Three Roads To Quantum Gravity

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GLOSSARY 229 event In relativity theory, something that happens at a particular point of space and moment of time. exclusion principle see Pauli exclusion principle. fermion A particle whose angular momentum comes in integer multiples of one-half of Planck's constant. Fermions satisfy the Pauli exclusion principle. Feynman diagram A depiction of a possible process in the interaction of several elementary particles. <strong>Quantum</strong> theory assigns to each diagram the probability amplitude for that process to occur. The total probability is proportional to the square of the sum of the amplitudes of the possible processes, each of which is depicted by a Feynman diagram. ®eld A physical entity that is described by specifying the value of some quantity at every point of space and time; examples are the electric and magnetic ®elds. future The future, or causal future, of an event consists of all those events that it can in¯uence by sending energy or information to it. future light cone For a speci®c event, all other events that can be reached from it by a signal travelling at the speed of light. Since the speed of light is the maximum speed at which energy or information can travel, the future light cone of an event marks the limits of the causal future of that event. See also light cone. general theory of relativity Einstein's theory of gravity, according to which gravity is related to the in¯uence the distribution of matter has on the causal structure of spacetime. graph A diagram consisting of a set of points, called vertices, connected by lines, called edges. See also lattice. Hawking radiation The thermal radiation black holes are predicted to give off, having a temperature which is inversely proportional to the black hole's mass. Hawking radiation is caused by quantum effects. hidden variables Conjectured degrees of freedom which underlie the statistical uncertainties in quantum theory. If there are hidden variables, then it is possible that the uncertainties in quantum theory are just the result of
230 THREE ROADS TO QUANTUM GRAVITY our ignorance about the values of the hidden variables and are not fundamental. horizon For each observer in a spacetime, the surface beyond which they cannot see, or receive any signals from. Examples are black hole horizons. information A measure of the organization of a signal. It is equal to the number of yes/no questions whose answers could be coded in the signal. knot theory A branch of mathematics concerned with classifying the different ways of tying a knot. lattice A space consisting of a ®nite number of points, with nearby points connected by lines called edges. A lattice is often, but not always, distinguished from a graph in that a lattice is a graph with a regular structure. An example of a lattice is shown in Figure 22. lattice theory A theory in which space or spacetime is considered to be a lattice. light cone All the events that can be reached by light signals travelling to the future, or coming from the past, from a single event. We may therefore distinguish between the future light cone, which contains events that can be reached by light travelling into the future, and the past light cone, which contains events that can be reached by light travelling from the past. link Two curves link in three-dimensional space if they cannot be pulled apart without passing one through the other. loop A circle drawn in space. loop quantum gravity An approach to quantum gravity in which space is constructed from the relationships between loops, originally derived by applying quantum theory to the formulation of general relativity discovered by Sen and Ashtekar. many-worlds interpretation An interpretation of quantum theory according to which the different possible outcomes of an observation of a quantum system reside in different universes, all of which somehow coexist. M theory The conjectured theory which would unify the different string theories.
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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 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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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
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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 218 and 219: EPILOGUE: 209 And quantum gravity i
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 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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