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Alchemy, natural philosophy, and early modern physics proposed the existence of a medium of the æther (also spelled ether, from the Greek word (αἰθήρ) aether, meaning "upper air" or "pure, fresh air" ), a space-filling substance or field, thought to be necessary as a transmission medium. The assorted aether theories embody the various conceptions of this "medium" and "substance". This early modern aether has little in common with the aether of classical elements from which the name was borrowed.
Although hypotheses of the Æther vary somewhat in detail they all have certain characteristics in common. Essentially it is considered to be a physical medium occupying every point in Space, including within material bodies. A second essential feature is that its properties gives rise to the electric, magnetic and gravitational potentials and determines the propagation velocity of their effects. Therefore the speed of light and all other propagating effects are determined by the physical properties of the Æther at the relevant location, analogous to how gaseous, liquid and solid media affect the propagation of sound waves.
The Æther is considered the global reference frame for the Universe and thus velocities are all absolute relative to its rest frame. Therefore, in this view, any physical consequences of those velocities are considered as having an absolute, ie real effects.
Recent Æther theories (see section below on protoscience links) of velocity effects, phenomenon of gravitation and planetary motion (i.e. the angular momentum), creation of proton, of stars (neutron stars too) and planets, etc., exist but are not generally accepted by the mainstream scientific community.
Additional recommended knowledge
Isaac Newton disproved the æther "vortex theory" of planetary motion but later proposed a "new" æther, exceptionally fluid, whose density was affected by the local density of matter and local gravitational field strength (see: Optiks). Newton also said that he did not know whether his new æther should be particulate or not  - if it were particulate, the particles would have to be incredibly small, even smaller than light-corpuscles. 
The basic idea of the æther as a physical transmission medium is simple, and like all media, if it exists, must have fundamental properties including a pressure, mass density, and temperature. Further, if compressible, it will also exhibit a characteristic finite propagation speed, c, at which all transfer of momentum and energy through it can be carried from one physical location to another. Compressibility also means that there will also be a distinct coefficient of compressibility (and its inverse, a distinct modulus), a characteristic impedance, and the ability to create and sustain wave activity. Any other properties, including ponderable matter and the specific characteristics of waves are solely dependent upon specifics arising from these basics.
As can be seen from historical timelines  referenced above, up until the early part of the twentieth century æther played a central and dominant role in the development and evolution of all of theoretical physics.
During the 19th century the most basic and fundamental physical characteristics known were those pertaining to electric, magnetic, and luminous (light) phenomena. The focus of theoretical development focused upon these phenomena and integrating them into a single common framework. Based upon Faraday's meticulous findings James Clerk Maxwell succeeded brilliantly in doing so. His model was based upon Helmholtz's æther vortex model and is described in detail in his 1861-62 series of articles titled On the Physical Lines of Force. Because of this, the aether concept was commonly referred to as luminiferous aether during this period. For the individuals of this time, which included Lord Kelvin, J. J. Thomson, and P. G. Tait, the ether was hypothesized to be a non-compressible, inviscid fluid.
Motion and the preferred frame
During the 19th Century attention was also focused on the interaction of electro-magnetic phenomena with matter. It was in this area of late 19th Century physics that certain unresolvable contradictions arose, leading to the emergence of Einstein's Relativity theories. At the time it was commonly assumed by many that ponderable matter (mass having a rest value & inertia) was distinctly different, and was embedded, or enveloped in the all pervasive æther. By logical extension, movement of such objects should require it to plow through this æther, and this in turn, should create a drag reaction in the æther. If the material object is not moving the pressure exerted by æther is equal in all directions (isotropic). This condition is called the rest frame of the æther.
It was logical therefore to attempt to measure the speed of matter through the æther. The motion of the Earth was considered to be of sufficient magnitude that its speed could be determined. The expected difference was calculated based upon the assumptions that; 1) light speed was independent of Earth's motion (or matter in general), and, 2) the matter in the measuring equipment is independent and unaffected this movement. When these assumptions are valid it was also demonstrated that this rest frame would have preferential properties making it physically different from all others. Thus this condition is also known as the preferred frame. The resulting geometrical calculation formed the basis for the expectation of a positive result, and expected lower bound value that should be seen.
An experiment testing this hypothesis was first performed by Albert Michelson in 1881. It produced a null result. It was repeated in 1887 in collaboration with Edward Morley and is known today as the Michelson-Morley experiment. To date all such experiments have failed to demonstrate the expected positive result.
Since it is improbable that any medium would not itself react to the movement of such a foreign embedded body, the idea of a stationary æther can be effectively ruled out. However, like swirling your hand in water, if the medium has any viscosity it will experience drag and form a circulation, which, over time, acts to reduce the relative speed and drag between the body and the medium. The final resulting magnitude is dependent upon the assumption of viscosity, and leads to many variants of the theory, each with slightly different drag coefficients and rules for how matter should interact with light. The number of competing theories of this type made keeping track of all the resulting predictions rather difficult. As with string theory today, there seemed to be too many options, and with the proper ad-hoc choice of coefficient values, it seemed that one could predict almost anything. These, as a group, are known as partially dragged æther theories.
Similar experiments have included:
Lorentz's ether and special relativity
The Lorentz ether theory ("LET") or Lorentzian electrodynamics (1904) made use of the Lorentz-FitzGerald contraction hypothesis - it suggested that an object moving through the aether was contracted in its direction of motion by a special ratio now named after Lorentz: According to this theory, an inertial observer would be incapable of measurng their absolute motion, so that their measurements would comply with the principle of relativity ("PoR"). Nevertheless, in Lorentz's theory, which is completely consistent with Maxwell's theory, the state of a material system is not independent of it motion relative to the medium.
Einstein's special theory of relativity ("SR", 1905) rederived Lorentz' relationships by declaring that all observers could claim that lightspeed was absolutely fixed in their own inertial frame. Where LET said that the variation of velocity of light through the aether was undetectable, SR used this undetectability to reject the concept of an underlying aether and replaced a notional state of aether motion with it with the concept of the inertial frame. SR is now generally considered to be the modern replacement for LET.
Acceleration effects still implied the existence of some physical property to spacetime, and if (like Mach), one decided that acceleration and rotation effects should be the result of interactions between distant masses, acceleration and rotation also had to be capable of distorting light-beam geometry, and, by implication, distorting spacetime itself. If these properties were absolute, then the properties of spacetime forced behaviours onto matter without accepting any back-reaction (like a form of "absolute" aether), a behaviour that Einstein referred to as "an inherent epistemological defect". But if the effects were purely relative, then forcing matter to move in a way that spacetime did not like should cause a distortion in spacetime ("space tells matter how to move, matter tells space how to bend").
See also: History of special relativity
Main article: Mechanical explanations of gravitation
From the 16th until the late 19th century, gravitational phenomena had also been modeled utilizing an aetherial concept. The most well-known concept is Le Sage's theory of gravitation. Other concepts were made by Isaac Newton, Bernhard Riemann, Lord Kelvin etc.
According to Rughede  physical space is the universal aether space, and gravity is a process of enduring exchange of radiant energy between all astrophysical bodies.
Aether and general relativity
"Aether and the theory of relativity" was a title used by Einstein in a lecture on general relativity and aether theory. Einstein said that general relativity's gravitational field parameters could be said to have all the usual properties of an aether except one: it was not composed of particulate bodies that could be tracked over time, and so it could not be said to have the property of motion.  The general attitude to this amongst physicists today seems to be that Einstein's comments don't count because they stretch the idea of aether theory too far: it is argued that a "non-particulate" aether theory is not really an aether theory, or at least, it doesn't correspond to the idea of "historical" aether theory that is currently taught.
Such a view, however, contradicts the continuum concept of space-time and fields and Einstein's statements in "Aether and the Theory of Relativity", May 5th, 1920:
"More careful reflection teaches us, however, that the special theory of relativity does not compel us to deny ether." and "To deny the ether is ultimately to assume that empty space has no physical qualities whatever".
As to the idea of "historical" aether theories it has to be decided, which "historical" theory out of many may adequately be currently taught?
Aether and quantum mechanics
Quantum mechanics can be used to describe spacetime as being "bitty" at extremely small scales, fluctuating and generating particle pairs that appear and disappear incredibly quickly. Instead of being "smooth", the vacuum is described as looking like "quantum foam". It has been suggested that this seething mass of virtual particles may be the equivalent in modern physics of a particulate aether.
In physics there is no concept considered exactly analogous to the aether. However, dark energy is sometimes called quintessence due to its similarity to the classical aether. Modern physics is full of concepts such as free space, space foam, Planck particles, quantum wave state (QWS), zero-point energy, quantum foam, and vacuum energy.
The Einstein-aether theory
Although it is by no means widely accepted, the most popular aether theory today is the Einstein æther theory, also known as Æ-theory. This theory was pioneered by Ted Jacobson among others. It is a generally covariant theory that comes equipped with a preferred temporal vector field called the æther field, which is the preferred time direction. Christopher Eling, Ted Jacobson and David Mattingly review this theory in their article Einstein Æther Theory.
All æther theories break the Lorentz symmetry of the theory down, at least, to the special orthogonal group of rotations. This symmetry breaking implies the existence of an associated Goldstone boson. Some experimental signatures of such a boson were analyzed by Nima Arkani-Hamed, Hsin-Chia Cheng, Markus Luty and Jesse Thaler in Universal Dynamics of Spontaneous Lorentz Violation and a New Spin-Dependent Inverse-Square Law Force.
External articles, references, and further readings
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Aether_theories". A list of authors is available in Wikipedia.|