Gauge Symmetry as the Origin of Spin-Orbit Coupling of Light

Abstract

Dual symmetry is an intrinsic property of Maxwell's equations, corresponding to a global U(1) symmetry in vacuum, with helicity as the associated conserved quantity. In this paper, we investigate light propagation in a spin-degenerate medium using a field-theoretical approach and introduce an effective gauge field A_s that emerges from the localization of dual symmetry. Within the geometric optics approximation, we show that the helical trajectory of light rays reveals this gauge field as a manifestation of spin-orbit coupling. Although orbital-orbit coupling also arises in such systems, the spin-orbit interaction possesses deeper physical significance, as it originates from the intrinsic dual symmetry embedded in Maxwell's equations.