n. (Astronomy, Cosmology) The theory that the known universe originated in an explosive event (the big bang) in which all of the matter and energy of the universe was contained in a single point and began to rapidly expand and evolve, starting as high-energy particles and radiation, and, as it cooled over time, evolving into ordinary subatomic particles, atoms, and then stars and galaxies. According to this theory, the four-dimensional space-time continuum which we perceive as our universe continues to expand to the present time, but it is unknown whether the expansion will continue indefinitely or eventually stop or even reverse, possibly leading to a contraction to a single point sometimes referred to as the “big crunch”. The competing “Steady-state Theory” gradually lost favor in the 1980's and 1990's. See also big bang. [ PJC ]

n. (Math., Engineering) That branch of Mathematics and Engineering which deals with the design, identification and analysis of systems with a view towards controlling them, i. e., to make them perform specific tasks or make them behave in a desired way. [ GG ]

. (Philol.) The theory which maintains that the primitive elements of language are reflex expressions induced by sensory impressions; that is, as stated by Max Müller, the creative faculty gave to each general conception as it thrilled for the first time through the brain a phonetic expression; -- jocosely so called from the analogy of the sound of a bell induced by the stroke of the clapper. [ Webster 1913 Suppl. ]

. A political theory current in the 1960's, according to which the conversion of one country in South Asia to communism will start a sequential process causing all Asian countries to convert to Communism. The apparent assumption was that an Asian country politically aligned with the West was as politically unstable as a domino standing on edge. It was used by some as a justification for American involvement in the Vietnam war, 1964-1972. [ PJC ]

n. A branch of mathematics that deals with strategies for maximizing gains or minimizing losses in competitive situations having defined constraints and involving random factors. Game theory is used for modelling and analysis of various decision-making situations such as military strategy or business policy. The theoretical models study the interactions among opposing entities called "players," where different kinds of situation can arise, for which the probabilities of occurence are known. Also known is the set of decisions each player can take. When a player takes a decision he makes a gain or incurs a loss. Based on the available knowledge each player tries to adopt a "strategy" so as to maximize his gains. The entire procedure constitutes a game.
Syn. -- theory of games. [ WordNet 1.5 ]

. 1. (Biol.) The theory that living organisms can be produced only by the development of living germs. Cf. Biogenesis, Abiogenesis. [ Webster 1913 Suppl. ]

2. (Med.) The theory which attributes contagious and infectious diseases, suppurative lesions, etc., to the agency of germs, i.e. pathogenic microorganisms. The science of bacteriology was developed after this theory had been established. [ Webster 1913 Suppl. +PJC ]

n. (Theoretical physics) Any of a class of physics theories that attempts to explain the electroweak forces, stong force, and gravitation within a single mathematical conceptual scheme. In the 1990's string theory and superstring theory were prominent examples. Abbreviated GUT, plural GUTs. [ PJC ]

n. (Math., Telecommunications) The science which studies the capacity of systems to contain, store, and transmit information{ 2 and 4 }, and the factors such as noise and channel capacity that may affect the rate or accuracy of information transmission and reception. [ PJC ]

n. Malthus' theory that population increase would outpace increases in the means of subsistence; Malthusianism. See Malthusian. [ WordNet 1.5 ]

. (Physiol. Chem.) A theory proposed by P. Ehrlich as a chemical explanation of immunity phenomena. In brief outline it is as follows: Animal cells and bacteria are complex aggregations of molecules, which are themselves complex. Complex molecules react with one another through certain of their side chains, but only when these side chains have a definite correspondence in structure (this accounts for the specific action of antitoxins). [ Webster 1913 Suppl. ]

n. (Physics) A mathematical theory for describing the properties of fundamental particles, which represents the particles as one-dimensional string-like objects, which exist in the normal four dimensions of space-time plus additional dimensions, the total dimensions being ten, eleven, or twenty-six depending on the version of the theory. The properties of fundamental particles in string theory and their manner of interaction with each other depend upon the modes of vibration of the strings{ 17 }. The attractiveness of this theory rests in part on its ability to provide a unified treatment of gravity as well as the three other basic forces of nature, in a manner consistent with quantum mechanics. The great difficulty of doing the calculations required by the theory, however, has thus far (1999) made it impossible to calculate the observable properties, such as the mass, of known particles, such as the electron, proton, mesons, quarks, and neutron; thus there is as yet no experimental verification for the theory. The most popular version of the theory depends on a mathematical property called supersymmetry, and the theory derived form this principle is properly called superstring theory, a term which is often used interchangeably with string theory. See also string{ 17 }. [ PJC ]

n.; pl. Theories [ F. théorie, L. theoria, Gr. &unr_; a beholding, spectacle, contemplation, speculation, fr. &unr_; a spectator, &unr_; to see, view. See Theater. ] 1. A doctrine, or scheme of things, which terminates in speculation or contemplation, without a view to practice; hypothesis; speculation. [ 1913 Webster ]

☞ “This word is employed by English writers in a very loose and improper sense. It is with them usually convertible into hypothesis, and hypothesis is commonly used as another term for conjecture. The terms theory and theoretical are properly used in opposition to the terms practice and practical. In this sense, they were exclusively employed by the ancients; and in this sense, they are almost exclusively employed by the Continental philosophers.” Sir W. Hamilton. [ 1913 Webster ]

2. An exposition of the general or abstract principles of any science; as, the theory of music. [ 1913 Webster ]

3. The science, as distinguished from the art; as, the theory and practice of medicine. [ 1913 Webster ]

4. The philosophical explanation of phenomena, either physical or moral; as, Lavoisier's theory of combustion; Adam Smith's theory of moral sentiments. [ 1913 Webster ]

Atomic theory,
Binary theory, etc. See under Atomic, Binary, etc. [ 1913 Webster ]

Syn. -- Hypothesis, speculation. -- Theory, Hypothesis. A theory is a scheme of the relations subsisting between the parts of a systematic whole; an hypothesis is a tentative conjecture respecting a cause of phenomena. [ 1913 Webster ]

. (Chem. & Physics) The theory, advanced by Thomson (Lord Kelvin) on the basis of investigation by Helmholtz, that the atoms are vortically moving ring-shaped masses (or masses of other forms having a similar internal motion) of a homogeneous, incompressible, frictionless fluid. Various properties of such atoms (
vortex atoms) can be mathematically deduced. This theory is now (1998) obsolete, and has been superseded by quantum mechanics, which provides more accurate and detailed explanations of atomic behavior. [ Webster 1913 Suppl. +PJC ]