Abstract

The observation, in hadronic collisions, of "ideal fluid" type
behavior in systems of a comparatively small number of particles,
presents a conceptual puzzle, since the way we usually derive
hydrodynamics is via approximating "many" particles as a continuum.
I will argue that making sense of this requires re-deriving
relativistic hydrodynamics as a "bottom-up" theory, with no reference
to microscopic physics except the local emergence of a thermalized
system.
We attempt to do this using basic statistical mechanics, and find the
appearance of a gauge-like redundancy hidden within relativistic
dissipative hydrodynamics, arising from the fluctuation-dissipation
theorem.
This might lead to the
apparently counter-intuitive conclusion that in the small viscosity
limit it might indeed be that smaller systems could thermalize faster.

Based on https://arxiv.org/abs/2109.06389
and
https://arxiv.org/abs/2007.09224