ETHER. – A real entity, extended, yet lacking the more 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 e. (elβέρ). This hypothetical substance was also of great importance in modern physics, as the substratum of luminous phenomena and subsequently of the electromagnetic and gravitational fields.
In the seventeenth century, parallel to the ancient corpuscular interpretations of light (v. OTTICA), and in particular to that adopted and revised by I. Newton, an interpretation of the wave type was proposed. Light, according to its colour, was to be conceived as waves of varying length propagating through a hypothetical medium called e., in a manner perfectly analogous to that in which, very usefully, sound is conceived according to its pitch, as waves of varying length propagating through the air. The enormous speed of light made it necessary to attribute to the hypothetical matter constituting this e. properties so inconceivably different from those of all known matter that no attempt was made to discuss them; it was simply called an imponderable matter endowed with those properties. For more than a century the Newtonian corpuscular theory prevailed, and little or nothing was said of e.; but increasingly important interference phenomena were about to restore the wave theory to favour when the discovery of polarization phenomena definitively revealed the inability of the wave theory to interpret them and brought about its downfall—yet the downfall of the wave theory as it was then conceived, not of all wave theories. Indeed, A. Fresnel (1821) showed that it was sufficient to resort to a transverse wave theory, rather than to the preceding 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.
But the transverse wave theory, that is, a theory which admitted the propagation of waves in directions perpendicular to the plane in which the perturbations producing them occurred, implied the necessity of supposing that e., rather than behaving as an elastic fluid, had to vibrate like a rigid body, because only in this way could the velocity of propagation of the waves reach the speed of light.
The attempts of physicists, including some of the greatest, to overcome this evidently awkward situation were numerous, but fruitless; nevertheless, the advantages of Fresnel’s theory were such that it was universally adopted and yielded only half a century later, in the face of Maxwell’s electromagnetic theory, which indeed implied the hypothesis of an e., but without having to impose upon it any mechanical characteristics.
But the admission of Maxwellian electromagnetic e., like that of the preceding elastic, fluid, and rigid e., still implied a serious difficulty. Every body must evidently be at rest or in motion with respect to it, whereas experimental physics shows that this cannot be detected either by experiments of a mechanical nature or by experiments of an electromagnetic or otherwise optical nature. This is the difficulty that gave rise to the first theory of synachronian relativity (v. RELATIVISMO), which, according to some, would have eliminated the notion of e. from physics. Nevertheless, especially with regard to the further developments of the theory itself, a prudent reserve on this matter is advisable today.
In the scientific world, however, the tendency is becoming increasingly manifest to unify e. and space into a single entity (space, physical space, cosmic matter), which would resolve the contradictory properties now attributed to e. (v. SPAZIO).