THE FERMI-PASTA-ULAM LATTICE Background The Fermi-Pasta-Ulam lattice is named after the experiments

The Fermi-Pasta-Ulam lattice is named after the experiments performed by Enrico Fermi, John Pasta, and Stanislaw Ulam in 1954-5 on the Los Alamos MANIAC computer, one of the first electronic computers. As reported in Ulam’s autobiography [Uh], Fermi immediately suggested using the new machine for theoretical work, and it was decided to start by studying the vibrations of a string under the influence of nonlinear internal forces. Nonlinearity makes the computations very difficult and the problem could not be attacked by standard mathematical methods. However, physical intuition suggested that the motion of such a string would eventually “thermalize”. The purpose of the experiment was to investigate the rate of thermalization. For the computer calculations, the string was approximated by a finite sequence of point particles with nearest-neighbour interactions — a lattice model.