Probably not overly controversial, but still a conjecture. That is, nobody has shown how exactly classical physics emerges from quantum mechanics (specifically,

Probably not overly controversial, but still a conjecture. That is, nobody has shown how exactly classical physics emerges from quantum mechanics (specifically, the measurement problem). Note that this is a separate and more fundamental issue from how simple quantum systems under certain conditions can show quasi-classical behavior, e.g. the "coherent state" of a quantum oscillator, or the common "let h-bar go to zero"