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This article is about the aether as a hypothesis in physics. For other uses, see aether (disambiguation).

In the late 19th century the luminiferous aether ("light-bearing aether"), or ether, was a substance postulated to be the medium for the propagation of light. Later theories including special relativity suggested that an aether did not have to exist, and today the concept is considered an obsolete scientific theory.

(The word "aether" stems via Latin from the Greek αιθηρ, 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.)

Contents 1 The history of light and aether 2 Aether and classical mechanics 3 Experiments 4 End of aether? 5 Continuing adherents 6 Aether conceptions 7 See also 8 References 9 External links

The history of light and aether

See also timeline of luminiferous aether.

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:

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."

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.

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. 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.

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.

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, massless, completely transparent, non-dispersive, incompressible, continuous, and without viscosity.

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. 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.

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.

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."

Paul Langevin argued in 1911 that absolute effects from velocity change or acceleration (such as radiation) demonstrate the existence of an aether, and as additional illustration he mentioned the absolute effect of velocity change on time dilation with his twins example. This example would later lead to the twin paradox.

Einstein's later general theory of relativity (1916) also attributed tangible physical properties to space in order to agree with Ernst Mach's idea that all forms of motion should be "relative", and the general theory arguably implemented its gravitational field as an updated, relativistic, nonparticulate aether (i.e. "the aether of general relativity" - Einstein, 1920). But by this time, people were increasingly associating the term "aether theory" with discredited and superceded theories predating special relativity, and modern theorists now tend to prefer talking about their work in terms of the expected properties of "the metric", "space" or "vacuum", rather than those of "the aether" or "the medium".

It must be noted that Einstein disagreed with Lorentz about his stationary ether hypothesis, for after agreeing with Lorentz in his 1920 Leiden speech about the ether that "according to the general theory of relativity space without ether is unthinkable; for in such space there [...] would be no propagation of light", he concluded his speech with the words: "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", and later Einstein clarified his ether concept by stating that "it must now be remembered that there is an infinite number of spaces, which are in motion with respect to each other".

Meanwhile, from his 1913 experiment with an interferometer in uniform rotation, Georges Sagnac concluded that "in the ambient space, light is propagated with a velocity V0, independent of the movement as a whole of the luminous source O and the optical system. That is a property of space which experimentally characterizes the luminiferous ether."

In agreement with Sagnac, Herbert Ives -- the first one to positively measure the effect of speed on clock rates -- 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."

Similarly, Dirac concluded in 1951 in an article in Nature, titled "Is there an ether?": "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."

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.

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".

But the same was not true for light. Since Maxwell's mathematics demanded a single, universal, speed for the propagation of light, following the same logic meant that the aether must have a single, universal, set of properties. Carrying out experiments with light on that same aircraft would have to show some sort of effect, because the light was not moving relative to the aircraft, but to the universal aether. 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 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. A third experiment, Michelson-Gale-Pearson experiment, was a modification of the Michelson-Morley experiment, as it used two rectangles, one much larger than the other instead of the two "arms" of the original Michelson-Morley version. The results consisted of 269 measurements that showed an effect of -0.04 to +0.55 fringes, where were seen by many as evidence of the rotational effects, and support for the existence of the aether, in apparent contradiction to the earlier Michelson-Morley experiment.

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.

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, which would lower the magnitude of the measurement, perhaps enough to explain MM's experiments. 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 his 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 notes no such effect around other planets.

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 "confiming special relativity".

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:

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):

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.

Other positive results included Sagnac in 1913, and Michelson and Gale 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.

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 ether. 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; 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 ether wind, light would travel faster in one direction than in the perpendicular direction. This would fit with predictions by Lorentz whose theory for light predict equivalent results to those of the special theory of relativity only in a vacuum. [1]

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.

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. For Einstein (but not for Lorentz) the transformation also implied something much more radical, 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".

Continuing adherents

Most current physicists do not see a need to have a medium for light to propagate. The combination of relativity and quantum mechanics seems to render the concept unnecessary. However, this doesn't mean it doesn't exist (just that it doesn't have to), and there remain a number of problems in modern physics that would be simplified with such a concept.

A few physicists (like Dayton Miller and Edward Morley) continued research on the aether for some time, and occasionally researchers still explore these concepts. While it is not difficult to create aether theories consistent with the Michelson-Morley experiment, it is much harder to remain consistent with all of the related 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.

Although the vast majority of modern physicists reject all aether-based theories, the intuitive appeal of a causal background for "relativistic" effects cannot be denied.

In a paper of 1958, G. Builder concluded that "the observable effects of absolute accelerations and of absolute velocites must be described to iteraction 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."

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."

A number of new ether concepts have been proposed in recent years.

For example, 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 therefore incompatible with Lorentz invariance, a symmetry of special theory of relativity. Its existence therefore potentially disagrees with the Michelson-Morley-like experiments.

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".

Aether conceptions

• Aether theories
• Aether (classical element)
• Aether and general relativity
• Aether drag hypothesis
• Aetherometry
• Etheric plane

See also

• History of special relativity
• Preferred frame

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 0716743450.
• J. Larmour, "A Dynamical Theory of the Luminiferous Medium". Transactions of the Royal Society, 1885-86.
• 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
• G. Builder, "Ether and Relativity", Australian Journal of Physics 11 (1958), p.279
• P. Dirac "Is there an ether?", Nature 168 (1951), p.906
• 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

External links

• What is the experimental basis of Special Relativity? - includes a lengthy list of aether experiments, among others
• The Ether of Space - Lord Rayleigh's address
• Modern scientific theories of the ancient aether - a categorised compendium of articles relating to the emergence of scientific theories that reference the aether, or quantum foam of space.
• Ether and the Theory of Relativity - Albert Einstein's 1920 inaguration address at the University of Leyden (actually delivered on 27 October 1920).