Superpositions are statistical distributions. Only statistics are wholly predictable. Individual events are wholly unpredictable.
You can have two statistical views of the same experiment in which each individual event is different but the statistics agree.
You can have a distribution of views in which events are somewhat different to varying degrees and the statistics still agree.
Each view will experience itself as unique in specifics while agreeing that the distribution of events is identical.
That doesn't solve the problem.
It's hard to get random processes to define an identical distribution without having classical mechanics as a foundation.
Basic QM hand-waves this away with "Here's some math that defines the distribution and..." But that's like "Starting with this universe we can..."
It so happens that the statistics correlate spatially with various field potentials. Which is interesting, but... Why is there a distribution at all? Why are there creation/destruction operators? Where does the "code" that defines this mechanism live? How do particles know they should follow it?
Do particles even exist, or are they just something random and constrained a field does every so often? How do fields know the physical configuration of an experiment has changed and sometimes that information appears to travel FTL? (Even though it can't be used for signalling.)
How does a particle keep track of how many other entities it's entangled with and in what ways? Where does that information live? (Bell suggests it's not inside the particle itself. So where is it?)
Is all of this shaped by some kind of hidden causal propagation mechanism which also defines how relativity works?
And so on. A complete explanation would answer all of these questions - and others - with ease. Clearly we're nowhere near that.
> How does a particle keep track of how many other entities it's entangled with and in what ways? Where does that information live? [...] A complete explanation would answer all of these questions - and others - with ease.
I disagree with this framing. Fundamental laws of physics are always going to have unanswered questions of this type. Given any set of rules you can ask "But what explains those rules? What are they built out of? How are they enforced?". Sometimes those questions will have answers and lead you deeper, but for truly foundational laws you'll be wasting your time. It'd be like asking "Where is the true platonic number 2 located? Is it in Canada?".
You can ask the same questions of classical mechanics, of course. We tend not to because it agrees with our intuitions, but you can. For example, where would a classical particle store its velocity? For that matter, where would it store its position? It's circular to say it stores its position where it is! Clearly a "true" theory of classical mechanics would answer these very important questions.
Concretely, there are a variety of ways of writing programs that act like quantum mechanics, that differ wildly on how the state is represented. This detail is simply not pinned down by the postulates of QM. That being said, what all these programs do have in common is that they are actually tracking information related the state; that the state is ontic.
So pick your favorite state representation: state vector, density matrices, Feynman paths, whatever, they all work! That doesn't mean they're describing things that aren't real, it just means there's many ways of correctly describing reality; such convenience!
I should probably add that I know everything I've said isn't convincing to a skeptic. Ultimately it comes down to: I know I can think of the quantum state as being really real, and that will work totally fine. I find that style of thinking is effective for me, and intuitively compelling, so I do it.
Every once in awhile I'll run into someone pointing out the philosophically fraught underpinnings of assuming reality is real or whatever, and I basically won't care because my goal is to be effective; not to be Descartes-level-certain about everything.
An example of something that would make me care is if QBism contained some key conceptual trick that made problems easier, and gradually many papers started using it because of this advantage. Or, of course, if there was an experiment distinguishing between interpretations.
I think Strilanc’s second point is that apparently according to QBism two agents may have different quantum descriptions for a system and we cannot say that one is more correct than the other because the theory rejects that there is a “correct” description.
Superpositions are statistical distributions. Only statistics are wholly predictable. Individual events are wholly unpredictable.
You can have two statistical views of the same experiment in which each individual event is different but the statistics agree.
You can have a distribution of views in which events are somewhat different to varying degrees and the statistics still agree.
Each view will experience itself as unique in specifics while agreeing that the distribution of events is identical.
That doesn't solve the problem.
It's hard to get random processes to define an identical distribution without having classical mechanics as a foundation.
Basic QM hand-waves this away with "Here's some math that defines the distribution and..." But that's like "Starting with this universe we can..."
It so happens that the statistics correlate spatially with various field potentials. Which is interesting, but... Why is there a distribution at all? Why are there creation/destruction operators? Where does the "code" that defines this mechanism live? How do particles know they should follow it?
Do particles even exist, or are they just something random and constrained a field does every so often? How do fields know the physical configuration of an experiment has changed and sometimes that information appears to travel FTL? (Even though it can't be used for signalling.)
How does a particle keep track of how many other entities it's entangled with and in what ways? Where does that information live? (Bell suggests it's not inside the particle itself. So where is it?)
Is all of this shaped by some kind of hidden causal propagation mechanism which also defines how relativity works?
And so on. A complete explanation would answer all of these questions - and others - with ease. Clearly we're nowhere near that.