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 around 1640. Until that time, forces between two bodies that are not in direct contact were assumed to act through space—by action at a distance. Descartes replaced this explanation by one based on an intermediate medium (ether) consisting of vortices that transmit forces between non-touching bodies.
The ether concept became especially predominant in the 19th century by the work of Young and Fresnel who revived Huygens' wave theory of light. They replaced Newton's light corpuscles by waves propagating through the ether. 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 direction of the propagation of the waves. Following Young and Fresnel, Maxwell assumed that 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 in closed rooms the same 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 physicists 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".
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 Hendrik Antoon 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.
Middle ages
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 philosophy was introduced into Western Europe in the 13th century by scholastics as Albertus Magnus (ca. 1200 – 1280) and Thomas of Aquino (1225 – 1274). After Aristotelianism was accepted by the Church, Aristotle's views on the nature of motion were incorporated into medieval natural philosophy: a heavy object has its natural place in the center of the universe (which before Copernicus was the center of the Earth) and a light object has its natural place in the sphere of the Moon. Ergo, a stone falls downward and smoke rises upward.
Descartes
René 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 touching 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). The Cartesian theory of light is—in the eyes of the modern beholder—rather convoluted. In the first place it is assumed that the speed of light is infinite and yet light is seen 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.
Hooke, Huygens and Newton
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 scientific giant Isaac Newton (1642 – 1727). Newton started his career as a strict adherent of ether theory. He wrote in 1672 and 1675 (as summarized in Ref. [1] 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.[2] 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 an object falls downward because of its natural place 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. This is exemplified by the resistance that Michael Faraday met when he cast doubt on the concept in relation to electric and magnetic forces. As the 18th century did not see much development in the theory of light, Newton's corpuscular theory was generally accepted, although it was forgotten that he had stated that light particles travel through ether. In short, ether was not of foremost interest to most 18th century natural philosophers.
Young and Fresnel
Ether re-entered the forefront of physics when Thomas Young revived the wave theory of light (1800). He noticed that Newton's theory had problems with the interference of light and with light falling under an angle on the surface of water. Light falling on water is partly refracted and partly reflected and wave theory can account elegantly for this, while corpuscular theory cannot. Young's theory was adapted an extended by his French contemporary Augustin-Jean Fresnel. Both workers recognized that stellar aberration needed to be explained by wave theory.
Stellar aberration was discovered by James Bradley in 1725, who explained his discovery in 1727 by noting that the velocity of the stellar light, c′, which is observed on earth, is the resultant of the velocity of light, c, with respect to an absolute frame attached to the fixed stars minus the velocity of the earth v with respect the absolute frame. The vectors c′≡ c−v and c make a small angle, the aberration angle. Thus, Bradley transformed the speed of light-particles from an absolute frame to a frame attached to the moving earth. Young made a first step in incorporating stellar aberration into the wave theory when he assumed in 1804 that the ether is at absolute rest, that is, ether is the absolute frame in which light waves propagate with velocity c and in which the fixed stars are standing still.
In passing, we note that it was known in the early 19th century that light goes through transparent material, such as glass, with a speed, cg, lower than c (speed in vacuum). This is expressed by the index of refraction n of the material being larger than unity. Remember that this index is the ratio of the two speeds: n ≡ c/cg. Inspired by Bradley's theory of stellar aberration and the positing by Young of a constant speed of light in absolute ether, François Arago performed in 1810 some experiments. By means of mirrors he guided light through glass lenses and prisms in two ways: (i) light propagating parallel to the speed v of the earth and (ii) antiparallel to v. He was able to measure the resultant speed which he expected to be in the two respective cases:
However, he did not observe this, or any other effect of the speed of the earth. Eight years later Fresnel thought about this experiment and made the assumption that ether is "dragged" along by the glass. He derived a "drag factor" (1−1/n2), so that the speeds that should be observed are
In 1851 Armand Hippolyte Louis Fizeau was able to confirm this result experimentally by guiding light through water.
(To be continued)