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

     [\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.

Disadvantages and Critics

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.

Ether theory postulate experiments

   * Bradley experiment - aberration of starlight
   * Lodge experiment - ether drag
   * Fresnel experiment - drag coefficient
   * Fizeau experiment - drag coefficient
   * Airy experiment - water-filled telescope

Timeline

     1818 - Augustin Fresnel's Wave Theory of Light.
     1820 - Discovery of SimŽon 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).