Hartle, James B. and Murray Gell-Mann
A striking feature of our fundamentally indeterministic quantum universe is its quasiclassical realm — the wide range of time, place and scale in which the deterministic laws of classical physics hold. Our quasiclassical realm is an emergent feature of the fundamental theories of our universe’s quantum state and dynamics. There are many types of quasiclassical realms our Universe could exhibit characterized by different variables, different levels of coarse-graining, different locations in spacetime, different classical physics, and different levels of classicality.We propose a measure of classicality for quasiclassical realms, We speculate on the observable consequences of different levels of classicality especially for information gathering and utilizing systems (IGUSes) such ourselves as observers of the Universe.