4: The Ether as Transmitter of Force

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4: The Ether as Transmitter of Force
And what "Contact" is like.

For my own part, considering the relation of a vacuum to the magnetic force, and the general character of magnetic phenomena external to the magnet, I am much more inclined to the notion that in the transmission of the force there is such an action, external to the magnet, than that the effects are merely attraction and repulsion at a distance. Such an action may be a function of the aether; for it is not unlikely that, if there be an aether, it should have other uses than simply the conveyance of radiation. ---FARADAY.

OUR commonest experience on this planet is to see one body act on another. You see a horse pulling a cart, or a magnet pulling a bit of iron, or the earth pulling down an apple: you also see a golf-club or the head or the boot of a football player acting on a ball, or a gun acting on a bird: or you find yourself lifting a hat or opening a window or throwing a stone. In fact in the material universe you experience little else than the action of one body on another. But there is an obvious apparent difference between the instances; in one set there appears to be contact between the two bodies; in the other set they appear to act on each other at a distance. And action at a distance seems to require more explanation than when there is contact. You may think of some other familiar but instructive instances of action at a distance. You can act on a distant dog by whistling to it, or by throwing a stick into water for the dog to fetch out; you can act on a distant person by either shouting or writing him a letter or telegraphing to him. But in all such cases you know, or ought to know, that there must be a medium, connecting you and the distant person, through which the action is transmitted: and it may occur to you to ask whether action is ever direct, whether there is always something akin to a projectile or to a connecting string, whether in fact a medium of communication is always involved.

This was a question which puzzled Sir Isaac Newton, and therefore may well puzzle us. The earth not only acts on stones and apples, it acts on the moon, across apparently empty space, certainly across space empty of matter. The sun similarly controls all the planets, and must be acting even on the most distant stars---although no effect is in that last case perceptible. Those who have contemplated closely all these actions have been led to ask themselves whether the apparent difference between them is an appearance or a reality, whether bodies always act on each other through a medium, and whether there is ever genuine contact. The result of cogitation has been to promulgate two opposition theses: one to the effect that a body cannot act where it is not: the other that true contact does not exist. In other words, that in cases of apparent contact there is really an interval, though an extremely minute one, separating the two bodies, across which infinitesimal interval the force has somehow to be transmitted.

At first sight this denial of real contact seems an unnecessary complication, reducing the more obvious cases to the more mysterious kind. For contact seems intelligible, whereas action at a distance seems to demand explanation. Let us consider the two theses further.

First "a body cannot act where it is not." That sounds in accordance with common sense. But then the question arises: Where is it? Is the magnet limited to the bit of steel that you handle and see, which is plainly limited or localized in space, or does it carry with it a field of force, which extends outwards in an unlimited manner, though with rapidly diminishing intensity? This field of force would then be essentially the magnet, though an intangible and supersensuous part of it. No one can deny that a magnet acts on a distant piece of iron; but it is quite possible to say that this only shows that the magnet extends as far as the iron, that its field of force reaches the iron, and that it is that extension of the magnet that produces the effect. This idea would answer the question, whether a body can act where it is not, in the negative; but would add that in some sense the body must extend beyond its obvious boundary, so that it really reaches everything on which it can act.

So Newton endowed the earth and the sun and planets with gravitational fields of force, which were an essential though not an obvious part of their constitution; and it becomes merely a question of convenience whether we say that the field of force of the earth reaches to the moon, or whether we say that the earth acts on some intervening medium, and that this medium transmits the action to the moon. The facts are clear and certain enough. The action is as if the earth attracted the moon. All can agree to that; though Newton felt, and many others feel, bound to look for some mechanism or medium of communication which renders such attraction---such action at a distance---possible. It is only the best mode of expression which is doubtful; and we realize that in order to express the action completely we must know more about the nature of gravitation.

Now take the other thesis, that "contact does not exist." At first sight that looks like nonsense. You see a book resting on a table, you see a locomotive propelling a truck, or a nurse pushing a perambulator: contact between material bodies is obvious. But when you contemplate the matter closely and realize that matter is composed of atoms, and that each atom is composed of electric charges, you perceive that what you call contact is not so simple. If in an ordinary solid you have to admit that the ultimate particles of which it is composed are separated from each other in the same sort of fashion, and by much the same relative distances, as the planets in the solar system are separated from each other---i.e. with intervening spaces great in proportion to their size---you naturally begin to doubt whether two different bodies can ever be more closely in touch than are the particles of the same body. And even in cases of impact, as when a hammer strikes a nail, or a ball rebounds, or two billiard balls kiss, you begin to wonder (if you are philosophically inclined) what the contact, atomically considered, is really like---whether it is real or only an appearance.

Astronomy gives us a hint in this connexion. You know perhaps that a comet coming out of the depths of space approaches the sun with constantly increasing speed, getting quicker and quicker as well as brighter as it comes nearer, that it then as it were rebounds from the sun and travels back into space whence it came, having acquired a conspicuous tail as the result of the collision. And if we were looking at the comet from a great distance, without being able to analyse exactly what was happening, we could very well liken it to the bounce of an indiarubber ball: the comet would appear to have struck the sun and rebounded. But we, being astronomically "near" the scene of action, are able to analyse this large-scale phenomenon more closely; we can tell (or astronomers can) that the comet has never struck the body of the sun, it has all the time been linked to it with gravitational hands across, as in a sort of Ladies' Chain, till it has been swirled round in a violently curved path and so been flung away again, travelling back by reason of its own momentum.

Here then is a collision which we can analyse: and we naturally ask whether an atomic collision (which is on so minute a scale that we cannot so easily examine it) is after all of the same nature. We find, or Sir Ernest Rutherford finds, that it is of just the same nature. Electrified particles, such as constitute the atoms of matter, join hands across, not in a gravitational but in an electrical way, and swing each other round in exactly that cometary fashion, if their charges are of opposite sign. If their charges are of similar sign, so that they repel each other, the two particles do not approach so close: they rebound at a greater distance. They still do not strike each other; there is, as it were, an elastic cushion between them; but the ultimate effect is precisely the same. An object or a particle approaching a repulsive centre slows down instead of hurrying up, storing up elastic energy in the medium between them, which then drives it back in a symmetrical path in the general direction whence it came. It takes the same identical path, seen from a distance, as if it had been attracted instead of repelled.

There are thus among electrified particles two different kinds of collision, the attractive and the repulsive kind; and these produce just the same effect, though for opposite reasons. No one seeing the result could tell which kind of collision had happened, whether it was of the attracting or the repelling variety. In neither case has there been any contact between the bodies themselves; the forces acting may have been violent, but they have been forces between the fields of the two bodies, or, in other words, there has been a strong deflecting force exerted in the medium between them.

A collision and rebound of this kind is an etherial phenomenon, conducted according to the law of the inverse square, just as in the case of a comet. There is no contact between the material particles themselves; in that sense it may be truly said that contact does not exist. There is always an elastic cushion which keeps the bodies apart: they both modify the ether in their neighbourhood; and a book resting on a table is really reposing on a cushion of ether.

Two pieces of glass or of metal pressed together do not really touch; if they did they would cohere; they remain separated by a microscopic distance. If they are pressed very tightly together and are very clean, or if a film of cement is placed between them, they may adhere and become one piece. Even then they are not really in contact, but they are closer now than the microscopic interval represented by a crack; they are now as close together as are the particles of any single body, their distance apart is molecular and ultra-microscopic. But even then they are cohering by their fields of force: the ultimate particles do not touch. Cohesion is an etherial force, pretty certainly either electric or magnetic, or both: the particles now no longer rebound; they have, as it were, entered into combination and attained a position of equilibrium.

But could it ever happen that two particles really collided? It could not happen if they were similarly charged, unless one was moving at a tremendous pace; they would usually repel each other so violently that they must rebound before they touched. But if they were oppositely charged, so as to attract, a real collision might occasionally occur; it would be very rare under ordinary circumstances, and something striking would be bound to happen. They might, as it were, obliterate each other, and we should have to ask what has become of their energy. They could not really knock each other into non-existence: depend upon it that never happens; energy is never destroyed. Some effect would remain; and it so happens that we know what that effect would be. A wave or pulse of energy would be generated in the ether: a luminous or X-ray pulse would be emitted by the collision, and would immediately start out in all directions with the speed of light. It is believed that in some of the stars such real collisions actually occur, and that this accounts for their violent radiation, which is of an intensity far greater than anything we experience on the earth.

Every collision or sudden change of speed generates some radiation. The molecular collisions with which we are familiar in a fire are of a gentle character: no destruction of matter occurs in them, at least no destruction of the permanent kind of matter which our instruments enable us to weigh and measure: only the extra energy of their motion is transformed into radiation. Moving matter has an adventitious or extra mass associated with it, and that kind of temporary mass can be shattered off into radiation by the ordinary encounter between their fields of force. Indeed, that is precisely how all radiation is generated.

In a wireless aerial the electrons are rushing along and being reflected. They are thus put into a state of large-scale oscillation, and the waves emitted are comparable to the dimensions of the aerial. Small waves, such as we call "light," are generated by oscillations of molecular dimensions; and X-rays are generated by oscillations or collisions of atomic dimensions, or sometimes even of the still smaller dimensions of an electron. There is no other way of generating radiation: it is all generated by sudden changes in the speed of particles, and is an outcome of their close relation or interaction with the ether. The ether, as it were, stands by, always ready to pick up any loose energy and broadcast it with the speed of light.