ETHER

whether space is a mere geometrical abstraction, or whether it has definite physical properties which can be investigated, is a question which in one form or another has often been debated. as to the parts which are occupied by matter, that is by a substance which appeals to the senses, there has never been any doubt; and the whole of science may be said to be an investigation of the properties of matter. but from time to time attention has been directed to the interven- ing portions of space from which sensible matter is absent; and this also has physical properties, of which the complete tnvesti- gation has hardly begun. these physical properties do not appeal directly to the senses, and are therefore comparatively obscure; but there is now no doubt of their existence, even among those who still prefer to use the term space. buta space endowed with physical properties is more than a geometrical abstraction, and is most conveniently thought of as a substantial reality, to which therefore some other name is appropriate. the term used is unimportant, but long ago the term ether was invented; it was adopted by isaac new- ton, and is good enough for us. the term ether therefore con- notes a genuine entity filling all space, without any break or cavity anywhere, the one omnipresent physical reality, of which there is a growing tendency to perceive that everything in the material universe consists; matter itself being in all probability one of its modifications. many attempts have been made to state the properties of such a substance in terms of material analogies, and all these attempts have shown signs of weakness and may be said to have failed. the properties of the ether are too fundamental to be stated in terms of something else. there have been tendencies at different times to invent ethers or effluvia with special qualities to account for specific phenom- ena. these attempts were long ago discarded, and are now re- garded as absurd. but that space has physical properties is a definite fact of experience, provided experience is extended to include inferences and deductions and is not limited to direct sensual perception. what we perceive directly are length, breadth and height, modified here and there by a resistance or obstruction which we call matter, and combined with the ele- ment of time or duration, as exhibited and measured by the ether motion of matter, with speeds that can be directly apprehended. but in addition to all that mass of common experience, the free unobstructed space is modified by the neighbourhood of matter; so that there exists everywhere a gravitational potential varying inversely with the distance from its appropriate portion of matter; the result of which is that matter tends to move from places of lower to places of higher potential, as if some force were driving the masses of matter together. civil engineering is constantly concerned with this fact; and on this basis the whole of the older astronomy has been worked out in detail. atoms of matter—the atoms of matter are not quiescent, even when a mass appears stationary, but are in a state of rapid quivering motion; and these motions are not independent of each other, but are interrelated and connected by additional and special disturbances which they communicate to the space or medium in which they occur. and about these supplementary disturbances our sense organ, the eye, has given us a mass of indirect information. these disturbances, though generated by matter, are not conveyed or transmitted by matter. they travel at a rate depending on innate properties of space; or rather, as we feel bound to say, on the physical properties of the substantial reality which fills space; thereby telling us some- thing definite about those properties, though in a form difficult of apprehension and one which is not fully expressible in terms of any of the familiar properties of matter. thus the different masses of matter, even though separated by great distances, are not isolated or independent of each other. they are connected gravitationally, and they are connected optically. the energies of the earth, of which we constantly make use, are derived from the sun, and have travelled across the intervening 92,000,000 m. of empty space at a perfectly known and definite rate, with which rate matter has nothing to do. there may be uncertainty as to what exactly it is that is travelling; but the fact that it is travelling energy is certain. all that matter does is to generate this radiant energy at one end and absorb it at the other. concerning the processes of generation and absorption a good deal is now known. moreover not only is the speed of travel of the transmitted disturbance known, but also the fact that it is a periodic disturbance, expressible mathematically in exact analogy with a wave equation. wherefore the disturbance may be spoken of without further hypothesis as ether “waves,” the generic name for which is radiation, a small range of this radta- tion being visible light. radiation is generated by some cataclysm or collision or other violent and sudden disturbance in the atoms of matter. when radiation encounters matter (unless it be merely reflected or passed on) it can throw the multitude of atoms into the con- fused motion we call “ heat,” and produce other remarkable and chemical effects. thus an ether is necessary for the purpose of transmitting what is called gravitational force between one piece of matter and another, and for the still more important and universal purpose of transmitting waves of radiation be- tween one piece of matter and another however small and dis- tant they be. electric and magnetic propertics—in addition to those two functions, other properties have been discovered, notably the properties called electric and magnetic. atoms of matter are electrically constituted, and accordingly tend to attract each other with a force which is the source of chemical affinity; with the result that molecules and other aggregates are formed, of which the structure is studied in the science of chemistry. moreover the molecules themselves attract each other by a residual affinity, giving the familiar shape of crystals and other solids, the particles of which are held together in regular packing across ultra-microscopic intervals by what is called cohesion, for which likewise the ether must be held responsible. for, as newton forcefully said in other words, it is absurd to imagine one piece of matter acting mechanically on another at a distance, whether that distance be large or small, without some interven- ing mechanism. the continuous medium which fills space there- fore is not only the vehicle of gravitation and light, but is also ether the instrument for cohesion and chemical! affinity and for elec- tric and magnetic attractions and repulsions. the intimate structure of the ether will probably be expressi- ble and partially understood in terms of the phenomena of electricity and magnetism: for electric and magnetic influences which can be the subject of experiment are transmitted per- fectly through vacuum, that is, across space empty of matter. they represent primarily properties of the ether, and are only made manifest to our senses by means of matter. it was in terms of electricity and magnetism that clerk maxwell was able to explain the phenomenon of light. a close study of elec- tro-magnetism, that is, of the interaction between electric and magnetic disturbances, showed that they must combine into a wave equation, the waves being transmitted at a rate calculable from purely electric and magnetic considerations. this velocity turned out to be the velocity of light; and so in 1865 the true theory of light was born. not that it is anything like complete. we know too little of the electric and magnetic properties of the ether to be able to picture them exactly. what we do know is that light is an electro-magnetic phenomenon, and that it is entirely dependent on the properties of the ether. the ether involves or possesses properties expressible by two fundamental constants; one of them regulates the force of attraction between two electrified bodies, and the other the force of attraction between two mag- nets. neither constant by itself is as yet known. but the value of the constants multiplied together is known: it was discovered by clerk maxwell, and is the reciprocal of the square of the velocity of light. in other words, the combination of the electric and magnetic properties of the ether enables it to transmit waves at a rate equal to the inverse geometric mean of its two constants. so far we have been dealing with things which have been known for some time. but the subject is so fundamental and important that a recapitulation in other terms seemed advisa- ble. it now remains to deal with the later progress which has been made in investigating the properties of this extraordinary non-material but physical substance. perhaps “ substance ” is hardly the right term, for, though exceedingly substantial in one sense, it makes no appeal to the senses and is therefore unlike any substance we know. in the oth edition of the encyclopedia britannica an attempt was made to estimate the elasticity and the density of the ether, on the strength of a certain hypothesis made by lord kelvin. in the 11th edition (see 1.292) this estimate was repeated, and it was hinted that the hypothesis might be erroneous and the values obtained exceedingly wrong. everything tends to confirm that conclusion. strictly speaking the very terms elasticity and density, which are terms applicable to matter, may be inappli- cable to the ether without re-definition; if used they must be understood in a formal sense. the properties of the ether are not likely to be expressible in terms of matter; but, as we have no better clue, we must proceed by analogy, and we may apolo- getically speak of the elasticity and density of the ether as representing things which, if it were matter, would be called by those names. what these terms really express we have not yet fathomed; but if, as is now regarded as very probable, atomic matter is a structure in ether, there is every reason for saying that the ether must in some sense be far denser than any known material substance. the densest known matter, or matter of highest inertia, is found in some of the stars; the barely visible companion of sirius having been found, on converging grounds of evidence, to be more than 1,000 times as dense as lead. the ether must exceed even that startling amount; indeed there are sound arguments for regarding it as a million times denser. the fundamental substance is not likely to be filmy and un- substantial. recent discoveries have represented the atom of matter as composed of minute electric charges, which fill hardly any of the space inside the atom, so that it is as porous as a solar system. the great bulk of an atom is occupied only by a few electrons; so that it is by no means impenetrable to particles, which if they 1027 fly through it at sufficient speed, can escape being entangled and absorbed. matter therefore is comparatively a gossamer structure, subsisting in a very substantial medium. an estimate of the substantiality of the medium can be made from its mag- netic energies, and it comes out almost incredibly large. if it is right to express it in terms of material properties (which is doubtful) its inertia comes out as of the order of 1,000 tons per cubic millimetre. while as to the elasticity, that is still more enormous, since it is equal to the density multiplied by the square of the velocity of light. these values are barely conceivable, being so much higher than anything of which we have sensual experience. but still they are definite and finite, and are capable of being measured and expressed; so the ether is a physical substance, with proper- ties which can in time be ascertained; and if the estimate above given of the source of the vast energies involved is wrong (as it is sure to be inadequately and incompletely worded) subsequent investigation can correct it. meanwhile we may assume that there is some truth underlying these modes of expression, a truth which we cannot at present formulate any better. the constants embodying the physical properties of the ether though so huge are not infinite, and it seems to have certain very simple and perfect properties. it is perfectly transparent, it dissipates no energy; otherwise the stars and the spiral nebu- lae could not be seen at their gigantic distances across space. there is no friction between matter and ether, otherwise a por- tion of matter isolated from the rest would cool down, and the planets would not continue forever in their courses unperturbed. the ether has nothing of what we call in matter viscosity or fluid friction. there is no real heat in the ether, nor any sound; nothing but one simple type of propagation of effects by waves goes on in free space, and that with a definite unchangeable velocity which is known as the velocity of light, the one funda- mental and so to say absolute velocity in the universe. the velocity of light-—the question arises as to what that velocity can be due to. the most probable surmise or guess at present is that the ether is a perfectly incompressible continuous fluid, in a state of fine-grained vortex motion, circulating with that same enormous speed. for it has been partly, though as yet incompletely, shown that such a vortex fluid would transmit waves of the same general nature as light waves—i.e., periodic disturbances across the line of propagation—and would transmit them at a rate of the same order of magnitude as the vortex or circulation speed. there remains indeed a question of stabil- ity to be safeguarded, but in these days of quanta (see (quantum theory) stability considerations are apt to be deferred. thus it appears possible that some day an extended hydrodynamics of a perfect fluid will explain all the physical properties of the material universe.! this motion of a structure due to vortex circulation in a per- fect fluid may be regarded by some as too material an idea, and it may have to be discarded; but it is the nearest approach that can be suggested to a pictorial image of the etheric constitution. certainly no structureless fluid could transmit actual radiation. and certainly the ether is continuous and without viscosity or any dissipation of energy, and so in many respects is like an ideal fluid. more than that we cannot say, except speculatively, about its constitution. meanwhile we must assume that the ether has a substantial- ity and a wave-conveying structure beyond our present clear imaginings, with parts of it modified in an unknown way into electrons and protons; that of these the atoms of matter are built up; and that the whole of material activity consists in the interactions of these minute electric charges, connected as they are by their lines of force and by radiation. these electric charges, and the aggregates which they have built up are subject to what we experience or recognise as loco- motion. the ether itself is stationary. whether it is really infinite in extent or whether though boundless, like the surface 1 lord kelvin, “the vortex theory of ether,” phil. afag. (1887) and afath. and phys. papers, vol. iv. and passim. g. f. fitzgerald, proc. rey. dub. soc. (1899), or collected papers, pp. 154, 238, 472. 1028 of a sphere, it is nevertheless finite, are questions which we can- not at present answer. there ts no doubt that it extends beyond the farthest visible stellar object, and for all practical purposes is infinite. there is very little doubt that matter is not an alien substance, but is essentially composed of it, being built up of the electrons and protons whose constitution has not yet been ascertained, but which must somehow be constituted of ether, perhaps in some sense analogous to that in which a knot in a piece of string is constructed of string, or a vortex in air is com- posed of air, or the fibre of a muscle is still essentially flesh. einsiein’s theory —the theory of relativity (see rela- tivity) has led some people—not many of the leaders of thought —to doubt if the ether can really exist. it may be useful there- fore at the present time in this supplementary article to explain in what way the equations connected with that theory are to be understood physically. newton expressed the laws binding the planets and suns together in terms of a hypothetical force acting between them, the same kind of force as we experience when a weight is supported above the earth; which force may therefore be taken as a fact of experience. but though the force is a fact, it is not explained: any expression in terms of action at a dis- tance is necessarily incomplete. einstein was led by considerations of relativity to formulate a law of gravitation, not in terms of force or of action at a dis- tance, but in terms of something in space, that is, in the ether, which results in a tendency of bodies to approach each other. it might be called a warp in space, or it might be called by other names: the names do not matter. the thing that has to be expressed is that the presence of matter modifies its whole neighbourhood, causing a gravitational potential, as has been previously said. and, until we know more about its intimate nature, the action of this modification is best expressed in terms of a diflerential equation which secks to formulate abstractly, without physical hypothesis, the essence of what is really hap- pening. none of the arguments which necessitate the existence of a medium are affected, but no name for it need be used, nor need the idea of a medium be introduced, for mathematical pur- poses. mathematicians are quite able to work with abstract equations about quantities without physical implications or conceptions, as long as they remain purely mathematicians. they can reduce even geometry to arithmetic. in a complete expression for the enlarged geometric interval between two points, the element of time must be introduced as well as the element of space, because they may be moving points. in other words geometry must be enlarged into kinematics, in order to express activities. the interval or line element between two neighbouring points may be expressed in polar co-ordinates m,0,o thus: ds?=—dr?—(rd6@)?—(r sin edg)?+ crdt?, a mode of expression devised by minkowski, an enthusiast for this kind of four-dimensional treatment, where the fundamental etheric velocity ¢ is introduced as a coefficient able to turn time into imaginary space, icdt. the emphasis on c, as an absolute geometric constant, is perhaps the most remarkable part of the einstein-minkowski conception, as a preparation for the build- ing erected upon it. but einstein took a further step, introducing the gravitation potential as something which would modify the motions of mat- ter, and introduced it not only into the element of time (as newton might have done if he had used that notation) but into the element of radial distance also; so that if the points are in the field of a mass of matter m the minkowski equation is:— ds? = — y} (dr)? — (rd@)}?— (r sin dg)? vctdt? where y=1—2p/c’, p being the gravitation potential at the place considered; which, if caused by a mass at a distance r, is p=*"/,, with & asthe newtonian gravitation constant. here the coefficient » occurs twice. if it occurred in the ! term only it would be a mode of stating newton’s theory of astronomy, in differential instead of integral fashion; but this v occurs in the r term also, as a result of the isotropy of the four- fold medium contemplated in this gravitational theory. this equation when elaborated gives, strangely enough, the out- standing progression of the perihelion of mercury, and it also ether gives the double deflection for a ray of light passing near the sun (doubled because the co-efficient » occurs twice), which has since been perhaps confirmed quantitatively by observation. it likewise gives the shift of the spectral lines emanating from an exceedingly massive body, which has now been confirmed beyond the reach of reasonable controversy by observations on light coming from the companion of sirius, which eddington has astonishingly proved to be by far the most compact and densest material body at present known to science. the beauty of these results is overwhelming; but the idea that any mathematical scheme is more than a powerful method of exploration, and that a universe can be thus constructed in which physical explanations can be dispensed with, involves too simple and anthropomorphic a view of nature. the things cal- culated, and the things observed, cannot exhaust reality; an ex- planation is bound to be sought, and ultimately attained, in terms of the partially recognised but largely unexplored proper- ties of the entity which fills space." locomotion of matter —the locomotion of matter is perhaps the commonest fact of experience, and it scems strange that it should be in need of explanation. but since the atom of matter is composed of electric charges, the locomotion of those charges has to be considered more in detail. an electric charge in motion constitutes an electric current, and the path of every electric current is surrounded by rings of magnetic force. this magnetic field confers inertia or momentum upon the moving charge; so that mechanical impulse is necessary to start it moving; and also to stop the motion. if not stopped it will continue to move uniformly in a straight line until it encounters some deflecting or retarding agency. but though locomotion can thus be stated and worked out elec- tromagnetically, that cannot be regarded as an ultimate explanation of so familiar and apparently simple a thing. moving matter ts known to have kinetic energy; and the familiar expression 410" is the type of its measure. but when we come to analyse this expression here are difficulties about it which the theory of relativity has hrought out and emphasised. for when we try to specify the velocity of a body, in order to calculate its energy, we find it difficult to say what that velocity really is: we can only specify it with reference to something else, commonly with reference to the earth. but the earth itself is moving. [fence $iv* does not give the absolute energy, hut only the energy relative to the earth or other frame of reference, as newton implicitly recognised. what the velocity of a body is in space we have no means at present of ascertaining, having no unt- versal standard of reference; and accordingly the usual expressions, though practically useful, are by no means ultimately satisfactory. nor can a statement in terms of electromagnetism be considered as ultimate, the fact is that locomotion docs not seem to be a property of the ether, which appears to be affected by one speed and one speed only, namely, what we suppose to be the speed of its internal circulation and are familiar with as the velocity of light. how then can a par- ticle of ether, however modified, move from one place to another? the analogy of a loose knot slipping along a string may be helpful. an electron even at rest has intrinsic energy, its electrostatic energy of constitution, which can be expressed in various ways, and which, when expressed in terms of mass and speed, is ntoc?, nto being its inertia at rest. its static energy is thus expressible as equivalent to that of a particle of certain mass 19 or 21%, moving with the speed c—the speed of light. to assist ideas, it might be thought of as a spinning motion; at any rate not locomotion. when the particle is moved, the natural idea would be that this velocity c is increased, or that some addition is made to it. but accorcing to the doctrine of relativity that 1s impossible: the velocity cisconstant. the thing that changes is not c, but m. and the energy of a moving body is muc?, where mt 1s greater than mo. and as the speed of motion increases, #1 increases too; until at high speeds it is very great, and, as the speed of light is approached, tends to become infinite. so that when an identified portion of ether is in locomotion, it is not the speed that is changed, but the amount of modified ether associated with that identified moving portion. and what we observe as the kinetic energy of the particle is (#1—mo)c or cdm, this is what we have hitherto recognised and called 42%, an expression which is relative, and which is not exactly applicable 10 great velocities, such as we encounter in vacuum tubes and in radioactivity generally. ‘to avoid any appearance of dogmatism on difficult subjects, it may be well to say that some eminent mathematicians are still of opinion that the einstein formulation is not quite satisfactory. for instance, larmor, phil. afag. (1923) claims that it only gives half values, and expects that some slight modification of newtonian and maxwellian theory will be found able to cover the above slight experimental deviations from what was previously known. the rest of this article must be treated as a i1entative attempt to indicate ideas that cannot be thoroughly expressed at the present date except by symbols. ethics thus when we try to took at locomotion absolutely, we have to admit that varying speed means varying amounts of sub- stance in the identified portion of matter we are attending to. if the earth were to move quicker it would be more massive: and this increase of mass would appeal to us as locomotion. it is as if the normal constitutional circulation trended or drifted in one direction, so as to constitute perceptible or available energy. the same idea may be expressed magnetically by calling attention to the magnetic field surrounding a moving charge. at high speeds the magnetic field is strong; more substance is involved in it: and the additional spin (if that is the right term, for magnetism is usually thought of as a kind of spin) accounts for the additional energy. why it should appeal to us as loco- motion, and what the real meaning of locomotion is, are not so clear. this is only an illustration of the difficulty we experience when we come to probe the simplest thing to its depths. we have grown accustomed to certain aspects of the universe given us by our senses, but we do not fundamentally understand them. and when we come to probe the meaning of things deeply enough, we find ourselves up against difficulties of conception, toward the elucidation of which our senses give hardly any aid. what we are used to is mechanical movements; but the effort to explain things ultimately in that way is not easy, and may turn out to be not possible. , meanwhile we take refuge in expressing these things in terms of electricity and magnetism; which is a step toward an explana- tion, and is useful in bringing out the difficulties which underlie every ultimate and absolute statement. the attempted abso- lute expression for static electric energy, mc? with the inertia m as the only variable, is a legitimate mathematical expression of electric facts. but the real meaning of ¢ is, at present, a hypothesis: and what the real meaning of #7 is, must be regarded as still less known. both these factors must have reference to the ether, and until we know more about the constitution of the ether we must be content to remain in a condition of pro- visional ignorance. we are led to regard the material universe as a substantial reality in various stages or varieties of internal activity. we may try to think of this activity as akin to a fine- grained vortex circulation in a continuous, incompressible, per- fect fluid: beyond that we cannot at present go; nor are we clear about the exact meaning of these terms when applied to a medium of unknown constitution. when we understand the real and ultimate nature of electricity andl magnetism we may hope to proceed further. till then we must be content with proximate explanations. (oo 1)