An Etymological Dictionary of Astronomy and Astrophysics
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The local → inertial frame and the → inertia of any body results from the distribution of all matter in the Universe. This principle has been neither confirmed nor refuted.

→ Mach number; → number.

The Newton's equations of motion, if they hold in any → reference frame, they are valid also in any other reference frame moving with uniform velocity relative to the first.

→ Newtonian; → principle; → relativity.

A quantum mechanical principle according to which no two identical → fermions can share the same → quantum state. Also known as → exclusion principle.

In honor of Wolfgang Pauli (1900-1958), Austrian theoretical physicist, who formulated the principle in 1925; → exclusion; → principle.

The → assumption, adopted by the → steady-state theory, that all observers, everywhere at all times, would view the same large-scale distribution of matter in the → Universe in all regions and in every direction. In contrast to the → cosmological principle, the perfect cosmological principle adds the assumption that the Universe does not change with time on the large scale.

→ perfect; → cosmological; → principle.

A fundamental, primary assumption, or general law from which others are derived.

From M.E., from O.Fr. principe, from L. principium "a beginning, first part," from princeps "first, chief, prince," literally "that takes first," from primus "first" + root of capere "to take."

Parvaz "origin, root, stock" (as used in particular by Ferdowsi); cf. Av. fra-vāza- "drawing from; leading onward," from Av. fra-, frā- "before; forward, forth" (fratəma- "first, front," pouruua- "first," fra-cara- "preceding;" O.Pers. fra- "forward, forth;" Mid.Pers. fra-; cf. Skt. pra- "before, formerly," prathama- "earliest, initial," pūrva- "first;" Gk. pro; L. pro; O.E. fyrst "foremost," superlative of fore) + vaz- "to draw, guide; bring; possess; fly; float," vazaiti "guides, leads" (cf. Skt. vah- "to carry, drive, convey," vahati "carries," pravaha- "bearing along, carrying," pravāha- "running water, stream, river;" L. vehere "to carry;" O.E. wegan "to carry;" O.N. vegr; O.H.G. weg "way," wegan "to move," wagan "cart;" M.Du. wagen "wagon;" PIE base *wegh- "to drive;" see also → flight).

Newton's third law of motion, which states that for every action there must be an equal and opposite reaction.

→ principle; → action; → reaction.

The second postulate in Einstein's theory of → special relativity whereby the → velocity of light in → vacuum has the same absolute value in all → inertial reference frames irrespective of the direction and speed of propagation of the light source. It should be emphasized that this constancy of the speed of light holds among → reference frames moving uniformly with respect to each other. An observer accelerated with respect to a light source will measure a speed of light that is smaller than the speed of light measured in a → rest frame. See also → principle of relativity.

→ principle; constancy, noun related to → constant.

In special relativity, the principle that the laws of physics take the same mathematical form in all inertial frames of reference.

→ principle; → covariance.

In → general relativity the principle which states that in the immediate proximity of an → accelerating system the acceleration is physically equivalent to → gravitational force. This principle also implies the equivalence of → gravitational mass and → inertial mass. Same as the → equivalence principle. See also → Einstein's elevator.

→ principle; → equivalence.

The second principle of → formal logic introduced in Aristotle's theory of the → syllogism: A statement is either → true or → false. In other words, two → contradictory → propositions cannot both be true. The truth of one implies the falsehood of the other. Also called law of excluded middle and → principle of excluded third.

→ principle; → exclude; → middle.

Same as → principle of excluded middle.

→ principle; → exclude; → third.

The first principle of → formal logic introduced in Aristotle's theory of the → syllogism: If a statement is true then it is true. Also called → law of identity.

→ principle; → identity.

The principle that, for a system whose total mechanical energy is conserved, the path to be taken for the system from one configuration to another is the one whose action has the least value relative to all other possible paths and from the same configurations. Also called Maupertuis' principle, least-action principle.

→ principle; → least; → action.

The third principle of → formal logic introduced in Aristotle's theory of the → syllogism: No statement can be both → true and → false at the same time. Also called → law of non-contradiction.

→ principle; → non-; → contradiction.

One consequence of the theory of → special relativity, according to which no two events separated by a distance greater than their separation in time multiplied by the → speed of light may have a → causal influence on each other. Violation of this principle leads to → paradoxes, such as that of an → effect preceding its → cause.

→ principle; → relativistic; → causality.

The first postulate in Einstein's theory of → special relativity whereby all the laws of physics are the same in every → inertial reference frame. In other words, no physical measurement can distinguish one inertial reference frame from another. See also → principle of constancy.

→ principle; → relativity.

The requirement employed by Einstein in his relativity theories, that the equations describing the laws of physics are the same in all frames of reference. This statement and that of the constancy of the speed of light constitute the founding principles of special relativity.

Relativity; → principle.

An empirical rule discovered before the advent of quantum mechanics which states that it is possible to find pairs of spectral lines, which have the property that the sum of their wavenumbers is also an observed spectral line.

Named after Walther Ritz (1878-1909), a Swiss theoretical physicist; → combination; → principle.

A version of the → anthropic principle that claims that the → Universe must be suitable for the formation of → intelligent life at some point. Compared with the → weak anthropic principle, this version is very controversial. Its implications are highly speculative from a scientific viewpoint.

→ strong; → anthropic; → principle.

1) Math.: The principle concerned with homogeneous and non-homogeneous → linear differential equations, stating that two or more solutions to a linear equation or set of linear equations can be added together so that their sum is also a solution.
2) One of the most fundamental principles of → quantum mechanics which distinctly marks the departure from classical concepts. It holds that any linear superposition of → wave functions is also a possible wave function. Simply put, the state of an object is all its possible states simultaneously, as long as we do not look to check. It is the measurement itself that causes the object to be limited to a single possibility. The superposition principle is rooted in the linearity of → Schrödinger's equation. Hence if two solutions of the wave function, ψ₁ and ψ₂, are known, other solutions, of the form: ψ = a₁ψ₁ + a₂ψ₂ also represent possible states of the system.

→ superposition; → principle.