Speaker
Rajeev Singh (Goethe University, Frankfurt)
Title of Talk
Relativistic Fluids Beyond Equilibrium: Frames, Fluctuations, and Spin from Heavy-Ion Collisions to Curved Spacetime
Abstract:
Relativistic hydrodynamics provides a universal macroscopic description of matter whenever microscopic interactions rapidly establish local equilibrium. Its applications range from the quark-gluon plasma created in heavy-ion collisions to relativistic matter evolving in astrophysical and curved-spacetime environments. Despite its broad success, several fundamental questions remain: How should the hydrodynamic variables be defined? Which collective modes represent physical excitations, and which arise from a particular choice of variables? How can thermal fluctuations be incorporated consistently? Can additional degrees of freedom, such as spin polarization, themselves become hydrodynamic?
In this seminar, I will address these questions. I will first discuss hydrodynamic-frame transformations and show how local redefinitions of temperature and fluid velocity leave the genuine long-wavelength modes unchanged while modifying, or even eliminating, certain non-hydrodynamic modes. This leads naturally to the Density Frame, a formulation of relativistic viscous hydrodynamics that is first order in time, contains no auxiliary dynamical fields, and reproduces kinetic-theory evolution accurately within the hydrodynamic regime. I will then describe a stochastic formulation based on the Metropolis algorithm, in which random momentum exchanges between neighbouring fluid cells are accepted according to their entropy change. This provides a direct numerical realization of viscous dynamics and thermal fluctuations while conserving energy and momentum. Its formulation using the 3+1 decomposition of spacetime also makes the connection with general coordinates transparent. Finally, I will discuss how hydrodynamics can be enlarged to include spin polarization as an independent collective degree of freedom. Such an extension predicts propagating spin modes and provides a framework for studying the polarization of particles produced in relativistic nuclear collisions. Together, these developments illustrate how symmetry, conservation laws, statistical mechanics, and effective field theory combine to describe relativistic matter across widely different physical scales..
Venue
Physikalisches Institut, INF 226
Goldene Box
Schedule:
Talk: 16:30 – 17:15
Questions & Discussion: 17:15 – 17:30
Pizza & Drinks: 17:30
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