In physics, aether theories (also known as ether theories) propose 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?" 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. However, a series of increasingly complex experiments had been carried out in the late 1800s like the Michelson–Morley experiment in an attempt to detect the motion of Earth through the aether, and had failed to do so. 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