Do quantum entangled particle pairs actually interact?

Question

I believe this question could help simplify the accepted understanding of quantum entanglement for myself and others with little to no real physics background.

So my understanding of one way to create entangled photons is basically the duplication of one photon into a pair with opposite spins. If that is the case, the superposition that describes the paired photons with opposite spins would have the particles at opposing vertical points on the wave function. So revealing, through measurement, what spin one of the photons is, isn’t communicating anything between the two particles, it simply necessitates that the other part of the pair, that has been opposite from the beginning, is still just that. Is my way of understanding here flawed or is there some type of interaction between the two particles that I’m missing?

Spooky action at a distance, only seems spooky to me if there is truly some kind of interaction going on. Instead of these photons just kind of humming along to the same tune.

Answer 1

When photons are entangled, their properties (like spin) are correlated, not necessarily "opposite," but linked in a specific way. For example, if you measure one photon to have a certain spin, the other photon's spin will be determined to maintain that correlation, no matter the distance between them.

You're right that there's no direct "communication" between the particles after measurement. Instead, quantum mechanics says the particles exist in a superposition of possible states until one is measured. When you measure one, the entanglement ensures that the other particle’s state is instantly known. This isn't an interaction or signal traveling between them—it's a manifestation of the entangled state that was created when the pair was born.

The "spooky action" (as Einstein called it) refers to how this instant correlation seems to defy the idea of local interactions, but there's no faster-than-light communication; it's just how entanglement works in quantum mechanics.

So, the key idea: No hidden interaction happens between the photons after they're separated—it's all about the nature of the entangled state from the start.

Answer 2

Well, the problem is when you design an experiment in which two far away observers measure the two different spins, but where each of the two far away observers decide randomly in which of two different directions to measure the spin.

In such a case there are correlations between the measurements that cannot be explained by a classical model in which you start with pairs of particles in different combination of spins in different directions. This is doable in the example you gave, which only measures one spin direction. In your example, initial pairs of particles with opposite spins are enough to explain the observations.

The more complex experiment that I mentioned, instead, introduce correlations between the measurements of the two particles that cannot be explained in that way, or in any classical local way. It can be explained in many different weird ways though, and one of them is that the particles are somehow connected, and the the angle in which we measure the spin of one of the particles, is enough information to use an algorithm that can generate these correlations.

This is generally not accepted because it would transfer information (not useful to us but enough to generate the correlations) at a speed faster than light. Some theories argue that they are connected by something similar to wormholes.

Physicists are not in consensus on how to interpret these results, I hope AI will help us find better ideas soon, perhaps a surprisingly simple explanation.

Answer 3

You haven't accurately described the problem with entanglement. Suppose you have two particles $S_1$ and $S_2$. For each particle you can measure two quantities $X$ and $Z$. The quantity $X$ has two possible measurement results $1$ and $-1$. The quantity $Z$ also has two possible measurement results $1$ and $-1$. We'll call the $X,Z$ for $S_1$ by the name $X_1,Z_1$ and the $X,Z$ for $S_2$ by the name $X_2,Z_2$.

In a suitable entangled state if Alice measures $X_1$ the probability of each possible outcome is $1/2$, and if Alice measures $Z_1$ the probability of each possible outcome is $1/2$. If Bob measures $X_2$ the probability of each possible outcome is $1/2$, and if Bob measures $Z_2$ the probability of each possible outcome is $1/2$.

The real problem comes up when you compare the measurement results. If Alice measures $X_1$ and Bob measures $X_2$ when the results are compared they will be found to be the same. If Alice measures $Z_1$ and Bob measures $Z_2$ when the results are compared they will be found to be the same. If Alice measures $Z_1$ and Bob measures $X_2$ when the results are compared they will match with a probability of 1/2: that is, if Alice gets $1$ when she measures $Z_1$ the comparison will find that $X_2$ has value $1$ with probability 1/2 and $-1$ with probability $1/2$. If Alice measures $X_1$ and Bob measures $Z_2$ when the results are compared they will match with a probability of 1/2.

So the issue is that the probability that the results will match depends on what measurement was done on each particle. Now, you write:

So revealing, through measurement, what spin one of the photons is, isn’t communicating anything between the two particles, it simply necessitates that the other part of the pair, that has been opposite from the beginning, is still just that.

But if this was the correct explanation how could the probability of a match depend on whether Alice and Bob happen to measure the same quantity on both systems?

The standard view is that you shouldn't ask for an explanation of entanglement correlations or anything else about quantum theory. This leads people to neglect even stating the problem clearly because that risks encroaching on the taboo against explanations of quantum theory.

I will give the only actual explanation of entanglement correlations I have found in the literature. Quantum equations of motion describe the evolution of measurable physical quantities in terms of matrices called observables whose eigenvalues are the possible measurement results. Large systems typically don't do weird quantum stuff because copying information out of a system suppresses interference - this is called decoherence:

https://arxiv.org/abs/1911.06282

But decoherent systems are still governed by quantum theory and still have observables. Those observables can carry quantum information as long as that information doesn't change the probabilities of measurement results. In entanglement experiments the observables of "classical" systems, i.e. - decoherent systems, carry quantum information that gives rise to the relevant correlations by local interactions when measurement results are compared:

https://arxiv.org/abs/quant-ph/9906007

https://arxiv.org/abs/1109.6223