  Heavy ion collisions at ultrarelativistic energies offer the opportunity to
study the irreversibility of multiparticle processes. Together with the
many-body decays of resonances, the multiparticle processes cause the system to
evolve according to Prigogine's steady states rather than towards statistical
equilibrium. These results are general and can be easily checked by any
microscopic string-, transport-, or cascade model for heavy ion collisions. The
absence of pure equilibrium states sheds light on the difficulties of thermal
models in describing the yields and spectra of hadrons, especially mesons, in
heavy ion collisions at bombarding energies above 10 GeV/nucleon.
