Causality in space-like separated collapse of entangled pairs

Question

Suppose two spacelike-separated measurement events take place on an entangled pair of particles, both of which events, in isolation, would cause the entanglement to collapse.

It is impossible to say which event occurs "first" and therefore it is impossible to say that either event causes the collapse by itself.

The cause of the collapse cannot be "both measurement events", because either one in isolation would have caused the collapse.

So, what does cause the collapse? It must have a definite, objective, attributable cause - otherwise why does it happen?

Answer 1

One can distinguish among three things in the context of this question: (1) the experimental results, (2) the mathematical theory with which one predicts what one would see in such an experiment, and (3) the interpretation of the theory. The first two combine into what we call the scientific method. The third cannot be confirmed and is therefore not considered part of the scientific method. However, that does not mean people cannot and does not ponder these interpretations.

In the context of the question, the idea of collapse lies in the third category. There is currently no known way to confirm that there really is such a thing as quantum collapse.

As for the predictions and the experiment, when such observations are made as space-like separated events, which implies that neither could have dictated the outcome of the other, then one would simply find that whatever is observed is always mutually consist. How this works and why it works this way we cannot say with certainty.

Answer 2

Magic can be used to coordinate but not to communicate.

Quantum entanglement lives in this middle ground. To resolve your paradox we must understand this middle ground.

Classical correlations: Suppose a machine splits a coin in half, separating the head and tail halves. It gives one half to Alice and the other to Bob, sealed tightly inside a box. Alice and Bob then separate. Once Alice opens her box, she immediately knows that Bob, who is lightyears away has the opposite face. However, each box already had it's face predetermined so nothing unusual happens. This is an example of local hidden variables. There is no causality.

Communication: After separation, Alice chooses whether she wants a heads or tails. Her box sends a radio signal to Bob's box which set's itself to the opposite of Alice's choice. This can only happen for causal time-like separations because of the radio signal.

Entanglement: Like the case of classical correlations, Alice can't choose whether she gets a heads (up) or a tails (down). No matter what she does she won't affect Bob. But unlike the classical choice, the entangled particles do what classical local hidden-variables (coins in boxes) can't, such as violating Bells inequality and playing coordination games.

Non-locality means that if we were to model the quantum state on the computer we would need to have Alice and and Bob modify and/or measure the same shared state no matter how far they are apart. However, they are restricted in what they can do to/with this joint state so they can't use it as a channel to send information. In addition, it does not matter who goes first, so no matter what frame we choose (which affects who is first if they are spacelike-separated), Alice and Bob's results will be the same. There is no paradox.