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Quantum gravity workshop at perimeter institute

by john baez
Tags: gravity, institute, perimeter, quantum, workshop
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Nov22-04, 10:18 PM
P: 134
Quote Quote by john baez
This quantum gravity workshop at the Perimeter Institute was
really cool:

It was their first conference in the new building. I'm writing an
issue of This Week's Finds about some things I learned that made me
much more optimistic about quantum gravity! But for now you can
look at this talk I gave, which reflects the more pessimistic mood I
had going into this conference - it focused on problems rather than solutions:

The problem of dynamics in quantum gravity
Here is a revised version of interest:
Dec8-04, 07:33 PM
Sci Advisor
PF Gold
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Quote Quote by john baez
... quantum gravity workshop at the Perimeter Institute...

the papers presented at that conference are gradually being posted on ArXiv.

today there were two that appeared. by Daniele Oriti and by V.Husain and O.Winkler. In both cases the titles of the papers are virtually identical to those of the talks given at the Halloween conference at perimeter

The Feynman propagator for quantum gravity: spin foams, proper time, orientation, causality and timeless-ordering
Daniele Oriti
8 pages; to appear in the Proceedings of the DICE 2004 Workshop "From Decoherence and Emergent Classicality to Emergent Quantum Mechanics"

"We discuss the notion of causality in Quantum Gravity in the context of sum-over-histories approaches, in the absence therefore of any background time parameter. In the spin foam formulation of Quantum Gravity, we identify the appropriate causal structure in the orientation of the spin foam 2-complex and the data that characterize it; we construct a generalised version of spin foam models introducing an extra variable with the interpretation of proper time and show that different ranges of integration for this proper time give two separate classes of spin foam models: one corresponds to the spin foam models currently studied, that are independent of the underlying orientation/causal structure and are therefore interpreted as a-causal transition amplitudes; the second corresponds to a general definition of causal or orientation dependent spin foam models, interpreted as causal transition amplitudes or as the Quantum Gravity analogue of the Feynman propagator of field theory, implying a notion of ''timeless ordering''."

Quantum black holes
Viqar Husain, Oliver Winkler
4 pages

"Using a recently developed quantization of spherically symmetric gravity coupled to a scalar field, we give a construction of horizon operators that allow a definition of general, fully dynamical quantum black holes. These operators capture the intuitive idea that classical black holes are defined by the presence of trapped surfaces, that is surfaces from which light cannot escape outward. They thus provide a mechanism for classifying quantum states of the system into those that describe quantum black holes and those that do not. We find that quantum horizons fluctuate, confirming long-held heuristic expectations. We also give explicit examples of quantum black hole states. The stage is thus set for addressing all the puzzles of black hole physics in a fully quantized dynamical setting."

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