Does Physics Live in the Activations? Localizing Physical Quantities in Video Diffusion Models

Video generation models produce strikingly realistic sequences and are increasingly proposed as world models, yet recent benchmarks reveal pronounced deficits in their physical reasoning. This raises the question of whether these models internalize physical principles or merely reproduce familiar motion patterns. We address this by probing internal representations of video DiTs for simulator-derived ground-truth physical quantities spanning kinematic motion and rigid-body dynamics under gravity and contact. We find that these quantities are linearly decodable with high accuracy early in the denoising process, substantially outperforming a baseline decoded directly from the model's own noised latents—indicating that the relevant physical information is actively constructed during denoising, rather than already present in the input. Additionally, we show that activations at on-object tokens carry the relevant information and that quantities defined over multiple frames are readable from single latent-frames. Hence, information is sharply localized within the token sequence and is computed globally but stored locally. The probes further show partial extrapolation, transferring unchanged to scene variations and object configurations they were never fit on, so what they read is not simply a correlate of the training scenes. When fitted directly in the raw activation space, the probing directions can serve as steering vectors to change the model's output.