, and I am sure other physicists have different points of view.

Answer 2

It's a bit of a complicated question. The simple answer is that there is no consensus, but there's more to know than that.

Many have thought about this to the extent that one does after one or two times learning about Bell's Inequalities. After this level of depth, generally it is agreed that it is possible for the universe to be local as long as it is "realistic", or value-indefinite. This is roughly what many to most physicists will explain if asked about the topic.

If you only pick from people who have spent a lot of time thinking about it, a larger percent of physicists believe that there must be some type of non-locality. However I wouldn't go as far as to call it a consensus. To be transparent, my own view is also that there is some type of non-locality, though I think I've been pretty objective in my description.

Note that just because there is disagreement doesn't mean that the answer is subjective. I think there are real answers to whether the universe has been shown to be non-local, the arguments are just somewhat subtle and different physicists have ended their train of thought in different places.

One thing to consider is that we may live in a world where fundamentally there is no clear definition of what locality even is at the microscopic level. What I mean by this is that if everything were to be fundamentally described by wave functions, and wave functions don't assign properties to locations in space ($x \in \mathbb{R}^3$) but rather to locations in configuration space ($x_1, x_2, x_3, ... \in \mathbb{R}^3\times \mathbb{R}^3\times \mathbb{R}^3...)$, it becomes hard to even define what locality even is. Because when properties aren't assigned to individual points, what's your definition of what it means to be local?