  Neutrons and x-rays are coherent probes, and their coherent properties are
used in scattering experiments. Only coherent scattering probes can elucidate
collective molecular motions. While phonons in crystals were studied for half a
century now, the study of collective molecular motions in soft-matter and
biology is a rather new but upcoming field. Collective dynamics often determine
material properties and interactions, and are crucial to establish
dynamics-function relations. We review properties of neutrons and x-rays and
derive the origin of coherent and incoherent scattering. Taking molecular
motions in membranes and proteins as example, the difference between coherent
and incoherent dynamics is discussed, and how local and collective motions can
be accessed in x-ray and neutron scattering experiments. Matching of coherent
properties of the scattering probe may become important in soft-matter and
biology because of (1) the missing long ranged order and (2) the large length
scales involved. It is likely to be important in systems, where fluctuating
nanoscale domains strongly determine material properties. Inelastic scattering
can provide very local structural information in disordered systems. Inelastic
neutron scattering experiments point to a coexistence of short-lived nanoscale
gel and fluid domains in phospholipid bilayers in the range of the gel-fluid
phase transition, which may be responsible for critical behavior and determine
elastic properties.
