Strange properties of quantum systems and possible interpretations

Quantum Mechanics (QM) applies successfully to all phenomena at the atomic, nuclear and subnuclear levels, and all experiments carried out so far have confirmed the validity of the theory. However, various conceptual difficulties are still present and different interpretations have been proposed. In the orthodox formulation, a central role is played by the interaction between the physical system and the observer, that is between the microworld and the macroscopic measuring instruments. In principle, there are two different possibilities: either the domains of validity of QM and classical physics are completely separated, or the properties of the macroscopic objects are reducible to those of their quantum constituents. In the first case, one does not know where to put the boundary between the two domains, while in the second case there is the problem of reconciling the probabilistic character of QM with the deterministic laws of classical physics. After a brief introduction recalling the main principles of QM, the above and related problems will be discussed and different interpretations will be examined.