The thesis deals with the issue of the classical limit of quantum mechanics. In particular, the problem is analyzed and discussed in the framework of the de Broglie-Bohm pilot wave interprétation of quantum mechanics, also known as Bohmian mechanics. The classical limit is the problem of deriving Newtonian mechanics from quantum mechanics. Newtonian mechanics has a clear ontology: individual particles moving along definite trajectories in space through time. Given the interpretational puzzles of quantum mechanics, this problem becomes extremely difficult to solve in the standard framework. The main motivation for dealing with Bohmian mechanics thus relies on the fact that this is a quantum theory with a clear interprétation: a général N-particle system is represented by a wave function and an actual configuration of N point-particles. The ontology of Bohmian mechanics is therefore represented by a time-dependent wave function and individual particles moving along definite trajectories in space through time. Thus, Bohmian mechanics shares a common ontology with Newtonian mechanics.
The classical limit problem, in the Bohmian framework, amounts to showing that:
1. The wave function "disappears" (i.e., is not manifest) in the classical limit;
2. The Bohmian trajectories of macroscopic objects reduce to Newtonian trajectories. Regarding the first point, the thesis shows that the wave function does not become manifest in the classical limit, due to the effective factorization of the wave function. It is pointed out that the interaction and entanglement with the environment (decoherence) plays a fundamental rôle in this transition, leading to the emergence autonomous subsystems described by well- localized effective wave functions. Regarding the second point, the thesis offers a comparison and an évaluation of several stratégies that have been proposed in the literature for deriving approximately Newtonian trajectories starting from the Bohmian ones.
The last part of the thesis concerns the ontology of the wave function in Bohmian mechanics. In particular, it is presented a metaphysical investigation on the meaning of the effective wave function, which is the relevant part of the universal wave function in the classical limit. The major metaphysical stances are discussed, and it is finally suggested that the wave function might be regarded as a new type of field (a multi-field) in 3D space.