The Speed of Light Is Not Fundamental: Signal Limits from Causal Stability Alone

A universal limit on signal propagation underpins modern physics, yet its origin is typically assumed rather than explained. In this work, we demonstrate that a strict upper bound on signal speed can emerge without presupposing spacetime, geometry, time, causality, or physical law. Using a fully pre-geometric, non-metric causal network subjected to continuous entropy injection, we show that stable causal organization necessarily enforces a finite propagation limit. Signals exceeding this threshold destabilize causal coherence and are selectively eliminated, while slower modes persist. The resulting constraint is invariant across perturbations and arises without external enforcement. These findings suggest that finite signal speed is not a fundamental axiom of nature, but an inevitable consequence of causal survivability, with spacetime structure emerging secondarily from deeper causal constraints.