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User:Reddi/Luminiferous aether

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The Luminiferous aether

The Earth moving along its orbit
through the
"medium" of light

In the late 19th century the luminiferous aether ("light-bearing aether"), or ether, was postulated to be the medium for the propagation of light, which had been shown by Young to behave as a wave (later confirmed by 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.

Existence

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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.

As of 2004, Jean-Louis Naudin and Patrick Cornille carried out experiments which, properly controlled, obtained positive results. Confirmation by having the experiments repeated by an third-party independent group have not proceeded.

Disadvantages and Critics

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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 incompressible for the propogation of light, it had to offer no resistance to the movement of the planets.

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 which light to travel through.

Some classic field physicists (like Dayton Miller and Edward Morley) continued research on the aether for some time, and occasionally researchers still explore these concepts. 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.

Unfortunately, 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. Modern analysis of aether must be consistent with all of the experiments testing phenomena.

Aether theory postulate experiments

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  • Bradley experiment - aberration of starlight
  • Lodge experiment - aether drag
  • Fresnel experiment - drag coefficient
  • Fizeau experiment - drag coefficient
  • Airy experiment - water-filled telescope

Experimental Considerations of the Aether

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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.

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 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 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):

 

An alternative experiment that tests for the existence of the aether is the Trouton Noble experiment. The experiment attempted to detect the motion of the ether in the 1903. The 1903 experiment, which like Michelson-Morley, obtained a null result. Some recent experiments which, properly controlled, obtained positive results. [1] [2] Confirmation by having the experiments repeated by an third-party independent group have not proceeded.

Timeline

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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).
1984 - Torr and Kolen find a cyclic phase shift between two atomic clocks, but the distance between is relatively short (0.5 km) and their clocks of the less-precise rubidium type
1991 - Over a six-month period, Roland DeWitte finds, over a 1.5-km underground coaxial cable, a cyclic component in the phase drift between higher-precision cesium-beam clocks on more-or-less the same meridian; the period equals the sidereal day [3][4]

Further readings

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

Classical References

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