Luminiferous Aether Aether And Classical Mechanics

In the late 1700s a series of experiments on diffraction clearly demonstrated the wave nature of light. Light was initially assumed to be a longitudinal wave, like sound, and by analogy to mechanical waves, physicists assumed that light required a medium for propagation. This was somewhat problematic, as it appeared the space between the Sun and Earth was empty, making it difficult for the Sun's light to reach us. Christiaan Huygens hypothesized that space was not truly empty, but instead filled with a light gas, the aether (a Greek word for the substance used to make the sun and stars).

In 1820 Young and Fresnel showed that, in order to account for the polarisation of light, it had to behave as a transverse wave. However, a transverse wave required the propagating medium to behave as a solid, as opposed to a gas. To account for the apparent incompatibility between this and the free movement of the planets, Stokes suggested that the aether might be (by analogy with pine pitch) rigid at very high frequencies and fluid at lower speeds. In order to account for the absence of longitudinal waves, Cauchy suggested that the aether had negative compressibility; but George GreenThis article is about the scientist George Green. For the place called George Green in Berkshire, England see George Green, Berkshire''. George Green ( July 1793 31 May, 1841), British scientist who wrote An Essay on the Applications of Mathematical Analy pointed out that such a fluid would be unstable.

Later, Maxwell's equationsMaxwell's 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. Introduction Maxwell's four equations express, respective showed that light is an electromagnetic waveElectromagnetic radiation is a combination of oscillating electric and magnetic fields in perpendicular orientation to each other, moving through space, effectively transporting energy from one place to another. Visible light is a form of electromagnetic. Maxwell's equations required that all electromagnetic waves in vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also propagate at a fixed speed, cCherenkov effect in a "swimming pool" nuclear reactor. The effect is due to electrons moving faster than the speed at which light moves in water. The speed of light (denoted as c reputedly from the Latin celeritas "speed", and also known as Einstein's con. 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.

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.

1 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.

2 Experiments

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 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.

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. Eventually one such theory rose to the top, the concept of "aether drag". In this theory the aether was dragged by large masses, so that that local speed was always that of the nearest massive object, in this case the Earth. This introduced the new magical property that while it was dragged about by mass, it could not create any drag in return, or planetary orbits would be wrong.

Aether drag was contradicted by the observations of stellar aberration, a change in angle of light from a star due to the Earth's motion, by James Bradley in 1725 and again by George Airy 1871, which were not consistent with an aether that moved with the Earth. This theory was also eventually ruled out in a clever experiment that ran a Michelson-Morley experiment between two massive steel plates, massive enough that the drag should have been measurable.

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 considered statistically unimportant.

3 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.

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".

4 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 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, Einstein is the only scientist most people can name. 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.

5 See also

6 References

Banesh Hoffman, Relativity and Its Roots (Freeman, New York, 1983).
Michael Janssen, 19th Century Ether Theory, Einstein for Everyone course at UMN (2001).

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