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.
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