### Irreversibility Revisited; my way

As far as we know, the microscopic components of matter obey dynamical laws that are time reversal invariant. In other words, for every initial condition that leads to a certain evolution forward in time, such as a positive current, there corresponds an initial condition that leads to the opposite evolution, again forward in time. How comes that macroscopic objects appear to behave in a clear irreversible fashion, as expressed by the second law of thermodynamics? This question has been in the physicists minds since the early days of statistical physics, and it has received various answers, that often contradict each other. Some would argue that, in fact, the microscopic dynamics are not reversible; others would reply that this is not necessary for macroscopic irreversibility, hence one should not rely on such microscopic symmetry effects. This leads then to the so called "past hypothesis" and to the need of special initial conditions for our Universe. But, what is irreversibility? How is it connected to entropy? How does it enter a physics reasoning? Which predictions does it lead to? I will summarize the main points of view and I will conclude with personal observations, based on recent developments concerning open systems, and on a principle stated by Feynman, that may require a new perspective on all these issues.

**Friday, 14 June 2019, ore 14:30 — Aula Magna "Tullio Regge" **