Luminiferous aether

In the late 19th century the luminiferous aether ("light-bearing aether"), or ether, was postulated to be the medium for the propagation of light. Light was initially assumed to be a longitudinal wave (in analogy with sound) which could propagate in gases. 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. 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 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 Green pointed out that such a fluid would be unstable. All these ideas were based on analogy with existing known materials and fluids. Later, Maxwell's equations showing that light is an electromagnetic wave. By analogy to mechanical waves, physicists assumed that electromagnetic waves required a medium for propagation, and hypothesized the aether. Aether was thought to be a fluid which was transparent, non-dispersive, incompressible, continuous, and without viscosity. This idea of an aether has since been rejected by the vast majority of scientists.

Other than its apparently unusual mechanical properties, the existence of a medium for light should mean that the velocity of light would be relative to the medium, so that a moving observer would see an altered velocity of light, but this was not consistent with later experiments. More concretely, 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. However, the assumption of such a fixed reference frame were contradicted by the Michelson-Morley experiment in 1887, which was unable to detect the effect that motion of the Earth through the postulated aether should have had on the speed of light. This experiment led to Albert Einstein's theory of special relativity, which assumes that the speed of light is constant in all reference frames.

Experimental considerations of the aether

The key difficulty with the aether hypothesis arose from the juxtaposition of the two well-established theories of non-relativistic Newtonian dynamics and of Maxwell's electromagnetism. Under a Galilean transformation the equations of Newtonian dynamics are invariant, whereas those of electromagnetism are not. Thus at any point there should be one special coordinate system, at rest relative to the local aether, relative to which Maxwell's equations assume their usual form. Motion relative to this aether should therefore be detectable.

The most famous attempt to detect this relative motion was the Michelson-Morley experiment in 1887, which produced a null result. To explain this apparent contradiction the Lorentz-Fitzgerald contraction hypothesis was proposed but the aether theory was finally abandoned when the Galilean transformation and the dynamics of Newton were modified by Albert Einstein's theory of relativity and when many experiments subsequent to Michelson-Morley failed to find any evidence of aether. Most current physicists do not see a need to have a medium for light to propagate.

(Even before the Michelson-Morley experiment, there were severe problems with the aether theory due to the apparently incompatible properties it had to possess: although it had to be rigid and incompressible for the propagation of light, it had to offer no resistance to the movement of the planets.)

One possible explanation of the Michelson-Morley result was that the Earth "dragged" the ether along with it, so that it is fixed for an Earthbound observer. However, this was contradicted by the observations of stellar abberation (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 ether that moved with the Earth.

Another experiment purporting to show effects of an ether 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 ether along, with a portion of the medium's velocity, but that understanding was rejected after Veltmann demonstrated that the index n in Fresnel's formula depended upon the wavelength of light (so that the ether 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):

Another experiment that also attempted to detect the motion of the ether was the 1903 Trouton-Noble experiment, which like Michelson-Morley obtained a null result.

Continuing adherents

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 scientists reject all aether-based theories, the aether's mystic appeal continues to draw pseudoscientific proponents and protoscientific aspirants.

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.

References

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