PNAS paper shows a tiny billiard-ball system is a universal computer — undecidability lives in two dimensions
eigensteve · x · 2026-09-11
A team publishing in PNAS demonstrates that a minimal billiard-ball system can run a universal computer: simple collision dynamics suffice to simulate any Turing machine, meaning undecidability already exists in dimension two. Timed amid the Navier–Stokes buzz, the authors frame it as a landmark for computational irreducibility in physical systems — mechanical systems can encode fully undecidable problems, not just chaos.
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