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## PowerPedia:Aether

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: See Directory:Aether for an index of Aether companies, applications, theories.

The assorted aether theories embody the various conceptions of a "There was an error working with the wiki: Code[22]" and "There was an error working with the wiki: Code[23]". The Aether of the classical elements is a concept, historically, used in science and in philosophy. There was an error working with the wiki: Code[92], There was an error working with the wiki: Code[93], and early modern Physics proposed the existence of a medium of the æther (also spelled ether, from the Greek word ????? (aith?r), meaning "upper air" or "pure, fresh air" There was an error working with the wiki: Code[1]), a space-filling substance or field, thought to be necessary as a There was an error working with the wiki: Code[94].

The factual accuracy of this article or section may be compromised due to out-of-date information.

You can improve the article by updating it. Since 2008, a workable and testable Super Relativity 'æther' has surfaced that redefines this to comply with the works of Maxwell, Lorentz and Einstein.

#### Introduction

The term aether, æther or ether may refer to the original meaning as the personification of the "upper sky", space and heaven, in Greek mythology.

In Science, engineering, and philosophy, the Aether of classical elements is a concept, historically, used in science (as a medium) and in philosophy (as a substance) and this includes a number of Aether theories in alchemy, natural philosophy, and modern physics which suppose a "fifth element". The Luminiferous aether, in early physics considered to be the medium through which light propagates. This should not be confused with "There was an error working with the wiki: Code[95]", a class of chemical compounds, or There was an error working with the wiki: Code[96] (which has the common name "ether").

In Spirituality, Etheric plane was a finer grade of matter, "ether" in addition to the solids, liquids, and gases - which permeates the subatomic structure of the earth and its atmosphere. The Etheric body, a sort of life force body or aura that constitutes the "blueprint" of the physical body, and which sustains the physical body.

Plato described the aether as that which God used in the delineation of the universe in There was an error working with the wiki: Code[97]There was an error working with the wiki: Code[2]. 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 occuping 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 that location which acts in a manner analogous to 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 research and modern derivatives) 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 all in the "There was an error working with the wiki: Code[98]" There was an error working with the wiki: Code[99].

#### Classic element

Aether (also spelled ether) is a concept used in ancient and There was an error working with the wiki: Code[24] as a There was an error working with the wiki: Code[25]. The aether was believed to be the substance which filled the region of the universe above the There was an error working with the wiki: Code[26]. There was an error working with the wiki: Code[27] distinct from the other four, There was an error working with the wiki: Code[28], There was an error working with the wiki: Code[29], There was an error working with the wiki: Code[30], and There was an error working with the wiki: Code[31]. Aether was also called Quintessence (from quinta essentia, "fifth element"). Quintessence was also supposed to be a definition of pure energy. Its force is imagined to be like a lightning. This element also has the power of life. Its There was an error working with the wiki: Code[100] was the There was an error working with the wiki: Code[101].

##### Mythological origins

The word 'aether' (?????) in There was an error working with the wiki: Code[32] breathed by mortals (also personified as a deity, There was an error working with the wiki: Code[33], the son of There was an error working with the wiki: Code[34]). It corresponds to the concept of There was an error working with the wiki: Code[102] in Hindu philosophy. It is related to There was an error working with the wiki: Code[3] "to incinerate"There was an error working with the wiki: Code[4], also intransitive "to burn, to shine" (related is the name Aithiopes (There was an error working with the wiki: Code[103]ns), meaning "people with a burnt (black) visage". See also There was an error working with the wiki: Code[104].

##### Fifth element

Plato's Timaeus posits the existence of a fifth element (corresponding to the fifth remaining Platonic solid, the dodecahedron) called quintessence, of which the cosmos itself is made. There was an error working with the wiki: Code[35] There was an error working with the wiki: Code[36] as the "fifth element" (the There was an error working with the wiki: Code[105]), on the principle that the four terrestrial elements were subject to change and moved naturally in straight lines while no change had been observed in the celestial regions and the heavenly bodies moved in circles. In Aristotle's system aether had no qualities (was neither hot, cold, wet, nor dry), was incapable of change (with the exception of change of place), and by its nature moved in circles.There was an error working with the wiki: Code[5] Medieval scholastic philosophers granted aether changes of density, in which the bodies of the planets were considered to be denser than the medium which filled the rest of the universe.There was an error working with the wiki: Code[6] There was an error working with the wiki: Code[106] stated that the aether was of the character that it was "subtler than light". Fludd cites the There was an error working with the wiki: Code[107] view of There was an error working with the wiki: Code[108], concerning the aether as penetrative and non-material.There was an error working with the wiki: Code[7]

#### Newtonian æther

There was an error working with the wiki: Code[109] 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: There was an error working with the wiki: Code[110]). Newton also said that he did not know whether his new æther should be particulate or not There was an error working with the wiki: Code[8] - if it was particulate, the particles would have to be incredibly small, even smaller than light-corpuscles. There was an error working with the wiki: Code[9]

#### Luminiferous æther

The basic idea of the æther as a physical There was an error working with the wiki: Code[37], at which all transfer of There was an error working with the wiki: Code[38], and the ability to create and sustain There was an error working with the wiki: Code[111] activity. Any other properties, including ponderable There was an error working with the wiki: Code[112] and the specific characteristics of waves are solely dependent upon specifics arising from these basics.

As can be seen from historical timelines There was an error working with the wiki: Code[10], 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. In the late 19th century,There was an error working with the wiki: Code[11] luminiferous aether ("light-bearing Aether (classical element)") was the term used to describe a medium for the propagation of There was an error working with the wiki: Code[39] ?????, from a root meaning "to kindle/burn/shine", which signified the substance thought in ancient times to fill the upper regions of space, beyond the clouds.

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 There was an error working with the wiki: Code[113]'s meticulous findings There was an error working with the wiki: Code[114] succeeded brilliantly in doing so. His model was based upon There was an error working with the wiki: Code[115]'s æther vortex model and is described in detail in his 1861-62 series of articles titled On the Physical Lines of Force.There was an error working with the wiki: Code[12] Because of this, the aether concept was commonly referred to as Luminiferous aether during this period.

##### The history luminiferous aether

See also There was an error working with the wiki: Code[116].

There was an error working with the wiki: Code[117] had assumed that light was made up of numerous small particles, in order to explain features such as its ability to travel in straight lines and reflect off surfaces. This theory was known to have its problems although it explained reflection well, its explanation of There was an error working with the wiki: Code[118] and There was an error working with the wiki: Code[119] was less pleasing. In order to explain refraction, in fact, Newton's Opticks (1704) postulated an "Aethereal Medium" transmitting vibrations faster than light, by which light (when overtaken) is put into "Fits of easy Reflexion and easy Transmission" (causing refraction and diffraction). Newton believed that these vibrations were related to things like heat radiation, saying:

:Is not the Heat of the warm Room convey'd through the Vacuum by the Vibrations of a much subtiler Medium than Air, which after the Air was drawn out remained in the Vacuum? And is not this Medium the same with that Medium by which Light is refracted and reflected, and by whose Vibrations Light communicates Heat to Bodies, and is put into Fits of easy Reflexion and easy Transmission?

The modern understanding, of course, is that heat radiation is light, but Newton considered them two different phenomena (believing heat vibrations to be excited "when a Ray of Light falls upon the Surface of any pellucid Body"). He wrote that "I do not know what this Aether is", but that if it consists of particles then they must be "exceedingly smaller than those of Air, or even than those of Light: The exceeding smallness of its Particles may contribute to the greatness of the force by which those Particles may recede from one another, and thereby make that Medium exceedingly more rare and elastick than Air, and by consequence exceedingly less able to resist the motions of Projectiles, and exceedingly more able to press upon gross Bodies, by endeavoring to expand itself."

There was an error working with the wiki: Code[120], prior to Newton, had hypothesized that light itself was a wave propagating through an Aether, but Newton rejected this idea. The main reason for his rejection stemmed from the fact that both men could apparently only envision light to be a longitudinal wave, like sound and other There was an error working with the wiki: Code[121]s in gases and fluids. However, longitudinal waves by necessity have only one form for a given propagation direction, rather than two There was an error working with the wiki: Code[122]s as in a transverse wave, and thus they were unable to explain the phenomenon of There was an error working with the wiki: Code[123] (where two polarizations of light are refracted differently by a crystal). Instead, Newton preferred to imagine non-spherical particles (or "corpuscles") of light with different "sides" that give rise to birefringence. A further reason why Newton rejected light as waves in a medium, however, was because such a medium would have to extend everywhere in space, and would thereby "disturb and retard the Motions of those great Bodies" (the planets and comets) and thus "as it [light's medium] is of no use, and hinders the Operation of Nature, and makes her languish, so there is no evidence for its Existence, and therefore it ought to be rejected."

In 1720 There was an error working with the wiki: Code[124] carried out a series of experiments attempting to measure There was an error working with the wiki: Code[125]. Although he failed to detect any parallax (thereby placing a lower limit on the distance to stars), he discovered another effect, There was an error working with the wiki: Code[126], an effect which depends not on position (as in parallax), but on speed. He noticed that the apparent position of the star changed as the Earth moved around its orbit. Bradley explained this effect in the context of Newton's corpuscular theory of light, by showing that the aberration angle was given by simple vector addition of the Earth's orbital velocity and the velocity of the corpuscles of light (just as vertically falling raindrops strike a moving object at an angle). Knowing the Earth's velocity and the aberration angle, this enabled him to estimate the speed of light. To explain stellar aberration in the context of an ether-based theory of light was regarded as more problematic, because it requires that the ether be stationary even as the Earth moves through it &ndash precisely the problem that led Newton to reject a wave model in the first place.

However, a century later, There was an error working with the wiki: Code[40] and There was an error working with the wiki: Code[41] revived the wave theory of light when they pointed out that light could be a There was an error working with the wiki: Code[42] still assumed, however, that like mechanical waves, light waves required a medium for There was an error working with the wiki: Code[43], and thus required Huygens' idea of an aether "gas" permeating all space.

However a transverse wave apparently required the propagating medium to behave as a solid, as opposed to a gas or fluid. The idea of a solid that did not interact with other matter seemed a bit odd, and There was an error working with the wiki: Code[127] suggested that perhaps there was some sort of "dragging", or "entrainment", but this made the aberration measurements difficult to understand. He also suggested that the absence of longitudinal waves suggested that the aether had negative compressibility but There was an error working with the wiki: Code[128] pointed out that such a fluid would be unstable. There was an error working with the wiki: Code[129] became a champion of the entrainment interpretation, developing a model in which the aether might be (by analogy with pine pitch) rigid at very high frequencies and fluid at lower speeds. Thus the Earth could move through it fairly freely, but it would be rigid enough to support light.

Later, There was an error working with the wiki: Code[44]. The apparent need for a propagation medium for such There was an error working with the wiki: Code[45] can be seen by the fact that they consist of perpendicular electric (E) and magnetic (B or H) waves. The E waves consist of undulating dipolar electric fields, and all such dipoles appeared to require separated and opposite electric charges. Electric charge is an inextricable property of There was an error working with the wiki: Code[130], so it appeared that some form of matter was required to provide the alternating current that would seem to have to exist at any point along the propagation path of the wave. Propagation of waves in a true vacuum would imply the existence of Electric fields without associated Electric charge, or of electric charge without associated matter. Albeit compatible with Maxwell's equations, There was an error working with the wiki: Code[131] of electric fields could not be demonstrated in vacuum, because all methods of detecting electric fields required electrically charged matter.

In addition, Maxwell's equations required that all electromagnetic waves in There was an error working with the wiki: Code[46]. As this can only occur in one There was an error working with the wiki: Code[132] in Newtonian physics (see There was an error working with the wiki: Code[133]), the aether was hypothesized as the absolute and unique frame of reference in which Maxwell's equations hold. That is, the aether must be "still" universally, otherwise c would vary from place to place. Maxwell himself proposed several mechanical models of aether based on wheels and gears and There was an error working with the wiki: Code[134] even constructed a working model of one of them. These models were non-trivial especially because they had to agree with the fact that the electromagnetic waves are There was an error working with the wiki: Code[135] but never There was an error working with the wiki: Code[136].

Nevertheless, by this point the mechanical qualities of the aether had become more and more magical: it had to be a There was an error working with the wiki: Code[47], and continuous at a very small scale.

Contemporary scientists were aware of the problems, but aether theory was so entrenched in physical law by this point that it was simply assumed to exist. In 1908 There was an error working with the wiki: Code[48] to the There was an error working with the wiki: Code[49] as saying that "aether is little more than a nominative case of the verb to undulate". Others criticized it as an "English invention", although Rayleigh jokingly corrected them to state it was actually an invention of the Royal Institution.

By the early 20th Century, aether theory was in trouble: A series of There was an error working with the wiki: Code[50] had been carried out in the late 1800s to try to detect the motion of earth through the aether, and had failed to do so. A range of proposed aether-dragging theories could explain the null result but these were more complex, and tended to use arbitrary-looking coefficients and physical assumptions. Lorentz and Fitzgerald offered a more elegant solution to how the motion of an absolute aether could be undetectable (length contraction), but if their equations were correct, the new There was an error working with the wiki: Code[137] (1905) could generate the same mathematics without referring to an aether at all. Aether fell to There was an error working with the wiki: Code[138].

##### luminiferous aether mechanics

The key difficulty with the aether hypothesis arose from the juxtaposition of the two well-established theories of Newtonian dynamics and Maxwell's electromagnetism. Under a There was an error working with the wiki: Code[139] the equations of Newtonian dynamics are There was an error working with the wiki: Code[140], whereas those of electromagnetism are not. Basically this means that while physics should remain the same in non-accelerated experiments, light would not follow the same rules because it is travelling in the universal "aether frame". Some effect caused by this difference should be detectable.

A simple example concerns the model on which aether was originally built: sound. The speed of propagation for mechanical waves, the There was an error working with the wiki: Code[141], is defined by the mechanical properties of the medium. For instance, if one is in an There was an error working with the wiki: Code[142], you can still carry on a conversation with the person beside you because the sound of your words are travelling along with the air inside the aircraft. This effect is basic to all Newtonian dynamics, which says that everything from sound to the trajectory of a thrown baseball should all remain the same in the aircraft as sitting still on the Earth. This is the basis of the Galilean transformation, and the concept of frame of reference.

But the same was not true for light, since Maxwell's mathematics demanded a single universal speed for the propagation of light, based, not on local conditions, but on two measured properties that were assumed to be the same throughout the universe. If these numbers did change, there should be noticeable effects in the sky stars in different directions would have different colors, for instance. Certainly they would remain constant within a small volume, inside the aircraft in our example for instance, which implies that light would not follow along with the aircraft (or the Earth) in a fashion similar to sound. Nor could light change media, for instance, using the atmosphere while near the Earth. It had already been demonstrated that if this were so, the sky would be colored in different directions as the light moved from the still medium of the aether to the moving medium of the Earth's atmosphere, causing diffraction.

Thus at any point there should be one special coordinate system, "at rest relative to the aether". Maxwell noted in the late 1870s that detecting motion relative to this aether should be easy enough &ndash light travelling along with the motion of the Earth would have a different speed than light travelling backward, as they would both be moving against the unmoving aether. Even if the aether had an overall universal flow, changes in position during the day/night cycle, or over the span of seasons, should allow the drift to be detected.

##### Experiments

Numerous experiments were carried out in the late 1800s to test for this "aether wind" effect, but most were open to dispute due to low accuracy. Measurements on the speed of propagation were so inaccurate that comparing two speeds to look for a difference was essentially impossible. The famous There was an error working with the wiki: Code[143] instead compared the source light with itself after being sent in different directions, looking for changes in phase in a manner that could be measured with extremely high accuracy. The publication of their result in 1887, the There was an error working with the wiki: Code[144], was the first clear demonstration that something was seriously wrong with the "absolute aether" concept. A series of experiments using similar but increasingly sophisticated apparatus all returned the null result as well. A conceptually different experiment that also attempted to detect the motion of the aether was the 1903 There was an error working with the wiki: Code[145], which like Michelson-Morley obtained a null result.

It is important to understand what "null result" means in this context. It does not mean there was no motion detected rather it means that the results produced by the experiment were not compatible with the assumptions used to devise it. In this case the MM experiment showed a small positive velocity causing a movement of the fringing pattern of about 0.01 of a fringe however it was too small to demonstrate the expected aether wind effect due to the earth's (seasonally varying) velocity which would have required a shift of 0.4 of a fringe, and the error was small enough that the value may have indeed been zero. More modern experiments have since reduced the possible value to a number very close to zero, about 10-15.

These "aether-wind" experiments led to its abandonment by some scientists, and to a flurry of efforts to "save" aether by assigning it ever more complex properties by others. Of particular interest was the possibility of "aether entrainment" or "aether drag", which would lower the magnitude of the measurement, perhaps enough to explain MMX results. However, as noted earlier, aether dragging already had problems of its own, notably aberration. A more direct measurement was made in the There was an error working with the wiki: Code[146], which ran a complete MM experiment with one of the "legs" placed between two massive lead blocks. If the aether was dragged by mass then this experiment would have been able to detect the drag caused by the lead, but again the null result was found. Similar experiments by Hoek placed one leg in a heavy vat of water. The theory was again modified, this time to suggest that the entrainment only worked for very large masses or those masses with large magnetic fields. This too was shown to be incorrect when There was an error working with the wiki: Code[147] noted no such effect around other planets.

Another, completely different, attempt to save "absolute" aether was made in the There was an error working with the wiki: Code[51] in 1932 as they concluded that the interferometer's arm contracts and also the frequency of its light source "very nearly" varies in the way required by relativity.There was an error working with the wiki: Code[13]

Another experiment purporting to show effects of an aether was There was an error working with the wiki: Code[52]'s 1851 experimental confirmation of There was an error working with the wiki: Code[53]'s 1818 prediction that a medium with There was an error working with the wiki: Code[148] n moving with a velocity v would increase the speed of light traveling through the medium in the same direction as v from c/n to:

:\frac{c}{n} + \left( 1 - \frac{1}{n^2} \right) v

That is, movement adds only a fraction of the medium's velocity to the light (predicted by Fresnel in order to make There was an error working with the wiki: Code[54] in 1907 to be an approximation, valid for v much smaller than c, for the correct relativistic formula to add the velocities v (medium) and c/n (rest frame):

:\frac{c/n + v}{1 + \frac{v c/n} {c^2}} \approx \frac{c}{n} + \left( 1 - \frac{1}{n^2} \right) v + O\left(\frac{v^2}{c^2}\right).

Variations on these themes continued for the next 30 years. Positive results were reported by several of the key researchers, including additional experiments by Michelson, Morley and There was an error working with the wiki: Code[149]. Miller reported positive results on several occasions, but of a magnitude that required further modifications to the drag or contraction theories. During the 1920s a slew of increasingly accurate experiments returned the null result, and the positives were generally attributed to experimental errors. Other positive results included Sagnac in 1913, and the There was an error working with the wiki: Code[150] in 1925. This effect that is known as There was an error working with the wiki: Code[151] is nowadays used in optical gyroscopes and shows that rotation is similarly "absolute" for light as it is for pendulums. Sagnac regarded this as evidence for the aether.There was an error working with the wiki: Code[14]

#### End of aether?

Aether theory was dealt another blow when the Galilean transformation and Newtonian dynamics were both modified by There was an error working with the wiki: Code[152]'s There was an error working with the wiki: Code[153], giving the mathematics of Lorentzian electrodynamics a new, "non-aether" context. Like most major shifts in scientific thought, the move away from aether theory did not happen immediately but, as experimental evidence built up, and as older scientists left the field and their places were taken by the young, the concept lost adherents.

Einstein based his special theory on Lorentz's earlier work, but instead of suggesting that the mechanical properties of objects changed with their constant-velocity motion through an aether, he took the somewhat more radical step of suggesting that the math was a general transformation, and that the Galilean transformation was a "special case" that worked only at the low speeds we had studied up to that time. By applying the transformation to all inertial frames of reference, he demonstrated that physics remained invariant as it had with the Galilean transformation, but that light was now invariant as well. With the development of special relativity, the need to account for a single universal frame had disappeared -- and aether went along with it, or so it seemed.

For Einstein the Lorentz transformation implied a radical conceptual change: that the concept of position in space or time was not absolute, but could differ depending on the observer's location and speed. This "oddness" of Einstein's interpretation led to special relativity being considered highly questionable for some time. All of this left the problem of light propagation through a vacuum. However, in another paper published the same month, Einstein also made several observations on a then-thorny problem, the There was an error working with the wiki: Code[55]"There was an error working with the wiki: Code[15]

Lorentz on his side continued to use the aether concept. In his lectures of around 1911 he pointed out that what "the theory of relativity has to say", "can be carried out independently of what one thinks of the aether and the time". He reminded his audience of the fact that "whether there is an aether or not, electromagnetic fields certainly exist, and so also does the energy of the electrical oscillations" so that, "if we do not like the name of "aether", we must use another word as a peg to hang all these things upon." He concluded that "one cannot deny the bearer of these concepts a certain substantiality".There was an error working with the wiki: Code[16]

There was an error working with the wiki: Code[154] was a strong supporter of special relativity but argued in 1911 that absolute effects from velocity change or acceleration (such as radiation) demonstrate the existence of an aether. As additional illustration he discussed the absolute effect of velocity change on There was an error working with the wiki: Code[155] on two space travelers. This example would later lead to the There was an error working with the wiki: Code[156].

In the meantime Einstein changed his opinion about the aether concept. In a lecture meant for his inauguration at the There was an error working with the wiki: Code[157] in 1920, Einstein stressed that space is "endowed with physical quantities"There was an error working with the wiki: Code[17] He held that general relativity attributed tangible physical properties to space including some kind of medium for light, although not a material one. Shortly before his lecture in Leyden in 1920 he admitted in the paper: "Grundgedanken und Methoden der Relativitätstheorie in ihrer Entwicklung dargestellt":

: "Therefore I thought in 1905 that in physics one should not speak of the ether at all. This judgement was too radical though as we shall see with the next considerations about the general theory of relativity. It moreover remains, as before, allowed to assume a space-filling medium if one can refer to electromagnetic fields (and thus also for sure matter) as the condition thereof ".

Also Michelson, who received the Nobel Prize in physics in 1907 for his optical studies, stated that even if relativity is here to stay we don't have to reject the aether. (Minneapolis Morning Tribune of April 14, 1923, p 21) Some other physicists who published their support for modern aether concepts were There was an error working with the wiki: Code[56], There was an error working with the wiki: Code[158] and Geoffrey Builder. Ives was the first to positively measure the effect of speed on clock rates. He wrote in 1940 in a paper in Science:

:"I have considered the popular claim that the ether has been "abolished" [...]. Reverting to experimental findings I have reviewed the experiment of Sagnac, having in mind the claim that the ether can not be detected experimentally. I have asserted that, in the light of the experimentally found variation of clock rate with motion, this experiment does detect the ether."

G. Builder asserted in a paper of 1958 that "there is therefore no alternative to the ether hypothesis" Builder wrote: "the observable effects of absolute accelerations and of absolute velocities must be described to interaction of bodies and physical systems with some absolute inertial system. [...] Interaction of bodies and physical systems with the universe cannot be described in terms of There was an error working with the wiki: Code[57], since this is untenable. There is therefore no alternative to the ether hypothesis." Professor Sherwin supported in 1960 the "philosophical point of view" of Ives and Builder about the aether because of his own conclusion that clocks are "literally slowed down by the speed itself". Sherwin wrote in 1960: "One is led therefore to the conclusion that clocks having a velocity in an inertial frame are literally slowed down by the speed itself. It is this very deduction which makes the generally accepted prediction regarding the "There was an error working with the wiki: Code[159]" unacceptable to Dingle, but which has led both Ives and Builder to consider interpretations of special relativity in which an ether plays an important role, at least from the philosophical point of view."

Also Dirac stated in 1951 in an article in Nature, titled "Is there an ether?" that "we are rather forced to have an ether" Dirac wrote about his theory: "We have now the velocity at all points of space-time, playing a fundamental part in electrodynamics. It is natural to regard it as the velocity of some real physical thing. Thus with the new theory of electrodynamics we are rather forced to have an ether." The large majority of "mainstream" scientists disagreed with such views.

Today, the majority of physicists hold that there is no need to imagine that a medium for light propagation exists. They believe that neither Einstein's general theory of relativity nor quantum mechanics have need for it and that there is no evidence for it. As such, a classical aether is an unnecessary addition to physics that violates the principle of There was an error working with the wiki: Code[58] of special relativity, There was an error working with the wiki: Code[160], relativistic Quantum mechanics, and so on. As outlined earlier, these conditions are often contradictory, making such a task inherently difficult.

Nevertheless the intuitive appeal of a causal background for "relativistic" effects cannot be denied. Some physicists hold that there remain a number of problems in modern physics that are simplified by an aether concept, so that Occam's razor doesn't apply. A very small number of physicists (like There was an error working with the wiki: Code[161]There was an error working with the wiki: Code[18] and There was an error working with the wiki: Code[162]) continued research on the aether for some time, and researchers such as Harold AspdenThere was an error working with the wiki: Code[19] still promote the concept. A number of new aether concepts have been proposed in recent years. However, these aethers differ considerably from the classical luminiferous aether.

Maurizio Consoli of the Italian National Institute of Nuclear Physics in Catania, Sicily, argues in Physics Letters A (vol 333, p 355) that any Michelson-Morley type of experiment carried out in a vacuum will show no difference in the speed of light even if there is an aether. According to him, electroweak theory and quantum field theory suggest that light could appear to move at different speeds in different directions in a medium such as a dense gas in contradiction with special relativity the speed of light would be sensitive to motion relative to an ether and the refractive index of the medium.

Consoli and Evelina Costanzo propose an experiment with laser light passing through cavities filled with a relatively dense gas. With the Earth passing through an aether wind, light would travel faster in one direction than in the perpendicular direction.There was an error working with the wiki: Code[20] Consoli and Constanzo have not run the proposed experiment. The mathematical treatment of their paper does not use the relativistic dragging coefficient to account for the speed of light in a moving medium, and most physicists regard this as an elementary error that leads to their incorrect conclusions.

Their paper is very similar to another paper by Reg Cahill ("R.T. Cahill, A New Light-Speed Anisotropy Experiment: Absolute Motion and Gravitational Waves Detected, in Progress in Physics, vol 4 , 2006" ), another proponent of an experiment that would detect the elusive "preferential frame". Cahill claims to have detected absolute motion with respect to a preferential frame but his paper suffers from the same mathematical shortcomings as the Consoli-Constanzo paper as well as from lack of experimental error bars in his experimental data processing. Consequently, their research had no impact on the physics community.

##### Outside the scientific community

Some adherents of There was an error working with the wiki: Code[163] claim that the Michelson-Morley experiment proves that the Earth is stationary which in turn causes them to explain the universe in terms of an aether or "firmament". Many of these ideas are related to fundamentalist interpretations of Christianity. (see Science and Scripture)

#### 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 the arena that, in the late 19th Century, trouble arose. At the time it was commonly assumed by many that ponderable There was an error working with the wiki: Code[59] reaction in the æther. If the material object is not moving the pressure exerted by æther is equal in all directions (There was an error working with the wiki: Code[164]). 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 There was an error working with the wiki: Code[165]. The resulting geometrical calculation formed the basis for the expectation of a positive result, and expected lower bound value that should be seen

##### Aether drag hypothesis

The aether drag hypothesis was an early attempt to explain the way experiments such as Arago's experiment showed that the There was an error working with the wiki: Code[60], that remains attached to things as they move. If this is the case then, no matter how fast the earth moves around the sun or rotates on its axis, light on the surface of the earth would travel at a constant velocity.

The primary reason the aether drag hypothesis is considered invalid is because of the occurrence of There was an error working with the wiki: Code[61] demonstrated that stellar aberration occurs even when a telesope is filled with water. It seems that if the aether drag hypothesis were true then stellar aberration would not occur because the light would be travelling in the aether which would be moving along with the telescope.

If you visualize a bucket on a train about to enter a tunnel and a drop of water drips from the tunnel entrance into the bucket at the very center, the drop will not hit the center at the bottom of the bucket. The bucket is the tube of a telescope, the drop is a photon and the train is the earth. If aether is dragged then the droplet would be traveling with the train when it is dropped and would hit the center of bucket at the bottom.

However, some modified versions of the hypothesis are still held by some dissidents that argue that aether drag may happen on a global (or larger) scale and the aberration is merely transferred into the entrained "bubble" around the earth which then faithfully carries the modified angle of incidence directly to the observer. This larger entrainment effect was believed by some scientists such as There was an error working with the wiki: Code[166] who continued the search for aether many years after the widespread acceptance of relativity.

The amount of stellar aberration, \alpha is given by:

: tan(\alpha) = v \delta t / c \delta t

So:

: tan(\alpha) = v / c

The speed at which the earth goes round the sun, v = 30 km/s, and the speed of light is c = 299,792,458 m/s which gives \alpha = 20.5 seconds of arc every six months. This amount of aberration is observed and this contradicts the aether drag hypothesis. In 1818 There was an error working with the wiki: Code[62] introduced a modification to the aether drag hypothesis that only applies to the interface between media. This was accepted during much of the nineteenth century but has now been replaced by There was an error working with the wiki: Code[167] (see below).

##### Historical importance

The aether drag hypothesis is historically important because it was one of the reasons why Newton's corpuscular theory of light was replaced by the wave theory and it is used in early explanations of light propagation without relativity theory. It originated as a result of early attempts to measure the speed of light. In 1810 There was an error working with the wiki: Code[168] realised that variations in the refractive index of a substance predicted by the corpuscular theory would provide a useful method for measuring the velocity of light. These predictions arose because the There was an error working with the wiki: Code[169] of a substance such as glass depends on the ratio of the velocities of light in air and in the glass. Arago attempted to measure the extent to which corpuscles of light would be refracted by a glass prism at the front of a telescope. He expected that there would be a range of different angles of refraction due to the variety of different velocities of the stars and the motion of the earth at different times of the day and year. Contrary to this expectation he found that that there was no difference in refraction between stars, between times of day or between seasons. All Arago observed was ordinary stellar aberration.

In 1818 There was an error working with the wiki: Code[170] examined Arago's results using a wave theory of light. He realised that even if light were transmitted as waves the refractive index of the glass-air interface should have varied as the glass moved through the aether to strike the incoming waves at different velocities when the earth rotated and the seasons changed.

Fresnel proposed that the glass prism would carry some of the aether along with it so that "..the aether is in excess inside the prism". He realised that the velocity of propagation of waves depends on the density of the medium so proposed that the velocity of light in the prism would need to be adjusted by an amount of 'drag'. The velocity of light v_n in the glass without any adjustment is given by:

: v_n = c / n

The drag adjustment v_d is given by:

: v_d = v (1 - \frac {\rho_e}{\rho_g})

Where \rho_e is the aether density in the environment, \rho_g is the aether density in the glass and v is the velocity of the prism with respect to the aether. The factor (1 - \frac {\rho_e}{\rho_g}) can be written as (1 - \frac{1}{n^2}) because the refractive index, n, would be dependent on the density of the aether. This is known as the Fresnel drag coefficient. The velocity of light in the glass is then given by:

: V = \frac {c}{n} + v (1 - \frac{1}{n^2})

This correction was successful in explaining the null result of Arago's experiment. It introduces the concept of a largely stationary aether that is dragged by substances such as glass but not by air. Its success favoured the wave theory of light over the previous corpuscular theory.

The Fresnel drag coefficient was confirmed by an There was an error working with the wiki: Code[63]. Water was passed at high speed along two glass tubes that formed the optical paths of the interferometer and it was found that the fringe shifts were as predicted by the drag coefficient. The There was an error working with the wiki: Code[171] predicts the result of the Fizeau experiment from the velocity addition theorem without any need for an aether. If V is the velocity of light relative to the Fizeau apparatus and U is the velocity of light relative to the water and v is the velocity of the water:

: U = \frac {c}{n}

: V = \frac {c/n + v}{1 + v/nc}

which, if v/c is small can be expanded using the binomial expansion to become:

: V = \frac {c}{n} + v (1 - \frac{1}{n^2})

This is identical to Fresnel's equation. It may appear as if Fresnel's analysis can be substituted for the relativistic approach, however, more recent work has shown that Fresnel's assumptions should lead to different amount of aether drag for different frequencies of light and violate There was an error working with the wiki: Code[172] (see Ferraro and Sforza (2005)). The aether drag hypothesis was one of the arguments used in an attempt to explain the There was an error working with the wiki: Code[173] before the widespread acceptance of the There was an error working with the wiki: Code[174].

##### Empirical falsification

An experiment testing this hypothesis was first performed by There was an error working with the wiki: Code[175] in 1881. It produced a There was an error working with the wiki: Code[176]. It was repeated in 1887 in collaboration with There was an error working with the wiki: Code[177] and is known today as the There was an error working with the wiki: Code[178]. 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 There was an error working with the wiki: Code[64] 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 There was an error working with the wiki: Code[179] 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:

There was an error working with the wiki: Code[180]

There was an error working with the wiki: Code[181]

There was an error working with the wiki: Code[182]

##### Lorentz's ether and special relativity

The There was an error working with the wiki: Code[65]: According to this theory, an inertial observer would be incapable of measurng their absolute motion, so that their measurements would comply with the There was an error working with the wiki: Code[183] ("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.

There was an error working with the wiki: Code[66]'s There was an error working with the wiki: Code[184] ("SR", 1905) rederived Lorentz' relationships by declaring that all observers could claim that lightspeed was absolutely fixed in their own There was an error working with the wiki: Code[185]. Where LET said that constant velocity through the aether was undetectable, SR used this undetectability to reject the concept of an underlying aether as superfluous, and replaced a notional state of aether motion with it with the concept of the There was an error working with the wiki: Code[185]. 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 There was an error working with the wiki: Code[67]), 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 There was an error working with the wiki: Code[68], 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 There was an error working with the wiki: Code[69] ("space tells matter how to move, matter tells space how to bend").

See also: There was an error working with the wiki: Code[187]

#### Modern derivatives

Modern understanding of There was an error working with the wiki: Code[70] and various scientific experiments of There was an error working with the wiki: Code[71] due to its similarity to the classical aether. Modern physics is full of concepts such as There was an error working with the wiki: Code[188], There was an error working with the wiki: Code[189], There was an error working with the wiki: Code[190]s, There was an error working with the wiki: Code[191] (QWS), Zero-point energy, There was an error working with the wiki: Code[192], and There was an error working with the wiki: Code[193].

##### Aether and quantum mechanics

There was an error working with the wiki: Code[72] that appear and disappear incredibly quickly. Instead of being "smooth", the vacuum is described as looking like "There was an error working with the wiki: Code[194]". It has been suggested that this seething mass of There was an error working with the wiki: Code[195] may be the equivalent in modern physics of a particulate aether.

##### Gravitational aether

By the late 1800s, gravitational phenomena had also been modeled utilizing an aetherial concept. This concept is known today as There was an error working with the wiki: Code[73] and There was an error working with the wiki: Code[74]. While Le Sage's theory is still studied by some researchers, it is not regarded as a viable theory within the mainstream scientific community.

##### The Einstein-aether theory

"Aether and the theory of relativity"http://www.tu-harburg.de/rzt/rzt/it/Ether.html 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. There was an error working with the wiki: Code[21] 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.

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 There was an error working with the wiki: Code[75] and describes a There was an error working with the wiki: Code[76] and a unit timelike There was an error working with the wiki: Code[77]: that in which the vector field lies along the frame's time direction. The presence of the preferred frame violates There was an error working with the wiki: Code[78]. 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. There was an error working with the wiki: Code[79] review this theory in their article Einstein Æther Theory.

All æther theories break the There was an error working with the wiki: Code[196] of the theory down, at least, to the There was an error working with the wiki: Code[197] of There was an error working with the wiki: Code[198]s. This symmetry breaking implies the existence of an associated There was an error working with the wiki: Code[199]. Some experimental signatures of such a boson were analyzed by There was an error working with the wiki: Code[200], There was an error working with the wiki: Code[201], There was an error working with the wiki: Code[202] and There was an error working with the wiki: Code[203] in Universal Dynamics of Spontaneous Lorentz Violation and a New Spin-Dependent Inverse-Square Law Force.

##### History

Einstein-æther theories were popularized by There was an error working with the wiki: Code[204] in a series of papers, such as Singularity Prevention and Broken Lorentz Symmetry in the There was an error working with the wiki: Code[205]. In addition to the metric of There was an error working with the wiki: Code[206] these theories also included a There was an error working with the wiki: Code[207] which intuitively corresponded to a universal notion of There was an error working with the wiki: Code[208]. Such a theory will have a preferred There was an error working with the wiki: Code[209], that in which the universal time is the actual time. The dynamics of the scalar field is identified with that of an There was an error working with the wiki: Code[210] which is at rest in the preferred frame. This is the origin of the name of the theory, it contains Einstein's gravity plus an æther.

Einstein-æther theories returned to prominence at the turn of the century with the paper Gravity and a Preferred Frame by There was an error working with the wiki: Code[80]. Their theory contains less information than that of Gasperini, instead of a scalar field giving a universal time it contains only a unit There was an error working with the wiki: Code[211] which gives the direction of time. Thus observers who follow the æther at different points will not necessarily age at the same rate in the Jacobson-Mattingly theory.

The existence of a preferred, dynamical time vector breaks the There was an error working with the wiki: Code[212] of the theory, more precisely it breaks the invariance under There was an error working with the wiki: Code[213]s. This symmetry breaking may lead to a There was an error working with the wiki: Code[214] for the graviton which would alter long distance physics, perhaps yielding an explanation for recent There was an error working with the wiki: Code[215] data which would otherwise be explained by a There was an error working with the wiki: Code[216]. The effect of breaking Lorentz invariance on There was an error working with the wiki: Code[217] has

a long history leading back at least to the work of There was an error working with the wiki: Code[218] and There was an error working with the wiki: Code[219] in 1939. Recently it has regained popularity with, for example, the paper Effective Field Theory for Massive Gravitons and Gravity in Theory Space by There was an error working with the wiki: Code[220], There was an error working with the wiki: Code[221] and There was an error working with the wiki: Code[222]. Einstein-æther theories provide a concrete example of a theory with broken Lorentz invariance and so have proven to be a natural setting for such investigations.

##### Consistency

It is still not known whether Einstein-æther theories exist as quantum theories. One immediate concern might be that the time vector, which breaks Lorentz invariance, will lead to There was an error working with the wiki: Code[81]s with a wrong-sign There was an error working with the wiki: Code[82] and so could have ruined the There was an error working with the wiki: Code[223] of the There was an error working with the wiki: Code[224].

##### The action

The action of the Einstein-æther theory is generally taken to consist of the sum of the There was an error working with the wiki: Code[83] may be written as the There was an error working with the wiki: Code[225] of a local There was an error working with the wiki: Code[226]:

:::S=\frac{1}{16\pi G_N}\int d^4x\sqrt{-g}\mathcal L

where GN is There was an error working with the wiki: Code[84] with There was an error working with the wiki: Code[227]. The Lagrangian density is

::\mathcal L=-R-K^{ab}_{mn}\nabla_a u^m\nabla_bu^n-\lambda (g_{ab}u^au^b-1).

Here R is the There was an error working with the wiki: Code[228], \nabla is the There was an error working with the wiki: Code[229] and the tensor K is defined by

::K^{ab}_{mn}=c_1g^{ab}g_{mn}+c_2\delta^a_m\delta^b_n+c_3\delta^a_n\delta^b_m+c_4u^au^bg_{mn}.

Here the ci are dimensionless adjustable parameters of the theory.

##### Stars

Several spherically symmetric solutions to æ-theory have been found. Most recently There was an error working with the wiki: Code[85] or else two asymptotic regions of spacetime, resembling a There was an error working with the wiki: Code[230] with but with no There was an error working with the wiki: Code[231]. They have argued that static stars must have static æther solutions, which means that the æther points in the direction of a timelike There was an error working with the wiki: Code[232].

##### Black holes problems

However this is difficult to reconcile with static black holes, as at the There was an error working with the wiki: Code[233] there are no timelike Killing vectors available and so the black hole solutions cannot have static æthers. Thus when a star collapses to form a black hole, somehow the æther must eventually become static even very far away from the collapse. In addition the There was an error working with the wiki: Code[234] does not obviously satisfy the There was an error working with the wiki: Code[235], one needs to make recourse to the equations of motion. This is in contrast with theories with no æther, where this property is independent of the equations of motion.

However, in non-moving ether this might be avoided. Aether density, as an addition, to tension, might be used to satisify the There was an error working with the wiki: Code[236].

##### Experimental constraints

In Universal Dynamics of Spontaneous Lorentz Violation and a New Spin-Dependent Inverse-Square Law Force There was an error working with the wiki: Code[237], There was an error working with the wiki: Code[238], There was an error working with the wiki: Code[239] and There was an error working with the wiki: Code[240] have examined experimental consequences of the breaking of boost symmetries inherent in æther theories. They have found that the resulting There was an error working with the wiki: Code[241] leads to, among other things, a new kind of There was an error working with the wiki: Code[242]. In addition that have argued that spin sources will interact via a new inverse square law force with a very unusual angular dependence. They suggest that the discovery of such a force would be very strong evidency for an æther theory, although not necessarily that of Jacobson, et al.

##### Higgs field

The Higgs field, named after the United Kingdom There was an error working with the wiki: Code[86], mediated by the There was an error working with the wiki: Code[87] and There was an error working with the wiki: Code[88] There was an error working with the wiki: Code[89] (the There was an error working with the wiki: Code[90] in Switzerland, which as of 2006 is still under construction, There was an error working with the wiki: Code[243] scientists will try to look for particle interactions characteristic of the Higgs Field.

The Higgs boson is a hypothetical massive Scalar field theory There was an error working with the wiki: Code[91], There was an error working with the wiki: Code[244] and There was an error working with the wiki: Code[245].

##### Theoretical details

The particle called Higgs boson is in fact the qua