Measurement dynamic-unit system example

There is some difficulty with using an intuitive notion of complexity, as we have been. How can we decide more precisely what we mean by "complexity" of a system The initial intuition is that any system is more complex than any of its proper subsystems. The solar system example shows that this intuition fails when the focus with which the system is considered changes. An alternative intuition is that the complexity of a system might be measured by the number and diversity of its component atomic entities (in this sense, an entity is atomic in any consideration of a structure if we do not inquire into its substructure). This also fails in that it does not deal well with the fact that a static structure, or an instantaneous snapshot of a structure, is a less complex entity than a dynamic structure that contains the same atomic units. There may be an information theoretic measure of the complexity of a system in terms of the number of bits of information required to specify its observable properties at the required focus. 

The above illustrations and discussion lead us to several general conclusions concerning the use of neutron spectroscopy in the study of torsional vibrations (and other large-amplitude modes) in molecular systems. First, the neutron technique-since it involves the interaction of neutrons with vibrating nuclei and is especially sensitive to large amplitude motions—can for appropriate molecules be an ideal complement for optical spectroscopy. Neutron spectroscopy, however, is hampered somewhat by the available instrumental resolution ( 10 cm-1) and by the inherent recoil resolution broadening in fluid-phase spectra. In addition, present accessibility of instrumentation for the neutron method (for low k molecular spectroscopy) is limited. For example, there are only a few reactors in the United States where appropriate instruments and intensity exist for such measurements (neutron sources and instrumentation amenable to the study of crystal and liquid structure and interatomic and intermolecular dynamics are more accessible). These factors make it imperative that studies of molecular systems be chosen with some care. 

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