The Ether of s.p.a.ce.
by Oliver Lodge.
PREFACE
Investigation of the nature and properties of the Ether of s.p.a.ce 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_.
INTRODUCTION
"Ether or aether (a???? probably from a??? I burn,) a material substance of a more subtle kind than visible bodies, supposed to exist in those parts of s.p.a.ce which are apparently empty."
So begins the article "Ether," written for the ninth edition of the _Encyclopaedia 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 aether suggests a far more subtle or penetrating and ultra-material kind of substance.
Newton employs the term for the medium which fills s.p.a.ce--not only s.p.a.ce which appears to be empty, but s.p.a.ce 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 Gla.s.s 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 aethereal Medium than in any Fluid, which fills all s.p.a.ce 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 _aether_ (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 s.p.a.ce, can pa.s.s 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 ma.s.sive 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 s.p.a.ce, 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 Inst.i.tution; 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 const.i.tution of the Ether, and of the way in which portions of it are modified to form the atoms or other const.i.tuent 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 const.i.tution of the aether, there can be no doubt that the interplanetary and interstellar s.p.a.ces 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 s.p.a.cE
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 circ.u.mference 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 s.p.a.ce eight minutes before striking our eyes, there must necessarily be in s.p.a.ce 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 s.p.a.ce.
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 sh.o.r.e. 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
dy/dt = v dy/dx;
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 s.p.a.ce 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 ill.u.s.trated 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 s.p.a.ce 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 ill.u.s.trate the meaning of this statement, by reviewing some fundamental electrical facts in the light of these a.n.a.logies:--
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 a.n.a.logous 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 a.n.a.logous to permitting a discharge of the jar--permitting the strained dielectric to recover itself--the electrostatic disturbance to subside.