A spin foam is a "mousse de spin" On Friday Rovelli is giving a symposium talk on spin foams and he was Etera Livine's thesis director. My uninformed guess is that Rovelli will talk about Livine's thesis and in particular chapter 8 (Covariant loop gravity) which reflects potentially important work by Alexandrov, some of which Livine co-authored. This work extends the symmetry of loop gravity to SL(2,C) and seems to get rid of the Immirzi parameter (!). The thesis is at http://arxiv.org/gr-qc/0309028 Quite recent, another reason it would seem like a good thing to bring up at the Strings Meets Loops symposium this Friday. Here is a quote from the abstact, giving links to a paper of Alexandrov/Livine among other things: ------------quote from abstract-------- 'Thu, 4 Sep 2003 Boucles et Mousses de Spin en Gravite Quantique Etera R. Livine 165 pages, in French; PhD Thesis 2003, Centre de Physique Theorique CNRS-UPR 7061 (France) I review the formalism of loop quantum gravity, in both its real and complex formulations, and spin foam theory which is its path integral counterpart. Spin networks for non-compact groups are introduced (following hep-th/0205268) to deal with gauge invariant structures based on the Lorentz group. The whole formalism is studied in details in three dimensions in both its canonical formulation (loop gravity) and its spin foam formulation. The main output (following gr-qc/0212077) is the discreteness of timelike intervals and the continuous character of spacelike distances even at the quantum level. Then it is explained how to extend these considerations to the 4-dimensional case. I review the Barrett-Crane model, its geometrical interpretation, its link with general relativity and the role of causality. It is shown to be the history formulation of a covariant canonical formulation of loop gravity (following gr-qc/0209105), whose link with standard loop quantum gravity is discussed. Similarly to the 3d case, spacelike areas turn out continuous. Finally, ways of extracting informations from the non-perturbative spin foam structures are discussed." ------------------end quote--------- BTW Livine seems to think it matters that time is discrete but space is continuous---a result from Freidel/Livine/Rovelli http://arxiv.org/gr-qc/0212077 Why should this result be so important? It is admittedly odd.