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Encyclopedia > Luminiferous aether
The luminiferous aether: it was hypothesised that the Earth moves through a "medium" of aether that carries light

In the late 19th century luminiferous aether ("light-bearing aether") was the term used to describe a medium for the propagation of light. Later theories including special relativity were formulated without the aether concept, and today the aether is considered to be an obsolete scientific theory. Aether wind diagram for Michelson Morley experiment; by me for Wiki File links The following pages link to this file: Luminiferous aether Michelson-Morley experiment User:Reddi/Luminiferous aether Categories: GFDL images ... Aether wind diagram for Michelson Morley experiment; by me for Wiki File links The following pages link to this file: Luminiferous aether Michelson-Morley experiment User:Reddi/Luminiferous aether Categories: GFDL images ... Chinese Wood (æœ¨) | Fire (ç«) | Earth (åœŸ) | Metal (é‡‘) | Water (æ°´) Hinduism and Buddhism The Pancha Mahabhuta (The Five Great Elements) Vayu/Pavan (Air/Wind) Agni/Tejas (Fire) Akasha (Aether) Prithvi/Bhumi (Earth) Ap/Jala (Water) Aether (also spelled ether) is a concept used in ancient and medieval science as a substance. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength. ... The special theory of relativity was proposed in 1905 by Albert Einstein in his article On the Electrodynamics of Moving Bodies. Some three centuries earlier, Galileos principle of relativity had stated that all uniform motion was relative, and that there was no absolute and well-defined state of rest... An obsolete scientific theory is a scientific theory that was once commonly accepted but (for whatever reason) is no longer considered the most complete description of reality by mainstream science; or a falsifiable theory which has been shown to be false. ...

The word "aether" stems via Latin from the Greek αἰθήρ, 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. Latin is an ancient Indo-European language originally spoken in Latium, the region immediately surrounding Rome. ...

## The history of light and aether GA_googleFillSlot("encyclopedia_square");

See also timeline of luminiferous aether. the Timeline of Luminiferous Aether begins in the late 19th century with the concept of the aether (light-bearing aether), or ether, as a medium for electromagnetic propagation. ...

Isaac Newton 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 refraction and diffraction 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: Sir Isaac Newton, FRS (4 January 1643 â€“ 31 March 1727) [OS: 25 December 1642 â€“ 20 March 1727][1] was an English physicist, mathematician, astronomer, alchemist, and natural philosopher who is generally regarded as one of the greatest scientists and mathematicians in history. ... The straw seems to be broken, due to refraction of light as it emerges into the air. ... To meet Wikipedias quality standards, this article or section may require cleanup. ...

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."

Christiaan Huygens, 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 mechanical waves in gases and fluids. However, longitudinal waves by necessity have only one form for a given propagation direction, rather than two polarizations as in a transverse wave, and thus they were unable to explain the phenomenon of birefringence (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." Christiaan Huygens Christiaan Huygens (pronounced in English (IPA): ; in Dutch: ) (April 14, 1629â€“July 8, 1695), was a Dutch mathematician and physicist; born in The Hague as the son of Constantijn Huygens. ... A wave which needs a medium in order to propagate itself. ... In electrodynamics, polarization (also spelled polarisation) is the property of electromagnetic waves, such as light, that describes the direction of their transverse electric field. ... A calcite crystal laid upon a paper with some letters showing the double refraction Birefringence, or double refraction, is the decomposition of a ray of light into two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of material, such as calcite crystals, depending on...

In 1720 James Bradley carried out a series of experiments attempting to measure stellar parallax. Although he failed to detect any parallax (thereby placing a lower limit on the distance to stars), he discovered another effect, stellar aberration, 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 – precisely the problem that led Newton to reject a wave model in the first place. James Bradley (March 1693 â€“ July 13, 1762) was an English astronomer, Astronomer Royal from 1742. ... Parallax (Greek: &#960;&#945;&#961;&#945;&#955;&#955;&#945;&#947;&#942; = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of said observer. ... Aberration of light (also referred to as astronomical aberration or stellar aberration) is an astronomical phenomenon defined as an apparent motion of the heavenly bodies; stars describing more or less elliptic annual orbits, according to the latitude of the star; consequently at any moment the star appears to be displaced...

However, a century later, Young and Fresnel revived the wave theory of light when they pointed out that light could be a transverse wave rather than a longitudinal wave—the polarization of a transverse wave (like Newton's "sides" of light) could explain birefringence, and in the wake of a series of experiments on diffraction the particle model of Newton was finally abandoned. Physicists still assumed, however, that like mechanical waves, light waves required a medium for propagation, and thus required Huygens' idea of an aether "gas" permeating all space. Thomas Young, English scientist Thomas Young (June 13, 1773 â€“ May 10, 1829) was an English scientist, researcher, physician and polymath. ... Augustin Fresnel Augustin-Jean Fresnel (pronounced [] in AmE, [] in French) (May 10, 1788 â€“ July 14, 1827), was a French physicist who contributed significantly to the establishment of the theory of wave optics. ... A light wave is an example of a transverse wave. ... The first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density. ... The word propagation can mean: Multiplication or increase, as by natural reproduction. ...

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 Augustin-Louis Cauchy 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 George Green pointed out that such a fluid would be unstable. George Gabriel Stokes 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. Augustin Louis Cauchy Augustin Louis Cauchy (August 21, 1789 &#8211; May 23, 1857) was a French mathematician. ... The title page to George Greens original essay on what is now known as Greens theorem. ... George Gabriel Stokes Sir George Gabriel Stokes, 1st Baronet (13 August 1819â€“1 February 1903) was an Anglo-Irish mathematician and physicist. ...

Later, Maxwell's equations showed that light is an electromagnetic wave. Maxwell's equations required that all electromagnetic waves in vacuum propagate at a fixed speed, c. As this can only occur in one reference frame in Newtonian physics (see Galilean-Newtonian relativity), 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 George FitzGerald 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 transverse but never longitudinal. Maxwells equations (sometimes called the Maxwell equations) are the set of four equations, attributed to James Clerk Maxwell, that describe the behavior of both the electric and magnetic fields, as well as their interactions with matter. ... Electromagnetic radiation can be conceptualized as a self propagating transverse oscillating wave of electric and magnetic fields. ... Look up Vacuum in Wiktionary, the free dictionary. ... The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness. In metric units, c is exactly 299,792,458 metres per second (1,079,252,848. ... A frame of reference in physics is a set of axes which enable an observer to measure the aspect, position and motion of all points in a system relative to the reference frame. ... In general, the principle of relativity is the requirement that the laws of physics be the same for all observers. ... George FitzGerald George Francis FitzGerald, or Fitzgerald, (3 August 1851 â€“ 22 February 1901) was a professor of natural and experimental philosophy (i. ... The term transverse means side-to-side, as opposed to longitudinal, which means front-to-back. In automotive engineering, the term transverse refers to an engine in which the crankshaft is oriented side-to-side relative to the wheels of the vehicle. ... The term, longitudinal means front-to-back or top-to-bottom as opposed to transverse which means side-to-side. In automotive engineering, the term, longitudinal refers to an engine in which the crankshaft is oriented along the long axis of the vehicle, front to back. ...

Nevertheless, by this point the mechanical qualities of the aether had become more and more magical: it had to be a fluid in order to fill space, but one that was millions of times more rigid than steel in order to support the high frequencies of light waves. It also had to be massless and without viscosity, otherwise it would visibly effect the orbits of planets. Additionally it appeared it had to be completely transparent, non-dispersive, incompressible, and continuous at a very small scale. A subset of the phases of matter, fluids include liquids, gases, plasmas and, to some extent, plastic solids. ... The pitch drop experiment at the University of Queensland. ... In fluid mechanics, an incompressible fluid is a fluid whose density (often represented by the Greek letter &#961;) is constant: it is the same throughout the field and it does not change through time. ...

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 Oliver Lodge gave a speech in behalf of Lord Rayleigh to the Royal Institution on this topic, in which he outlined its physical properties, and then attempted to offer reasons why they were not impossible. Nevertheless he was also aware of the criticisms, and quoted Lord Salisbury 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. Vanity Fair cartoon. ... Lord Rayleigh The Right Honourable John William Strutt, 3rd Baron Rayleigh (aka Lord Rayleigh) (12 November 1842 â€“ 30 June 1919) was a British physicist who (with William Ramsay) discovered the element argon, an achievement that earned him the Nobel Prize for Physics in 1904. ... The Royal Institution of Great Britain was set up in 1799 by the leading British scientists of the age, including Henry Cavendish and its first president George Finch, the 9th Earl of Winchilsea, for diffusing the knowledge, and facilitating the general introduction, of useful mechanical inventions and improvements; and for... The Most Honourable Robert Arthur Talbot Gascoyne-Cecil, 3rd Marquess of Salisbury, KG, GCVO, PC (3 February 1830â€“22 August 1903), known as Lord Robert Cecil before 1865 and as Viscount Cranborne from 1865 until 1868, was a British statesman and Prime Minister. ...

By the early 20th Century, aether theory was in trouble: A series of increasingly complex experiments 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 special theory of relativity (1905) could generate the same mathematics without referring to an aether at all. Aether fell to Occam's Razor. Special relativity (SR) or the special theory of relativity is the physical theory published in 1905 by Albert Einstein. ... William of Ockham Occams razor (also spelled Ockhams razor) is a principle attributed to the 14th-century English logician and Franciscan friar William of Ockham (Guilhelmi Ockam and Guillermi de ockam in Latin [1]). Originally a tenet of the reductionist philosophy of nominalism, it is more often taken...

## Aether and classical 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 Galilean transformation the equations of Newtonian dynamics are invariant, 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. The Galilean transformation is used to transform between the coordinates of two coordinate systems in a constant relative motion in Newtonian physics. ... Invariant may have meanings invariant (computer science), such as a combination of variables not altered in a loop invariant (mathematics), something unaltered by a transformation invariant (music) invariant (physics) conserved by system symmetry This is a disambiguation page &#8212; a navigational aid which lists other pages that might otherwise share...

A simple example concerns the model on which aether was originally built: sound. The speed of propagation for mechanical waves, the speed of sound, is defined by the mechanical properties of the medium. For instance, if one is in an airliner, 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". The speed of sound is a term used to describe the speed of sound waves passing through an elastic medium. ... An Airbus A340 airliner operated by Air Jamaica An airliner is a large fixed-wing aircraft whose primary function is the transportation of paying passengers. ...

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 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 – 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 Michelson-Morley experiment 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 null result, 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 Trouton-Noble experiment, which like Michelson-Morley obtained a null result. The Michelson-Morley experiment, one of the most important and famous experiments in the history of physics, was performed in 1887 by Albert Michelson and Edward Morley at what is now Case Western Reserve University, and is considered by some to be the first strong evidence against the theory of... Generally, a null result is a result which is null (nothing): that is, the absence of an observable result. ... The Trouton-Noble experiment attempted to detect motion of the Earth through the luminiferous aether, and was conducted in 1901â€“1903 by Frederick Thomas Trouton (who also developed the Troutons ratio) and H. R. Noble. ...

It is important to understand what "null result" means in this context. It does not mean there was no motion detected, 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 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 Hamar experiment, 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 Oliver Joseph Lodge noted no such effect around other planets. The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... Vanity Fair cartoon. ...

Another, completely different, attempt to save "absolute" aether was made in the Lorentz-Fitzgerald contraction hypothesis, which posited that everything was affected by travel through the aether. In this theory the reason the Michelson-Morley experiment "failed" was that it contracted in length in the direction of travel. That is, the light was being affected in the "natural" manner by its travel though the aether as predicted, but so was the experiment itself, cancelling out any difference when measured. Even Lorentz was not very happy with this suggestion, although it did neatly solve the problem. Later this idea received additional support from the Kennedy-Thorndike experiment in 1932, as Kennedy and Thorndike concluded that both a Lorentz contraction as well as time dilation occur, thus "confirming special relativity". The Lorentz-FitzGerald contraction hypothesis was proposed by George FitzGerald and independently proposed and extended by Hendrik Lorentz to explain the negative result of the Michelson-Morley experiment, which attempted to detect Earths motion relative to the luminiferous aether. ... The Kennedy-Thorndike experiment (Experimental Establishment of the Relativity of Time), first conducted in 1932, is a modified form of the Michelson-Morley experimental procedure. ... Time dilation is the phenomenon where the observed time rate of an observers reference frame is different from that of a different reference frame. ...

Another experiment purporting to show effects of an aether was Fizeau's 1851 experimental confirmation of Fresnel's 1818 prediction that a medium with refractive index 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: Armand Hippolyte Louis Fizeau Physicist Armand Hippolyte Louis Fizeau (September 23, 1819-1896), French physicist, was born in Paris. ... Augustin Fresnel Augustin-Jean Fresnel (pronounced [] in AmE, [] in French) (May 10, 1788 â€“ July 14, 1827), was a French physicist who contributed significantly to the establishment of the theory of wave optics. ... The refractive index (or index of refraction) of a material is the factor by which the phase velocity of electromagnetic radiation is slowed in that material, relative to its velocity in a vacuum. ...

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

That is, movement adds only a fraction of the medium's velocity to the light (predicted by Fresnel in order to make Snell's law work in all frames of reference, consistent with stellar aberration). This was initially interpreted to mean that the medium drags the aether along, with a portion of the medium's velocity, but that understanding was rejected after Wilhelm Veltmann demonstrated that the index n in Fresnel's formula depended upon the wavelength of light (so that the aether could not be moving at a wavelength-independent speed). With the advent of special relativity, Fresnel's equation was shown by Laue 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): Snells law is the simple formula used to calculate the refraction of light when travelling between two media of differing refractive index. ... The wavelength is the distance between repeating units of a wave pattern. ... Max von Laue (October 9, 1879 - April 24, 1960) was a German physicist, who studied under Max Planck. ...

$frac{c/n + v}{1 + frac{v c/n} {c^2}} approx frac{c}{n} + left( 1 - frac{1}{n^2} right) v + Oleft(frac{v^2}{c^2}right).$

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 Dayton Miller. 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. The factual accuracy of this article is disputed. ...

Other positive results included Sagnac in 1913, and the Michelson-Gale-Pearson experiment in 1925. This effect that is known as Sagnac effect is nowadays used in optical gyroscopes and shows that rotation is similarly "absolute" for light as it is for pendulums. The Michelson-Gale-Pearson experiment is a modified version of the Michelson-Morley experiment which tests the aether drag along the rotating frame of Earth. ... The Sagnac effect manifests itself in an experimental setup called ring interferometry. ...

## End of aether?

Aether theory was dealt another blow when the Galilean transformation and Newtonian dynamics were both modified by Albert Einstein's special theory of relativity, 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. Albert Einstein, photographed in 1947 by Oren J. Turner. ... Special relativity (SR) or the special theory of relativity is the physical theory published in 1905 by Albert Einstein. ...

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.

In his lectures of around 1911, Lorentz explained his continued use of his aether concept by pointing 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 to the bearer of these properties a certain substantiality, and if so, then one may, in all modesty, call true time the time measured by clocks which are fixed in this medium, and consider simultaneity as a primary concept."

For Einstein however, 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 photoelectric effect. In this work he demonstrated that light can be considered as particles that have a "wave like nature". Particles obviously do not need a medium to travel, and thus, neither did light. This was the first step that would lead to the full development of quantum mechanics, in which the wave-like nature and the particle-like nature of light are both considered to be simplifications of what is "really happening". An excellent summary of Einstein's thinking about the aether hypothesis, relativity and light quanta may be found in his 1909 lecture Über die Entwicklung unserer Anschauungen über das Wesen und die Konstitution der Strahlung, available in its English translation The Development of Our Views on the Composition and Essence of Radiation. The photoelectric effect is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays. ... Fig. ...

Nevertheless, in a lecture meant for his inauguration at the University of Leiden in 1920, after first stating that: "There can be no space nor any part of space without gravitational potentials", Einstein concluded the following: "Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it." Thus general relativity implies an ether, but Einstein disagreed with Lorentz's stationary ether concept. Unexpected as it is for physicists to have differing opinions on such a fundamental concept as the ether, it must be acknowledged that Einstein was the one who changed his opinion. Shortly before his lecture in Leyden in 1920 he confessed 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 ". Leiden University in the city of Leiden, is the oldest university in the Netherlands. ...

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 Occam's razor. William of Ockham Occams razor (also spelled Ockhams razor) is a principle attributed to the 14th-century English logician and Franciscan friar William of Ockham (Guilhelmi Ockam and Guillermi de ockam in Latin [1]). Originally a tenet of the reductionist philosophy of nominalism, it is more often taken...

Even Michelson himself, who received the Nobel Prize in physics in 1907 for his optical studies stated for Minneapolis Morning Tribune of April 14, 1923, p 21, that even if relativity is here to stay we don't have to reject the aether.

Moreover, it is hard to develop an aether theory that is consistent with all experiments of modern physics. Any new theory of aether must be consistent with all of the experiments testing phenomena of special relativity, general relativity, relativistic quantum mechanics, and so on. As outlined earlier, these conditions are often contradictory, making such a task inherently difficult. In the scientific method, an experiment (Latin: ex-+-periri, of (or from) trying), is a set of actions and observations, performed in the context of solving a particular problem or question, to support or falsify a hypothesis or research concerning phenomena. ... A phenomenon (plural: phenomena) is an observable event, particularly something special (literally something that can be seen, derived from the Greek word phainomenon = observable). ... General relativity (GR) is the geometrical theory of gravitation published by Albert Einstein in 1915 [1][2]. It unifies special relativity and Isaac Newtons law of universal gravitation with the insight that gravitation is not due to a force but rather is a manifestation of curved space and time... Fig. ...

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 Dayton Miller and Edward Morley) continued research on the aether for some time, and occasionally researchers still promote the concept 1. The factual accuracy of this article is disputed. ... Edward Morley (1887). ...

In a paper of 1958, G. Builder concluded that "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 Mach's hypothesis, since this is untenable. There is therefore no alternative to the ether hypothesis." This article is in need of attention from an expert on the subject. ...

In agreement with this, Professor 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 "clock paradox" 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." The twin paradox, sometimes called the clock paradox, stems from Paul Langevins 1911 thought experiment in special relativity: one of two twin brothers undertakes a long space journey with a high-speed rocket at almost the speed of light, while the other twin remains on Earth. ...

More recently (2005), Unnikrishnan remarked: "the entire voluminous and elaborate writings on the twin clock problem can all be replaced by the single-sentence resolution that the clocks age with Lorentz factors corresponding to their velocity relative to the preferred frame of the matter-filled universe."

A number of new aether concepts have been proposed in recent years. However, these aethers differ considerably from the classical luminiferous aether.

In a controversial quantum approach to gravity called loop quantum gravity, spacetime is filled with a structure called the spin foam. Much like aether, it picks a privileged reference frame and is incompatible with Lorentz invariance, a symmetry of special theory of relativity. Its existence therefore potentially disagrees with the Michelson-Morley-like experiments. Loop quantum gravity (LQG), also known as loop gravity and quantum geometry, is a proposed quantum theory of spacetime which attempts to reconcile the seemingly incompatible theories of quantum mechanics and general relativity. ... In physics, a spin foam is a four-dimensional graph made out of two-dimensional faces that represents one of the configurations that must be summed to obtain Feynmans path integral (functional integration) describing the alternative formulation of quantum gravity known as loop gravity or loop quantum gravity. ... Lorentz covariance is a term in physics for the property of space time, that in two different frames of reference, located at the same event in spacetime but moving relative to each other, all non-gravitational laws must make the same predictions for identical experiments. ... Special relativity (SR) or the special theory of relativity is the physical theory published in 1905 by Albert Einstein. ...

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. [1]

## Outside the scientific community

Some adherents of modern geocentrism 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. The term modern geocentrism refers to a belief currently held by certain groups that the Earth is the center of the universe and does not move. ...

## Aether concepts

Alchemy, natural philosophy, and early modern physics proposed the existance of aether (also spelled ether, from the Latin word aether, meaning upper air [1]), a space-filling substance or field, thought to be necessary as a transmission medium. ... Chinese Wood (æœ¨) | Fire (ç«) | Earth (åœŸ) | Metal (é‡‘) | Water (æ°´) Hinduism and Buddhism The Pancha Mahabhuta (The Five Great Elements) Vayu/Pavan (Air/Wind) Agni/Tejas (Fire) Akasha (Aether) Prithvi/Bhumi (Earth) Ap/Jala (Water) Aether (also spelled ether) is a concept used in ancient and medieval science as a substance. ... The aether drag hypothesis was an early attempt to explain the way experiments such as Aragos experiment showed that the speed of light is constant. ...

To meet Wikipedias quality standards, this article or section may require cleanup. ... A superseded, or obsolete, scientific theory is a scientific theory that was once widely used, but due to emergence of more accurate present theory is no longer as popular as it used to be. ... In theoretical physics, a preferred or priveleged frame is usually a special hypothetical frame of reference in which the laws of physics might appear to be identifiably different to those in other frames. ... // Galactic time NGC 4414, a typical spiral galaxy alike our Milky Way Galactic time, not to confuse with siderial time, is the time that is described by our spin relative to the center of the galaxy. ...

## References

• Banesh Hoffman, Relativity and Its Roots (Freeman, New York, 1983).
• Michael Janssen, 19th Century Ether Theory, Einstein for Everyone course at UMN (2001).
• Isaac Newton, Opticks (1704). Republished 1952 (Dover: New York), with commentary by Bernard Cohen, Albert Einstein, and Edmund Whittaker.
• Tipler, Paul; Llewellyn, Ralph (2002). Modern Physics (4th ed.). W. H. Freeman. ISBN 0-7167-4345-0.
• J. Larmour, "A Dynamical Theory of the Luminiferous Medium". Transactions of the Royal Society, 1885-86.
• Albert Einstein (1909) The Development of Our Views on the Composition and Essence of Radiation, Phys. Z., 10, 817-825. (review of aether theories, among other topics)
• Albert Einstein, "Ether and the Theory of Relativity" (1920), republished in Sidelights on Relativity (Dover, NY, 1922) [2]
• Albert Einstein, "Ideas and Opinions" pp. 281, 362. ISBN 0-517-88440-2
• Langevin, P. (1911) "L’évolution de l’espace et du temps", Scientia, X, p31
• G. Builder, "Ether and Relativity", Australian Journal of Physics 11 (1958), p.279
• P. Dirac "Is there an ether?", Nature 168 (1951), p.906 [3]
• H. Ives "The measurement of velocity with atomic clocks", Science Vol.91 (1940), p.65
• H.A. Lorentz, "The Principle of Relativity for uniform translations (1910-1912)", Lectures on Theoretical Physics Vol.III, 1931 (authorised translation of the Dutch version of 1922)
• G. Sagnac, E. Bouty, "The Luminiferous Ether Demonstrated by the Effect of the Relative Motion of the Ether in an Interferometer in Uniform Rotation"(in French), Comptes Rendus (Paris) 157 (1913), p.708-710
• C. Sherwin, "Some recent Experimental Tests of the "Clock Paradox"", Physical Review 120 no.1 (1960), p.17-21
• Kostro, Ludwik (2000). Einstein and the Ether. Montreal, Apeiron. ISBN 0-9683689-4-8.
• Unnikrishnan, C (2005), Current Science, Vol. 89, NO. 12, p.2009

Washington Avenue Bridge at night The University of Minnesota, Twin Cities, almost always abbreviated U of M, and sometimes referred to as The U by locals, is the oldest and largest part of the University of Minnesota system. ...

Results from FactBites:

 Luminiferous aether (3666 words) In the late 19th century luminiferous aether ("light-bearing aether") was the term used to describe a medium for the propagation of light. This was initially interpreted to mean that the medium drags the aether along, with a portion of the medium's velocity, but that understanding was rejected after Wilhelm Veltmann demonstrated that the index n in Fresnel's formula depended upon the wavelength of light (so that the aether could not be moving at a wavelength-independent speed). Aether theory was dealt another blow when the Galilean transformation and Newtonian dynamics were both modified by Albert Einstein's special theory of relativity, giving the mathematics of Lorentzian electrodynamics a new, "non-aether" context.
 Luminiferous aether - Wikipedia, the free encyclopedia (3893 words) In the late 19th century luminiferous aether ("light-bearing aether") was the term used to describe a medium for the propagation of light. This was initially interpreted to mean that the medium drags the aether along, with a portion of the medium's velocity, but that understanding was rejected after Wilhelm Veltmann demonstrated that the index n in Fresnel's formula depended upon the wavelength of light (so that the aether could not be moving at a wavelength-independent speed). Aether theory was dealt another blow when the Galilean transformation and Newtonian dynamics were both modified by Albert Einstein's special theory of relativity, giving the mathematics of Lorentzian electrodynamics a new, "non-aether" context.
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