Of course, here a problem of extreme difficulty confronts us. When by varying external conditions at will we can produce variations in a magnitude, we may maintain that this magnitude is relative to surrounding conditions; for instance, the weight of a body varies with its distance from the earth. When, on the other hand, nothing that we can do appears to produce the slightest effect, we claim that the magnitude is absolute; mass in classical science was a case in point. Obviously, however, a test of this sort has a purely negative value for the simple reason that many of the external conditions lie beyond our power to vary. We cannot, for instance, annihilate the stars or the sun or the electrons. Thus, whereas a magnitude which has been established as relative will presumably remain a relative for all time to come, the same measure of assurance can no longer be claimed for our determinations of absolute quantities. Nevertheless, unless we are to fall into a state of complete agnosticism, we are compelled to establish a difference between quantities which appear absolute in the present state of our knowledge and those which are known to be relative. If we prefer, therefore, we may refer to absolutes as “relative-absolutes,” where the word “relative” implies the limitations of our present means of investigation.
It is possible to present Newton’s arguments in favour of absolute space in a number of different ways. The following illustrations may serve to clear up a few additional points. Consider a circular disk of gigantic proportions around whose central axis the stars would appear to be rotating in a clockwise direction with an angular velocity
. We shall assume the disk to be inhabited by beings living around its centre; and we shall further suppose that dense clouds conceal the stars from their gaze, so that there would be no incentive for them even to suspect that their disk might be in rotation. These men would naturally refer all motion to their disk, just as, prior to Copernicus and Galileo, men were wont to refer all motions to the earth’s surface.
Suppose, now, that they were to perform mechanical experiments. They would soon discover that no object could remain motionless on the disk unless it were placed at the very centre or else fixed by artificial means. In particular, all objects originally fixed to some non-central point, then suddenly abandoned, would start moving away radially from the centre, then gradually follow a curved course, circling clockwise along an expanding spiral. In certain cases, however, a body might describe a true circle round the centre; but whenever this circular motion occurred, it would always be directed clockwise, and, furthermore, its angular velocity would invariably be given by a certain constant quantity co. Then again, if billiard balls were shot out in all directions with equal initial speeds from some non-central point, no two of their trajectories would be alike; not the slightest symmetry would be observed, and the clockwise motions would always be present. It may well be realised that for men who claimed space to be homogeneous and everywhere the same, this dissymmetry in the motions of bodies would be hard to account for.
Let us assume that just above the first disk a second one is rotating relatively to the first in a clockwise direction with angular velocity
. The inhabitants of this second disk would find that bodies remained at rest wherever placed on their disk, that bodies set in motion would pursue straight courses with constant speeds, and, in short, that the asymmetry characteristic of the lower disk would give place to perfect symmetry of motion, no special direction being privileged. Furthermore, whereas the inhabitants of the lower disk would have the greatest difficulty in returning to the centre if peradventure they ever wandered away from it, and whereas they would always feel the action of unsymmetrical forces when they moved about, the inhabitants of the upper disk could move as they pleased and never be subjected to the action of forces.
Such would be the facts of the case. Now, if motion were relative, we could not assume that there existed any absolute difference between the motions of the two disks; either disk would be rotating, but only with reference to the other disk taken as standard. Yet here we see that whether or not a metaphysical difference is assumed to exist, it is quite certain that a vast physical difference is present in the conditions reigning on the two disks.
And so we are naturally led to enquire: What causes this dissymmetry in a homogeneous isotropic space? It is always preferable in science to search for causes in phenomena that are, so to speak, palpable, rather than in invisible agencies, for less guesswork is involved. In the present case, however, no visible cause can be countenanced. True, if the cloud: were to lift, it might strike the inhabitants of the lower disk that, with respect to their world, the stars were rotating clockwise with an angular velocity