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The Grip of Gravity The Quest to Understand the Laws of Motion and Gravitation Gravity is the most enigmatic of all known forces of nature. It controls everything, from ocean tides to the expansion of the Universe. The search for the laws of motion and gravitation started over two thousand years ago. The reader is taken on an exciting journey through the subsequent centuries, identifying the blind alleys, profound insights and flashes of inspiration that have punctuated this search. Despite the fantastic progress that has been made, the true nature of gravity is still a mystery and the book attempts to show how the current developments in string theory (perhaps the ‘Theory of Everything’) may lead to a new and radical interpretation of gravity. This book describes the fundamental concepts and developments, and the experiments, both performed and planned, to increase our understanding of gravity and the natural phenomena in which gravity is the principal player. prabh akar gondhal e kar is an astrophysicist with major research interest in the interstellar medium and active galactic nuclei. He was formerly head of the Space Astronomy Group at the Rutherford Appleton Laboratory. prabhakar gondhalekar The Grip of Gravity The Quest to Understand the Laws of Motion and Gravitation CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 2RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521803168 © Cambridge University Press 2001 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2001 This digitally printed first paperback version 2005 A catalogue record for this publication is available from the British Library Library of Congress Cataloguing in Publication data Gondhalekar, Prabhakar, 1942– The grip of gravity: the quest to understand the laws of motion and gravitation / Prabhakar Gondhalekar. p. cm. Includes bibliographical references and index. ISBN 0 521 80316 0 1. Gravitation – History. 2. Motion – History. I. Title. QC178.G56 2001 531´.14–dc21 00-065149 ISBN-13 978-0-521-80316-8 hardback ISBN-10 0-521-80316-0 hardback ISBN-13 978-0-521-01867-8 paperback ISBN-10 0-521-01867-6 paperback Contents Preface vii Aristotle 1 Greek science 2 Alexandrian science Islamic science 19 12 Kepler 25 The Copernican revolution 28 The professional astronomer 35 Marriage of mathematics and observations 42 Galileo 51 Mechanical philosophy 55 Birth of experimental physics 61 Newton 77 Origins of dynamics 81 Laws of motion 91 Universal gravitation 98 Newton’s universe 101 Laws of Kepler and Newton 102 Conserved quantities 104 Action 108 After the Principia 111 Einstein 119 Speed of light 124 What is a straight line? 126 Doubts about Newtonian mechanics 131 v vi contents Theory of special relativity 142 Theory of general relativity 155 Choice between Newton and Einstein 170 Alternatives to general relativity 173 Consequences of general relativity 176 Dicke 189 Universal constant of gravitation G 191 Inverse square law 203 Gravitational redshift 207 Deflection of light by gravitational field 211 The ‘fourth test’ or the time delay of radio waves Equivalence Principle 223 Gravitomagnetism 233 Gravitational waves 238 Hubble & Eddington 247 Figure of planets 247 Tides 249 Ice ages 251 The universe – an expanding fire-ball 254 Galaxies, quasars and clusters of galaxies 275 Dark matter 286 The birth, life and de