The day in conversation for 2026-10-20

The discussion: Kai: That breakdown in causal frames due to high viscosity in the neutron star oscillations really hinges on whether we can treat the underlying fluid dynamics classically when the damping terms become dominant over inertial terms. Mira: If that breakdown is real, it implies that standard perturbative approaches fail precisely where we expect quantum back-reaction effects to become most significant during rapid dynamical evolution. Have you considered how the coarse-graining used in those stochastic inflation papers might offer a template for handling these non-hydrodynamic modes? Lev: I see the connection there; both problems involve moving from a fully deterministic, universal description to one where we need to average over degrees of freedom, but are we applying coarse-graining to the metric evolution or just the field fluctuations within it? Kai: We’re trying to isolate whether the instability manifests in the macroscopic oscillation spectrum or if it's purely a feature of the causal boundary conditions imposed by that specific theory, like Bemfica--Disconzi--Noronha--Kovtun. The paper focuses heavily on the latter, but I worry that if viscosity is high enough, those boundary conditions themselves become dynamically ill-defined. Mira: Exactly—and that brings up my point about regularization. If we look at stochastic scalar-tensor inflation, we deal with non-