State of the art latent dynamics world models achieve impressive performance for robot planning. But do they actually encode the physics of the world? In this paper, we show that popular world models today achieve strong results on tasks that reduce to image similarity but forget key details about objects in the scene like mass, deformation, friction, and bounciness. We achieve this by defining precisely what ought to be encoded in such a world model and introducing two new evaluation methods that can directly tease physical properties and easily scale to new ones. Our evaluation, which we call Roz, includes 10 contrastive tasks and 3 continuous tasks, totaling 1734 individual scenarios. We validate our eval on control tasks. On our own model, AnonWM, we find that performance roughly improves with training steps, while not necessarily being correlated with reconstruction MSE. We argue current model failures are a symptom of evaluation strategies (probes and planners) that are opaque, model-specific, and high-compute. Roz requires only a model that can ingest still frames or video, a way to extract latents, and a distance function between them, and it can be run in one pass. We hope future evaluations will adopt Roz's evaluation techniques. More broadly, we hope Roz can be a case study for a future where benchmarks allow rapid iteration and comparison across models.

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