gravity and the relation
between spin foams and the continuum.
The LQG workshop entitled ``Frontiers of Loop Quantum Gravity" had 25 participants and the discussions centered among five topics of current interest: Phenomenology, Semiclassical Issues, Hamiltonian Constraint, Spin Foams and Future Directions. Each session was three hour long, with 2 short presentations and lots of discussion. On Friday Jan 30th, the workshop started with a presentation by Daniel Sudarsky on recent results that show that a low energy effective description in terms of a Lorentz violating theory (with a preferred frame) is already ruled out by existing experimental and theoretical bounds. This leads to a subtler possibility for manifestations of the discreetness coming from the Planck scale in the form of a Double Special Relativity (DSR) framework. L. Freidel gave a review talk on the two kinds of DSR theories which he called DSR-1 and DSR-2, and their prospects for becoming physically viable.
The Session on Semiclassical issues had two short presentations. Hanno Sahlmann gave a nice summary of the current approaches to construct semi-classical states of the theory, from the now classical weave states, gauge coherent states and shadow states to statistically generated states. Abhay Ashtekar gave a presentation on possible physical applications that the semiclassical states might try to attack; in particular the construction of states that approximate the Minkowski vacuum and the definition of effective potentials for scattering scenarios.
Spin Foams was the topic of the next session on Sat. Jan 31st, where Alejandro Perez gave a status report on the Barret Crane model. In particular, there was discussion on recent progress on the proper definition of the path integral measure for the Plebanski action and its possible relation to the elimination of bubble divergences when the symmetries of the system are properly addressed. The other important issue that was discussed was the dominance of degenerate configurations in the asymptotics of the terms that compose the state sum model. Louis Crane discussed possible new mathematical descriptions for spin foam models via an n-categorical approach.
In the Hamiltonian Constraint session, Abhay Ashtekar discussed the status of the so-called Thiemann-like Hamiltonians. Are they clinically dead? The consensus after the discussion was that they are still alive, and no fatal flaw has been found in the approach, even when there is still no agreement on the right way of finding a projector on physical states. Thomas Thiemann gave an introduction to the Master Constraint program, where the particular proposal for finding such operator seems to be very promising. It was agreed upon that this approach might shed light on defining the projector (via a spin foam model).
In the Morning of Sun Feb 1st Jorge Pullin gave a summary of the
results that he and R. Gambini have found in defining consistent
discretizations for quantum and classical gravity. In particular,
he discussed some of its applications to quantum cosmology and
decoherence. Abhay Ashtekar summarized what had been discussed in
the workshop in the Concluding Remarks, and a general discussion
followed. In particular, there was discussion about different
possible discrete frameworks, observables associated to particles
and the cosmological constant in gravity and the relation
between spin foams and the continuum.
The general agreement between the participants was that the meeting was very productive for clarifying (and reaching consensus) on several conceptual and technical issues, and in defining direction in which we should focus our attention in the future. There was such excitement that this was dubbed the ``First NAFTA meeting on LQG" and the participants somewhat committed to followups to this workshop that would take place in Canada, the USA and México on a rotating basis.
On Monday Feb 2nd, the Ryan-fest was held at UNAM, to celebrate both Mike's 60th birthday and 30 years of his arrival to México. The program was divided into technical talks and more informal `anecdotic' ones: Abhay Ashtekar gave a seminar on Loop Quantum Cosmology followed by a talk on Non-commutative Quantum Cosmology by Octavio Obregon and later on by Richard Matzner on Linearizations and Hyperbolicity in Numerical Relativity. On the less technical side there were talks by Marcos Rosenbaum who described Mike's path during this last 30 years and his influence in starting and consolidating the gravity group at UNAM. Roberto Sussman gave us a student's perspective of Mike as a teacher in the `old times', and Jorge Pullin took us to a Journey of discovery through the eyes of Mike's papers of all times.
The final event was a two-day celebration for Marcos Rosenbaum who was responsible for the mere existence of the Institute for Nuclear Sciences (and the Gravity Department) as a research center, of which he was the director for 16 years. There were two kinds of talks: those by present and past University Officers (including the present and a former Rector), who highlighted Prof. Rosenbaum's service to the University at large. The second series of talks were of a scientific nature and were divided into the two main areas of research of Prof. Rosenbaum: General Relativity and Mathematical Physics. Among the invited speakers were Stanley Deser, Richard Matzner, Octavio Obregon and Raymundo Bautista. These talks were complemented by talks from `local' people such as M. Alcubierre, C. Chyssomalakos, M. Ryan, A. Turbiner and the author of this note.
What this series of events made clear for the participants was
that the Gravity Department at UNAM
http://www.nuclecu.unam.mx/~gravit, after only 30 years of its
birth, is now coming of age, fit and running. We wish the
department, and both Professors Ryan and Rosenbaum many more years
of healthy and productive life.