An Etymological Dictionary of Astronomy and Astrophysics
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The average velocity of a charged particle in a plasma in response to an applied electric field.

→ drift; → velocity.

A macroscopic process that occurs in a → fluid because of the relative motions induced by the non-uniform → turbulent motions of the fluid. Also known as turbulent → diffusion. Eddy diffusion may occur in an atmosphere if it is unstable against turbulence. It dominates the atmosphere below the homopause. See also → molecular diffusion.

→ eddy; → diffusion.

Same as → Einstein model.

→ Einstein; → theory; → specific heat.

A diffraction phenomenon resulting from the passage of electrons through matter, analogous to the diffraction of visible light. This phenomenon is the main evidence for the existence of waves associated with elementary particles; → de Broglie wavelength.

→ electron; → diffraction.

An important physical process occurring in stars, which is the relative separation of the various → chemical elements. It is caused by → gravitational settling and → thermal diffusion, on the one hand, and → radiative levitation on the other. This process, which was described by Michaud (1970) to account for the abundance anomalies observed in → chemically peculiar  → A star, is now recognized as occuring in all types of stars. Its influence on the observed → chemical abundances is extremely variable, however, due to competing macroscopic motions like → convective  → mixing or rotation-induced → turbulence. In the Sun, no observable abundance anomalies are expected from element diffusion, as the time scale of the process is longer than the solar lifetime. However the small induced → depletion of → helium and → heavy elements by about 20% is detectable through → helioseismology. Such detections are more difficult in stars, as only global → oscillation modes can be detected, in contrast to the Sun, where local oscillations of the surface can be analyzed (Théado et al., 2005, A&A 437, 553).

→ element; → diffusion.

1) The hypothetical motion of the supposed → ether relative to the Earth. The → Michelson-Morley experiment found no ether drift. An analogy is given by a boat drifting in a fast-flowing river due to the river's current. With the same power, the speed would be slower when sailing across the stream. Moreover, in order to reach directly opposite the starting point, the boat would have to be headed somewhat upstream. See also the → ether drag.
2) The motion of the Earth relative to the → rest frame defined by the → cosmic microwave background radiation.

→ ether; → drift.

If N(x,y) is a → function of two → independent variables, then dN = (∂N/∂x)dx + (∂N/∂y)dy is the exact differential.

→ exact; → differential.

A → differential equation composed of → continuous  → differentiable functions for which certain conditions are fulfilled. The equation M(x,y)dx + N(x,y)dy = 0 is called exact if M(x,y) and N(x,y) are continuous differentiable functions for which the following relationship is fulfilled: ∂M/∂y = ∂N/∂x, and ∂M/∂y and ∂N/∂x are continuous in some region.

→ exact; → differential; → equation.

A symbol of → predicate logic which expresses that the statements within its scope are → true for at least one instance of something. The symbol ∃ is used as the existential quantifier. Existential quantifiers are normally used in logic in → conjunction with → predicate symbols, which say something about a → variable or → constant, in this case the variable being quantified (→ quantify, → quantification).

→ existential; → quantifier.

→ extraterrestrial life.

→ exo-; → life.

Life that may exist and originate outside the planet Earth.

→ extraterrestrial; → life.

Philosophy of science: The concept according to which a proposition or theory cannot be scientific if it does not admit consideration of the possibility of its being false. According to Karl Popper (1902-1994), falsifiability is the crucial feature of scientific hypotheses. Any theory not falsifiable is said to be unscientific.

→ falsifiable; → -ity.

The quality of something that can be falsified. → falsifiability.

→ falsify; → -able.

1) To show or prove that a theory is incorrect or false.
2) To alter something in order to deceive.

Verb from → false.

A classification scheme for distinguishing a → radio galaxy from an → active galaxy based on their → radio frequency  and → luminosity and their kpc-scale appearance. Analyzing a sample of 57 radio galaxies from the → 3CR catalogue, which were clearly resolved at 1.4 GHz or 5 GHz, Fanaroff & Riley (1974) discovered that the relative positions of regions of high and low → surface brightness in the → lobes of extragalactic → radio sources are correlated with their radio luminosity. They divided the sample into two classes using the ratio R_FR of the distance between the regions of highest surface brightness on opposite sides of the central galaxy or quasar, to the total extent of the source up to the lowest brightness contour in the map. → Fanaroff-Riley Class I (FR-I) , → Fanaroff-Riley Class II (FR-II). The boundary between the two classes is not very sharp, and there is some overlap in the luminosities of sources classified as FR-I or FR-II on the basis of their structures. The physical cause of the FR-I/II dichotomy probably lies in the type of flow in the → radio jets.

Bernard L. Fanaroff and Julia M. Riley, 1974, MNRAS 167, 31P; → classification.

A → differential equation containing only the first → derivative. For example, dy/dx = 3x and 2y(dy/dx) + 3x = 5.

→ first; → order; → differential; → equation.

A manifold with a → Riemannian metric that has → zero → curvature.

→ flat; → manifold.

A method used to test the → image quality of → mirrors and → lenses. The test is performed by moving a knife edge laterally into the → image of a small → point source. The → eye, or a → camera, is placed immediately behind the knife edge, and the → exit pupil of the system is observed.

Named after the French physicist Léon Foucault (1819-1868), who invented the method; → knife; → edge; → test.

An undesired progressive change in an oscillator's frequency with time.

→ frequency; → drift.

The change in the frequency of a wave motion due to the → Doppler effect.

→ frequency; → shift.