The Use of Models

Models, or in other words, theories in science, and their use and relevance do not constitute a subject that technologists worry about. I mentioned them here 

Models are works of fiction, not truths or images of real processes. They are at most dispensable prototruths and acceptable only if they point the way to new facts to be established experimentally. An understandable wish for certainty tends to ignore this, as can be witnessed in heated debates. Scientific models such as quantum chemistry and equilibrium thermodynamics can sometimes explain what has been observed, but as tools for prediction in technology such models have as much use as astrology sometimes they work and sometimes they don t. 

Many simplifying models developed in physics and physical chemistry have been very successful. They simplify the system by transforming strongly interacting units into separate weakly interacting ones. Not too complicated systems thereby become simple collections of independent units and are calculable. Examples are  

Normal vibrations are constructs that make sense of infrared spectra. Atoms in lattices are strongly bound to each other and their vibrations are strongly coupled to those of their neighbors. The atomic displacements along the three axes cannot be isolated from each other. However, if the displacements are carefully combined into groups to form normal vibrations then these are to a good approximation independent of one another. 

Quasi-electrons are electrons in solids with properties other than those in vacuum. In a condensed phase electrons interact with each other and with the nuclei. In order to deal with them as independent free units their interactions with the environment must be eliminated (or at least substantially reduced). This is done by transforming the interactions into novel particle properties. For example, when the motion of an electron in a solid is impeded by atoms the electron seems to be heavier, and as a result the electron is transformed into a free quasi-electron with a higher mass. The properties have changed under the transformation from those of free electrons in vacuum to those of quasi-electrons in a solid and so have their identities. 

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