An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The idea that the existence of → life and, in particular, our presence as → intelligent  → observers, → constrains the nature of the → Universe. It is an attempt to explain the observed fact that the → fundamental constants of nature are just right or fine-tuned to allow the Universe and life to exist. This is not however a "principle." See also → weak anthropic principle, → strong anthropic principle. Compare → Copernican principle.

Anthropic, from Gk. anthropikos "human," from anthropos "human being, man," → anthropo-; → principle.

Parvaz, → principle; ensân-hasti, from ensân, → anthropo-, + Mod.Pers. hasti "existence, being," Mid.Pers. astih, O.Pers. astiy; Av. asti "is," O.Pers./Av. root ah- "to be;" cf. Skt. as-; Gk. esti; L. est; PIE *es-.

A body immersed totally or partially in a liquid is buoyed up by a force equal to the weight of the liquid displaced by the body. → buoyancy.

Archimedes of Syracuse (c. 287 BC - c. 212 BC), Greek mathematician and inventor; → principle.

Arašmidos altered form of Archimedes in classical Ar. texts; parvaz, → principle.

The → diffraction pattern for an → aperture is the same as the diffraction pattern for its → complementary aperture.

→ Babinet compensator; → principle.

The principle that cause must always precede effect.

→ causality; → principle.

Physical principle, put forward by Niels Bohr in 1928, that a complete knowledge of phenomena on atomic dimensions requires a description of both wave and particle properties.

→ complementarity; → principle.

1) Physics: A basic statement that there should be no "special" observers to explain the phenomena. The principle is based on the discovery by Copernicus that the motion of the heavens can be explained without the Earth being in the geometric center of the system, so the Aristotelian/Ptolemaic assumption that we are observing from a special position can be given up.
2) Exobiology: By extension, human beings and the Earth are not at the centre of the → Universe and therefore are not "special". Life would therefore be commonplace. Compare → anthropic principle.

→ Copernican model; → principle.

The principle first put forward by N. Bohr according to which the behavior of quantum mechanical laws reduce to classical laws in the limit of large quantum numbers.

→ correspondence; → principle.

The → hypothesis that on → large scales the → Universe is → isotropic and → homogeneous, that is, it appears the same at all places and, from any one place, looks the same in all directions. See also → perfect cosmological principle.

Introduced by E.A. Milne in 1933; → cosmological; → principle.

The statement that a moving body can be brought to a → static equilibrium by applying an imaginary inertia force of the same magnitude as that of the accelerating force but in the opposite direction. More specifically, when a body of mass m is moving with a uniform acceleration a under the action of an external force F, we can write: F = m . a, according to Newton's second law. This equation can also be written as: F - ma = 0. Therefore, by applying the force -ma, the body will be considered in equilibrium as the sum of all forces acting on it is zero. Such equilibrium is called → dynamic equilibrium. Owing to this principle, dynamical problems can be treated as if they were statical.

Named after the French mathematician and philosopher Jean le Rond d'Alembert (1717-1783), who introduced the principle in his Traité de dynamique (1743).

→ d'Alembert's principle.

→ d'Alembert's principle; → Lagrangian.

The → equivalence principle as stated by Einstein, on which is based the theory of → general relativity. It comprises the three following items:
1) The → weak equivalence principle is valid.
2) The outcome of any local non-gravitational experiment is independent of the velocity of the freely-falling → reference frame in which it is performed. Also known as → local Lorentz invariance.
3) The outcome of any local non-gravitational experiment is independent of where and when in the Universe it is performed. Also called → local position invariance.

→ Einstein; → equivalence; → principle.

A fundamental concept of physics, put forward by A. Einstein, that states that gravitational and inertial forces are of a similar nature and indistinguishable. In other words, acceleration due to gravity is equivalent to acceleration due to other forces, and gravitational mass is the same as inertial mass. Same as the → principle of equivalence.

→ equivalence; → principle.

In → quantum mechanics, the principle according to which no → two → fermions of the same kind may simultaneously → occupy the same → quantum state. Also known as → Pauli exclusion principle.

→ exclusion; → principle.

The path taken by a ray of light going from one point to another through any set of media is such that the time taken is a minimum. This principle governs the light propagation and determines the geodesics of optical paths.

Put forward by Pierre de Fermat (1601-1665), French mathematician, born at Beaumont-de-Lomagne; → principle

Of all the possible paths along which a → dynamical system can move from one configuration to another within a specified time interval (consistent with any constraints), the actual path followed is that which minimizes the time integral of the → Lagrangian function. Hamilton's principle is often mathematically expressed as δ∫Ldt = 0, where L is the Lagrangian function, the integral summed from t₁ to t₂, and δ denotes the virtual operator of Lagrangian dynamics and the → calculus of variations.

→ Hamiltonian function; → principle.

The uncertainty in the measurement of the position and momentum of an elementary particle. The more precisely one quantity is known, the less certain the precision of the other. A similarly linked pair of quantities is the time and energy content in a volume of space.

Named after Werner Heisenberg (1901-1976), the German physicist who in 1927 derived the uncertainty principle. In 1932 he was awarded the Nobel Prize in Physics; uncertainty, from → un- "not" + → certainty; → principle.

Every point of a → wavefront may be considered as a center of a secondary disturbance which gives rise to spherical wavelets, and the wavefront at any later instant may be regarded as the envelope of these wavelets. This statement suffices to account for the laws of → reflection and → refraction, and the approximately straight line propagation of light through large apertures, but it fails to account for → diffraction, the deviations from exact straight line propagation of light. Huygens' principle was later extended by Fresnel and led to the formulation of → Huygens-Fresnel principle, which is of great importance in the theory of diffraction.

→ Huygens; → principle.

A development of → Huygens' principle stating that every point on a → wavefront acts, at a given instant, as a source of outgoing secondary spherical waves. The secondary wavelets mutually interfere and the resulting net light amplitude at any position in the outgoing wavefront is the vector sum of the amplitudes of all the individual wavelets. Using this principle, Fresnel calculated with a high accuracy the distribution of light in → diffraction patterns. The Huygens-Fresnel principle was put on a firm theoretical basis by Kirchhoff and expressed as an integral derived from the → wave equation.

→ Huygens; → Fresnel diffraction; → principle.

The vector → impulse of the → resultant force on a particle, in any time interval, is equal in magnitude and duration to the vector change in momentum of the particle: ∫F dt = mv₂ - mv₁. The impulse-momentum principle finds its chief application in connection with forces of short duration, such as those arising in collisions or explosions. Such forces are called → impulsive forces.

→ impulse; → momentum; → principle.

A change in one of the variables (such as temperature, pressure, and concentration of various species) that describe a system at equilibrium produces a shift in the position of the equilibrium that counteracts the effect of this change.

Named after the French chemist and engineer Henry Louis Le Chatelier (1850-1936); → principle.