In the 19th century most physicists accepted the old idea that all of space is filled with an elusive substance called the “luminiferous ether”. It was well known that. You were a sensical theory, luminiferous ether. And physicists love when things make sense. But science need not make sense—the universe. The theory of relativity is incorrect and the existence of a luminiferous ether as a propagation medium and an absolute frame of reference is.
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Luminiferous aether or ether   “luminiferous”, meaning “light-bearing”was the postulated medium for the propagation of light.
The assumption of a spatial plenum of luminiferous aether, rather than a spatial vacuum, provided the theoretical medium that was required by wave theories of light.
The aether hypothesis was the topic of considerable debate throughout its history, as it required the existence of an invisible and infinite material with no interaction with physical objects.
As the nature of light was explored, especially in the 19th century, the physical qualities required of an aether became increasingly contradictory. By the late s, etner existence of the aether was being questioned, although there was no physical theory to replace it. The negative outcome of the Michelson—Morley experiment suggested that the aether did not exist, a finding that was confirmed in subsequent experiments through the s.
Light and the luminiferous ether (video) | Khan Academy
This led to considerable theoretical work to explain the propagation of light without an aether. A major breakthrough was the theory of relativitywhich could explain why the experiment failed to see aether, but was more broadly interpreted to suggest that it was not needed. The Michelson-Morley experiment, along with the blackbody radiator and photoelectric effectwas a key experiment in the development of modern physicswhich includes both relativity and quantum theorythe latter of which explains the wave-like nature of light.
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 steams moving in a determinate course between the north pole and the south”.
Christiaan Huygens hypothesized that light is a wave propagating through an aether. He and Isaac Newton could only envision light waves as being longitudinalpropagating like sound and other mechanical waves in fluids.
However, longitudinal waves necessarily have only one form for a given propagation direction, rather than two polarizations like transverse wave.
Thus, longitudinal waves can not explain birefringencein 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 [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”.
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 reflect off surfaces. Newton imagined that 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.
Newton believed that these vibrations were related to heat radiation:. 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?
In luminuferous to the modern understanding that heat radiation and light are both electromagnetic radiationNewton 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”.
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 ouminiferous motions of Projectiles, and exceedingly more able to press upon gross Bodies, by endeavoring to expand itself. InJames Bradley luminifferous out a series of experiments attempting to measure stellar parallax by taking measurements of stars at different luminlferous of the year.
luminiferpus 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 luminiferuos orbital circumference of the Earth around the sun. He failed lumiiferous 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 luniniferous 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, this enabled him to estimate the speed of luminifreous. 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 sther 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. A century later, Thomas Young 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 luminiferoous explain birefringence, and in the wake of a series of experiments on diffraction the particle model of Newton was finally abandoned.
Physicists assumed, moreover, that like mechanical waves, light waves required a medium for propagationand 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 gas or fluid. The idea of luminifrrous 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 luminferous the absence of ethet waves suggested that the aether had negative compressibility. 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 luminiferois aether might be by analogy with pine pitch rigid at very high frequencies and fluid at lower speeds. Thus the Earth could move through it fairly freely, but it would be rigid enough to support light.
InWilhelm Eduard Weber and Rudolf Kohlrausch measured the numerical value of the ratio of the electromagnetic unit of charge to the electrostatic unit of charge. They found that the ratio equals the product of the speed of light and the square luminifeorus of two. 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.
Luminiferous aether – Wikipedia
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 paper Ulminiferous 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 luminiferoua 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 luminiferouz of Weber and Kohlrausch’s result ofand 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 FizeauMaxwell concluded that light consists in undulations of the same medium that is the luminifrrous of electric and magnetic phenomena.
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 inentitled ” A Dynamical Theory of the Electromagnetic Field “, in which the details of the luminiferous medium were less explicit. The apparent need for a propagation medium for such Hertzian 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 matterso 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 fields without associated electric chargeor 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 luminiiferous waves in vacuum propagate at a fixed speed, c. As this can only occur in one reference frame in Newtonian physics see Galilean-Newtonian relativitythe 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 etuer 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. By this point the mechanical qualities of the aether had become more and more magical: It also had to be luminiferoud and without viscosityotherwise it would visibly affect the orbits of planets. Additionally it appeared it had to be completely transparent, non-dispersive, incompressibleand continuous at a very small scale.
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. Contemporary scientists were aware of the problems, but aether ethher was so entrenched in physical law by luminlferous point etyer it was simply assumed to exist.
In Oliver Lodge gave a speech on behalf of Lord Rayleigh  to the Royal Institution on this topic, in which he outlined its physical properties, and then attempted to offer reasons why they were not impossible. Nevertheless, he was also aware of the criticisms, and quoted Lumimiferous Salisbury as saying that “aether is little more than a nominative case of the verb to undulate “.
Others criticized it as an “English invention”, although Luminifeous jokingly stated it was actually an invention of the Royal Institution. By the early 20th century, aether theory was in trouble. A series of increasingly complex experiments had been carried out in the late 19th century to etyer to detect the luminifegous 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 contractionbut if their equations were correct, the new special theory of relativity could generate the same mathematics without referring to an aether at all.
Aether fell to Occam’s Razor. The two most important models, which were aimed to describe the relative motion of the Earth and aether, were Augustin-Jean Fresnel ‘s model of the nearly stationary aether including a partial aether drag determined by Fresnel’s dragging coefficient, [A 4] and George Gabriel Stokes ‘ [A 5] model of complete aether drag.
The latter theory was not considered as correct, since it was not compatible with the aberration of lightand the auxiliary hypotheses developed to explain this problem were not convincing. Also, subsequent experiments as the Sagnac effect also showed that this model is untenable. That is, movement adds only a fraction fther the medium’s velocity to the light predicted by Fresnel in order to make Snell’s law work in all frames of reference, consistent with stellar ehter. This was initially interpreted to mean that the medium drags the aether along, with a portion of the medium’s velocity, but that understanding became very luminiferohs after Wilhelm Veltmann demonstrated that the index n in Fresnel’s formula depended upon the wavelength of light, so that the aether could not be moving at a wavelength-independent speed.
This implied that there must be a separate aether for each of the infinitely many frequencies.
The key difficulty with Fresnel’s aether hypothesis arose from the juxtaposition of the two well-established theories of Newtonian dynamics and Maxwell’s electromagnetism. Under a Galilean transformation the equations of Newtonian dynamics are invariantwhereas those of electromagnetism are not. Basically this means that while physics should remain the same in non-accelerated experiments, light would not follow the same rules because it is travelling in the universal “aether frame”.
Some effect caused by this difference should be detectable. A simple example concerns the model on which aether was originally built: The speed of propagation for mechanical waves, the speed of soundis defined by the mechanical properties of the medium. This explains why a person hearing an explosion underwater and quickly surfacing can hear it again as the slower travelling sound arrives through the air.
Similarly, a traveller on an airliner can still carry on a conversation with another traveller because the sound of words is travelling along with the air inside the aircraft. This effect is basic to all Newtonian dynamics, which says that everything from sound to the trajectory of a thrown baseball should all remain the same in the aircraft flying at least at a constant speed as if still sitting on the ground.
This is the basis of the Galilean transformation, and the concept of frame of reference. But the same was not supposed to be true for light, since Maxwell’s mathematics demanded a single universal speed for the propagation of light, based, not on local conditions, but on two measured properties, the permittivity and permeability of free space, that were assumed to be the same throughout the universe.
If these numbers did change, there should be noticeable effects in the sky; stars in different directions would have different colours, for instance.