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

Arising from internal forces or causes; independent of external agencies; self-acting.

From L.L. spontaneus "willing, of one's free will," from L. (sua) sponte "of one's own accord, willingly," of unknown origin.

Sarxod, literally "by himself/herself," from sar "head" (soru, sorun "horn;" karnâ "a trumpet-like wind instrument," variant sornâ "a wind instrument;" Mid.Pers. sar "head," sru "horn;" Av. sarah- "head," srū- "horn, nail;" cf. Skt. śiras- "head, chief;" Gk. kara "head," karena "head, top," keras "horn;" L. cornu "horn," cerebrum "brain;" P.Gmc. *khurnaz (Ger. Horn, Du. horen; cognate with E. horn, as above, from PIE *ker- "head, horn;" O.E. horn "horn of an animal," also "wind instrument;" E. horn); PIE base *ker- "head, horn, top, summit") + xod "self" (Mid.Pers. xwad "self; indeed;" Av. hva- "self, own").

The self-ignition of a substance that produces sufficient heat within itself, by a slow oxidation process, for ignition to take place without the need for an external high-temperature source. The produced heat energy is absorbed by the substance raising its temperature slowly until the → ignition temperature is reached. Same as spontaneous ignition.

→ spontaneous; → combustion.

The emission of electromagnetic radiation from an atom or molecule that does not depend on the presence of external fields.

→ spontaneous; → emission.

A physical phenomenon whereby a symmetric system becomes permanently asymmetric. A simple example is a ball lying on top of a hill in equilibrium. The hill-ball system is symmetric about the vertical axis through the top of the hill. Moreover, there is no preferred horizontal direction to the system. However, its state is unstable, since the slightest perturbing force will cause the ball to roll down the hill in some particular direction. The system becomes permanently asymmetric because the ball will not roll uphill by itself. Symmetry breaking is found in several fields of physics, for example in → magnetism (→ ferromagnetism), → thermodynamics (→ crystallization), and → particle physics, where it constitutes the basis of → electroweak interactions. In cosmology, according to the → Big Bang model, the fundamental forces of the Universe split off from one another in a form of spontaneous symmetry braking. If a single, unified force existed with a certain symmetry just after the Big Bang, if that symmetry were somehow broken so that the unified force were fractured, then the result might be several fundamental forces. See also → grand unified theory, → theory of everything, → phase transition.

→ spontaneous; → symmetry; → break.

An → atomic transition that gives rise to a → spontaneous emission.

→ spontaneous; → transition.