Any ether theorists out there willing to make predictions?

However, IMHO, there are two objections to such ethers. The
first is essentially philosophical: such ethers require the
ether "atoms" to interact at a distance. How is this interaction
to be explained?

If it is OK for ether atoms to interact at a
distance without further explanation, then why is it necessary
to invoke an ether to explain interaction between particles
of ordinary matter?

The other is quantisation of light. ....
How can this be explained by classical mechanical waves in
an ether?

an ether theory needs to do more than simply reproduce the results
of classical electrodynamics.

Ether theories, on the other hand, proved remarkably resilient
and could be modified to accomodate the new evidence. This
will be explored further in part 3.

Abstract

This is the second section of a discussion in three posts
on theories of light. This part is about ether theories of
light; the first part was about emission theories, and the
final part is about light in the framework of the special
theory of relativity.

The ether is often dealt with in accounts of optics,
electromagnetics, or relativity by the simple, but
insufficient, "the Michelson-Morley experiment proved that
there was no ether". While appealing in its simplicity,
this fails to capture either the scientific or the
historical aspects. A closer look at why conventional
physics abandoned the ether is worthwhile.

Introduction

...[Snip]... With the benefit of hindsight, knowing that
light is an electromagnetic wave, we can treat the
postulated luminiferous and electromagnetic ethers as
identical from the beginning.

The earliest ether theory that can be called "scientific"
in the modern sense is perhaps that of Huygens (see Crew
1900 for a translation of Huygens work). Into the corpuscule
wave debate stepped Huygens, who rejected corpuscular theories
on the basis that counter-propagating rays of light do not
disturb each other. Transmission of light through a vacuum
then shows that the luminiferous medium - the ether - is not
ordinary matter.

Huygens concluded that the ether, like gases and liquids, was
composed of small particles, but composed of much smaller
ether molecules, able to freely move among the particles of
gases and liquids, and also able to penetrate the pores of
solid bodies, which must "have a structure similar to that of
a sponge." Due to the great speed at which light waves travel,
the ether molecules must be extremely hard.

Young (1802) successfully explained most known optical phenomena
in terms of waves in a highly elastic ether. Young noted the
smallness of the wavelength of light (accurately) and the
exceedingly high frequencies of the waves (inaccurately, since
he assumed a value for the speed of light approximately half
the true value).

Difficulties remained however - polarisation of light was
unexplainable while light waves were assumed to be pressure
waves in the ether, analagous to sound waves in air. The key
element in the general acceptance of the wave theory of light
was the explanation of polarised light by Fresnel in terms of
transverse waves in the ether (Frankel 1976).

That light consists of transverse waves, and only transverse waves,
has some important consequences for ether theories:

... an ether that behaves like an atomic gas, with "ether atoms"
that basically bounce off each other during collisions can only
support longitudinal waves, not transverse waves.

Thus, Huygen's gas-like ether was ruled out.

An ether, then, must be either fundamentally continuous, or, if
"atomic", consist of particles that can interact at a distance.

Electromagnetism

The early history of electromagnetic theory and experiment
following Oersted's discovery shows very rapid progress, with
Ampere, Biot, Savart, and Faraday rapidly completing the
foundations that would be needed for a unified theory of
electromagnetism.

While continental attempts at explaining electromagnetic
phenomena were based firmly on action-at-a-distance in true
Newtonian style,

Faraday was picturing electromagnetism in terms of lines of
force ...

... connecting charges. Incidentally, Faraday also discovered
some phenomena that complicated the early simple ether theory
of light, such as Faraday rotation.

Following the beginning made by Faraday, and work by W. Thomson,
Maxwell (1861-1862, 1865) then obtained a unified theory of
electromagnetism based on the mechanical properties of the ether.
Maxwell (1865) showed that the speed of electromagnetic waves
in the ether was equal to the speed of light, ...

... leading to the conclusion that the luminiferous medium and
the electromagnetic ether were the one and same thing.

Maxwell (1861-1862) explicitly used vortices in a fluid to model
magnetic fields, showing the properties that the fluid and the
vortices ...

... therein must have if this was to be the mechanical explanation
of magnetic fields. Despite his success, there remained some severe
problems; it is worth quoting Maxwell on this:

- We have yet given no answer to the questions, "How
are these vortices set in rotation?" and "Why are
they arranged according to the known laws of force
about magnets and currents?" These questions are
certainly of a higher order of difficulty than either
of the former; and I wish to separate the suggestions
I may offer by way of provisional answer to them, from
[the answers to the earlier questions].-

In Maxwell's later work, he appears to step back from his earlier
model:

I have on a former occasion attempted to describe a particular
kind of motion and a particular kind of strain, so arranged as
to account for the phenomena. In the present paper I avoid any
hypothesis of this kind (Maxwell 1865)

The theory proposed in the preceding pages is evidently of a
provisional kind, resting as it does on unproved hypotheses
relating to the nature of molecular vortices, and the mode in
which they are affected by the displacement of the medium. We
must therefore regard any coincidence with observed facts as of
much less scientific value in the theory of the magnetic
rotation of the plane of polarization than in the electromagnetic
theory of light, which, though it involves hypotheses about the
electric properties of media, does not speculate as to the
consitution of their molecules. (Maxwell 1873)

Despite these reservations, Maxwell remained firmly committed to the
idea of a medium, closing his Treatise (1873) with "Hence all these
theories lead to the conception of a medium in which the propagation
takes place".

The downfall of the ether

Early experiments and observations (Fizeau-Fresnel ether drag and
aberration) suggested that the ether was "stationary". It is worth
pointing out that, at the time, it was believed that our galaxy
was quite small, and, furthermore, that our (small) galaxy might
constitute the entire universe. "Stationary", therefore, should be
taken to mean stationary relative to the universe as a whole.
This, then, suggested the possibility of determining the motion
of the earth through the ether by electromagnetic or optical means.
Maxwell (1880) himself noted the possibility, and the attempts
to actually measure the motion of the earth are well-known
(eg the Michelson-Morley experiment, Trouton-Noble, etc), as
is the fact that all yielded a null result. (Let us not be
diverted by the Miller arguments! Even if Miller's result
was a genuine non-null result, it still rules out a
stationary ether.)

Contrary to the modern simplification that ether was therefore
proved to exist, active experimentation to determine the
properties of the ether continued (eg looking for ether drag in
the vicinity of moving massive bodies), as did theoretical
investigations (for example, Larmor's monumental work). As is
quite well known, investigations of this nature led to Lorentz
developing a theory that explained all known observations and
experiments to date.

Special relativity offered a simpler alternative to the rather
ad hoc modifications to ether theories to accomodate the
"experimental foundations of special relativity", and it is
within reason to state that special relativity eliminated the
need for ether. This is, however, historically inaccurate, ...

... at least in a strict sense. Lorenz had already formulated
a theory of electromagetism, including the identification of
light as an electromagnetic wave, without recourse to ether.
Lorenz's work, however, does not seem to have attracted much
notice (Keller 2002).

The actual downfall of the ether was more complex. Firstly,
EInstein's theory of special relativity attracted little
attention in England until 1920, and work on ether proceeded
in the meantime. This led to some serious theoretical difficulties:
if the ether was continuous, then the interaction of matter
with the ether would result in the rapid cooling of matter
(Jeans 1905, 1922) - essentially the ultraviolet catastrophe
in ether guise. The inability of the continuous ether to deal
with such difficulties led to its demise.

Possible ethers?

As noted above, "atomic" gas-like ethers are ruled out,

...and continuous ethers are ruled out.

What remains - perhaps the possibility of an "atomic" fluid
or solid ether. If fluid, the ether must be able to support
transverse waves, which, perhaps, suggests the possibility of
a superfluid.

One practical problem remains: the absence of longitudinal
waves; while solids and superfluids can indeed support
transverse waves, they also support longitudinal waves,
which are not observed.

An interesting idea is that atomic ethers are experimentally
testable - there must be a lower limit to the wavelength
of undulations in the ether. Any ether theorists out there
willing to make predictions? If we are moving through such an
ether, then presumably the lower limit is also anisotropic.

However, IMHO, there are two objections to such ethers. The
first is essentially philosophical: such ethers require the
ether "atoms" to interact at a distance. How is this
interaction to be explained?

If it is OK for ether atoms to interact at a distance without
further explanation, then why is it necessary to invoke an
ether to explain interaction between particles of ordinary
matter?

The other is quantisation of light. Consider the emission
of light by a dipole. Classically, the light is emitted in
the radiation pattern of a dipole oscillator. The wave spreads
out in space. Quantumly, if a single photon is emitted, only
a single photon is detected, and the single photon is detected
at a point, not spread out in the manner of the classical wave.
How can this be explained by classical mechanical waves in
an ether? With classical electrodynamics having essentially the
status of a high-photon-number approximation to QED, an ether
theory needs to do more than simply reproduce the results
of classical electrodynamics.

Conclusion

This leads to an interesting final conclusion: the photon
hypothesis strongly suggest that some kind of emission theory
of light should be correct. However, old corpuscular theories
of light and the emission theories suggested in opposition to
special relativity failed to match observation and experiment.
Ether theories, on the other hand, proved remarkably resilient
and could be modified to accomodate the new evidence. This
will be explored further in part 3.

Abstract

This is the second section of a discussion in three posts on
theories of light. This part is about ether theories of
light; the first part was about emission theories, and the final
part is about light in the framework of the special theory of
relativity.

The ether is often dealt with in accounts of optics,
electromagnetics, or relativity by the simple, but
insufficient, "the Michelson-Morley experiment proved that
there was no ether". While appealing in its simplicity, this
fails to capture either the scientific or the historical aspects.
A closer look at why conventional physics abandoned the ether
is worthwhile.



Bibliography

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Young and Fresnel, New York: American Book Company.

Fizeau, H. (1851), Sur les hypothèses relatives à l éther lumineux,
et sur une expérience qui paraît démontrer que le mouvement
des corps change la vitesse avec laquelle la lumière se propage
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Fizeau, H. (1859), Sur les hypothèses relatives à l éther lumineux
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Frankel, E. (1976), Corpuscular optics and the wave theory of light:
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Jeans, J. H. (1922), The present position of the radiation problem,
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Keller, O. (2002), Optical works of L. V. Lorenz, Progress in Optics
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Lorenz, L. (1863), Ueber die Theorie des Licht