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In the history of physics, aether theories (or ether theories) proposed the existence of a medium, a space-filling substance or field as a transmission medium for the propagation of electromagnetic or gravitational forces. Since the development of special relativity, theories using a substantial aether fell out of use in modern physics, and are now replaced by more abstract models.

This early modern aether has little in common with the aether of classical elements from which the name was borrowed. The assorted theories embody the various conceptions of this medium and substance.

Historical models

Luminiferous aether

Isaac Newton suggests the existence of an aether in the Third Book of Opticks (1st ed. 1704; 2nd ed. 1718): "Doth not this aethereal medium in passing out of water, glass, crystal, and other compact and dense bodies in empty spaces, grow denser and denser by degrees, and by that means refract the rays of light not in a point, but by bending them gradually in curve lines? ...Is not this medium much rarer within the dense bodies of the Sun, stars, planets and comets, than in the empty celestial space between them? And in passing from them to great distances, doth it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies towards one another, and of their parts towards the bodies; every body endeavouring to go from the denser parts of the medium towards the rarer?"^[1]

In the 19th century, luminiferous aether (or ether), meaning light-bearing aether, was a theorized medium for the propagation of light. James Clerk Maxwell developed a model to explain electric and magnetic phenomena using the aether, a model that led to what are now called Maxwell's equations and the understanding that light is an electromagnetic wave.^[2] Later, a series of increasingly careful experiments were carried out in the late 1800s, including the Michelson–Morley experiment, to try to detect the motion of Earth through the aether, but no drag was detected. A range of proposed aether-dragging theories could explain the null result but these were more complex, and tended to use arbitrary-looking coefficients and physical assumptions. Joseph Larmor discussed the aether in terms of a moving magnetic field caused by the acceleration of electrons.

Hendrik Lorentz and George Francis FitzGerald offered, within the framework of Lorentz ether theory, an explanation of how the Michelson–Morley experiment could have failed to detect motion through the aether. However, the initial Lorentz theory predicted that motion through the aether would create a birefringence effect, which Rayleigh and Brace tested and failed to find (Experiments of Rayleigh and Brace). All of those results required the full application of the Lorentz transformation by Lorentz and Joseph Larmor in 1904.^[3]^[4]^[5]^[6] Summarizing the results of Michelson, Rayleigh and others, Hermann Weyl would later write that the aether had "betaken itself to the land of the shades in a final effort to elude the inquisitive search of the physicist".^[7] In addition to possessing more conceptual clarity, Albert Einstein's 1905 special theory of relativity could explain all of the experimental results without referring to an aether at all. This eventually led most physicists to conclude that the earlier notion of a luminiferous aether was not a useful concept.

Mechanical gravitational aether

From the 16th until the late 19th century, gravitational effects had also been modeled using an aether. In a note at the end of his work "A Dynamical Theory of the Electromagnetic Field", Maxwell discussed a model for gravity based on a medium similar to the one he used for the electromagnetic field. He concluded that the medium would have "an enormous intrinsic energy" and would necessarily have to be diminished in areas of mass. He could not "understand in what way a medium can possess such properties" so he did not pursue it further.^[8] The most well-known formulation is Le Sage's theory of gravitation, although variations on the idea were entertained by Isaac Newton, Bernhard Riemann, and Lord Kelvin. For example, Kelvin published a note on Le Sage's model in 1873, in which he found Le Sage's proposal thermodynamically flawed and suggested a possible way to salvage it using the then popular vortex theory of the atom. Kelvin later concluded

This kinetic theory of matter is a dream, and can be nothing else, until it can explain chemical affinity, electricity, magnetism, gravitation, and the inertia of masses (that is, crowds) of vortices. Le Sage's theory might give an explanation of gravity and of its relation to inertia of masses, on the vortex theory, were it not for the essential aeolotropy of crystals, and the seemingly perfect isotropy of gravity. No finger post pointing towards a way that can possibly lead to a surmounting of this difficulty, or a turning of its flank, has been discovered, or imagined as discoverable.^[9]

None of those concepts are considered to be viable by the scientific community today.

Non-standard interpretations in modern physics

General relativity

Albert Einstein sometimes used the word aether for the gravitational field within general relativity, but the only similarity of this relativistic aether concept with the classical aether models lies in the presence of physical properties in space, which can be identified through geodesics. As historians such as John Stachel argue, Einstein's views on the "new aether" are not in conflict with his abandonment of the aether in 1905. As Einstein himself pointed out, no "substance" and no state of motion can be attributed to that new aether.^[10] Einstein's use of the word "aether" found little support in the scientific community, and played no role in the continuing development of modern physics.^[11]^[12]

Quantum vacuum

Quantum mechanics can be used to describe spacetime as being non-empty at extremely small scales, fluctuating and generating particle pairs that appear and disappear incredibly quickly. It has been suggested by some such as Paul Dirac^[13] that this quantum vacuum may be the equivalent in modern physics of a particulate aether. However, Dirac's aether hypothesis was motivated by his dissatisfaction with quantum electrodynamics, and it never gained support from the mainstream scientific community.^[14]

Physicist Robert B. Laughlin wrote:

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 [in special relativity] was that no such medium existed [...] 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. ... Relativity actually says nothing about the existence or nonexistence of matter pervading the universe, only that any 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 not accepted (taboo).^[15]

Pilot waves

Louis de Broglie stated, "Any particle, ever isolated, has to be imagined as in continuous "energetic contact" with a hidden medium."^[16]^[17] However, as de Broglie pointed out, this medium "could not serve as a universal reference medium, as this would be contrary to relativity theory."^[16]