Ether (physics)

The ether (also spelled aether) was a concept in physics made obsolete in 1905 by Einstein's theory of special relativity.

The idea of an ether was introduced into science by Descartes in the 1630s when he replaced action at a distance by an intermediate medium (ether) that transferred forces between interacting bodies by direct contact. The ether concept became especially predominant in the 19th century by the work of Young and Fresnel on the wave theory of light. They replaced Newton's light corpuscles by waves. In order to explain stellar aberration, first observed in the 1720s and then shown to be caused by the velocity of the Earth relative to the velocity of Newton's light corpuscles, Young (1804) assumed ether to be in a state of absolute rest. Maxwell showed in the 1860s that light waves are electromagnetic waves transverse (perpendicular) to the propagation direction of the waves. Maxwell inferred that the electromagnetic waves are vibrations of the 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; hence ether was thought to have solid-like properties. Since light behaves the same in closed rooms as in open fields and many materials conduct light, ether was assumed to fill up all of space and all of matter. Thus, at the end of the 19th century one had a picture of the ether as a quasi-rigid solid (not completely rigid because it can vibrate), luminiferous (light carrying) medium that is massless and transparent, at absolute rest and present everywhere.

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 scientific concept it evolved through three 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 scholastics as Albertus Magnus (ca. 1200 – 1280) and Thomas of Aquino (1225 – 1274). Aristotle's concept of natural motion was generally accepted in medieval 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 occult and therefore superseded; he believed instead 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). In this framework, Descartes' description 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 (1665) 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 gravitation—a typical action without apparent direct contact—in terms of ether particles that are 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 Huygens, whom he had known personally as a child.

Hooke's and Huygens' theories were obliterated (at least for over a century) by their contemporary, the scientific giant Isaac Newton (1642 – 1727). Newton started his career as a strict adherent of ether theory. He wrote in 1672 and 1675 (summarized in Ref. p.19): "All space is permeated by an elastic medium or aether, which is capable of propagating vibrations. This aether pervades the pores of all material bodies and is the cause of their cohesion; its density varies from one body to another, being greatest in the free interplanetary spaces". Newton suggested three mechanisms by which light may proceed through the ether. His second suggestion that light consists of "multitudes of unimaginable small and swift corpuscles of various sizes springing from shining bodies" was generally selected by later scientists. In 1675 Newton submitted a memorandum to the Royal Society in which, among other things, he explained gravity. He wrote that aether condenses continually in bodies such as the earth and therefore there is a constant downward stream of it that impinges on gross bodies and carries them along. Further Newton suggested in this memorandum that the resulting movement of aether holds the planets in closed orbits. However, later when writing the Principia (1687), Newton become more inclined toward considering gravity as an action at a distance. He realized that this would not be easily digested by his contemporaries, who had just freed themselves of the Aristotelian notion that the earth attracts an object because of the natural place of the latter in the universe. And indeed he was right, both Huygens and Leibniz were very critical of the idea of attraction. In the second edition of the Principia (1713), Newton defended his point of view by adding a "General Scholium" in which he attacked the vortex theory of Descartes, pointed out that his gravitational law was mathematically correct, that he did not know the deeper reason for it, and said Hypotheses non fingo (I don't make up hypotheses).

Because of Newtons' enormous influence on 18th century science, action at a distance was no longer seen as a problem. As a matter of fact, when Faraday around 1830 cast doubt on the application of the concept to electric and magnetic forces, he met enormous resistance. The 18th century did not see much development in the theory of light and Newton's corpuscular theory was generally assumed to be correct, although it was forgotten that Newton had stated that light particles travel through ether. In short, ether was beyond the horizon of most 18th century natural philosophers.

(To be continued)