THE ROMANES LECTURE 1922 _THE_ _THEORY OF RELATIVITY_ _and its_ _Influence on Scientific Thought_ BY ARTHUR STANLEY EDDINGTON M.A., F.R.S. _Plumian Professor of Astronomy, Cambridge_ _President of the Royal Astronomical Society_ DELIVERED IN THE SHELDONIAN THEATRE 24 MAY, 1922 OXFORD AT THE CLARENDON PRESS 1922 Has not a deeper meditation taught certain of every climate and age, that the where and the when so mysteriously inseparable from all our thoughts, are but superficial terrestrial adhesions to thought? CARLYLE, _Sartor Resartus_. Oxford University Press THE THEORY OF RELATIVITY In the days before Copernicus the earth was, so it seemed, an immovable foundation on which the whole structure of the heavens was reared. Man, favourably situated at the hub of the universe, might well expect that to him the scheme of nature would unfold itself in its simplest aspect. But the behaviour of the heavenly bodies was not at all simple; and the planets literally looped the loop in fantastic curves called epicycles. The cosmogonist had to fill the skies with spheres revolving upon spheres to bear the planets in their appointed orbits; and wheels were added to wheels until the music of the spheres seemed well-nigh drowned in a discord of whirling machinery. Then came one of the great revolutions of scientific thought, which swept aside the Ptolemaic system of spheres and epicycles, and revealed the simple plan of the solar system which has endured to this day. The revolution consisted in changing the view-point from which the phenomena were regarded. As presented to the earth the track of a planet is an elaborate epicycle; but Copernicus bade us transfer ourselves to the sun and look again. Instead of a path with loops and nodes, the orbit is now seen to be one of the most elementary curves--an ellipse. We have to realize that the little planet on which we stand is of no great account in the general scheme of nature; to unravel that scheme we must first disembarrass nature of the distortions arising from the local point of view from which we observe it. The sun, not the earth, is the real centre of the scheme of things--at least of those things in which astronomers at that time had interested themselves--and by transferring our view-point to the sun the simplicity of the planetary system becomes apparent. The need for a cumbrous machinery of spheres and wheels has disappeared.