The aether was a substance filling background space and thought to be required to explain how light waves could be propagated through space, as all wave transmission requires a medium. Its existence has been postulated a medium for the propagation of electricity, light, and gravitation throughout ages, from the Ancient Greeks, to Newton, to the scientists of the Victorian Era. Aristotle called aether the Fifth Element.
James Clerk Maxwell, the father of Electromagnetic Theory and a figure widely considered to be one of the greatest scientists to have lived, utilizes aether in his treatise:
“ In several parts of this treatise an attempt has been made to explain electromagnetic phenomena by means of mechanical action transmitted from one body to another by means of a medium occupying the space between them. The undulatory theory of light also assumes the existence of a medium. We have now to show that the properties of the electromagnetic medium are identical with those of the luminiferous medium. ”
—James Clerk Maxwell, A Treatise on Electricity and Magnetism/Part IV/Chapter XX (Source)
Aether in Modern Science
It is popularly believed that Einstein abolished the aether in science with his Theory of Special Relativity. However, the following quotes suggest a different story.
“ It is ironic that Einstein’s most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise was that no such medium existed…. Einstein… utterly rejected the idea of ether and inferred from its nonexistence that the equations of electromagnetism had to be relative. But this same thought process led in the end to the very ether he had first rejected, albeit one with some special properties that ordinary elastic matter does not have. The word “ether” has extremely negative connotations in theoretical physics because of its past association with opposition to relativity. This is unfortunate because, stripped of these connotations, it rather nicely captures the way most physicists actually think about the vacuum.
In the early days of relativity the conviction that light must be waves of something ran so strong that Einstein was widely dismissed. Even when Michelson and Morley demonstrated that the earth’s orbital motion through the ether could not be detected, opponents argued that the earth must be dragging an envelope of ether along with it because relativity was lunacy and could not possibly be right…. Relativity actually says nothing about the existence or nonexistence of matter pervading the universe, only that such matter must have relativistic symmetry.
It turns out that such matter exists. About the time relativity was becoming accepted, studies of radioactivity began showing that the empty vacuum of space had spectroscopic structure similar to that of ordinary quantum solids and fluids. Subsequent studies with large particle accelerators have now led us to understand that space is more like a piece of window glass than ideal Newtonian emptiness. It is filled with “stuff” that is normally transparent but can be made visible by hitting it sufficiently hard to knock out a part. The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo. ”
—Robert B. Laughlin (1993 Nobel laureate in physics), "A Different Universe: Reinventing Physics from the Bottom Down", 2005, pp. 120-121).
“ According to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there would not only be no propagation of light, but also no possibility of existence for standards of space and time (measuring rods and clocks), nor therefore any space-time intervals in the physical sense ”
—Albert Einstein, “Geometry and Experience,” in Sidelights on Relativity, 1983, p. 30
“ …in 1905 I was of the opinion that it was no longer allowed to speak about the ether in physics. This opinion, however, was too radical, as we will see later when we discuss the general theory of relativity. It does remain allowed, as always, to introduce a medium filling all space and to assume that the electromagnetic fields (and matter as well) are its states…once again “empty” space appears as endowed with physical properties, i.e., no longer as physically empty, as seemed to be the case according to special relativity. One can thus say that the ether is resurrected in the general theory of relativity….Since in the new theory, metric facts can no longer be separated from “true” physical facts, the concepts of “space” and “ether” merge together. ”
—Albert Einstein, “Grundgedanken und Methoden der Relativitätstheorie in ihrer Entwicklung dargestellt,” Morgan Manuscript, EA 2070, as cited in Ludwik Kostro, Einstein and the Ether, 2000, p. 2.
“ Modern science has its roots in ancient Greek philosophy. This philosophy, as we know, used the word “ether” to designate the particular kind of matter that filled the universe. This term was used throughout the history of philosophy and science, and it was also current at the beginning of this century. A resumption of its use at the dawn of this new century is now a fact. Since, according to the General Theory of Relativity and other modern branches of physics, the space and time of the universe do not constitute a vacuum, but a structured material plenum characterized by different physical quantities, the historical and traditional word “ether” is the most appropriate to express these features of the universe. ”
—Ludwik Kostro, Einstein and the Ether, 2000, pp. 186-187.
“ Certainly, from the standpoint of the special theory of relativity, the ether hypothesis appears at first to be an empty hypothesis. In the equations of the electromagnetic field there occur, in addition to the densities of the electric charge, only the intensities of the field. The career of electromagnetic processes in vacuo appears to be completely determined by these equations, uninfluenced by other physical quantities. The electromagnetic fields appear as ultimate, irreducible realities, and at first it seems superfluous to postulate a homogeneous, isotropic ether-medium, and to envisage electromagnetic fields as states of this medium.
But on the other hand there is a weighty argument to be adduced in favour of the ether hypothesis. To deny the ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view. For the mechanical behaviour of a corporeal system hovering freely in empty space depends not only on relative positions (distances) and relative velocities, but also on its state of rotation, which physically may be taken as a characteristic not appertaining to the system in itself. In order to be able to look upon the rotation of the system, at least formally, as something real, Newton objectivises space. Since he classes his absolute space together with real things, for him rotation relative to an absolute space is also something real. Newton might no less well have called his absolute space 'Ether'; what is essential is merely that besides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real...
It is true that Mach tried to avoid having to accept as real something which is not observable by endeavouring to substitute in mechanics a mean acceleration with reference to the totality of the masses in the universe in place of an acceleration with reference to absolute space. But inertial resistance opposed to relative acceleration of distant masses presupposes action at a distance; and as the modern physicist does not believe that he may accept this action at a distance, he comes back once more, if he follows Mach, to the ether, which has to serve as medium for the effects of inertia. But this conception of the ether to which we are led by Mach's way of thinking differs essentially from the ether as conceived by Newton, by Fresnel, and by Lorentz. Mach's ether not only conditions the behaviour of inert masses, but is also conditioned in its state by them.
Mach's idea finds its full development in the ether of the general theory of relativity. ”
—Albert Einstein, an address delivered on May 5th, 1920, in the University of Leyden.
“ Einstein's new kind of ether was the metrical tensor field. He thus started to adhere to this new ether. He named it "Mach's ether" or simply "ether," and supplied the same reasons that Poincare had provided in his writings as to why we should adhere to the ether (we need the ether in order to remove absolute rotation and action-at-a-distance: see my papers "Poincare's ether"). Einstein thus returned to the 19th century concept of the ether, but stripped of it its most important characteristic: a medium being in absolute rest. One could still pose the perplexing question: Was Einstein's ether endowed with any properties independent of the masses in it? For if it did possess such properties then there was actually no difference between Einstein and Poincare's ether. Einstein did not give a definitive answer to the above question in his (1920) lecture. ”
—"Einstein's Ether: D. Rotational Motion of the Earth," Galina Granek, Department of Philosophy, Haifa University, Mount Cannel, Haifa 31905, Israel, Apeiron, Vol. 8, No. 2, April 2001, p. 64.
“ It would have been more correct if I had limited myself, in my earlier publications, to emphasizing only the non-existence of an ether velocity, instead of arguing the total non-existence of the ether, for I can see that with the word ether we say nothing else than that space has to be viewed as a carrier of physical qualities. ”
—Einstein in a 1919 letter to Lorentz
“ A few words about the gravitational ether, and the ether concept in general may be in place here. The ether hypothesis was thought to be buried by the Michelson-Morley experiment, but today it is more alive than ever, in the form of the CBR [Cosmic Background Radiation]: experiments capable of finding the ether were not possible in the 1880s, but were possible in the 1960s. In a sense, the electromagnetic ether has always been observed – as the heat of the Sun (since as pointed out, CBR is reprocessed photons)…. All the main cosmological, astrophysical and physical facts: the gravity and Olbers paradoxes, redshift effects and CBR, gravitation and radiation, and the existence of particles can be conceived in the framework of this ether concept. ”
—“Action-at-a-Distance and Local Action in Gravitation,” in Pushing Gravity, ed., Matthew Edwards, pp. 157-159., 1991 (Archive)
“ Dirac in 1951 published a Letter to Nature titled "Is There an Aether?"(2) in which he showed that the objections to an aether posed by Relativity were removed by Quantum Mechanics, and that in his reformulation of electrodynamics the vector potential was a velocity.(3) He concludes the Letter with 'We have now the velocity(2) at all points of space-time, playing a fundamental part in electrodynamics. It is natural to regard it as the velocity of some real physical thing. Thus with the new theory of electrodynamics we are rather forced to have an aether'. ”
—An Aether Model of the Universe (Archive)
“ Everything becomes clear if the idea that particles always have a position in space through time is brought back…. According to my current thinking, the particle is always located within a physical wave….The movement of the particle is assumed to be the superposition of a regular movement… and of a Brownian movement due to random energy exchanges which take place between the wave and a hidden medium, which acts as a subquantum thermostat. The point of prime importance in this model is that at each moment the particle occupies a well-defined position in space, and this re-establishes the clear meaning which the configuration space had in classical mechanics. ”
—Louis de Broglie, “Waves and Particles,” Physics Bulletin, 22, February 1971
“ Later in our treatise we will find that the very ether Louis de Broglie desired offers a solution to the wave/particle conundrum that has hampered modern science since de Broglie first discovered that electrons produce waves. Any particle that moves through a medium will, indeed, create waves. In fact, a return to ether will help solve one of the most mysterious and perplexing problems in Quantum Mechanics today, the phenomenon of “entanglement” – the spooky connection between pairs of photons, electrons or atoms even though they are separated by great distances. Perhaps this was why John Stewart Bell, the inventor of Bell’s Theorem to answer the phenomenon of entanglement, stated in a BBC radio interview: “Yes, the idea that there is an ether…that is a perfectly coherent point of view." ”
—Ludwik Kostro, Einstein and the Ether, p. 154, citing M. Jammer’s, “John Stewart Bell and the Debate on Significance of his Contributions to the Foundations of Quantum Mechanics,” in Bell’s Theorem and the Foundations of Modern Physics, eds. A. Van der Merwe, F. Felleri, G. Tarozzi, Singapore, 1992, p. 5; also cited in P. C. W. Davies and J. R. Brown, eds., The Ghost in the Atom, 1986, pp. 49-50.
“ According to accepted theory, free space is a vacuum. If this is so, how can it exhibit impedance? But it does. The answer, of course, is that there is no such thing as a vacuum, and what we call free space has structure. The impedance equals 376+ ohms. ”
—"Space Must Be Quantizied", Robert Moon, professor emeritus in physics at University of Chicago, 21st Century, 1988, p. 26
“ Today the vacuum is recognized as a rich physical medium....A general theory of the vacuum is thus a theory of everything, a universal theory. It would be appropriate to call the vacuum "ether" once again. ”
—"The Philosophy of Vacuum" , 1991, Simon W. Saunders, Harvey R. Jr. Brown, p 251.
“ In this paper we review the evolution of the concept of “vacuum” according to different theories formulated in the last century, like Quantum Mechanics, Quantum Electrodynamics, Quantum Chromodynamics in Particle Physics and Cosmology. In all these theories a metastable vacuum state is considered which transforms from one state to another according to the energy taken into consideration. It is a “fluid” made up by matter and radiation present in the whole Universe, which may be identified with a modern definition of ether. ”
—"The Vacuum as Ether in the Last Century", M. Barone, Foundations of Physics Vol. 34, 2004 (Source)
“ Prior to the twentieth century, physics tried to explain how Nature works. Over the twentieth century, and especially in the last half, we got much more ambitious - now we’re uncovering what Nature is. The foundation is an entity I call the Grid. The Grid fills space, and is full of spontaneous activity. In some ways it resembles the old idea of “ether”. But the Grid is highly evolved ether, ether on steroids if you like, with many new features. We have some wonderful ideas waiting to be tested. There are good reasons to think that the Universe is a multilayered multicolored superconductor; that all four known forces can be brought together in a unified theory; that seemingly hopelessly different kinds of matter are just different aspects of one all-embracing stuff. I anticipate that the next few years will be a new Golden Age in fundamental physics. ”
—Frank Wilczek, Professor of Physics at MIT, Nobel Prize winner of 2004, author of the book "THE LIGHTNESS OF BEING: Mass, Ether, and the Unification of Forces" (Basic Books; September 2, 2008) in a Q&A regarding his book. (Archive)
“ At the beginning of the twentieth century, Albert Einstein replaced the aether theory by relativity, but a twenty-first century aether is still puzzling physicists today. This modern aether is the quantum vacuum. The aether was thought to be an all-penetrating mysterious substance that carries light through space like air carries sound. Take away all light, and the aether would still be there, defining a universal frame of reference. Now, according to quantum field theory, the state of absolute darkness, the vacuum state, is still a physical state filling space throughout, similar to the aether. There is an important difference though: one does not notice motion at uniform speed relative to the quantum vacuum, but, as we describe in this chapter, during acceleration the vacuum glows, although slightly, causing friction. Furthermore, as Stephen Hawking predicted in 1974 (Hawking, 1974), the quantum vacuum should also cause black holes to evaporate, because at the event horizon particles are created from nothing, at the expense of the black hole's mass. ”
—Ulf Leonhardt, Essential Quantum Optics, 2010, p. 189 (Source)
“ The concept of ether surfaced decades before scientists knew of quantum mechanics and some very fundamental symmetry principles of the microscopic world. Because of the huge change in knowledge, the historic word ether is not used anymore since it is a historical concept based on classical, not quantum physics.
Today's equivalent of the ether is the vacuum!
Physicists have come to realize that vacuum is not just empty space and the absence of things. The laws of quantum physics and experimental observations clearly show that many physical phenomena are explained by the fact that the vacuum has certain physical properties, including vacuum energy and quantum fluctuations: particles and antiparticles can appear and then disappear after a short period of time. ”
—Fermilab Q&A, Concept of ether in explaining forces, 2014 (Archive)
- Michelson-Morley Experiment - Light velocity experiment which suggests a lack of Earth's motion around the Sun
- Sagnac Experiment - Experiments which show that light's velocity is indeed affected by detector motion
- Airy's Failure - An experiment which suggests that the stars are in motion, rather than the Earth