Fr.: expérience Fermi-Pasta-Ulam
A computer experiment that was aimed to study the → thermalization process of a → solid. In other words, the goal was to see whether there is an approximate → equipartition of energy in the system, which would mean that the motion is → chaotic. Using computer simulation, Fermi-Pasta-Ulam studied the behavior of a chain of 64 mass particles connected by → nonlinear springs. In fact, they were looking for a theoretical physics problem suitable for an investigation with one of the very first computers, the he MANIAC (Mathematical Analyzer, Numerical Integrator and Computer). They decided to study how a → crystal evolves toward → thermal equilibrium by simulating a chain of particles, linked by a quadratic interaction potential, but also by a weak nonlinear interaction. Fermi-Pasta-Ulam assumed that if the interaction in the chain were nonlinear, then an exchange of energy among the normal modes would occur, and this would bring forth the equipartition of energy, i.e. the thermalization. Contrary to expectations, the energy revealed no tendency toward equipartition. The system had a simple quasi-periodic behavior, and no → chaoticity was observed. This result, known as the Fermi-Pasta-Ulam paradox, shows that → nonlinearity is not enough to guarantee the equipartition of energy (see, e.g., Dauxois et al., 2005, Eur. J. Phys., 26, S3).
E. Fermi, J. Pasta, S. Ulam, 1955, Los Alamos report LA-1940; → problem.