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8: Electromagnetism

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« on: April 10, 2024, 08:13:14 am »

Chapter Eight: Electromagnetism
How radiation is generated: and its problems.

There is weighty argument to be adduced in favour of the ether hypothesis. To deny the ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view. According to the general theory of relativity space is endowed with physical qualities ; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time. . . . But this ether may not be thought of as endowed with the quality characteristic of ponderable media. . . . The idea of motion [locomotion] may not be applied to it.---Sidelights on Relativity, by PROF. EINSTEIN.

Every disturbance of the aether, including radiation as one type of disturbance, is originated by translatory motion of electrons through the aether. . . . The æther is a perfect fluid endowed with rotational elasticity.---SIR JOSEPH LARMOR.

A high speed electron has additional mass by reason of its motion. When it is stopped, this mass is thrown off as radiation. This will be a quantum, greater or less according to the speed: its amount can be reckoned; it should correspond with the energy of the voltage-drop that produced the motion. When radiation throws off an electron, this mass is restored to it and that determines its speed.---O. J. L.

It is well known that magnetism can be produced by electricity, or more accurately that a magnetic field can be excited by an electric current. We have now to try to enter on the more intimate detail of the process.

When we were speaking of electrons as the basis of electric charge, we were not satisfied to accept the electrons as units given without explanation: we tried to speculate as to their nature, and see if we could imagine some structure in the Ether which would account for them. Let us try to do something of the same sort with magnetism: we may not succeed, but at least we can state certain undoubted facts. What I say about an electron must be true of a proton also, with change of sign; but it will be sufficient if we attend to one: and at present we know less about a proton than about an electron.

An electron at rest has nothing magnetic about it: it has a field of electric force; lines of force radiate from it in all directions: it seems to have a centre or nucleus in a certain locality, but its lines of force stretch out everywhere. An electron is capable of locomotion;---locomotion is one characteristic of every form of matter;---it is doubtful if the unmodified ether is capable of locomotion, at any rate we don't know how to set it moving. But we do know how to move an electron, its field of force gives us a handle: it is easy to move, it is the most mobile thing we know: it has extremely little inertia, and the smallest force will set it going very quickly.

When it moves, what happens? Its line of motion is surrounded by magnetic rings: it, as it were, threads magnetic loops. It does not generate those loops, it rearranges pre-existent loops: its line of motion is surrounded by magnetism, as an umbrella might be surrounded by an indiarubber ring. The faster it moves, the more those rings open out, the bigger they grow, until they become conspicuous. A moving electron is surrounded by a magnetic field.

This has important consequences, extraordinarily important: and if we knew all that was happening, and exactly what the rings were like, we should know a great deal more than we do. But we know some of the consequences: one is that an electron displays the fundamental property of matter, the property of inertia, the property of going on until it is stopped: once set in motion, its motion will continue; it will continue to move in a straight line with uniform velocity, unless deflected by external agency. An electron has a certain massiveness; although so minute it is not a geometrical point, it is a substantial thing: it can deal a blow to an obstacle put in its way. It has a certain amount of energy; every moving body has energy: the energy of a cannon-ball or of a motor-car is conspicuous, you cannot stop it dead without violence: put an obstacle in the way, something happens.

The energy of a moving electron can be calculated. If it is stopped, what becomes of that energy? Well, what becomes of the energy of a cannon-ball when that is stopped? That is more complex: a great moving mass is not a simple thing like an electron. We know that the target is damaged, that the air is disturbed in such a way as to cause waves of sound to travel out from the impact, and that if the blow is violent enough, the ether is disturbed too, and there is a flash of light. Moreover all the molecules are set quivering: in other words a lot of heat is produced.

Does anything of the same sort happen when a quickly moving electron is stopped by a target, that is by some heavy atom which gets in the way? The energy must go somewhere. When the motion is stopped, the magnetic rings cease to be. What becomes of them? Do they go out of existence? Certainly not. Do they shrink up to infinitesimal size, as they might if the electron were checked gradually? No, if the impact is violent enough, the rings do not shrink, they rather expand: the electric and the magnetic fields, which had previously existed quietly together, now combine into an Ether disturbance. They travel out as waves, not a series of waves but a pulse, a shell of wave, rapidly expanding with the speed of light. And they carry with them the momentum of their origin. The moving electron had extra mass,---temporary matter,---and that extra mass, when it stops, is converted into a quantum of radiation.

Such wave-like shells were discovered by Röntgen; they are known as X-rays. It is a most instructive phenomenon, one that can be followed in some detail. The quicker the electron was moving, the stronger and sharper will be the shell of radiation. If an electron had been travelling at what, for it, must be called a moderate speed, a few thousand miles a second, it emits what are called "soft X-rays," i.e. the kind which are not very penetrating, which can be stopped or absorbed by fairly soft tissues: whereas if it had been moving say a hundred thousand miles a second, or something approaching the speed of light, the X-rays will be exceedingly penetrating, able to go through bone and iron, what are called "hard X-rays." Thus we are getting on familiar ground. The energy of the wave will be comparable with the energy of the electron before its stoppage; a quantum of energy passes into the Ether: there is no waste in sound, and not much in heat, it nearly all goes out as radiation.

Is the electron destroyed in the process? No. The electric charge remains, and has to be carried away; wires are provided for the purpose: the target must not be insulated, or it will gradually get charged by the torrent of electrons and decline to receive any more. The charge itself is uninjured: whence then comes the energy? It comes from the magnetic field surrounding the moving electron, and surrounding it only while it is moving: this it is which is broken up and destroyed, or rather not destroyed, but distributed as a wave into space, turned into an etherial quiver. The process is typical of the production of radiation generally: there is probably no other radiation except that which is produced by the impact of electrons and protons. Electric charges clashing together excite radiation: this is probably true even in the comparatively gentle chemical processes of combustion and flame.

But now comes an important though perhaps rather difficult point. The magnetic field surrounding the electron possessed energy; it possessed momentum, it enabled the electron to continue in motion, it is responsible for its inertia; it probably contains the clue to the meaning of inertia. In what form is the energy in a magnetic loop or ring? We do not know: we can make a surmise, based on certain evidence; but experiment has not yet answered. The great mathematician, Sir Joseph Larmor, who holds the Chair of Newton at Cambridge, has surmised that round a magnetic loop the Ether is circulating,---not necessarily quickly, but circulating,---as a curtain-ring might be spinning in its own plane, or like the rim of a spinning-top or fly-wheel; and that the energy is represented by this circulation. I said that the Ether was not capable of locomotion, or at least that we had no means of getting hold of it and moving it from place to place. That remains true, but a spin is not locomotion. There is no locomotion about a spinning-top, if it is merely spinning; it has gone to sleep, it is stationary. Such motion is sometimes called "stationary motion"; it is not apparent unless you try to stop it. When you try to stop it, something happens: bring something in contact with the edge, the top flies away. Stop the magnetic circulation in a loop, it too flies away,---not like the top, but by expansion; thereby a disturbance is generated in the Ether which we call either X-rays or light, according to circumstances.

What generated this spinning motion in the Ether? What was the origin of these spinning loops? No one knows: Science cannot answer the question of "origin" or genesis. We have no means of generating such loops: they are there: all we can do is to open them out and make them apparent. They can be opened out quietly, as in an electro-magnet; or they can open out violently, as in radiation.

It must be understood that the loops are a reality. What is going on in them is a speculation: it still remains for experiment to ascertain; to confirm or contradict our guess. One way that I have suggested of verifying the supposed circulation is to make a very strong magnetic field, and send light along it, first one way and then the other, or rather both ways simultaneously, and see whether one is accelerated and the other retarded. The experiment is a difficult one, and has not yet been adequately made. I have indeed made the experiment with considerable care but with insufficient power, and so far without result, that is without detecting the circulation. But there are good reasons for that; if the Ether has the density which I attribute to it, the circulation in any artificial field is almost too slow to be detected. The experiment requires to be repeated on a very much larger scale, if it is to give any result. Meantime we must leave this part of the subject as a speculation, and say no more about it here.

We are sure of this,---that radiation is produced by atomic or electronic collision; i.e. by sudden contacts at high speed. And we have already studied the nature of 'contact' (see Chapter Four) and found that a collision is of an astronomical or etheric order, that atoms of matter are never in contact, there is always a sort of elastic cushion which keeps them apart, and that by interaction of etheric fields radiation is generated, and generated at the expense of a temporary kind of matter produced electromagnetically by motion.

I am here only touching the fringe of a vast subject, the connexion between ether and matter, and the temporary kind of matter which is consumed in producing radiation, and on which the size of the quantum depends. We are on the borderland of knowledge, and must walk warily.

There are strange unexplained facts which we see looming ahead of us. Radiation has been studied for a century in the full light of a wave-theory, but it still contains outstanding puzzles. It is emitted and absorbed in packets, in quanta; radiation simulates some of the properties of matter, it carries momentum and exerts pressure; who knows what its exact relation to matter is? Sometimes we feel as if radiation were a half-way stage between ether and matter---something half-way between the free and the modified ether. Matter is discontinuous. Is radiation discontinuous too? Is light discontinuous? Is it a kind of matter which is bound to travel at a fixed speed, and is tolerated by the ether only at that speed; or only otherwise tolerated if it be modified, as when its energy is imparted or converted into an electric charge? Strange notions these, but we have to get used to strange notions. Whatever the truth may be it will be profoundly significant when we grasp it. The singular difficulty and the intense interest of the problems before us are equalled only by the ingenuity with which they are being attacked.

Meanwhile the only way to progress safely is to study the facts, especially the obscure facts. Somehow light bears traces of its origin from quivering electric units; and it exhibits those traces forcibly when it encounters similar units. Matter excites radiation at its own expense: does radiation ever, anywhere, give rise to matter? The eternity of the cosmos seemed at one time doubtful, because of the dissipation of energy: now there is some glimpse of a way out. Matter tends to fall together gravitationally: but radiation tends to spread to the confines of the universe. And yet, however diluted, it retains its vigour. When quanta are thus isolated, what are they? What becomes of them? There is some deep meaning even in the speed with which light travels: no greater speed seems possible. In that speed, and in the newly presented puzzles about the connexion between ether and matter, and about the nature of radiation,---problems which are beginning to take definite shape,---we are down among the foundations of material being. We stand as it were enthralled by the revelation which is dawning upon us. Confronted with a majestic vision of Reality, we---like those other explorers on their first view of the Pacific Ocean---have

  "Look'd at each other with a wild surmise---
      Silent, upon a peak in Darien."

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