In physics, the term “aether” has encompassed various different notions over time. Physicists have considered different aethers as “subtle substances distinct from matter and capable of providing or transmitting effects between bodies.” These diverse effects include the trajectories of planets (according to Descartes), the transmission of gravitational force (Isaac Newton), the transportation of light (from Descartes and Robert Hooke to Newton and others until the early 20th century), the conveyance of electric and magnetic force, and later, electromagnetism, even involving the creation of electric charge in certain bodies. Additionally, it involves the creation of a repulsive force around bodies, counteracting gravity (as explained by Pierre-Simon de Laplace in the study of gaseous phenomena).
The theoretical study of the luminiferous aether (light transmitter) led to the development of the concept of elasticity. It allowed for the prediction of certain experimental results by considering light as a wave transmitted by an aether viewed as a fluid with various properties (explored by Christian Huyghens, and Augustin Fresnel, among others). This medium, not directly accessible to experimentation, also served as a platform to compare inductive and deductive methods (a debate particularly active between John Stuart Mill and William Whewell).
Once James Clerk Maxwell‘s equations were established, multiple attempts were made to formulate a theory of an aether that mechanically transports these electromagnetic waves (with Maxwell being the first to attempt this). However, none of these attempts succeeded in accounting for all the properties of electromagnetism, even in the simplest cases. The experiments of Michelson and Morley on the optics of moving bodies led to abstract considerations in such theories (explored by Hendrik Lorentz, and Joseph Larmor). The advent of special relativity put an end to these endeavors, relegating questions about the aether to general considerations, particularly regarding fluctuations in the quantum vacuum.
Aether in Antiquity
Originally, Aether was a primordial god in Greek mythology, personifying the upper parts of the sky and its brilliance. This concept has persisted in classical poetic language, where “aether” is used to describe a pure sky. Empedocles, who introduced the classical theory of four elements, frequently mentions the ether as a distinct entity. Plato, in Timaeus (58d), refers to aether as “the purest form of air.” Aristotle, in his treatise “On the Heavens,” introduces a new element existing only in the celestial sphere, which moves in a circular motion without needing an external force.
“It is absolutely necessary that there exists a simple body whose nature is to move in circular translation, in accordance with its own nature… Beyond the bodies that surround us here below, there exists another body, separated from them, and possessing a nature more noble the more it is distant from those in our world.” — (Aristotle, On the Heavens, I, 2).
This innovation, causing some confusion, eventually became associated with aether, although Aristotle did not use the word to describe it. As the “most noble” element, the celestial element is never referred to by Aristotle as the fifth. During the Hellenistic period, when Aristotle’s texts were more or less withdrawn from circulation, different interpretations merged what he termed “the first body” with aether and also with the substance of the soul. In the early centuries, a sort of consensus confirmed the confusion. For example, according to Sextus Empiricus, the notion of aether dates back to the Pythagorean Ocellus (or possibly Philolaus):
“Ocellus of Lucania and Aristotle, in addition to the four elements, added a fifth body, endowed with circular motion, which they think is the matter of celestial bodies.” — (Sextus Empiricus, Against the Mathematicians, X, 316).
Pseudo-Plutarch gives a similarly approximate version:
“Aristotle holds that the supreme god is a separate form, based on the roundness and sphere of the universe, which is an aethereal and celestial body. He calls it the fifth body: and this celestial body, being divided into several spheres of coherent natures and separated only by intelligence, he considers each of these spheres to be an animal composed of body and soul, of which the body is ethereal, moving in a circular manner, and the immobile rational soul is the cause of movement, according to action.”
The ideas of the Stoics strongly influenced the understanding of aether, and they had a considerable impact on Latin expositions. Cicero (Tusculan Disputations, I, 10) adds that Aristotle argues that the soul “originates” from this fifth element, also called quintessence. The Stoic Cleanthes considers aether to be the supreme god.
A synoptic image of the prevailing confusion has been depicted by André-Jean Festugière: “In the oldest authors, ‘aether’ refers to the sky (Homer, The Iliad, 412; Hesiod, Works and Days, 18)… The word ‘aether’ had already been used by Empedocles, but to denote atmospheric air, as opposed to fog… Anaxagoras was the first to distinguish between air and aether, but what he referred to as aether was fire (fragments 59 A 43, 59 A 73)… From Plato’s Phaedo onward, the space between the air and the fixed sky (the region of fire) becomes aether, the abode of the astral gods. Plato attributes to aether its specific characteristic of being always in motion.
Aether is considered a kind of air, the purest kind. Plato distinguishes three kinds of aether: upper air, atmospheric air, foggy air… It is with the Epinomis and Aristotle’s On Philosophy [an early work], two contemporary works [around 350 BCE], that we see the emergence of the notion of aether as the fifth body. Epinomis mentions aether as the fifth body for the first time (981c6), as a kind of subtler and purer air: aether is not the abode of the stars (that is fire), but, like air, it is the dwelling place of translucent demonic beings, who serve as intermediaries between humans and the visible gods, the stars.
Fragments from Aristotle’s On Philosophy show that the notion of aether as the fifth body holds a significant place. The Ancients unanimously regarded Aristotle as the inventor of the doctrine of aether as the fifth element. Aristotle always follows aether, fire, air, water, and earth, and this order prevails, with aether (not fire) considered the material of the stars and the element where they reside. The soul is in perpetual motion because it is drawn from the aether that is always in motion. Finally, this Aristotelian aether is warmth, the principle of warmth, and thus of life.”
Recent Pythagoreans, in the Pythagorean Memorabilia (3rd century BCE), seem to admit three aethers: 1) the hot (solar, astral, and divine fire), 2) the cold (air), and 3) the dense (water, serum, liquid, blood…), and two kinds of souls: 1) a soul made of warm aether, intellect (corresponding to animal life), and 2) a soul made of a mixture of two aethers, warm and cold, vapor, the vegetative soul (corresponding to non-living, that is, non-sentient and non-mobile).
In the late 1st century, Xenarchus wrote a treatise Against the Fifth Element, the knowledge of which becomes essential in any discussion of aether. Cesare Cremonini wrote a refutation in 1616, and Galileo mentioned it as well. Meanwhile, aether or quintessence became an important concept for alchemists. They interpreted the incorruptible element defined by Aristotle through its absence of a counterpart. Discussions about it continued until the 19th century, particularly among metaphysicians.
Gravitational Aether in Pre-Relativistic Physics
René Descartes developed vortex mechanics to explain that the movement of planets is due to large vortices of aether (a subtle substance composed of tiny transparent globules) filling space and carrying and maintaining them on their trajectories. This qualitative physics justified planetary motion mechanistically, refuting the existence of the void, which he regarded as nothingness. This same aether was supposed to instantly transmit light in the form of pressure.
After rejecting Descartes’ vortex theory around 1680, Isaac Newton formulated his theory of universal gravitation, where the gravitational force is transmitted instantaneously from one body to another, over any distance and through space, whether empty or not.
Newton, while satisfied with the effectiveness of his theory, was not content with a situation where a force is transmitted through a vacuum. In a letter to Richard Bentley in 1692, Newton stated, “That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it.”
Thus, in the General Scholium of Book III of the Principia, he conceives a “kind of extremely subtle spirit that permeates all solid bodies,” adding that “it is by the force and action of this spirit that the particles of bodies attract each other”: a mechanical aether filling space and enabling the transmission of gravitational force.
This aether mediates gravitational force but is not subject to it and appears exempt from the characteristics and physical principles stated in the Principia. Newton supported this view based on theological considerations, stating that space is the sensorium Dei, a sort of sensory organ of God that allows him to transmit influences from one body to another. This aether has always remained an underlying hypothesis, not intervening in calculations and having the status of a reassuring assumption regarding the coherence of the theory. For Newton, this aether was the same as the one transmitting light, considered to be composed of particles of different sizes transmitting oscillations to the aether, creating colors.
Luminiferous Aether in Pre-Relativistic Physics
Until the advent of special relativity, physicists developed theories of luminiferous aether, a medium that scatters light and is considered a wave. The challenge was to create a coherent theory accounting for all observations made on light, while no experiment highlighted the properties of the aether considered as a fluid or physical medium.
René Descartes (who did not clearly formulate the notion of waves), Robert Hooke, and Christian Huygens assumed that, like sound in air or waves on the surface of a liquid medium, light propagated through a fluid: the aether. The aether, subtle and undetectable as it did not impede any bodies, was supposed to fill the universe, as the light from the stars reaches us.
Newton considered an aether that met the stringent requirements of transmitting gravitational force: exempt from being subject to the same principles as ordinary matter, endowed with an active role assimilated to God’s intervention in the natural world. This aether was sometimes equated with Hermann Boerhaave’s fire in the early 18th century, an imponderable substance penetrating space and bodies, with a repulsive power opposed to the attractive power of bodies.
In the 18th century, theories of subtle aether were developed, explaining electrical and magnetic phenomena, optics, as well as heat and chemistry, often modeling them on Newton’s and Boerhaave’s ideas. Benjamin Franklin explained the electrification and electrical discharge of bodies by the presence of an aether composed of particles attracted to bodies but repelling each other. For John Canton, the aether was the air itself. For André-Marie Ampère, a universal imponderable aether composed of two oppositely charged electricities explained the ponderomotive force between electrical circuits.
In 1801, Thomas Young developed the interpretation of light as the vibration of an aether to account for phenomena related to diffraction using the interference of waves. However, his model did not explain polarization, and this imponderable aether (like Benjamin Thompson’s for heat and Humphry Davy’s for electrochemistry) did not attract much interest from scientists at a time when many properties were explained by attractive forces between particles of matter (capillarity, solid cohesion, chemical reactions, etc.). Pierre-Simon de Laplace hypothesized an aether called caloric, producing a repulsive force between particles of matter, allowing the corpuscular theory of light to be consistent with Huygens’ double refraction.
From 1830 onwards, Augustin Fresnel’s theory, stating that light is a ripple of an aether, prevailed. To account for polarization, which Young’s theory could not explain, he had to consider the aether as solid and elastic. This model allowed for the prediction of several unexpected effects (such as circular polarization and conical refraction). The study of solid and elastic aethers, whose vibrations form light, became a research theme until the end of the 19th century. Augustin-Louis Cauchy, studying elasticity, found an expression for the speed of propagation of transverse waves (light being a transverse wave in Fresnel’s theory), and James MacCullagh derived the laws of crystal optics from a Lagrange function of the aether.
This elastic body had strange properties: it had to be almost infinitely rigid to transmit light from distant stars, while offering zero resistance to the movement of material objects (since the Earth orbits the Sun without being slowed down). George Gabriel Stokes showed that it was sufficient for the aether to have a slight viscosity to allow bodies to pass through it slowly, and he thus rediscovered the partial entrainment coefficient of the aether by refractive bodies that Fresnel had already proposed to explain aberration. In 1851, Hippolyte Fizeau experimentally verified the value of this coefficient for moving water.
Inspired by magnetism, Hermann Helmholtz and William Thomson proposed aethers with rotational movements. For his work unifying electricity and magnetism, James Clerk Maxwell relied on Michael Faraday’s idea of a force field to eliminate the notion of action at a distance from these domains. The field or lines of force, is a kind of spatial distribution of the influence of a body, awaiting the presence of another body to influence it and changing at a finite speed without a material hypothesis about the aether filling geometric space and carrying this field. Maxwell then proposed an aether model that he wanted compatible with his theory of electromagnetism, especially the electromagnetic waves he had highlighted in his equations and identified with light: this aether was composed of “molecular vortices surrounded by free wheels, whose movement was analogous to electric current.”
Comparing Maxwell’s and MacCullagh’s aethers, George Francis Fitzgerald showed their analogies and refined their properties, making Maxwell’s aether a serious competitor to the solid and elastic luminiferous aether. Heinrich Hertz’s experiments confirmed that Maxwell’s electromagnetic waves were similar in many respects to light waves, and therefore bolstered confidence in Maxwell’s aether. However, it was never successful enough to account for all the properties of optics and electromagnetism, “despite the considerable efforts of scientists.”
In 1887, the result of the Michelson-Morley experiment on the optics of moving bodies contradicted the predictions of all aether theories. Hendrik Lorentz and Joseph Larmor, each in their own way, tried to conceive more abstract aether theories to account for these results. In the early 20th century, physicists’ opinions were divided, with some questioning the existence of the aether, while others—far more numerous—were confident in its reality.
“We care little whether the aether exists in reality; that is the concern of metaphysicians. The essential for us is that everything happens as if it exists, and that this hypothesis is convenient for explaining phenomena. After all, do we have any other reason to believe in the existence of material objects? This is also just a convenient hypothesis; only it will never cease to be so, whereas a day will undoubtedly come when the aether will be rejected as unnecessary.” — Henri Poincaré, Science and Hypothesis (Chapter 12)
Aether After 1905
In 1905, Albert Einstein proposed his theory of special relativity, where the aether is absent, and the speed of light is the same for all inertial frames, stating that there was no need for the concept of aether in physics.
1905: Einstein Denies the Existence of Aether
Before the publication of his early works, Einstein studied Lorentz’s theory of aether through the books of Paul Drude, giving it particular attention. This luminiferous aether remains in absolute rest and constitutes a preferential frame in which electromagnetic phenomena occur, and in which light has a constant speed. Soon, he rejects this concept of a preferential frame because he deems it introduces an unacceptable asymmetry between the laws of mechanics, which do not depend on a frame of reference, and the theory of electromagnetism.
In 1905, Einstein proposed the theory of special relativity, postulating the total equivalence of the laws of physics, including electromagnetics, regardless of the frame of reference. This implies the constancy of the speed of light, regardless of the frame of reference, and renders the concept of aether meaningless. From this moment until 1916, Einstein denied any reality to the concept of aether. However, convincing physicists of the absence of aether proves challenging, especially Lorentz, who remains unconvinced. According to Kostro, it was Lorentz’s insistence that led Einstein to a new position from 1916 onwards. In the meantime, facing opposition in 1909, he attempted to justify the absence of aether with a new argument using a wave-particle duality of light, for which he proposes an improved theory: for him, the energy and momentum of light are carried by an autonomous quantum that needs no support or aether.
In 1913, Einstein developed the general theory of relativity. Initially, he finds additional reasons in this theory to completely abandon the aether; he writes to Ernst Mach, “It becomes absurd to attribute physical attributes to space,” and the arbitrary way in which space and time variables are chosen “strips space of the last vestiges of reality.” However, Lorentz’s conception of aether, as presented by Drude, precisely implied attributing physical characteristics, and thus a certain “reality,” to space. For Einstein, the case for the aether is settled.
1916: Einstein Admits the Existence of a Certain Form of Ether
However, this position changed from 1916 onwards, influenced by a correspondence with Lorentz and controversies with the German physicist Philipp Lenard. In June 1916, Lorentz sends a long letter to Einstein congratulating him on the discovery of the general theory of relativity, for which he shows great enthusiasm, and tries to demonstrate that this theory can be reconciled with the concept of a stationary aether.
Einstein quickly responds in detail to Lorentz’s arguments, acknowledging for the first time the possibility of introducing a new concept of aether. However, he vigorously rejects the stationary nature of the aether defended by Lorentz, i.e., the conception of a rigid medium with its own reference frame in which it is at rest, as this contradicts the principle of relativity. Instead, he admits the possibility of an aether that is not a medium with a state of motion and therefore does not violate the principle of relativity. This “new aether” would have a state that determines the motion of physical objects, whose metric behavior would be described by the tensor. However, Einstein did not consider these ideas mature enough and published nothing regarding this “new aether” for over two years.
In July 1917, Lenard published an article titled “Principle of Relativity, Aether, Gravitation,” attempting to show that the general theory of relativity has recycled the concept of aether by renaming it “space” and that this theory does not stand without the concept of aether. In response to this article, Einstein published in November 1918 his first article explaining his new positions regarding the aether: “Dialogue Concerning the Accusations Against the Theory of Relativity.” In this response, he concedes to Lenard that the general theory of relativity implies attributing physical properties to space. However, he denies that this signifies a return to Lorentz’s aether, possessing a defined state of motion.
Hermann Weyl summarized in 1922 the fundamental difference between Einstein’s “new aether” and that defended by Lenard and why the “aether” of general relativity is not the same as Lorentz’s: “The old aether of the theory of light was a substantial medium, a three-dimensional continuum, with each point P at any time t at a defined location p in space; the ability to distinguish and track the evolution in space of a certain point in the aether over time is a fundamental point,” … “this aether is now and forever rigid and is not influenced by matter.” The aether of general relativity is a medium but not substantial, lacking “points” whose movement in space can be tracked. It endows space with a “field of states,” possessing physical reality, interacting with matter, and being influenced by it.
1920: Einstein’s “New Aether,” the Leyden Discourse
However, this was not enough to calm the vehement anti-Einstein campaigns, led notably by Lenard and Ernst Gehrcke, which reached their peak during the year 1920. Far from appeasing criticisms against general relativity and the principle of relativity, Einstein’s acceptance of a certain form of aether seems to provide fodder for his adversaries. Pressed by these attacks and encouraged by Lorentz, Einstein decided to officially communicate about his “new aether” during his inaugural address at the University of Leyden on October 17, 1920, titled “Aether and the Theory of Relativity,” which constitutes his first major work on aether.
In this discourse, Einstein begins by presenting the historical reasons that led physicists to imagine the existence of aether, which, according to him, are twofold: the problem of action at a distance and the discovery of the wave properties of light. The problem of action at a distance emerged with Newton’s theory of gravitation, where the inevitable question arises of whether attractive forces propagate instantaneously and at a distance, without a transport medium, or instead gradually through a medium. The second hypothesis implies the existence of an aether.
On the other hand, the development of the theory of electromagnetism by Maxwell and Lorentz also led to imagining the existence of an aether, but no mechanical model proves consistent with experience. Einstein then describes Lorentz’s work on aether, one of the only aether theories compatible with experience. In this theory, the aether lacks any mechanical property and, present both in matter and in a vacuum, is a mere support for electromagnetic waves. On the other hand, matter is devoid of any electromagnetic property, playing a role in electromagnetism solely because matter particles can possess an electric charge, which is the only thing capable of moving. But Einstein points out that Lorentz strips the aether of all mechanical properties except one: its immobility.
In the second part of the discourse, Einstein then shows that the theory of special relativity removes this last mechanical property of the aether, completing, according to him, the movement initiated by Lorentz. According to this theory, the laws of physics (especially the laws of electromagnetism) are identical in all inertial reference frames moving rectilinearly and uniformly relative to each other. There is, therefore, no physical reason to distinguish a particular reference frame in which the aether is stationary, introducing an asymmetry not justified or detected by any physical experiment. But Einstein admits that he was wrong to conclude the non-existence of the aether:
“But careful consideration teaches us, nevertheless, that the principle of special relativity does not imply denying the existence of the aether. We could admit the existence of an aether; only we must refrain from attributing a state of motion to it, that is, we must abstain from the last mechanical attribute that Lorentz left it.”
Einstein then explains, in the third part of the discourse, that the idea of an aether can return to attribute physical properties (other than mechanical or kinematic) to space, and that space — even devoid of matter — cannot be considered truly empty. Citing the example of Mach’s principle, which attributes inertial forces such as centrifugal force to distant masses, he states the need for a medium to transmit the gravitational interaction of these distant masses, while emphasizing a crucial difference between this medium and all aethers imagined until then:
“This conception of an aether, to which the Machian approach leads, differs in an essential aspect from the aethers of Newton, Fresnel, or Lorentz. Mach’s aether not only conditions the behavior of inert masses but is also conditioned, concerning its state, by them.”
In the last part of the discourse, Einstein explains how these Machian ideas contributed to the development of general relativity and how the notion of aether can evolve with this latter theory. He also describes the relations of this aether with gravitational and electromagnetic interactions. The state of the relativistic aether is entirely determined at each point by its local interaction with matter and with the immediately adjacent points of the aether, following the principle of locality cherished by Einstein. The state of Lorentz’s aether, on the contrary, is defined only by itself and — in the absence of an electromagnetic field — is the same everywhere.
Einstein insists that one cannot imagine a region of space devoid of gravitational potential because it is this potential that locally defines the metric of any region of space according to the theory of general relativity. However, according to Einstein, a region of space can be conceived entirely without an electromagnetic field, and electromagnetism therefore only maintains a secondary, non-fundamental relationship with the aether insofar as (according to Einstein’s theories and reflections at that time on the unification of electromagnetic and gravitational forces) matter particles influencing the relativistic ether are considered condensations of the electromagnetic field.
Einstein concludes his exposition on the aether with the following summary:
“In summary, according to the theory of general relativity, space is endowed with physical properties, and in this sense, therefore, there exists an aether. According to the theory of general relativity, a space without aether is inconceivable because, in such a space, not only would there be no propagation of light but also no possibility of existence for a standard space and time (measured by rules and clocks), and therefore no space-time intervals in the physical sense of the term. However, this aether cannot be conceived as endowed with the qualities of ponderable media and as consisting of parts with a trajectory in time. The idea of motion cannot be applied to it.”
Aether in Contemporary Physics
In the 21st century, the perplexing properties or characteristics attributed by contemporary physics to the vacuum (Higgs field, vacuum energy, dark energy) strangely resemble the mysterious properties of aether. However, physicists emphasize that this does not mean a return to hypotheses from before 1905.