Thermalization and Entropy Increase in Closed Macroscopic Quantum Systems

Physikalisches Kolloquium

Date:	11/02/2026, 2:15 PM - 4:45 PM
Category:	colloquium
Location:	Hubland Süd, Geb. P1 (Physik), Röntgen-Hörsaal, Online
Organizer:	Fakultät für Physik und Astronomie, Theoretische Physik I, Prof. Dr. Ronny Thomale
Speaker:	Prof. Dr. Roderich Tumulka

Am 2. November 2026 findet um 14:15 Uhr im Röntgen-Hörsaal des Physikalischen Instituts und onlina via Zoom das nächste Physikalische Kolloquium mit einem Vortrag zum Thema „Thermalization and Entropy Increase in Closed Macroscopic Quantum Systems“ von Prof. Dr. Roderich Tumulka (Eberhard-Karls-Universität Tübingen, Fachbereich Mathematik) statt.

Abstract

In the 20th century, it was a common idea that thermal equilibrium and entropy in quantum statistical mechanics would have to refer to mixed states, and thus to ensembles or open systems. In the 21st century, a number of researchers have explored a different approach: that thermodynamic behavior can also occur in a single, closed, macroscopic quantum system in a pure state. This approach is particularly connected with the „eigenstate thermalization hypothesis“. I will discuss this approach and this hypothesis, what is meant here by thermalization, and some recent developments. I will also consider an extension of the concept of Boltzmann entropy (logarithm of the number of micro states consistent with a macro state) to such a system by setting S(nu) = log dim H_nu, where H_nu is the subspace of the system's Hilbert space H comprising the wave functions compatible with macro state nu. Since a general wave function in H is a superposition of contributions from different H_nu's, it is not obvious what could even be meant by saying that this kind of entropy increases with time. I will propose such a meaning and explore the question under which conditions this increase occurs.