Field-theory spin and momentum in water waves

TL;DR

This paper demonstrates both theoretically and experimentally that the fundamental concepts of spin and momentum from relativistic field theory naturally manifest in water surface waves, revealing their microscopic mechanical origins.

Contribution

It provides the first direct experimental observation linking field-theory spin and momentum concepts to classical water waves, illustrating their universality and physical reality.

Findings

Belinfante-Rosenfeld construction applies to water waves

Canonical momentum relates to Stokes drift

Spin arises from subwavelength circular water particle motion

Abstract

Spin is a fundamental yet nontrivial intrinsic angular-momentum property of quantum particles or fields, which appears within relativistic field theory. The spin density in wave fields is described by the theoretical Belinfante-Rosenfeld construction based on the difference between the canonical and kinetic momentum densities. These quantities are usually considered as abstract and non-observable per se. Here, we demonstrate, both theoretically and experimentally, that the Belinfante-Rosenfeld construction naturally arises in gravity (water surface) waves. There, the canonical momentum is associated with the generalized Stokes-drift phenomenon, while the spin is generated by subwavelength circular motion of water particles. Thus, we directly observe these fundamental field-theory properties as microscopic mechanical properties of a classical wave system. Our findings shed light onto the nature of spin and momentum in wave fields, demonstrate the universality of relativistic field-theory concepts, and offer a new platform for their studies.