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Luminiferous aether

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, non-dispersive, incompressible, continuous, and without viscosity. This idea of an aether has since been rejected by the vast majority of scientists.

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

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 and when many experiments subsequent to Michelsom-Morley failed to find any evidence of aether. Most current physicists do not see a need to have a medium for which light to travel through.

An alternative experiment that tests the existence of the aether is the Trouton Noble experiment.

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

There remain some modern proponents of aether theory. Its mystic appeal draws pseudoscientific proponents. Its intuitive appeal draws protoscientific proponents. Its conservative history draws classical field proponents.

It rather easy to create aether theories which conform to the null result of the Michelson-Morley experiment, but it becomes increasing difficult to create theories that are consistent with all of the related experiments which are consistent with no aether. Modern analysis of aether must be consistent with all of the experiments testing phenomena.

Timeline

1818 - Augustin Fresnel's Wave Theory of Light.
1820 - Discovery of Siméon Poisson's "Bright Spot", supporting the Wave Theory.
1873 - James Maxwell's Treatise on Electricity and Magnetism.
1878 to 1880 - Maxwell suggests absolute velocity of Earth in aether may be optically detectable.
1881 - Albert Abraham Michelson publishes first interferometer experiment.
1881 - Hendrik Antoon Lorentz finds Michelson's calculation have errors (i.e., doubling of the expected fringe shift error).
1882 - Michelson acknowledges his interpretation errors.
1887 - Michelson and Edward Williams Morley experiment produces the famous null results.
1887 to 1888 - Heinrich Hertz verifies the existence of electromagnetic waves.
1889 - George Francis FitzGerald proposes the Contraction Hypothesis.
1895 - Lorentz proposes independently another Contraction Hypothesis.
1905 - Miller and Morley's experiment data is published. Test of the Contraction Hypothesis has negative results. Test for aether dragging effects produces null result. Albert Einstein introduces the special theory of relativity.
1919 - Arthur Eddington's Africa eclipse expedition is conducted and appears to confirm the general theory of relativity.
1921 - Dayton Miller conducts aether drift experiments at Mount Wilson. Miller performs tests with insulated and non-magnetic interferometers and obtains positive results.
1921 to 1924 - Miller conducts extensive tests under controlled conditions at Case University.
1924 - Miller's Mount Wilson repeats experiments and yields a positive result.
1925 - Michelson and Gale perform the Pearson experiment producing a null result while attempting to detect the effect of Earth's rotation on the velocity of light. Null result predicted by both relativity and aether theory.
1925 April - Meeting of the National Academy of Sciences.
Arthur Compton explains the Stokes aether drag problems.
Miller Presents his positive results of the aether drag.
1925 December - American Association for the Advancement of Science meeting.
Miller proposes two theories to account for the positive result. It consists of a modified aether theory and a slight departure from the Contraction Hypothesis.
1926 - Roy J. Kennedy produces a null result. Auguste Piccard and Ernest Stahel at Mont Rigi produce a null result.
1927 - K. K. Illingworth produces a null result.
1927 - Mount Wilson conference.
Miller talks of partial entrainment
Michelson talks about aether drag and altitude differential effects
1929 - Michelson and F. G. Pease perform the Pearson experiment and produce a null result.
1930 - Von Georg Joos produces a null result.
1934 - Joos publishes on the Michelson-Gale Results, stating that it is improbable that aether would be entrained by translational motion and not by rotational motion.
1955 - R. S. Shankland, S. W. McCuskey, F. C. Leone, and G. Kuerti perform a debated analysis of Miller's positive results. Shankland, who led the study, reports statistical fluctuations in the readings and systematic temperature disturbances (both allegations have been later disproven).

Classical References

1. Maxwell, Collected Papers, H. A. Lorentz, Archives Neerlandaises, xxi. 1887, and xxv. 1892
2. Versuch einer Theorie der electrischen und optischen Erscheinungen in bewegten Korpern (Leyden, 1895)
3. "Elektrodynamik " and " Elektronentheorie " in the Encyk. der Math. Wissenschaften, Band v. 13, 14
4. O. Lodge, " On Aberration Problems," Phil. Trans. 1893 and 1897
5. J. Larmor, Phil. Trans. 1894-95-97, and a treatise, Aether and Matter (1900) p. 262
6. P. K. L. Drude, A. Schuster, R. W., General physics of the aether;
7. Collected Papers of Lord Rayleigh