Fermi-Pasta-Ulam problem

The Fermi-Pasta-Ulam problem involves studying how energy distributes in a system of connected particles (like masses linked by springs). Physicists expected that, over time, energy would spread evenly among all particles, leading to thermal equilibrium. However, they found that energy often remained confined in a few modes, causing the system to behave in unexpected, quasi-repetitive ways. This challenged assumptions about how systems reach equilibrium and helped spark developments in nonlinear dynamics and chaos theory, revealing that complex systems can have surprising, persistent patterns instead of always becoming completely random.