Transcriber’s Notes: Underscores “_” before and after a word or phrase indicate _italics_ in the original text. Equal signs “=” before and after a word or phrase indicate =bold= in the original text. Small capitals have been converted to SOLID capitals. Illustrations have been moved so they do not break up paragraphs. Typographical and punctuation errors have been silently corrected. HARPER’S MODERN SCIENCE SERIES THE A B C OF RELATIVITY BY BERTRAND RUSSELL AUTHOR OF “THE PRINCIPLES OF MATHEMATICS” “PROPOSED ROADS TO FREEDOM” AND “WHY MEN FIGHT” PUBLISHERS HARPER & BROTHERS NEW YORK AND LONDON THE A B C OF RELATIVITY Copyright, 1925, by Harper & Brothers Printed in the United States of America _Contents_ CHAPTER PAGE I. TOUCH AND SIGHT: THE EARTH AND THE HEAVENS 1 II. WHAT HAPPENS AND WHAT IS OBSERVED 14 III. THE VELOCITY OF LIGHT 28 IV. CLOCKS AND FOOT RULES 43 V. SPACE-TIME 58 VI. THE SPECIAL THEORY OF RELATIVITY 71 VII. INTERVALS IN SPACE-TIME 91 VIII. EINSTEIN’S LAW OF GRAVITATION 111 IX. PROOFS OF EINSTEIN’S LAW OF GRAVITATION 131 X. MASS, MOMENTUM, ENERGY AND ACTION 144 XI. IS THE UNIVERSE FINITE? 163 XII. CONVENTIONS AND NATURAL LAWS 177 XIII. THE ABOLITION OF “FORCE” 192 XIV. WHAT IS MATTER? 206 XV. PHILOSOPHICAL CONSEQUENCES 219 THE A B C OF RELATIVITY CHAPTER ONE: TOUCH AND SIGHT: THE EARTH AND THE HEAVENS