Three Roads To Quantum Gravity

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KNOTS, LINKS AND KINKS 141 FIGURE 29 Two pictures of quantum spacetimes. Each event in the quantum spacetime is a simple change in the quantum geometry of space, corresponding to one of the moves shown in Figure 28. According to loop quantum gravity, this is what spacetime looks like if we examine it on a time scale of 10 743 of a second and a length scale of 10 733 of a centimetre. The upper picture shows a single elementary move. The lower one shows a combination of two elementary moves. [From C. Rovelli, `The projector on physical states in loop quantum gravity', gr-qc/9806121.]
142 THREE ROADS TO QUANTUM GRAVITY FIGURE 30 Another of the elementary moves for quantum transitions among spin networks, together with the spacetime picture that represents it. [From R. de Pietri, `Canonical loop quantum gravity and spin foam models', gr-qc/ 9903076.] Is the theory right? We do not know yet. In the end, it will be decided by experiments designed to test the predictions the theory makes about the discreteness of area and volume and other measures of spacetime geometry. I do want to emphasize that although it follows directly from the combination of the principles of general relativity and quantum theory, loop quantum gravity does not need to be the complete story to be true. In particular, the main predictions of the theory, such as the quantization of area and volume, do not depend on many details being correct, only on the most general assumptions drawn from quantum theory and relativity. The predictions do not constrain what else there can be in the world, how many dimensions there are or what the fundamental symmetries are. In particular, they are completely consistent with the basic features of string theory, including the existence of extra dimensions and supersymmetry. I know of no reason to doubt their truth. Of course, in the end experiment must decide. But can we really hope for experimental con®rmation of the structure of space on the Planck scale, 20 orders of magnitude smaller than the proton? Until very recently most of us were sceptical about whether we might see such tests in our lifetime. But
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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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Page 180 and 181: THE HOLOGRAPHIC PRINCIPLE 171 gener
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Page 190 and 191: HOW TO WEAVE A STRING 181 thing!" I
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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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