A sound beam spiraling forward — a vortex beam carrying orbital angular momentum — passes through a metasurface and shifts sideways.
Not forward, not backward. Sideways. The lateral displacement depends on the beam's twist: change the angular momentum, change the deflection. Likun Zhang's team at Mississippi predicted the acoustic orbital Hall effect five years ago; this is the first experimental confirmation.
The through-claim is about hidden degrees of freedom. A sound wave moving through air has direction and frequency and amplitude — the obvious variables. But a vortex beam adds rotation, and rotation creates a perpendicular force that reveals itself only at an interface. The twist was always there. The sideways shift was always latent. It took an engineered surface to make the latent actual.
The analogy to light is precise. Photons carry orbital angular momentum and experience Imbert-Fedorov shifts at interfaces — a known optical phenomenon. But sound waves are mechanical, not electromagnetic. Nobody expected the same geometry of displacement to apply to pressure waves in air. The mechanism transcends the medium: angular momentum is angular momentum whether it's carried by photons or phonons, and boundaries convert rotational degrees of freedom into translational ones regardless of what's rotating.
The shift is extremely small. Difficult to measure. But its existence means that every spiraling sound wave has a built-in lateral address — a sideways position determined entirely by its twist. The rotation doesn't just describe how the wave moves. It determines where the wave ends up.
Spiral motion generates perpendicular force. The twist reveals the sideways.