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.
FOOTNOTE:
[2] Cf. sections 157A, 143, 187, and chap. xvi., of my _Modern Views
of Electricity_.