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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? ...
These generally covariant theories describes a spacetime endowed with both a metric and a unit timelike vector field named the aether. The aether in this theory is "a Lorentz-violating vector field" [1] unrelated to older luminiferous aether theories; the "Einstein" in the theory's name comes from its use of Einstein's general relativity ...
The path difference can be any fraction of the wavelength, depending on the angle and speed of the aether wind. To prove the existence of the aether, Michelson and Morley sought to find the "fringe shift". The idea was simple, the fringes of the interference pattern should shift when rotating it by 90° as the two beams have exchanged roles.
These theories were supported by the results of the Michelson–Morley experiment in which evidence for the motion of aether was conclusively absent. [23] The results of the experiment influenced many physicists of the time and contributed to the eventual development of Einstein's theory of special relativity .
Lorentz (1892a) set the foundations of Lorentz aether theory, by assuming the existence of electrons which he separated from the aether, and by replacing the "Maxwell–Hertz" equations by the "Maxwell–Lorentz" equations. In his model, the aether is completely motionless and, contrary to Fresnel's theory, also is not partially dragged by matter.
The so-called Sagnac effect was considered excellent evidence for aether at the time, but was later explained via general relativity. Good explanations based on SR also exist. 1914 – Walther Zurhellen uses observations of binary stars to determine if the speed of light is dependent on movement of the source.
The aether hypothesis was the topic of considerable debate throughout its history, as it required the existence of an invisible and infinite material with no interaction with physical objects. As the nature of light was explored, especially in the 19th century, the physical qualities required of an aether became increasingly contradictory.
A 1933 portrait of E. T. Whittaker by Arthur Trevor Haddon. The book was originally written in the period immediately following the publication of Einstein's Annus Mirabilis papers and several years following the early work of Max Planck; it was a transitional period for physics, where special relativity and old quantum theory were gaining traction.