|A History of the Aether in Physical Theory
- In Greek mythology Aether is
one of the first-born
elemental gods. Aether is
the personification of the
upper sky beyond the
clouds. He is the upper air
that the Greek Gods
breathe. Aristotle taught
that the physical world was
made up of four elements:
air, earth, fire and water.
The elements communicated
via a “subtle” medium, a fifth
element: the aether.
Descartes, similar in conception to Aristotle's theory that the aether tied the four elements
together, hypothesized that the ether, a very dense medium of very small particles, pervaded all
space. He reasoned that no bodies of matter could act upon one another through empty space.
This ether medium, he wrote, transmitted forces from one object to another by collisions of the
particles. Descartes' universe was a purely mechanical universe—he believed all phenomena to
be the result of collisions of matter.
Using his ether model, Descartes believed the planets are pushed along their orbits by a
cosmologically-sized vortex motion of the ether. In his “Solar Vortex Theory of Gravity” he
theorized that the Earth is located within an ether vortex whose center was the sun. The
rotating motion of the sun's ether-vortex pushed earth in such a way as to make it orbit it.
However, it is unclear what kind of vortex Descartes had in mind. Newton interpreted Descartes
theory as implying circular vortex.
Descartes' Solar Vortex Theory of Gravity
It is unclear what kind of vortices
Descartes had in mind. Newton
interpreted Descartes theory as implying
circular vortices. The orbits of the planets
around the sun depicted here were
circular vortices of the fluid aether.
Hooke described light as a mechanical wave; and, like all mechanical waves, a medium or
material is required for it to propagate. Huygens revised and expanded Hooke's wave theory of
light and hypothesized that light is a wave propagating through the ether. His conception of the
ether was similar to Descartes. He believed it penetrates all matter and space and was made of
Huygens' reasoned that a propagating wave of light at any instant conforms to the
envelope of spherical wavelets emanating from every point on the wavefront at the prior instant.
Huygens further hypothesized that light propagates as a longitudinal compression wave. In a
longitudinal wave the displacement of the medium-particles is parallel to the direction of travel of
the wave; there are regions of high pressure, compression, and regions of low pressure,
rarefaction. In a light wave, Huygens argued that the ether molecules move forwards and
backwards; where they are compressed together, a compression results; where they are forced
further apart, there is a rarefaction.
This figure shows "Huygens'
Principle" which states that each
point on wavefront of light acts
as a point source for a new
wavefront (i.e., like a minor "light
Combining these two aspects of Huygens' wave theory of light, that each point on a wavefront
(compression area) acts as a point-source for a new wavefront, and that it propagates as a
longitudinal wave, gives the following physical conception:
Upon studying Descartes' theory of gravity, Newton reasoned that a circular vortex theory of
gravity would cause planets' orbits to be circular. This violates Kepler's Laws of Planetary Motion,
which empirically quantify the fact that planetary orbits are elliptical not circular in nature. Here is a
direct quote from Newton's Principia: “Hence it is manifest that the planets are not carried round in
corporeal vortices; for, according to the Copernican hypothesis, the planets going round the sun
revolve in ellipses, having the sun in their common focus; and by radii drawn to the sun describe
areas proportional to the times. But now the parts of a vortex can never revolve with such a
With the demise of Descartes' (circular) solar vortex theory of gravity, and Newton's empirical
success quantifying the action of gravity, the ether was now hypothesized to exist only to act as a
medium for the propagation of light. Thus it became known as the “luminiferous ether.” However,
Newton expressly stated that he merely gave an empirical definition of gravity and not an
explanation of what gravity actually is (“hypothesis non fingo” or “I don't make up hypotheses”).
He did not believe gravity to be caused by a “force of attraction” that acted instantaneously over a
distance without a material cause. Here is a direct quote from his Principia: “I have not as yet been
able to discover the reason for these properties of gravity from phenomena, and I do not feign
hypotheses. For whatever is not deduced from the phenomena must be called a hypothesis; and
hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no
place in experimental philosophy. In this philosophy particular propositions are inferred from the
phenomena, and afterward rendered general by induction.” Newton is, however, somewhat to
blame for using the word “attraction” to describe gravity; and his followers to culpable for
assuming by this that an actual force of attraction, or “pulling force” was meant.
Enraptured with the empirical success of Newton's equations caused inquiry into the actual
physical cause of gravity to be neglected. In fact, this began a long trend amongst scientists of
pursuing the mathematical empirical success of physical theories and ignoring their necessary
physical conceptional counterpart. It follows that, since Newton provided no alternate explanation
of gravity without the use of an ether concept, he and scientists erred when they relegated the
ether to be a mere medium of light.
Newton, in support of his own hybrid theory of light, disputed Huygens' theory. Since
Huygens' theory only supported the propagation of light-waves (or compression pulses of the
ether) in one direction, parallel to that of the movement of the ether-particles, it could not explain
the phenomenon of birefringence, or double refraction. When light passes through a crystal it is
split into two beams. There is no way Huygens' longitudinal compression wave could split
“transversely” in this way—that is, the compression pulse could not also move perpendicular to the
direction of motion of the ether particles. Light, then, could not be strictly a longitudinal wave.
State of the Aether Concept
- Descartes attempts to explain all phenomena mechanically, as direct interactions (collisions)
between matter. He introduced the idea of "plenum" in space, a moving aether, to explain,
among other things, gravity (solar [circular] vortex theory of gravity).
- Huygens' uses the aether to explain light as a longitudinal compression wave of aether
particles whereby each point on the wave front (compressed area) acts as a source of a new
wavelet (or compression pulse)
- Newton disproves Descartes' solar (circular) vortex of gravity. Newton disputes Huygens'
wave theory of light since it could not explain the phenomenon of double refraction.
- Where this leaves the aether after Newton: it is not abandoned as a theory because light
still exhibits many wavelike properties. However, to describe other phenomena, like gravity,
it is not used. Thus, after Newton the aether becomes the "luminiferous aether," believed
only to be significant as a medium through which the disturbance, light, could propagate
A strictly longitudinal wave cannot account
for the phenomenon of birefringence.
Summary of this page:
- The Aether in Greek Mythology
- Descartes' introduction of the aether as a physical theory: the solar vortex theory of
- Huygens' wave theory of light as a disturbance of the aether medium
- Newton's proof against Descartes' theory of gravity.
- Newton's disputation of Huygens' wave theory of light.
- The luminiferous aether: the aether is restricted to an explanation only of the
propagation of light through space and to explain its wavelike properties.
|Young and Fresnel to Maxwell
In a longitudinal wave the particle displacement is parallel to the direction of wave propagation.
The animation below shows a one-dimensional longitudinal plane wave propagating down a
tube*. The particles do not move down the tube with the wave; they simply oscillate back and
forth about their individual equilibrium positions. Pick a single particle and watch its motion. The
wave is seen as the motion of the compressed region (i.e., it is a pressure wave), which moves
from left to right.
* Note: waves/particles of wave in a three-dimensional medium, not confined by six sides
of a tube, do not propagate/oscilalte in a strictly longitudinal direction.
|Updated with important revisions 9.29.09
This figure shows the kinematics of a longitudinal wave.