An Etymological Dictionary of Astronomy and Astrophysics
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1) A theory which studies how, in certain nonlinear systems, there may be paths and shifts in behavior dependent on small changes in circumstances or the current position of the system.
2) Math.: The study of the behavior of a solution of a nonlinear problem in the neighborhood of a known solution, particularly as a parameter varies.

→ bifurcation; → theory.

→ Big Bang; → theory.

A mechanical model put forward before Newton's theory of gravity to explain the revolution of the planets around the Sun. Descartes in his 1644 Principia Philosophiae postulated that the space between the Sun and the planets is filled with matter in the form of a fluid. The fluid rotates in countless whirlpools, one for each planet, thus carrying the planets along in their flow. The vortices vary in size and are contiguous as well as nested. Descartes believed that two objects can exert force on each other only when they are in physical contact. This is why he postulated that space is filled with matter. Newton refuted the vortex theory, using the principle of → action at a distance on which relies his → law of universal gravitation.

→ Cartesian; → vortex; → theory.

A theory that deals with the concept of → category and generalizes the → set theory.

→ category; → theory.

The theory of unpredictable behavior that can arise in systems obeying deterministic scientific laws.

→ theory; → chaos.

The theory that studies distributions of → energy, → matter, and other physical quantities under circumstances where their discrete nature is unimportant. Classical field theory traditionally includes → Newtonian mechanics, Maxwell's → electromagnetic theory, and Einstein's theory of → general relativity. The main scope of classical field theory is to construct the mathematical description of → dynamical systems with an infinite number of degrees of freedom. The word "classical" is used in contrast to those field theories that incorporate → quantum mechanics (→ quantum field theory). Classical field theories are usually categorized as → non-relativistic and → relativistic.

→ classical; → field; → theory.

Newton's theory according to which light is made up of point-like particles without any mass. It failed to explains several phenomena: simultaneous reflection and refraction at a semi-transparent boundary, interference, diffraction and polarization. Moreover, it requested that the speed of light be greater in a denser medium than in a rarer medium; this prediction is contrary to experimental results. In 1924 Louis de Broglie postulated that matter has not only a corpuscular nature but also a wave nature, and subsequent experiments confirmed de Broglie's model.

Corpuscular, adj. from → corpuscle; → theory, → light.

One possible explanation for → spiral arms, first put forward by B. Lindblad in about 1925 and developed later by C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material structures, but regions of somewhat enhanced density, created by → density waves. Density waves are perturbations amplified by the self-gravity of the → galactic disk. The perturbation results from natural non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters. Density waves rotate around the → galactic center and periodically compress the disk material upon their passage. If the spiral arms were rigid structures rotating like a pinwheel, the → differential rotation of the galaxy would wind up the arms completely in a relatively short time (with respect to the age of the galaxy), → winding problem. Inside the region defined by the → corotation radius, density waves rotate more slowly than the galaxy's stars and gas; outside that region they rotate faster.
As the density waves rotate, they are overtaken by the individual stars and nebulae/molecular clouds that are rotating around the galaxy at a higher rate. The molecular clouds passing through the density wave are subjected to compression because it is a region of higher density. This triggers the formation of clusters of new stars, which continue to move through the density wave.
The short-lived stars die, most likely as supernovae, before they can leave the spiral density wave. But the longer-lived stars that are formed pass through the density wave and eventually emerge on its front side and continue on their way as a slowly dissipating cluster of stars. Density wave theory explains much of the spiral structure that we see, but there are some problems. First, computer simulations with density waves tend to produce very orderly "grand design" spirals with a well-defined, wrapped 2-arm structure. But there are many spiral galaxies that have a more complex structure than this (→ flocculent spiral galaxy). Second, density wave theory assumes the existence of spiral density waves and then explores the consequences.
See also: → stochastic self-propagating star formation.

→ density; → wave; → theory.

A theory in which specification of the initial value of all relevant variables of the system is sufficient to calculate the past values and to predict the future values of such variables for any arbitrary time. Moreover, it is possible, for any arbitrary time, to assign a value to all the variables characterizing the system. In quantum mechanics, the time evolution of the → wave function, governed by the → Schrodinger equation, is deterministic. Quantum mechanics, however, is a non deterministic theory because of the probabilistic nature of the predictions for the values of the → observables of a quantum system.

→ deterministic; → theory.

Branch of magnetohydrodynamics concerned with self-excitation of magnetic fields in any large rotating mass of conducting fluid in motion (usually turbulent). Self-exciting dynamo action is believed to account for magnetic fields at the planetary, stellar, and galactic scales.

→ dynamo; → theory.

Same as → Einstein model.

→ Einstein; → theory; → specific heat.

The description of combined electric and magnetic fields mainly by → Maxwell's equations. Same as → electromagnetism.

→ electromagnetic; → theory.

The theory describing light as a wave phenomenon resulting from the combination of two electric and magnetic fields vibrating transversely and mutually at right angles. → electromagnetic radiation; → electromagnetic wave; → Maxwell's equations.

→ electromagnetic; → theory; → light.

The theory that describes the Galactic dynamics, that is the orbits of stars and gas clouds in the → Galactic disk, as well as the spiral → density wave. Formulated by Bertil Lindblad (1895-1965), the epicyclic theory assumes that orbits are circular with small deviations. Star orbits are described by the superposition of two motions: i) a rotation of the star (epicenter) around the Galactic center at the circular angular velocity, Ω, and ii) a retrograde elliptical motion at → epicyclic frequency, κ. The epicyclic motion in the Galactic plane occurs in a retrograde sense to conserve → angular momentum. In general Ω and κ are different and, therefore, orbits do not close. However, seen by an observer who rotates with the epicenter, orbits are closed ellipses.

→ epicyclic; → theory.

1) A theory which uses the concept of → field to describe physical phenomena. It consists of two types: → classical field theory and → quantum field theory.
2) Math.: A branch of mathematics which studies the → fields.

→ field; → theory.

A field theory in which it is possible to perform a transformation without altering any measurable physical quantity.

→ gauge; → theory.

Any physical theory that unites the strong, electromagnetic, and weak interactions at high energy. It is hoped that GUTs can ultimately be extended to incorporate gravity. → theory of everything.

M.E. graunt, from O.Fr. grant, grand, from L. grandis "big, great," also "full-grown;" unified, p.p. of → unify; → theory.

Negâré, → theory; yegâneš, verbal noun of yegânestan, → unify; bozorg→ great.

The branch of → mathematics dealing with → graphs. In particular, it involves the ways in which sets of points (→ vertex) can be connected by lines or arcs (→ edge).

→ graph; → theory.

A theory that treats gravity as a field rather than a force acting at a distance.

→ gravitational; → field.

A branch of mathematics concerned with structures called → groups and the description of their properties. Group theory provides a powerful formal method of analyzing abstract and physical systems in which → symmetry is present. It has a very considerable use in physics, especially → quantum mechanics, notably in analyzing the → eigenstates of energy of a physical system.

→ group; → theory.