The Layman’s Last Doubt
The final answer to any series of questions is inevitably “because the world is so constructed.” The things we are content to leave on that basis are those to which we are accustomed, and which we therefore think we understand; those for which this explanation leaves us unsatisfied are those which are new and unfamiliar. Newton told us that the world of three-dimensional space with one-dimensional time superposed was so constructed that bodies left to themselves would go on forever in a straight line at constant speed. We think we understand this, but our understanding consists merely of the unspoken query, “Why, of course; what is there to prevent?” The Greeks, an intelligent people, looked at this differently; they would have met Newton with the unanimous demand “Why so; what is there to keep them going?” So if, in seeking an explanation of anything, we come sooner than we had expected to the finality “Because the world is so constructed,” let us not feel that we have been cheated.
IX
THE PRINCIPLE OF RELATIVITY
A Statement of What it is All About, in Ideas of One Syllable
BY HUGH ELLIOT
CHISLEHURST, KENT, ENGLAND
The invariance of the laws of nature was one of the most popular themes of nineteenth century philosophy. For it was not till last century that general acceptance was accorded to the doctrine of the “Uniformity of Law,” adumbrated in ancient times by Epicurus and Lucretius. It is now a cardinal axiom of science that the same cause in the same conditions is always followed by the same effect. There exists in nature no indeterminate element; all things are governed by fixed laws, and the discovery of these laws is the main business of science.
It is necessary to guard against reading into this statement an erroneous idea of the content of a “law of nature.” Such a law is of course not an enactment of any sort; and it is not even to be thought of as an actual explanation of the how and why of the phenomena with which it has to do. It really is nothing but an expression of our belief in the pronouncement of the preceding paragraph, that like conditions do produce like results. It is a prediction based on past experience, and is of value merely in that past experience leads us to credit its accuracy. The composite essay beginning on page 19 discusses this question of the reality of natural laws, and should be consulted in connection with the present contribution.—Editor.
This great philosophic principle was derived of course from the study of natural science; i.e., from observations and experiments conducted upon the earth. Their comprehensiveness is therefore limited by the fact that the observer is always in a state of rest, or nearly so, as compared with the earth. All observers upon the earth are moving through space at the same velocity; and it was possible to argue that the uniformity of law might only hold good, when experiments were conducted at this velocity. An observer moving at very different velocity might discover that the laws of nature under these new conditions were somewhat different.
Such a view could indeed never be very plausible, for motion is only a relative conception. Imagine a universe consisting of infinite “empty” space, in which there is poised a single material body. How shall we determine whether this body is at rest, or whether it is moving at high or low velocity through space? It is never getting nearer to anything or farther from anything, since there is no other body for it to get nearer to or farther from. If we say it is moving at a uniform velocity of a thousand miles a second, our statement really has no significance. We have no more reason for affirming that it is in motion than we have for affirming that it is at rest. In short, there is no such thing as absolute motion; the conception of motion only arises when there are two or more bodies changing their position relatively to one another. This is what is meant by the relativity of motion. It seemed therefore improbable that the laws of nature would be different if the observer were moving at high velocity; for the movement of the observer is not an absolute quantity, but merely a statement of his relation to other bodies, and if there were no other bodies, the statement itself would be meaningless.