Was Einstein "spooky action at a distance" about entanglement or about wave function collapse?

Question

I've been watching Sabine's videos and this is my understanding:

There is no "spooky action at a distance" based simply on entanglement. Entanglement is a correlation. There is no communication between the two entangled particles. And thus, there is no spooky action at a distance.

There is "spooky action at a distance" when the wave function collapses. Up until the time of the collapse, the properties of a particle are probabilities. After collapse, the properties can be determined. However, there is no way to know "why" the wave function collapses into a specific state because either (1) the wave function is not just a mathematical model, it is reality or (2) measurement devices cause the collapse.

Am I wrong with the above understanding?

Answer 1

I guess that this is the blog post/video that you're referring to. Hossenfelder argues that Einstein's "spukhafte Fernwirkung" referred to (more or less) wave function collapse, and not to entanglement. I think she's right. Conifold argued the same thing in a HSM.SE answer.

What bothered Einstein was that in, e.g., a single-slit experiment with a single particle, the particle is always detected at only one point on the screen. If the particle is spread out across a large area before the detection, as the Schrödinger equation suggests, then there is no way it could avoid the possibility of being detected at more than one location except by prearrangement (hidden variables) or some sort of faster-than-light coordination (ghostly remote effects, to translate the phrase a bit differently than usual). This experiment doesn't satisfy the normal definition of entanglement. It's not quite the same thing as wave function collapse either, but it's closely related to it.

The issue was not what counts as a measurement device or what triggers collapse to happen, since even if you solved that problem, the nonlocality problem would remain.

Answer 2

The original EPR paper is available on the web. The key example (on which all else depends) is described in Section 2, with the entangled state described in equation (7). Without non-trivial entanglement, the entire paper would be pointless.

The reason that entanglement strikes people as "spookier" than wave function collapse is this: In a non-entangled state, the possible outcomes of experiments are random variables, with a well-defined joint probability distribution. In an entangled state, the possible outcomes of experiments do not obey any joint probability distribution and so cannot be random variables in the classical sense.

Nobody ever thought that randomness was spooky. Randomness without random variables understandably strikes some people as odd.

Answer 3

There are several issues here, regardless of which of Hossenfelder's works you are referring to. Here is a paper that summarizes her position well (keep in mind I think her position on Superdeterminism is horribly flawed, but that's a different story):

Rethinking Superdeterminism Hossenfelder and Palmer, 2019.

The short version is that she seeks to maintain locality (through some to-be-determined theory featuring Superdeterministic elements). Specifically: she wants to "understand the apparent nonlocality of quantum physics" because "this experience [i.e. actually performed experiments] misleads us". So naturally she would argue against "spooky action at a distance" (shorted here to "AAAD") under any name you might give it. I will use that phrase because that is the terminology of the question being asked, although quantum nonlocality is a more common term.

There is no "spooky action at a distance" based simply on entanglement. Entanglement is a correlation. There is no communication between the two entangled particles. And thus, there is no spooky action at a distance.

The above is a fair representation of her position. However, I will explain the serious problems in those few sentences. While the following statements may be disputed if you follow a particular quantum interpretation, I would say they are a fair representation of the views of most physicists in the area of entanglement:

a. Entanglement is the primary theoretical and experimental element of arguments in favor of AAAD.

b. Entanglement is evidenced by correlations*. However: within the framework of QM, it is much more than just "correlation". Specifically, systems of 2 entangled photons (which is the example I will keep returning to) are usually created by parametric down conversion (PDC or SPDC). Such quantum systems are indivisible and cannot be properly described within the quantum formalism as a system consisting of 2 independent particles. Such a system (entangled pair of photons) is often called a "biphoton" to highlight this important distinction. A biphoton is in a Fock state of N=2, one in which there is (usually) maximal entanglement. These are often referred to as "Bell States" (there are 4 permutations of these), and these are routinely produced as described in at least 10,000 papers on the subject in the past 20 years.

So no, calling these experiments studies of "just correlations" would be a gross misuse of the language of physics*. Entangled systems are the only quantum systems that can produce predictions that cannot be reproduced using Product state statistics. Product state statistics are those in which system is described as a Product of 2 separate components. (If there were 2 separate components, obviously they would not be evidence of AAAD.)

c. As a way to highlight point b., please keep in mind experiments that have demonstrated: Biphotons (a system of 2 entangled photons) can be created at a distance. 2 previously uncorrelated photons can be entangled that have never existed within a common backward light cone. If that is not AAAD, I don't know what is. See for example: High-fidelity entanglement swapping with fully independent sources which states: "A successful entanglement swapping procedure will result in photons 1 and 4 being entangled, although they never interacted with each other." One of the authors of this paper shared the 2022 Nobel for this and other works.

And guess what, the evidence for this result is based on violations of one of the Bell Inequalities (this one being CHSH - one of its creators also shared a Nobel). Again, the correlations are evidence of entanglement. A little thought should convince you that if these magical correlations appear in places too far away for any kind of classical (non-AAAD) action to occur, that would be difficult to explain to any reasonable person.

d. Is there any communication between the "individual" components of an entangled system? No one knows the answer to this question. QM does not supply us an answer as to what the mechanism might look like. It "merely" provides a formalism which has allowed scientists to correctly predict experimental outcomes that demonstrate AAAD.

If you believe Hossenfelder et al, you must ignore this powerful evidence. You also stated:

There is "spooky action at a distance" when the wave function collapses. Up until the time of the collapse, the properties of a particle are probabilities. After collapse, the properties can be determined. However, there is no way to know "why" the wave function collapses into a specific state...

e. It is not generally agreed that there is something called "collapse", although it is almost impossible to describe QM without referring to this. If there is collapse, then surely it requires AAAD. You might logically assume that measuring A causes remote (nonlocal) collapse that determines B's state uniquely. But...

It is also helpful to know what collapse is not. There is no evidence whatsoever that a measurement on photon A - occurring first - causes a change to photon B (rather than B's later measurement causing a change to A instead). If you think this is a far-fetched point, think again. The problem is there are plenty of experiments that demonstrate conclusively that this could NOT be the case. For example, you can entangle photons AFTER they are already detected (see Delayed Choice experiments to understand this). Sounds impossible, but again a Nobel was awarded for the successful demonstration of this. Further, it is possible to entangle photons that have never even co-existed. That tears the heart out of more traditional views of collapse. See for example: Entanglement Between Photons that have Never Coexisted "Using entanglement swapping between two temporally separated photon pairs we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime."

f. As to the "why" of apparently random quantum outcomes: No interpretation actually postulates a way for that to be predicted in advance, and certainly QM does not explain this except as a given.

-DrC

*Correlations coupled with matching theory are the backbone of science, not just physics (or quantum physics). Virtually every experiment on entanglement features a presentation of the expected results per theory, compared to actual observation. What else could you possibly ask for?