I’m not sure if we can say much about a classical universe “in practice” because in practice we do not live in a classical universe. I imagine you could have perfect information if you looked at some simple classical universe from the outside.
For classical universes with complete information you have Newtonian dynamics. For classical universes with incomplete information about the state you can still use Newtonian dynamics but represent the state of the system with a probability distribution. This ultimately leads to (classical) statistical mechanics. For universes with incomplete information about the state and about its evolution (“category 3a” in the paper) you get quantum theory.
[Important caveat about classical statistical mechanics: it turns out to be a problem to formulate it without assuming some sort of granularity of phase space, which quantum theory provides. So it’s all pretty intertwined.]
I’m not sure if we can say much about a classical universe “in practice” because in practice we do not live in a classical universe. I imagine you could have perfect information if you looked at some simple classical universe from the outside.
For classical universes with complete information you have Newtonian dynamics. For classical universes with incomplete information about the state you can still use Newtonian dynamics but represent the state of the system with a probability distribution. This ultimately leads to (classical) statistical mechanics. For universes with incomplete information about the state and about its evolution (“category 3a” in the paper) you get quantum theory.
[Important caveat about classical statistical mechanics: it turns out to be a problem to formulate it without assuming some sort of granularity of phase space, which quantum theory provides. So it’s all pretty intertwined.]