Three Roads To Quantum Gravity

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EPILOGUE: 209 And quantum gravity is nothing if not high risk. The unfortunate lack of experimental tests means that relatively large groups of people may work for decades only to ®nd that they have completely wasted their time, or at least done little but eliminate what at ®rst seemed to be attractive possibilities for the theory. Measured sociologically, string theory seems very healthy at the moment, with perhaps a thousand practitioners; loop quantum gravity is robust but much less populous, with about a hundred investigators; other directions, such as Penrose's twistor theory, are still pursued by only a handful. But thirty years from now all that will matter is which parts of which theory were right. And a good idea from one person is still worth hundreds of people working incrementally to advance a theory without solving its fundamental problems. So we cannot allow the politics of the academy too much in¯uence here, or we shall all end up doing one thing. If that happens, then a century from now people may still be writing books about how quantum gravity is almost solved. If this is to be avoided, all the good ideas must be kept alive. Even more important is to maintain a climate in which young people feel there is a place for their ideas, no matter how initially unlikely or how far from the mainstream they may seem. As long as there is still room for the young scientist with the uncomfortable question and the bright idea, I see nothing to prevent the present rapid rate of progress from continuing until we have a complete theory of quantum gravity. I should like to close this book by sticking out any part of my neck which is not yet exposed, and making a few predictions about how the problem of quantum gravity will in the end be solved. I believe that the huge progress we have made in the last twenty years is best illustrated by the fact that it is now possible to make an educated guess about how the last stages of the search for quantum gravity will go. Until recently we could have done no more than point to a few good ideas that were not obviously wrong. Now we have several proposals on the table that seem right enough and robust enough, and it is hard to imagine that they are completely wrong. The picture I have presented in this book was
210 THREE ROADS TO QUANTUM GRAVITY assembled by taking all those ideas seriously. In the same spirit, I offer the following scenario of how the present revolution in physics will end. . Some version of string theory will remain the right description at the level of approximation at which there are quantum objects moving against a classical spacetime background. But the fundamental theory will look nothing like any of the existing string theories. . Some version of the holographic principle will turn out to be right, and it will be one of the foundational principles of the new theory. But it will not be the strong version of the principle I discussed in Chapter 12. . The basic structure of loop quantum gravity will provide the template for the fundamental theory. <strong>Quantum</strong> states and processes will be expressed in diagrammatic form, like the spin networks. There will be no notion of a continuous geometry of space or spacetime, except as an approximation. Geometrical quantities, including areas and volumes, will turn out to be quantized, and to have minimum values. . A few of the other approaches to quantum gravity will turn out to play signi®cant roles in the ®nal synthesis. Among them will be Roger Penrose's twistor theory and Alain Connes's non-commutative geometry. These will turn out to give essential insights into the nature of the quantum geometry of spacetime. . The present formulation of quantum theory will turn out to be not fundamental. The present quantum theory will ®rst give way to a relational quantum theory of the kind I discussed in Chapter 3, which will be formulated in the language of topos theory. But after a while this will be reformulated as a theory about the ¯ow of information among events. The ®nal theory will be non-local or, better, extra-local, as space itself will come to be seen only as an appropriate description for certain kinds of universe, in the same way that thermodynamic quantities such as heat and temperature are meaningful only as averaged descriptions
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To my parents Pauline and Michael C
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I POINTS OF DEPARTURE
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CHAPTER 2 .........................
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II WHAT WE HAVE LEARNED
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CHAPTER 7 .........................
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CHAPTER 9 .........................
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CHAPTER 11 ........................
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Page 178 and 179: CHAPTER 12 ........................
Page 180 and 181: THE HOLOGRAPHIC PRINCIPLE 171 gener
Page 182 and 183: THE HOLOGRAPHIC PRINCIPLE 173 FIGUR
Page 184 and 185: THE HOLOGRAPHIC PRINCIPLE 175 the g
Page 186 and 187: THE HOLOGRAPHIC PRINCIPLE 177 In th
Page 188 and 189: CHAPTER 13 ........................
Page 190 and 191: HOW TO WEAVE A STRING 181 thing!" I
Page 192 and 193: HOW TO WEAVE A STRING 183 other. Th
Page 194 and 195: HOW TO WEAVE A STRING 185 What is r
Page 196 and 197: HOW TO WEAVE A STRING 187 for a pic
Page 198 and 199: HOW TO WEAVE A STRING 189 in Einste
Page 200 and 201: HOW TO WEAVE A STRING 191 p in the
Page 202 and 203: HOW TO WEAVE A STRING 193 may be th
Page 204 and 205: WHAT CHOOSES THE LAWS OF NATURE? 19
Page 216 and 217: EPILOGUE: .........................
Page 220 and 221: EPILOGUE: 211 of systems containing
Page 222 and 223: POSTSCRIPT 213 These cosmic rays ar
Page 224 and 225: POSTSCRIPT 215 Three explanations h
Page 226 and 227: POSTSCRIPT 217 light, which is the
Page 228 and 229: POSTSCRIPT 219 This is good news in
Page 230 and 231: POSTSCRIPT 221 But there is a secon
Page 232 and 233: POSTSCRIPT 223 Chopin Soo and Marti
Page 234 and 235: POSTSCRIPT 225 speed of light with
Page 236 and 237: GLOSSARY 227 cannot be greater than
Page 238 and 239: GLOSSARY 229 event In relativity th
Page 240 and 241: GLOSSARY 231 Newton's gravitational
Page 242 and 243: GLOSSARY 233 spin The angular momen
Page 244 and 245: SUGGESTIONS FOR FURTHER READING ...
Page 246 and 247: SUGGESTIONS FOR FURTHER READING 237
Page 248 and 249: SUGGESTIONS FOR FURTHER READING 239
Page 250 and 251: INDEX .............................
Page 252 and 253: INDEX 243 black hole, 70, 73±4, 17
Page 254 and 255: INDEX 245 black hole formation, 189
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Three Roads To Quantum Gravity

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