Superfluidity and superconductivity are two remarkable
phenomena in which, at low temperatures, materials abruptly gain the ability to
flow without friction. Microscopic quantum theories of these phases of matter
were constructed in blockbuster papers of Lev Landau (1940) and John Bardeen,
Leon Cooper, and J. Robert Schrieffer
(1957). The actual explanation of
the flow, however, is rooted in a
Einstein paper of 1924 that introduces a condensate, a quantum configuration describing a finite
fraction of the particles in the system.
Superfluidity can then be understood in terms of the wave
function for this configuration, which necessarily extends over a finite
fraction of the system. Neither
blockbuster paper mentions Einstein or the crucial idea of a condensate wave
function. The reasons for this omission are mooted.