# Optical Current, Momentum and Angular Momentum in Anisotropic Materials exposed to Detailed Balance

## TL;DR

This paper explores the theoretical behavior of radiative momentum and angular momentum in anisotropic materials under isotropic noise, revealing unexpected effects like persistent momentum and circulating Poynting vectors predicted by classical Maxwell equations.

## Contribution

It demonstrates that bi-anisotropic materials exhibit finite radiative momentum and angular momentum effects under isotropic noise, connecting classical theory with recent quantum electrodynamics results.

## Key findings

- Persistent radiative momentum in bi-anisotropic materials
- Induction of angular momentum via Faraday rotation
- Circulating Poynting vector around matter

## Abstract

This work investigates the theory behind a thought experiment that intends to measure momentum and angular momentum of matter exposed to `isotropic radiative noise'. Radiative momentum has been a controversial subject for decades. The momentum of isotropic noise is intuitively expected to be zero. We formulate the general features of the isotropic noise such as equipartition of energy and the vanishing of integrated Poynting vector. We demonstrate that in bi-anisotropic materials, a finite radiative momentum persists that performs work on the matter when its parameters change slowly. We find that Faraday rotation in the scattering of the radiative noise induces an angular momentum along the applied external magnetic field. Also, a Poynting vector starts circulating around the matter, raising the question whether it really describes the energy flow. These effects are small and hard to measure in any real experiment. Yet, they are surprising predictions of the classical, macroscopic Maxwell equations, and make contact with the outcome of recent QED calculations done for the quantum vacuum.