Editing Luminiferous aether (section)

==History of light and aether==
{{See also|Timeline of luminiferous aether}}

===Particles vs. waves===
{{main|Wave–particle duality}}
In the 17th century, [[Robert Boyle]] was a proponent of an aether hypothesis. According to Boyle, the aether consists of subtle particles, one sort of which explains the absence of vacuum and the mechanical interactions between bodies, and the other sort of which explains phenomena such as magnetism (and possibly gravity) that are, otherwise, inexplicable on the basis of purely mechanical interactions of macroscopic bodies, "though in the ether of the ancients there was nothing taken notice of but a diffused and very subtle substance; yet we are at present content to allow that there is always in the air a swarm of streams moving in a determinate course between the north pole and the south".<ref>Robert Boyle, ''The Works of the Honourable Robert Boyle'', ed. Thomas Birch, 2nd edn., 6 vols. (London, 1772), III, 316; quoted in E. A. Burtt, ''The Metaphysical Foundations of Modern Science'' (Garden City, New York: Doubleday & Company, 1954), 191–192.</ref>

[[Christiaan Huygens]]'s ''[[Treatise on Light]]'' (1690) hypothesized that light is a wave propagating through an aether. He and [[Isaac Newton]] could only envision light waves as being [[longitudinal wave|longitudinal]], propagating like sound and other [[mechanical wave]]s in [[fluid]]s. However, longitudinal waves necessarily have only one form for a given propagation direction, rather than two [[polarization (waves)|polarization]]s like a [[transverse wave]]. Thus, longitudinal waves can not explain [[birefringence]], in which two polarizations of light are refracted differently by a crystal. In addition, Newton rejected light as waves in a medium because such a medium would have to extend everywhere in space, and would thereby "disturb and retard the Motions of those great Bodies" (the planets and comets) and thus "as it {{bracket|light's medium}} is of no use, and hinders the Operation of Nature, and makes her languish, so there is no evidence for its Existence, and therefore it ought to be rejected".<ref>{{cite book |title=The Metaphysical Foundations of Modern Science |author1=Edwin Arthur Burtt |edition=illustrated, unabridged, reprinted |publisher=Courier Corporation |year=2003 |isbn=978-0-486-42551-1 |page=270 |url=https://books.google.com/books?id=G9WBMa1Rz_kC}} [https://books.google.com/books?id=G9WBMa1Rz_kC&pg=PA270 Extract of page 270]</ref>

Isaac Newton contended that light is made up of numerous small particles. This can explain such features as light's ability to travel in straight lines and [[Reflection (physics)|reflect]] off surfaces. Newton imagined light particles as non-spherical "corpuscles", with different "sides" that give rise to birefringence. But the particle theory of light can not satisfactorily explain [[refraction]] and [[diffraction]].<ref>{{cite book |title=Experimental Mechanics of Solids |author1=Cesar A. Sciammarella |author2=Federico M. Sciammarella |edition= |publisher=John Wiley & Sons |year=2012 |isbn=978-1-119-97009-5 |page=146 |url=https://books.google.com/books?id=aVP6tpNXAf4C}} [https://books.google.com/books?id=aVP6tpNXAf4C&pg=PT146 Extract of page 146]</ref> To explain refraction, Newton's Third Book of ''[[Opticks]]'' (1st ed. 1704, 4th ed. 1730) postulated an "aethereal medium" transmitting vibrations faster than light, by which light, when overtaken, is put into "Fits of easy Reflexion and easy Transmission", which caused refraction and diffraction. Newton believed that these vibrations were related to heat radiation:

<blockquote>Is not the Heat of the warm Room convey'd through the vacuum by the Vibrations of a much subtiler Medium than Air, which after the Air was drawn out remained in the Vacuum?  And is not this Medium the same with that Medium by which Light is refracted and reflected, and by whose Vibrations Light communicates Heat to Bodies, and is put into Fits of easy Reflexion and easy Transmission?<ref group=A name=newton />{{rp|349}}</blockquote>

In contrast to the modern understanding that heat radiation and light are both [[electromagnetic radiation]], Newton viewed heat and light as two different phenomena. He believed heat vibrations to be excited "when a Ray of Light falls upon the Surface of any pellucid Body".<ref group=A name=newton />{{rp|348}} He wrote, "I do not know what this Aether is", but that if it consists of particles then they must be <blockquote>exceedingly smaller than those of Air, or even than those of Light: The exceeding smallness of its Particles may contribute to the greatness of the force by which those Particles may recede from one another, and thereby make that Medium exceedingly more rare and elastic than Air, and by consequence exceedingly less able to resist the motions of Projectiles, and exceedingly more able to press upon gross Bodies, by endeavoring to expand itself.<ref group=A name=newton />{{rp|352}}</blockquote>

===Bradley suggests particles===
In 1720, [[James Bradley]] carried out a series of experiments attempting to measure [[stellar parallax]] by taking measurements of stars at different times of the year. As the Earth moves around the Sun, the apparent angle to a given distant spot changes. By measuring those angles the distance to the star can be calculated based on the known orbital circumference of the Earth around the Sun. He failed to detect any parallax, thereby placing a lower limit on the distance to stars.

During these experiments, Bradley also discovered a related effect; the apparent positions of the stars did change over the year, but not as expected. Instead of the apparent angle being maximized when the Earth was at either end of its orbit with respect to the star, the angle was maximized when the Earth was at its fastest sideways velocity with respect to the star. This effect is now known as [[stellar aberration]].

Bradley explained this effect in the context of Newton's corpuscular theory of light, by showing that the aberration angle was given by simple vector addition of the Earth's orbital velocity and the velocity of the corpuscles of light, just as vertically falling raindrops strike a moving object at an angle. Knowing the Earth's velocity and the aberration angle enabled him to estimate the speed of light.

Explaining stellar aberration in the context of an aether-based theory of light was regarded as more problematic. As the aberration relied on relative velocities, and the measured velocity was dependent on the motion of the Earth, the aether had to be remaining stationary with respect to the star as the Earth moved through it. This meant that the Earth could travel through the aether, a physical medium, with no apparent effect – precisely the problem that led Newton to reject a wave model in the first place.

===Wave-theory triumphs===
A century later, [[Thomas Young (scientist)|Thomas Young]]{{efn|Young ascribed aether to [[caloric theory]], pairing light and heat, and cited passages from Newton such as: "A luminiferous ether pervades the Universe, rare and elastic in a high degree," and:<blockquote>Is not the heat conveyed through the vacuum by the vibration of a much subtiler medium than air? And is not this medium the same with that medium by which light is refracted and reflected, and by whose vibration light communicates heat to bodies, and is put into fits of easy reflection, and easy transmission?<ref>{{cite book |last=Gillispie |first=Charles Coulston |author-link=Charles Coulston Gillispie |title=The Edge of Objectivity: An Essay in the History of Scientific Ideas |year=1960 |publisher=Princeton University Press |isbn=0-691-02350-6 |page=[https://archive.org/details/edgeofobjectivit00char/page/408 408] |url=https://archive.org/details/edgeofobjectivit00char/page/408 }}</ref></blockquote>}} and [[Augustin-Jean Fresnel]] revived the wave theory of light when they pointed out that light could be a transverse wave rather than a longitudinal wave; the polarization of a transverse wave (like Newton's "sides" of light) could explain birefringence, and in the wake of a series of experiments on diffraction the particle model of Newton was finally abandoned. [[Physicist]]s assumed, moreover, that, like mechanical waves, light waves required a medium for [[Wave propagation|propagation]], and thus required Huygens's idea of an aether "gas" permeating all space.

However, a transverse wave apparently required the propagating medium to behave as a solid, as opposed to a fluid. The idea of a solid that did not interact with other matter seemed a bit odd, and [[Augustin-Louis Cauchy]] suggested that perhaps there was some sort of "dragging", or "entrainment", but this made the aberration measurements difficult to understand. He also suggested that the ''absence'' of longitudinal waves suggested that the aether had negative compressibility. [[George Green (mathematician)|George Green]] pointed out that such a fluid would be unstable. [[George Gabriel Stokes]] became a champion of the entrainment interpretation, developing a model in which the aether might, like pine pitch, be [[dilatant]] (fluid at slow speeds and rigid at fast speeds). Thus the Earth could move through it fairly freely, but it would be rigid enough to support light.

===Electromagnetism===
In 1856, [[Wilhelm Eduard Weber]] and [[Rudolf Kohlrausch]] measured the numerical value of the ratio of the electrostatic unit of charge to the electromagnetic unit of charge. They found that the ratio between the [[electrostatic unit of charge]] and the [[Abcoulomb|electromagnetic unit of charge]] is the speed of light ''c''.<ref>{{Cite book |last=Schwartz |first=Melvin |title=Principles of Electrodynamics |publisher=Dover Publications, Inc. |year=1987 |isbn=978-0-486-65493-5 |edition=Revised |pages=106–107}}</ref> The following year, [[Gustav Kirchhoff]] wrote a paper in which he showed that the speed of a signal along an electric wire was equal to the speed of light. These are the first recorded historical links between the speed of light and electromagnetic phenomena.

[[James Clerk Maxwell]] began working on [[Michael Faraday]]'s [[lines of force]]. In his 1861 paper ''[[:Image:On Physical Lines of Force.pdf|On Physical Lines of Force]]'' he modelled these magnetic lines of force using a sea of molecular vortices that he considered to be partly made of aether and partly made of ordinary matter. He derived expressions for the dielectric constant and the magnetic permeability in terms of the transverse elasticity and the density of this elastic medium. He then equated the ratio of the dielectric constant to the magnetic permeability with a suitably adapted version of Weber and Kohlrausch's result of 1856, and he substituted this result into Newton's equation for the speed of sound. On obtaining a value that was close to the speed of light as measured by [[Hippolyte Fizeau]], Maxwell concluded that light consists in undulations of the same medium that is the cause of electric and magnetic phenomena.<ref group=B name=whitt /><ref group=B name=janb /><ref group=B name=darrigol /><ref group=B name=schaffner />

Maxwell had, however, expressed some uncertainties surrounding the precise nature of his molecular vortices and so he began to embark on a purely dynamical approach to the problem. He wrote another paper in 1864, entitled "[[A Dynamical Theory of the Electromagnetic Field]]", in which the details of the luminiferous medium were less explicit.<ref group=A name=maxb /> Although Maxwell did not explicitly mention the sea of molecular vortices, his derivation of [[Ampère's circuital law]] was carried over from the 1861 paper and he used a dynamical approach involving rotational motion within the electromagnetic field which he likened to the action of flywheels. Using this approach to justify the electromotive force equation (the precursor of the [[Lorentz force]] equation), he derived a wave equation from a set of eight equations which appeared in the paper and which included the electromotive force equation and [[Ampère's circuital law]].<ref group=A name=maxb /> Maxwell once again used the experimental results of Weber and Kohlrausch to show that this wave equation represented an electromagnetic wave that propagates at the speed of light, hence supporting the view that light is a form of electromagnetic radiation.

In 1887–1889, [[Heinrich Rudolf Hertz|Heinrich Hertz]] experimentally demonstrated the electric magnetic waves are identical to light waves. This unification of electromagnetic wave and optics indicated that there was a single luminiferous aether instead of many different kinds of aether media.<ref>{{Cite journal |url=https://en.wikisource.org/wiki/Popular_Science_Monthly/Volume_66/November_1904/The_Fundamental_Concepts_of_Physical_Science |first=Edward L. |last=Nichols |title=The Fundamental Concepts of Physical Science |journal=Popular Science Monthly |volume=66 |date=November 1904}}</ref>

The apparent need for a propagation medium for such [[Heinrich Rudolf Hertz|Hertzian waves]] (later called [[Radio wave|radio waves]]) can be seen by the fact that they consist of orthogonal electric (E) and magnetic (B or H) waves. The E waves consist of undulating dipolar electric fields, and all such dipoles appeared to require separated and opposite electric charges. Electric charge is an inextricable property of [[matter]], so it appeared that some form of matter was required to provide the alternating current that would seem to have to exist at any point along the propagation path of the wave. Propagation of waves in a true vacuum would imply the existence of [[electric field]]s without associated [[electric charge]], or of electric charge without associated matter. Albeit compatible with Maxwell's equations, [[electromagnetic induction]] of electric fields could not be demonstrated in vacuum, because all methods of detecting electric fields required electrically charged matter.

In addition, Maxwell's equations required that all electromagnetic waves in [[vacuum]] propagate at a fixed speed, ''[[speed of light|c]]''. As this can only occur in one [[frame of reference|reference frame]] in Newtonian physics (see [[Galilean invariance|Galilean relativity]]), the aether was hypothesized as the absolute and unique frame of reference in which Maxwell's equations hold. That is, the aether must be "still" universally, otherwise ''c'' would vary along with any variations that might occur in its supportive medium. Maxwell himself proposed several mechanical models of aether based on wheels and gears, and [[George Francis FitzGerald]] even constructed a working model of one of them. These models had to agree with the fact that the electromagnetic waves are transverse but never longitudinal.

===Problems===
By this point the mechanical qualities of the aether had become more and more magical: it had to be a [[fluid]] in order to fill space, but one that was millions of times more rigid than steel in order to support the high frequencies of light waves. It also had to be massless and without [[viscosity]], otherwise it would visibly affect the orbits of planets. Additionally it appeared it had to be completely transparent, non-dispersive, [[incompressible fluid|incompressible]], and continuous at a very small scale.<ref>{{Cite book |last=Yousef |first=Mohamed Haj |url=https://books.google.com/books?id=9_WEDwAAQBAJ&dq=Luminiferous+aether+Additionally+it+appeared+it+had+to+be+completely+transparent%2C+non-dispersive%2C+incompressible%2C+and+continuous+at+a+very+small+scale&pg=PA73 |title=Duality of Time: Complex-Time Geometry and Perpetual Creation of Space |date=2018-01-01 |publisher=Mohamed Haj Yousef |isbn=978-1-5395-7920-5 |language=en}}</ref> Maxwell wrote in ''Encyclopædia Britannica'':<ref group=A name=maxa />

<blockquote>Aethers were invented for the planets to swim in, to constitute electric atmospheres and magnetic effluvia, to convey sensations from one part of our bodies to another, and so on, until all space had been filled three or four times over with aethers. ... The only aether which has survived is that which was invented by Huygens to explain the propagation of light.</blockquote>

By the early 20th century, aether theory was in trouble. A series of [[#Negative aether-drift experiments|increasingly complex experiments]] had been carried out in the late 19th century to try to detect the motion of the Earth through the aether, and had failed to do so. A range of proposed aether-dragging theories could explain the null result but these were more complex, and tended to use arbitrary-looking coefficients and physical assumptions. Lorentz and FitzGerald offered within the framework of [[Lorentz ether theory]] a more elegant solution to how the motion of an absolute aether could be undetectable (length contraction), but if their equations were correct, the new [[special theory of relativity]] (1905) could generate the same mathematics without referring to an aether at all. Aether fell to [[Occam's Razor]].<ref group=B name=whitt /><ref group=B name=janb /><ref group=B name=darrigol /><ref group=B name=schaffner />