Acknowledgements

Eugenio Carminati, Carlo Doglioni, Emiliano Di Luzio, Roberto Gambini, Carlo Nicolai, Simone Sciamanna, and Mario Tozzi are gratefully acknowledged since they significantly contributed to shape the ideas presented in this review. Etta Patacca Paolo Scandone and Vittorio Zucconi are also thanked for fruitful discussion.