Aether drag hypothesis

In the 19th century, the theory of the luminiferous aether as the hypothetical medium for the propagation of light waves was widely discussed.

The aether hypothesis arose because physicists of that era could not conceive of light waves propagating without a physical medium in which to do so.

Albert Einstein's special theory of relativity (1905) excludes aether as a mechanical medium.

[1][2][3] In 1810, François Arago realised that variations in the refractive index of a substance predicted by the corpuscular theory would provide a useful method for measuring the velocity of light.

Arago attempted to measure the extent to which corpuscles of light would be refracted by a glass prism at the front of a telescope.

Contrary to this expectation, he found that there was no difference in refraction between stars, between times of day or between seasons.

He realised that even if light were transmitted as waves the refractive index of the glass-air interface should have varied as the glass moved through the aether to strike the incoming waves at different velocities when the Earth rotated, and the seasons changed.

The velocity of light in the glass is then given by: This correction was successful in explaining the null result of Arago's experiment.

It introduces the concept of a largely stationary aether that is dragged by substances such as glass but not by air.

However, this theory is considered to be refuted for the following reasons:[1][2][3] For George Stokes (1845) the model of an aether which is totally unaffected or only partially affected by moving matter was unnatural and unconvincing, so he assumed that the aether is completely dragged within and in the vicinity of matter, partially dragged at larger distances, and stays at rest in free space.

[7][8][9][10] Also Heinrich Rudolf Hertz (1890) incorporated a complete aether drag model within his elaboration of Maxwell's theory of electromagnetism, to bring it into accord with the Galilean principle of relativity.

[14] Another version of Stokes' model was proposed by Theodor des Coudres and Wilhelm Wien (1900).

[15] And to save Stokes's explanation of aberration, Max Planck (1899) argued in a letter to Lorentz, that the aether might not be incompressible, but condensed by gravitation in the vicinity of Earth, and this would give the conditions needed for the theory of Stokes ("Stokes-Planck theory").

It is also compatible with Hammar's and Michelson–Morley experiment, as the aether is completely dragged by the large mass of Earth.

[11] And from a theoretical side it was noted by Hendrik Antoon Lorentz, that the Stokes-Planck hypothesis requires that the speed of light is not affected by a density increase of 50,000 times of the aether.

[1][16] Since Lorentz was forced to abandon Stokes' hypothesis, he chose Fresnel's model as a starting point.

[citation needed] He was able to reproduce Fresnel's dragging coefficient in 1892, though in Lorentz's theory it represents a modification of the propagation of light waves, not the result of any aether entrainment.

So, it was Albert Einstein (1905), who recognized that it is only required to assume the principle of relativity and the constancy of the speed of light in all inertial frames of reference, in order to develop the theory of special relativity and to derive the complete Lorentz transformation.

[18] As shown by Max von Laue (1907), special relativity predicts the result of the Fizeau experiment from the velocity addition theorem without any need for an aether.

is the velocity of the water: which, if v/c is small can be expanded using the binomial expansion to become: This is identical to Fresnel equation.

[22][23] The controversial Allais effect is not predicted by general relativity and his experimental results have been disputed.

[24]: 211 [25][26] In modern physics (which is based on the theory of relativity and quantum mechanics), the aether as a "material substance" with a "state of motion" no longer plays any role.

However, frame-dragging as predicted by general relativity, in which rotating masses distort the spacetime metric, causing a precession of the orbit of nearby particles, does exist.

Lodge's ether machine. Light from a sensitive common path interferometer was guided between the rapidly rotating disks.
Complete aether dragging is inconsistent with the phenomenon of stellar aberration. In this illustration, imagine the stars to be infinitely distant. Aberration occurs when the observer's velocity has a component that is perpendicular to the line traveled by the light incoming from the star. As seen in the animation on the left, the telescope must be tilted before the star will appear in the center of the eyepiece. As seen in the animation of the right, if the aether is dragged in the vicinity of the Earth, then the telescope must be pointed directly at the star for the star to appear in the center of the eyepiece.