Three Roads To Quantum Gravity

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THE UNIVERSE IS MADE OF PROCESSES, NOT THINGS 55 Ball bounces just in Sam and Olga collide Sue slams it Olga returns it again Sue just gets it back Sam is confused Sue cheers Olga at the net returns Marlo hits it back Sam returns Sue serves FIGURE 7 One volley in a tennis game, represented by the causal relations of a few of its events. future of a particular event in Figure 7. A causal universe is not a series of stills following on, one after the other. There is time, but there is not really any notion of a moment of time. There are only process that follow one another by causal necessity. It makes no sense to say what such a universe is. If one wants to talk about it, one has no alternative but to tell its story. One way to think of such a causal universe is in terms of the transfer of information. We can think of the content of each arrow in Figures 6 to 8 as a few bits of information. Each event is then something like a transistor that takes in information from events in its past, makes a simple computation and sends the result to the events in its future. A computation is
56 THREE ROADS TO QUANTUM GRAVITY Future of Olga's second return Ball bounces just in Sam and Olga collide Sue slams it Olga returns it again Sue just gets it back Sam is confused Sue cheers Olga at the net returns Marlo hits it back Sam returns Sue serves Past of Olga's second return FIGURE 8 The future and past of Olga's second return. Note that Sam being confused is in neither set of events. then a kind of story in which information comes in, is sent from transistor to transistor, and is occasionally sent to the output. If we were to remove the inputs and outputs from modern computers, most of them would continue to run inde®nitely. The ¯ow of information around the circuits of a computer constitutes a story in which events are computations and causal processes are just the ¯ow of bits of information from one computation to the next. This leads to a very useful metaphor ± the universe as a kind of computer. But it is a computer in which the circuitry is not ®xed, but can evolve in time as a consequence of the information ¯owing through it.
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To my parents Pauline and Michael C
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Page 76: II WHAT WE HAVE LEARNED
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CHAPTER 9 .........................
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CHAPTER 11 ........................
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CHAPTER 12 ........................
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THE HOLOGRAPHIC PRINCIPLE 171 gener
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THE HOLOGRAPHIC PRINCIPLE 173 FIGUR
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THE HOLOGRAPHIC PRINCIPLE 175 the g
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THE HOLOGRAPHIC PRINCIPLE 177 In th
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CHAPTER 13 ........................
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HOW TO WEAVE A STRING 181 thing!" I
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HOW TO WEAVE A STRING 183 other. Th
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HOW TO WEAVE A STRING 185 What is r
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HOW TO WEAVE A STRING 187 for a pic
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HOW TO WEAVE A STRING 189 in Einste
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HOW TO WEAVE A STRING 191 p in the
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HOW TO WEAVE A STRING 193 may be th
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WHAT CHOOSES THE LAWS OF NATURE? 19
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EPILOGUE: .........................
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EPILOGUE: 209 And quantum gravity i
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EPILOGUE: 211 of systems containing
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POSTSCRIPT 213 These cosmic rays ar
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POSTSCRIPT 215 Three explanations h
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POSTSCRIPT 217 light, which is the
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POSTSCRIPT 219 This is good news in
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POSTSCRIPT 221 But there is a secon
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POSTSCRIPT 223 Chopin Soo and Marti
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POSTSCRIPT 225 speed of light with
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GLOSSARY 227 cannot be greater than
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GLOSSARY 229 event In relativity th
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GLOSSARY 231 Newton's gravitational
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GLOSSARY 233 spin The angular momen
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SUGGESTIONS FOR FURTHER READING ...
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SUGGESTIONS FOR FURTHER READING 237
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SUGGESTIONS FOR FURTHER READING 239
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INDEX .............................
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INDEX 243 black hole, 70, 73±4, 17
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INDEX 245 black hole formation, 189
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Three Roads To Quantum Gravity

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