Discussion group on 6 June 2019, Ingram Building, Rm. 110

I will talk about new methods to compute the shear and the bulk viscosity in strongly interacting quantum gases. These quantities determine the damping and dissipation of the collective dynamics of a quantum gas (such as the collective oscillations or the expansion from a trap). There is a significant effort to measure the viscosities since quantum gases are said to form a “perfect fluid’’, in which the viscosities are anomalously small (the only other example of such a fluid is the quark-gluon plasma at a vastly different energy scale). In particular, the shear viscosity comes close to saturating a shear viscosity-to-entropy bound from string theory. I will discuss why the theoretical calculations of the viscosity is very intricate and present a way to overcome this at high temperatures by means of a cluster expansion that is represented in terms of Feynman diagrams. For the shear viscosity, I outline how a connection to results from a Boltzmann kinetic transport equation is made, and I comment on a new result for the bulk viscosity that differs from kinetic theory.