The Fourth Dimension

by Charles Howard Hinton

We have seen that his matter must be supposed to have an extension, though a very small one, in the third dimension. And thus, in the small particles of his matter, three-dimensional movements may well be conceived to take place. Of these movements he would only perceive the resultants. Since all movements of an observable size in the plane world are two-dimensional, he would only perceive the resultants in two dimensions of the small three-dimensional movements. Thus, there would be phenomena which he could not explain by his theory of mechanics—motions would take place which he could not explain by his theory of motion. Hence, to determine if we are in a four-dimensional world, we must examine the phenomena of motion in our space. If movements occur which are not explicable on the suppositions of our three-dimensional mechanics, we should have an indication of a possible four-dimensional motion, and if, moreover, it could be shown that such movements would be a consequence of a four-dimensional motion in the minute particles of bodies or of the ether, we should have a strong presumption in favour of the reality of the fourth dimension.

By proceeding in the direction of finer and finer subdivision, we come to forms of matter possessing properties different to those of the larger masses. It is probable that at some stage in this process we should come to a form of matter of such minute subdivision that its particles possess a freedom of movement in four dimensions. This form of matter I speak of as four-dimensional ether, and attribute to it properties approximating to those of a perfect liquid.

Deferring the detailed discussion of this form of matter to Chapter VI., we will now examine the means by which a plane being would come to the conclusion that three-dimensional movements existed in his world, and point out the analogy by which we can conclude the existence of four-dimensional movements in our world. Since the dimensions of the matter in his world are small in the third direction, the phenomena in which he would detect the motion would be those of the small particles of matter.

Suppose that there is a ring in his plane. We can imagine currents flowing round the ring in either of two opposite directions. These would produce unlike effects, and give rise to two different fields of influence. If the ring with a current in it in one direction be taken up and turned over, and put down again on the plane, it would be identical with the ring having a current in the opposite direction. An operation of this kind would be impossible to the plane being. Hence he would have in his space two irreconcilable objects, namely, the two fields of influence due to the two rings with currents in them in opposite directions. By irreconcilable objects in the plane I mean objects which cannot be thought of as transformed one into the other by any movement in the plane.

Instead of currents flowing in the rings we can imagine a different kind of current. Imagine a number of small rings strung on the original ring. A current round these secondary rings would give two varieties of effect, or two different fields of influence, according to its direction. These two varieties of current could be turned one into the other by taking one of the rings up, turning it over, and putting it down again in the plane. This operation is impossible to the plane being, hence in this case also there would be two irreconcilable fields in the plane. Now, if the plane being found two such irreconcilable fields and could prove that they could not be accounted for by currents in the rings, he would have to admit the existence of currents round the rings—that is, in rings strung on the primary ring. Thus he would come to admit the existence of a three-dimensional motion, for such a disposition of currents is in three dimensions.

Now in our space there are two fields of different properties, which can be produced by an electric current flowing in a closed circuit or ring. These two fields can be changed one into the other by reversing the currents, but they cannot be changed one into the other by any turning about of the rings in our space; for the disposition of the field with regard to the ring itself is different when we turn the ring, over and when we reverse the direction of the current in the ring.

As hypotheses to explain the differences of these two fields and their effects we can suppose the following kinds of space motions:—First, a current along the conductor; second, a current round the conductor—that is, of rings of currents strung on the conductor as an axis. Neither of these suppositions accounts for facts of observation.

Hence we have to make the supposition of a four-dimensional motion. We find that a four-dimensional rotation of the nature explained in a subsequent chapter, has the following characteristics:—First, it would give us two fields of influence, the one of which could be turned into the other by taking the circuit up into the fourth dimension, turning it over, and putting it down in our space again, precisely as the two kinds of fields in the plane could be turned one into the other by a reversal of the current in our space. Second, it involves a phenomenon precisely identical with that most remarkable and mysterious feature of an electric current, namely that it is a field of action, the rim of which necessarily abuts on a continuous boundary formed by a conductor. Hence, on the assumption of a four-dimensional movement in the region of the minute particles of matter, we should expect to find a motion analogous to electricity.

Now, a phenomenon of such universal occurrence as electricity cannot be due to matter and motion in any very complex relation, but ought to be seen as a simple and natural consequence of their properties. I infer that the difficulty in its theory is due to the attempt to explain a four-dimensional phenomenon by a three-dimensional geometry.

In view of this piece of evidence we cannot disregard that afforded by the existence of symmetry. In this connection I will allude to the simple way of producing the images of insects, sometimes practised by children. They put a few blots of ink in a straight line on a piece of paper, fold the paper along the blots, and on opening it the lifelike presentment of an insect is obtained. If we were to find a multitude of these figures, we should conclude that they had originated from a process of folding over; the chances against this kind of reduplication of parts is too great to admit of the assumption that they had been formed in any other way.