The Kaprekar Horizon: How the Digits 0–9 Force Reality to Halt or Loop

Kaprekar's constant 6174 is routinely taught as a mathematical curiosity. We show it is something far deeper: an empirical signature of structural completion in a finite alphabet. When a deterministic operation iterates over a bounded digit set {0,1,…,9}, the pigeonhole principle guarantees that trajectories cannot diverge infinitely. They must either converge to a fixed point or enter a limit cycle. We demonstrate this across all digit lengths: 3-digit numbers halt at 495, 4-digit numbers halt at 6174, and 5+ digits enter stable oscillations. Using polar phase portraits, we visualize the topological transition from spiraling attractor basins to orbital rings. We further prove that only contractive operations (subtraction, division) yield finite structure, while expansive operations (addition, multiplication) diverge or overflow. A dedicated division test confirms that discrete integer dynamics cannot produce transcendentals (π, e); they quantize into integer attractor basins. Kaprekar dynamics are not a number theory parlor trick. They are laboratory proof that a finite alphabet forces reality to halt or loop. Infinity is mathematically impossible.