Aether theories

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?

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. 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. 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". 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.

Electric aether

Before any connection was made between light and electromagnetism, an electric aether was proposed to explain electrostatic phenomena. Johann Euler, developed a model for electrostatic forces in 1757 based on fluid dynamics in the aether, before the introduction of Coulomb's law. Johann's father, Leonhard Euler hypothesized in 1760 that the luminiferous aether could explain electric phenomena.

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. 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 historical note on Le Sage's model in 1872, noting that Le Sage's proposal disagreed with conservation of energy. Kelvin suggested a possible way to salvage it using the Kelvin's vortex theory of the atom. That theory was extended by JJ Thomson but ultimately abandoned as not productive.

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