|A History of the Aether in Physical Theory - Part II
|Summary of this page:
- Young and Fresnel propose light to be a transverse wave of the aether.
- The mechanics of a transverse wave explained.
- The phenomenon of polarization explained in terms of the transverse wave theory of light.
- James Bradley's discovery of "stellar aberration" seems to necessitate the aether be fixed and immobile.
- Maxwell's discovery of light as an "electromagnetic" wave seems to necessitate that the aether be a
- Maxwell declares that the speed of light must be constant to agree with his four equations of
electromagnetism. This assumptions requires the aether to fixed and immobile.
Young and Fresnel, to account for the phenomenon of double refraction, or birefringence, and polarization
proposed that, instead of being a longitudinal wave, light travels as a transverse wave. But to allow such
transverse waves, the aether needed to have enough rigidity to supply the forces to oppose the distortions
produced by the waves. In other words, the aether, if a mechanical system, could not be a fluid; it had to be
Click Here for a description of the mechanical properties of a transverse wave. This page also explains why
the aether could not support such a wave (impossible restoring forces).
Light is proposed to be a transverse wave to explain the phenomenon of polarization. Click Here for a
description of this phenomenon.
While the transverse wave theory of light accounts for the phenomena of birefringence and the
polarization of light it required some seemingly impossible conditions of the aether. They are:
- First, the propagating three-dimensional medium of the aether, as opposed to the "two-
dimensional medium" of water, would have to behave like a solid to support the high
frequencies of light. In fact, it would have to be six million times denser than steel. This is
not a problem in itself until the aether is postulated to be motionless. How would it be
possible for the aether to move through the aether, which had to extremely dense to
support the high frequencies of light, without constantly losing momentum from the
friction, or "aether drag"?
- Secondly, and much more seriously, the aether would have to have an impossible
"restoring force" or negative compressibility as explained on the Transverse Wave page.
|Transverse Wave Theory of Light: New Problem for Aether Theory
It was discovered that any moving magnetic charge produces an electric field; and any moving electric
charge produces a magnetic field. Light was discovered to produce both kinds of fields, and thus, was
called an electromagnetic (EM) wave. That light has this property seemed to necessitate that light be a
transverse wave to propagate perpendicular, and mutually inclusive, electric and magnetic waves. But,
as I already mentioned with respect to Young and Fresnel, the idea of the ether providing the
necessary conditions to propagate this kind of complex transverse wave is highly dubious..
Maxwell derived his electromagnetic equations for a reference frame that was stationary with
respect to the ether. It was reasonable to do because the ether at that time was believed to be
itself stationary. It is fluid dynamic principle that the velocity of waves always remain constant with
respect to the medium through which it is traveling. The velocity of electromagnetic waves, then,
with respect to reference frames that are stationary in the medium, found to be constant. Maxwell
abbreviated the velocity for electromagnetic waves—the velocity he derived by positing a
stationary ether—as “c”. But his equations do not necessitate that the speed of light of be
constant in all inertial frames; just that it be constant relative to the inertial frame through which it
is traveling. In other words, the speed of light can be (and is) variable.
Maxwell, a firm believer in the existence of the ether, albeit a motionless one, proposed an
experiment to detect it. If the ether is motionless, then all objects, such as planets, must
experience a drag force when they move through it. On earth we should be able to detect an ether
“wind” as the earth moves through it. The inverse motion of the ether with respect to the earth is
often called the “ether drift”. Shortly before his death, he argued that the only way to measure the
earth's velocity with respect to the ether in a laboratory experiment is to look for variations in the
velocity of light traveling back and forth between two mirrors.
Wave: Based on
waves, this is
what such a wave
is believed to look
|The Electromagnetic Transverse Wave Theory of Light
|The Constant Speed of Light
The Michelson-Morley Experiment
The Michelson-Morley experiment was designed to discover the monolithic aether. A null-result
was produced: no aether drag, no monolithic aether, was discovered. This was taken as definitive
proof that the aether does not exist.
Key Assumption of Einstein's Special Theory of Relativity
Einstein based his entire Special Theory of Relativity on the assumption that the speed of light is
constant in all inertial frames of reference. The STR was postulated to supplant the aether
entirely. It led to such infamous logical paradoxes as the relativity of time, mass, length of objects,
and all inertial frames of reference. It also re-opened the interpretation of light. Einstein's
explanation of the photoelectric effect hypothesized light to be a particle. This supposition led to
the very illogical theory that light is both a wave (of emptiness) and a particle at the same time.
|Crucial Consequences of "c"
|Go to the
page to learn how
the EM transverse
wave theory of
light is used to
mechanics of this
Bradley discovered the phenomenon of stellar aberration in 1725. At this time Newton's corpuscular theory of
light was still considered viable. Bradley
Bradley's Interpretation of Stellar Aberration
Bradley’s interpretation, based on the Newtonian corpuscular concept of light, accounted quite well for the basic
phenomenon of stellar aberration. However, if light consists of ballistic corpuscles their speeds ought to depend
on the relative motion between the source and observer, and these differences in speed ought to be detectable,
whereas no such differences were found. Experiments suggest that the speed of light is independent of the
motion of the source (star), so they tended to support the wave theory of light, rather than the corpuscular
Attempts at an Aether-based Explanation of Stellar Aberration
With the revival of the aether wave theory of light at the beginning of the 19th century through the work of
Young and Fresnel, attempts were made to explain stellar aberration in terms of the aether. The simple
explanation of aberration in the particle theory can be adapted to the wave theory if one can still represent the
velocity of a star’s light with respect to a terrestrial observer by the simple vector diagram shown to the right. In
a wave-theoretic setting, the velocities added in this diagram must be interpreted as velocities with
respect to the luminiferous aether, the medium in which the light waves were thought to propagate. Any motion
of the aether lying between the star and the terrestrial observer would affect light waves travelling between
them. So, the motion of the light relative to the earth would be more complicated and the simple explanation of
stellar aberration borrowed from the particle theory would fail. In other words, stellar aberration seemed to call
for a completely stationary or, as we prefer to call it, immobile ether.
Stokes and the Dragged Aether
In 1845 George Gabriel Stokes (1819–1903) nonetheless attempted to account for stellar aberration on the
basis of a theory in which the earth drags along the ether in its vicinity. The attempt involves careful
consideration of how the wave fronts of stellar light change direction upon entering the earth’s ether
atmosphere. On Stokes’ account, rather than an apparent motion, the light ray really is “refracted” during its
passage through the ether. Stokes was interested in such an alternative account of aberration because he
believed the hypothesis of an immobile ether to be highly implausible. The new picture of the aether after Young
and Fresnel--the aether as a solid to restore the distortions of a transverse wave--is difficult to reconcile with
the hypothesis of an immobile ether undisturbed by the motion of matter. It was much more natural to assume
that matter drags along the ether. It was Stokes who became the champion of this view. He put forward a
model of the aether that has been described as the “Silly Putty” model. Stokes’ aether behaves as a rigid solid
for the high-frequency oscillations constituting light and as a fluid for the relatively slow motion of celestial
bodies travelling through it. The latter motion, however, no longer leaves the ether undisturbed. At the earth’s
surface, the ether will be at rest with respect to it. The price that Stokes had to pay for his more realistic model
of the ether was therefore a more complicated explanation of aberration.
Although many attempts were, and still are being, made to explain stellar aberration in terms of a monolithic
aether, primarily because of "aether drag"--they all are flawed. A monolithic aether, or aether drag on earth,
cannot account for the phenomenon of stellar aberration.
Appearance Relative to Observer vs. Actual Refraction
it's important to realize that stellar aberration was and still is proposed to be only an apparent aberration of the
star's position relative to the observer. It is not considered to be an actual refractive phenomenon
independent of what observer's appear to see relative to their movement on earth.
In a transverse wave the particle displacement is perpendicular to the direction of wave propagation.
The animation below shows a one-dimensional transverse plane wave propagating from left to right.
The particles do not move along with the wave; they simply oscillate up and down about their
individual equilibrium positions as the wave passes by.