EgoLite-G: Restoring the Physical Gauge in Egocentric Token World Models

Token-based egocentric world models canonicalize every modality in its own first-frame camera frame. We show that this design silently removes the physical gauge—the gravity direction and the height of the wearer above the support surface—on which world-frame body pose depends. We introduce EgoLite-G, a gauge-state egocentric world model that keeps the gauge as an explicit, persistent state: gravity from the head-worn IMU and a take-level support height estimated from the head trajectory alone. The state enters a masked-token world model through zero-initialized gauge embeddings and is trained with privileged, noise-perturbed heights so that the same network is deployed with legal estimates. On the official EE4D-Motion validation set (99 unseen participants, 44,046 windows, scored once after all predictions were frozen), EgoLite-G lowers world MPJPE to 130.4 mm: 17.2 mm below a budget-matched fine-tuned ReViV, 19.3 mm below the strongest pre-registered baseline, and 5.6 mm below EgoAllo given the very same floor estimate, while matching EgoAllo supplied with a label-derived floor. Controlled studies show that the gain is caused by the gauge itself: shuffling gravity at inference costs 15.9 mm, and body error decreases monotonically as the gauge becomes more accurate.