ETHER. - A real entity, extended, yet lacking the most common qualities of sensible material entities, such as weight, impermeability, etc., and filling the celestial spaces. The hypothesis of its existence dates back to the most remote Greek antiquity, from which it received the name of *aether* (ἀήρ). This hypothetical substance has also held great importance in modern physics, as the substratum of luminous phenomena and later of electromagnetic and gravitational fields.
In the 17th century, alongside ancient corpuscular interpretations of light (see OPTICS), and in particular the one adopted and revised by I. Newton, an interpretation of an undulatory type was proposed. Light, depending on its color, was to be conceived as waves of varying length propagating in a hypothetical medium called *aether*, in a manner perfectly analogous to how sound, depending on its pitch, is conceived as waves of varying length propagating in air. The enormous speed of light necessitated attributing to this hypothetical matter, constituting the *aether*, properties so inconceivably different from those of all known matter that it did not stop to discuss them, and it was simply called an imponderable matter endowed with those properties. For over a century, Newton’s corpuscular theory prevailed, and little or nothing was said about the *aether*; but the increasingly important phenomena of interference were about to restore the wave theory to favor when the discovery of polarization phenomena definitively highlighted the inability of the wave theory to explain them and led to its abandonment. However, this was not the abandonment of all wave theories, but only of the wave theory as then conceived. Indeed, A. Fresnel (1821) showed that it sufficed to resort to a transverse wave theory, rather than the previous longitudinal wave theory, to make possible not only the interpretation of polarization phenomena but also a systematic interpretation of all optical phenomena far superior to that provided by the corpuscular theory.
Yet the transverse wave theory, i.e., a theory that admits the propagation of waves in the direction perpendicular to the plane in which the disturbances producing them occur, implied the necessity of supposing that the *aether*, instead of vibrating like an elastic fluid, had to vibrate like a rigid body, because only thus could the propagation speed of the waves reach the speed of light.
The attempts of physicists, even of some great men, to overcome this situation of evident discomfort were numerous, but in vain; however, the advantages of Fresnel’s theory were such that it was universally adopted and yielded only half a century later, before Maxwell’s electromagnetic theory, which indeed implied the hypothesis of an *aether*, but without imposing upon it any mechanical characteristics.
Yet even the admission of the Maxwellian electromagnetic *aether*, like that of the previous elastic, fluid, and rigid *aethers*, still implied a serious difficulty. Every body must evidently be in a state of rest or motion relative to it, whereas experimental physics shows that neither through mechanical experiments nor through electromagnetic or optical experiments is it possible to ascertain this. It is this difficulty that gave rise to the advent of Einstein’s first theory of relativity (see RELATIVITY), which, according to some, would have eliminated the notion of *aether* from physics. However, especially with regard to the further developments of the same theory, a prudent reserve is advisable on this point.
Yet in the scientific world, the tendency to unify the *aether* and space into a single entity (space, physical space, cosmic matter) that would resolve the contradictory properties now attributed to the *aether* is becoming increasingly evident (see SPACE).