The point of the EPR experiment is that when we observe A at L we will observe A' at L'. And when we observe B at L we will observe B at B'. In other words "collapse is instantaneous".
In the many worlds interpretation, nothing is actually happening faster than light. At L we split into an A and a B observer. At L' we split into an A' and a B' observer. And then when those observers meet up, QM just describes how those observers match up to generate two consistent stories.
However this is a very, very important for theoretical physics. General Relativity is an inherently local theory. QM is inherently nonlocal. In decades of trying, nobody has figured out how to come up with a theory that bridges them.
But to add, there are at least ideas, it is not the case that we made no progress at all. I would like to highlight the ER=EPR conjecture [1] by Leonard Susskind and Juan Maldacena. It roughly states that entangled particles are connected by worm holes and that they are therefore not actually separated in space. Personally I prefer a less sensationalist sounding description, that entanglement defines what it means for points in space to be close together or far apart. This avoids constantly visualizing pairs of particle connected by glowing worm holes from science fiction movies.
My understanding is that there is a growing consensus on this
Think about it, think about all physics you have ever done, in school or wherever. You have probably never dealt with a situation where something was created or destroyed. You need variables to describe all the things you are dealing with, how do you add new variables or get rid of some old variables halfway in your calculation when something is created or destroyed?
This is what those fields in Fock space are for, they are mathematical tools to keep track of the number of particles you have in specific states. You do not start with a field and discover that it has quanta behaving like particles, you start with particles and just describe them with fields in order to be able to handle particle creation and annihilation.
Here is Nima Arkani-Hamed giving the Salam lectures 2012, a five day series on the state of fundamental physics. I marked [1] about 30 seconds where he is very clear about this. You can also start here [2] and watch about 10 minutes of this lecture where he quickly redevelops the ideas of quantum field theory.
Even if you can not follow all the details, you should easily be able to follow the explanation where the fields come from and why. The particles vs fields issue is explicitly discussed three or four times within those 10 minutes. You may of course want to watch the entire lecture series, it is really good and is pretty non-technical in the beginning. The last day I linked to is actually the exception if I remember correctly, the first four days are mostly without math.
Furthermore you can still speak of particles just as you can speak of atoms, because it's useful to call the bumps something other than bumps all the time.
Here are some quotes from a piece on the experiment I mentioned above. from https://www.sciencenews.org/blog/context/entanglement-spooky...
> Einstein insisted that the handedness of the glove must be determined in advance by some physical law. He was perplexed by the possibility that Alice’s choice of hand could have something to do with whether Bob’s glove would fit his fingers.
> Suppose you prepared entangled photons and sent them to Alice and Bob in such a way that if Alice measured hers to be vertically polarized, she instantly knows that Bob’s will be horizontally polarized. . . .
> . . . The photon does not have an orientation until Alice detects it. Same for Bob’s. But once Alice makes a measurement, the outcome of Bob’s measurement is certain.
> Here’s where a lot of confusion clouds entanglement commentary. Contrary to what you might have read in a magazine with “New” and “Yorker” in the title, Alice’s measurement does not “instantaneously” influence Bob’s photon. No signal is sent, no influence transmitted. For all Alice knows, Bob might have measured his photon first. In fact, if the measurements are made at nearly the same time, there might be no objective way to say who made the first measurement. (A space traveler flying along at nearly the speed of light might see Bob’s measurement first, while another traveler flying in a different direction would see Alice’s first.)
Say you have a pair of entangled spins in a superposition of the two anti-parallel states first spin up and second spin down or first spin down and second spin up with equal probability for both states, i.e. each spin is up or down with a probability of 50 % but the two spins always have opposing orientations.
If you measure one of the spins, then there is no time, no matter how far the spins are separated, in which you could measure the second spin to have the same orientation as the one you just measured because the result of the measurement somehow had not yet enough time to reached the second spin, supposedly because this state change is limited to travel at the speed of light.
It is as if the spins had secretly picked on of the two possible outcomes when they were created but you just do not know which one until you perform a measurement. But this is not the case, this is what is called a local hidden variable theory and is ruled out by Bell test experiments. The spins have no definite orientation until you measure the first one but after the first measurement both spins instantaneously have a definit orientation no matter how far separated.
https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_m...
[...] there is no time, no matter how far the spins are separated, in which you could measure the second spin to have the same orientation as the one you just measured [...]
If an interpretation makes use of non-locality to explain this fact is a different matter, but experimentally it looks exactly like measurements instantaneously affecting entangled partners.