Phonons and Forces: Momentum versus Pseudomomentum in Moving Fluids

TL;DR

This paper explains the concept of pseudomomentum in wave physics, especially in moving fluids, and demonstrates how it can be used to understand forces and conservation laws using relativistic formalism.

Contribution

It provides a pedagogical framework linking pseudomomentum, forces, and conservation laws in fluid dynamics through relativistic analogies.

Findings

Pseudomomentum can be used to compute real forces in wave-medium interactions.

Relativistic formalism simplifies the derivation of conservation laws in moving fluids.

Application to sound waves illustrates the practical relevance of pseudomomentum concepts.

Abstract

I provide a pedagogical introduction to the notion of pseudomomentum for waves in a medium, and show how changes in pseudomomentum may sometimes be used to compute real forces. I then explain how these ideas apply to sound waves in a fluid. When the background fluid is in motion, the conservation laws for pseudomomentum and pseudoenergy are most easily obtained by exploiting the acoustic metric and the formalism of general relativity.