Three Roads To Quantum Gravity (Science Masters)
Lee Smolin
Language: English
Pages: 256
ISBN: 0465078362
Format: PDF / Kindle (mobi) / ePub
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to questions, but by the questions that are asked. Everett's relative-state form of the theory must therefore be supplemented by a theory of why what we observe corresponds to the answers to certain questions, and not to an in®nite number of other questions. Several people have attempted to deal with this, and some progress has been made using an idea called decoherence. A set of questions is called decoherent if there is no chance that a de®nite answer to one is a superposition of de®nite
not just a question of waiting long enough for light from a distant region to reach us. We could be right next to a black hole, yet never be able to see what observers inside it can see, however long we waited. All observers have their own hidden region. The hidden region of each observer consists of all those events that they will not be able to receive information from, no matter how long they wait. Each hidden region will include the interiors 71 72 T H R E E R O A D S T O Q U A N T U M G R
quantum gravity, and in particular to quantum black holes. For what we have seen in the last few chapters is that black holes are systems which may be AREA AND INFORMATION described by the laws of thermodynamics. They have a temperature and an entropy, and they obey an extension of the law of increase of entropy. This allows us to raise several questions. What does the temperature of a black hole actually measure? What does the entropy of a black hole really describe? And, most importantly,
light cones. But it also has more, for we can draw slices through it that correspond to moments of time. As in relativity theory, there are many different ways of slicing an evolving spin network, so as to see it as a succession of states evolving in time. Thus, the picture of spacetime given by loop quantum gravity agrees with the fundamental principle that in the theory of relativity there are no things, only processes. John Wheeler used to say that on the Planck scale spacetime would no longer
in some cases, quite surprising ideas have been discovered, which I believe are up to the task of answering these questions. I believe that they provide the conceptual frame- THE QUEST FOR QUANTUM GRAVITY work that is allowing us to take the next step ± to proceed to a quantum theory of gravity. It has also happened that someone on this third road discovered a mathematical structure which at ®rst seemed unconnected to anything else. Such results are often dismissed by the more conservative
