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I would like to discuss a puzzle I'm wondering about in loopy approaches to quantum gravity.
The puzzle is roughly as follows:
The Immirzi parameter (IP) is a strange constant that appears in loopy approaches. Calculations of black hole entropy in loopy approaches seem to need to fix the IP to be a particular value to agree with semi-classical entropy calculations. On the other hand, http://arxiv.org/abs/0809.2280 suggests that the IP plays little role in establishing the semi-classical limit of certain spin foam models. So what gives? Have I misunderstood something elementary? Will we find that the IP is given by the black hole value via other means? If not, what do the loopy approaches describe when the IP is not tuned to the black hole value? It almost seems impossible to have a semi-classical limit of GR for all IP, but only be able to reproduce the semi-classical black entropy for a particular IP.
Please let's keep the discussion to the science as much as possible. Thanks!
The puzzle is roughly as follows:
The Immirzi parameter (IP) is a strange constant that appears in loopy approaches. Calculations of black hole entropy in loopy approaches seem to need to fix the IP to be a particular value to agree with semi-classical entropy calculations. On the other hand, http://arxiv.org/abs/0809.2280 suggests that the IP plays little role in establishing the semi-classical limit of certain spin foam models. So what gives? Have I misunderstood something elementary? Will we find that the IP is given by the black hole value via other means? If not, what do the loopy approaches describe when the IP is not tuned to the black hole value? It almost seems impossible to have a semi-classical limit of GR for all IP, but only be able to reproduce the semi-classical black entropy for a particular IP.
Please let's keep the discussion to the science as much as possible. Thanks!