March 15, 1997 This Week's Finds in Mathematical Physics (Week 99) John Baez Life here at the Center for Gravitational Physics and Geometry is tremendously exciting. In two weeks I have to return to U. C. Riverside and my mundane life as a teacher of calculus, but right now I'm still living it up. I'm working with Ashtekar, Corichi, and Krasnov on computing the entropy of black holes using the loop representation of quantum gravity, and also I'm talking to lots of people about an interesting 4-dimensional formulation of the loop representation in terms of "spin foams" - roughly speaking, soap-bubble-like structures with faces labelled by spins. Here are some papers I've come across while here: 1) Lee Smolin, The future of spin networks, in The Geometric Universe: Science, Geometry, and the Work of Roger Penrose, eds. S. Hugget, Paul Tod, and L.J. Mason, Oxford University Press, 1998. Also available as gr-qc/9702030 I've spoken a lot about spin networks here on This Week's Finds. They were first invented by Penrose as a radical alternative to the usual way of thinking of space as a smooth manifold. For him, they were purely discrete, purely combinatorial structures: graphs with edges labelled by "spins" j = 0, 1/2, 1, 3/2, etc., and with three edges meeting at each vertex. He showed how when these spin networks become sufficiently large and complicated, they begin in certain ways to mimic ordinary 3-dimensional Euclidean space. Interestingly, he never got around to publishing his original paper on the subject, so it remains available only if you know someone who knows someone who has it: | |
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