Ether, or Æther. Many physical phenomena are supposed to be due to the propagation of a state of stress or motion through a medium filling all space. Such a medium is called an ether. The theory of propagation through an ether is the exact opposite of the theory of direct action at a distance. Newton said that it was inconceivable to him that any one who was capable of thinking correctly in such matters could admit that direct action at a distance was possible. He suggested that Gravitation (q.v.) might be due to diminution of pressure in a fluid filling space in the neighbourhood of material bodies. Sir W. Thomson has shown that this diminution of pressure would be accounted for if we suppose that each particle of matter is a source at which an incompressible fluid is being constantly created at a rate proportional to the mass of the particle, the fluid being also constantly annihilated at an infinite distance. Or, conversely, we might suppose that the fluid is created at infinity, and absorbed and annihilated at each particle, at a rate proportional to its mass. In either case the motion of the fluid would be greatest in the neighbourhood of material bodies; but it is a known fact in hydrodynamics that, in a fluid, the pressure is least where the speed of motion is greatest. Thus the state of stress assumed by Newton would be accounted for.
Light and the so-called radiant heat are supposed to consist in waves of transverse vibrations which are propagated through the ether. Indeed, the phenomena of interference (see LIGHT) cannot be explained in any other way. Similarly, electric and magnetic effects are supposed to be due to the propagation of some state of stress through an ether. But it would be utterly unscientific to fill space over and over again with a new medium for every special phenomenon which has to be explained in this way, and it seems probable that one medium can explain all. Maxwell has assumed the existence of, and has investigated the state of stress in, a medium through which electro-magnetic action is propagated. The equations which he obtains are identical in form with the equations of motion of an elastic solid, and the rate of propagation of an undulation is, in his theory, numerically equal to the ratio of the electro-magnetic and electrostatic units. This ratio is found by experiment to be numerically equal to the speed of propagation of light. Thus the medium which Maxwell assumed in order to explain electro-dynamic phenomena is identical, in his electro-magnetic theory of light, with the luminiferous medium; and the theory gives an explanation of reflection, refraction (single and double), and various other phenomena. Additional support has lately been given to the theory by the experiments of Hertz and others, which prove the existence of these electro-magnetic undulations, and show that they have properties identical with those of light. See ELECTRICITY, and LIGHT.
The ether seems to be of the nature of an elastic solid; and, in order to account for the immense rapidity of its vibrations when radiation passes through it, its rigidity must be excessively large compared with its density. It may be asked how, if this be so, the earth can move through the ether at the rate of nearly a million miles per day. But, if we consider that shoemaker's wax is so brittle a solid that it splinters under the blow of a hammer, and that it yet flows slowly like a liquid into the crevices of a vessel in which it is placed, and that bullets sink slowly down through it, and corks float slowly up through it, the motion of the earth through the ether does not seem so incomprehensible. The bullet moving through the wax experiences great resistance to its motion when it has to move, say, an inch in some weeks; but if we give it some years to move an inch, the resistance would be very small. So it may be that the motion of planets through the ether is relatively much the same as that of the bullet moving with excessive slowness through the wax. From magneto-optic phenomena it seems certain that something of the nature of molecular rotation is going on in the ether (see MATTER, and VORTEX).
There is no evidence of the existence of a condensational-rarefactional wave (as in the case of sound in air) in the ether. In the electro-magnetic theory it is got rid of from its velocity being infinite. In a theory of the ether recently advanced by Thomson, in which it is supposed to have negative compressibility, and to be made stable by being infinite or having rigid boundaries, the speed of propagation of the condensational-rarefactional wave is zero. The question of a contractile ether had been previously considered by Green, but was dismissed by him with the statement (erroneous, as Thomson has shown) that it is essentially unstable. The theories of an ether of zero or positive compressibility lead to results which are inconsistent with experiment. On the other hand, Thomson's theory leads to the known experimental results, as also does the electro-magnetic theory.