Light-Driven Motion in Microgravity: From Radiation Momentum to Equilibrium Re-Finding

Light can induce translation and rotation in microgravity without propellant. In mainstream physics, this effect is described through conservation of linear and angular momentum carried by electromagnetic radiation. While the language of “radiation pressure” is correct at a macroscopic level, it can obscure the underlying physical mechanism. This standalone note reframes light-driven motion as an equilibrium problem: illumination drives the coupled light–matter system into a new dynamical steady state, and the observed force or torque emerges from how the system reconfigures to satisfy conservation laws. The document presents standard solar-sail and optical-torque derivations, clarifies why pulsed illumination can outperform continuous light under equal-energy or equal-average-power constraints, and introduces a USP Field Theory bridge interpretation. In the USP view, illumination re-tunes the system’s frequency-mismatch envelope (Δf), and repeated re-tuning with biased relaxation pathways can accumulate net motion without violating conservation principles.