The Universe in a Nutshell ALSO A BLACK HOLES BY STEPHEN BRIEF AND HISTORY BABY HAWKING OF T I M E U N I V E R S E S AND O T H E R ESSAYS The Universe LONDON • NEW YORK • in TORONTO a • Nutshell SYDNEY • AUCKLAND A Book Laboratory Book T R A N S W O R L D PUBLISHERS - Uxbridge R o a d , London W 5SA a division of T h e Random House Group Ltd R A N D O M H O U S E A U S T R A L I A ( P T Y ) LTD 20 Alfred Street, Milsons Point, Sydney, N e w South Wales , Australia R A N D O M H O U S E N E W Z E A L A N D LTD 18 Poland R o a d , Glenfield, Auckland , N e w Zealand R A N D O M H O U S E S O U T H AFRICA (PTY) LTD Endulini, 5a Jubilee R o a d , Parktown , South Africa Published 0 by Bantam Press a division of Transworld Publishers Copyright © Stephen Hawking 0 Original illustrations © 0 by M o o n r u n n e r Design Ltd UK and T h e B o o k Laboratory ™ Inc T h e right of Stephen Hawking to be identified as the author of this work has been asserted in accordance with sections 77 and 78 of the Copyright Designs and Patents Act 8 A catalogue record for this book is available from the British Library ISBN All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of the publishers Printed in Great Britain by Butler and Tanner Ltd, Frome, Somerset 10 FOREWORD CHAPTER ~ vii ~ page A Brief History of Relativity How Einstein laid the foundations of the two fundamental theories of the twentieth century: general relativity and quantum theory CHAPTER ~ page T h e Shape of Time Einstein's general relativity gives time a shape How this can he reconciled with quantum theory CHAPTER ~ page T h e Universe in a Nutshell The universe has multiple histories, each of which is determined by a tiny nut CHAPTER ~ page 1 Predicting the Future How the loss of information in black holes may reduce our ability to predict the future CHAPTER ~ page Protecting the Past Is time travel possible? Could an advanced civilization go back and change the past? CHAPTER ~ page 5 Our Future? Star Trek or Not? How biological and electronic life will go on developing in complexity at an ever increasing rate CHAPTER ~ page Brane New World Do we live on a brane or are we just holograms? Glossary Suggested further readings Acknowledgments Index T H E Stephen Hawking in 2001,©StewartCohen vi U N I V E R S E I N A N U T S H E L L F O R E W O R D FOREWORD I HADN'T EXPECTED MY POPULAR BOOK, A Brief History of Time, to be such a success It was on the London Sunday Times bestseller list for over four years, which is longer than any other book has been, and remarkable for a book on science that was not easy going After that, people kept asking when I would write a sequel I resisted because I didn't want to write Son of Brief History or A Slightly Longer History of Time, and because I was busy with research But I have come to realize that there is room for a different kind of book that might be easier to understand A Brief History of Time was organized in a linear fashion, with most chapters following and logically depending on the preceding chapters This appealed to some readers, but others got stuck in the early chapters and never reached the more exciting material later on By contrast, the present book is more like a tree: Chapters and form a central trunk from which the other chapters branch off The branches are fairly independent of each other and can be tackled in any order after the central trunk They correspond to areas I have worked on or thought about since the publication of A Brief History of Time Thus they present a picture of some of the most active fields of current research Within each chapter I have also tried to avoid a single linear structure The illustrations and their captions provide an alternative route to the text, as in The Illustrated Brief History of Time, published in 9 ; and the boxes, or sidebars, provide the opportunity to delve into certain topics in more detail than is possible in the main text vii T H E U N I V E R S E I N A N U T S H E L L In 8 , when A Brief History of Time was first published, the ultimate Theory of Everything seemed to be just over the horizon How has the situation changed since then? Are we any closer to our goal? As will be described in this book, we have advanced a long way since then But it is an ongoing journey still and the end is not yet in sight According to the old saying, it is better to travel hopefully than to arrive Our quest for discovery fuels our creativity in all fields, not just science If we reached the end of the line, the human spirit would shrivel and die But I don't think we will ever stand still: we shall increase in complexity, if not in depth, and shall always be the center of an expanding horizon of possibilities I want to share my excitement at the discoveries that are being made and the picture of reality that is emerging I have concentrated on areas I have worked on myself for a greater feeling of immediacy The details of the work are very technical but I believe the broad ideas can be conveyed without a lot of mathematical baggage I just hope I have succeeded I have had a lot of help with this book I would mention in particular Thomas Hertog and Neel Shearer, for assistance with the figures, captions, and boxes, Ann Harris and Kitty Ferguson, who edited the manuscript (or, more accurately, the computer files, because everything I write is electronic), Philip Dunn of the Book Laboratory and Moonrunner Design, who created the illustrations But beyond that, I want to thank all those who have made it possible for me to lead a fairly normal life and carry on scientific research Without them this book could not have been written Stephen Hawking Cambridge, May 2, 0 viii F O R E W O R D Quantum mechanics M-theory P-branes General relativity 10-dimensional membranes Superstrings 11-dimensional supergravity Black holes B R A N E N E W W O R L D (FIG ) The brane world picture of the origin of the universe differs from that discussed in Chapter 3, because the slightly flattened four-dimensional sphere, or nutshell, is no longer hollow but is filled by a fifth dimension 197 T H E U N I V E R S E I N A N U T S H E L L o f s p a c e t i m e c a n b e e n c o d e d o n its boundary S o m a y b e w e t h i n k we live in a f o u r - d i m e n s i o n a l world b e c a u s e we are s h a d o w s cast on t h e b r a n e by w h a t is h a p p e n i n g in the interior of t h e b u b b l e H o w e v e r , from a positivist viewpoint, o n e c a n n o t ask: w h i c h is reality, brane or bubble? T h e y are both m a t h e m a t i c a l m o d e l s that describe t h e observations O n e is free to use w h i c h e v e r m o d e l is m o s t c o n v e n i e n t W h a t is outside the brane? T h e r e are several possibilities (Fig ) : 198 B R A N E N E W W O R L T h e r e may be n o t h i n g outside A l t h o u g h a b u b b l e of steam D (FIG ) has water outside it, this is just an a n a l o g y to help us visualize t h e origin of the universe O n e could imagine a m a t h e m a t i c a l model that was just a brane with a higher-dimensional space inside but absolutely n o t h i n g outside, not even e m p t y s p a c e O n e can calculate what the mathematical m o d e l predicts w i t h o u t r e f e r e n c e to what is outside O n e could have a mathematical model in which the outside of a bubble was glued to the outside of a similar bubble T h i s model is actu- I A brane/bubble with a higher- dimensional space inside with nothing outside ally mathematically equivalent to the possibility discussed above that there is nothing outside the bubble but the difference is psychological: Identify people feel happier being placed in the center of spacetime rather than on its edge; but for a positivist, possibilities and are the same T h e bubble m i g h t expand into a space that was not a mirror image of what was inside the b u b b l e T h i s possibility is different from the two discussed a b o v e and is m o r e like t h e case of boiling water O t h e r bubbles could form and e x p a n d If t h e y c o l l i d e d and merged with the b u b b l e in w h i c h we lived, the results c o u l d be cat- A possibility in which the outside of astrophic It has even been suggested that the big b a n g itself may a brane/bubble is glued to the outside have produced by a collision b e t w e e n branes of another bubble Brane world models like this are a h o t t o p i c of research T h e y are highly speculative but t h e y offer new kinds of b e h a v i o r that c a n b e tested b y observation T h e y c o u l d explain w h y gravity seems t o be so weak Gravity might be quite strong in t h e fundamental t h e ory but the spreading of the gravitational force in t h e extra dimensions would mean it would be weak at large distances on the brane on which we live A c o n s e q u e n c e of this would be that the Planck length, t h e smallest distance to w h i c h we can p r o b e without c r e a t i n g a b l a c k hole, would be quite a lot larger than it would appear from t h e weakness of gravity on our four-dimensional b r a n e T h e smallest Russian doll wouldn't be so tiny after all and m i g h t be within t h e reach of particle accelerators of the future In fact we m i g h t already have discovered the smallest doll, the fundamental Planck length, if the U S hadn't g o n e through a fit of feeling p o o r in 9 and canceled the S S C ( S u p e r c o n d u c t i n g S u p e r C o l l i d e r ) even t h o u g h i t was half built O t h e r particle accelerators such as t h e L H C (Large A brane/bubble expands into a space which is not the mirror image of what is inside O t h e r bubbles could form and expand in such a scenario 199 T H E U N I V E (FIG ) Layout of the LEP tunnel showing the existing infrastructure and the future construction of the Large Hadron Collider in Geneva, Switzerland 200 R S E I N A N U T S H E L L B R A N E N E W W O R L D 201 T H 202 E U N I V E R S E I N A N U T S H E L L G L O S S A R Y 203 T H 204 E U N I V E R S E I N A N U T S H E L L G L O S S A R Y 205 T H 206 E U N I V E R S E I N A N U T S H E L L G L O S S A R Y 207 T H 208 E U N I V E R S E I N A N U T S H E L L S U G G E S T E D F U R T H E R R E A D I N G S 209 T H 210 E U N I V E R S E I N A N U T S H E L L ... beams at right angles to each other As the Earth's orbit and in a direction at right angles to it Earth rotates on its axis and orbits the Sun, the apparatus moves through the ether with varying... spacetime which is curved Einstein and Grossmann wrote a joint paper in in which they put forward the idea that what we think of as gravitational forces are just an expression of the fact that... have to be accelerating in opposite directions but staying at a constant distance from each other (Fig 1.11) But on his return to Zurich in 1912 Einstein had the brain wave of realizing that the