Ether (physics)

The light ether: the hypothesis was developed, today obsolete, that the whole space is occupied by an invisible and intangible “medium”, the ether, in which the waves of light are propagated and in which the celestial bodies are moved.

The luminiferous ether or ether («luminiferous» means 'bearer of light'), was the medium postulated for the propagation of the light. It was used to explain the apparently wave-based ability of light to propagate through empty space, something waves should not be able to do. The assumption of a luminiferous ether space plane, rather than a space vacuum, provided the theoretical medium required by wave theories of light.

The ether hypothesis was the subject of considerable debate throughout its history, as it required the existence of an invisible, infinite material with no interaction with physical objects. As the nature of light was explored, especially in the 19th century, the required physical qualities of an ether became increasingly contradictory. At the end of the 19th century, the existence of the ether was questioned, although there was no physical theory to replace it.

The negative result of the Michelson-Morley experiment (1887) suggested that the ether did not exist, a finding that was confirmed in later experiments up to the 1920s. This led to considerable theoretical work to explain the propagation of light without an ether One important advance was the theory of relativity, which could explain why the experiment failed to see the ether, but was interpreted more broadly to suggest that it was not necessary. The Michelson-Morley experiment, along with the blackbody radiator and the photoelectric effect, was a key experiment in the development of modern physics, which includes both relativity and quantum theory, the latter of which explains the wave nature of of the light.

The story of light and ether

Particles vs. Waves

In the 17th century, Robert Boyle was a supporter of an ether hypothesis. According to Boyle, the ether consists of subtle particles, one of which explains the absence of a vacuum and the mechanical interactions between bodies, and the other explains phenomena such as magnetism (and possibly gravity) that are otherwise inexplicable on Earth. the basis of the purely mechanical interactions of macroscopic bodies, “although in the ether of the ancients nothing more than a diffuse and very subtle substance was noticed; The vapors move in a certain course between the north and south poles."

Christiaan Huygens hypothesized that light is a wave propagating through an ether. He and Isaac Newton could only imagine light waves as longitudinal, propagating like sound and other mechanical waves in fluids. However, longitudinal waves necessarily have only one shape for a given direction of propagation, instead of two polarizations like the transverse wave. Therefore, longitudinal waves cannot explain birefringence, in which two polarizations of light are refracted differently by a crystal. Furthermore, Newton rejected light as waves in a medium because such a medium would have to extend everywhere in space, and therefore "perturb and retard the motions of those great Bodies" (the planets and comets) and therefore " since it is useless and hinders the functioning of Nature, and makes it languish, so there is no evidence of its existence, and therefore it must be rejected”.^{[citation required]}

Isaac Newton argued that light is made up of many small particles. This may explain features such as the ability of light to travel in straight lines and reflect off surfaces. Newton imagined light particles to be non-spherical "corpuscles", with different "sides" giving rise to birefringence. But the particle theory of light cannot satisfactorily explain refraction and diffraction. To explain refraction, Newton in Opticks (1704) postulated an "ethereal medium" transmitting vibrations faster than light, whereby light, when exceeded, is put into "Fits of easy Reflection." and easy Transmission”, which caused refraction and diffraction. Newton believed that these vibrations were related to heat radiation:

Is not the heat of the warm room transmitted through the vacuum by the vibrations of a medium much more subtle than the air, that after the air was extracted remained in the void? And is not this Media the same with that Media by which the Light refracts and reflects, and by what Vibrations the Light communicates the Calor to the Corps, and puts itself in Easy Reflection Adjustment and Easy Transmission?

In contrast to the modern understanding that heat radiation and light are electromagnetic radiation, Newton viewed heat and light as two different phenomena. He believed that heat vibrations were excited "when a Ray of Light falls on the Surface of any pellucid body". He wrote:

I do not know what this ether is, but if it is composed of particles, then they must be extremely smaller than those of the air, or even those of the light: the excessive smallness of their particles can contribute to the greatness of the force by which these particles can retreat from one another, and therefore make that medium much more rare and elastic than the air and therefore much less able to resist the movements of the projectiles,

Bradley suggests particles

In 1720, James Bradley carried out a series of experiments that attempted 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 of a given distant point changes. By measuring those angles, the distance to the star can be calculated based on the Earth's known orbital circumference around the Sun. He didn't detect any parallax, so he placed a lower bound on the distance to the stars.

During these experiments, Bradley also discovered a related effect; The apparent positions of the stars changed throughout the year, but not as expected. Instead of maximizing the apparent angle when Earth was at one end of its orbit relative to the star, the angle was maximized when Earth was at its fastest lateral velocity relative 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 angle of aberration was given by the simple vector sum of the orbital velocity of the Earth and the velocity of the corpuscles of light., much like vertically falling raindrops collide with a moving object at an angle. Knowing the speed of the Earth and the angle of aberration, this allowed him to estimate the speed of light.

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

The theory of waves triumphs

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 (as "sides" of Newton's light) could explain birefringence, and after a series of experiments on diffraction, Newton's particle model was finally abandoned. Furthermore, physicists assumed that, like mechanical waves, light waves required a medium for propagation and therefore required Huygens's idea of an ether "gas" pervading all space.

However, a transverse wave apparently requires the propagation medium to behave like a solid, as opposed to a gas or fluid. The idea of a solid not interacting with other matter seemed a bit strange, and Augustin-Louis Cauchy suggested that perhaps there was some kind of "drag", but this made aberration measurements difficult to understand. He also suggested that the absence of longitudinal waves suggested that the ether had negative compressibility. George Green pointed out that such a fluid would be unstable. George Gabriel Stokes became a proponent of the drag interpretation, developing a model in which the ether could be rigid at very high frequencies and fluid at lower velocities (by analogy with pine resin). Thus, the Earth could move through it quite freely, but it would be rigid enough to support light.^{[citation needed]}

Electromagnetism

In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch measured the numerical value of the ratio of unit electromagnetic charge to unit electrostatic charge. They found that the relationship is equal to the product of the speed of light and the square root of two. The following year, Gustav Kirchhoff wrote a paper showing that the speed of a signal along an electrical wire was equal to the speed of light. These are the first recorded historical links between the speed of light and electromagnetic phenomena.^{[citation needed]}

James Clerk Maxwell began working on Michael Faraday's lines of force. In his 1861 paper On Physical Lines of Force, he modeled these magnetic lines of force using a sea of molecular vortices that he considered to be made partly ether and partly ordinary matter. He derived expressions for the dielectric constant and magnetic permeability in terms of the transverse elasticity and density of this elastic medium. He then compared the relationship of the dielectric constant to magnetic permeability with a suitably adapted version of Weber and Kohlrausch's 1856 result, and substituted this result in Newton's equation for the speed of sound. Obtaining a value that was close to the measurements of the speed of light made by Hippolyte Fizeau and by Léon Foucault, Maxwell concluded that light consists of undulations in the same medium that is the cause of electrical and magnetic phenomena.

However, Maxwell had expressed some uncertainties about the precise nature of his molecular vortices, so he began to embark on a purely dynamical approach to the problem. He wrote another paper in 1864, entitled A Dynamic Theory of the Electromagnetic Field, in which the details of the luminiferous medium were less explicit. Although Maxwell did not explicitly mention the sea of molecular vortices, his derivation of Ampère's law of circulation was carried over from the 1861 paper and 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 forerunner of the Lorentz force equation), he derived a wave equation from a set of eight equations that appeared in the paper, including the electromotive force equation and the law of Ampère circulation. Maxwell once again used the experimental results of Weber and Kohlrausch to show that this wave equation represents an electromagnetic wave traveling at the speed of light, thus supporting the view that light It is a form of electromagnetic radiation.

The apparent need for a propagation medium for such hertzian waves can be seen from the fact that they consist of orthogonal electric (E) and magnetic (B o H). E-waves consist of wavy dipole electric fields, and all dipoles seemed to require separate and opposite electric charges. Electric charge is an inextricable property of matter, so it seemed that some kind of matter was required to provide the alternating current that seems to exist at any point along the path of wave propagation. The propagation of waves in a true vacuum would imply the existence of electric fields without associated electric charge, or electric charge without associated matter. While compatible with Maxwell's equations, the electromagnetic induction of electric fields could not be demonstrated in a vacuum, since all electric field detection methods require electrically charged material.

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

Problems

At this point, the mechanical qualities of the ether had become more and more magical: it had to be a fluid to fill space, but it was millions of times stiffer than steel to withstand the high frequencies of the waves. light. It also had to be massless and viscosityless, otherwise it would visibly affect the orbits of the planets. Also, it seemed like it had to be completely transparent, non-dispersive, incompressible, and continuous at a very small scale. Maxwell wrote in the Encyclopedia Britannica:

The ethers were invented so that the planets would swim, to constitute electrical atmospheres and magnetic effluences, to transmit sensations from one part of our body to another, and so on, until all the space had been filled three or four times with ethers.... The only ether that has survived is the invented by Huygens to explain the spread of light.

Contemporary scientists were aware of the problems, but at this point the ether theory was so entrenched in physical law that it was simply assumed to exist. In 1908 Oliver Lodge made a speech on behalf of Lord Rayleigh To the Royal Institution on this subject, in which he described its physical properties and then attempted to offer reasons why they were not impossible. However, he too was aware of the criticism and quoted Lord Salisbury as saying that "ether is little more than a nominative case of the verb to undulate". Others criticized it as an "English invention", though Rayleigh quipped that it was actually a Royal Institution invention.

At the beginning of the 20th century, the ether theory was in trouble. A series of increasingly complex experiments were carried out at the end of the 19th century to try to detect the movement of the Earth at through the ether, and they didn't. A number of proposed aether drag theories could explain the null result, but they were more complex and tended to use arbitrary-appearing coefficients and physical assumptions. Lorentz and FitzGerald offered in the framework of Lorentz's ether theory a more elegant solution to how the motion of an absolute ether 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 ether at all. The ether fell to Ockham's razor.

Relative motion between earth and ether

Aether Drag

The two most important models, whose goal was to describe the relative motion of the Earth and the ether, were Augustin-Jean Fresnel's (1818) model of the (almost) stationary ether that includes a partial resistance to the ether determined by the Fresnel drag coefficient. and George Gabriel Stokes' (1844) ether full drag model. The latter theory was not considered correct, since it was not compatible with light aberration, and the auxiliary hypotheses developed to explain this problem were not convincing. Furthermore, later experiments such as the Sagnac effect (1913) also showed that this model is untenable. However, the most important experiment supporting Fresnel's theory was Fizeau's 1851 experimental confirmation of Fresnel's 1818 prediction that a medium of refractive index n moving with velocity v would increase the speed of light traveling through the medium in the same direction as v from c/n to:

cn+(1− − 1n2)v.{displaystyle {frac {c}{n}}}+left(1-{frac {1}{n^{2}}}right)v.}

That is, the motion adds only a fraction of the speed of the medium to light (predicted by Fresnel to make Snell's law work in all reference frames, consistent with stellar aberration). This was initially interpreted as the medium dragging the ether with a fraction of the velocity of the medium, but that understanding became highly problematic after Wilhelm Veltmann showed that the rate n in Fresnel's formula depended of the wavelength of light, so the ether could not move at a velocity independent of the wavelength. This implied that there must be a separate ether for each of the infinite frequencies.

Negative ether drift experiments

The key difficulty with Fresnel's ether 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 invariant, while those of electromagnetism are not. Basically, this means that while the physics should remain the same in unaccelerated experiments, light won't follow the same rules because it's traveling in the universal "ether frame." Some effect caused by this difference should be detectable.

A simple example concerns the model on which ether was originally built: sound. The speed of propagation of mechanical waves, the speed of sound, is defined by the mechanical properties of the medium. Sound travels 4.3 times faster in water than in air. This explains why a person hearing an explosion underwater and quickly resurfacing may hear it again as the slower sound comes through the air. Similarly, a traveler on an airliner can still hold a conversation with another traveler because the sound of words travels with the air inside the plane. This effect is basic to all Newtonian dynamics, which says that everything from sound to the trajectory of a thrown baseball must remain the same in the flying plane (at least at a constant speed) as if it were still sitting on the ground. soil. This is the basis of the Galilean transformation, and the concept of reference frame.

But the same was not supposed to be true for light, since Maxwell's mathematics demanded a universally unique velocity for the propagation of light, based not on local conditions, but on two measured properties, permittivity and the permeability of free space, which were supposed to be the same throughout the universe. If these numbers were to change, there should be noticeable effects in the sky; Stars in different directions would have different colors, for example.

Therefore, at any point there must be a special coordinate system, "at rest relative to the ether." Maxwell noted in the late 1870s that detecting motion relative to this ether should be easy enough: light traveling along with the Earth's motion would have a different speed than light traveling backwards, since both they would move against the motionless ether. Even if the ether had a general universal flux, changes in position during the day/night cycle, or over the course of the seasons, should allow detection of drift.

First-order experiments

Although the ether is almost stationary according to Fresnel, its theory predicts a positive result of the ether drift experiments only of second order in v/c{displaystyle v/c}, because Fresnel's drag coefficient would cause a negative result of all optical experiments capable of measuring first order effects in v/c{displaystyle v/c}. This was confirmed by the following first-order experiments, which gave negative results. The following list is based on the description of Wilhelm Wien (1898), with additional changes and experiments according to the descriptions of Edmund Taylor Whittaker (1910) and Jakob Laub (1910):

• The experiments of François Arago (1810), to confirm whether the refraction, and therefore the aberration of light, is influenced by the movement of the Earth. Similar experiments were performed by George Biddell Airy (1871) through a water-filled telescope, and Eleuthère Mascart (1872).
• The Fizeau experiment (1860), to find if the rotation of the polarization plane through glass columns is modified by the Earth movement. He got a positive result, but Lorentz was able to prove that the results have been contradictory. DeWitt Bristol Brace (1905) and Strasser (1907) repeated the experiment more accurately and obtained negative results.
• Martin Hoek's experiment (1868). This experiment is a more precise variation of the Fizeau experiment (1851). Two rays of light were sent in opposite directions: one of them crosses a path full of resting water, the other follows a path through the air. According to Fresnel's drag coefficient, it got a negative result.
• Wilhelm Klinkerfues's experiment (1870) investigated whether there is an influence of the Earth's movement on the sodium absorption line. He obtained a positive result, but it was proved that it was an experimental error, because a repetition of the Haga experiment (1901) gave a negative result.
• The experiment of Ketteler (1872), in which two rays of an interferometer were sent in opposite directions through two tubes tilted together full of water. There were no changes in the interference strips. Later, Mascart (1872) showed that the interference strips of the polarized light in the calcite were not affected either.
• The experiment of Eleuthère Mascart (1872) to find a change of rotation of the plane of polarization in the quartz. No change of rotation was found when light rays had the direction of the Earth movement and then the opposite direction. Lord Rayleigh performed similar experiments with greater precision and also obtained a negative result.

In addition to these optical experiments, first-order electrodynamic experiments were also performed, which should have given positive results according to Fresnel. However, Hendrik Antoon Lorentz (1895) modified Fresnel's theory and showed that these experiments can also be explained by a stationary ether:

• Wilhelm Röntgen's experiment (1888), to find out if a loaded capacitor produces magnetic forces due to the Earth's movement.
• The experiment of Theodor des Coudres (1889), to discover if the inductive effect of two cable rolls on a third is influenced by the direction of the Earth movement. Lorentz showed that this effect is first cancelled by the electrostatic charge (produced by the Earth's movement) on the drivers.
• The Königsberger experiment (1905). The plates of a capacitor are in the field of a strong electromagnet. Because of the Earth movement, the plates should have been loaded. No such effect was observed.
• The experiment of Frederick Thomas Trouton (1902). A condenser was brought parallel to the Earth movement, and an impulse was assumed when the capacitor was loaded. The negative result can be explained by Lorentz's theory, according to which the electromagnetic moment compensates for the moment due to the Earth's movement. Lorentz was also able to demonstrate that the sensitivity of the apparatus was too low to observe that effect.

Second Order Experiments

The Michelson-Morley experiment compared time for light to be reflected from mirrors in two orthogonal directions.

While first-order experiments could be explained by a modified stationary ether, more precise second-order experiments were expected to produce positive results, however no such results could be found.

The famous Michelson-Morley experiment compared the light source to itself after being sent in different directions, looking for changes in phase in a way that could be measured with extremely high precision. In this experiment, his goal was to determine the velocity of the Earth through the ether. The publication of his result in 1887, the null result, was the first clear demonstration that something was seriously wrong with the ether hypothesis (the first Michelson's experiment in 1881 was not entirely conclusive). In this case, the MM experiment produced a fringe pattern shift of approximately 0.01 of a fringe, corresponding to a small velocity. However, it was inconsistent with the expected aether wind effect due to Earth speed (which varies seasonally) which would have required a change of 0.4 of a fringe, and the error was small enough that the value has been zero. Therefore, the null hypothesis, the hypothesis that there was no ether wind, could not be rejected. Since then, more modern experiments have reduced the possible value to a number very close to zero, around 10−17.

It is obvious why it has been done before it would be useless to try to solve the problem of the movement of the solar system through observations of optical phenomena on the surface of the Earth.

A. Michelson and E. Morley. "On the Relative Motion of the Earth and the Luminiferous Æther". Phil. Mag. S. 5. Vol. 24. No. 151. Dec. 1887.

A series of experiments with similar but increasingly sophisticated apparatus also returned the null result. Conceptually, different experiments that also attempted to detect the motion of the ether were the Trouton-Noble experiment (1903), whose aim was to detect torsion effects caused by electrostatic fields, and the experiments by Rayleigh and Brace (1902, 1904), to detect double refraction in various media. However, they all returned a null result, as Michelson-Morley (MM) did earlier.

These "ether wind" led to a series of efforts to "save" the ether by assigning it more and more complex properties, while only a few scientists, such as Emil Cohn or Alfred Bucherer, considered the possibility of abandoning the ether hypothesis. Of particular interest was the possibility of "ether drag" or 'ether drag', which would reduce the magnitude of the measurement, perhaps enough to explain the results of the Michelson-Morley experiment. However, as noted above, ether drag already had problems of its own, notably aberration. In addition, the interference experiments of Lodge (1893, 1897) and Ludwig Zehnder (1895), aimed to show whether the ether is dragged by various rotating masses, it did not show aether drag. A more precise measurement was made in the experiment of Hammar (1935), who performed a complete MM experiment with one of the "legs" placed between two massive lead blocks. If the ether was dragged en masse, this experiment would have been able to detect the drag caused by the wire, but again the null result was achieved. The theory was modified again, this time to suggest that drag only worked for very large masses or those with large magnetic fields. This was also shown to be incorrect by the Michelson-Gale-Pearson experiment, which detected the Sagnac effect due to the Earth's rotation (see ether resistance hypothesis).

An entirely different attempt to save the "absolute" ether was made in the Lorentz-FitzGerald contraction hypothesis, which postulates that everything was affected by the journey through the ether. In this theory, the reason the Michelson-Morley experiment "failed" was that the apparatus contracted in the direction of travel. That is, the light was being "naturally" affected by its journey through the ether as predicted, but so was the apparatus itself, canceling out any differences when measured. FitzGerald had deduced this hypothesis from an article by Oliver Heaviside. Without reference to an ether, this physical interpretation of relativistic effects was shared by Kennedy and Thorndike in 1932, as they concluded that the arm of the interferometer contracts and that the frequency of its light source "almost" varies in the manner required by relativity.

Similarly, the Sagnac effect, observed by G. Sagnac in 1913, was immediately seen as fully compatible with special relativity. In fact, the Michelson-Gale-Pearson experiment in 1925 was specifically proposed as a test to confirm the theory of relativity, although it was also recognized that such tests, which simply measure absolute rotation, are also consistent with non-relativistic theories.

During the 1920s, the experiments started by Michelson were repeated by Dayton Miller, who publicly proclaimed positive results on several occasions, although they were not large enough to be compatible with any known ether theory. However, other researchers were unable to duplicate Miller's results. Over the years, the experimental precision of such measurements has increased by many orders of magnitude, and no traces of any violation of Lorentz invariance have been found. (A later reanalysis of Miller's results concluded that he had underestimated variations due to temperature.)

Since Miller's experiment and its unclear results, there have been many more experimental attempts to detect the ether. Many experimenters have claimed positive results. These results have not gained much attention from mainstream science, as they contradict a large number of high-precision measurements, all of which were consistent with special relativity.

Lorentz ether theory

Between 1892 and 1904, Hendrik Lorentz developed an electron-ether theory, in which he introduced a strict separation between matter (electrons) and ether. In his model, the ether is completely immobile, and will not set in motion in the vicinity of accumulative matter. Contrary to previous electron models, the ether's electromagnetic field appears as a mediator between electrons, and changes in this field cannot propagate faster than the speed of light. A fundamental concept of Lorentz's theory in 1895 was the "corresponding states theorem" for the terms of order v/c. This theorem states that an observer moving relative to the ether makes the same observations as an observer at rest, after a suitable change of variables. Lorentz noted that it was necessary to change spatiotemporal variables by changing frames and introduced concepts such as the contraction of physical length (1892) to explain the Michelson-Morley experiment and the mathematical concept of local time (1895) to explain the light aberration. and Fizeau's experiment. This gave rise to the formulation of the so-called Lorentz transformation by Joseph Larmor (1897, 1900) and Lorentz (1899, 1904), so that (it was noted by Larmor) the full formulation of local time is accompanied by some kind of of time dilation. Of electrons moving in the ether. As Lorentz later pointed out (1921, 1928), he regarded the time indicated by clocks resting in the ether as "true" time, while local time was seen by him as a heuristic working hypothesis. and a mathematical trick. Therefore, modern authors consider Lorentz's theorem to be a mathematical transformation of a "real" that rests in the ether in a "dummy" in motion.

Lorentz's work was refined mathematically by Henri Poincaré, who formulated the Principle of Relativity on many occasions and tried to harmonize it with electrodynamics. He stated simultaneity only as a convenient convention that depends on the speed of light, so the constancy of the speed of light would be a useful postulate to make the laws of nature as simple as possible. In 1900 and 1904, he physically interpreted local Lorentz time as the result of clock synchronization by light signals. In June and July 1905 he declared the principle of relativity as a general law of nature, including gravitation. He corrected some Lorentz errors and proved the Lorentz covariance of electromagnetic equations. However, he used the notion of an ether as a perfectly undetectable medium and distinguished between apparent time and real time, so most historians of science argue that he did not invent special relativity.

Ether's End

Special Relativity

Aether theory received another blow when Galilean transformation and Newtonian dynamics were modified by Albert Einstein's theory of special relativity, giving the mathematics of Lorentzian electrodynamics a new "non-aether" context. Unlike Of the most important changes in scientific thought, special relativity was adopted by the scientific community remarkably quickly, consistent with Einstein's last comment that the laws of physics described by the Special Theory were " ready for discovery" in 1905. Max Planck's first defense of the theory, together with Hermann Minkowski's elegant formulation of it, did much to contribute to the rapid acceptance of special relativity among working scientists.

Einstein based his theory on Lorentz's earlier work. Rather than suggest that the mechanical properties of objects changed with their constant-velocity motion through an undetectable ether, Einstein proposed deducing the characteristics that any successful theory must possess in order to be consistent with the most basic and firmly established principles, regardless of Existence of a hypothetical ether. He found that the Lorentz transformation must transcend its connection to Maxwell's equations and must represent the fundamental relationships between space and time coordinates of inertial reference frames. In this way, he showed that the laws of physics remained invariant as they had with Galileo's transformation, but that light was now also invariant.

With the development of the special theory of relativity, the need to explain a single universal frame of reference had disappeared, and the acceptance of the theory of the century XIX of a luminiferous ether disappeared with her. For Einstein, the Lorentz transformation implied a conceptual change: that the concept of position in space or time was not absolute, but could differ depending on the location and velocity of the observer.

In addition, in another paper published the same month in 1905, Einstein made several observations about a thorny problem, the photoelectric effect. In this work, he showed that light can be considered as particles that have a "wave nature". Particles obviously don't need a medium to travel, and therefore neither does light. This was the first step that would lead to the full development of quantum mechanics, in which the wave nature and the luminous nature of light are considered valid descriptions of light. A summary of Einstein's thinking on the ether hypothesis, relativity, and quanta of light can be found in his 1909 lecture (originally in German) "The Development of Our Views on the Composition and Essence of Radiation".

Lorentz, on his side, continued to use the ether hypothesis. In his lectures around 1911, he pointed out that "what the theory of relativity has to say...can be carried out independently of what one thinks about the ether and time." He commented that "whether there is an ether or not, electromagnetic fields certainly exist, and so does the energy of electrical oscillations" so, "if we don't like the name 'ether', we should use another word like A peg to hang all this stuff on." He concluded that "the bearer of these concepts cannot be denied a certain substantiality."

Ether theories

Conjectures and proposals

According to the philosophical view of Einstein, Dirac, Bell, Polyakov, 't Hooft, Laughlin, de Broglie, Maxwell, Newton and other theorists, there could be a medium with physical properties that fill space «vacuum», an ether, allowing observed physical processes.^{[citation needed]}

Albert Einstein in 1894 or 1895: "The speed of a wave is proportional to the square root of the elastic forces that cause [its] propagation, and inversely proportional to the mass of the ether moved by these forces."

Albert Einstein in 1920: «We can say that, according to the general theory of relativity, space is endowed with physical qualities; in this sense, therefore, an ether exists. According to the general theory, space without ether is unthinkable; in such a space there would not only be no propagation of light, but also no possibility of existence for the standards of space and time (measuring rods and clocks), and therefore no space-time intervals in the physical sense. But this ether cannot be considered as endowed with the quality characteristic of separable media, consisting of parts that can be traced through time. The idea of motion cannot be applied to it."

Paul Dirac wrote in 1951: "Physical knowledge has come a long way since 1905, notably with the advent of quantum mechanics, and the situation [about the scientific plausibility of ether] has changed again. If one examines the question in the light of present knowledge, one finds that the aether is no longer ruled out by relativity, and good reasons can now be advanced for postulating an aether... We now have the velocity at all points in space- time, playing a fundamental role in electrodynamics. It is natural to think of it as the speed of a real physical thing. Therefore, with the new theory of electrodynamics [vacuum filled with virtual particles] we are forced to have an ether."

John Bell in 1986, interviewed by Paul Davies in The Ghost in the Atom, suggested that an ether theory could help resolve the EPR paradox by allowing a frame of reference in which the signals go faster than light. He suggests that the Lorentz contraction is perfectly consistent, not inconsistent with relativity, and could produce a theory of the ether perfectly consistent with the Michelson-Morley experiment. Bell suggests that the ether was wrongly rejected for purely philosophical reasons: "what is not observable does not exist" [p. 49]. Einstein found the non-ether theory simpler and more elegant, but Bell suggests that doesn't rule him out. In addition to arguments based on his interpretation of quantum mechanics, Bell also suggests resurrecting the ether because it is a useful pedagogical device. That is, many problems are more easily solved by imagining the existence of an ether.

Einstein commented that "God does not play dice with the universe." And those who agree with him are looking for a classical, deterministic theory of the ether, involving quantum-mechanical predictions as a statistical approximation, a theory of hidden variables. In particular, Gerard 't Hooft surmised that: "We must not forget that quantum mechanics does not really describe what kind of dynamical phenomena are actually occurring, but rather gives us probabilistic results. It seems to me extremely plausible that any reasonable theory for Planck-scale dynamics would lead to processes that are so complicated to describe, that one should expect apparently stochastic fluctuations in any approximation theory that describes the effects of all this at much larger scales. bigger. It seems quite reasonable first to prove a classical, deterministic theory for the Planck domain. "One might then speculate that what we today call quantum mechanics may be nothing more than an ingenious technique for handling these dynamics statistically." In their article Blasone, Jizba, and Kleinert "have attempted to substantiate G.'t Hooft's recent proposal that quantum theory is not a complete field theory, but is actually an emergent phenomenon arising from a higher level." deeper dynamics. The underlying dynamics are considered to be classical mechanics with singular Lagrangians with a suitable loss of information condition. With plausible assumptions about the actual nature of constraint dynamics, quantum theory is shown to emerge when the classical Dirac-Bergmann algorithm for constrained dynamics is applied. to the classical integral route[...] ».

Louis de Broglie, «If a hidden sub-quantum medium is assumed, knowledge of its nature would seem desirable. It is certainly quite complex in character. It could not serve as a universal means of reference, as this would be contrary to the theory of relativity."

In 1982, Ioan-Iovitz Popescu, a Romanian physicist, wrote that the ether is "a form of existence of matter, but differs qualitatively from substance (atomic and molecular radiation) (photons)". The fluid ether is "governed by the principle of inertia and its presence produces a modification of the geometry of space-time". Built on Le Sage's ultramundane corpuscles, Popescu's theory posits a finite Universe "filled with particles of extremely small mass, traveling chaotically at the speed of light" and material bodies "made up of particles called aetherons".

Sid Deutsch, a professor of electrical engineering and bioengineering, conjectures that a spherical "spinning ether particle" must exist to "carry electromagnetic waves", and derives its diameter and mass using the density of dark matter.

A Fermi degenerate fluid model, "consisting primarily of electrons and positrons" that results in the slowing of light "with time on the scale of the age of the universe," was proposed by Allen Rothwarf In a cosmological extension, the model was "extended to predict a slowing expansion of the universe".

Non-standard interpretations in modern physics.

General Relativity

Einstein sometimes used the word ether for the gravitational field within general relativity, but this terminology never gained wide support.

We can say that, according to the general theory of relativity, space is endowed with physical qualities; in this sense, therefore, there is an ether. According to the general theory of relativity, space without ether is unthinkable; because in that space there would not only be the propagation of light, but there would also be no possibility of existence for the standards of space and time (measuring bars and clocks), nor, therefore, no space-time interval in the physical sense. But this ether cannot be regarded as endowed with the quality feature of the media, as parts that can be traced over time. The idea of movement cannot be applied to it.

Quantum Vacuum

Quantum mechanics can be used to describe spacetime as non-vacuum at extremely small scales, fluctuating and generating pairs of particles that appear and disappear incredibly quickly. It has been suggested by some such as Paul Dirac that this quantum vacuum may be the modern physics equivalent of a particulate ether. However, Dirac's ether hypothesis was motivated by his dissatisfaction with quantum electrodynamics, and never gained support from the mainstream scientific community.

Robert B. Laughlin, Nobel Prize Winner in Physics, Professor of Physics at Stanford University, had this to say about the ether in contemporary theoretical physics:

It is ironic that Einstein's most creative work, the general theory of relativity, is reduced to conceptualizing space as a medium when his original premise [in special relativity] was that there was no such medium [...] The word "eter" has a lot of negative connotations in theoretical physics due to its past association with opposition to relativity. This is unfortunate because, devoid of these connotations, it captures quite well the way that most physicists really think about the void... Relativity actually says nothing about the existence or absence of matter that permeates the universe, only that such matter must have relativistic symmetry. [..] It turns out that such matter exists. About the time the relativity was accepted, radioactivity studies began to show that the vacuum of space had a spectroscopic structure similar to that of ordinary quantum solids and fluids. The later studies with large particle accelerators have led us to understand that space is more like a window glass piece than Newton's ideal vacuum. It is full of "things" that are usually transparent, but can be made visible if they are beaten with enough force to remove a part. The modern concept of space vacuum, confirmed every day by the experiment, is a relativistic ether. But we don't call it that because it's taboo.

Historical models

Luminiferous Aether

Isaac Newton suggests the existence of an ether in the Third Book of Optics (1718): «Not this ethereal medium, when passing through water, glass, crystal and other compact and dense bodies in empty spaces, grows more and more densely by degrees, and by that means, refracting light rays not at a point, but gradually bending them into curved lines... Isn't this means much rarer within the dense bodies of the Sun, the stars, the planets? and the comets, that in the celestial emptiness? Space between them? And passing from them to great distances, it does not become denser and denser, and therefore causes the gravity of those great bodies towards each other, and of their parts towards the bodies; every body strives to go from the densest parts of the middle to the rarest?".

In the 19th century, luminiferous ether (or ether), meaning light-containing ether, was a medium theorized for the propagation of light (electromagnetic radiation). However, at the end of the 19th century a series of increasingly complex experiments were carried out, such as the Michelson experiment -Morley, in an attempt to detect the movement of the Earth through the ether, and they did not. A number of proposed aether drag theories could explain the null result, but they were more complex and tended to use arbitrary-appearing coefficients and physical assumptions. Joseph Larmor discussed the ether in terms of a moving magnetic field caused by the acceleration of the electrons.

James Clerk Maxwell said of the ether, "In various parts of this treatise an attempt has been made to explain electromagnetic phenomena by means of mechanical action transmitted from one body to another by means of a medium occupying the space between them. The wave theory of light also assumes the existence of a medium. Now we have to show that the properties of the electromagnetic medium are identical to those of the luminiferous medium."

Hendrik Lorentz and George Francis FitzGerald offered in the framework of the Lorentz ether theory a more elegant solution to how the motion of an absolute ether could be undetectable (length contraction), but if their equations were correct, the theory of Albert Einstein's special relativity of 1905 could generate the same mathematics without referring to an ether at all. This led most physicists to conclude that this early modern notion of a luminiferous ether was not a useful concept. However, Einstein declared that this consideration was too radical and too anticipatory and that his theory of relativity still required the presence of a medium with certain properties.

Mechanical Gravitational Aether

From the 16th century to the end of the XIX, gravitational phenomena were also modeled using an ether. The best known formulation is Le Sage's theory of gravitation, although other models were proposed by Isaac Newton, Bernhard Riemann, and Lord Kelvin. None of these concepts is considered viable by the current scientific community.