On the Determinant Problem for the Relativistic Boltzmann Equation

TL;DR

This paper computes the Jacobian determinant for the relativistic Boltzmann collision map, providing bounds and examples of where it vanishes, highlighting challenges in extending Newtonian cancellation techniques to the relativistic setting.

Contribution

It explicitly calculates the complex Jacobian determinant for the relativistic collision map and analyzes its properties, revealing difficulties in applying Newtonian methods.

Findings

The Jacobian determinant can approach zero at specific points.

An upper bound for the Jacobian without singularities was established.

Numerical evidence shows the Jacobian has many points where it is effectively zero.

Abstract

This article considers a long-outstanding open question regarding the Jacobian determinant for the relativistic Boltzmann equation in the center-of-momentum coordinates. For the Newtonian Boltzmann equation, the center-of-momentum coordinates have played a large role in the study of the Newtonian non-cutoff Boltzmann equation, in particular, we mention the widely used cancellation lemma by Alexandre et al. (Arch. Ration. Mech. Anal. 152(4):327-355, 2000). In this article we calculate specifically the very complicated Jacobian determinant, in ten variables, for the relativistic collision map from the momentum $p$ to the post-collisional momentum $p'$; specifically we calculate the determinant for $p\mapsto u = \theta p'+\left(1-\theta\right)p$ for $\theta \in [0,1]$. Afterwards we give an upper-bound for this determinant that has no singularity in both $p$ and $q$ variables. Next we give an example where we prove that the Jacobian goes to zero in a specific pointwise limit. We further explain the results of our numerical study which shows that the Jacobian determinant has a very large number of distinct points at which it is machine zero. This generalizes the work of Glassey-Strauss (Transport Theory Statist. Phys. 20(1):55-68, 1991) and Guo-Strain (Comm. Math. Phys. 310(3):649-673, 2012). These conclusions make it difficult to envision a direct relativistic analog of the Newtonian cancellation lemma in the center-of-momentum coordinates.