A straight beam of light that isn't spinning can still force a microscopic particle to rotate.
April 23, 2026
Original Paper
Tunable Optical Torque by Asymmetry-Induced Spin-Hall Effect in Tightly Focused Spinless Gaussian Beams
arXiv · 2604.18514
The Takeaway
Gaussian beams are the most common type of laser light and usually have zero spin momentum. Breaking the symmetry of how this light is focused triggers a hidden Spin-Hall Effect that generates torque. This torque is strong enough to physically spin a microparticle without using specialized twisted light. Researchers previously believed that rotation required light with an inherent orbital or spin angular momentum. This discovery makes it possible to build simpler optical motors and micromachines for drug delivery inside the human body.
From the abstract
A linearly polarized Gaussian beam, carrying zero net spin angular momentum, is conventionally not expected to exert optical torque or induce rotational motion in birefringent microparticles. When such a beam is tightly focused, the constituent left- and right-circular polarization components separate spatially due to spin-orbit interaction, commonly known as the spin Hall effect of light. However, this separation is at wavelength scales and is also axially symmetric, resulting in zero net spin