When Quantum Talks Classical: a dialogue between quantum mechanics and gravity via black holes

This thesis is devoted to show how classical physics can naturally emerge from Quantum Mechanics (QM) as a macroscopic limit. In particular we make a dialogue between QM and gravity using Black Holes (BH) as paradigmatic examples. The quantum algebraic formalism is related to the classical geometrical one by generalized coherent states in order to introduce a formal procedure, the Large-N method, to represent the route from micro-to-macro as a quantum-to-classical crossover. The method is used to tackle the so-called problem of time, related to the different meaning given by QM and classical physics to the time parameter. We then point out how gravity seems to suggest BH as ideal clocks to solve the problem and identify such enigmatic bodies as specific macroscopic quantum systems. We finally show how this identification allows to describe some of the quantum properties related to BH evaporation, considering the "information paradox".