  Superfluids and superconductors are ordinary matter that show a very
surprising behavior at low temperatures. As their temperature is reduced,
materials of both kinds can abruptly fall into a state in which they will
support a persistent, essentially immortal, flow of particles. Unlike anything
in classical physics, these flows engender neither friction nor resistance. A
major accomplishment of Twentieth Century physics was the development of an
understanding of this very surprising behavior via the construction of
partially microscopic and partially macroscopic quantum theories of superfluid
helium and superconducting metals. Such theories come in two parts: a theory of
the motion of particle-like excitations, called quasiparticles, and of the
persistent flows itself via a huge coherent excitation, called a condensate.
Two people, above all others, were responsible for the construction of the
quasiparticle side of the theories of these very special low-temperature
behaviors: Lev Landau and John Bardeen. Curiously enough they both partially
ignored and partially downplayed the importance of the condensate. In both
cases, this neglect of the actual superfluid or superconducting flow interfered
with their ability to understand the implications of the theory they had
created. They then had difficulty assessing the important advances that
occurred immediately after their own great work.
  Some speculations are offered about the source of this unevenness in the
judgments of these two leading scientists.
