The Urysohn Ladder: Recursive Metric Contraction for Scalable Continual Learning
arXiv:2512.18471v2 Announce Type: replace Abstract: Continual learning systems face a fundamental geometric obstacle: as experience accumulates on a fixed-capacity manifold, covering numbers grow linearly with time, eventually forcing representational overlap and catastrophic interference. Prevailing approaches attack this problem by expansion - projecting into higher-dimensional spaces via kernels, overparameterization, or replay. We argue the solution is the opposite: contraction. We formalize abstraction as the Urysohn Ladder, a hierarchy of quotient maps that recursively collapse validated metric neighborhoods into compact tokens, converting unbounded ambient-space search into bounded navigation on a low-dimensional intrinsic scaffold. Geometrically, each collapsed token acts as a shortcut - a region of extreme metric contraction that bridges distant experiences, much like a wormhole in the representational manifold. We establish four results that collectively guarantee separability (metric contraction renders nonlinearly entangled structure linearly separable at each quotient level, and this separability propagates faithfully through the entire hierarchy), bounded capacity (covering numbers remain $O(1)$ per quotient level, independent of stream length), stability (parity-partitioned flow/scaffold subspaces enable unbounded plasticity without catastrophic interference), and scalability (inference cost scales with quotient distance, not ambient distance). We validate each claim empirically with pretrained models and real-world datasets. Moreover, we demonstrate the potential of Urysohn Ladder for scalable continual learning via scaffold amortization.