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Victor Weisskopf developed a statistical treatment of particle emission from highly excited heavy atomic nuclei. His 1937 paper compares the process with evaporation: the excitation energy is shared among many internal quantum states, and the available final states constrain which particles can escape with which energies. This connects nuclear level density with emission probabilities and approximately thermal-looking outgoing spectra. Nuclear evaporation supplies a way to predict the aggregate outcome of a complicated nuclear reaction without following every individual internal trajectory.^[ref]

A dilute condensed Bose system whose interaction is sufficiently weak for a small-depletion expansion. Its low-energy excitations are described by a Bogoliubov transformation rather than free particles. The homogeneous quadratic theory is controlled only while noncondensate occupation remains small compared with the condensate and its excitation energies are stable.