Ether

ETHER. - A real entity, extended, but lacking the most common qualities of sensible material entities, such as weight, impenetrability, etc., and filling the celestial spaces. The hypothesis of its existence goes back to the most remote Greek antiquity, from which it received the name of ether (αἰθήρ). This hypothetical substance was also of great importance in modern physics, as the substrate of luminous phenomena and then of the electromagnetic and gravitational fields.

In the 17th century, in parallel with the ancient corpuscular interpretations of light (v. OTTICA), and in particular with that adopted and reworked by I. Newton, an undulatory interpretation was put forward. Light, according to its color, would have to be conceived as waves of varying length propagating in a hypothetical medium called ether, in a manner perfectly analogous to that by which, very usefully, sound is conceived according to its pitch, as waves of varying length propagating in air. The enormity of the speed of light imposed the attribution to the hypothetical matter constituting this ether of properties so inconceivably different from those of all known matters that people did not stop to discuss them and limited themselves to calling it an imponderable matter endowed with those properties. For over a century the Newtonian corpuscular theory prevailed and little or nothing was said of the ether; but ever more important phenomena of interference were about to restore the undulatory theory to favor when the discovery of the phenomena of polarization definitively brought out the inability of the undulatory theory to interpret them and determined its fall; but the fall of the undulatory theory as it was then conceived, not that of all undulatory theories. In fact A. Fresnel (1821) showed that it was enough to have recourse to a transverse undulatory theory, rather than to the previous longitudinal undulatory theory, to make possible not only the interpretation of the phenomena of polarization but also a systematic interpretation of all optical phenomena far superior to that provided by the corpuscular theory.

But the transverse undulatory theory, that is, a theory that admitted the propagation of waves in the direction perpendicular to the plane in which the perturbations producing them occurred, implied the necessity of supposing that the ether, rather than as an elastic fluid, had to vibrate as a rigid body, because only thus could the speed of propagation of the waves reach the speed of light.

The attempts of physicists, including some of the greatest, to overcome this situation of evident discomfort were numerous, but vain; however, the advantages of Fresnel's theory were such that it was adopted everywhere and yielded only half a century later, before Maxwell's electromagnetic theory, which indeed implied the hypothesis of an ether, but without having to impose on it any characteristic of a mechanical nature.

But even the admission of the Maxwellian electromagnetic ether, like that of the previous elastic ethers, fluid and rigid, still implied a serious difficulty. Every body must evidently find itself in a state of rest or of motion with respect to it, whereas experimental physics shows that, neither with experiments of a mechanical nature, nor with experiments of an electromagnetic or otherwise optical nature, is it possible to ascertain this. This is the difficulty that gave rise to the advent of the first theory of synchronic relativity (v. RELATIVITÀ), which according to some would have eliminated the notion of ether from physics. But, especially as regards the further developments of the same theory, a prudent reservation is advisable on this point today.

However, the tendency to unify the ether and space into a single entity (space, physical space, cosmic matter) that would resolve the contradictory properties today attributed to the ether is becoming ever more manifest in the scientific world (v. SPAZIO).

Pier Paolo Straneo