**The Classical Limit of Quantum Mechanics: One World, Many Routes
**Davide Romano (LanCog, University of Lisbon)

19 February 2021, 16:00 | Online, via Zoom

**Abstract:** This talk is a presentation of my FCT research project: *The Emergence of the Classical World From Quantum Mechanics*, hosted by the Centre of Philosophy at the University of Lisbon. The project deals with the problem of the classical limit of quantum mechanics, which can be framed as follows: why does a collection of quantum systems (elementary particles, atoms, molecules) in ordinary macroscopic conditions generally form a classical object (a table, a chair, a human being)? Why does this transition happen, and under which conditions? Nowadays, the standard answer to this problem is given by decoherence theory: when a quantum system interacts with an external environment, it loses some of its characteristic quantum effects (it “decoheres”) and, when measured, it looks like a classical object. Nevertheless, there is no consensus in the literature on what has been really achieved by decoherence and the role it plays in the different interpretations of quantum mechanics. The project will investigate these issues, seeking to provide a comprehensive and ontologically clear account of the classical limit of quantum mechanics.

The talk will be divided into two parts. In the first part, I will present the merits and limits of decoherence. I will show that, despite the experimental success of decoherence, this theory (in the standard context) is unable to provide an explanation of the quantum to classical transition that goes beyond a pure instrumentalist approach. In the second part, I will briefly discuss the role and significance of decoherence theory in three different interpretations of quantum mechanics: the de Broglie–Bohm theory, the Ghirardi-Rimini-Weber (GRW) theory and the Everett/Many Worlds Interpretation. For each of these interpretations, indeed, not only can the problem of the classical limit be framed differently, but even decoherence itself plays either a different role or no role at all. Even though no definite results will be provided, I will suggest what further steps should be taken in order to develop a clear account of the classical limit.

**Free Attendance, but preregistration required:** https://cful.letras.ulisboa.pt/lancog/registration/