Ether (physics)

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Some ether ideas imply a seasonally changing ether wind

The ether ( Greek αἰθήρ ["aithḗr"], the (blue) sky ') is a hypothetical substance that was postulated as a medium for the propagation of light in the late 17th century . Later the concept from optics was also transferred to electrodynamics and gravity , especially to avoid assumptions based on long-range effects . Generally the ether was seen as the carrier of all physical processes.

These conceptual transfers resulted in insurmountable conceptual difficulties and contradictions to experimental results. The idea of ​​an ether could not be confirmed experimentally. Thus Maxwell's equations could never be brought completely into agreement with the mechanical ether models. Likewise, the aether had to be defined on the one hand as a material solid, on the other hand its resistance to the movement of the heavenly bodies should be imperceptibly low. The existence of both a dormant as well as that of an entrained aether was refuted by experiments and observations: The dormant aether was refuted by the Michelson-Morley experiment , and an ether entrainment contradicted the aberration of the light . Various auxiliary hypotheses that were introduced to save the concept contradicted themselves and also appeared to be arbitrary.

An ether does not play a role in the observable physical phenomena. An alternative concept, in which a medium connected to a state of motion is not required, was created with the special theory of relativity . With their help, the propagation of electromagnetic waves could for the first time be described without contradiction; for more reasons and motivations see the main article History of Special Relativity . Relativistic electrodynamics has meanwhile been merged with quantum mechanics ; the resulting relativistic quantum electrodynamics also does not require a carrier medium for the waves.

Early development of the light ether concept

The modern theories of ether go back to the Aristotelian theory of elements , which introduced the ether as a medium for the even circular movements of the stars.

Descartes, Hooke, Huygens

In modern times, René Descartes started from the following philosophical considerations: Matter is only characterized by expansion and, conversely, there is no expansion without matter. From this it follows that the entire "empty" space must be filled with matter. He combined this with the notion that all processes occur through direct contact with this matter, i.e. H. must be understood as close effects in the form of movement and pressure . He used this idea in 1637 in his theory of the nature of light by postulating spherical light particles, whereby the static pressure exerted by these closely pressed particles is to be understood as light. He succeeded (like Willebrord van Roijen Snell before him in 1621) in discovering the laws of refraction .

After Descartes there had been heated discussions about whether an empty room was conceivable. Blaise Pascal remarked: “Nature endures its demise sooner than the smallest empty space.” With this, Pascal criticized the postulate of contemporaries that they could have created a complete vacuum by reducing the air pressure. He was referring in particular to Evangelista Torricelli's assumption that he had created an empty space. Pascal pointed out that the absence of air is not automatically identical with a complete emptiness of space.

In contrast to Descartes' idea of ​​static pressure, Robert Hooke assumed a “ homogeneous medium ” in 1665 , in which light spreads in the form of pulses and vibrations in a straight line and at an even speed in all directions. Every light pulse can be viewed as an ever-increasing sphere, analogous to the propagation of waves on the water surface. This means that there is no transport of matter, rather only information about the state of motion is transmitted. The different areas of the pulses would have different speeds in the transition from one medium to the other, which is how Hooke replaced Descartes' explanation of refraction. Although his theory was a great step forward compared to Descartes, since he did not yet have the concepts of wave theory, he could not fully explain all the laws of refraction and reflection. His related theory of colors was also soon refuted by Newton.

Christiaan Huygens finally formulated the first complete wave theory of light in 1678–1690 ; According to his imagination, its light ether penetrated solid matter as well as the empty space of the universe . By devising a systematic description and explanation of wave phenomena , he was able to give an elegant explanation for reflection and refraction. This was seen as an important argument for the wave theory and thus for the ether.

Newton's criticism

Isaac Newton criticized that particle theory could not explain the polarization of light. Nevertheless, he assumed that light consists of particles or corpuscles in order to be able to interpret the linear propagation and reflection phenomena mechanically, whereby he made no special statements about the nature of these corpuscles. With this model, Newton was only able to explain the refraction and diffraction phenomena in an unsatisfactory manner. That is why he retained the corpuscular conception of light in his influential work Opticks (1704), but combined it with an ether, which was supposed to be responsible for the transfer of heat. This medium should lose some of its density in the vicinity of the matter ; the interaction of the corpuscles with this medium generates heat on the one hand and effects such as diffraction and refraction on the other. He wrote:

“Isn't the heat of a room transported by the vibrations of a much finer medium in the vacuum, which remains in the vacuum after the air has been evacuated? And isn't this medium the same as the one through which light is refracted and reflected and through whose vibrations the light transfers heat to bodies and is thereby put into states of easy reflection and transmission? "

Although Newton in the second book of his main work " Principia Mathematica (always on a ether hypothesis based)" all vortex theories had refuted to explain the planetary motions - a then widely accepted hypothesis was published in 1644 by Descartes - he never rejected the airwaves final, but known - last time in 1704 - in Opticks only:

"Because I don't know what the ether is."

Light ether as a solid

With exceptions such as Benjamin Franklin and Leonhard Euler , the corpuscle theory was preferred by most of the physicists of the time due to Newton's great authority . This was confirmed above all by James Bradley's discovery (1728) of the aberration of light , which could easily be related to the nature of particles.

It was not until 1800 to 1804 that Thomas Young was able to help the wave theory break through. Young was the first to show that the wave theory of light could explain some phenomena that could not be reconciled with Newton's corpuscle theory. So he explained z. B. Newton's rings through the principle of interference and was the first to carry out the double slit experiment , the result of which clearly spoke for the wave nature of light and thus for the existence of aether. Young was also unable to reconcile the effect of polarization with the wave model. In 1817 he also solved this problem by assuming that light waves behave like transverse waves - this was unusual because, in analogy to sound, light waves had been imagined as longitudinal waves .

After all , it was Augustin Jean Fresnel who gave an elaborated theory of optical phenomena based on light ether, which is still valid today. In the period from 1816 to 1819 he derived it from properties of the ether, following the example of mechanics . According to his theory the aether behaves towards transverse waves like an elastic solid . That means, in empty space the ether is at rest and the light spreads in all directions at the same speed.

The theories of the elastic ether (in different forms) was u. a. continued by Claude Louis Marie Henri Navier (1827), Augustin Louis Cauchy (1828), Siméon Denis Poisson (1828), James MacCullagh (1837), Franz Ernst Neumann (1837), George Green (1838). On the one hand, these models were very helpful and useful in the development of the theory of solids, but on the other hand, there were also many partly (from today's perspective) fantastic hypotheses about the mechanical ether constitution.

For example, MacCullagh's ether model from 1839 was based on mechanical rotations against absolute space in an elastic solid and resulted in equations of motion that correspond in their form exactly to the then unknown Maxwell equations. Despite this astonishing agreement, the model had to be discarded due to various contradictions in the explanation of optical phenomena. It was only 40 years later that George Francis FitzGerald pointed out that MacCullogh, with his equations presented in 1839, anticipated the Maxwell equations published in 1864 in a certain sense.

Electromagnetic ether

After various models of aether pressure had been developed in the 16th and 17th centuries to explain magnetism and electricity, the triumphant advance of Newton's theory of gravity led to the assumption that these phenomena would also act at a distance without aether. This is how the important theories of Charles Augustin de Coulomb and André-Marie Ampère emerged . It was already noted by Wilhelm Eduard Weber (1856) and others that the speed of light plays an important role in electromagnetism.

Theory of the molecular vortices of the ether according to James Clerk Maxwell : AB means an electric current from A to B, the small circles represent electric particles, the large spaces in between are the molecular vortices.

Michael Faraday was the first to interpret this connection . He concluded that there are lines of force in the aether which transmit the electromagnetic effects with finite speed. With the Maxwell equations , which James Clerk Maxwell had developed from 1861 to 1864, the unification of optics and electrodynamics was finally achieved. The ether thereby became the carrier of all electromagnetic phenomena including optics, of whose effectiveness Maxwell was firmly convinced. In the entry he wrote in the Encyclopædia Britannica , he summarizes at the end:

“Whatever difficulties we have to develop a consistent idea of ​​the nature of the ether: There can be no doubt that the interplanetary and interstellar space is not empty, but that both are filled with a material substance, which is certainly the most extensive and probably the most uniform Is matter that we know about. "

The link between the electrodynamic and optical phenomena was the speed of light, which was considered the limit speed relative to the ether. Maxwell himself and others formulated several mechanical ether models, such as B. the widely discussed model of the molecular vortex by Maxwell (right picture). As Maxwell himself noted, this could only explain partial aspects, because these models often contradicted each other - what was certain were the mathematical results that culminated in Maxwell's equations.

In addition to Maxwell, other researchers also set up various models. Those in which vortices were used to represent molecular and electromagnetic effects were particularly well known . Hermann von Helmholtz (1858) showed that vortex rings are indestructible in a perfect fluid. Kelvin (1867) then developed a theory in which the atoms of matter are just such eddies. The interactions of matter are then comparable to the interplay of smoke rings , which are constantly forming new connections. But even this theory had to be rejected because the connections could not remain stable. Another variant was Kelvin's vortex-sponge theory, in which both rotating and non-rotating parts work together in certain sections of the ether. Here, too, one did not get beyond analogies, so that in the end it was not possible to create a unified mechanical ether theory which explains the entire electromagnetic field and matter.

While British researchers adopted Maxwell's theory relatively quickly and developed it further (such as Joseph John Thomson , Oliver Heaviside , George Francis FitzGerald , John Henry Poynting , Joseph Larmor ), in German-speaking countries the theories at a distance in the sense of Weber and Neumann remained. That did not change until 1888, when Heinrich Hertz directly demonstrated the finite propagation speed of the electromagnetic forces predicted by Maxwell. He summed up the contemporary view of the ether:

“Take the electricity from the world, and the light disappears; take the light-bearing ether from the world, and the electrical and magnetic forces can no longer cross space. "

Hertz developed his electrodynamics of moving bodies between 1887 and 1890. Before or at the same time as Hertz, a similar theory had also been developed by Oliver Heaviside . What was important was the formulation of Maxwell's equations, which Hertz had based his theory on as a postulate, and which later had a great influence in the form of the “Maxwell-Hertz equations” - the equations finally being given their modern form by Heaviside.

The models also went into biophysics , for example in Mesmer's assumption of animal magnetism (mesmerism).

Problems of the ether theories


A fundamental problem of the theories of aether was that a mechanical aether would have to oppose a moving body with a resistance in the direction of movement. To solve this problem, George Gabriel Stokes (1845) suggested that the ether behaves in a similar way to pitch : it shatters when it is hit very quickly with a hammer. A heavy weight, on the other hand, sinks in like a viscous liquid. This could explain that with vibrations like those of light the aether behaves like an elastic solid, and with massive, slow objects like the planets like a liquid. In the meantime, investigations into the ether theory had led to the assumption that the ether substance must be about 1.5 · 10 11 times lighter than atmospheric air .

Other physicists were more radical: They assumed that the ether was the original matter and that the visible matter was only a form of stimulation of the ether. In analogy to vibrations that propagate through a medium at constant speed - the problem of resistance would no longer arise here. Some examples: According to William Thomson, 1st Baron Kelvin , the ether is a liquid, and matter can be understood as a vortex that propagates in the ether. According to Lorentz, matter is just a "modification" of the ether, according to Joseph Larmor it is to be understood as a torsion of the ether, and Paul Langevin defines it as a mere liquefaction of the ether, whereby these places of liquefaction move on and the ether solidifies again behind them.

Relative movement between ether and matter

According to Young, the aberration can only be explained within the framework of the ether theory if the ether is immobile.
Left: aberration with stationary ether
Right: no aberration with complete ether entrainment
(black lines: telescope)

The question of the relative movement between matter and ether arose. The aberration of light spoke loudly Young and Fresnel for the adoption of a resting or from matter uninfluenced ether and contradicted the complete Äthermitführung through matter.

Dormant ether

The above explanation of the aberration by means of resting ether only works if light also behaves like a particle in the ether. Since the light was viewed as a wave, the following problem arose: Due to the movement of the earth through the resting ether, there is no deflection of the wave planes at all - that is, the position of the wave front does not change and there is no aberration. The problem can be solved if the Poynting vector following from electromagnetic light theory is taken into account, which indicates the direction of the energy transport or the beam paths in the waves. This direction depends on the relative movement of the source and observer and consequently results in the aberration. A simpler explanation (which goes back to Fresnel) arises if it is taken into account that interference occurs when entering the lens and certain wave groups are “cut out” from the wave front. Since wave groups behave analogously to particles, the corresponding aberration also results from this.

As early as 1810, François Arago experimentally tested the possibility of an influence of the movement of a prism on the refraction of light, which should lead to a changed aberration angle, but the result was negative. Fresnel (1818) explained the result with the assumption that the speed of light in the bodies is modified by partial entrainment of the ether. This entrainment arises from the fact that the ether in the bodies is compressed and therefore a little denser, whereby precisely this excess of ether density - with the exception of the range of normal density - is carried along by the bodies. The drag coefficient (where the velocity of the medium and n is the refractive index ) is given by .

An exact confirmation of the drag coefficient was made possible by the Fizeau experiment by Hippolyte Fizeau (1851). He used an interferometer arrangement that measured the speed of light in water. Fizeau's result was more precisely confirmed by Michelson and Morley (1886). The weakness of Fresnel's explanation was that due to the dependence on the ether density there should also be a dependence of the coefficient on the color or frequency - which could not be correct. Fresnel's formula and the basic idea of ​​a dormant ether were accepted by many, but the exact processes in the ether, which resulted in the partial entrainment, remained unexplained or could only be treated very speculatively. Fresnel's theory could not be maintained later due to various experiments that contained quantities of the second order to v / c , but it formed the basis of the theory of the aether at rest, which Lorentz set up from 1892–1895 (see section "Lorentz's theory of ethers").

Complete ether entrainment

For George Gabriel Stokes (1845) and later also Hertz (1890) the idea of ​​an ether, which was hardly or not at all influenced by the movement of matter, was very unnatural. Also proceeding from an elastic ether, Stokes therefore represented the idea of ​​a complete entrainment of the ether inside and - decreasing with distance - also outside the body. In order to obtain the same effects as Fresnel for the explanation of the aberration of light and the Fresnel entrainment coefficient, Stokes had to introduce complicated additional hypotheses.

The main problem was the aberration of the light: While Young and Fresnel were able to derive the effect from the fundamental assumptions of almost dormant aether with a low entrainment coefficient (picture above, left), this seemed to be excluded with a completely entrained ether. Because here on the surface of the earth, or within a telescope, there is absolutely no relative movement between earth and aether, and consequently there is no reason for an aberration of the light (picture above, right). Stokes therefore had to assume that the aether was incompressible and nonetheless rotation-free when it was completely carried along on the earth's surface . These circumstances would lead to a refraction of the light in the aether carried along, which could reproduce the effect of the aberration. For the Fresnel entrainment coefficient and thus the explanation of the Arago experiment (and later the Fizeau experiment) he assumed that although the entire ether is carried along, the speed of the ether in the body is somewhat modified.

But Lorentz (1886) was able to show that Stokes' assumptions about aberration contradict themselves and the mechanical laws: all conditions cannot be fulfilled at the same time. Because of the contradictions and artificiality of these hypotheses, Stokes' theory could not prevail over Fresnel's successful theory.

Zero results of the ether wind experiments

First order experiments

Fresnel's entrainment coefficient had the consequence that in ether drift experiments, i.e. H. When trying to determine the relative speed of the earth and the ether, no positive results in the order of magnitude were to be expected, where v is the relative speed of earth and aether and c is the speed of light. This was confirmed by the following experiments, the following list being based on the description of Wilhelm Wien (1898), with changes and further experiments according to the descriptions by Edmund Taylor Whittaker (1910) and Jakob Laub (1910):

  • The Aragos experiment (1810), which was supposed to prove whether the refraction, and thus the aberration of the light of the fixed stars, is influenced by the movement of the earth. Similar attempts have been made u. a. employed by George Biddell Airy (1871) using a telescope filled with water, and Eleuthère Mascart (1872), who were also unable to determine any influence.
  • The experiment by Fizeau (1860) was intended to investigate whether the rotation of the plane of polarization through glass columns is changed by the movement of the earth. He received a positive result, but Lorentz was able to show that the results were contradicting themselves. DeWitt Bristol Brace (1904) and Strasser (1907) repeated the experiment with greater accuracy and actually found a negative result.
  • The experiment by Martin Hoek (1868): This is a more precise variant of the Fizeau experiment , where two beams of light are sent on opposite, rectangular paths, with water at rest in one arm of the experiment. Here, too, the Fresnel entrainment coefficient gives a negative result.
  • The experiment by Wilhelm Klinkerfues (1870) was intended to investigate whether the movement of the earth had an influence on the absorption line of sodium vapor. In fact, he managed to get a positive result. But it was obviously an observation error, because a repetition of the experiment by Haga (1901) gave a negative result.
  • In the experiment by Ketteler (1872), the two beams of an interferometer were sent in opposite directions through two tubes filled with water and inclined towards one another. There was no change in the interference fringes. And Mascart (1874) was able to show that the interference fringes also remained unaffected by polarized light in calcite plates.
  • The experiment by Eleuthère Mascart (1872) to prove the rotation of the plane of polarization in quartz showed no change in the rotation when the light rays once had the direction of the earth's motion and then the opposite direction. John William Strutt, 3rd Baron Rayleigh , carried out similar experiments in 1902 with significantly higher accuracy, and also got a negative result.

In addition, first-order electrodynamic experiments were carried out. The negative results of the following experiments can be explained with Lorentz's theory of the static ether:

  • The experiment by Wilhelm Conrad Röntgen (1888) was intended to prove whether a charged capacitor generates magnetic forces due to the movement of the earth.
  • The experiment by Theodor des Coudres (1889) was intended to determine whether the induction effect of two coils of wire on a third is influenced by the direction of the earth's movement. Lorentz showed that this effect is at most of the second order, since the electrostatic charge on the conductors caused by the movement of the earth cancels the effect of the first order.
  • Frederick Thomas Trouton's experiment (1902). Here a capacitor has been placed so that its plates are parallel to the movement of the earth. The negative result can be explained on the basis of the electromagnetic pulse resulting from Lorentz's theory.
  • The Königsberger experiment (1905). The plates of a capacitor are in the field of a strong electromagnet. Due to the movement of the earth in the ether, the plates should receive charges, which was not observed.

Second order experiments

In experiments that could have shown effects on the order of magnitude , according to the theories of Fresnel and Lorentz, positive results should necessarily be obtained. The Michelson-Morley experiment (1887) was the first experiment of this kind. It showed that the speed of the earth on the earth's surface is close to zero relative to the assumed aether, so that the aether, if present, is completely carried along. The result corresponded to about 5–8 km / s, which in view of the expected speed of 30 km / s could not be interpreted as an ether wind. In addition, various cosmic speeds (rotation of the Milky Way, relative movement to the rest system of cosmic microwave radiation) suggest a speed of approx. 368 km / s, which shows the insignificance of the result even more clearly. Further repetitions with lasers and measles carried out to date have actually produced completely zero results . Exceptions like the experiments of Dayton Miller (approx. 8-10 km / s) could not be confirmed, whereby various sources of error could be shown in Miller's experiment (see Michelson-Morley experiment # Further experiments ). Other experiments that could determine second-order quantities were the Trouton-Noble experiment (1902), the experiments of Rayleigh and Brace (1904), the Trouton-Rankine experiment (1908), and the Kennedy-Thorndike experiment (1932 ). All of these also delivered zero results.

The results of the second-order experiments were very strange from the point of view of the time, because they were only compatible with Stokes 'theory - but Lorentz had pointed out Stokes' errors in 1886. On the other hand, Fresnel's drag coefficient and thus the theory of the aether at rest had been confirmed very precisely by the first-order experiments, in direct contradiction to the result of the MM experiment.

Modification of Stokes' theory

The interference experiments by Oliver Lodge (1893, 1897) and Ludwig Zehnder (1895) had shown that the aether is not carried along by the movement of different masses. Lodge used rotating disks and was able to observe that the interference pattern was not influenced between the disks. (Later the Hammar experiment (1935) achieved even greater accuracy. Here, one arm was surrounded by a lead sleeve while the other was free. The result was also negative here.)

In order to avoid these problems, according to Theodor des Coudres and Wilhelm Wien (1898), the ether should be carried in proportion to the mass or gravity of the body. In the case of large masses like the earth, the entrainment would therefore be complete, which would have explained the negative results of experimental arrangements resting on the earth such as the Michelson-Morley experiment (1887). On the other hand, the positive results of arrangements moving on earth could also be explained, as in the Fizeau experiment (1851) or the Sagnac effect (1913), and also the negative results of Lodge etc., since in both cases the gravitational effect of the used Instruments are not sufficient to carry the ether sufficiently. But here too the same aberration problems arose as with Stokes. Nevertheless Max Planck (1899) tried to save this idea with the assumption that a condensation of the ether could take place in the vicinity of the earth through gravitation, so that the ether receives similar properties as Stokes needs for his theory ("Stokes-Planck theory" ). Lorentz (1899) pointed out that according to this assumption, even a 50,000-fold compression of the aether would have no significant influence on the electromagnetic phenomena - which is extremely unlikely.

As Georg Joos (1934) later explained, a complete entrainment through the earth contradicts the positive result of the Michelson-Gale-Pearson experiment (1925), which is a variant of the Sagnac experiments. Here an attempt was made to measure the rotation of the earth itself, i.e. H. In contrast to the usual Sagnac experiments, the arrangement rests on the earth; therefore a positive result would not be expected with a complete entrainment, because it is hardly conceivable that the aether should be influenced by the translation but not by the rotation of the earth.

Hertz's "Electrodynamics Moving Bodies" (1889), which also included a complete transport of ether, had the same problems. His theory was also rejected because it only gave correct results for conductors moving in an electromagnetic field , but not for moving insulators . As was established by Eichenwald (1903) and Wilson (1905), the effects of moving insulators only correspond to a partial dielectric shift, not a complete shift according to Stokes and Hertz.

From the ether to the theory of relativity

Because of the negative results of the second-order experiments, and because the idea of ​​the ether being completely carried along was exposed to too many difficulties, either Fresnel's theory of the (approximately) resting ether had to be modified or the ether idea had to be discarded altogether. With exceptions such as Emil Cohn (1901) or Alfred Bucherer (1903), the latter was hardly taken into account, since electrodynamics without classical ether seemed unthinkable for most. That is why the vast majority of physicists retained the etheric thought. Even Albert Einstein tried to involve young age (1894/1895) the ether in its deliberations. In 1905, these efforts resulted in his discarding the ether.

Lorentzian ether

Between 1892 and 1906, Hendrik Antoon Lorentz and Henri Poincaré developed a theory that combined Fresnel's ether theory with Maxwell's equations and the electron theory of Rudolf Clausius . Lorentz introduced a strict separation between matter ( electrons ) and ether. In his model the ether is assumed to be completely immobile . In this variant of an abstract electromagnetic aether, a mechanical explanation in the sense of the older aether models is dispensed with. Max Born then identified the Lorentz ether with Isaac Newton's absolute space . The state of this ether can be described in the sense of Maxwell-Lorentzian electrodynamics by the electric field E and the magnetic field H , whereby these fields were understood as stimulation states or vibrations in the ether caused by the charges of the electrons . In contrast to Clausius, who assumed that the electrons act on each other through remote action, the same electromagnetic field of the ether acts as a mediator between the electrons, in which effects can spread at the maximum speed of light. With his theory, Lorentz was able to theoretically explain the Zeeman effect , for which he received the Nobel Prize in 1902 . It should be mentioned that Joseph Larmor, at the same time as Lorentz, drafted a similar theory of electrons or ethers, which was based on a mechanical ether.

In Lorentz's ether theory (as well as in Larmor's theory) the contradiction to the Michelson-Morley experiment is resolved by introducing Lorentz transformations . The length contraction and time dilation are understood as processes to which scales and clocks moving relative to an ether are subject, while space and time remain unchanged. This means that these effects are viewed as asymmetrical, i.e. moving scales are actually shorter and clocks actually run more slowly. An observer in motion assesses resting scales identically as shorter and resting clocks as slower. This assessment is interpreted as a deception, since it is gained by the moving observer using falsified measuring instruments and clocks. The symmetry of the observations and thus the obvious validity of the phenomenological principle of relativity emphasized by Poincaré is interpreted as a consequence of a rather coincidental symmetry of the underlying dynamic processes. It prevents the possibility of determining one's own speed relative to the aether, and thus makes it a fundamentally inaccessible quantity in theory.

Special theory of relativity

In the special theory of relativity (SRT) Einstein succeeded in deriving the Lorentz transformation and the other parts of the theory solely from the assumption of two principles - the principle of relativity and the constancy of the speed of light. These principles were also used in part by Poincaré and Lorentz. Unfortunately, they did not realize that the theories were sufficient to establish a closed theory without the use of an ether or any assumed properties of matter. But this is exactly one of the most important conclusions of Einstein:

"The introduction of a" light ether "will prove to be superfluous as, according to the conception to be developed, neither an" absolute space "equipped with special properties is introduced, nor is a speed vector assigned to a point in empty space in which electromagnetic processes take place. "

In SRT, length contraction and time dilation are a consequence of the properties of space and time and not of material standards and clocks. The symmetry of these effects is therefore no coincidence, but a consequence of the equivalence of the observers, which is the basis of the theory as a principle of relativity. All quantities of the theory are experimentally accessible. Based on these principles, Einstein was then able to derive the equivalence of mass and energy . Hermann Minkowski's (1907) elaboration of Poincaré's (1906) idea of ​​a four-dimensional space-time continuum formed a considerable extension of the theory . All of this later resulted in the general theory of relativity with the inclusion of further principles .

Science historians such as Robert Rynasiewicz or Jürgen Renn are also of the opinion that considerations on quantum theory (as introduced by Planck (1900) and Einstein (1905)) also played a role in the rejection of the ether. These possible connections between Einstein's 1905 work ( Annus mirabilis ) regarding electrodynamics of moving bodies and the light quantum hypothesis were described by Renn as follows:

"S. 179: Einstein's considerations on the light quantum hypothesis also had, conversely, far-reaching consequences for his work on the electrodynamics of moving bodies, because it transformed his original experimental considerations on the abolition of ether into an inevitable prerequisite for his further research. "

This interpretation was based on analyzes of the 1905 work, from Einstein's letters, as well as a work from 1909. According to this assumption, several hypotheses decisively shaped by Einstein in 1905 (light quanta, equivalence of mass-energy, relativity principle, light constancy etc.) are mutually exclusive influenced and had the following consequences: that rays and fields can exist as independent objects, that there is no aether at rest, and that certain radiation phenomena favor an etherless corpuscle theory (which is also wrong on its own), while others spoke for the wave theory - whereby the SRT is compatible with both the wave and the particle concept.

Gravitational ether

Pressure theories

Aether was also used in an attempt to explain the law of gravitation by using basic mechanical processes such as B. Explain shocks without having to resort to the concept of action at a distance.

Nicolas Fatio de Duillier (1690) and Georges-Louis Le Sage (1748) proposed a corpuscle model with Le Sage gravity and used a shielding or shading mechanism. A similar model was developed by Hendrik Antoon Lorentz , who used electromagnetic rays instead of corpuscles. René Descartes (1644) and Christiaan Huygens (1690) used aether vortices to explain gravitation. Robert Hooke (1671) and James Challis (1869) assumed that every body emits waves in all directions and that these waves attract the other bodies. Isaac Newton (1675) and Bernhard Riemann (1853) suggested etheric currents which flow in the direction of the body and carry away the other bodies. Again Newton (1717) and Leonhard Euler (1760) proposed a model in which the ether in the vicinity of bodies loses density, which should lead to an attraction between them. William Thomson, 1st Baron Kelvin (1871) and Carl Anton Bjerknes (1871) designed a model in which each body sets the surrounding ether in pulsation, and tried to explain the electrical charges. These models could not prevail and are no longer regarded as useful explanations of gravity even today.

Einstein's new definition of the ether

The current standard model for describing gravitation without action at a distance is the General Theory of Relativity (ART) completed by Einstein in 1915 . In a letter to Einstein (1916) Lorentz now suspected that in this theory the ether was basically reintroduced. In his answer Einstein wrote that one could definitely speak of a “new ether”, but the concept of movement should not be applied to it. He continued this line of thought in several semi-popular works (1918, 1920, 1924, 1930).

In 1920 he wrote in his work "Aether and Relativity Theory" that the special theory of relativity does not necessarily exclude aether, since one has to ascribe physical qualities to space in order to explain effects such as rotation and acceleration. And in the general theory of relativity, space cannot be thought of without gravitational potential, which is why one can speak of a "gravitational ether" in the sense of an "ether of general relativity". This is fundamentally different from all mechanical ether models or Lorentz's ether, since (as already mentioned in the letter to Lorentz) the concept of movement cannot be applied to it:

“In the meantime, a closer reflection teaches that this denial of the ether is not necessarily required by the special principle of relativity. [...] According to the general theory of relativity, space is endowed with physical qualities; there is an ether in this sense. According to the general theory of relativity, a space without ether is unthinkable; for in such a place there would not only be no light propagation, but also no possibility of the existence of scales and clocks, thus also no spatial-temporal distances in the sense of physics. This ether, however, must not be thought of as being endowed with the property characteristic of ponderable media, namely that it consists of parts that can be traced through time; the concept of movement must not be applied to him. "

And in 1924, in his work “About the Aether”, Einstein used the term aether for every object with physical properties that existed outside of matter. Newton's absolute space is the “ether of mechanics”, which was later followed by the “ether of electrodynamics” by Maxwell and Lorentz with its absolute state of motion. The special theory of relativity also uses an “ether of electrodynamics”, but in contrast to Newton's absolute space or the classical light ether, there is no longer a preferred state of motion in this aether - however, a preferred state of acceleration must still be spoken of. The aether of the SRT, like the aether of electrodynamics, is to be designated as absolute, since the spatio-temporal or relativistic effects occurring in it are not co-determined by matter. This "absolute ether" was only abolished by the "ether of the general theory of relativity", where its properties are co-determined by matter:

"Even according to the special theory of relativity, the ether was absolute, because its influence on inertia and light propagation was thought to be independent of physical influences of any kind. [...] The ether of the general theory of relativity differs from that of classical mechanics, respectively. of the special theory of relativity by the fact that it is not 'absolute', but rather its locally variable properties are determined by ponderable matter. "

He finally summarized his new definition of the "ether" again:

“But even when these possibilities mature into real theories, we become of the ether, i. H. the continuum, endowed with physical properties, in which theoretical physics cannot do without; for the general theory of relativity, to whose fundamental point of view the physicists will probably always adhere, excludes an unmediated action at a distance; however, every theory of close effects presupposes continuous fields, thus also the existence of an 'ether'. "

The agreement of this relativistic ether concept with the classical ether models consisted only in the presence of physical properties in space. Therefore (e.g. according to John Stachel ) the assumption that Einstein's new concept of aether stands in contradiction to his previous rejection of the aether has to be denied. Because as Einstein himself explained, as required by the SRT, one can still not speak of a material ether in the sense of Newtonian physics, and the concept of movement cannot be applied to it either. Now this correspondence with the classical ether is too little for this new concept of ether to be able to establish itself in the professional world. Even in the context of ART, it is not used to this day.

German physics

The ether concept was later used in the context of the so-called German physics or misused for ideological reasons, as it was finally represented by the National Socialists. A mechanical and, above all, descriptive justification of physics was required here. Philipp Lenard already spoke (1923) of the ether, which is carried by the earth, and of the "primordial ether", which is not influenced by the movement of the earth. Lenard believed that he could explain both the (apparent) principle of relativity and gravity.

This theory could not even establish itself in circles of German physics, which was particularly expressed in the Munich Religious Discussion (1940), in which a certain approximation to the relativity and quantum theory was achieved.

Ether and modern physics

In addition to the aforementioned approaches by Einstein with regard to GTR, other physicists also tried to transfer the concept of ether into modern physics, for example Herbert E. Ives formulated a Lorentzian interpretation of the SRT. For some time Paul Dirac interpreted the Dirac lake postulated by him as a quantum mechanical ether. None of these formulations could prevail.

There are phenomena that are still seen by some physicists as analogies to the concept of ether. In his Nobel Prize lecture (2006), George F. Smoot mentioned that the frame of reference in which the cosmic microwave radiation is isotropic could be called ether (“New Ether Drift Experiments”). Smoot made it clear that there is no contradiction here to the SRT and the Michelson-Morley experiment, since this reference system is preferred only to simplify the description of the expansion of the universe. Opinions outside of the scientific mainstream continue to be represented by Nobel Prize winners Robert B. Laughlin and Frank Wilczek , according to which  one can speak of an ether in modern physics - especially with regard to the quantum vacuum .

Because the existence of the ether has been considered a scientific error for decades , it is hardly or not at all mentioned in most modern textbooks. In exceptional cases, the current doctrinal opinion at universities is expressed fairly clearly. Examples of this are the many statements and remarks in " Gerthsen ", a widely used German textbook on physics, also in the 2006 edition. In addition, there are still voices that support the theory of relativity or the rejection of the state of motion Reject ether, but these opinions no longer play a role in the professional world, see criticism of the theory of relativity .

Further etheric terms

Aether wave sign above the old main entrance of the former transmitter building of the Flensburg transmitter


  • Franz Exner : Lectures on the physical basics of the natural sciences . 1st edition. F. Deuticke, Vienna 1919 (contains 22 fully developed lectures on the ether of physics ).
  • Edmund Taylor Whittaker : A History of the Theories of Aether and Electricity . 1st edition. Longman, Green and Co., Dublin 1910 ( ).
  • Olivier Darrigol: Electrodynamics from Ampère to Einstein . Clarendon Press, Oxford 2000, ISBN 0-19-850594-9 .
  • Michel Janssen, John Stachel: The Optics and Electrodynamics of Moving Bodies . In: Max Planck Institute . 2004.
  • Kenneth F. Schaffner: Nineteenth-century aether theories , Oxford: Pergamon Press, 1972. (contains reproductions of several original works by famous physicists)
  • Max Born : Einstein's Theory of Relativity . Springer, Berlin / Heidelberg / New York 2003, ISBN 3-540-00470-X .
  • James Clerk Maxwell: Ether . In: Encyclopædia Britannica Ninth Edition . 8, 1878, pp. 568-572.
  • Walter Ritz : About the role of the ether in physics . In: Scientia 1908, No. VI: "Du rôle de l'éther en physique"
  • Albert Einstein : About the developments in our views on the nature and constitution of radiation (PDF; 2.3 MB) - main lecture on September 21, 1909 before the “Annual Meeting of German Natural Scientists and Doctors” in Salzburg - with many statements about the ether hypothesis as a overcome point of view . In: Physikalische Zeitschrift . 10, No. 22, 1909, pp. 817-825.
  • Gernot Böhme with Hartmut Böhme : Fire, water, earth, air: a cultural history of the elements . CH Beck, Munich 1996, (Taschenbuch 2004) - cultural history of the ether included (section: ether and light in modern physics , p. 158 ff).

References and comments

  1. Norman Sieroka : Philosophy of Physics . In: Philosophy Introductions. CH Beck knowledge . Munich 2014, ISBN 978-3-406-66794-7 , pp. 19 .
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  3. Hooke considered light to be a phenomenon of rapidly oscillating motion that “moves in all directions through a homogeneous medium, in the form of direct or straight lines that extend in all directions, like rays from the center of a sphere […] Each pulse each oscillation of the light body produces a ball which is over and over but greater infinitely volatile by the same principle as the waves or rings swell on the surface, in ever increasing circles about a point on it. "Quoted in the Times of 15. September 1893
  4. ^ Robert Hooke : Micrographia . 1665.
  5. Christiaan Huygens : Treatise on light (=  Ostwald's classic of the exact sciences . No. 20 ). 4th edition. Thun, 1996, ISBN 3-8171-3020-1 (French: Traité de la lvmière . Leide 1690. Translated by Rudolf Mewes, written around 1678, reprint of the 1885 edition).
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  9. See also the discussion in: The quasi-elastic body as an ether model - Section 15 of Chapter III. In: Arnold Sommerfeld : Mechanics of deformable media. 5th edition, edited and supplemented by Erwin Fues and Ekkehart Kröner. Geest & Portig, Leipzig 1964. (Lectures on theoretical physics; Volume 2, Ed. 5) p. 96 ff
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  11. Ether . In: Encyclopædia Britannica , Ninth Edition. on Wikisource . All of the original text of Maxwell's entry
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  19. a b Born, pp. 166-172 (literature).
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  41. Jürgen Renn: On the shoulders of giants and dwarfs. Einstein's unfinished revolution . Wiley-VCH, Weinheim 2006, ISBN 3-527-40595-X .
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  55. ^ The conversations between the philosophers of science from the Hugo Dingler circle and physicists invited by Wolfgang Finkelnburg (including Hans Kopfermann , Otto Scherzer , Carl Friedrich von Weizsäcker , Otto Heckmann , Georg Joos ) took place on November 15, 1940 in the Munich Medical Center, and were continued in Seefeld in Tirol in November 1942, see also Heisenbergs Krieg by Thomas Powers, in: Hoffmann and Campe 1993, p. 439. Finkelnburg himself describes the processes in a manuscript from 1946 The fight against party physics , from which one A copy found in Heisenberg's estate, reprinted in Physics and National Socialism by Klaus Hentschel (Ed.), In: Birkhäuser 1996, p. 339.
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  57. PAM Dirac: Is there an Aether? In: Nature. 168, 1951, pp. 906-907.
  58. ^ PAM Dirac: The position of the ether in the physics . In: Naturwissenschaftliche Rundschau . 6, 1953, pp. 441-446.
    PAM Dirac: Quantum mechanics and the aether . In: The Scientific Monthly . 78, 1954, pp. 142-146.
  59. ^ GF Smoot (2006): Cosmic Microwave Background Radiation Anisotropies: Their Discovery and Utilization (Nobel Prize Speech).
  60. Robert B. Laughlin: Farewell to the world formula , Chapter 10: The fabric of spacetime. , Piper Verlag, 2007, ISBN 978-3-492-04718-0 , pp. 184–192 (For quotes from the book, see Ether on Wikiquote or Robert B. Laughlin on Wikiquote ).
  61. ^ Frank Wilczek : Lightness of being: mass, ether, and the unification of forces. Basic books, New York 2008, ISBN 978-0-465-00321-1 , pp. 73-111 (Chapter 8: The Grid (Persistence of Ether) ), 228 (Glossary). For quotes from the book see Aether on Wikiquote .
  62. Reiner Ruffing: Small Lexicon of Scientific Errors , Gütersloher Verlagshaus 2011, ISBN 978-3-579-06566-3 , pp. 29–31
  63. In the 23rd edition (2006) of Gerthsen Physik see in particular pages 127, 177, 297, 447, 504, 519, 609, 677, 669, 913, 914


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Web links

Wikiquote: Ether  - Quotes
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