Ether (or translation of ether; English: ether or aether)

Ether is a substance envisioned by ancient Greek philosophers, a medium of propagation of imaginary electromagnetic waves, and is considered to be omnipresent.

In ancient Greece, ether referred to the blue sky or the upper atmosphere. In cosmology, ether was sometimes used to represent matter that occupies the space of celestial bodies.

Descartes in the 17th century was a philosopher who had a significant influence on the development of scientific ideas. He first introduced ether into science and gave it some mechanical properties.

In Descartes' view, all forces between objects must be transmitted through some intermediate media substance, and there is no effect beyond distance. Therefore, space cannot be empty, it is filled with medium substances such as ether. Although ether cannot be felt by human senses, it can transmit the effects of forces, such as magnetism and the moon's force on tides.

Later, Ether was largely associated with the theory of light waves as a load of light. The theory of light fluctuation was first proposed by Hooker and further developed by Huygens. For a considerable period (until the early 20th century), people's understanding of waves was limited to the mechanical vibration of a certain medium substance. This medium substance is called the load of waves, such as air is the load of sound waves.

Since light can propagate in a vacuum, Huygens proposed that the medium substance (ether) that loads light waves should fill all space including vacuum and penetrate into normal substances. In addition to being a load object for light waves, Huygens also uses ether to illustrate the phenomenon of gravity.

Although Newton disagreed with Hook's theory of light fluctuation, he also opposed the action beyond distance like Descartes and acknowledged the existence of ether. In his opinion, ether is not necessarily a single matter, so it can transmit various effects, such as the generation of different phenomena such as electricity, magnetism and gravity. Newton also believed that ether can propagate vibration, but the vibration of ether is not light, because the theory of light fluctuation at that time could not explain the polarization phenomenon of light, nor could it explain why light propagates linearly.

The 18th century was a period of decline of the Etherism. Because the French Cartesians rejected the inverse square law of gravity, Newton's followers rose against the Cartesian philosophical system, and thus joined the opposition together with the Etherism he advocated.

With the success of the inverse square law of gravity in celestial mechanics and the lack of actual results in the exploration of ether experiments, the view of over-distance action was popular. The theory of fluctuation of light was also abandoned, and the theory of particles was widely recognized. By the late 18th century, it was confirmed that the force between charges (and between magnetic poles) was also inversely proportional to the square of distance. Therefore, the concept of electromagnetic ether was also abandoned, and the view of over-distance action also dominated electricity.

In the 19th century, the ether theory was revived and developed, which began with optics first, mainly due to the work of Thomas Young and Fresnel. Yang used the interference of light waves to explain the Newtonian ring, and under the inspiration of experiments, he proposed a new view that light waves were transverse waves in 1817, solving the difficulty of the wave theory that the long-term polarization phenomenon of light could not be explained. Scientists gradually discovered that light is a wave, and most of the waves in life need to propagate medium (such as the transmission of sound waves requires the help of air, and the propagation of water waves is water, etc.). Influenced by traditional mechanical ideas, they imagined that there is a substance called ether everywhere in the universe, and it is this substance that plays a medium role in the propagation of light.

The assumption of ether actually represents the traditional view: the propagation of electromagnetic waves requires an "absolutely static" reference system, and when the reference system changes, the speed of light also changes.

However, according to Maxwell's system of equations, the propagation of electromagnetic waves does not require an "absolutely static" reference system, because both parameters in this equation are directionless scalars, so the speed of light remains unchanged in any reference system.

Where ε0 is the vacuum dielectric constant and μ0 is the vacuum permeability.

This "absolute static system" is the "Ether system". The speed of light measured by observers of other inertial systems should be the speed of light of the "Ether system", and the vector sum of the speed of this observer on the "Ether system".

The ether is omnipresent, without mass, and is absolutely still. According to the conjecture at that time, the ether fills the entire universe, and electromagnetic waves can propagate in it. Assuming that the sun is still in the ether system, since the earth orbits the sun, it has a velocity v relative to the ether. Therefore, if the speed of light is measured on the earth, the value measured in different directions should be different, the maximum is c+v and the minimum is cv. If the sun is not stationary in the ether system, the speed of light in different directions should also be different on the earth.

Fresnel successfully explained the diffraction phenomenon of light using the theory of fluctuation. The theoretical method he proposed (now often called the Huygens-Fresnel principle) can correctly calculate the diffraction pattern and explain the linear propagation phenomenon of light. Fresnel further explained the birefringence of light, which was a great success.

In 1823, based on Yang's theory that light waves are transverse waves, he proposed in 1818 that the assumption that the etheric density in transparent matter is proportional to its refractive index quadratic force, under certain boundary conditions, he introduced a famous formula for reflected light and refractive light amplitude, which well illustrates the results measured by Bruster several years ago from experiments.

An important theoretical work of Fresnel on ether is to derive the velocity formula of light in transparent objects moving relative to the ether reference system. In 1818, in order to explain Arago's experiment on the refractive behavior of starlight, he proposed based on Yang's idea that the density of ether in transparent matter is proportional to the quadratic refractive index of the matter. He also assumed that when an object moves relative to the ether reference system, the ether inside is only the part that exceeds the vacuum and is driven by the object (the ether part traction hypothesis). Using Fresnel's theory, it is easy to obtain the velocity of light in the moving object.

In the mid-19th century, some experiments were conducted to show the effects caused by the movement of the Earth relative to the etheric reference system, and to measure the speed of the Earth relative to the etheric reference system, but all of them came up with negative results. These experimental results can be explained from the Fresnel theory. According to the formula for the speed of light in the Fresnel motion medium, when the experimental accuracy only reaches a certain order of magnitude, the speed of the Earth relative to the etheric reference system will not be shown in these experiments, and none of the experiments at that time have reached this accuracy.

After the work of Young and Fresnel, the fluctuation of light established its position in physics. Subsequently, ether also gained status in electromagnetics, mainly due to the contributions of Faraday and Maxwell.

In Faraday's mind, the view that the role is to gradually pass through has a very strong position. He introduced force lines to describe magnetic and electrical functions. In his opinion, force lines are real existences, space is filled with force lines, and light and heat may be the horizontal vibration of force lines. He once proposed to use force lines instead of ether, and believed that matter atoms may be the force field that gathered near a point-shaped center. He wrote in 1851: "If the existence of light ether is accepted, it may be the loading of force lines." But Faraday's view was not accepted by theoretical physicists at that time.

By the early 1860s, Maxwell proposed the concept of displacement current and proposed a set of differential equations to describe the general laws of electromagnetic fields. This set of equations was later called Maxwell's system of equations. According to Maxwell's system of equations, it can be concluded that the disturbance of the electromagnetic field propagates in the form of waves, and the speed of electromagnetic waves in the air was 310,000 kilometers per second, which is consistent with the known speed of light in the air at that time, 315,000 kilometers per second within the experimental error range.

After pointing out that the propagation of electromagnetic disturbances is similar to that of light propagation, Maxwell wrote: "Light is the transverse vibration of the medium (referring to ether) that produces electromagnetic phenomena." Later, Hertz used experimental methods to confirm the existence of electromagnetic waves. The electromagnetic theory of light successfully explained the properties of light waves, so that ether not only gained a position in electromagnetics, but also unified the electromagnetic ether with light ether.

Maxwell also envisioned the use of the mechanical motion of ether to explain electromagnetic phenomena. In his 1855 paper, he compared the magnetic induction intensity to the speed of ether. Later, he accepted Tom Sun (i.e. Kelvin)'s view and changed it to a magnetic field representing rotation and an electric field representing translation.

He believed that the ether rotates around the magnetic line to form vortex elements, and there is a layer of charge particles between adjacent vortex elements. He also assumed that when these particles deviate from their equilibrium position, they will have a force on the matter in the vortex element to cause deformation of the vortex element, which represents an electrostatic phenomenon.

There is some correspondence between electric fields and displacement, which is not completely new. Tomson once compared the electric field to the displacement of ether. In addition, Faraday proposed earlier that when an insulating substance is placed in the electric field, the charge in it will be displaced. The difference between Maxwell and Faraday is that he believes that no matter whether there is or not, as long as there is an electric field, there will be a displacement of etheric charged particles, and the magnitude of the displacement is proportional to the intensity of the electric field. When the displacement of the charged particles changes with time, a current will be formed, which is what he calls the displacement current. For Maxwell, the displacement current is a real current, and now we know that only a part of it (polarization current) is the real current.

During this period, some other ether models were established, but ether theory also encountered some problems. First, if the light wave is a transverse wave, the ether should be an elastic solid medium. So why is the celestial body free from resistance when it runs? Someone has proposed an explanation: ether may be a plastic substance like wax or asphalt. It has enough elasticity for light vibrations as fast as it is like a solid, and for celestial body motions like a fluid.

In addition, in addition to transverse waves, there should generally be longitudinal waves in elastic media, but experiments have shown that there are no longitudinal light waves. How to eliminate longitudinal waves of ether and how to obtain the boundary conditions required to deduce the reflection intensity formula are a long-term debate on various ether models.

In order to adapt to the needs of optical, people assume some extraordinary properties of ether, such as the 1839 McCola model and the Cauchy model. In addition, since the refractive index is also different for different light frequencies, the traction coefficient will also be different for different frequencies. In this way, light of each frequency will have to have its own ether, etc. The seemingly contradictory properties of ether are actually beyond people's ability to understand.

From 1881 to 1884, Albert Michaelson and Edward Morey conducted the famous Michaelson-Morey experiment to measure the relative velocity of the earth and the ether. The experimental results show that there is no difference in the speed of light in different directions. This actually proves the principle that the speed of light is unchanged, that is, the speed of light in vacuum has the same value under any reference system, and has nothing to do with the relative velocity of the reference system, and the ether does not exist. Later, many experiments supported the above conclusion.

The theory of ether was once deeply rooted in people's minds for a period of historical period, deeply influenced the thoughts of physicists. The famous physicist Lorentz deduced the Lorentz transformation formula that meets the conditions for electromagnetic covariation, but could not abandon the view of ether.

In the 1890s, Lorentz proposed a new concept, and he attributed the electromagnetic properties of matter to the effects of electrons associated with atoms. As for the ether in matter, there is no difference between density and elasticity from the ether in vacuum. He also assumed that the ether in the object does not drive the ether in its movement. However, when the electrons in an object move with an object, they are not only subject to the force of the electric field, but also to the force of the magnetic field, and when the object moves, the dielectric motion current will appear, and the electromagnetic wave velocity in the moving matter is not the same as that in the stationary matter.

After considering the above effects, Lorentz also introduced Fresnel's formula for the speed of light in moving matter, and the difficulties encountered by Fresnel theory (light at different frequencies has different ethers) no longer exist. Lorentz can deduce the change of refractive index with frequency based on the forced vibration of bound electrons. Lorentz's above theory is called electron theory, and it has achieved great success.

The late 19th century can be said to be the heyday of the ether theory. However, in Lorentz's theory, the ether no longer has any other movements or changes except for the electromagnetic vibration of the load, so that it has almost degenerated into a certain abstract sign. Apart from being a load object and an absolute reference system for electromagnetic waves, it has lost all other concrete and vivid physical properties, which has created conditions for its decline.

As mentioned above, in order to measure the motion of the earth relative to the etheric reference system, the experimental accuracy must reach a very high order of magnitude. By the 1880s, the experiments conducted by McElson and Morey achieved this accuracy for the first time, but the result was still negative, that is, the earth does not move relative to the ether. Some other experiments have also obtained the same results, so the ether further lost its properties as an absolute reference system. This result made the principle of relativity widely recognized and extended to the entire field of physics.

In the late 19th and early 20th centuries, although some efforts were made to save the ether, after the establishment of the special theory of relativity, it was finally abandoned by physicists. People accepted the concept that electromagnetic fields themselves are a form of material existence, and the fields can propagate in the form of waves in a vacuum.

The establishment of quantum mechanics strengthens this view because people have found that the movement of atoms of matter and the particles such as electrons, protons and neutrons that make up them also have the attribute of waves. Waveness has become an aspect of the basic attribute of matter movement, and the narrow view that only understands waves as mechanical vibrations of a certain medium substance have been completely broken.

However, people's understanding continues to develop. After the mid-20th century, people gradually realized that vacuum is not absolute vacuum, and there are continuous fluctuations and falls (the generation of virtual particles and subsequent annihilation). This vacuum fluctuation and fall is a quantum effect of the interacting field.

Today, theoretical physicists further discovered that vacuum has more complex properties. The vacuum state represents the ground state of the field, which is degenerate, and the actual vacuum is a specific state among these degenerate states. Many of the symmetry damage observed in particle physics are caused by this special "orientation" of vacuum. The theory of electric weak unified interaction and electromagnetic interaction established on this view has achieved great success.

But Einstein boldly abandoned the theory of Ether, believed that the constant speed of light was the basic principle, and used this as one of the starting points to create the special theory of relativity. Although later facts proved that there was no ether, the ether hypothesis still left traces in our lives, such as Ethernet.

It seems that although the mechanical ether theory is dead, some spirits of the ether concept (no longer than distance, no vacuum in the sense of absolute emptiness) are still alive and have vigorous vitality.
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