why is superdeterminism not the universally accepted explanation of nonlocality?

Wouldn't retro-casuality have exactly the same outward appearance as superdeterminism to us? It is certainly more economical (in occam sense) - the 'conspiracy' in each case only has to go back in time as far as needed to achieve correct outcomes as opposed to setting it all up just before big bang. Also laws of physics are time-symmetrical already (:those that counts anyway:).

You're right, I am assuming that. But that's a harmless assumption to make. Perfect correlations for matching measurement settings is a consequence of quantum mechanics, so surely if a local realist theory wanted to match the predictions of QM then it would have to have perfect correlations for matching measurement settings. I highly doubt this is what ThomasT is disputing.

(You can, of course, be the fringe type of local realist who has a theory making predictions contrary to QM, but who believes that the only reason the experiments have proven QM right is that they're subject to various flaws, loopholes, and systematic biases. But as Bell tests become more sophisticated, that becomes an increasingly untenable positon, arguably even more so than superdeterminism.)

OK, let me give you an example of why you need special initial conditions.

In a deterministic theory, in order to predict the current behavior of any object, you need to know the initial conditions of the object, as well as the deterministic laws of the universe. In Newtonian mechanics, for example, you need to know the positions and velocities of all the particles at time t=0, and then F=ma will tell you the behavior of the particles at all later times.

Now let's consider what a local deterministic explanation of entanglement would look like. Let particles A and B be an entangled pair of photons, which are separated by a great distance and then sent through polarization detectors. We also have particles C and D: C tells the experimenter what angle he should set the polarizer that measures A, and D tells the experimenter how to set the polarizer that measures B. You can think of C and D as neurons in the brains of the experimenters if you like.

Now we find experimentally that the behavior of particle A through its measurement device is strongly correlated with the angle at which B's measurement device is set. And that angle is determined by particle D. So we have a correlation between the behavior of particles A and D.

But particles A and D are seperated by such a large distance, so they cannot communicate with each other to coordinate their behavior (unless you have a nonlocal theory like Bohmian mechanics which allow undetectable faster-than-light signalling between particles). So a local determinist has to conclude that A and D are correlated not based on a current relationship between the present states of A and D, which would be impossible, but based on a past relationship of the initial states of A and D.

This is what we mean by special initial conditions: A and D seemingly have nothing to do with each other. After all, it is A and B that were in the entangled state, and yet somehow we have to conclude that the initial conditions of A and D had to be specially set so that a correlation between A and D would be observed in the future. And instead of just D, we can have a large number of particles D1, D2, D3,... which together determine the measurement setting, so the initial state of particle A had to have been set based on the initial states of all these particles. And in the real world, almost all particles in the universe are interacting in some way with almost all other particles, so really the setting of measurement device depends on almost everything in the universe, from which we conclude that the initial conditions of the whole universe were specially set so that the right kind of correlation would be displayed billions of years later between particle A and the measuring device.

This is why superdeterminism is called "conspiratorial". That doesn't mean it's wrong, it just has some issues which make it rather difficult to construct a viable superdeterministic theory, but let me repeat that some potential first steps toward such a theory have already been taken by a few people.

Free will is not a necessary component of QM. So I certainly am not rejecting superdeterminism because of that. I reject superdeterminism as an explanation for Bell test results, and I do so for the reasons already stated.

Please bear in mind that there are no candidate superdeterministic theories to reject at this point, so it is a moot point in many ways. The reason I mention the amount of local information to be stored in every particle is because a candidate theory will end up postulating this (in some form or fashion) as a way to explain Bell test results. It is not necessary to assume free choice for measurement settings in any stage of the argument, but you must explain how (i.e. the exact mechanism, since we have no other reason to suspect it exists) the choice is propagated in a superdeterministic candidate.

Not so easy, I assure you. Which is again, the answer to your original question.

exactly. it should be called super determinism. it should be called universal causality.

if im the first with this this then i claim it. adrians universal causality

That is just NOT true in any meaningful sense. A particle has only a few observable elements: momentum, position, mass, charge, spin, color, etc. It would be instructive to state specifically how you would know ANY information about the past interactions by knowing these. Suppose the spin is +1. What does that tell you? Or momentum is 1.63 (units ignored) in direction XYZ? Not much history to be gained from that!

No, you need there to be a rich hidden internal structure. One that contains the entire initial conditions of the universe, like DNA. And this DNA would need to be in every particle so they know how to react during Bell tests.

also, has it been proven in a lab that time moves forward and isnt frozen or moving backwards? it seems like that is something our intuition has guided us to regard as truth. is there not enough intuition in the world to regard determinism as truth?

Again, I am not claiming that A and D could never have interacted. I am saying that the interaction of A and D would have to have occurred in just the right way so that they would demonstrate nonlocal correlations of just the right kind. And by similar arguments, you would have to conclude that at the beginning of the universe all the particles interacted with each other to set just the right initial states for each particle, so that all the Bell tests which would be performed in the entire history of the universe would get just the right results. That's what's called a conspiracy.

conservation of information. finiteness of information.... you keep throwing these things at me as if they are a way around the law of conservation of information. seriously?

Any deterministic theory has causality. But if you have some arbitrary deterministic theory and some arbitrary initial conditions, chances are you won't get the nonlocal correlations necessary to match the results of Bell tests. It's only if you have very specific initial conditions, conditions where the initial state of each particle is set based on the initial states of all other particles, that you get the right kind of nonlocal correlations. That's what makes superdeterminism conspiratorial.

Let me repeat, that does not mean superdeterminism is ruled out, it just means there are hurdles that any superdeterministic theory has got to face.

No, there isn't, but there is something conspiratorial about a particle behaving in just the right way based on the information it has about what other particles are going to do at just the right time and place. Again, particles interacting and transferring information is not regarded as a conspiracy. It's a conspiracy if their interaction induced some very special behavior on their part which leads to a Bell-type nonlocal correlation.

To repeat, in order to have Bell-type nonlocal correlations between A and D in a local deterministic theory, we need A and D not only to have interacted in the past, but to have interacted in just the right way so that they would get the right "conspiratorial" initial conditions so that they would display the right kind of nonlocal correlations years later.

Off topic, but Einstein definitely did not find the new radical notions of space and time he came up with intuitive. He was led to SR because he saw that the electrodynamics of moving bodies seemed to possess a greater degree of symmetry than their conventional description gave them credit for, and so he tried to redo the laws of kinematics so that Maxwell's equations could be shown to conform with the principle of relativity.

There's no way to know or demonstrate that information, or anything else, is instantaneously transferred from a to b. In fact, instantaneous propagation is a contradiction in terms. If a and b are changing instantaneously, then they're changing simultaneously. And there's nothing in our observations of our world, our universe, that suggests that simultaneous, spacelike separated, changes in a and b imply a causal relationship, or any sort of communication, between a and b. Rather, what this does imply is that a and b are part of a larger system, or that a and b have something in common due to a common cause.

Wrt some formulations (eg., inferred wrt standard QM and explicit wrt dBB interpretation) a and b can be said to change, or are explicitly encoded as changing, simultaneously. So, if one wants to give this some sort of pseudo mechanical meaning, then one might say that information is being instantaneously transferred between a and b. But this isn't really mechanics. It's just an assumption that can't be verified or falsified. Ie., a physically meaningless statement.

That's news to me. I would say that observations indicate that our universe behaves contrary to the notion of block time. That is, it's evolving and transitory. But that certain theoretical constructs/eventualities suggest block time. And, afaik, the theoretical stuff that suggests block time (or that contradicts observation) is more or less routinely disregarded/discarded.

Because it's unwarranted wrt extant observation and mainstream interpretation of theory.

Your title asks why superdeterminsim isn't universally accepted. My guess is that it's because superdeterminism doesn't refer to anything other than determinism. Determinism might not be universally accepted, but I think it's the predominant assumption and starting point wrt virtually all of the physical sciences.

Why not? Are you saying that the assumption of determinism implies action at a distance? Or superluminal propagations?

Ok. So far this is just determinism.

You've arbitrarily assumed a starting point (ie., initial conditions) that isn't influenced by past events. But we can just as well assume that wrt whatever you want to assume as a starting point there are antecedent events, ie., some prior history/conditions.

So, as far as I can tell, superdeterminism is a superfluous term, which actually just refers to determinism.

bells nonlocal correlations... come up with a billion of them... they are a non issue because they dont violate relativity and are therefore predetermined

Yes, I reread your reply. I still don't understand what differentiates superdeterminism from determinism. I think Demystifier also tried to explain it one time to me. That didn't do it for me either. Or what Bell or 't Hooft have to say about it. I mean, it just isn't clear to me what the word superdeterminism refers to that's different from what the word determinism refers to.