Aether (classical element)

According to ancient and medieval science, aether (Greek: αἰθήρ aithēr), also spelled æther or ether, also called quintessence, is the material that fills the region of the universe above the terrestrial sphere. The concept of aether was used in several theories to explain several natural phenomena, such as the traveling of light and gravity. In the late 19th century, physicists postulated that aether permeated all throughout space, providing a medium through which light could travel in a vacuum, but evidence for the motion of such a medium was not found in the Michelson–Morley experiment entitled, "On the Relative Motion of the Earth and the Luminiferous Ether."

The word αἰθήρ (aithēr) in Homeric Greek means "pure, fresh air" or "clear sky." In Greek mythology, it was thought to be the pure essence that the gods breathed, filling the space where they lived, analogous to the air breathed by mortals. It is also personified as a deity, Aether, the son of Erebus and Nyx in traditional Greek mythology. Aether is related to αἴθω "to incinerate," and intransitive "to burn, to shine" (related is the name Aithiopes (Ethiopians; see Aethiopia), meaning "people with a burnt (black) visage").

In Plato's Timaeus (58d) speaking about air, Plato mentions that "there is the most translucent kind which is called by the name of aether (αίθηρ)". but otherwise he adopted the classical system of four elements. Aristotle, who had been Plato's student at the Akademia, agreed on this point with his former mentor, emphasizing additionally that fire sometimes has been mistaken for aether. However, in his Book On the Heavens he introduced a new "first" element to the system of the classical elements of Ionian philosophy. He noted that the four terrestrial classical elements were subject to change and naturally moved linearly. The first element however, located in the celestial regions and heavenly bodies, moved circularly and had none of the qualities the terrestrial classical elements had. It was neither hot nor cold, neither wet nor dry. With this addition the system of elements was extended to five and later commentators started referring to the new first one as the fifth and also called it aether, a word that Aristotle had not used.

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