THE INEVITABLE ETHER
Ether is the fundamental postulate of physics. Its existence, nowise
apparent, is in all manner of ways implied. The properties that must
be assigned to it are certainly arduous of conception. We need the
aid of forced analogies to enable us to realize, even imperfectly and
indistinctly, the mode in which it discharges functions obviously
somehow discharged. But in the last resort everything is obscure;
if our thought-borings go deep enough, they always reach the
incomprehensible.
The original ether was the 'quintessence' of the ancients--a kind of
matter vaguely imagined as pure and incorruptible enough to serve for
the raw material of the heavenly bodies, the four common elements being
reserved exclusively for sublunary use. The distinction, however,
eventually broke down. All the spheres, from the _primum mobile_ to
the very surface of our low earth, are pervaded by a subtle mode of
action, demanding for its transmission machinery of a finer kind than
could be constructed out of gross everyday matter. The phenomena of
light, when they came to be attentively studied, imperatively required
a medium, universally diffused, evasive to sense, accessible only by
processes of reasoning. Hooke and Newton accordingly brought the ether
through the Horn Gate of dreamland into a region of reality, where it
became a subject of legitimate speculation to men of science. The task,
nevertheless, of definitely specifying its qualities was not taken
seriously in hand until the beginning of the nineteenth century, when
the establishment of the undulatory theory of light supplied tangible
holding-ground for ideas regarding the vehicle of propagation, and
rendered the ether a fixture of thought.
A great deal is demanded from it. We cannot afford to set up an
establishment of ethers; one factotum must suffice. Incongruous offices
are devolved upon it. It has to be Atlas and Mercury in one. It is
the universal supporter and the universal messenger. Whatever kind of
influence or form of energy can pass from world to world is conveyed
by its means. If 'action at a distance' be inadmissible (as Newton
himself held it to be), the pull of gravity must be exerted through
a medium; and common-sense insists upon its identification with the
transmitting medium of light, as well as upon the identification of
that with the transmitting medium of electricity. A genuine conformity
to these demands of reason is vouched for, not only by Hertz's
discovery that an electrical explosion starts an undulatory disturbance
indistinguishable, except in scale, from luminous waves; but also by
Dr. Lorentz's indicated conclusion that strains of the same ethereal
essence bear the all-pervading mandates of gravity. The unity of the
medium may, then, be regarded as finally ascertained; the complex
interactions of sundry different 'fluids' need no longer be taken into
account. To provide one with the capabilities implied by the services
we perceive it to render is, indeed, a sufficiently formidable task.
In popular apprehension the ether of space figures as a finer kind of
air. No idea could be more misleading. The elasticity by which air
transmits the longitudinal waves of sound is totally different from the
elasticity by which ether propagates the transversal waves of light.
Air yields to pressure; disturbance hence produces in it undulatory
condensations due to oscillations of the gaseous molecules along the
line in which the audible commotion travels. Ether, on the contrary,
appears to be entirely incompressible; it conveys no vibrations
directed lengthwise. Now this is extremely perplexing. We have no
experience of a kind of matter absolutely rigid to pressure, while
yielding, albeit with intense reluctance, to distortional stresses.
A jelly-like solid makes the nearest, though a very distant, approach
to fulfilling the indispensable conditions; and a solid ether was
accordingly in vogue until long past the middle of the nineteenth
century. For a solid it had very peculiar qualities; that, for
instance, of offering no resistance to motion. It was, in truth,
obviously a mere temporary expedient--a scientific fiction which
might pass muster until replaced by something corresponding less
distantly with the fundamental fact. At last, on the advent of the
electro-magnetic theory of light and the modified conceptions which it
brought in its train, the solid ether withdrew behind the scenes. Its
properties, though inconsistent and unconvincing, had not been chosen
arbitrarily; they were imposed by the necessities of the situation;
and when these varied, speculators naturally had recourse to fresh
inventions.
The most plausible is that of a medium neither solid, liquid, nor
gaseous in the ordinary sense, but in the ideal state of a 'perfect
fluid.' Out of such an ether Lord Kelvin, with exquisite ingenuity,
constructed his 'vortex-atoms,' which 'had their day and ceased to be.'
Other ideas now prevail. 'The present tendency of physical science,'
the late Mr. Preston wrote in 1890,[74] 'is to regard all the phenomena
of Nature, and even matter itself, as manifestations of energy stored
in the ether.' The more closely we look into the things around us,
the more strongly the persuasion is forced upon us that what we call
ether, electricity, and matter are really varied forms of one primal
substance.
Two comprehensive schemes of molecular physics, resting upon the basis
of this unifying thought, have lately been elaborated--one by Dr.
Larmor, the other by Professor Osborne Reynolds. They have nothing in
common beyond the largeness of their synthesis. In every respect they
are radically unlike, save in regarding the intangible ether as the
one material reality. Dr. Larmor, however, is not quite confidently
explanatory. He presents no cut-and-dried theory of the universe; its
haunting mysteries are not ignored in his efforts to rationalize them.
He is vividly aware of the difficulties besetting the endowment of the
ether with the type of elasticity which it is recognised to possess. He
can only surmise that it results from particular modes of motion--from
'kinetic stability' ensuing upon a special dynamical state. The medium
may thus be thought of as pervaded by 'a structure of tangled or
interlaced vortex filaments, which might resist deformation by forming
a stable configuration.'[75] But the details of any such scheme of
action are evidently far too intricate to be easily unravelled; what
concerns us here is to point out that no simple structureless fluid can
avail to maintain cosmical communications.
Reduced to its lowest terms, Dr. Larmor's conception of the ether is
that of a 'rotationally elastic medium.'[76] In other words, it resists
being turned round an axis. The forces continually acting upon it are
nevertheless of a gyratory nature; and hence arise strains, betrayed
to our apprehension by electrical manifestations. Each 'electron' is
held to be the nucleus of some kind of distortion or displacement,[77]
and carries with it, as it moves, a field of force. Out of these 'point
charges' material atoms are variously built up. They are 'structures in
the ether,' encompassed by 'atmospheres of ethereal strain,' not--as
they were formerly taken to be--'small bodies exerting direct action at
a distance on other atoms according to extraneous laws of force.'[78]
Obviously the new view brings to the front extremely subtle questions
regarding the nature of 'dynamical transmission'[79]--what the
propagation of energy essentially consists in, and by what mechanism
it is effected; and they are, for the present, unanswerable.
Electricity is, on the theory we are attempting to sketch, positive
or negative according to the direction of the originating strain.
A positive electron might be imagined to resemble a spiral nebula
of the right-handed sort, a negative one a left-handed spiral, or
_vice versâ_. The analogy is, perhaps, fanciful; yet it helps towards
obtaining a mental picture of objects which, insignificant and elusive
though they appear, may be the initials and ultimates of this strange
world.
The forces, at any rate, by which it is at present kept going are
evoked _ad libitum_ by the pioneers of modern research from the
ethereal plenum. The actualities of matter are potentialities in the
ether. 'All mass,' in Professor J. J. Thomson's opinion, 'is mass of
the ether, all momentum, momentum of the ether, and all kinetic energy,
kinetic energy of the ether.[80] Only if this be so,' he adds, 'the
density of the ether must be immensely greater than that of any known
substance.'
The condition is startling, but in dealing with such subjects we must
be prepared to meet with anomalies. They come, as the ghosts appeared
to Odysseus in Hades, at first one by one, then in an awe-inspiring
swarm. Yet, in spite of the perplexities they occasion, we can discern,
with growing sureness of insight, the amazing reality of the universal
medium. It is, in a manner, the only reality. For what is manifest to
sense is subject to change. We can conceive that the visible framework
of material existence might crumble and dissolve, like 'the baseless
fabric of a vision,' into seeming nothingness. But a substance that is
inapprehensible is, to our limited ideas, imperishable. The ether is
assuredly the seat of intense activities, which lie at the root, most
likely, of all the processes in Nature. An absolutely uniform medium,
however, can scarcely be imagined to energize or react. Some kind of
heterogeneity it must possess; and the heterogeneity produced, in Dr.
Larmor's view, by strains, is associated, in Professor Reynolds's
theory, with intrinsic texture.
The 'Sub-Mechanics of the Universe' are here made to depend upon the
fitting together of ineffably small, ideally rigid grains. A misfit
gives rise to matter, which might hence be defined as 'ether out of
joint'; and the misfit can be propagated endlessly from one range of
granules to the next. This propagation through the ether of an abnormal
arrangement of its constituent particles, without any transference of
the particles themselves, explains the phenomena of matter in motion. A
concrete existence belongs to the ether alone. It is composed of round
aboriginal atoms, the diameters of which measure the _seven hundred
thousand millionth part_ of the wave-length of violet light.[81] They
are packed closely together, yet not so closely but that free paths are
left to them averaging in length the _four hundred thousand millionth
part_ of their diameters.
This inconceivably small relative motion suffices, nevertheless, to
render the medium elastic; is, indeed, 'the only cause of elasticity
in the universe, and hence is the prime cause of the elasticity of
matter.' The medium so formed is ten thousand times denser than
water; it exerts a mean pressure of 750,000 tons on the square inch;
the coefficient of its transverse elasticity is 9 + 10^{24} (in C.G.S.
units); which gives a velocity of transmission identical with that
of light for vibrations of the same type, while longitudinal waves
are propagated 2·4 times more rapidly. The scheme further includes a
plausible rationale of gravity and of electrical effects; so that there
is much to warrant the claim of its author to have excogitated 'the one
and only conceivable purely mechanical system capable of accounting for
all the physical evidence, as we know it, in the universe.'
The machine, to be sure, lacks motive power; but that is a want which
no human ingenuity can supply. Its source is obscured in the primal
mystery of creation. And as regards the preliminary assumptions
required for the constitution of an atomic ether, inclined though we
might be to cavil at them, we should, perhaps, act more wisely in
following Dr. Larmor's advice by abstaining from attempts to explain
'the simple group of relations which have been found to define the
activity of the ether. We should rather rest satisfied,' he tells us,
'with having attained to their exact dynamical correlation, just as
geometry explores or correlates, without explaining, the descriptive
and metric properties of space.'[82] Yet one cannot help remarking
that the properties of space are not ordinarily modified to suit the
needs of demonstration, while those of the ethereal medium are varied
at the arbitrary discretion of rival cosmogonists. In the future, when
they come to be more clearly ascertained, they will, perhaps, form
the basis of a genuine new science. Already, the study of ethereal
physics excites profound interest and attention. Nor is it possible to
ignore the gathering indications that it will impose qualifications
upon principles consecrated by authority and hitherto regarded as
fundamental.
The grand modern tenet of the conservation of energy, for example,
may need a gloss; it may prove to be admissible only with certain
restrictions. The second bulwark of the scientific edifice is even more
seriously undermined. For the 'strain theory' of atomic constitution
necessarily includes the conception of opposite distortions
corresponding to positive and negative electricity. And the further
inference lies close at hand that these, by combining, may neutralize
one another. The coalescence, then, of a positive and negative electron
should result in the smoothing out of the complementary strains they
stand for; and there would ensue the annihilation of a pair of the
supposed ultimates of matter. The event might be called the statical
equivalent of the destruction of light through interference. That
its possibility should be contemplated even by the most adventurous
thinkers is a circumstance fraught with meaning as to the subversive
tendencies of recent research.
Already, in May, 1902, Professor J. A. Fleming[83] pointed out that 'if
the electron is a strain-centre in the ether, then corresponding to
every negative electron there must be a positive one. In other words,
electrons must exist in pairs of such kind that their simultaneous
presence at one point would result in the annihilation of both of
them.' The consequence thus viewed in the abstract finds concrete
realization, if Mr. Jeans's suggestion be adopted,[84] in the processes
of radio-activity, which possibly consist 'in an increase of material
energy at the expense of the destruction of a certain amount of matter.
There would, therefore, be conservation neither of mass nor of material
energy.'
No longer ago than at the opening of the present century such notions
would have been scouted as extravagant and paradoxical; now there
is no escape from giving them grave and respectful consideration.
Scientific reason has ceased to be outraged by hypotheses regarding
the disappearance of mass and the development of energy. Mass and
energy may, after all, be interchangeable; they are, at any rate, kept
less rigidly apart in our meditations than used formerly to be the
case. Nor can we assert with any confidence that partial subsidences
into or emergences from the surrounding medium are for either a sheer
impossibility; the universal framework, on the contrary, presents
itself to us in the guise of an iridescent fountain leaping upward
from, and falling back towards, the ethereal reservoir.
To the very brink of that mysterious ocean the science of the twentieth
century has brought us; and it is with a thrill of wondering awe that
we stand at its verge and survey its illimitable expanse. The glory
of the heavens is transitory, but the impalpable, invisible ether
inconceivably remains. Such as it is to-day, it already was when the
_Fiat Lux_ was spoken; its beginning must have been coeval with that
of time. Nothing or everything, according to the manner in which it
is accounted of, it is evasive of common notice, while obtrusive to
delicate scrutiny. Its negative qualities are numerous and baffling.
It has no effect in impeding motion; it does not perceptibly arrest,
absorb, or scatter light; it pervades, and may even share in the
displacements of gross matter; yet its motion (if it do move), is
without effect on the velocity of light.
Looking, however, below the surface of things, we find the
semi-fabulous quintessence to be unobtrusively doing all the world's
work. It embodies the energies of motion; is, perhaps, in a very real
sense, the true _primum mobile_; the potencies of matter are rooted in
it; the substance of matter is latent in it; universal intercourse is
maintained by means of the ether; cosmic influences can be exerted only
through its aid; unfelt, it is the source of solidity; unseen, it is
the vehicle of light; itself non-phenomenal, it is the indispensable
originator of phenomena. A contradiction in terms, it points the
perennial moral that what eludes the senses is likely to be more
permanently and intensely actual than what strikes them.
FOOTNOTES:
[Footnote 74: _Theory of Light_, second edition, p. 28.]
[Footnote 75: _Encyclopædia Britannica_, vol. xxv., p. 106.]
[Footnote 76: _Report of the British Association_, 1900, p. 626.]
[Footnote 77: _Æther and Matter_, p. 26.]
[Footnote 78: _Nature_, vol. xlii., p. 453.]
[Footnote 79: Larmor, _Report British Association_, 1900, p. 625.]
[Footnote 80: _Electricity and Matter_, p. 51.]
[Footnote 81: _The Structure of the Universe_, Rede Lecture, June 10,
1902, p. 14.]
[Footnote 82: _Nature_, June, vol. lxii., p. 451.]
[Footnote 83: _Proceedings of the Royal Institution_, vol. xvii., p.
177.]
[Footnote 84: _Nature_, vol. lxx., p. 101.]