Ether (physics)

In physics, the ether (also spelled aether) is a concept that was made obsolete by Einstein's theory of special relativity of 1905. The idea of an ether was introduced into science by René Descartes in the 1630s. It became especially predominant in the 19th century by the work of Fresnel in the 1810s on light and Maxwell in the 1860s on electromagnetic fields. In order to explain stellar aberration Fresnel had to assume a medium (ether) in a state of absolute rest relative to the fixed stars, and Maxwell showed that electromagnetic waves are transverse (perpendicular to the propagation direction) vibrations, which he inferred to be vibrations of ether.

In the 19th century it was known that transverse waves are not possible in a gas or a liquid, but only in a solid. It was believed that light, and electromagnetic waves in general, must be interpreted as vibrations of a medium that fills up all of space. Thus, one arrived at a picture of the ether as a quasi-rigid (not completely rigid because it can vibrate) luminiferous (light carrying) medium that is a massless transparent solid at rest with respect to the Earth and the stars.

Today, the concept of ether does not play a role any longer in physics, but in daily life the word lives on in connection with radio and television signals, which commonly are said to be transmitted "through the ether".

Short history

It is not really possible to speak of "the" ether, because as a concept it evolved through the centuries, from Descartes (1596 – 1650), who conceived it as a whirlpool of rotating chains of particles to Lorentz (1853 – 1928), who saw ether as a transparent massless solid at complete rest. Its only shared property, conserved through the centuries, is that it permeates all space and all matter, even the interstitial spaces between the atoms.

The name ether comes from ancient Greek αἰθήρ (aithèr) where it means the upper, radiating, air. Aristotle introduced it as a fifth element (quinta essentia) next to Earth, Fire, Water, and (sea-level) Air. Aristotelian thought was introduced into Western Europe in the 13th century by scholars as Albertus Magnus (ca. 1200 – 1280) and Thomas of Aquino (1225 – 1274). Aristotle's concept of natural motion was generally accepted in medieval natural philosophy: a heavy object has its natural place in the center of the universe (before Copernicus the center of the Earth) and a light object has its natural place in the sphere of the Moon. Therefore, a stone falls downward and smoke rises upward.

Descartes considered the medieval views on motion superseded and occult; he believed in contrast that all forces are transmitted by direct contact. With regard to the actions between bodies not in contact with each other, such as two magnets, or the influence of the Moon's position on the tides, he postulated that they must be in direct contact through intermediate contiguous matter. The force is transmitted through this matter—the ether—by two agencies, pressure and impact. Space, in Descartes' view, is a plenum occupied by an ether, which, imperceptible to the senses, is capable of transmitting forces on material bodies immersed in it. Descartes assumed that the ether particles are in constant motion, but, as there is no empty space for them to move to, he inferred that they move to places vacated by other ether particles. The particles participate then in the spinning motions of closed chains of particles (vortices). Descartes' description within this framework of the propagation of light is—in the eyes of the modern beholder—rather convoluted. In the first place he assumes the speed of light to be infinite and yet he sees light as a projectile whose velocity varies from one medium to another. The vehicle of light is matter of the "second kind", which is intermediate between vortex matter and ordinary, ponderable matter. This matter of the "second kind" forms globules and different rotational velocities of the globules give light of different colors.

The next event relevant to the history of ether is the publication (1667) of Micrographia by Robert Hooke (1635 – 1703). Hooke's description of the propagation of light is mechanical and in that sense it resembles that of Descartes. However, while the Cartesian hypothesis is a static pressure in the ether, Hooke's theory concerns a rapid vibrational motion of small amplitude. He introduces the idea of a wave front, which twelve years later (in 1679) was taken over by Christiaan Huygens (1629 – 1695), who greatly improved and extended the wave theory of light. Huygens inferred that the ether, in which light propagation takes place, penetrates all matter and is even present in the vacuum. Huygens' ether was, like Descartes', constituted of particles. Huygens interpreted gravity, a typical action without apparent direct contact, in terms of ether particles, rapidly rotating in the space surrounding the Earth. His rotating particles are reminiscent of the Cartesian vortices, which is not surprising as Descartes had had a strong influence on the young Christiaan, whom he had known personally as a child.

Hooke's and Huygens' theories were brought into oblivion (at least for over a century) by their contemporary, the scientific giant Isaac Newton (1642 – 1727).

(To be continued)