Three Roads To Quantum Gravity

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ACCELERATION AND HEAT 87 separating an observer from a region which is hidden from them, there is associated an entropy which measures the amount of information which is hidden behind it. This entropy is always proportional to the area of the horizon. These two laws are the basis for our understanding of quantum black holes, as we shall see in the next chapter.
CHAPTER 7 ............................................................................................ BLACK HOLES ARE HOT The reason why we have been considering an accelerating observer is that her situation is very similar to that of an observer hovering just above the horizon of a black hole. So the two laws we found at the end of the last chapter, Unruh's law and Bekenstein's law, can be applied to tell us what we see as we hover over a black hole. Applying the analogy, we can predict that an observer outside the black hole will see themself as embedded in a gas of hot photons. Their temperature must be related to the acceleration the engines need to deliver to keep the spacecraft hovering a ®xed distance above the horizon. Furthermore, the photons that this observer detects will be randomized because a complete description of them will require information that is beyond the horizon, coded in correlations between the photons she sees and photons that remain beyond the horizon (Figure 18). <strong>To</strong> measure this missing information she will attribute an entropy to the black hole. And this entropy will turn out to be proportional to the area of the horizon of the black hole. Although the analogy is very useful, there is an important difference between the two situations. The temperature and entropy measured by the accelerating observer are consequences of her motion alone. If she turns off her engines, the photons making up her horizon will catch up with her. She can then see into her hidden region. She no longer sees a hot gas of photons, so she measures no temperature. There is no missing information as she sees only empty space, which is
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I POINTS OF DEPARTURE
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CHAPTER 2 .........................
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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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