We discuss the first steps for solving what may be called great twin puzzles in physics today: the so-called quantum measurement problems on the one hand, and the emergence of the classical mechanics in quantum mechanics, on the other. These two classes of problems are often mixed up in the literature, making the whole questions look more mysterious and confusing than necessary. Actually they are - mostly - distinct and independent issues, better to be considered separately. At the same time, however, there are a couple of aspects linking the two classes of the problems, in a precise way.
First we discuss the solutions of the quantum measurement problems from a new perspective, based on two key ideas. The particle nature of the basic building blocks of our world, and the particularly subtle way factorization and entanglement interplay in the physical measurement processes, are the keys for eliminating the difficulties associated with Born’s rule. From our new perspective, the quantum fluctuations described by the wave function are real: they are there, independently of any experiments, and of human presence.
In the second part of the talk, Newton’s law is derived from quantum mechanics, for a macroscopic body in the vacuum, at finite temperatures. We first review three issues (Heisenberg’s uncertainty relations, absence of the diffusion, and decoherence): together, they explain why the CM of a macroscopic body at finite T has a well-defined classical trajectory. We then apply Ehrenfest’s theorem to derive Newton’s equation for it, under the gravitational forces, a harmonic potential, and a weak external electromagnetic fields.
Question of the boundaries between quantum and classical is briefly discussed and some final reflections will conclude the talk.