THE GENERAL PRINCIPLE OF RELATIVITY IN ITS PHILOSOPHICAL AND HISTORICAL ASPECT BY H. WILDON CARR, D. LITT. PROFESSOR OF PHILOSOPHY IN THS UNIVERSITY OF LONDON SECOND EDITION REVISED AND ENLARGED MACMILLAN AND CO., LIMITED ST. MARTINS STREET, LONDON 1922 PREFACE SINCE the publication of this book, a little more than a year ago, the interest in Einstein and the principle of relativity has very greatly increased. There are now a large number of popular ex positions, and the theory itself has undergone some notable advances in its philosophical, mathematical and physical application. In pure philosophy Lord Haldanes Reign of Relativity has applied it to the direct interpretation of the theory of knowledge. In mathematical physics the important work of Hermann Weyl, Space-Time-Matter, is an advance even on Einstein, and applies the principle not only to the geometry of masses, but to that of intermolecular move ments, and to electro-magnetic phenomena gener ally, thus making it embrace the physical world in its entirety. The main purpose of this book is to show the historical relations of the new principle to the old vi PREFACE philosophical problems and to the classical theories of space and time. I have, with the exception of a few important emendations, left the historical chapters unaltered, but I have added a new chapter on Einsteins Theory in order to show more clearly than the original work did the nature of the revolution which it effects in scientific method and in the concept of physical reality. I have to acknowledge the very kind assistance of Dr. G. B. Jeffery of University College, London. Many of the emendations are due to his suggestion. LONDON. March, 1922. CONTENTS CHAPTERI PAGE SPACE, TIME AND MOVEMENT i CHAPTER II EINSTEINS THEORY 24 CHAPTER III THE ANTINOMY OF MOVEMENT 54 CHAPTER IV ATOMS AND THE VOID 70 CHAPTER V THE VORTEX THEORY 87 CHAPTER VI THE PROBLEM OF GRAVITATION - - - - 107 viii CONTENTS CHAPTER VII 1 AGK LEIBNIZ AND THE THEORY THAT SPACE is THE ORDER OF COEXISTENCES 127 CHAPTER VIII THE MODERN SCIENTIFIC REVOLUTION AND ITS LEADERS 150 CHAPTER IX CONCLUSION IN WHAT SENSE is THE UNIVERSE INFINITE 188 INDEX 199 CHAPTER I SPACE, TIME AND MOVEMENT THE new theory of Einstein which is known as the general principle of relativity is perfectly simple when once it is understood and peculiarly difficult to understand. This arises from the fact that the human mind, in its ordinary attitude of reflection, and particularly in its well-balanced moods, subject to reason and superior to emotion, is always ready to revise its conclusions. When, however, it is required not merely to revise its conclusions but actually to amend its premises, a kind of mental giddiness is experi enced, a feeling of insecurity as though the firm ground on which its conclusions are based and from which they derive their whole strength had begun to shake and prove unstable. The wonderful structure of physical science, with the assurance consequent on the continual progress and constant acceleration of its advance in the 2 PRINCIPLE OF RELATIVITY last two centuries of the modern period, seems in jeopardy the moment real doubt is thrown on the concepts of absolute space and time and movement, which appear as its conditions. It is because these concepts are rejected by the new principle that the revolution in science is so profound and far-reaching. Space, time and movement seemdirect self-revealing realities and to the ordinary man the necessity of having theories about them is difficult to appreciate. There are indeed, as everyone knows, puzzling psychological problems and even perplexing philosophical questions con cerning them, but these all seem, when we reflect on them, to concern wholly and solely our knowledge, and the mistakes and illusions which may arise in regard to our knowledge...