View source for Fermi-Pasta-Ulam problem

The '''Fermi-Pasta-Ulam problem''' (1955) is the numerical experiment that broke the [[Ergodic hypothesis|ergodic hypothesis]]. Enrico Fermi, John Pasta, Stanisław Ulam, and Mary Tsingou simulated a one-dimensional chain of nonlinearly coupled oscillators, expecting the system to thermalize — to reach equilibrium and exhibit ergodic behavior. Instead, the energy cycled periodically among the normal modes, returning almost exactly to its initial state. The system remembered its initial conditions.\n\nThe FPU problem revealed that nonlinearity alone is not sufficient for ergodicity. The explanation came a decade later with the [[KAM theorem]]: the FPU system is nearly integrable, and most of its invariant tori survive the nonlinear perturbation, trapping trajectories in regular motion. The problem reshaped the foundations of statistical mechanics, forcing physicists to confront that the route to equilibrium is not universal but depends sensitively on the structure of the Hamiltonian.\n\n''The FPU problem is not a footnote in the history of computing. It is the experiment that proved intuition wrong — and in doing so, it exposed the vast gap between what physicists assumed about many-body systems and what those systems actually do.''\n\n[[Category:Physics]]\n[[Category:Systems]]