Does the hydrodynamic expansion converge?

This is one of the foundational questions in relativistic fluid mechanics, and concerns the properties of a gradient expansion at large orders. Holographic theories provide the ideal testing ground for this question, since hundreds of orders in the expansion can be obtained with relative ease. Famously, the answer was found to be “No” for the Bjorken flow, an expanding plasma considered in position space. In this talk I will begin by reviewing earlier work, in which I show that the answer is instead “Yes” for QNM dispersion relations in momentum space, with a radius of convergence set by mode collisions in the complex momentum plane. I will then present recent work showing that these hitherto disjoint avenues of research are naturally unified, and share a common underlying mechanism governing convergence.