Causality violations in realistic simulations of relativistic nuclear collisions

¹ Illinois Center for Advanced Studies of the Universe, Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
² Department of Physics, Pepperdine University, Malibu, CA 90263, USA

^* e-mail: christopher.plumberg@pepperdine.edu
^** e-mail: almaalol@illinois.edu
^*** e-mail: tdore2@illinois.edu
^**** e-mail: jn0508@illinois.edu
^† e-mail: jnorhos@illinois.edu

Published online: 1 March 2023

Abstract

We show that relativistic causality is violated in the early stages of state-of-the-art heavy-ion hydrodynamic simulations of nuclear collisions. Up to 75% of the initial fluid cells violate nonlinear causality constraints, while superluminal propagation is observed by up to 15% the speed of light. Only after 2-3 fm/c of evolution, do 50% of the fluid cells become definitely causal. Inclusion of pre-equilibrium evolution significantly reduces the number of acausal cells, but it does not eliminate them. Relativistic causality thus imposes constraints on the model parameter space of heavy-ion collision simulations.