Chapter 2 THE INTERSTELLAR ETHER AS A

CONNECTING MEDIUM

So far I have given a general idea of the present condition of the wave theory of light, both from its theoretical and from its experimental sides. The waves of light are not anything mechanical or material, but are something electrical and magnetic-they are in fact electrical disturbances periodic in space and time, and travelling with a known and tremendous speed through the ether of space. Their very existence depends upon the ether, and their speed of propagation is its best known and most certain quantitative property.

A statement of this kind does not even initially express a tithe of our knowledge on the subject; nor does our knowledge exhaust any large part of the region of discoverable fact; but the statement above made may be regarded as certain, although the absence of mechanics or ordinary dynamics about it removes it, or seems to remove it, from the category of the historically soundest and best worked department of Physical Science, viz. that explored by the Newtonian method. Though in truth there is every reason to suppose that we should have had Newton with us in these modern developments.

There is, I believe, a general tendency to underrate the certainty of some of the convictions to which natural philosophers have gradually, in the course of their study of nature, been impelled; more especially when those convictions have reference to something intangible and occult. The existence of a continuous space-filling medium, for instance, is probably regarded by most educated people as a more or less fanciful hypothesis, a figment of the scientific imagination,-a mode of collating and welding together a certain number of observed facts, but not in any physical sense a reality, as water and air are realities.

I am speaking purely physically. There may be another point of view from which all material reality can be denied, but with those questions physics proper has nothing to do; it accepts the evidence of the senses, regarding them as the tools or instruments wherewith man may hope to understand one definite aspect of the universe; and it leaves to philosophers, equipped from a different armoury, the other aspects which the material universe may-nay, must-possess.

By a physical "explanation" is meant a clear statement of a fact or law in terms of something with which daily life has made us familiar. We are all chiefly familiar, from our youth up, with two apparently simple things, motion and force. We have a direct sense for both these things. We do not understand them in any deep way, probably we do not understand them at all, but we are accustomed to them. Motion and force are our primary objects of experience and consciousness; and in terms of them all other less familiar occurrences may conceivably be stated and grasped. Whenever a thing can be so clearly and definitely stated, it is said to be explained, or understood; we are said to have "a dynamical theory" of it. Anything short of this may be a provisional or partial theory, an explanation of the less known in terms of the more known, but Motion and Force are postulated in physics as the completely known: and no attempt is made to press the terms of an explanation further than that. A dynamical theory is recognised as being at once necessary and sufficient.

Now, it must be admitted at once that of very few things have we at present such a dynamical explanation. We have no such explanation of matter, for instance, or of gravitation, or of electricity, or ether, or light. It is always conceivable that of some such things no purely dynamical explanation will ever be forthcoming, because something more than motion and force may perhaps be essentially involved. Still, physics is bound to push the search for an explanation to its furthest limits; and so long as it does not hoodwink itself by vagueness and mere phrases-a feebleness against which its leaders are mightily and sometimes cruelly on their guard, preferring to risk the rejection of worthy ideas rather than permit a semi-acceptance of anything fanciful and obscure-so long as it vigorously probes all phenomena within its reach, seeking to reduce the physical aspect of them to terms of motion and force,-so long it must be upon a safe track. And, by its failure to deal with certain phenomena, it will learn-it already begins to suspect, its leaders must long have surmised-the existence of some third, as yet unknown, category, by incorporating which the physics of the future may rise to higher flights and an enlarged scope.

I have said that the things of which we are permanently conscious are motion and force, but there is a third thing which we have likewise been all our lives in contact with, and which we know even more primarily, though perhaps we are so immersed in it that our knowledge realises itself later,-viz. life and mind. I do not now pretend to define these terms, or to speculate as to whether the things they denote are essentially one and not two. They exist, in the sense in which we permit ourselves to use that word, and they are not yet incorporated into physics. Till they are, they may remain more or less vague; but how or when they can be incorporated, is not for me even to conjecture.

Still, it is open to a physicist to state how the universe appears to him, in its broad character and physical aspect. If I were to make the attempt I should find it necessary for the sake of clearness to begin with the simplest and most fundamental ideas; in order to illustrate, by facts and notions in universal knowledge, the kind of process which essentially occurs in connection with the formation of higher and less familiar conceptions,-in regions where the common information of the race is so slight as to be useless.

Primary Acquaintance with the External World.

Beginning with our most fundamental sense I should sketch the matter thus:-

We have muscles and can move. I cannot analyse motion,-I doubt if the attempt is wise,-it is a simple immediate act of perception, a direct sense of free unresisted muscular action. We may indeed move without feeling it, and that teaches us nothing, but we may move so as to feel it, and this teaches us much, and leads to our first scientific inference, viz. space; that is, simply, room to move about. We might have had a sense of being jammed into a full or tight-packed universe; but we have not: we feel it to be a spacious one.

Of course we do not stop at this baldness of inference: our educated faculty leads us to realise the existence of space far beyond the possibility of direct sensation; and, further, by means of the direct appreciation of speed in connection with motion,-of uniform and variable speed,-we become able to formulate the idea of "time," or uniformity of sequence; and we attain other more complex notions-acceleration and the like-upon a consideration of which we need not now enter.

But our muscular sense is not limited to the perception of free motion: we constantly find it restricted or forcibly resisted. This "muscular action impeded" is another direct sense, that of "force"; and attempts to analyse it into anything simpler than itself have hitherto resulted only in confusion. By "force" is meant primarily muscular action not accompanied by motion. Our sense of this teaches us that space, though roomy, is not empty: it gives us our second scientific inference-what we call "matter."

Again we do not stop at this bare inference. By another sense, that of pain, or mere sensation, we discriminate between masses of matter in apparently intimate relation with ourselves, and other or foreign lumps of matter; and we use the first portion as a measure of the extent of the second. The human body is our standard of size. We proceed also to subdivide our idea of matter,-according to the varieties of resistance with which it appeals to our muscular sense,-into four different states, or "elements" as the ancients called them; viz. the solid, the liquid, the gaseous, and the etherial. The resistance experienced when we encounter one or other of these forms of material existence varies from something very impressive-the solid,-through something nearly impalpable-the gaseous,-up to something entirely imaginative, fanciful, or inferential, viz. the ether.

The ether does not in any way affect our sense of touch (i.e. of force); it does not resist motion in the slightest degree. Not only can our bodies move through it, but much larger bodies, planets and comets, can rush through it at what we are pleased to call a prodigious speed (being far greater than that of an athlete) without showing the least sign of friction. I myself, indeed, have designed and carried out a series of delicate experiments to see whether a whirling mass of iron could to the smallest extent grip the ether and carry it round, with so much as a thousandth part of its own velocity. These shall be described further on, but meanwhile the result arrived at is distinct. The answer is, no; I cannot find a trace of mechanical connection between matter and ether, of the kind known as viscosity or friction.

Why, then, if it is so impalpable, should we assert its existence? May it not be a mere fanciful speculation, to be extruded from physics as soon as possible? If we were limited for our knowledge of matter to our sense of touch, the question would never even have presented itself; we should have been simply ignorant of the ether, as ignorant as we are of any life or mind in the universe not associated with some kind of material body. But our senses have attained a higher stage of development than that. We are conscious of matter by means other than its resisting force. Matter acts on one small portion of our body in a totally different way, and we are said to taste it. Even from a distance it is able to fling off small particles of itself sufficient to affect another delicate sense. Or again, if it is vibrating with an appropriate frequency, another part of our body responds; and the universe is discovered to be not silent but eloquent to those who have ears to hear. Are there any more discoveries to be made? Yes; and already some have been made. All the senses hitherto mentioned speak to us of the presence of ordinary matter,-gross matter, as it is sometimes called,-though when appealing to our sense of smell, and more especially to a dog's sense of smell, it is not very gross; still, with the senses hitherto enumerated we should never have become aware of the ether. A stroke of lightning might have smitten our bodies back into their inorganic constituents, or a torpedo-fish might have inflicted on us a strange kind of torment; but from these violent tutors we should have learnt little more than a schoolboy learns from the once ever-ready cane.

But it so happens that the whole surface of our skin is sensitive in yet another way, and a small portion of it is astoundingly and beautifully sensitive, to an impression of an altogether different character-one not necessarily associated with any form of ordinary matter-one that will occur equally well through space from which all solid, liquid, or gaseous matter has been removed. Hold your hand near a fire, put your face in the sunshine, and what is it you feel? You are now conscious of something not arriving by ordinary matter at all. You are now as directly conscious as you can be of the etherial medium. True the process is not very direct. You cannot apprehend the ether as you can matter, by touching or tasting or even smelling it; but the process is analogous to the kind of perception we might get of ordinary matter if we had the sense of hearing alone. It is something akin to vibrations in the ether that our skin and our eyes feel.

It may be rightly asserted that it is not the etherial disturbances themselves, but other disturbances excited by them in our tissues, that our heat nerves feel; and the same assertion can be made for our more highly-developed and specialised sight nerves. All nerves must feel what is occurring next door to them, and can directly feel nothing else; but the "radiation," the cause which excited these disturbances, travelled through the ether,-not through any otherwise known material substance.

It should be a commonplace to rehearse how we know this. Briefly, thus: Radiation conspicuously comes to us from the sun. If any free or ordinary matter exists in the intervening space, it must be an exceedingly rare gas. In other words, it must consist of scattered particles of matter, some big enough to be called lumps, some so small as to be merely atoms, but each with a considerable gap between it and its neighbour. Such isolated particles are absolutely incompetent to transmit light. And, parenthetically, I may say that no form of ordinary matter, solid, liquid, or gaseous, is competent to transmit a thing travelling with the speed and subject to the known laws of light. For the conveyance of radiation or light all ordinary matter is not only incompetent, but hopelessly and absurdly incompetent. If this radiation is a thing transmitted by anything at all, it must be by something sui generis.

But it is transmitted,-for it takes time on the journey, travelling at a well-known and definite speed; and it is a quivering or periodic disturbance, falling under the general category of wave-motion. Nothing is more certain than that. No physicist disputes it. Newton himself, who is commonly and truly asserted to have promulgated a rival theory, felt the necessity of an etherial medium, and knew that light consisted essentially of waves.

Sight.

A small digression here, to avoid any possible confusion due to the fact that I have purposely associated together temperature nerves and sight nerves. They are admittedly not the same, but they are alike in this, that they both afford evidence of radiation; and, were we blind, we might still know a good deal about the sun, and if our temperature nerves were immensely increased in delicacy (not all over, for that would be merely painful, but in some protected region), we might even learn about the moon, planets, and stars. In fact, an eye, consisting of a pupil (preferably a lens) and a sunken cavity lined with a surface sensitive to heat, could readily be imagined, and might be somewhat singularly effective. It would be more than a light recorder, it could detect all the etherial quiverings caused by surrounding objects, and hence would see perfectly well in what we call "the dark." But it would probably see far too much for convenience, since it would necessarily be affected by every kind of radiation in simple proportion to its energy; unless, indeed, it were provided with a supply of screens with suitably selected absorbing powers. But whatever might be the advantage or disadvantage of such a sense-organ, we as yet do not possess one. Our eye does not act by detecting heat; in other words, it is not affected by the whole range of etherial quiverings, but only by a very minute and apparently insignificant portion. It wholly ignores the ether waves whose frequency is comparable with that of sound; and, for thirty or forty octaves above this, nothing about us responds; but high up, in a range of vibration of the inconceivably high pitch of four to seven hundred million million per second-a range which extremely few accessible bodies are able to emit, and which it requires some knowledge and skill artificially to produce-to those waves the eye is acutely, surpassingly, and most intelligently sensitive.

This little fragment of total radiation is in itself trivial and negligible. Were it not for men, and glow-worms, and a few other forms of life, hardly any of it would ever occur, on such a moderate-sized lump of matter as the earth. Except for an occasional volcano, or a flash of lightning, only gigantic bodies like the sun and stars have energy enough to produce these higher flute-like notes; and they do it by sheer main force and violence-the violence of their gravitative energy-producing not only these, but every other kind of radiation also. Glow-worms, so far as I know, alone have learnt the secret of emitting the physiologically useful waves, and none others.

Why these waves are physiologically useful-why they are what is called "light," while other kinds of radiation are "dark," is a question to be asked, but, at present, only tentatively answered. The answer must ultimately be given by the Physiologist; for the distinction between light and non-light can only be stated in terms of the eye, and its peculiar specialised sensitiveness; but a hint may be given him by the Physicist. The etherial waves which affect the eye and the photographic plate are of a size not wholly incomparable with that of the atoms of matter. When a physical phenomenon is concerned with the ultimate atoms of matter, it is often relegated at present to the field of knowledge summarised under the head of Chemistry. Sight is probably a chemical sense. The retina may contain complex aggregations of atoms, shaken asunder by the incident light vibrations, and rapidly built up again by the living tissues in which they live; the nerve-endings meanwhile appreciating them in their temporarily dissociated condition. A vague speculation! Not to be further countenanced except as a working hypothesis leading to examination of fact; but, nevertheless, the direction in which the thoughts of some physicists are tending-a direction towards which many recently discovered experimental facts point.[2]

Gravitation and Cohesion.

It would take too long to do more than suggest some other functions for which a continuous medium of communication is necessary. We shall argue in Chapter VIII that technical action at a distance is impossible. A body can only act immediately on what it is in contact with; it must be by the action of contiguous particles,-that is, practically, through a continuous medium, that force can be transmitted across space. Radiation is not the only thing the earth feels from the sun; there is in addition its gigantic gravitative pull, a force or tension more than what a million million steel rods, each seventeen feet in diameter, could stand (see Chap. IX). What mechanism transmits this gigantic force? Again, take a steel bar itself: when violently stretched, with how great tenacity its parts cling together! Yet its particles are not in absolute contact, they are only virtually attached to each other by means of the universal connecting medium-the ether,-a medium that must be competent to transmit the greatest stresses which our knowledge of gravitation and of cohesion shows us to exist.

Electricity and Magnetism.

Hitherto I have mainly confined myself to the perception of the ether by our ancient sense of radiation, whereby we detect its subtle and delicate quiverings. But we are growing a new sense; not perhaps an actual sense-organ, though not so very unlike a new sense-organ, though the portions of matter which go to make the organ are not associated with our bodies by the usual links of pain and disease; they are more analogous to artificial teeth or mechanical limbs, and can be bought at an instrument-maker's.

Electroscopes, galvanometers, telephones-delicate instruments these; not yet eclipsing our sense-organs of flesh, but in a few cases coming within measurable distance of their surprising sensitiveness. And with these what do we do? Can we smell the ether, or touch it, or what is the closest analogy? Perhaps there is no useful analogy; but nevertheless we deal with it, and that closely. Not yet do we fully realise what we are doing. Not yet have we any dynamical theory of electric currents, of static charges, and of magnetism. Not yet, indeed, have we any dynamical theory of light. In fact, the ether has not yet been brought under the domain of simple mechanics-it has not yet been reduced to motion and force: and that probably because the force aspect of it has been so singularly elusive that it is a question whether we ought to think of it as material at all. No, it is apart from mechanics at present. Conceivably it may remain apart; and our first additional category, wherewith the foundations of physics must some day be enlarged, may turn out to be an etherial one. And some such inclusion may have to be made before we can attempt to annex vital or mental processes. Perhaps they will all come in together.

Howsoever these things be, this is the kind of meaning lurking in the phrase that we do not yet know what electricity or what the ether is. We have as yet no dynamical explanation of either of them; but the past century has taught us what seems to their student an overwhelming quantity of facts about them. And when the present century, or the century after, lets us deeper into their secrets, and into the secrets of some other phenomena now in course of being rationally investigated, I feel as if it would be no merely material prospect that will be opening on our view, but some glimpse into a region of the universe which Science has never entered yet, but which has been sought from far, and perhaps blindly apprehended, by painter and poet, by philosopher and saint.

Note on the Spelling of Ethereal.

The usual word "ethereal" suggests something unsubstantial, and is so used in poetry; but for the prosaic treatment of Physics it is unsuitable, and etheric has occasionally been used instead. No just derivation can be given for such an adjective, however; and I have been accustomed simply to spell etherial with an i when no poetic meaning was intended. This alternative spelling is not incorrect; but Milton uses the variant "ethereous," in a sense suggestive of something strong and substantial (Par. Lost, vi, 473). Either word, therefore, can be employed to replace "ethereal" in physics: and in succeeding chapters one or other of these is for the most part employed.

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