[wpietri]

I so appreciate this explanation, as I have wondered about this for years. Thanks!

My understanding of (real) probabilities is that they're really about our ignorance of a system. E.g., if I'm flipping a coin but put very little spin on it, it's easy for a human to predict what side will show. As we put more spin on it, the probability of guessing correctly goes down, but that's about perceptual and cognitive limitations.

My (layman's) understanding of quantum mechanics, though, is that there are no hidden variables. In which case, I couldn't understand why anybody would call the quantum stuff "probability". It's a relief to know that they're fundamentally different.

[ccvannorman]

I read once that you can look at QM without randomness by comparing it to interference waves, similar to what happens if you sprinkle water on the surface of a pond -- the result looks chaotic but it is 100% deterministic, and QM can be viewed in this way too.