Minkowski and Galilei/Newton Fluid Dynamics: A Geometric 3+1 Spacetime Perspective

TL;DR

This paper develops a unified geometric 3+1 spacetime framework for fluid dynamics that encompasses both Minkowski and Galilei/Newton spacetimes, emphasizing the role of inertia over energy.

Contribution

It introduces a geometric 3+1 spacetime perspective on fluid dynamics that unifies relativistic and non-relativistic cases using a common kinetic theory approach.

Findings

Reinterprets the stress-energy tensor as a stress-inertia tensor.

Derives conservative fluid equations in geometric form applicable to both spacetimes.

Provides a conceptual bridge to general relativistic hydrodynamics.

Abstract

A kinetic theory of classical particles serves as a unified basis for developing a geometric $3+1$ spacetime perspective on fluid dynamics capable of embracing both Minkowski and Galilei/Newton spacetimes. Parallel treatment of these cases on as common a footing as possible reveals that the particle four-momentum is better regarded as comprising momentum and \textit{inertia} rather than momentum and energy; and consequently, that the object now known as the stress-energy or energy-momentum tensor is more properly understood as a stress-\textit{inertia} or \textit{inertia}-momentum tensor. In dealing with both fiducial and comoving frames as fluid dynamics requires, tensor decompositions in terms of the four-velocities of observers associated with these frames render use of coordinate-free geometric notation not only fully viable, but conceptually simplifying. A particle number four-vector, three-momentum $(1,1)$ tensor, and kinetic energy four-vector characterize a simple fluid and satisfy balance equations involving spacetime divergences on both Minkowski and Galilei/Newton spacetimes. Reduced to a fully $3+1$ form, these equations yield the familiar conservative formulations of special relativistic and non-relativistic hydrodynamics as partial differential equations in inertial coordinates, and in geometric form will provide a useful conceptual bridge to arbitrary-Lagrange-Euler and general relativistic formulations.