A STUBBORNLY PERSISTENT ILLUSION A STUBBORNLY PERSISTENT ILLUSION THE ESSENTIAL SCIENTIFIC WRITINGS OF ALBERT EINSTEIN RUNNING PRESS PHILADELPHIA (cid:129) LONDON © 2007 by Stephen Hawking All rights reserved under the Pan-American and International Copyright Conventions Printed in the United States This book may not be reproduced in whole or in part, in any form or by any means, electronic or mechanical, includ- ing photocopying, recording, or by any information storage and retrieval system now known or hereafter invented, with- out written permission from the publisher. 9 8 7 6 5 4 3 2 1 Digit on the right indicates the number of this printing First paperback edition published in 2009 Library of Congress Control Number: 2007935658 ISBN-13 978-0-7624-3564-7 Cover design by Bill Jones Interior design by Bill Jones and Aptara, Inc. Edited by Jennifer Kasius with David Goldberg Typography: Adobe Garamond This book may be ordered by mail from the publisher. Please include $2.50 for postage and handling. But try your bookstore first! Running Press Book Publishers 2300 Chestnut Street Philadelphia, PA 19103-4371 Visit us on the web! www.runningpress.com TEXT CREDITS Selections from THE EVOLUTION OF PHYSICS: Reprinted with the per- mission of Simon & Schuster Adult Publishing Group, from The Evo- lution of Physics by Albert Einstein and Leopold Infeld. Copyright © 1938 by Albert Einstein and Leopold Infeld. Copyright © renewed 1966 by Albert Einstein and Leopold Infeld. All Rights Reserved. Selection from THE MEANING OF RELATIVITY: Courtesy of Princeton University Press. Autobiographical notes reprinted by permission of Open Court Publish- ing Company, a division of Carus Publishing Company, Peru, IL, from A. Einstein: Autobiographical Notes translated and edited by Paul Arthur Schilpp, first published in Albert Einstein: Philosopher-Scientist in The Library of Living Philosophers Series Volume VII, (c) 1949, 1951, 1970, 1979 by The Library of Living Philosophers, Inc., and the Estate of Albert Einstein. Selections from OUT OF MY LATER YEARS: Reprinted with permis- sion from The Philosophical Library, New York. v C ONTENTS INTRODUCTION BY STEPHEN HAWKING ix THE PRINCIPLE OF RELATIVITY 1 RELATIVITY, THE SPECIAL AND GENERAL THEORY 125 SIDELIGHTS ON RELATIVITY 235 SELECTION FROM THE MEANING OF RELATIVITY, “SPACE AND TIME IN PRE-RELATIVITY PHYSICS” 263 SELECTIONS FROM THE EVOLUTION OF PHYSICS, “RELATIVITY, FIELD” AND “QUANTA” 283 AUTOBIOGRAPHICAL NOTES 337 SELECTIONS FROM OUT OF MY LATER YEARS 383 INDEX 457 vii INTRODUCTION BY STEPHEN HAWKING Afew years ago the world celebrated the 100th anniversary of Einstein’s miracle year, the year in which he revolutionized physics in multiple ways with a series of astonishing new ideas that brought about profound changes in the way physicists view the universe. Human intuition tells us that space is a stage upon which the events of our lives play out, that time is governed by a universal clock. But in 1905 and the decade that followed, Einstein showed that space and time do not have identical meanings for observers sitting in a chair and those flying on a plane, those orbiting with us on earth, those having tea somewhere in the Virgo cluster, or those being sucked into a black hole. Einstein’s ideas once stunned physicists. Today they are automati- cally incorporated into the equations and formalism learned by every undergraduate physics major. As long as those ideas stand up, Einstein wrote in one of the articles in this collection, the Germans will call him a “German savant,” and the English will call him a “Swiss Jew.” But if his ideas were ever discredited, he wrote, he would be a “Swiss Jew” for the Germans and a “German savant” for the English. Today there are few physicists left who remember Einstein as a living, breath- ing, and witty human being. Today his ideas of space and time inter- twined are ingrained in popular culture, and described by writers several generations down. But the most lucid, not to mention enter- taining, proponent of Einstein’s ideas has always been Einstein himself. As he describes in this volume, Einstein’s 1905 special theory of relativity grew out of a simple observation. The theory of electro- magnetism discovered by James Clerk Maxwell in the 1860s showed that whether you are moving toward or away from a beam of light, the light will always approach you at the same speed. This is not true of our experience in the everyday world. If you race away from an onrushing train you will survive for a few more seconds than if you race toward it (assuming that you don’t get the idea to jump sideways). ix INTRODUCTION In the former case the train’s speed of approach will be the difference between its speed and your speed (relative to the track). In the latter case its speed of approach will be the sum of your speeds. The same, according to Maxwell’s theory, does not apply to the light emitted from the train’s headlamps. How could the speed of light not appear slower in the former case and faster in the latter? By speed we mean distance traveled divided by the time of the trip. And so, Einstein realized, if we are to take Maxwell’s theory at face value, we must alter our ideas of space and time. They are not fixed and unchanging, but adjust according to the observer, bending or stretching in just the way necessary to keep the speed of light con- stant. The same bending and stretching means of course that the speed at which the train itself approaches is also not the simple sum or dif- ference I described above. But at speeds far less than the speed of light the difference in adding and subtracting derived by Einstein has only negligible effect. The same chain of logic, when taken further, requires also the equivalence of mass and energy, the reason that we can have atomic energy, and, unfortunately, atomic weapons. The details of Einstein’s reasoning, and the simple algebra behind it, are explained nowhere better than as found here, in Einstein’s own words. Einstein’s theory of general relativity also grew from a simple observation. In Newton’s laws of motion there appears a quantity called the mass, which determines how easily an object accelerates when a force is applied. A massive truck is far more difficult to bring to speed than a far less massive Volkswagen. In Newton’s day three forces were known: electricity, magnetism, and gravity. The resistance to changing velocity in Newton’s laws of motion does not depend on which of those forces is applied. But Newton also discovered a law governing one of those forces, the force of gravity. In that law there appears another quantity which determines the amount of gravita- tional pull an object exerts, and the amount of gravitational pull it feels when in the presence of another object. That quantity is also called the mass. The two definitions of mass play quite different roles, but they are both called mass for good reason: it turns out they are x