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According to ancient and medieval science aether (Greek αἰθήρ aithēr ), also spelled æther or ether,  is the material that fills the region of the universe above the terrestrial sphere. From the time of Aristotle until the late 16th century, aether was known instead as quintessence and was thought to have a rigid crystalline structure. Tycho Brahe discovered a new star in the night sky and proved that comets exist outside the sphere of the moon, which disproved the perfect crystalline theory of quintessence; this discovery brought about the term aether, given to the mysterious medium that many posited must fill the void between celestial bodies in space.

Mythological origins
The word αἰθήρ (aithēr) in Homeric Greek means "pure, fresh air" or "clear sky", imagined in Greek mythology to be the pure essence where the gods lived and which they breathed, analogous to the air breathed by mortals (also personified as a deity, Aether, the son of Erebus and Nyx). It is related to αἴθω "to incinerate", also intransitive "to burn, to shine" (related is the name Aithiopes (Ethiopians), meaning "people with a burnt (black) visage"). See also Empyrean.

Fifth element
In Plato's Timaeus (55d) speaking about air, Plato mentions that "there is the most translucent kind which is called by the name of aether (αίθηρ)". Aristotle, who had been Plato's student at the Akademia, disagreed with his former mentor and added aether to the system of the classical elements of Ionian philosophy as the "fifth element" (the quintessence). In his theory the terrestrial sphere was composed of 4 elements: earth, water, air, and fire, which desire to move in this order to the center of the Earth. Outside of this terrestrial sphere existed celestial bodies held in place in crystalline spheres of "quintessence." He postulates that the four terrestrial elements were subject to change and moved naturally in straight lines. In Aristotle's system aether had no qualities (was neither hot, cold, wet, or dry), was incapable of change (with the exception of change of place), and by its nature moved in circles, and had no contrary, or unnatural, motion. Medieval scholastic philosophers granted aether changes of density, in which the bodies of the planets were considered to be more dense than the medium which filled the rest of the universe. Robert Fludd stated that the aether was of the character that it was "subtler than light". Fludd cites the 3rd-century view of Plotinus, concerning the aether as penetrative and non-material. See also Arche.

Quintessence, as used in alchemy
The quintessence, or fifth element, was a term used by medieval alchemists for a substance similar or identical to that thought to make up the heavenly bodies. It was proposed that a little of the quintessence was present in things on earth, meaning that things on earth could be affected by what happened in the heavens. This theory was developed in the text “The testament of Lullius”, attributed to Raymond Lull and written in the early 14th century. Alchemy then dealt with the isolation and use of this fifth element.

The idea spread with rapidity through Europe and was popular with later alchemists, especially of the medical sort. This can be seen in “The book of Quintessence”, a 15th-century English translation of a continental text. In it, the quintessence is used as a medicine for man’s illnesses, and instructions are given for making it from seven times distilled alcohol. The term has over the years become synonymous with elixirs, medicine or the philosopher’s stone itself.

Legacy
With the 18th century physics developments some physical models known as "aether theories" made use of a similar concept as an explanation for the propagation of electromagnetic or gravitational forces. As early as the 1670s, Newton used the idea of aether to help match observations to strict mechanical rules of his physics. However, the early modern aether had little in common with the aether of classical elements from which the name was borrowed. These aether theories are considered to be scientifically obsolete, as the development of special relativity showed that Maxwell's equations do not require the aether for the transmission of these forces. However, Einstein himself noted that his own model which replaced these theories could itself be thought of as an aether, as it implied that the empty space between objects had its own physical properties.

Despite the early modern aether models being superseded by general relativity, occasionally some physicists have attempted to reintroduce the concept of an aether in an attempt to address perceived deficiencies in a current physical model. One proposed model of dark energy has been named "quintessence" by its proponents, in honor of the classical element.

Aether and light
According to the wave theory of light proposed by Christiaan Huygens, light travelled in the form of longitudinal waves via an "omnipresent, perfectly elastic medium having zero density, called aether". Later, when it was proved that the nature of light wave is transverse instead of longitudinal, Huygens' theory was replaced by subsequent theories proposed by Maxwell, Einstein and de Broglie, which rejected the existence and necessity of aether to explain the various optical phenomena.

Aether and gravitation
Aether has been used in various gravitational theories as a medium that helps explain gravitation and what causes it. It was used in one of Sir Isaac Newton's first published theories of gravitation. In his aether model, Newton describes aether as a medium that "flows" continually downward toward the Earth's surface and is partially absorbed and partially re-emitted. This "circulation" of aether is what he associated the force of gravity with to help explain the action of gravity in a non-mechanical fashion. This theory described aether as a medium that was dense within objects and rare without of them. As particles of denser aether interacted with the rare aether they were attracted back to the dense aether much like cooling vapors of water are attracted back to each other to form water. This change is what caused the pull of gravity to take place, according to this early theory, and allowed an explanation for action at a distance instead of action through direct contact. Newton also explained this changing rarity and density of aether in his letter to Robert Boyle in 1679. He illustrated aether and its field around objects in this letter as well and used this as a way to inform Boyle about his theory. Though Newton eventually changed his theory to one involving force and the laws of motion, his starting platform for the modern understanding of gravity came from his original aether model for gravitation.