Quantum mechanics is protean, in the sense that you can keep changing the representation, in fact at every time you can change the representation, and the representation has a huge freedom in change of representation, in change of basis. Besides that, even if you're given the basis at every single time, there's a huge freedom of coarse-graining. So the number of possible realms is gigantic, and yet we seem to use almost always a realm that can be called hydrodynamic. It’s described by ranges of values of operators, which are integrals of conserved or almost conserved densities over small volumes of space, and spaced at small intervals of time. And the volumes of space are chosen to be large enough for some kind of internal equilibrium to occur, but small enough to, well, to allow the realm to be maximal; that would be one way to say it.

here is a Chinese professor who seems to understand well

当然，不是每一位量子力学博士教授都能回答如下三个问题(可能有点咬文嚼字)

4. 在某一状态上的动量分布可以取任意大的数值，是否意味作粒子的速度会超光速?

5. 定态波函数是局域的(localized)吗?

6. 定态波函数是非局域的(nonlocal)吗?