On the Relativistic Origin of spin: A Case for the "Rest Angular Momentum"

TL;DR

This paper argues that spin is fundamentally a relativistic property arising from spacetime symmetry, specifically the Poincaré group, rather than solely a quantum mechanical phenomenon, emphasizing the concept of 'rest angular momentum.'

Contribution

It provides a relativistic perspective on the origin of spin, linking it to spacetime symmetry and the Poincaré group, offering a new conceptual understanding.

Findings

Spin emerges from relativistic wave equations and Thomas precession.

Photons' spin is formulated within electrodynamics as a relativistic property.

Spin is inherently a consequence of spacetime symmetry, not just quantum mechanics.

Abstract

The intrinsic angular momentum, or spin, is a cornerstone of modern physics with profound applications from nuclear magnetic resonance to spintronics. While its mathematical structure within quantum theory is well-defined, its fundamental origin is often less emphasized. This paper revisits the genesis of spin by examining its emergence in relativistic wave equations, its role in the Thomas precession, and its formulation for massless photons in electrodynamics. It is argued that these foundational elements collectively demonstrate that spin is inherently a consequence of relativistic spacetime symmetry, rather than a purely quantum mechanical property. Consequently, the term "rest angular momentum" offers a more conceptually accurate description, highlighting its origin as an intrinsic property manifest even in an object's rest frame, as dictated by the Poincar\'e group.