HARPER'S LIBRARY _of_ LIVING THOUGHT
[Illustration: HARPER'S LIBRARY _of_ LIVING THOUGHT]
"Wie Alles sich zum Ganzen webt
Eins in dem andern wirkt und lebt!"
THE ETHER
OF
SPACE
BY
SIR OLIVER
LODGE, F.R.S.
HARPER &
BROTHERS
LONDON & NEW YORK
[Illustration: FIG. 15. View of Ether machine complete and in action.
(See Chapter V, and figs. 12 and 13.)
_Frontispiece_]
THE ETHER OF
SPACE
BY
SIR OLIVER LODGE, F.R.S.
_D. Sc. Lond., Hon. D. Sc. Oxon. et Vict._
_LL.D. St. Andrews, Glasgow, and Aberdeen_
_Vice-President of the Institution of Electrical Engineers_
_Rumford Medallist of the Royal Society_
_Ex-President of the Physical Society of London_
_Late Professor of Physics in the University College of Liverpool_
_Honorary Member of the American Philosophical Society of Philadelphia;
of the Manchester Philosophical Society; of the Batavian_
_Society of Rotterdam; and of the Academy of Sciences of Bologna_
_Principal of the University of Birmingham_
ILLUSTRATED
LONDON AND NEW YORK
HARPER & BROTHERS
45 ALBEMARLE STREET, W.
1909
_Copyright, 1909, by Harper & Brothers
All rights reserved_
PREFACE
Investigation of the nature and properties of the Ether of Space has
long been for me the most fascinating branch of Physics, and I welcome
the opportunity of attempting to make generally known the conclusions
to which I have so far been led on this great and perhaps
inexhaustible subject.
OLIVER LODGE.
UNIVERSITY OF BIRMINGHAM,
_March, 1909_.
TO THE FOUNDERS OF
UNIVERSITY COLLEGE, LIVERPOOL,
ESPECIALLY TO THOSE BEARING THE NAMES
OF RATHBONE AND OF HOLT
THIS BOOK IS INSCRIBED
CONTENTS
CHAPTER PAGE
INTRODUCTION. GENERAL AND HISTORICAL xi
I. THE LUMINIFEROUS ETHER AND THE
MODERN THEORY OF LIGHT 1
II. THE INTERSTELLAR ETHER AS A CONNECTING
MEDIUM 13
III. INFLUENCE OF MOTION ON VARIOUS
PHENOMENA 29
IV. EXPERIMENTS ON THE ETHER 44
V. SPECIAL EXPERIMENT ON ETHERIAL
VISCOSITY 67
VI. ETHERIAL DENSITY 80
VII. FURTHER EXPLANATIONS CONCERNING
THE DENSITY AND ENERGY OF THE ETHER 87
VIII. ETHER AND MATTER 98
IX. STRENGTH OF THE ETHER 115
X. GENERAL THEORY OF ABERRATION 127
APPENDIX 1. ON GRAVITY AND ETHERIAL
TENSION 143
APPENDIX 2. CALCULATIONS IN CONNEXION
WITH ETHER DENSITY 146
APPENDIX 3. FRESNEL'S LAW A SPECIAL CASE
OF A UNIVERSAL POTENTIAL FUNCTION 152
LIST OF ILLUSTRATIONS
FIG.
_Illustrations of Aberration._
1. Cannon shots 35
2. Boats or Waves 36
3. Lighthouse beams 37
4. Ray through a moving stratum 40
5. Wave-fronts in moving medium 41
6. Normal reflexion in moving medium 43
_Experiments on Ether drift._
7. Interference Kaleidoscope 51
8. Hoek's experiment 53
9. Experiment of Mascart and Jamin 54
10. Diagram of Michelson's experiment 61
_Illustrations of Ether Machine_ (Lodge).
11. Diagram of course of light 69
12. General view of whirling part of Ether Machine 72
13. General view of optical frame 73
14. Drawing of optical details 74
15. View of Ether Machine in action _Frontispiece_
16. Appearance of interference bands and micrometer wires 76
17. Iron mass for magnetisation 77
18. Appearance of bands 76
19. Arrangement for electrification 78
INTRODUCTION
"Ether or Æther (αιθηρ probably from αιθω I burn,) a material
substance of a more subtle kind than visible bodies, supposed to exist
in those parts of space which are apparently empty."
So begins the article "Ether," written for the ninth edition of the
_Encyclopædia Britannica_, by James Clerk Maxwell.
The derivation of the word seems to indicate some connexion in men's
minds with the idea of Fire: the other three "elements," Earth, Water,
Air, representing the solid, liquid, and gaseous conditions of
ordinary matter respectively. The name Æther suggests a far more
subtle or penetrating and ultra-material kind of substance.
Newton employs the term for the medium which fills space--not only
space which appears to be empty, but space also which appears to be
full; for the luminiferous ether must undoubtedly penetrate between
the atoms--must exist in the pores so to speak--of every transparent
substance, else light could not travel through it. The following is an
extract from Newton's surmises concerning this medium:--
"Qu. 18. If in two large tall cylindrical Vessels of Glass
inverted, two little Thermometers be suspended so as not to
touch the Vessels, and the Air be drawn out of one of these
Vessels, and these Vessels thus prepared be carried out of a
cold place into a warm one; the Thermometer _in vacuo_ will
grow warm as much and almost as soon as the Thermometer which
is not _in vacuo_. And when the vessels are carried back into
the cold place, the Thermometer _in vacuo_ will grow cold
almost as soon as the other Thermometer. Is not the Heat of the
warm Room conveyed through the Vacuum by the Vibrations of a
much subtiler Medium than Air, which after the Air was drawn
out remained in the Vacuum? And is not this Medium the same
with that Medium by which Light is [transmitted], and by whose
Vibrations Light communicates Heat to Bodies?... And do not
the Vibrations of this Medium in hot Bodies contribute to the
intenseness and duration of their Heat? And do not hot Bodies
communicate their Heat to contiguous cold ones by the
Vibrations of this Medium propagated from them into the cold
ones? And is not this Medium exceedingly more rare and subtile
than the Air, and exceedingly more elastic and active? And doth
it not readily pervade all bodies? And is it not (by its
elastic force) expanded through all the Heavens?"
"Qu. 22. May not Planets and Comets, and all gross Bodies,
perform their motions more freely, and with less resistance in
this Æthereal Medium than in any Fluid, which fills all Space
adequately without leaving any Pores, and by consequence is much
denser than Quick-silver and Gold? And may not its resistance be
so small, as to be inconsiderable? For instance; if this _Æther_
(for so I will call it) should be supposed 700000 times more
elastic than our Air, and above 700000 times more rare; its
resistance would be above 600000000 times less than that of
Water. And so small a resistance would scarce make any sensible
alteration in the Motions of the Planets in ten thousand Years."
That the ether, if there be such a thing in space, can pass readily
into or through matter is often held proven by tilting a mercury
barometer; when the mercury rises to fill the transparent vacuum.
Everything points to its universal permeance, if it exist at all.
But these, after all, are antique thoughts. Electric and Magnetic
information has led us beyond them into a region of greater certainty
and knowledge; so that now I am able to advocate a view of the Ether
which makes it not only uniformly present and all-pervading, but also
massive and substantial beyond conception. It is turning out to be by
far the most substantial thing--perhaps the only substantial thing--in
the material universe. Compared to ether the densest matter, such as
lead or gold, is a filmy gossamer structure; like a comet's tail or a
milky way, or like a salt in very dilute solution.
To lead up to and justify the idea of the reality and substantiality,
and vast though as yet largely unrecognised importance, of the Ether
of Space, the following chapters have been written. Some of them
represent the expanded notes of lectures which have been given in
various places--chiefly the Royal Institution; while the first chapter
represents a lecture before the Ashmolean Society of the University of
Oxford in June, 1889. One chapter (viz. Chap. II) has already been
printed as part of an appendix to the third edition of _Modern Views
of Electricity_, as well as in the _Fortnightly_ and _North American
Reviews_; but no other chapters have yet been published, though parts
appear in more elaborate form in Proceedings or Transactions of
learned societies.
The problem of the constitution of the Ether, and of the way in which
portions of it are modified to form the atoms or other constituent
units of ordinary matter, has not yet been solved. Much work has been
done in this direction by various mathematicians, but much more
remains to be done. And until it is done, some scepticism is
reasonable--perhaps laudable. Meanwhile there are few physicists who
will dissent from Clerk Maxwell's penultimate sentence in the article
"Ether" of which the beginning has already been quoted:--
"Whatever difficulties we may have in forming a consistent idea
of the constitution of the æther, there can be no doubt that
the interplanetary and interstellar spaces are not empty, but
are occupied by a material substance or body, which is
certainly the largest, and probably the most uniform body of
which we have any knowledge."
THE ETHER OF SPACE
CHAPTER I
THE LUMINIFEROUS ETHER AND THE MODERN THEORY OF LIGHT
The oldest and best known function for an ether is the conveyance of
light, and hence the name "luminiferous" was applied to it; though at
the present day many more functions are known, and more will almost
certainly be discovered.
To begin with it is best to learn what we can, concerning the
properties of the Interstellar Ether, from the phenomena of Light.
For now wellnigh a century we have had a wave theory of light; and a
wave theory of light is quite certainly true. It is directly
demonstrable that light consists of waves of some kind or other, and
that these waves travel at a certain well-known velocity,--achieving a
distance equal to seven times the circumference of the earth every
second; from New York to London and back in the thirtieth part of a
second; and taking only eight minutes on the journey from the sun to
the earth. This propagation in time of an undulatory disturbance
necessarily involves a medium. If waves setting out from the sun exist
in space eight minutes before striking our eyes, there must
necessarily be in space some medium in which they exist and which
conveys them. Waves we cannot have, unless they be waves in something.
No ordinary matter is competent to transmit waves at anything like the
speed of light: the rate at which _matter_ conveys waves is the
velocity of sound,--a speed comparable to one-millionth of the speed
of light. Hence the luminiferous medium must be a special kind of
substance; and it is called the ether. The _luminiferous_ ether it
used to be called, because the conveyance of light was all it was then
known to be capable of; but now that it is known to do a variety of
other things also, the qualifying adjective may be dropped. But,
inasmuch as the term 'ether' is also applied to a familiar organic
compound, we may distinguish the ultra-material luminiferous medium by
calling it the Ether of Space.
Wave-motion in ether, light certainly is; but what does one mean by
the term wave? The popular notion is, I suppose, of something heaving
up and down, or perhaps of something breaking on a shore. But if you
ask a mathematician what he means by a wave, he will probably reply
that the most general wave is such a function of _x_ and _y_ and _t_
as to satisfy the differential equation
d²y/dt² = v² d²y/dx²;
while the simplest wave is
y = a sin (x-vt).
And he might possibly refuse to give any other answer.
And in refusing to give any other answer than this, or its equivalent
in ordinary words, he is entirely justified; that _is_ what is meant
by the term wave, and nothing less general would be all-inclusive.
Translated into ordinary English the phrase signifies, with accuracy
and comprehensive completeness, the full details of "a disturbance
periodic both in space and time." Anything thus doubly periodic is a
wave; and all waves--whether in air as sound waves, or in ether as
light waves, or on the surface of water as ocean waves--can be
comprehended in the definition.
What properties are essential to a medium capable of transmitting
wave-motion? Roughly we may say two: _elasticity_ and _inertia_.
Elasticity in some form, or some equivalent of it,--in order to be
able to store up energy and effect recoil; inertia,--in order to
enable the disturbed substance to overshoot the mark and oscillate
beyond its place of equilibrium to and fro. Any medium possessing
these two properties can transmit waves, and unless a medium possesses
these properties in some form or other, or some equivalent for them,
it may be said with moderate security to be incompetent to transmit
waves. But if we make this latter statement one must be prepared to
extend to the terms elasticity and inertia their very largest and
broadest signification, so as to include any possible kind of
restoring force, and any possible kind of persistence of motion,
respectively.
These matters may be illustrated in many ways, but perhaps a simple
loaded lath, or spring, in a vice will serve well enough. Pull it to
one side, and its elasticity tends to make it recoil; let it go, and
its inertia causes it to overshoot its normal position. That is what
inertia is,--power of overshooting a mark, or, more accurately, power
of moving for a time even against driving force,--power to rush
uphill. Both causes together make it swing to and fro till its energy
is exhausted. This is a disturbance simply periodic in time. A regular
series of such springs, set at equal intervals and started vibrating
at regular intervals of time one after the other, would be periodic in
space too; and so they would, in disconnected fashion, typify a wave.
A series of pendulums will do just as well, and if set swinging in
orderly fashion will furnish at once an example and an appearance of
wave motion, which the most casual observer must recognise as such.
The row of springs obviously possesses elasticity and inertia; and any
wave-transmitting medium must similarly possess some form of
elasticity and some form of inertia.
But now proceed to ask what is this Ether which in the case of light
is thus vibrating? What corresponds to the elastic displacement and
recoil of the spring or pendulum? What corresponds to the inertia
whereby it overshoots its mark? Do we know these properties in the
ether in any other way?
The answer, given first by Clerk Maxwell, and now reiterated and
insisted on by experiments performed in every important laboratory in
the world, is:--
The elastic displacement corresponds to electrostatic
charge,--roughly speaking, to electricity.
The inertia corresponds to magnetism.
This is the basis of the modern electromagnetic theory of light.
Let me attempt to illustrate the meaning of this statement, by
reviewing some fundamental electrical facts in the light of these
analogies:--
The old and familiar operation of charging a Leyden jar--the storing
up of energy in a strained dielectric--any electrostatic charging
whatever is quite analogous to the drawing aside of our flexible
spring. It is making use of the elasticity of the ether to produce a
tendency to recoil. Letting go the spring is analogous to permitting a
discharge of the jar--permitting the strained dielectric to recover
itself--the electrostatic disturbance to subside.
In nearly all the experiments of electrostatics etherial elasticity is
manifest.
Next consider inertia. How would one illustrate the fact that water,
for instance, possesses inertia--the power of persisting in motion
against obstacles--the power of possessing kinetic energy? The most
direct way would be, to take a stream of water and try suddenly to
stop it. Open a water tap freely and then suddenly shut it. The
impetus or momentum of the stopped water makes itself manifest by a
violent shock to the pipe, with which everybody must be familiar. This
momentum of water is utilised by engineers in the "water-ram."
A precisely analogous experiment in Electricity is what Faraday called
"the extra current." Send a current through a coil of wire round a
piece of iron, or take any other arrangement for developing powerful
magnetism, and then suddenly stop the current by breaking the circuit.
A violent flash occurs, if the stoppage is sudden enough, a flash
which means the bursting of the insulating air partition by the
accumulated electromagnetic momentum. The scientific name for this
electrical inertia is "self-induction."
Briefly we may say that nearly all electromagnetic experiments
illustrate the fact of etherial inertia.
Now return to consider what happens when a charged conductor (say a
Leyden jar) is discharged. The recoil of the strained dielectric
causes a current, the inertia of this current causes it to overshoot
the mark, and for an instant the charge of the jar is reversed; the
current now flows backwards and charges the jar up as at first; back
again flows the current; and so on, charging and reversing the charge,
with rapid oscillations, until the energy is all dissipated into heat.
The operation is precisely analogous to the release of a strained
spring, or to the plucking of a stretched string.
But the discharging body, thus thrown into strong electrical
vibration, is imbedded in the all-pervading ether; and we have just
seen that the ether possesses the two properties requisite for the
generation and transmission of waves, viz.: elasticity, and inertia or
density; hence just as a tuning fork vibrating in air excites aërial
waves, or sound, so a discharging Leyden jar in ether excites etherial
waves, or light.
Etherial waves can therefore be actually produced by direct electrical
means. I discharge here a jar, and the room is for an instant filled
with light. With light, I say, though you can see nothing. You can
see and hear the spark indeed--but that is a mere secondary
disturbance we can for the present ignore--I do not mean any secondary
disturbance. I mean the true etherial waves emitted by the electric
oscillation going on in the neighbourhood of the recoiling dielectric.
You pull aside the prong of a tuning fork and let it go: vibration
follows and sound is produced. You charge a Leyden jar and let it
discharge: vibration follows and light is excited.
It is light, just as good as any other light. It travels at the same
pace, it is reflected and refracted according to the same laws; every
experiment known to optics can be performed with this etherial
radiation electrically produced,--and yet you cannot see it. Why not?
For no fault of the light, the fault (if there be a fault) is in the
eye. The retina is incompetent to respond to these vibrations--they
are too slow. The vibrations set up when this large jar is discharged
are from a hundred thousand to a million per second, but that is too
slow for the retina. It responds only to vibrations between 400
billion and 700 billion per second. The vibrations are too quick for
the ear, which responds only to vibrations between 40 and 40,000 per
second. Between the highest audible and the lowest visible vibrations
there has been hitherto a great gap, which these electric oscillations
go far to fill up. There has been a great gap simply because we have
no intermediate sense organ to detect rates of vibration between
40,000 and 400,000,000,000,000 per second. It was therefore an
unexplored territory. Waves have been there all the time in any
quantity, but we have not thought about them nor attended to them.
It happens that I have myself succeeded in getting electric
oscillations so slow as to be audible,--the lowest I had got in 1889
were 125 per second, and for some way above this the sparks emit a
musical note; but no one has yet succeeded in directly making electric
oscillations which are visible,--though indirectly every one does it
when they light a candle.
It is easy, however, to have an electric oscillator which vibrates 300
million times a second, and emits etherial waves a yard long. The
whole range of vibrations between musical tones and some thousand
million per second, is now filled up.
With the large condensers and self-inductances employed in modern
cable telegraphy, it is easy to get a series of beautifully regular
and gradually damped electric oscillations, with a period of two or
three seconds, recorded by an ordinary signalling instrument or siphon
recorder.
These electromagnetic waves in space have been known on the side of
theory ever since 1865, but interest in them was immensely quickened
by the discovery of a receiver or detector for them. The great though
simple discovery by Hertz, in 1888, of an "electric eye," as Lord
Kelvin called it, made experiments on these waves for the first time
easy or even possible. From that time onward we possessed a sort of
artificial sense organ for their appreciation,--an electric
arrangement which can virtually "see" these intermediate rates of
vibration.
Since then Branly discovered that metallic powder could be used as an
extraordinarily sensitive detector; and on the basis of this
discovery, the 'coherer' was employed by me for distant signalling by
means of electric or etheric waves; until now when many other
detectors are available in the various systems of wireless telegraphy.
With these Hertzian waves all manner of optical experiments can be
performed. They can be reflected by plain sheets of metal,
concentrated by parabolic reflectors, refracted by prisms, and
concentrated by lenses. I have made, for instance, a large lens of
pitch, weighing over three hundredweight, for concentrating them to a
focus.[1] They can be made to show the phenomenon of interference, and
thus have their wave-length accurately measured. They are stopped by
all conductors, and transmitted by all insulators. Metals are opaque;
but even imperfect insulators, such as wood or stone, are strikingly
transparent; and waves may be received in one room from a source in
another, the door between the two being shut.
The real nature of metallic opacity and of transparency has long been
clear in Maxwell's theory of light, and these electrically produced
waves only illustrate and bring home the well-known facts. The
experiments of Hertz are, in fact, the apotheosis of Maxwell's theory.
* * * * *
Thus, then, in every way, Clerk Maxwell's brilliant perception or
mathematical deduction, in 1865, of the real nature of light is
abundantly justified; and for the first time we have a true theory of
light,--no longer based upon analogy with sound, nor upon the supposed
properties of some hypothetical jelly or elastic solid, but capable of
being treated upon a substantial basis of its own, in alliance with
the sciences of Electricity and of Magnetism.
Light is an electromagnetic disturbance of the ether. Optics is a
branch of electricity. Outstanding problems in optics are being
rapidly solved, now that we have the means of definitely exciting
light with a full perception of what we are doing, and of the precise
mode of its vibration.
It remains to find out how to shorten down the waves--to hurry up the
vibration until the light becomes visible. Nothing is wanted but
quicker modes of vibration. Smaller oscillators must be used--very
much smaller--oscillators not much bigger than molecules. In all
probability--one may almost say certainly--ordinary light is the
result of electric oscillation in the molecules or atoms of hot
bodies, or sometimes of bodies not hot--as in the phenomenon of
phosphorescence.
The direct generation of _visible_ light by electric means, so soon as
we have learnt how to attain the necessary frequency of vibration,
will have most important practical consequences; and that matter is
initially dealt with in a section on the Manufacture of Light, § 149,
in Chapter XIV of _Modern Views of Electricity_. But here we abandon
further consideration of this aspect of our great subject.
FOOTNOTE:
[1] See Lodge and Howard, _Philosophical Magazine_ for July, 1889. See
also _Phil. Mag._, August, 1888, page 229.