Virtual particleA virtual particle is a theoretical transient particle that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle. The concept of virtual particles arises in the perturbation theory of quantum field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. A process involving virtual particles can be described by a schematic representation known as a Feynman diagram, in which virtual particles are represented by internal lines.
Necking (engineering)In engineering and materials science, necking is a mode of tensile deformation where relatively large amounts of strain localize disproportionately in a small region of the material. The resulting prominent decrease in local cross-sectional area provides the basis for the name "neck". Because the local strains in the neck are large, necking is often closely associated with yielding, a form of plastic deformation associated with ductile materials, often metals or polymers.
HomeomorphismIn the mathematical field of topology, a homeomorphism (, named by Henri Poincaré), also called topological isomorphism, or bicontinuous function, is a bijective and continuous function between topological spaces that has a continuous inverse function. Homeomorphisms are the isomorphisms in the —that is, they are the mappings that preserve all the topological properties of a given space. Two spaces with a homeomorphism between them are called homeomorphic, and from a topological viewpoint they are the same.
Hierarchy of genresA hierarchy of genres is any formalization which ranks different genres in an art form in terms of their prestige and cultural value. In literature, the epic was considered the highest form, for the reason expressed by Samuel Johnson in his Life of John Milton: "By the general consent of criticks, the first praise of genius is due to the writer of an epick poem, as it requires an assemblage of all the powers which are singly sufficient for other compositions." Below that came lyric poetry, and comic poetry, with a similar ranking for drama.
