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Luminiferous aether

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In the late 19th century the luminiferous aether ("light-bearing aether") was invoked as the medium for the propagation of light, when it was discovered, from Maxwell's equations, 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, undispersive, incompressible, continuous, and without viscosity.

Other than the question of propagation, the aether was intended to solve the problem that Maxwell's equations require that electromagnetic waves propagate at a fixed speed, c. As this can only occur in one reference frame according to Newtonian physics (see Galilean-Newtonian relativity[?]), the aether was hypothesized as the absolute and unique frame of reference in which Maxwell's equations hold. Later it was regarded as the seat of all electromagnetic energy and attempts were made to describe matter in terms of vortices in this fluid.

Many experiments were conducted to prove the existence of aether. It appeared to be verified by Fresnel's determination that the velocity of light relative to the aether on passing through a medium of refractive index n and velocity v (in the same direction) is

$\frac{c}{n} = \left( 1 - \frac{1}{n^2} \right) v$

and in the Airy experiment[?] on aberration. However, this theory required that matter moving through the aether should modify the velocity of the aether and that because of dispersion the relative velocity of medium and aether would be different for different wavelengths, thus requiring a different aether for each wavelength of light.

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, which is part of the basis of quantum mechanics. The modern standard model of physics explains how light waves can travel through vacuum without needing an aether to describe light in dual terms, being both a wave (the field) and a particle (called a photon).

Some classic field physicists (like Dayton Miller and Edward Morley) continued research on the aether.

There remain a few modern proponents of aether theory. It's intuitive and mystic appeal draws pseudoscientific proponets and It's conservative history draws classical field proponets . It is possible in principle to create aether theories which conform to the null-result of the Michelson-Morley experiment (though at the expense of Occam's razor sometimes). Simple aether-based physical theories are easier to understand than quantum mechanical theories. But simple aether theories fail to explain certain phenomena, requiring layers of complication which may end up reformulating much of Quantum electrodynamics but can also be intellectually stimulating. Plasma cosmologist equate the spacetime vacuum medium as the aether.

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