The Causal Archipelago: Communication and Coexistence in a Light-Bounded Universe

For three decades, cosmic silence has been interpreted through Hanson's Great Filter and Bostrom's existential hazard hypothesis as an empirical warning of biological rarity or civilizational self-destruction. This deduction rests on an unexamined premise: treating civilizational longevity as synonymous with astronomical visibility. Recent work in theoretical astrobiology (incorporating percolation, steady-state settlement, and ecological thermodynamics) has established that interstellar expansion is non-homogeneous, leaving large uncolonized voids and favouring homeostatic equilibria over exponential growth. Building upon this synthesis, we formalize the physical-optical, relativistic, and epistemological constraints that enforce observational isolation, a framework termed the Causal Archipelago. Directed optical signalling requires prior coordinates because diffraction collimation partitions the sky into ∼10¹⁴ pointing cells, yielding a blind interception probability of ∼10⁻¹⁵. Within the galactic plane, star-planet contrast (C_opt ∼ 10⁻¹⁰) and dust extinction restrict atmospheric characterization to a spectroscopic horizon of d_obs ≤ 1,000 ly, obscuring >99.97% of the Milky Way. Combined with light latency (3d/c) and duty cycles (η ∼ 10⁻⁶), deliberate exchange fails across galactic distances for realistic lifespans (L ∼ 10³–10⁴ yr). Extending this analysis extragalactically via M/G/∞ queueing theory, we show that the Bayes factor relating silence to civilizational collapse is unity (Λ ≈ 1.0). Placing these physical bounds alongside Weinberg's clearing of teleological expectations reframes cosmic silence: far from signalling an empty universe or catastrophe, technological civilizations persist as an archipelago of autonomous, self-sufficient local worlds.