The luminiferous aether: it was hypothesised that the Earth moves through a "medium" of aether that carries light
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 Einstein's Theory of Relativity, demonstrated that an aether did not have to exist, and today the concept is considered "quaint".
(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.)
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 [sic] and easy Transmission" (causing refraction and diffraction). Newton believed that these vibrations were related to heat radiation: "Is not the Heat of the warm Room convey'd through the Vacuum by the Vibrations of a much subtiler [sic] 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 [sic] 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."
The models of Huygens and Newton continued to be debated, nevertheless. There was evidence that seemed to provide excellent support for an aether medium of light. In the 1720 James Bradley carried out a series of experiments in order to attempt to measure stellar parallax. Although he failed to measure parallax, his measurements placed a lower limit on the distance to stars. Much more interesting was his discovery of another effect, stellar aberration, an effect based not on position (as in parallax), but in speed. He noticed that the effect changed the apparent position of the star in space as the Earth moved around its orbit, thereby demonstrating that light had a finite speed, as well as the fact that the Earth really is moving. Bradley's results make perfect sense in an aether-filled universe. In this case the effect is being caused by the Earth's movement through the aether. The light, travelling "straight" in the aether, hits the Earth at an angle, thereby moving the image of the star.
A century later, however, Young and Fresnel realized 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.
Needless to say scientists of the late 19th century were growing increasingly unhappy with an edifice they had picked up "by default" from work a century earlier. In the late 1800s a series of increasingly complex experiments were carried out to try to see if the aether could be found. If it couldn't, Occam's razor should be used, and the concept discarded.
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 math 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.
Numerous experiments were carried out in the late 1800s to test this effect, but most were arguable 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 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, it means that the measured result could not support the hypothesis. If your hypothesis is that yellow and red make blue, you will get the null result when you try it; orange is not blue even though it is a color. In this case the MM experiment showed a small positive velocity; however it was too small to demonstrate aether wind, 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 experiments on aether led alternately to its abandonment by many scientists, or to a flurry of efforts to "save" aether by introducing even more magical 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 which showed a drift that was 10 times less than expected. 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.
Another, completely different, attempt to save 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. Eventually this idea was also ruled out in the Kennedy-Thorndike experiment in 1932.
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. Other positive results included Sagnac in 1913, and Michelson and Gale in 1925. During the 1920s a slew of increasingly accurate experiments returned the null result, and the positives were generally attributed to experimental errors.
End of aether
The aether theory was finally abandoned when the Galilean transformation and Newtonian dynamics were both modified by Albert Einstein's theory of relativity. Like most major shifts in scientific thought, this 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 theory lost its adherents.
Einstein based his work on Lorentz's earlier work, but instead of suggesting that it changed the mechanical properties of objects in an aether, he took the somewhat more radical step of suggesting that the math was actually a general transformation -- that the Galilean transformation was actually a "special case" that worked only at the low speeds we had studied up to that time. By applying the transformation to all frames of reference, he demonstrated that physics remained invariant as it had with the Galilean transformation, but that light was now invariant as well.
The null results were no longer odd at all, they made perfect sense. The need for a single universal frame disappeared -- and aether along with it. However the transformation also implied something much more radical, that the concept of position in space or time was not absolute, these formerly theoretical constructs had very real physical qualities, that would differ depending on the observer's location and speed. This "oddness" of his work led to it being considered highly questionable for some time.
All of this left the problem of propagation through a vacuum, however, the original reason for aether being introduced. But 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 it was most likely that light consisted of particles that had a "wave like nature". Particles 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".
Most current physicists do not see a need to have a medium for light to propagate. The combination of relativity and quantum mechanics render the concept unneeded. 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.
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.
For some reason, relativity is considered by many non-scientists to be "disturbing". The most likely reason for this is the massive publicity the theory receives: the only scientist most people can name is Einstein. For this reason there have been continued efforts to re-introduce a consistent aether theory in order to be able to abandon relativity. Although the vast majority of modern scientists reject all aether-based theories, the aether's mystic appeal continues to draw pseudoscientific proponents and protoscientific aspirants.
A final possibility to explain the Michelson-Morley experiments is that the earth is stationary in the center of the universe. This is the basis of the theory of modern geocentrism.
- 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.
- "ether", Oxford English Dictionary, 2nd edition (1989).