The preprint is available on SSRN: The Aether Is Radiation: Absolute Time and the End of Propagation (October 02, 2026). http://dx.doi.org/10.2139/ssrn.7553138

 

 

 

The Aether Is Radiation:

Absolute Time and the End of Propagation

 

 

Juliet Zhong

Independent Researcher | London, United Kingdom | October 2026

ORCID: 0009-0006-5099-3671

Abstract

The aether of modern optics was a set of functions assigned to one name: the medium through which light propagates, the rest frame against which the Earth's motion could be measured, and the bearer of absolute space and time. Behind all three stood one assignment: light is a thing that propagates through or within something. Newton's 1675 Hypothesis already makes light something of a different kind from the aether, propagated from lucid bodies, and derives all things from the aether by condensation. The paper traces the assignment through Fresnel, Maxwell and Lorentz, and shows that Einstein's removal of the aether in 1905 retained it as the propagation of light at the invariant speed c, which no later ether removed. With earlier results that c is a constant of the registering material domain rather than the speed of a propagating entity, and that the radiation domain has neither relative time nor space, the paper derives that radiation in its own domain does not propagate: radiation is not in the aether; radiation is the aether, and both are Newton's absolute time. Newton could not join his aether and his absolute time because he possessed no distinction between dimensional levels and no structure of spacetime. Three consequences follow. No medium of propagation is required; no aether wind and no preferred frame of light propagation exist to be measured; and absolute time requires no bearer, because absolute time, the aether and radiation are one thing under three names. Three failure conditions are stated.

 

Keywords: aether; luminiferous ether; Michelson–Morley experiment; Lorentz ether theory; absolute time; speed of light; radiation domain; history of physics; foundations of relativity

 

 

1. Two Aethers in Newton and the Optical Aether Tradition

The word aether is older than optics. Greek aithēr named the bright upper air in which the gods dwelt, and Aristotle made it the fifth element, the substance of the celestial spheres, which alone among the elements moves naturally in circles and is subject to neither generation nor decay (Aristotle, De Caelo I.2–3). Descartes denied the void altogether and filled the heavens with a subtle matter whose vortices carried the planets around the Sun (Descartes 1644). Neither conception was an aether of light. Aristotle's aether was a celestial body with its own natural motion, and Descartes' subtle matter was matter, distinguished from ordinary bodies by the fineness of its parts; both were refuted on their own terms, Aristotle's by the comets and novae of the sixteenth century and Descartes' vortices by Newton's demonstration in the second book of the Principia that a fluid vortex cannot reproduce Kepler's laws. The aether that concerns the foundations of physics is a later and narrower conception, formed in seventeenth-century optics, and its history is the history of one ontological assignment.

The assignment is already explicit in Newton. In December 1675 he sent to the Royal Society An Hypothesis explaining the Properties of Light, a text he presented as a hypothesis for the reader to accept or not, and which contains, side by side, two structures that later history did not keep together (Newton 1675/1959). The first is a theory of the aether as the source of bodies. Newton supposed an aethereal medium "much of the same constitution with air, but far rarer, subtler, and more strongly elastic", and went on to suggest that "perhaps the whole frame of nature may be nothing but various contextures of some certain aethereal spirits or vapours, condensed as it were by precipitation", concluding: "Thus perhaps may all things be originated from aether." The second is a theory of light, and it separates light from the aether in the plainest terms. Light, in Newton's hypothesis, "is neither aether, nor its vibrating motion, but something of a different kind propagated from lucid bodies"; the aether is excited by the rays and its vibrations in turn act on them, but the two are of different kinds.

The two structures are logically independent, and the remainder of the history can be read by following them separately. The first makes the aether the substrate from which gross matter is formed by condensation. The second makes light a thing of one kind and the aether a thing of another, related by mutual action. Newton did not identify the aether with light, and he did not identify it with the absolute time and absolute space of the Scholium in the Principia, which he defined without reference to any medium (Newton 1687/1999). Whatever is said below about the relation of these three, light, aether and absolute time, is the present paper's reading, not Newton's claim. What Newton's text establishes is that light, at the beginning of the modern conception, is already a thing propagated from lucid bodies through a medium, and that a quite different idea, that matter is condensed aether, is present beside it.

Newton returned to the aether in the Queries he added to the second English edition of the Opticks in 1717 (Newton 1730). Query 18 asks whether the heat of a warm room is not conveyed through the vacuum by the vibrations of a medium much subtler than air, which remains when the air is drawn out. Query 21 asks whether this medium is not much rarer within the dense bodies of the Sun, stars, planets and comets than in the empty celestial spaces between them, and whether, growing denser with distance from them, it does not cause bodies to move from the denser parts of the medium towards the rarer, which would be gravity. The same Query estimates the medium's properties. Since vibrations of the medium must outrun light, which moves above 700,000 times faster than sound, Newton concluded that the elastic force of the medium in proportion to its density must exceed that of air in proportion to its density by more than 700,000 × 700,000 times, and Query 22 adds that such a medium, more than 700,000 times as elastic and more than 700,000 times as rare as air, would resist the planets so little that their motions would scarcely alter sensibly in ten thousand years. The estimate is a historical record of what Newton required of the aether, a medium extreme in rarity and elasticity, and it is not reconstructed here by modern data. The order of magnitude it demands is nonetheless instructive: the energy density of the cosmic microwave background, the dominant radiation in the present universe, is about 4.2 × 10⁻¹⁴ J/m³, a mass equivalent of about 4.7 × 10⁻³¹ kg/m³, some thirty orders of magnitude rarer than air. The comparison is a numerical analogy and not a test of Newton's theory: radiation supplies a background as rarefied as anything his Query required, and rarer by far.

Query 21 has a second feature that bears on the structure examined below. Newton's aether is rarer inside and near bodies and denser away from them, and bodies are urged from the denser region towards the rarer. In structure this is a gradient of an ambient substrate, depleted by the presence of matter, producing a push towards the depletion. A previous proposal derived the inverse-square law from exactly this structure, with the ambient substrate taken to be radiation and the depletion taken to be the radiation shadow cast by matter, which on that account is itself condensed radiation (Zhong 2026a). The correspondence is structural and is recorded as such. Newton did not anticipate the proposal; the proposal shares with his Query the form of a push towards a depletion in an ambient field, and differs from it in what the field is and in why matter depletes it.

The propagation of light through the aether became the foundation of optics once light was understood as a wave, because a wave was then understood to be a motion of something. Huygens had already treated light as a pulse transmitted through an aethereal matter of hard elastic particles (Huygens 1690). Young's interference experiments, and Fresnel's demonstration that polarisation requires light to be a transverse vibration, made the medium more definite and more strange: transverse waves require shear rigidity, so the luminiferous aether had to be an elastic solid, rigid enough to carry vibrations at 3 × 10⁸ m/s and yet offering no detectable resistance to the planets (Young 1804; Whittaker 1951; Schaffner 1972). The medium's properties were inferred from the behaviour of light, and its existence was inferred from the fact that light was a wave. Neither inference was questioned, because the assignment behind them, light as a motion of something other than itself, was not seen as an assignment at all.

The assignment brought with it a second function. If light travels through a medium at a fixed speed relative to the medium, an observer moving through the medium should find the speed of light different in different directions, and the medium supplies a frame of rest against which that motion is defined. Arago's observation in 1810 that starlight refracted by a prism showed no effect of the Earth's motion led Fresnel, in a letter to Arago of 1818, to propose that a transparent body moving through the aether drags the aether within it partially, with the coefficient 1 − 1/n² for a body of refractive index n, which made all first-order effects of the Earth's motion on optical experiments undetectable (Fresnel 1818). Fizeau measured the speed of light in flowing water in 1851 and found the partial drag that Fresnel had predicted (Fizeau 1851). Airy filled a telescope with water in 1871 and found stellar aberration unchanged, as Fresnel's coefficient required (Airy 1871). Stokes proposed the opposite arrangement in 1845, an aether dragged completely by the Earth near its surface, and Lodge's experiments of 1893 with rapidly rotating steel discs, which produced no detectable drag on light passing between them, told against it (Stokes 1845; Lodge 1893). The two theories disagreed about how the aether moved and agreed that light moved through it. The aether had by then acquired the role it would keep for half a century: the stationary frame of absolute rest, which first-order experiments could not detect because the partial drag exactly compensated the effects that motion through it would otherwise produce.

Maxwell's theory gave the medium a third role and a definite number. In 1865 Maxwell showed that the equations of the electromagnetic field admit transverse waves propagating at 1/√(ε₀μ₀), a speed computed from electrical and magnetic constants measured on the bench, and that this speed agreed with the measured speed of light (Maxwell 1865). Light was an electromagnetic disturbance of the medium, and the electrical constants of the vacuum were properties of the medium. In his article "Ether" for the ninth edition of the Encyclopaedia Britannica, Maxwell wrote that whatever difficulties attend the conception of its constitution, there can be no doubt that the interplanetary and interstellar spaces are not empty but occupied by a material substance or body, which is certainly the largest and probably the most uniform body of which we have any knowledge (Maxwell 1878). Maxwell also observed that the Earth's motion through this body could in principle be detected by its second-order effect on the round-trip time of light, though he doubted that any terrestrial method was sensitive enough. The medium of propagation, the frame of rest and the bearer of the electromagnetic field had become one body.

The same decades revived Newton's other structure. In 1867 William Thomson proposed that atoms are vortex rings in a perfect fluid filling space, permanent because vortex motion in an ideal fluid can be neither created nor destroyed, and distinguished from one another by the knotting and linking of their rings (Thomson 1867). Matter, on this theory, is not a substance placed in the aether but a form of motion of the aether itself. Larmor took the idea further in Aether and Matter, where electrons are centres of rotational strain in the aether and ordinary matter is built from them, so that the aether is the only substance and matter a configuration of it (Larmor 1900). These were the most developed physical theories of the century in which the aether was made the source of bodies, and they placed the condensation of the aether into matter at the centre of physics, as Newton's Hypothesis had placed it at the margin. They also kept the propagation of light intact: in Thomson's and Larmor's theories light remained a wave travelling through the medium of which atoms were knots. The two structures Newton had set side by side were by then both in use, but their compatibility was not made a question. If matter is a configuration of the aether, and light is a disturbance propagated through the aether, then matter and light are both forms of one substrate; and the one thing that set light apart, its travelling through that substrate at a speed, was precisely what the century's experiments were about to make unintelligible.

Michelson undertook the measurement Maxwell had doubted. His interferometer compared the round-trip times of light along two perpendicular arms, and his paper of 1881 stated its premise without qualification: the aether is at rest, and the Earth moves through it (Michelson 1881). With Morley he repeated the experiment in 1887 with an arm length extended to about 11 metres by repeated reflection, floating the apparatus on mercury so that it could be rotated (Michelson and Morley 1887). The expected shift of the fringes on rotation, for the Earth's orbital speed of about 30 km/s, was about four-tenths of a fringe. The observed shift was a small fraction of that, consistent with zero, and the result was confirmed with increasing precision through the following decades; Kennedy and Thorndike later varied the arm lengths to test a different combination of the same effects and again found nothing (Kennedy and Thorndike 1932; Swenson 1972). Dayton Miller, repeating the experiment on Mount Wilson in the 1920s, reported a small periodic shift that he attributed to an aether wind; a later reanalysis of his data by Shankland and colleagues traced the effect to temperature variations across the apparatus, and no subsequent experiment has reproduced it (Shankland et al. 1955). The long search for the wind is itself evidence of how firmly the assignment was held: a null result was read, for four decades, as a wind not yet found, because light without a medium to travel through was not a conception available to be tested.

The theory that accommodated the null result was Lorentz's, and its structure must be stated accurately. FitzGerald and Lorentz each proposed, in 1889 and 1892, that a body moving through the aether is contracted along the direction of motion by the factor √(1 − v²/c²) (FitzGerald 1889; Lorentz 1892). In 1895 Lorentz introduced the local time, a modified time coordinate in which the equations of electrodynamics for a moving system take the same form as for a system at rest, to first order in v/c (Lorentz 1895). Larmor reached the full transformation, including the slowing of moving processes, by 1900 (Larmor 1900), and in 1904 Lorentz gave the transformation in the form that bears his name, valid for any velocity below that of light (Lorentz 1904). Poincaré named the transformations, showed that they form a group, and interpreted the local time as the time read by clocks synchronised by light signals exchanged under the assumption that light's speed is the same in both directions (Poincaré 1900; Poincaré 1906). In Lorentz's theory the null result of Michelson and Morley is explained by the transformation properties of moving systems, the contraction together with the associated transformation of time and of the electromagnetic field, which make every optical and electrical experiment performed within a moving system come out as if the system were at rest (Janssen 2002). The aether remained, at rest, and so did a true time, the time of the aether frame, of which the local time was a calculational auxiliary. Lorentz later wrote that the chief cause of his failure to arrive at Einstein's theory was his clinging to the idea that the variable t only can be considered as the true time, and that his local time must be regarded as no more than an auxiliary mathematical quantity (Lorentz 1916).

From Newton's Hypothesis to Lorentz's theory of electrons, the functions of the aether changed. It was a source of bodies and a cause of gravity for Newton, an elastic solid for Fresnel, the bearer of the electromagnetic field for Maxwell, and a frame of absolute rest made undetectable by the transformation properties of matter for Lorentz. One thing did not change. In every version light was a thing that travelled, and the aether was what it travelled through. What changed with Einstein was whether the aether was needed, and the question is whether the assignment changed with it.

2. The Aether Removed and the Assignment Retained

Before Einstein, Poincaré had already loosened the aether's hold. In La Science et l'hypothèse he wrote that whether the aether exists matters little and is the concern of metaphysicians; what matters is that everything happens as if it existed, and that the hypothesis is convenient for the explanation of phenomena (Poincaré 1902). The judgement was methodological. It treated the aether as a hypothesis to be kept or dropped according to its convenience, and it left untouched the conception of light that had made the hypothesis convenient in the first place.

Einstein's paper of 1905 removed the aether by declaring it superfluous. Its opening section states that the introduction of a "light aether" will prove superfluous, since the view to be developed requires no "absolutely stationary space" and assigns no velocity vector to any point of empty space at which electromagnetic processes occur (Einstein 1905). From two postulates, that the laws of physics take the same form in all inertial frames and that light in vacuum propagates with a definite speed c independent of the motion of its source, Einstein derived the transformation that Lorentz had reached through electrodynamics, and with it the relativity of simultaneity, the dilation of moving clocks and the contraction of moving rods, now as properties of space and time rather than of matter moving through a medium. The frame of absolute rest was gone, and with it the aether wind, which was no longer hidden but absent. Lorentz's true time was gone as well: every inertial frame's time had the same standing.

What Einstein retained was the second postulate. Light propagates with the speed c. The aether no longer carried the propagation, but the propagation remained, and with it the conception of light as something that travels from one place to another at a speed. The postulate is a statement about propagation; its content is that the speed of the propagation is the same in all inertial frames. The removal of the medium did not remove the thing that had been supposed to propagate through it. It removed only the frame relative to which the propagation had been supposed to have its speed, and replaced it with the requirement that the speed be the same relative to every frame. The question that had been answered by the aether, relative to what does light move at c, was answered by declaring that it moves at c relative to everything, which is the postulate, and is not an answer to why.

Einstein himself did not regard the matter as closed. In his inaugural lecture at Leiden in 1920, Ether and the Theory of Relativity, he argued that general relativity endows space with physical qualities, since the metric field determines the behaviour of rods and clocks and is in turn determined by matter, and that in this sense space without ether is unthinkable (Einstein 1920/1922). The ether he admitted was carefully restricted. It may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time; the idea of motion may not be applied to it. Einstein's ether of 1920 was a substrate without a state of motion, which is to say a substrate without a rest frame, and it was the gravitational field itself (Kostro 2000). It did not carry light. Light still propagated, now through a space whose physical qualities the ether named.

Dirac proposed in 1951 that an aether should be reintroduced on quantum-mechanical grounds (Dirac 1951). In classical physics an aether at rest at each point would define a velocity at each point and therefore a preferred frame, in conflict with relativity. In quantum mechanics, Dirac argued, the velocity of the aether at a point need not be definite; it can be distributed over all values in a way that is Lorentz invariant, so that no frame is preferred, and the vacuum of quantum electrodynamics might be such a state. Wilczek has since argued that the ensemble of quantum fields that fills space, which he calls the Grid, is the aether of modern physics in all but name, a medium that is everywhere, has structure and properties, and is the substance of which particles are excitations (Wilczek 1999; Wilczek 2008). The quantum vacuum, on this view, is the aether returned, and the electromagnetic field is one of its components.

The modern aether, however, is stranger than its proponents' word for it suggests, and its strangeness bears directly on the question of propagation. The fields of quantum field theory are operator-valued distributions that have no values at points; the photon admits no position operator of the Newton–Wigner kind; and the particle content of a field state depends on the state of motion of the system that registers it, so that an accelerated detector registers a thermal bath where an inertial detector in the same vacuum registers nothing (Newton and Wigner 1949; Unruh 1976). A photon is registered only through its interaction with matter: in Glauber's theory of photodetection the counting rate of an ideal absorptive detector is governed by the positive-frequency part of the field, which annihilates the photon that is counted (Glauber 1963). A previous analysis drew from these results the conclusion that the quantum fields are not a medium located in three-dimensional space, that they carry no proper duration, and that they become located, dated facts only when matter registers them (Zhong 2026b). If that conclusion is right, the Grid is not a medium in space through which its excitations travel. It is a structure that is not in space, registered in space by matter. Wilczek's aether and the substrate identified below are then the same substrate described from two sides, and what separates the two descriptions is exactly the question at issue here: whether its excitations propagate.

The modern experimental record treats frames and propagation separately, and the distinction is essential to what follows. Smoot, Gorenstein and Muller detected a dipole anisotropy in the cosmic microwave background, a variation of its temperature across the sky of a few parts in a thousand, which is the Doppler signature of the Solar System's motion relative to the frame in which the background is isotropic (Smoot, Gorenstein and Muller 1977); the Planck satellite has measured the corresponding velocity at about 370 km/s (Planck Collaboration 2020). The frame defined by the background is physical and measurable. It is not, however, a frame of light propagation. The background radiation is a population of photons last scattered by matter about 380,000 years after the hot early phase, and the frame in which it is isotropic is the frame of that population, determined by the distribution of the material sources from which it was released. The laws of physics are not different in that frame, and the propagation of light is not referred to it. The strongest modern tests of the isotropy of light propagation, rotating optical resonators compared over long periods, find no dependence of the speed of light on direction to a level of about one part in 10¹⁷ (Herrmann et al. 2009), and the wider body of tests constrains departures from Lorentz symmetry across many sectors to similarly small bounds (Mattingly 2005; Kostelecký and Russell 2011). There is a frame of the radiation population and there is no frame of light propagation.

A second modern line reopened the comparison between Lorentz and Einstein on its own terms. Bell argued that the Lorentzian pedagogy, in which the contraction of a moving body is derived from the response of its constituent forces to motion, is a legitimate and illuminating route to special relativity, and that the difference between it and Einstein's route is one of philosophy and style rather than of physics (Bell 1976). Brown developed this into a systematic account in which the relativistic behaviour of rods and clocks is explained by the symmetries of the laws governing the matter of which they are made, so that the geometry of Minkowski spacetime is a codification of that behaviour rather than its cause (Brown 2005). Janssen traced the historical relation between the two theories and argued that the difference lies in where each locates the explanation of the common symmetry (Janssen 2002). These neo-Lorentzian accounts retain one of two things. Some retain a preferred frame, empirically undetectable; others retain the contraction as a physical response of matter to its motion. What they share with Einstein is the propagation of light at c. The present analysis differs from them on all three points: it retains no preferred frame, it does not need contraction as a compensating mechanism, and it does not retain the propagation.

The conception of light as a thing that travels was reinforced by the way its speed has been measured. Rømer inferred a finite speed in 1676 from the delays of the eclipses of Jupiter's moon Io, which ran behind their predicted times when the Earth was farther from Jupiter (Rømer 1676). Fizeau's toothed wheel in 1849 and Foucault's rotating mirror in 1862 measured the time light took to travel to a distant mirror and back, and every later terrestrial determination, through Michelson's and the cavity and laser measurements that preceded the 1983 definition, has the same structure: light is emitted by matter at one event and absorbed by matter at another, and the speed is computed from a distance measured by material rods and a duration measured by a material clock. What these measurements register is a pair of material events and the interval between them as the material domain counts it. Special relativity states what the light itself contributes to that interval. The emission event and the absorption event are separated by a null interval, ds² = 0, and the proper time elapsed along the light's worldline between them is zero, whatever the distance and whatever the coordinate duration (Taylor and Wheeler 1992). The finite duration belongs to the clocks at the two ends. Between emission and absorption, on the light's side of the interval, nothing elapses. The measurements of c have always been measurements of the material domain's interval between two material events, interpreted as the speed of a thing that crossed it.

Every return of the aether since 1905, and every departure from it, has kept one element fixed. Einstein's postulate is a postulate about the propagation of light. Einstein's ether of 1920 made the physical structure of spacetime the substrate within which light propagation and material measurement occur. Dirac's aether is a state of the field through which light propagates. Wilczek's Grid, on the reading given here, is a medium whose excitations, photons among them, propagate through it. The neo-Lorentzian accounts explain the behaviour of rods and clocks and take the propagation of light at c as given. The functions of the aether have been dropped, restricted and restored; the assignment behind them, that radiation is a thing which travels at a speed through or within something, has not been examined.

The examination requires a structural statement, made here once. The universe is organised as a six-level structural hierarchy, labelled S6D through S1D, whose levels are distinguished by function and by the character of their proper time rather than by a count of spatial coordinates. S6D is the original source of all radiation in the universe, and is undetectable and unknowable. S5D is the radiation field, where the undifferentiated radiations of S6D interact and are differentiated into different configurations of radiation; S4D is the quantum field, which projects these configurations onto S3D. These three levels constitute the radiation domain, which has no relative time, no space and no mass; motion, propagation and dynamics arise only when radiation enters the material domain. S3D, S2D and S1D constitute the material domain: S3D is macroscopic matter, S2D the atomic level, and S1D the smallest indivisible unit form of matter. S4D is not the four-dimensional spacetime of general relativity. The whole of spacetime, with its metric, its light cones, its clocks and its rods, belongs to S3D.

From here the argument proceeds on the paper's own account, and its inputs are results argued in previous work. The constant c is the rate at which temporal ordering is denominated in spatial extension, the conversion constant between the ordering parameter of physics and spatial extension; on that reading the light postulate is a consequence rather than a postulate, and the 1983 definition of the metre through c institutionalises an order the physics had already reached (Zhong 2026c). The calibrated value of c is the S3D expression of the differential between S4D and S3D: c is a structural constant of the registering material domain, the scale at which that domain takes up what arrives from the level above, and not the velocity of a propagating entity (Zhong 2026d). The universe divides into a radiation domain and a material domain with τ > 0, a partition derived from Newton's distinction between absolute and relative time and from the special-relativistic result that the proper time of light vanishes while that of every massive body is strictly positive (Zhong 2026e). The vanishing proper time of light is the trace, within S3D, of a domain that has no relative time at all. The quantum fields of physics are the structure of S4D, which projects onto S3D, and particles are the registered facts of that structure in S3D (Zhong 2026b).

On these inputs the assignment loses its referent, and this is the paper's derivation. Propagation is a relation between a thing and the places it occupies in succession: a thing propagates if it is at one place and then at another, and its speed is the ratio of the distance between the places to the duration between the occupations. Both quantities, the distance and the duration, are quantities of the material domain, measured by material rods and clocks. The radiation domain has no relative time in which a succession of places could occur and no space in which places could be distinguished; duration and extension are both quantities of S3D, to which the radiation domain does not belong. Radiation in its own domain therefore does not propagate, does not move, and has no speed. What the material domain measures as c is the scale at which the material domain registers what reaches it from S4D, a property of the registering domain, identical for every observer in S3D because every such observer belongs to the same domain. Light in S3D, the electromagnetic flux that instruments detect, is S4D radiation that has entered the material domain without taking material form, and its measured speed is the material domain's reading, not a velocity it possesses in its own domain.

The sentence on which the whole history of the aether rests, that light travels through a medium, then has no application in the radiation domain. Nothing there travels, so nothing there requires a medium to travel through. The question of what the medium is, which occupied physics from Huygens to Lorentz, has no answer, not because the medium is undetectable but because the relation it was introduced to support does not hold. And the separation of light from the aether, Newton's "something of a different kind", receives its ground: the substrate that physics sought under the name of aether is the radiation domain, which is radiation itself and not a container that radiation occupies, and light in S3D is that radiation as the material domain registers it. The difference Newton saw is a difference of level, not a difference between a substrate on one side and a travelling thing on the other. Radiation is not in the aether. Radiation is the aether. What remains is to determine what becomes of each of the three functions the aether was made to carry.

3. Radiation as the Aether: Three Consequences

The first function was propagation, and its consequence is the simplest: with no propagation in the radiation domain, no medium of propagation is required. The elastic solid of Fresnel, with its paradoxical rigidity and transparency, was required to carry transverse vibrations from place to place; Maxwell's material body filling interstellar space was required to carry the electromagnetic disturbance. Both were demanded by the conception of light as a motion of something across space. In the radiation domain, nothing moves across space, and the demand lapses. The classical electromagnetic field equations remain unchanged. They describe the electromagnetic field as the material domain registers it, and the speed 1/√(ε₀μ₀) they contain is the material domain's scale of registration: the permittivity and permeability of the vacuum are, on this reading, constants of the domain in which electromagnetic phenomena are registered, not properties of a substance occupying it.

The present SI records this status in its own structure. Since 2019 the speed of light, the Planck constant and the elementary charge have fixed numerical values, and the magnetic constant μ₀, which was exact under the previous definition of the ampere, has become a measured quantity, determined through the fine-structure constant, with ε₀ then determined from ε₀μ₀ = 1/c² (BIPM 2019). The vacuum constants are no longer interpreted as the elastic and inertial parameters of a mechanical medium; they are unit-dependent electromagnetic constants that express the response scale of electromagnetic measurements in the units the material domain uses to register them. Nothing in the definition of the units refers to a substance occupying space.

The same consequence answers a question Einstein's postulate left standing. Every velocity in physics is the velocity of something relative to something, and changes with the frame; c alone does not. Einstein made the exception a postulate, and the postulate is among the best-tested statements in science, but a postulate is a statement that the exception holds, not an account of why. If c is the velocity of light relative to a medium, its invariance is a mystery that Lorentz explained by compensation; if c is the velocity of light relative to every frame, its invariance is a postulate. If c is not a velocity of light at all but the scale at which the material domain registers what reaches it from S4D, its invariance is what is to be expected. A change of inertial frame changes an observer's state of motion within S3D. It does not change the observer's domain, and the scale belongs to the domain. Every observer measures the same c because every observer in S3D registers by the same domain's scale, in the way that every user of one currency finds the same denomination however they spend it. The invariance is not a property granted to one velocity among many; it is the mark of a quantity that was never a velocity.

The delay of light, which has always been the primary evidence that light travels, receives a definite reading on this account. Light from a distant galaxy arrives long after it was emitted, as the material domain counts years, and radar echoes return from the planets after the round-trip times that astronomers use to measure their distances. None of this is denied. A previous analysis showed that operational distance is receiver-side proper time multiplied by c, as radar ranging and the SI definition of the metre both make explicit, and that the waiting between an emission and its absorption belongs to the registering system and not to the relation connecting the two bodies, along which no proper duration accumulates (Zhong 2026f). The delay is real, and it is a quantity of the material domain: the duration counted by the receiver's clock between events that the material domain orders. The radiation that connects emitter and absorber contributes no duration of its own to that count, and the inference from the delay to a thing travelling across the intervening space is the inference the measurements of c have always made, from an interval the material domain registers to a motion the radiation performs.

The second function was the frame of rest, and its consequence is that there is no aether wind to measure. The aether wind is a relative motion between the Earth and a medium distinct from light, through which light moves at a fixed speed relative to the medium. Where radiation is the aether, there is no medium distinct from radiation, and there is no relative motion between the Earth and such a medium, because there is no such medium. The null result of Michelson and Morley acquires on this reading the status of a result that could not have been otherwise. Three readings of it can now be set side by side, and each is consistent with every measurement made. On Lorentz's reading, the Earth moves through a stationary aether, and the transformation properties of moving systems, the contraction of the apparatus and the associated transformation of time and field, make the effect of the motion undetectable. On Einstein's reading, the null result follows from the postulate that light has the speed c in every inertial frame, and the aether is superfluous. On the present reading, there was no wind because there was no medium, distinct from the light, for the Earth to move through.

The three readings make the same predictions, and the third is an interpretation, not a new experimental claim. Its claim on attention is that it requires fewer assumptions than the first and answers a question the second leaves open. Lorentz's reading requires two things: a medium, and a set of transformation properties of matter which ensure that the medium is never detected. A medium whose every effect is cancelled by the behaviour of the instruments that would detect it is a medium posited and then hidden, and the second assumption exists only to protect the first. Einstein's reading dispenses with the medium but must postulate the invariance of c, which is the exception the medium had been introduced to avoid. The present reading dispenses with the medium and derives the invariance, at the cost of identifying the aether with radiation and the measured c with the material domain's registration scale; the null result then requires neither a cancellation nor a postulate. The Lorentz contraction is not thereby removed from physics. It is the measured behaviour of moving rods and clocks, and it remains exactly what special relativity says it is. What changes is its role. It is not a compensating mechanism that hides a medium, but the form the invariance of c takes in the material domain's measurements of length and time, the same invariance read off rods rather than off light.

The identification might be taken for Wilczek's under a new name, or for a neo-Lorentzian account without a preferred frame. It is neither, and the difference is exact. Wilczek's Grid, as summarised here, is a medium that fills space and whose excitations move through it at speeds limited by c, so that the photon, on his account, is a disturbance propagating through the Grid as Maxwell's waves propagated through the aether. The neo-Lorentzian accounts explain the behaviour of rods and clocks dynamically and take the propagation of light at c as the given against which that behaviour is measured. Both keep the assignment in its modern form: radiation is a thing travelling within something. The present identification withdraws it. The substrate is not in space, it does not travel, and the speed c belongs to the material domain that registers it. The consequence is not a new prediction, since all three accounts reproduce the measurements of special relativity, but a different answer to what those measurements are measurements of.

The modern frame of the cosmic background, which might seem to restore an aether frame, confirms the distinction instead. The background radiation is radiation registered in S3D, a population of photons released by matter and detected by matter, and its rest frame is the frame of that population, fixed by the material distribution from which it decoupled. An individual photon has no rest frame, by the massless dispersion relation, and a population of photons has one only as a population, in the way that a gas of molecules has a frame in which its mean momentum vanishes. The existence of that frame is a fact about how radiation in S3D is distributed, not a fact about the radiation domain, which has no location in S3D and therefore no frame within it. The laboratory has a measured velocity of about 370 km/s relative to the background frame, while rotating-resonator experiments independently constrain any directional anisotropy of the speed of light to about one part in 10¹⁷, and the two facts together show the separation directly: the frame of the radiation population is measurable, and the propagation of light is not referred to it. Dirac's proposal reached the same separation by a different route, since a velocity distributed over all values in a Lorentz-invariant way defines no frame; but Dirac's aether still had a velocity to be distributed, and the present reading has none to distribute.

The third function was the bearing of absolute time and absolute space, and its consequence is that absolute time requires no mechanical bearer. Newton defined absolute time without reference to any medium, as flowing equably of itself and without relation to anything external, and he called it by another name duration (Newton 1687/1999). The nineteenth century gave it a physical location by identifying the frame of absolute rest with the aether, so that absolute time became, in practice, the time of the aether frame: Lorentz's true time. When the aether frame was abolished, absolute time was abolished with it, as though the two were one thing. On the present reading the nineteenth century was right to join them and wrong about what it joined. Newton's marks of absolute time, that it exists of itself and from its own nature, without relation to anything external, are met by radiation in its own domain and by nothing in the material domain: radiation there has no relative time and no space, so no motion of matter can alter it and no frame can be assigned to it. Absolute time is therefore not a condition that radiation is in, and not a quantity that a substance carries. Absolute time is radiation, and the aether is radiation; the three names denote one thing. Absolute time is what Newton explicitly distinguished from relative, apparent and common time, "any sensible and external measure (precise or imprecise) of duration by means of motion", which is the time of clocks (Newton 1687/1999). Newton's flowing is the material domain's reading of it, since flow belongs to clocks, and the invariance of c measured in every laboratory is the mark it leaves on the material domain's registration. Absolute time requires no bearer, because it is the substrate that the aether was introduced to be. Lorentz's attachment to a true time was, on this reading, an attachment to something real placed in the wrong location: he located absolute time in a frame of the material domain, the rest frame of the aether, when it is radiation, which has no frames.

Newton's aether and Newton's absolute time are, on this reading, one thing that Newton described twice. The aether of the Hypothesis and the Queries is invisible, fills every space including the space within bodies, persists when the air is withdrawn, is rarer within and near dense bodies, and is the source from which all bodies are condensed. Each of these is a property of radiation as it is present at every point of S3D, where electromagnetic radiation across the whole spectrum, almost all of it invisible, pervades and penetrates matter and is depleted by the matter that absorbs it. The absolute time of the Scholium exists of itself and from its own nature, without relation to anything external, which is radiation in its own domain. Newton could not join the two descriptions. He had one space and one time, both belonging to the world of bodies, and no distinction between a level that is registered in space and time and a level that has neither relative time nor space; the aether therefore had to be a substance located in space, and absolute time a flow without substance. The distinctions he lacked became available only with the vanishing proper time of light and the structure of spacetime (Einstein 1905; Minkowski 1909), together with the dimensional hierarchy drawn on here. With them, the aether is radiation as it is present throughout the material domain, absolute time is radiation as it is in its own domain, and the two are one. Lorentz's aether points to the same referent: the bearer of the electromagnetic field and the location of true time is radiation, and what he added to it, a frame of absolute rest, is the one property radiation cannot have.

Newton's two structures can now be assigned their outcomes. His separation of light from the aether, light as something of a different kind, is retained as the difference between radiation in its own domain and radiation as the material domain registers it; what the present reading withdraws is the propagation he attached to it, light as a thing propagated from lucid bodies through the medium. His condensation of aethereal substance into gross bodies is retained. The proposal already recalled treats matter as condensed radiation and radiation as released matter, on the ground that mass and radiation are both energy and that the two are routinely interconverted. The interconversion is standard physics: an electron and a positron annihilate into photons, and photons of sufficient energy create electron–positron pairs, so that the boundary between the sector that carries proper duration and the sector that does not is crossed by interaction (Breit and Wheeler 1934; Zhong 2026g). The mass of ordinary matter confirms the same relation from a different direction, since almost all of the mass of the proton and neutron is the energy of the massless gluon field and of the quarks bound by it, not the sum of intrinsic masses (Wilczek 2008). On the present account this is the differentiation of radiation into matter: the radiation domain, by projection onto S3D, takes material form, and what takes material form carries mass and proper duration. Newton's "thus perhaps may all things be originated from aether" is, with the aether identified as radiation, a statement of this relation in seventeenth-century terms. The condensation and the separation were never consequences of one another, and the history of physics kept the separation in the form of propagation and discarded the condensation. The present reading keeps both and discards the propagation.

The ether Einstein admitted in 1920 was the gravitational field, a substrate that gives space its metrical properties and has no state of motion. Newton's aether of Query 21 was a substrate whose depletion near bodies produces gravity. On the present account these two conceptions converge on the same substrate. A previous proposal derived the inverse-square law and the weak-field metric of general relativity from the shadowing of the ambient radiation field by matter, which is condensed radiation, and read the curvature of spacetime as the geometric encoding of that radiative configuration (Zhong 2026a). If gravity is the effect of matter's shadow in the radiation that constitutes the aether, then Einstein's gravitational ether and Newton's gravitating aether are both descriptions of radiation in S3D as matter shadows it, one written in geometry and one in the language of a rarer and denser medium. The identification is the previous proposal's, and it is recalled here only for what it implies about the aether: the substrate that bears gravity and the substrate that is radiation need not be two.

Lorentz and Einstein were solving different problems under different assignments of where the absolute belongs, and the comparison is not a ranking. Lorentz placed the absolute in the medium: an aether at rest and a true time belonging to its frame, with matter so constituted that neither could be detected. His assignment cost him two hidden structures and gained him a physical picture in which the transformation was a property of matter. Einstein removed the medium and placed the only remaining absolute in the speed of light, the invariant on which both frames agree, at the cost of accepting that invariance as a postulate. The present reading places the absolute in radiation itself, which is absolute time, and places c in the material domain as the scale at which that domain registers the radiation that reaches it. The cost of this assignment is the identification of the aether with radiation, which is an ontological claim and is stated as one. What it gains is that neither a hidden medium nor an unexplained postulate is required, and that Newton's absolute time, Lorentz's true time and Einstein's invariant are recognised as three readings of radiation, each made from a different location.

The account would fail under three conditions. The first is decidable and already strongly constrained: if a preferred frame of light propagation were detected, as an anisotropy of the speed of light or an aether wind correlated with the laboratory's motion, including a dependence of the speed of light on direction relative to the rest frame of the cosmic background, the absence of any medium distinct from radiation would be refuted. Every experiment from Michelson and Morley to the rotating resonators has found no such effect, now at the level of one part in 10¹⁷. The second concerns the null interval: if radiation propagation were found to carry a nonzero invariant proper duration, for instance through a nonzero photon mass, radiation would belong to the sector that carries proper time and the claim that it does not propagate in its own domain would fail; current photon-mass bounds constrain this condition tightly without eliminating it (Goldhaber and Nieto 2010). The third is operational: if a measurement were established that shows c to be a property of radiation independent of the registering material domain, the identification of c as a constant of that domain would fail; the condition is stated without presupposing how such a measurement could be made.

4. Conclusion

The history of the aether is not the history of a mistaken entity discarded by better physics. It is the history of an assignment that was retained through every change in the entity. Newton set light beside the aether as something of a different kind, propagated through it; Fresnel made the aether an elastic solid through which light vibrated, and a frame of rest; Maxwell made it the bearer of the electromagnetic field and computed the speed of light from its constants; Lorentz kept it at rest and made it undetectable by the transformation properties of matter. Einstein removed the medium and kept the propagation, and the ethers of Einstein in 1920, of Dirac and of the quantum vacuum restored a substrate within which light was still taken to propagate. Throughout, radiation was a thing that travels at a speed through or within something else. On the account given here, radiation in its own domain does not travel, since that domain has neither relative time nor space, and c is a constant of the material domain in which radiation is registered. The assignment then has no referent. No medium of propagation is required, no aether wind or preferred frame of propagation exists to be detected, and absolute time is radiation itself rather than the time of a mechanical medium. Newton's conjecture that all things may be formed from aether by condensation survives as the differentiation of radiation into matter, and his aether and his absolute time, which he could not join, are one: radiation. Radiation is not in the aether. Radiation is the aether, and the aether is absolute time.

 

 

 

 

 

Declarations

Funding: This research received no external funding.

Conflicts of interest: The author declares no conflicts of interest.

Data availability: No new data were generated or analysed in this study.

Author contributions: Juliet Zhong: conceptualisation, methodology, formal analysis, writing.

Declaration of generative AI and AI-assisted technologies in the manuscript preparation process: During the preparation of this work, the author used Claude (Anthropic), ChatGPT (OpenAI), and Gemini (Google) for assistance with drafting text, checking calculations, locating references, and language editing. The author reviewed and edited the output as needed and takes full responsibility for the content of the published article.

 

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