Action learning: score whole trajectories with no forces or simulator, 2000× lower error

LeoTZ03 · x · 2026-10-24

Sarah Liaw, with Yilun Du and Yuanqi Du, introduces action learning — learning a stochastic action that scores how plausible an entire trajectory is, directly from observed trajectories, with no known forces, potential energy function, or simulator likelihood. Results: over 2,000× lower error than local regression recovering dynamics in a double well system; detects a hidden 0.01% perturbation in the Sun's mass from a 31-body orbital system; a single learned molecular action reuses across trajectory completion, steering, constrained optimization, and temperature transfer. Framed as a new paradigm for reasoning about trajectory data.

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