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 bodies at a distance.

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 are transparent to 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. 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 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 corpuscular theory had problems with the interference of light and with the refraction and reflection of light falling under an angle on the surface of water. Wave theory can account elegantly for both effects, 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 – 1726 . A year later (1727) he explained his discovery in the framework of corpuscular theory. Bradley noted that the velocity of the stellar light observed on earth, c_E, is the resultant of the absolute velocity of light, c, expressed with respect to a frame attached to the fixed stars, minus the velocity of the earth v relative to the same absolute frame. The vectors c_E≡ c−v and c make a small angle, the aberration angle. Thus, Bradley transformed the speed of the light-corpuscles from an absolute frame to a frame attached to the moving earth. While doing this, he derived that the aberration angle is proportional to the ratio v/c, a ratio that was to become very important in 19th century physics. The speed of the earth, v, is about 3×10⁴ m/s and c ≈ 3×10⁸ m/s, so that v/c ≈ 10⁻⁴.

In 1804 Young made a first step in explaining stellar aberration by the wave theory when he assumed that the ether is at absolute rest, that is, ether offers the absolute reference frame used by Bradley. In the absolute frame light propagates with speed c (speed in vacuum).

It was known in the early 19th century that light travels through a transparent material on earth—such as a block of glass—with a speed, c_g, lower than c. This is expressed by the index of refraction n of the material being larger than unity. Remember that the index of refraction is the ratio of the two speeds: n ≡ c/c_g. The refractive properties of a prism depend, through Snell's law, on n and hence on the speed of light c_g in the glass. François Arago was of the rather obvious opinion that only the velocity of light relative to the prism should enter Snell's law. Inspired by Bradley's theory of stellar aberration, he performed in 1810 telescopic observations of the speed of stellar light. He mounted a prism on a telescope and observed stars situated at different angles in the sky, exhibiting different aberration angles. According to the Newton/Bradley theory, the light rays from the stars at different angles have different velocities relative to the prism; these should be observable in the refraction patterns of the prism. Arago got a null result, he did not observe any effects; this null outcome was the first of several to come in the next century.

The largest differences in speed of light on earth can be expected when a fixed star is on the horizon, and the earth travels parallel to its light rays, toward or away from the star. The absolute speed of light (measured by an observer motionless in the ether) is,

$c=c_{\mathrm {g} }+v\equiv {\frac {c}{n}}+v,$

where v is the speed of the earth; v is positive when the earth goes in the same direction as the stellar light and negative if it goes in opposite direction. Note that c=c_g +v is a special case of the vector equation c=c_E+v, introduced above.

In 1818 Fresnel gave some thoughts about incorporating Arago's observations in the wave theory of light. He made the assumption that ether is "dragged" along by the glass of the prism, so that the relative ether-glass speed is reduced. The ether in a transparent body is entrained with velocity v(1-1/n²) when the body itself moves with velocity v with respect to the absolute ether. The "ether drag factor" (1−1/n²) reduces the absolute speed of light, which becomes according to Fresnel,

$c={\frac {c}{n}}+v(1-{\frac {1}{n^{2}}}).$

Fresnel showed that inclusion of this drag factor into the theory gives contributions to Arago's results that start with (v/c)², too small to be observable.

In 1851 Armand Hippolyte Louis Fizeau was able to confirm Fresnel's drag factor experimentally by guiding light through flowing water.

For a while wave theory had difficulties in explaining birefringence (double refraction) and the associated phenomenon of polarization of light. Around 1820 Fresnel was able to account for these effects by assuming that light in a crystal is a transverse wave (a vibration perpendicular to the propagation direction). In analogy he inferred that light propagating through the ether consists also of transverse waves. This was confirmed by Maxwell's theory of 1861.

Ether in the second half of the 19th century

Michael Faraday, one of the fathers of electromagnetism, had a strong dislike of hypothetical entities for which no convincing experimental evidence existed. As a consequence, he was skeptical about the existence of the ether. But at the same time he was opposed to all action-at-a-distance theories, which he tried to replace by field theories. James Clerk Maxwell, who tread in Faraday's footsteps and accepted the physical reality of fields, formulated a mathematical theory of electromagnetic waves that he inferred to propagate through the luminiferous ether. The difference between Faraday's conception of a field without an ether and Maxwell's conception of a field with an underlying ether is subtle and not easy to understand for a modern physicist. The ether, in the view of Maxwell and almost all physicists at that time, filled all space and had many of the characteristics of a polarizable dielectric.

Maxwell was of the opinion that all terrestrial optical experiments aimed at determining Fresnel's ether drag, which give effects of order (v/c)², were not sensitive enough to detect the influence of the drag. He, thought, however, that observations of astronomical events, such as the eclipses of Jupiter's satellites, are of order v/c and could conceivably be seen. Albert Abraham Michelson disagreed with this and considered it possible to build an interferometer that was sensitive enough to detect effects of the order (v/c)². He designed an instrument while he was on leave in Berlin (1881) and tried to determine the speed v of the earth with respect to the ether dragged by the earth. The method he used was to compare the times it takes for light to travel the same distance either parallel or transversely to the earth's orbital motion relative to the ether, that is, he tried to measure the speed of "ether wind". His conclusion was that the speed of ether wind is zero. Lorentz found an error in Michelson's theory of the experiment and was dubious about the interpretation of the result. Lord Rayleigh (John William Strutt), who was a strict believer in ether, urged Michelson to repeat the experiment. So, Michelson, who in the mean time was appointed at Case School of Applied Science in Cleveland, Ohio, repeated the experiment in collaboration with Edwin Williams Morley, a chemist from Western Reserve University, also in Cleveland. They built an new interferometer and in August 1887 they measured again a null effect.

Maxwell's theories were difficult and did not find much response until Heinrich Rudolf Hertz proved experimentally between 1885 and 1888 the existence of the electromagnetic waves as predicted by Maxwell. (Hertz produced waves of wavelengths from a centimeter to a meter, much longer than the wavelengths of visible light, which are on the order of 500 nm). As a 19th century physicist, who had studied under Hermann von Helmholtz, Hertz stayed committed to the ether throughout his professional life. His conviction of the ether’s importance developed throughout his career, intensified during his research on electromagnetic waves, and finally became his chief preoccupation during the final years of his life. After Hertz's empirical confirmation of Maxwell’s electromagnetic waves, it was universally assumed that the existence of ether was established. The experiments of Hertz had an overwhelming impact on physics. They occurred in the formative years of a generation of physicists who dedicated themselves to electromagnetism and who raised the ether to the status of one of the basic building blocks of nature. In a letter of Oliver Heaviside, another of the founding father of electromagnetic theory, to Hertz on August 14, 1889, we find this passage:

Then there is the vexed question of the motion of the ether. Does it move when "bodies" move through it, or does it remain at rest? We know that there is an ether: the question is therefore a legitimate physical question that must be answered. I am in hopes that extensions of your researches will supply material for an answer. As for the structure of the ether itself, that is a far more difficult matter, and one that can never, it seems to me, be answered otherwise than speculatively.

(To be continued)