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v3.3.3
Accurate modelling of relativistic fluids under extreme conditions—such as those encountered in heavy ion collisions and astrophysical phenomena like neutron star mergers—requires a theoretical framework that extends beyond the limitations of first- and second-order dissipative fluid dynamics. While second-order theories, particularly the Israel-Stewart formalism, represent a critical improvement over first-order approaches by restoring causality and stability, they may fall short in far-from-equilibrium regimes where rapid expansion, large gradients, and strong coupling effects dominate.
This talk motivates the necessity of pursuing fluid dynamic theories beyond second order. We highlight three key reasons: (i) second-order approaches serve as the minimal bridge between classical and relativistic regimes, (ii) higher-order corrections depend on lower-order terms and may impose additional constraints on equilibrium behaviour, and (iii) essential couplings between dissipative processes—such as heat flow, shear, and bulk viscosities—are only fully realized at third order and beyond.
We will explore recent theoretical advancements, including third-order frameworks derived from kinetic theory and the entropy principle, and discuss their applications in modelling quark-gluon plasma evolution, early-time dynamics in heavy ion collisions, and relativistic transport in astrophysical systems. By addressing these needs, beyond-second-order fluid dynamics offers a more complete and predictive framework for studying complex relativistic systems across energy scales and environments.