SOME RECENT THEORIES OF THE ETHER*

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W. A. SHENSTONE

THINK I can scarcely contrive a more fitting preface to an article on "the ether" than the two quotations which follow. They indicate in the fewest possible words how far we have traveled since the days when "the ether" was invented by Huygens, for the simple purpose of ac- 5 counting for the propagation of light.

The first of these quotations is taken from the late Professor Preston's book on Light, and it runs as follows: "The present tendency indeed of physical science is to regard all the phenomena of Nature, and even of matter 10 itself, as manifestations of energy stored in 'the ether."" The second comes from a "Silliman Lecture," delivered at Yale University, by Professor J. J. Thomson, about three years ago. On one view of the constitution of matter, said Professor Thomson, “All mass is mass of the ether, 15 all momentum, momentum of the ether, and all kinetic kinetic energy energy, of the ether. These two extracts will sufficiently explain the appearance of an article on this subject in the Cornhill Magazine. For, if they truly represent, in any considerable degree, the present 20 trend of physical speculation, could any scientific topic be more important or more interesting?

Only as recently as the year 1894, when he was President of the British Association, at its last meeting at Oxford, the late Marquis of Salisbury told the assembled parlia- 25 ment of science that at present we know absolutely nothing about this all-pervading entity, the ether, except this one fact that it can be made to undulate, and that it performs even this solitary function in an abnormal fashion which *The Cornhill Magazine, 1905.

has caused infinite perplexity. It is my object to tell something about the present state of our knowledge of this elusive entity, and to indicate, as far as I may, the lines followed by some recent speculations concerning its 5 nature and its relations to those other manifestations named by us matter and electricity.

First, let us consider how it has come about that this hypothetical medium called, or I should say recalled, into existence a century ago by Dr. Thomas Young for the 10 single purpose of explaining the phenomena of light, now plays so dominating a part as that assigned it in the two passages quoted above.

I need hardly remind my readers that the notion that there exists an invisible intangible material, filling all that 15 part of space not occupied by ordinary matter, is one of the oldest in science. But many of them may not know that, at one time, ethers were created by men of science almost as plentifully as blackberries by a blackberry bush, that they were called into existence in every difficulty with 20 almost reckless profusion. Ethers have been invented, as Clerk Maxwell has said, "for the planets to swim in, to constitute electric atmospheres and magnetic effluvia, to convey sensations from one part of our body to another, till all space was filled several times over with ethers,” 25 with the result that science in the end turned restive under this "multiplication of entities," this constant piling up, so to speak, of the ethereal population of space and, after a period of reaction during which it became almost a point of honor to discard the assistance of ethers, now 30 contents itself with a single ether-viz., that invented by Huygens in 1690-to explain the propagation of light. But this single ether, as we shall see, has to carry a heavy burden and to perform many and sometimes incongruous functions. It is, as Miss Agnes Clerke has wittily re

marked, at once the universal provider and universal messenger. It is Atlas and Mercury rolled into one.

It may be said, I think, in a general way, that our single ether owes its survival to the unwillingness of science to admit the possibility of “action at a distance,” 5 its unwillingness to admit, for example, that gravity is a primary property of masses incapable of explanation, and acting at all distances across empty space; for it follows from this that when the undulatory theory of light was established by Young and Fresnel in the early part of the 10 last century, the conception of a luminiferous ether was accepted as a necessary part of the theory. How could waves of light and heat emitted, for example, by the sun reach the earth unless some medium capable of undulating occupied the interstellar space between them? For if 15 waves travel from the sun to the earth, then is it not evident that these waves must be waves of something or waves in something? Or, to look at the matter from another point of view, if light be a manifestation of energy, which is ex hypothesi indestructible, and if it be not carried to us 20 by minute particles, as Newton supposed, then what becomes of it during the eight minutes which elapse between the moment when it leaves the sun, and that at which it reaches the earth's atmosphere? Where is it stored during those eight minutes when it is neither on the sun nor on 25 the earth? The answer to these questions is this: The missing energy is in the ether, and the propagation of light across the interstellar space, and anywhere and everywhere, depends upon waves in this ether which fills all space and permeates all matter.

Most of us will agree that, if we accept the undulatory theory of light, we are bound to admit the existence of some medium such as the ether. But when we attempt to form a mental picture of this ether, even if we neglect ·

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for the moment all its properties except its optical properties, we find ourselves in difficulties, for none of us have ever met with anything like it before. It cannot be a gas, for light passes through an exhausted vessel, and through the 5 interstellar void, which we believe contains no gas; and for similar reasons, still less can it be regarded as a liquid or a solid, though it must be incompressible and resist cutting even more strongly than steel itself. One thing, as Lord Salisbury said, we do know about the ether. If 10 it exists at all, it can undulate. We feel we tread solid ground here, for if the ether could not undulate, then it could not transmit the vibrations which we call light. The ethereal undulations which constitute light must differ widely from the motions which originate the waves of the 15 sea, or the aerial disturbances known as sound, and the elasticity of the ether must be of a different order from that of the familiar gases, liquids, and solids. Air yields to pressure, and sound depends upon oscillations of its particles backward and forward along the line of pro20 pagation of the audible disturbances. The ether, on the other hand, must be regarded as incompressible; for the properties of light require us to assume that light-waves are not produced by the compression and rarefaction of a medium like the air, that they are not waves such as 25 might be produced, for example, if the separate type on

this page should presently begin to oscillate backward and forward from left to right, and right to left, along the lines of print, but transverse waves such as we should have before us if the type were to swing upward and 30 downward across the lines so as to produce more or less the effect suggested by the following diagram.

has

Now, it is difficult to picture a substance which we cannot weigh, which is as rigid as steel to pressure, and yet yields, to some extent, to the distortional stresses that will account for the propagation in it of luminous undulations. Nor are our difficulties diminished when we remember that we 5 must conceive this bewildering substance as filling all space, permeating the inmost recesses of all matter, solid, liquid, and gaseous (for in its absence how could these transmit light and other electromagnetic disturbances?); rigid, as I have said, as steel, and yet permitting material 10 particles like grains of sand or the earth to move freely through it. Clearly the most learned of us has no experience to appeal to here. How can we draw a mental picture of such stuff as this? Think of men blind from their birth groping their way through a world they have 15 never seen, and you will have some conception of the difficulties which stand in our way.

But on fuller thought you will see also that the problem may not be altogether beyond our powers. The blind man with his stick learns much that is true about the world 20 he lives in-sufficient, in fact, to enable him to construct in his mind a useful, if imperfect, hypothesis or working model of his invisible environment; and so, similarly, with the resources at our command, limited though they may be, why should we not discover a great deal about this 25 ether which we can neither see nor feel, but which exists, as we are convinced, in us and around us?

"The picture we may form, like a blind man's model of his unseen world, may be, at first, but an imperfect and colorless reproduction of the reality. But what of that? 30 It will grow more true and more perfect, and in time may · even gain something corresponding to color, if we press on. But while the task of forming a clear idea or mental picture of the ether constitutes one of the most difficult