More than correlation

I've been somewhat puzzled over the last couple of (no, actually, several) years as to the commentary coming from some physicists/science commentators active on social media - in particular Sabine Hossenfelder and "Nick Lucid" (the guy who does Science Asylum videos) about quantum entanglement.

The issue boils down to the degree to which you can consider entanglement a matter of simple correlation - because if it was, there is no mystery about it at all. (Basically, if it was just a question of two entangled particulars having complimentary properties from when they were created, and that measuring one lets you know the property of the other, no matter how far away it is - that is readily understandable.)

Yet this is the impression that Sabine gives in this tweet today:

The misunderstanding that stands in the way of progress in the foundations of physics: Believing that two entangled particles are physically linked.

Fact is: If you have two entangled particles and you do something to one of them, what happens to the other particle is: Nothing. Why? Because interactions in quantum physics are local. 

Entanglement is a *correlation* between two particles, it is not an interaction. Despite what you have heard, entangled particles are not nonlocally linked. No, they are not. They can't be, because... interactions in quantum physics are local! 

You wouldn't believe how often even physicists get this wrong -- though it's mathematically obvious. Act with a unitary on one of the particles, what happens to the other one is: NOTHING!

The reason people get so confused about this, I believe, is that they think the measurement process is an interaction. It should be an interaction! After all, a detector is made of particles. But in quantum mechanics it is not.   

That's the famous measurement problem of quantum physics.

The measurement process in quantum physics reveals the value of an observable. If you have two correlated observables -- like with entangled particles -- then revealing one in one location tells you something about the other.

But did this revelation actually do something to the particle? Well, some physicists think yes, others not. This alone should tell you that there is zero evidence that it's the case! 

The idea that a measurement on one of a pair of entangled particles does something to the other is what Einstein called "spooky action at a distance". He argued it is not physical. (I think he was right and I continue to be baffled that anyone thinks otherwise.)

But: It is not the entanglement that is a "spooky action"! Because there is no action in entanglement. It's a correlation. Of course correlations can be nonlocal. If two of you look at this post at the same time, the pixels on your screen are nonlocally correlated. Entanglement is nonlocal for the same reason. 

Einstein's "spooky action" is the idea that the measurement process is physical, rather than just a revelation of the properties of the particle. There is no evidence that supports the idea that spooky action is real. There is no evidence that if you do anything to one in a pair of entangled particles anything happens to the other. 

I am convinced that we'll not make progress in the foundations of physics until people in the field understand why Einstein criticized quantum mechanics. 

 Nick Lucid has made what sounds like a similar point:  entangled particles remain part of the same "system"- hence it is not that measuring one has to mean information is being sent to the other to tell it how to behave on measurement.  But talking like this still sounds very much like it is reducing the matter to one of "correlation".

Now, on Curt Jaimungal's very good "Theories of Everything" channel, I've watched a bit of Jacob Barandes, and in one video he explains how quantum entanglement is much "stronger" than mere correlation.  I will post the whole video here:  

 

The comments following both of these items are interesting:   many after Sabine's tweet interpret it as meaning their hunch that it's a simple matter of correlation has always been correct.   But surely this is misleading:  it would never be treated as a big mystery at all if it was only a matter of correlation.

One comment after the Barandes video is of interest to me:

@wmstuckey

Jacob did a great job explaining the EPR and Bell papers. Foundations of quantum mechanics (QM) has been my area of research since 1994 and he's right when he says many (most?) people misunderstand what EPR and Bell are saying in just the way he describes. Well done, Jacob. 

It may surprise you to know there is a way to resolve the "EPR paradox," i.e., solve the mystery of quantum entanglement, without having to resort to nonlocal or superdeterministic or retro causal mechanisms; and QM is as complete as possible in this solution to the mystery. How can that be? You simply view QM as a "principle theory" (Einstein's terminology), exactly like Einstein did to solve the mystery of length contraction in 1905. Accordingly, the relativity principle justifies the observer-independence of the speed of light c and length contraction follows as a kinematic fact, not a dynamical effect due to the luminiferous aether. Ironically, the mystery of quantum entanglement can be solved exactly the same way. Quantum information theorists have shown that the relativity principle justifies the observer-independence of Planck's constant h and quantum entanglement follows as a kinematic fact, not a dynamical effect due to some nonlocal or superdeterministic or retro causal mechanism. You can read about this in "Einstein's Entanglement: Bell Inequalities, Relativity, and the Qubit" (Oxford UP, 2024).  

 Now, Sabine does get some pushback in tweets too:

Richard Behiel
@RBehiel
·
11h
The problem is that the observed patterns of correlations are impossible, unless something trippy is going on. Your pixels-on-screen analogy is the classic hidden variable argument, the intuitive starting point which Bell showed is, shockingly, not how nature works. That’s the problem.

It’s true that it’s not an “action” at a distance, you’re right to point out that the word is misleading, but somehow the universe does seem to know, at one particle, something about the way in which the other was measured. It is apparently a nonlocal, superluminal (non-signaling) effect.

I used to think hidden variables might explain the spooky behavior, but Bell’s theorem shows they’d have to be nonlocal, which is still spooky. I’ve really been diving into the math behind his argument, in preparation for an upcoming video, and I just don’t see any non-spooky explanation here. Quantum entanglement is indeed a deep mystery.

And there is this:

mmiguel6288

@mmiguel6288

·

14h

The Bell tests have experimentally verified non-locality in the universe - and this inescapably shows up in every interpretation if one is being intellectually honest. Ignoring evidence via shut-up and calculate or abandoning objective reality are stances that fundamentally oppose the spirit of science despite their unfortunate popularity over the past century. In the Copenhagen interpretation, the shared wavefunction between the two particles instantaneously and non-locally collapses when one is measured even if the particles are on opposite sides of the galaxy. This collapse is an instance of non-locality, persisting even when realism is abandoned. In Many Worlds, the entire universe and all of space, non-locally branches based on the measurement. Branching is non-local, this persists even when singular objective reality is abandoned. In Bohmian mechanics it is admitted to be non-local and the universal wavefunction evolves non-locally. This is non-locality under single objective reality. In superdeterminism, some unexplained filtering mechanism foresaw all possible non-local correlation violations and crafted the initial conditions of the universe to avoid all of those without any mechanism besides this non-local prediction filtering mechanism. Non-local quantum correlation violations shows up as the triggering condition to filter out a possible initial configuration. You can say these aren't interactions because collapsing, branching, wavefunction evolution, and initial conditions filtering don't involve momentum/energy transfer, but that doesn't change the fact that the Bell experiments have shown that there is undeniably something non-local going on in the universe. Momentum transfer is irrelevant to this fact and it is misleading to say disregard the experimentally verified nonlocality that is happening by artificially constraining the set of things under consideration to be only those that involve momentum/energy transfer.

Now, I know that Sabine thinks that superdeterminism is probably the key to it all - she has pointed out many times that it is an assumption of Bell that the scientist has a "free choice" as to what to measure.  Hence in the thread after her tweet we get this:

As I say, we all know the one she thinks must be false.   

But, I think one could ask - is superdeterminism "spooky" in its own way?   And how do you tell the difference between superdeterminism and backwards causation from the future?   

A decent thread on why scientists are sceptical of superdeterminism appeared at Reddit a few years ago, and I think it makes many decent points.

So, how do I end this post?   Perhaps in this unsatisfactory manner which was determined since the beginning of the universe?   

Update:  Youtube reminded this morning that Sabine herself had done a video a few years back which explained that it's not a simple matter of correlation.   (The explanation of why, is, however, hard to follow.   And Barandes similarly doesn't explain well - or at all really!).   But I do get the feeling that it "suits" Sabine to now play up the use of "correlation", and I don't think she is being clear or upfront as to why.  Here's the old video: 

 

Update 2: I had forgotten about Frank Tipler, with his deep allegiance to Many Worlds theory, thought that it readily solves the non locality problem. A pity, though, that most physicists think he's kind of nuts, especially with the book explaining Jesus as creating miracles with high tech science! From his 2014 paper Quantum nonlocality does not exist:

Significance

I show that quantum nonlocality is an artifact of the assumption that observers obey the laws of classical mechanics, whereas observed systems obey quantum mechanics. Locality is restored if observed and observer both obey quantum mechanics, as in the many-worlds interpretation (MWI). Using the MWI, I show that the quantum side of Bell’s inequality is entirely local. Thus, experiments confirming “nonlocality” are actually confirming the MWI. The mistaken interpretation of Bell’s inequality depends on the idea that the wave function is a probability amplitude, but the MWI holds that the wave function is a world density amplitude. Assuming the wave function is a world density amplitude, I derive the Born interpretation directly from Schrödinger’s equation.

Abstract

Quantum nonlocality is shown to be an artifact of the Copenhagen interpretation, in which each observed quantity has exactly one value at any instant. In reality, all physical systems obey quantum mechanics, which obeys no such rule. Locality is restored if observed and observer are both assumed to obey quantum mechanics, as in the many-worlds interpretation (MWI). Using the MWI, I show that the quantum side of Bell’s inequality, generally believed nonlocal, is really due to a series of three measurements (not two as in the standard, oversimplified analysis), all three of which have only local effects. Thus, experiments confirming “nonlocality” are actually confirming the MWI. The mistaken interpretation of nonlocality experiments depends crucially on a question-begging version of the Born interpretation, which makes sense only in “collapse” versions of quantum theory, about the meaning of the modulus of the wave function, so I use the interpretation based on the MWI, namely that the wave function is a world density amplitude, not a probability amplitude. This view allows the Born interpretation to be derived directly from the Schrödinger equation, by applying the Schrödinger equation to both the observed and the observer.

He has no concern at all about what an odd view of the true nature of reality it provides.  This is from the conclusion section of the paper (my bold):

I have given several powerful arguments for the MWI: the restoration of locality of physics and the true origin of the Born interpretation. The main difficultly that many physicists have with the MWI is the required existence of the analogs of themselves. However, every time physicists measure a frequency and verify the quantum expectation value in the Bell inequality, they are actually seeing the effect of the analogs of themselves making the same measurements of the electron spin. The language of the frequency interpretation of probability has prevented physicists from seeing what is actually happening. It has prevented physicists from realizing that they are actually observing the effects in our universe of the other universes of the multiverse.