The Ether of Space Part 7

Another mode of expressing the result is that the difference of magnetic potential applied, namely, a drop of two million C.G.S. units of magnetic potential, does not hurry light along it by so much as 1/50th part of a wave-length.

There may be reasons for supposing that some much slower drift or conveyance than this is really caused in the ether by a magnetic field; but if so, the ether must be regarded as so excessively dense that the amount of such a drift for any practicable magnetic field seems almost hopelessly beyond experimental means of detection.

CHAPTER VI

ETHERIAL DENSITY

This leads us to enter upon the question of whether it is possible to determine with any approach to accuracy the actual density or ma.s.siveness of the ether of s.p.a.ce, compared with those forms of matter to which our senses have made us accustomed.

The arguments on which an estimate may be made of the density or ma.s.siveness of the ether as compared with that of matter depend on the following considerations, the validity of which again is dependent upon an electrical theory of matter. In this theory, or working hypothesis, an a.s.sumption has to be made: but it is one for which there is a large amount of justification, and the reasons for it are given in many books,--among others in my book on _Electrons_, and likewise at the end of the new edition of _Modern Views of Electricity_, also in my _Romanes Lecture_, published by the Clarendon Press in 1903. Put briefly, the a.s.sumption is that matter is composed, in some way or other, of electrons; which again must be considered to be essentially peculiarities, or singularities, or definite structures, in the ether itself. Indeed, a consideration of electrons alone is sufficient for the argument, provided it be admitted that they have the ma.s.s which experiment shows them to possess, and the size which electrical theory deduces for them: the basis of the idea--which, indeed, is now experimentally proved--being that their inertia is due to their self-induction,--i.e. to the magnetic field with which they must be surrounded as long as they are in motion.

The ma.s.s, or inertia, of an electron is comparable to the thousandth part of that of the atom of hydrogen. Its linear dimension, let us say its diameter, is comparable to the one-hundred-thousandth part of what is commonly known as molecular or atomic dimension; which itself is the ten-millionth part of a millimetre.

Hence, the ma.s.s and the bulk of an electron being known, its density is determined, provided we can a.s.sume that its ma.s.s is all dependent on what is contained within its periphery. But that last a.s.sumption is one that quite definitely cannot be made: its ma.s.s is for the most part outside itself, and has to be calculated by magnetic considerations. (See Appendix 2.)

These details are gone into in my paper in the _Philosophical Magazine_ for April, 1907, and in Chapter XVII of _Modern Views of Electricity_. But without repeating arguments here, it will suffice to say that although the estimates may be made in various ways, differing entirely from each other, yet the resulting differences are only slight; the calculated densities come out all of the same order of magnitude, namely, something comparable to 10 C.G.S.

units,--that is to say, a million million grammes per cubic centimetre, or, in other words, a thousand tons to the cubic millimetre.

But, throughout, we have seen reason to a.s.sert that the ether is incompressible; arguments for this are given in _Modern Views of Electricity_, Chapter I. And, indeed, the fundamental medium filling all s.p.a.ce, if there be such, _must_, in my judgment, be ultimately incompressible; otherwise it would be composed of parts, and we should have to seek for something still more fundamental to fill the interstices.

The ether being incompressible, and an electron being supposed composed simply and solely of ether, it follows that it cannot be either a condensation or a rarefaction of that material, but must be some singularity of structure, or some portion otherwise differentiated. It might, for instance, be something a.n.a.logous to a vortex ring, differentiated kinetically, i.e. by reason of its rotational motion, from the remainder of the ether; or it might be differentiated statically, and be something which would have to be called a strain-centre or a region of twist, or something which cannot be very clearly at present imagined with any security; though various suggestions have been made in that direction.

The simplest plan for us is to think of it somewhat as we think of a knot on a piece of string. The knot differs in no respect from the rest of the string, except in its tied-up structure; it is of the same density with the rest, and yet it is differentiated from the rest; and, in order to cease to be a knot, would have to be untied--a process which as yet we have not learned how to apply to an electron.

If ever such a procedure becomes possible, then electrons will thereby be resolved into the general body of the undifferentiated ether of s.p.a.ce,--that part which is independent of what we call "matter."

The important notion for present purposes is merely this: that the density of the undifferentiated or simple ether, and the density of the tied-up or be-knotted or otherwise modified ether const.i.tuting an electron, are one and the same. Hence the argument above given, at least when properly worked out, tends to establish the etherial density as of the order 10 times that of water.

There ought to be nothing surprising (though I admit that there is something very surprising) in such an estimate; inasmuch as many converging lines of argument tend to show that ordinary matter is a very porous or gossamer-like substance, with inters.p.a.ces great as compared with the s.p.a.ces actually occupied by the nuclei which const.i.tute it. Our conception of matter, if it is to be composed of electrons, is necessarily rather like the conception of a solar system, or rather of a milky way; where there are innumerable dots here and there, with great inters.p.a.ces between. So that the average density of the whole of the dots or material particles taken together,--that is to say, their aggregate ma.s.s compared with the s.p.a.ce they occupy,--is excessively small.

In the vast extent of the Cosmos, as a whole, the small bulk of actual matter, compared with the volume of empty s.p.a.ce, is striking--as we shall show directly; and now on the small scale, among the atoms of matter, we find the conditions to be similar. Even what we call the densest material is of extraordinarily insignificant ma.s.siveness as compared with the unmodified ether which occupies by far the greater proportion of its bulk.

When we speak of the density of _matter_, we are really though not consciously expressing the group-density of the modified ether which const.i.tutes matter,--not estimated per unit, but per aggregate; just as we might estimate the group or average density of a cloud or mist.

Reckoned per unit, a cloud has the density of water; reckoned per aggregate, it is an impalpable filmy structure of hardly any density at all. So it is with a cobweb, so perhaps it is with a comet's tail, so also with the Milky Way, with the cosmos,--and, as it now turns out, with ordinary matter itself.

For consider the average density of the material cosmos. It comes out almost incredibly small. In other words, the amount of matter in s.p.a.ce, compared with the volume of s.p.a.ce it occupies, is almost infinitesimal. Lord Kelvin argues that ultimately it must be really infinitesimal (_Philosophical Magazine_, Aug., 1901, and Jan., 1902), that is to say that the volume of s.p.a.ce is infinitely greater than the total bulk of matter which it contains. Otherwise the combined force of gravity--or at least the aggregate gravitational potential--on which the velocity generated in material bodies ultimately depends, would be far greater than observation shows it to be.

The whole visible universe, within a parallax of 1/1000 second of arc, is estimated by Lord Kelvin as the equivalent of a thousand million of our suns; and this amount of matter, distributed as it is, would have an average density of 16 10? grammes per c.c. It is noteworthy how exceedingly small is this average or aggregate density of matter in the visible region of s.p.a.ce. The estimated density of 10?

c.g.s. means that the visible cosmos is as much rarer than a "vacuum"

of a hundred millionths of an atmosphere, as that vacuum is itself rarer than lead.

It is because we have reason to a.s.sert that any ordinary ma.s.s of matter consists, like the cosmos, of separated particles, with great intervening distances in proportion to their size, that we are able to maintain that the aggregate density of ordinary stuff, such as water or lead, is very small compared with the continuous medium in which they exist, and of which all particles are supposed to be really composed. So that lead is to the ether, as regards density, very much as the "vacuum" above spoken of is to lead. The fundamental medium itself must be of uniform density everywhere, whether materialised or free.

CHAPTER VII

FURTHER EXPLANATIONS CONCERNING THE DENSITY AND ENERGY OF THE ETHER

A reader may suppose that in speaking of the immense density or ma.s.siveness of ether, and the absurdly small density or specific gravity of gross matter by comparison, I intend to signify that matter is a _rarefaction_ of the ether. That, however, is not my intention.

The view I advocate is that the ether is a perfect _continuum_, an absolute _plenum_, and that therefore no rarefaction is possible. The ether inside matter is just as dense as the ether outside, and no denser. A material unit--say an electron--is only a peculiarity or singularity of some kind in the ether itself, which is of perfectly uniform density everywhere. What we "sense" as matter is an aggregate or grouping of an enormous number of such units.

How then can we say that matter is millions of times rarer or less substantial than the ether of which it is essentially composed? Those who feel any difficulty here, should bethink themselves of what they mean by the average or aggregate density of any discontinuous system, such as a powder, or a gas, or a precipitate, or a snowstorm, or a cloud, or a milky way.

If it be urged that it is unfair to compare an obviously discrete a.s.semblage like the stars, with an apparently continuous substance like air or lead,--the answer is that it is entirely and accurately fair; since air, and every other known form of matter, is essentially an aggregate of particles, and since it is always their average density that we mean. We do not even know for certain their individual atomic density.

The phrase "specific gravity or density of a powder" is ambiguous. It may mean the specific gravity of the dry powder as it lies, like snow; or it may mean the specific gravity of the particles of which it is composed, like ice.

So also with regard to the density of matter, we might mean the density of the fundamental material of which its units are made--which would be ether; or we might, and in practice do, mean the density of the aggregate lump which we can see and handle; that is to say, of water or iron or lead, as the case may be.

In saying that the density of matter is small,--I mean, of course, in the last, the usual, sense. In saying that the density of ether is great,--I mean that the actual stuff of which these highly porous aggregates are composed is of immense, of wellnigh incredible, density. It is only another way of saying that the ultimate units of matter are few and far between--i.e. that they are excessively small as compared with the distances between them; just as the planets of the solar system, or worlds in the sky, are few and far between,--the intervening distances being enormous as compared with the portions of s.p.a.ce actually occupied by lumps of matter.

It may be noted that it is not unreasonable to argue that the density of a _continuum_ is necessarily greater than the density of any disconnected aggregate: certainly of any a.s.semblage whose particles are actually composed of the material of the _continuum_. Because the former is "all there," everywhere, without break or intermittence of any kind; while the latter has gaps in it,--it is here, and there, but not everywhere.

Indeed, this very argument was used long ago by that notable genius Robert Hooke, and I quote a pa.s.sage which Professor Poynting has discovered in his collected posthumous works and kindly copied out for me:--

"As for _matter_, that I conceive in its essence to be immutable, and its essence being expatiation determinate, it cannot be altered in its quant.i.ty, either by condensation or rarefaction; that is, there cannot be more or less of that power or reality, whatever it be, within the same expatiation or content; but every equal expatiation contains, is filled, or is an equal quant.i.ty of _materia_; and the densest or heaviest, or most powerful body in the world contains no more materia than that which we conceive to be the rarest, thinnest, lightest, or least powerful body of all; as gold for instance, and _aether_, or the substance that fills the cavity of an exhausted vessel, or cavity of the gla.s.s of a barometer above the quicksilver. Nay, as I shall afterwards prove, this cavity is more full, or a more dense body of aether, in the common sense or acceptation of the word, than gold is of gold, bulk for bulk; and that because the one, viz. the ma.s.s of aether, is all aether: but the ma.s.s of gold, which we conceive, is not all gold; but there is an intermixture, and that vastly more than is commonly supposed, of aether with it; so that vacuity, as it is commonly thought, or erroneously supposed, is a more dense body than the gold as gold. But if we consider the whole content of the one with that of the other, within the same or equal quant.i.ty of expatiation, then are they both equally containing the _materia_ or body."--[_From the Posthumous Works of Robert Hooke, M.D., F.R.S., 1705, pp. 171-2_ (_as copied in Memoir of Dalton, by Angus Smith_).]

Newton's contemporaries did not excel in power of clear expression, as he himself did; but Professor Poynting interprets this singular attempt at utterance thus:--"All s.p.a.ce is filled with equally dense _materia_. Gold fills only a small fraction of the s.p.a.ce a.s.signed to it, and yet has a big ma.s.s. How much greater must be the total ma.s.s filling that s.p.a.ce."

The tacit a.s.sumption here made is that the particles of the aggregate are all composed of one and the same continuous substance, --practically that matter is made of ether; and that a.s.sumption, in Hooke's day, must have been only a speculation. But it is the kind of speculation which time is justifying, it is the kind of truth which we all feel to be in process of establishment now.[6]

We do not depend on that sort of argument, however; what we depend on is experimental measure of the ma.s.s, and mathematical estimate of the volume, of the electron. For calculation shows that however the ma.s.s be accounted for--whether electrostatically or magnetically, or hydrodynamically--the estimate of ratio of ma.s.s to effective volume can differ only in a numerical coefficient, and cannot differ as regards order of magnitude. The only way out of this conclusion would be the discovery that the negative electron is not the real or the main matter-unit, but is only a subsidiary ingredient; whereas the main ma.s.s is the more bulky positive charge. That last hypothesis however is at present too vague to be useful. Moreover, the ma.s.s of such a charge would in that case be unexplained, and would need a further step; which would probably land us in much the same sort of etherial density as is involved in the estimate which I have based on the more familiar and tractable negative electron. (See Appendix 2.)

It may be said why a.s.sume any finite density for the ether at all? Why not a.s.sume that, as it is infinitely continuous, so it is infinitely dense--whatever that may mean--and that all its properties are infinite? This might be possible were it not for the velocity of light. By transmitting waves at a finite and measurable speed, the ether has given itself away, and has let in all the possibilities of calculation and numerical statement. Its properties are thereby exhibited as essentially finite--however infinite the whole extent of it may turn out to be. Parenthetically we may remark that "gravitation" has not yet exhibited any similar kind of finite property; and that is why we know so little about it.

ETHERIAL ENERGY.

Instead then of saying that the density of the ether is great, the clearest mode of expression is to say that the density of matter is small. Just as we can say that the density of the visible cosmos is small, although in individual lumps its density is comparable to that of iron or rock.

At the risk of repet.i.tion, I have explained this over again, because it is a matter on which confusion may easily arise. The really important thing about ether is not so much its density, considered in itself, as the energy which that density necessarily involves, on any kinetic theory of its elasticity. For it is not impossible--however hopeless it may seem now--that a modic.u.m of that energy may some day be partially utilised.

Lord Kelvin's incipient kinetic theory of elasticity is a complicated matter, and I will only briefly enter upon it. But before doing so, I want to remove an objection which is sometimes felt, as to the fluid and easily permeable character of a medium of this great density,--that is to say, as to the absence of friction or viscosity--the absence of resistance to bodies moving through it. As a matter of fact there is no necessary connexion whatever between density and viscosity.

'Density' and 'Viscosity' are entirely different things; and, if there is no fluid friction, a fluid may have any density you please without interposing any obstacle to constant velocity. To _acceleration_ it does indeed oppose an obstacle, but that appears as essentially a part of the inertia or ma.s.siveness of the moving body. It contributes to its momentum; and, if the fluid is everywhere present, it is impossible to discriminate between, or to treat separately, that part of the inertia which belongs to the fluid displaced, and that part which belongs to the body moving through it,--except by theory.

As for the elasticity of the ether, that is ascertainable at once from the speed at which it transmits waves. That speed--the velocity of light--is accurately known, 3 10 centimetres per second. And the ratio of the elasticity or rigidity to the density is equal to the square of this speed;--that is to say, the elasticity must be 9 10 times the density; or, in other words, 10 C.G.S. units.

That is an immediate consequence of the estimate of density and the fact of the velocity of light; and if the density is admitted, the other cannot be contested.

But we must go on to ask, To what is this rigidity due? If the ether does not consist of parts, and if it is fluid, how can it possess the rigidity appropriate to a solid, so as to transmit transverse waves?

To answer this we must fall back upon Lord Kelvin's kinetic theory of elasticity:--that it must be due to rotational motion--intimate fine-grained motion throughout the whole etherial region--motion not of the nature of locomotion, but circulation in closed curves, returning upon itself,--vortex motion of a kind far more finely grained than any waves of light or any atomic or even electronic structure.

Now if the elasticity of any medium is to be thus explained kinetically, it follows, as a necessary consequence, that the speed of this internal motion must be comparable to the speed of wave propagation;--that is to say that the internal squirming circulation, to which every part of the ether is subject, must be carried on with a velocity of the same order of magnitude as the velocity of light.