  Quasielastic neutron scattering is a powerful tool for the study of
non-periodic motions in condensed matter as a detailed line shape analysis can
give information about the geometry and rate of the scatterers' displacements.
Unfortunately, there are also a number of artifacts which can masquerade as
signatures of motions and can therefore lead to erroneous results. Their
influence on the evaluation of the motions of the phospholipid
dimyristoylphosphatidylcholine (DMPC) is discussed. On a 60 ps time scale, the
long-range motion of the molecules has a flow-like character with similar
velocities above and below the main phase transition. It is proposed that the
concepts of dynamical heterogeneities and "floppy modes" developed in glass
physics provide a framework to explain the observed behaviour.
