Events

The Beth experiment: 90 Years On—CANCELLED

Etienne Brasselet, Research Director, Centre National de la Recherche Scientifique

THIS EVENT HAS BEEN CANCELLED

Abstract

Optical angular momentum—encompassing both spin, associated with polarization, and orbital, linked to spatial phase structure—has become a cornerstone of modern optics. Yet, its mechanical detection traces back to Richard Beth’s 1935-36 experiments [1], which revealed that light can exert a measurable torque on matter. This seminal demonstration, using a torsion pendulum, marked the birth of optical angular momentum optomechanics. Ninety years later, the torsion pendulum remains a powerful and evolving platform to explore angular momentum transfer from light to matter, from classical to quantum regimes, and from spin to orbital degrees of freedom.

This talk will revisit the historical roots of Beth’s experiment, trace the scientific advances it inspired across the electromagnetic spectrum, and highlight recent progress on its orbital counterpart [2]. The story reveals how a simple idea—light twisting matter—continues to shape our understanding of wave-matter interactions.

[1] R. A. Beth, Direct detection of the angular momentum of light, Physical Review 48, 471 (1935); R. A. Beth, Mechanical detection and measurement of the angular momentum of light, Physical Review 50, 115 (1936).

[2] E. Brasselet, Torsion pendulum driven by the angular momentum of light: Beth’s legacy continues, Advanced Photonics 5, 034003 (2023).

[5] Nature Reviews Physics, 1-17 (2025)

Biography

Etienne Brasselet is a Research Director at CNRS and leads the SINGULAR group at the Laboratoire Ondes et Matière d’Aquitaine, University of Bordeaux, in France. His research explores how structured waves interact with structured matter, connecting optics, acoustics, and mechanics through concepts of topology and vector fields.