Luminiferous aether

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

The luminiferous aether: it was hypothesised that the Earth moves through a "medium" of aether that carries light
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including special relativity were formulated without the aether concept, and today the aether is considered to be a superseded scientific theory.

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.

The history of light and 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 – precisely the problem that led Newton to reject a wave model in the first place.

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. It also had to be massless and without viscosity, otherwise it would visibly affect the orbits of planets. Additionally it appeared it had to be completely transparent, non-dispersive, incompressible, 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 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.

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.

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, 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.


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.

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 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.

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 and it was a first step towards relativity theory. Without referral to an ether, this physical interpretation of relativistic effects was shared by Kennedy and Thorndike 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.

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:

\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):

\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 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 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. Sagnac regarded this as evidence for the aether

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, 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 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". A summary of Einstein's thinking about the aether hypothesis, relativity and light quanta may be found in his 1909 (originally German) lecture " The Development of Our Views on the Composition and Essence of Radiation"

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".

Paul Langevin 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 time dilation on two space travelers. This example would later lead to the twin paradox.

In the meantime Einstein changed his opinion about the aether concept. In a lecture meant for his inauguration at the University of Leiden in 1920, Einstein stressed that space is "endowed with physical quantities" 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 in 1923 that even if relativity is here to stay we don't have to reject the aether. Some other physicists who published their support for modern aether concepts were Herbert Ives, Paul Dirac 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" 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"

Also Dirac stated in 1951 in an article in Nature, titled "Is there an ether?" that "we are rather forced to have an ether"

The large majority of scientists disagreed with such views.

Continuing adherents

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.

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.

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 such as Harold Aspden 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.

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.

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. 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 similarly flawed 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 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.

Aether concepts

  • Aether theories
  • Aether (classical element)
  • Aether drag hypothesis

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