This series consists of talks in the area of Quantum Gravity. http://pirsa.org/podcast/S005 Science 2009 http://blogs.law.harvard.edu/tech/rss en-ca Thu, 08 Jan 2009 07:15:26 -0500 sbradwell@perimeterinstitute.ca Thu, 08 Jan 2009 07:15:26 -0500 G 180 sbradwell@perimeterinstitute.ca Steve Bradwell's - Podcast Generator Superselection rules are limitations on the physically realizable quantum operations that can be carried out by a local agent. For example, it is impossible to cre­ate or destroy an isolated particle that carries locally conserved charges, such as an electrically charged particle, a fermion, or (in a two­ dimensional medium) an anyon. Recently, Popescu has suggested that su­perselection rules might have interesting implications for the security of quantum cryptographic protocols. The intuitive idea behind this suggestion is that superselec­tion rules could place inescapable limits on the cheat­ing strategies available to the dishonest parties, thus en­hancing security. Might, say, unconditionally secure bit commitment be possible in worlds (perhaps including the physical world that we inhabit) governed by suitable su­perselection rules? An affirmative answer could shake the foundations of cryptography. The purpose of this paper is to answer Popescu's in­triguing question. Sadly, our conclusion is that superse­lection rules can never foil a cheater who has unlimited quantum­ computational power. Dominic Mayers http://streamer.perimeterinstitute.ca/mp3/3a22f6b6-9997-4bf9-a7ce-4fbc10cb6562.mp3 Science http://streamer.perimeterinstitute.ca/mp3/3a22f6b6-9997-4bf9-a7ce-4fbc10cb6562.mp3 Fri, 15 Aug 2003 10:50:00 -0400 Quantum mechanics, information theory, and relativity theory are the basic foundations of theoretical physics. The acquisition of information from a quantum system is the interface of classical and quantum physics. Special relativity imposes severe restrictions on the transfer of information between distant systems. Various applications will be presented. Asher Peres http://streamer.perimeterinstitute.ca/mp3/2c863a58-06c3-40ec-b5dd-e985d3764733.mp3 Science http://streamer.perimeterinstitute.ca/mp3/2c863a58-06c3-40ec-b5dd-e985d3764733.mp3 Fri, 15 Aug 2003 12:10:00 -0400 Shamit Kachru http://streamer.perimeterinstitute.ca/mp3/400b2120-50c4-49c8-b203-2159438c1b12.mp3 Science http://streamer.perimeterinstitute.ca/mp3/400b2120-50c4-49c8-b203-2159438c1b12.mp3 Thu, 07 Oct 2004 10:55:00 -0400 Lee Smolin http://streamer.perimeterinstitute.ca/mp3/27e4adab-9116-4794-babe-5754f1e59598.mp3 Science http://streamer.perimeterinstitute.ca/mp3/27e4adab-9116-4794-babe-5754f1e59598.mp3 Sun, 31 Oct 2004 17:20:00 -0500 Robert Oeckl http://streamer.perimeterinstitute.ca/mp3/492c2dd0-ea84-48e8-aa63-8842f9b71e8f.mp3 Science http://streamer.perimeterinstitute.ca/mp3/492c2dd0-ea84-48e8-aa63-8842f9b71e8f.mp3 Tue, 02 Nov 2004 14:00:00 -0500 Frederik Schuller http://streamer.perimeterinstitute.ca/mp3/48a7b09c-1745-4aa8-9839-bca1da38c313.mp3 Science http://streamer.perimeterinstitute.ca/mp3/48a7b09c-1745-4aa8-9839-bca1da38c313.mp3 Tue, 23 Nov 2004 16:05:00 -0500 Sven Bachman http://streamer.perimeterinstitute.ca/mp3/fa579117-ab64-4860-80cc-8e25a810dd05.mp3 Science http://streamer.perimeterinstitute.ca/mp3/fa579117-ab64-4860-80cc-8e25a810dd05.mp3 Tue, 30 Nov 2004 16:05:00 -0500 Arundhati Dasgupta http://streamer.perimeterinstitute.ca/mp3/23789d78-76b4-44a1-b205-4587b80cc47e.mp3 Science http://streamer.perimeterinstitute.ca/mp3/23789d78-76b4-44a1-b205-4587b80cc47e.mp3 Tue, 07 Dec 2004 16:05:00 -0500 Chris van den Broeck http://streamer.perimeterinstitute.ca/mp3/1e64beed-a950-466c-9692-f85025230871.mp3 Science http://streamer.perimeterinstitute.ca/mp3/1e64beed-a950-466c-9692-f85025230871.mp3 Thu, 09 Dec 2004 16:00:00 -0500 Saurya Das http://streamer.perimeterinstitute.ca/mp3/8b649889-67b5-47da-822d-e7fc0988e3b8.mp3 Science http://streamer.perimeterinstitute.ca/mp3/8b649889-67b5-47da-822d-e7fc0988e3b8.mp3 Tue, 14 Dec 2004 14:15:00 -0500 Robert Brout http://streamer.perimeterinstitute.ca/mp3/68ad508f-f1af-4b5a-a13e-148a2c4d4ee9.mp3 Science http://streamer.perimeterinstitute.ca/mp3/68ad508f-f1af-4b5a-a13e-148a2c4d4ee9.mp3 Thu, 03 Feb 2005 16:00:00 -0500 James Bjorken http://streamer.perimeterinstitute.ca/mp3/eb9b7ee9-4af3-466d-82d7-b30e45f0e941.mp3 Science http://streamer.perimeterinstitute.ca/mp3/eb9b7ee9-4af3-466d-82d7-b30e45f0e941.mp3 Fri, 04 Feb 2005 13:30:00 -0500 A joint Guelph-Waterloo Gravity Group/Perimeter Institute Seminar --------------------------------------------------------------------------- Observational evidence suggests that the large scale dynamics of the universe is presently dominated by dark energy, meaning a non-luminous cosmological constituent with a negative value of the pressure to density ratio w, which would be unstable if purely fluid, but could be stable if effectively solid with sufficient rigidity. It was suggested by Bucher and Spergel that such a solid constituent might be constituted by an effectively cold (meaning approximately static) distribution of cosmic strings with w=-1/3, or membranes with the observationally favoured value w=-2/3, but it was not established whether the rigidity in such models actually would be sufficient for stabilisation. For cases (exemplified by an approximately O(3) symmetric scalar field model) in which the number of membranes meeting at a junction is even (though not if it is odd) it is easy to obtain an explicit evaluation of the rigidity to density ratio, which is shown to 3/15 in both string and membrane cases, and it is confirmed that this is indeed sufficient for stabilisation. Brandon Carter http://streamer.perimeterinstitute.ca/mp3/eb57fc7a-ccf4-4a1e-9ba1-ee5c87c9a890.mp3 Science http://streamer.perimeterinstitute.ca/mp3/eb57fc7a-ccf4-4a1e-9ba1-ee5c87c9a890.mp3 Thu, 24 Mar 2005 12:00:00 -0500 Observers agree that a citizen of Ohio had much larger voting power than a citizen of Texas or California in the recent US presidential election. Why is it so? A brief introduction to the theory of voting will be provided. We analyze the voting power of a member of a voting body, or of a person which elects his representative, who will take part in the voting on her behalf. The notion of voting power is illustrated by examples of the systems of voting in the European Council. We propose a representative voting system based on the square root law of Penrose. Using statistical approach and considering fictitious countries with randomly chosen populations we study the problem of selecting an optimal quota. Karol Zyckowski http://streamer.perimeterinstitute.ca/mp3/f84d5836-2183-4819-b545-ad936a941b18.mp3 Science http://streamer.perimeterinstitute.ca/mp3/f84d5836-2183-4819-b545-ad936a941b18.mp3 Thu, 09 Jun 2005 02:15:00 -0400 Maya Paczuski http://streamer.perimeterinstitute.ca/mp3/2f800588-2d96-4c83-acac-f33cb907e4b4.mp3 Science http://streamer.perimeterinstitute.ca/mp3/2f800588-2d96-4c83-acac-f33cb907e4b4.mp3 Thu, 16 Jun 2005 14:15:00 -0400 Pasha Bolokhov http://streamer.perimeterinstitute.ca/mp3/189cb903-b973-47e4-8ae0-ed0af9addfb7.mp3 Science http://streamer.perimeterinstitute.ca/mp3/189cb903-b973-47e4-8ae0-ed0af9addfb7.mp3 Tue, 26 Jul 2005 10:00:00 -0400 Elisa Manrique http://streamer.perimeterinstitute.ca/mp3/e35f7f74-63e9-44a4-8a4f-e004e17a0554.mp3 Science http://streamer.perimeterinstitute.ca/mp3/e35f7f74-63e9-44a4-8a4f-e004e17a0554.mp3 Wed, 16 Nov 2005 11:00:31 -0500 Antonello Scardicchio http://streamer.perimeterinstitute.ca/mp3/643be3d6-48ed-488b-bba9-f80ea51b0cc1.mp3 Science http://streamer.perimeterinstitute.ca/mp3/643be3d6-48ed-488b-bba9-f80ea51b0cc1.mp3 Fri, 09 Dec 2005 14:00:46 -0500 The phenomenology of quantum gravity can be examined even though the underlying theory is not yet fully understood. Effective extensions of the standard model allow us to study specific features, such as the existence of extra dimensions or a minimal length scale. I will talk about some applications of this approach which can be used to make predictions for particle- and astrophysics, and fill in some blanks in the puzzle of quantum gravity. A central point of this investigations is the physics of black holes. I will comment on possible ways to proceed and on the missing pieces I find most important to look for. Sabine Hossenfelder http://streamer.perimeterinstitute.ca/mp3/9f12ab18-d102-40cc-a4bc-c33a9dd4d889.mp3 Science http://streamer.perimeterinstitute.ca/mp3/9f12ab18-d102-40cc-a4bc-c33a9dd4d889.mp3 Tue, 13 Dec 2005 16:00:59 -0500 We calculate analytically the highly damped quasinormal mode spectra of generic single-horizon black holes using the rigorous WKB techniques of Andersson and Howls. We thereby provide a firm foundation for previous analysis, and point out some of their possible limitations. The numerical coefficient in the real part of the highly damped frequency is generically determined by the behavior of coupling of the perturbation to the gravitational field near the origin, as expressed in tortoise coordinates. This fact makes it difficult to understand how the (in)famous ln(3) could be related to the quantum gravitational microstates near the horizon. Ramin Daghigh http://streamer.perimeterinstitute.ca/mp3/88da9b4b-49e6-4dab-aa2b-4cbdf3a23f89.mp3 Science http://streamer.perimeterinstitute.ca/mp3/88da9b4b-49e6-4dab-aa2b-4cbdf3a23f89.mp3 Thu, 12 Jan 2006 11:00:35 -0500 Space-time measurements and gravitational experiments are made by the mutual relations between objects, fields, particles etc... Any operationally meaningful assertion about spacetime is therefore intrinsic to the degrees of freedom of the matter (i.e. non-gravitational) fields and concepts such as ``locality'' and ``proximity'' should, at least in principle, be definible entirely within the dynamics of the matter fields. We propose to consider the regions of space just as general ``subsystems''. By writing the Hilbert space of the matter fields as a generic tensor product of subsystems we analyse the evolution of a state vector on an information theoretical basis and discuss general principles to recover a posteriori the usual space-time relations. We apply such principles to generic interacting second quantized models with a finite number of fermionic degrees of freedom. Finally, we discuss the possible role of gravity in this framework. Federico Piazza http://streamer.perimeterinstitute.ca/mp3/33c1a709-379a-4892-b498-b13aa66a1002.mp3 Science http://streamer.perimeterinstitute.ca/mp3/33c1a709-379a-4892-b498-b13aa66a1002.mp3 Thu, 19 Jan 2006 11:00:51 -0500 Lisa Randall http://streamer.perimeterinstitute.ca/mp3/d1f3e257-b688-4f58-ae83-b904d868b372.mp3 Science http://streamer.perimeterinstitute.ca/mp3/d1f3e257-b688-4f58-ae83-b904d868b372.mp3 Thu, 02 Mar 2006 13:00:01 -0500 It is a standard axiom of quantum mechanics that the Hamiltonian H must be Hermitian because Hermiticity guarantees that the energy spectrum is real and that time evolution is unitary. In this talk we examine an alternative formulation of quantum mechanics in which the conventional requirement of Hermiticity is replaced by the more general and physical condition of space- time reflection (PT) symmetry. We show that if the PT symmetry of H is unbroken, Then the spectrum of H is real. Examples of PT-symmetric non-Hermitian Hamiltonians are $H=p^2+ix^3$ and $H=p^2-x^4$. Amazingly, the energy levels of these Hamiltonians are all real and positive despite the ``wrong'' sign in the $x^4$ potential! We show that such PT-symmetric Hamiltonians specify physically acceptable quantum-mechanical theories in which the norms of states are positive and time evolution is unitary. To do so we demonstrate that a Hamiltonian that has an unbroken PT symmetry also possesses a new physical symmetry that we call C. Using C, we construct an inner product whose associated norm is positive definite. The result is a new class of consistent complex quantum theories. In effect, we have extended and generalized quantum mechanics into the complex domain. We then discuss PT-symmetric quantum field theories. PT-symmetric scalar field-theoretic Hamiltonians corresponding to the above quantum-mechanical Hamiltonains have interaction terms $igphi^3$ and $-gphi^4$. The latter Theory is interesting because (1) it is asymptotically free and (2) the expectation value of $phi$ is nonzero. (Thus, such a theory might be useful in describing the Higgs sector.) PT symmetry resolves the long-standing problem of ghosts in the Lee model. When the renormalized coupling constant in this model increases past a critical value, the Hamiltonian ceases to be Hermitian and a negative-norm ghost state appears. At this transition the Hamiltonian becomes PT-symmetric, and the ghost is a physical particle. PT-symmetric QED and the PT-symmetric massive Thirring model will also be discussed. Finally, we mention recent papers which suggest that PT-symmetry may provide insight into cosmological problems. Carl Bender http://streamer.perimeterinstitute.ca/mp3/37a090ac-7f2e-42f6-b31c-800b999e89c3.mp3 Science http://streamer.perimeterinstitute.ca/mp3/37a090ac-7f2e-42f6-b31c-800b999e89c3.mp3 Thu, 13 Apr 2006 11:00:35 -0400 We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell. One of the Euler-Lagrange equations of this theory guarantees its metricity. We show that quantum corrections generate a counterterm that destroys this metricity property, and that there are no other counterterms, at least at the one-loop level. There is a new coupling constant that controls the non-metric character of the theory. Its beta-function can be computed and is negative, which shows that the non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological ``constant'' and may provide a mechanism for naturally small ``dark energy''. Kirill Krasnov http://streamer.perimeterinstitute.ca/mp3/3b5953d2-726c-4300-9d65-2cee833072d4.mp3 Science http://streamer.perimeterinstitute.ca/mp3/3b5953d2-726c-4300-9d65-2cee833072d4.mp3 Thu, 30 Nov 2006 13:30:00 -0500 An ingredient in recent discussions of curvature singularity avoidance in quantum gravity is the "inverse scale factor" operator and its generalizations. I describe a general lattice origin of this idea, and show how it applies to the Coulomb singularity in quantum mechanics, and more generally to lattice formulations of quantum gravity. The example also demonstrates that a lattice discretized Schrodinger or Wheeler-DeWitt equation is computationally equivalent to the so called "polymer" quantization derived from loop quantum gravity. Viqar Husain http://streamer.perimeterinstitute.ca/mp3/7175d78f-8069-4ef0-bcc1-5484414474c7.mp3 Science http://streamer.perimeterinstitute.ca/mp3/7175d78f-8069-4ef0-bcc1-5484414474c7.mp3 Thu, 02 Aug 2007 14:00:00 -0400 In 3d quantum gravity, Planck's constant, the Planck length and the cosmological constant control the lack of (co)-commutativity of quantities like angular momenta, momenta and postion coordinates. I will explain this statement, using the quantum groups which arise in the 3d quantum gravity but avoiding technical details. The non-commutative structures in 3d quantum gravity are quite different from those in the deformed version of special relativity desribed by the kappa-Poincare group, but can be related to the latter by an operation called semi-dualisation. I will explain this operation, and make comments on its possible physical significance. The talk is based on joint work with Shahn Majid. Bernd Schroers http://streamer.perimeterinstitute.ca/mp3/f2ed8a24-3998-40fc-8d9a-f4d2a2a19093.mp3 Science http://streamer.perimeterinstitute.ca/mp3/f2ed8a24-3998-40fc-8d9a-f4d2a2a19093.mp3 Thu, 20 Sep 2007 14:00:00 -0400 There is a deep relation between Loop Quantum Gravity and notions from category theory, which have been pointed out by many researchers, such as Baez or Velhinho. Concepts like holonomies, connections and gauge transformations can be naturally formulated in that language. In this formulation, the (spatial) diffeomorphisms appear as the path grouopid automorphisms. We investigate the effect of extending the diffeomorphisms to all such automorphisms, which can be viewed as "distributional diffeomorphisms". We also give a notion of "categorial holonomy-flux-algebra", and present the construction of the automorphism-invariant Hilbert space for abelian gauge groups, which will be entirely combinatorial. Benjamin Bahr http://streamer.perimeterinstitute.ca/mp3/d589cdab-2cd7-48fa-9df0-72a39a81c586.mp3 Science http://streamer.perimeterinstitute.ca/mp3/d589cdab-2cd7-48fa-9df0-72a39a81c586.mp3 Thu, 15 Nov 2007 14:00:00 -0500 We show that the matrix-model for noncommutative U(n) gauge theory actually describes SU(n) gauge theory coupled to gravity. The nonabelian gauge fields as well as additional scalar fields couple to a dynamical metric G_ab, which is given in terms of a Poisson structure. This leads to a gravity theory which is naturally related to noncommutativity, encoding those degrees of freedom which are usually interpreted as U(1) gauge fields. Essential features such as gravitational waves and the Newtonian limit are reproduced correctly. UV/IR mixing is understood in terms of an induced gravity action. The framework appears suitable for quantizing gravity. Harold Steinacker http://streamer.perimeterinstitute.ca/mp3/bbff5761-a895-434f-a27f-d3db634fa103.mp3 Science http://streamer.perimeterinstitute.ca/mp3/bbff5761-a895-434f-a27f-d3db634fa103.mp3 Thu, 24 Jan 2008 13:30:00 -0500 There has been a dream that matter and gravity can be unified in a fundamental theory of quantum gravity. One of the main philosophies to realize this dream is that matter may be emergent degrees of freedom of a quantum theory of gravity. We study the propagation and interactions of braid-like chiral states in models of quantum gravity in which the states are (framed) four-valent spin networks embedded in a topological three manifold and the evolution moves are given by the dual Pachner moves. There are results for both the framed and unframed case. We study simple braids made up of two nodes which share three edges, which are possibly braided and twisted. We find three classes of such braids, those which both interact and propagate, those that only propagate, and the majority that do neither. These braids may serve as fundamental matter content. Yidun Wan http://streamer.perimeterinstitute.ca/mp3/bbf8e97e-0769-4125-9f4f-0f8a028c5fca.mp3 Science http://streamer.perimeterinstitute.ca/mp3/bbf8e97e-0769-4125-9f4f-0f8a028c5fca.mp3 Thu, 31 Jan 2008 14:00:00 -0500 Loop Quantum Gravity and Deformation Quantization Abstract: We propose a unified approach to loop quantum gravity and Fedosov quantization of gravity following the geometry of double spacetime fibrations and their quantum deformations. There are considered pseudo--Riemannian manifolds enabled with 1) a nonholonomic 2+2 distribution defining a nonlinear connection (N--connection) structure and 2) an ADM 3+1 decomposition. The Ashtekar-Barbero variables are generalized and adapted to the N-connection structure which allows us to write the general relativity theory equivalently in terms of Lagrange-Finsler variables and related canonical almost symplectic forms and connections. The Fedosov results are re-defined for gravitational gauge like connections and there are analyzed the conditions when the star product for deformation quantization is computed in terms of geometric objects in loop quantum gravity. We speculate on equivalence of quantum gravity theories with 3+1 and 2+2 splitting and quantum analogs of the Einstein equations. Sergiu Vacaru http://streamer.perimeterinstitute.ca/mp3/036b0512-12bf-41dd-843b-3730734ea042.mp3 Science http://streamer.perimeterinstitute.ca/mp3/036b0512-12bf-41dd-843b-3730734ea042.mp3 Thu, 07 Feb 2008 14:00:00 -0500 The principles of Quantum Mechanics and of Classical General Relativity imply Uncertainty Relations between the different spacetime coordinates of the events, which yield to a basic model of Quantum Minkowski Space, having the full (classical) Poincare' group as group of symmetries. The four dimensional Euclidean distance is a positive operator bounded below by a constant of order one in Planck units; the area operator and the four volume operator are normal operators - the latter being a Lorentz invariant operator with pure point spectrum - whose moduli are also bounded below by a constant of order one. While the spectrum of the 3 volume operator includes zero. These findings are in perfect agreement with the physical intuition suggested by the Spacetime Uncertainty Relations which are implemented by the Algebra of Quantum Spacetime. The formulations of interactions between quantum fields on Quantum Spacetime will be discussed. The various approaches to interactions, equivalent to one another on the Minkowski background, yield to different schemes on Quantum Spacetime, with the common feature of a breakdown of Lorentz invariance due to interactions. In particular one of these schemes will be discussed and motivated, which leads to fully Ultraviolet-Finite theories. Quantum fields will depend on the quantum coordinates, but, in presence of Gravity, the commutators of the coordinates might in turn depend on the quantum fields, giving rise to a quantum texture where fields and spacetime coordinates cannot be separated. Possible deep physical consequences will be outlined. Sergio Doplicher http://streamer.perimeterinstitute.ca/mp3/25872437-5aba-44dc-b641-955101f0d2d7.mp3 Science http://streamer.perimeterinstitute.ca/mp3/25872437-5aba-44dc-b641-955101f0d2d7.mp3 Thu, 14 Feb 2008 14:00:00 -0500 The problem of time is studied in a toy model for quantum gravity: Barbour and Bertotti's timeless formulation of non-relativistic mechanics. We quantize this timeless theory using path integrals and compare it to the path integral quantization of parameterized Newtonian mechanics, which contains absolute time. In general, we find that the solutions to the timeless theory are energy eigenstates, as predicted by the usual canonical quantization. Nevertheless, the path integral formalism brings new insight as it allows us to precisely determine the difference between the theory with and without time. This difference is found to lie in the form of the constraints imposed on the gauge fixing functions by the boundary conditions. In the stationary phase approximation, the constraints of both theories are equivalent. This suggests that a notion of time can emerge in systems for which the stationary phase approximation is either good or exact. As there are many similarities between this model of classical mechanics and general relativity, these results could provide insight to how time might be emergent in a theory of quantum gravity. Sean Gryb http://streamer.perimeterinstitute.ca/mp3/b0396c59-acb1-44d9-b235-40a9f9d3d4fb.mp3 Science http://streamer.perimeterinstitute.ca/mp3/b0396c59-acb1-44d9-b235-40a9f9d3d4fb.mp3 Thu, 10 Apr 2008 14:00:00 -0400 We analyze the trans-Planckian problem and its formulation in the context of cosmology, black-hole physics, and analogue models of gravity. In particular, we discuss the phenomenological approach to the trans-Planckian problem based on modified, locally Lorentz-breaking, dispersion relations (MDR). The main question is whether MDR leave an detectable imprint on macroscopic physics. In the framework of the semi-classical theory of gravity, this question can be unambiguously answered only through a rigorous formulation of quantum field theory on curved space with MDR. In this context, we propose a momentum-space analysis of the Green's function, which will hopefully lead to the correct renormalization of the stress tensor. Massimiliano Rinaldi http://streamer.perimeterinstitute.ca/mp3/33aa7588-3a16-42da-9c40-fb5e9cb2ef83.mp3 Science http://streamer.perimeterinstitute.ca/mp3/33aa7588-3a16-42da-9c40-fb5e9cb2ef83.mp3 Thu, 24 Apr 2008 14:00:00 -0400 This talk presents some recent results in renormalizable noncommutative quantum field theory. After introducing the renormalization group approach in the commutative setting I will procede to its generalization to the simplest noncommutative model, $phi_4^{star 4}$ on the Moyal space. The well known phenomenon of ultraviolet/infrared mixing is cured by adding a harmonic potential term to the free action. Under the new renormalization group, adapted to the noncommutative geometry, this model turns out to be renormalizable to all orders in perturbation theory. Moreover it is {f asymptotically safe} at all orders in perturbation theory. The consequences of this results are discussed. Razvan Gurau http://streamer.perimeterinstitute.ca/mp3/eb74e671-73ba-4251-bbb2-05b8e86bc6f6.mp3 Science http://streamer.perimeterinstitute.ca/mp3/eb74e671-73ba-4251-bbb2-05b8e86bc6f6.mp3 Thu, 08 May 2008 11:00:00 -0400 TBA Donald Marolf http://streamer.perimeterinstitute.ca/mp3/372e87a8-d18d-4543-9409-738ca0144523.mp3 Science http://streamer.perimeterinstitute.ca/mp3/372e87a8-d18d-4543-9409-738ca0144523.mp3 Thu, 08 May 2008 14:00:00 -0400 For quantum gravity, the requirement of metric positivity suggests the use of noncanonical, affine kinematical field operators. In view of gravity's set of open classical first class constraints, quantization before reduction is appropriate, leading to affine commutation relations and affine coherent states. The anomaly in the quantized constraints may be accommodated within the projection operator approach, which treats first and second class quantum constraints in an equal fashion. Functional integral representations are derived for expressions both with and without constraint imposition. As with all coherent state formulations, close contact between the classical and quantum theories is maintained throughout. Perturbative nonrenormalizability is understood as a partial hard-core behavior of the interaction, which as soluble models suggest, leads to a perturbative formulation, not about the traditional free theory, but rather about a suitable pseudofree theory that properly incorporates the essence of the hard core. John Klauder http://streamer.perimeterinstitute.ca/mp3/73d39d11-bc49-40c1-b93d-3c15eb3847ec.mp3 Science http://streamer.perimeterinstitute.ca/mp3/73d39d11-bc49-40c1-b93d-3c15eb3847ec.mp3 Thu, 15 May 2008 14:00:00 -0400 This talk is concerned with the existence of spectral triples in quantum gravity. I will review the construction of a spectral triple over a functional space of connections. Here, the *-algebra is generated by holonomy loops and the Dirac type operator has the form of a global functional derivation operator. The spectral triple encodes the Poisson structure of General Relativity when formulated in terms of Ashtekars variables. Finally I will argue that the Hamiltonian of General Relativity may emerge from the construction via the requirement that inner automorphisms vanish on the vacuum sector. Jesper Grimstrup http://streamer.perimeterinstitute.ca/mp3/a8f61716-ff3e-4f53-9506-0ccfcff951eb.mp3 Science http://streamer.perimeterinstitute.ca/mp3/a8f61716-ff3e-4f53-9506-0ccfcff951eb.mp3 Thu, 29 May 2008 04:00:00 -0400 Graphity models are characterized by configuration spaces in which states correspond to graphs and Hamiltonians that depend on local properties of graphs such as degrees of vertices and numbers of shortcycles. It has been argued that such models can be useful in studying how an extended geometry might emerge from a background independent dynamical system. As statistical systems, graphity models can be studied analytically by estimating their partition functions or numerically by Monte Carlo simulations. In this talk I will present recent results obtained using both of these approaches. In particular, I will describe the transition between the high and low temperature regimes and arguethat matter degrees of freedom must play an important role in order for the graph states dominating in the low temperature regime to resemble interesting extended geometries. Tomasz Konopka http://streamer.perimeterinstitute.ca/mp3/7d5460be-afd2-468f-8877-1d688e27c17d.mp3 Science http://streamer.perimeterinstitute.ca/mp3/7d5460be-afd2-468f-8877-1d688e27c17d.mp3 Thu, 12 Jun 2008 14:00:00 -0400 The speculation that Dark Energy can be explained by the backreaction of present inhomogeneities on the evolution of the background cosmology has been increasingly debated in the recent literature. We demonstrate quantitively that the backreaction of linear perturbations on the Friedmann equations is small but is nevertheless non-vanishing. This indicates the need for an improved averaging procedure capable of averaging tensor quantities in a generally covariant way. We present an averaging process which decomposes the metric into Vielbeins selected employing a variational principle, and parallel-transports them to a single point at which they can be averaged. The functionality of the process is discussed in specific 2-d examples, and its application to 3-surfaces and metric recovery in cosmology is outlined. Juliane Behrend http://streamer.perimeterinstitute.ca/mp3/4b937c45-a003-473f-8b7a-7f6f797bd73b.mp3 Science http://streamer.perimeterinstitute.ca/mp3/4b937c45-a003-473f-8b7a-7f6f797bd73b.mp3 Thu, 14 Aug 2008 14:00:00 -0400 TBA Tim Hsieh,Francesca Vidotto,Alexandru Dafinca,Robert Mooney http://streamer.perimeterinstitute.ca/mp3/09a47a6c-2a71-4229-8a27-f67e8691f7da.mp3 Science http://streamer.perimeterinstitute.ca/mp3/09a47a6c-2a71-4229-8a27-f67e8691f7da.mp3 Wed, 20 Aug 2008 16:00:00 -0400 Federico Piazza http://streamer.perimeterinstitute.ca/mp3/1963080a-248b-475b-836f-28ff88c278eb.mp3 Science http://streamer.perimeterinstitute.ca/mp3/1963080a-248b-475b-836f-28ff88c278eb.mp3 Thu, 04 Sep 2008 16:00:00 -0400 Federico Piazza http://streamer.perimeterinstitute.ca/mp3/1963080a-248b-475b-836f-28ff88c278eb.mp3 Science http://streamer.perimeterinstitute.ca/mp3/1963080a-248b-475b-836f-28ff88c278eb.mp3 Thu, 04 Sep 2008 16:00:00 -0400 During the last two decades Alain Connes developed Noncommutative Geometry, which allows to unify two of the basic theories of modern physics: General Relativity and the Standard Model of Particle Physics. In the noncommutative framework the Higgs boson, which had previously to be put in by hand, and many of the ad hoc features of the standard model, appear in a natural way. The aim of my talk is to motivate this unification from basic physical principles and to give a flavour of its derivation. I will give an overview of the basic tools such as almost-commutative spectral triples and the spectral action principle. The latter allows to derive the Einstein-Hilbert Lagrangian and the Standard Model Lagrangian together with a set of relations among the Standard Model parameters. Christoph Stephan http://streamer.perimeterinstitute.ca/mp3/b6ff8926-7789-4c7f-aacc-21502db27cbe.mp3 Science http://streamer.perimeterinstitute.ca/mp3/b6ff8926-7789-4c7f-aacc-21502db27cbe.mp3 Thu, 18 Sep 2008 14:00:00 -0400 TBA Julian Barbour http://streamer.perimeterinstitute.ca/mp3/f0785886-0285-490e-b976-7098e45dc6e7.mp3 Science http://streamer.perimeterinstitute.ca/mp3/f0785886-0285-490e-b976-7098e45dc6e7.mp3 Thu, 25 Sep 2008 14:00:00 -0400 The standard Hamiltonian formulation of (first order) gravity breaks manifest covariance both in its retention of the Lorentz group as a local gauge group and in its discrepant treatment of spacelike and timelike diffeomorphisms. Here we promote more covariant alternatives for canonical quantum gravity that address each of these problems, and discuss the implications for both the classical and the quantum theory of gravity. By retaining the full local Lorentz group, one gains significant insight into the geometric and algebraic properties of the Hamiltonian dynamics. As an example, we discuss the possibility of computing the internal spin angular momentum of asymptotically flat spacetimes, which may lead to insight into the nature of spin in quantum gravity. By treating the spacelike and timelike diffeomorphisms on equal footing, using techniques from geometric quantization we find a new representation of the quantum constraints where the total Hamiltonian is kinematical in the same vein as the Gauss and diffeomorphism constraints. Finally, we discuss the possibility of a manifestly 4-dimensional symplectic form on the Lagrangian phase space. Andy Randono http://streamer.perimeterinstitute.ca/mp3/2acf2f1b-3e03-49f5-8b11-3684a699cb43.mp3 Science http://streamer.perimeterinstitute.ca/mp3/2acf2f1b-3e03-49f5-8b11-3684a699cb43.mp3 Thu, 02 Oct 2008 16:30:00 -0400 The general boundary state formulation is a key tool for extracting the semiclassical limit of nonpertubative theories of quantum gravity. In this talk I will discuss how this formalism works in the context of four-dimensional quantum Regge calculus with a general triangulation. A Gaussian boundary state selects a classical internal solution and peaks the path integral on it. As a result boundary observables, in particular the two-point function, can be computed order by order in a semiclassical asymptotic expansion. When the same methods are applied to a modified Regge theory that substitutes the exponential of the action by its cosine at each simplex in the triangulation, as conjectured from the semiclassical limit of spin foam models, the contributions from the sign-reversed terms are suppressed and the results match those of conventional Regge calculus. This talk is based on the results published in arXiv:0808.1107 [gr-qc]. Alejandro Satz http://streamer.perimeterinstitute.ca/mp3/a1fe341a-5a25-4498-9ead-2aa6d37eef54.mp3 Science http://streamer.perimeterinstitute.ca/mp3/a1fe341a-5a25-4498-9ead-2aa6d37eef54.mp3 Fri, 10 Oct 2008 14:00:00 -0400 A modified version of the double potential formalism for the electrodynamics of dyons is constructed. Besides the two vector potentials, this manifestly duality invariant formulation involves four additional potentials, scalar potentials which appear as Lagrange multipliers for the electric and magnetic Gauss constraints and potentials for the longitudinal electric and magnetic fields. In this framework, a static dyon appears as a Coulomb-like solution without string singularities. Dirac strings are needed only for the Lorentz force law, not for Maxwell's equations. The magnetic charge no longer appears as a topological conservation law but as a surface integral on a par with electric charge. The theory is generalized to curved space. As in flat space, the string singularities of dyonic black holes are resolved. As a consequence all singularities are protected by the horizon and the thermodynamics is shown to follow from standard arguments in the grand canonical ensemble. Glenn Barnich http://streamer.perimeterinstitute.ca/mp3/d1e3ea13-2372-4d47-a4a3-1deef1428495.mp3 Science http://streamer.perimeterinstitute.ca/mp3/d1e3ea13-2372-4d47-a4a3-1deef1428495.mp3 Thu, 16 Oct 2008 14:00:00 -0400 In an asymptotically anti-de Sitter space, three-dimensional topologically massive gravity has some remarkable properties, which suggest interesting applications to quantum gravity. Unfortunately, though, the theory appears to be unstable, even at the special 'chiral' value of the coupling. I will discuss recent work, and recent controversies, in this field. Steve Carlip http://streamer.perimeterinstitute.ca/mp3/b1faf903-52f1-46d8-a7dd-91f6cc1770fb.mp3 Science http://streamer.perimeterinstitute.ca/mp3/b1faf903-52f1-46d8-a7dd-91f6cc1770fb.mp3 Thu, 23 Oct 2008 14:00:00 -0400 After reviewing Wilson's picture of renormalization, and the associated Exact Renormalization Group, I will show that no (physically acceptable) non-trivial fixed points exist for scalar field theory in D>=4. Consequently, an asymptotic safety scenario is ruled out, and the triviality of the theory is confirmed. Oliver Rosten http://streamer.perimeterinstitute.ca/mp3/9b0b487a-8f70-4422-95a3-053fba8bdfbb.mp3 Science http://streamer.perimeterinstitute.ca/mp3/9b0b487a-8f70-4422-95a3-053fba8bdfbb.mp3 Thu, 30 Oct 2008 14:00:00 -0400 We discuss the possibility that spacetime geometry may be an emergent phenomenon. This idea has been motivated by the Analogue Gravity programme. An 'effective gravitational field' dominates the kinematics of small perturbations in an Analogue Model. In these models there is no obvious connection between the 'gravitational' field tensor and the Einstein equations, as the emergent spacetime geometry arises as a consequence of linearising around some classical field. After a brief introduction on this topic, we present our recent contributions to the field. Silke Weinfurtner http://streamer.perimeterinstitute.ca/mp3/a68a0358-2e22-42b4-aeee-1bc4f676337a.mp3 Science http://streamer.perimeterinstitute.ca/mp3/a68a0358-2e22-42b4-aeee-1bc4f676337a.mp3 Thu, 06 Nov 2008 14:00:00 -0500 The semiclassical-quantum correspondence (SQC) is a new principle which has enabled the explicit solution of the quantum constraints of GR in the full theory in the Ashtekar variables for gravity coupled to matter. The solutions, which constitute the physical space of states implementing the quantum dynamics of GR in the Dirac procedure, include a special class of states known as the generalized Kodama states (GKod). The GKodS can be seen as an analogue of the pure Kodama state (Kod) when quantum gravity (QGRA) is coupled to matter fields quantized on the same footing. The criterion for finiteness stems from a precise cancellation of the ultraviolet singularities stemming from the quantum Hamiltonian constraint, allowing for an exact solution. This signifies the following developments for 4D QGRA: (i) Equivalence among the Dirac, reduced phase, geometric and path integration approaches to quantization for GKods; (ii) A generalization of topological field theory to include matter fields via the instanton representation of GKod; (iii) A possible mechanism to establish 4D QGRA, via tree networks, as a renormalizable theory (iv) A direct link from QGRA to Minkoswki spacetime physics, which would enable tests of 4D QGRA without the necessity to access the Planck scale (v) A third-quantized analogy to second quantized spin network states implementing the quantum dynamics of GR. The aforementioned algorithm is designed to construct explicit solutions to the constraints of the full theory by inspection, while implementing any desired ‘boundary’ conditions on the states necessary to reduce to the appropriate semiclassical limit. Conversely, the finite states of 4D QGRA can place severe restrictions on phenomena occurring in the weak gravitational limit below the Planck scale. While we demonstrate this for the GKodS in this talk, the procedure can be applied to obtain a family of states labeled by two arbitrary functions of position, which possess the requisite Hilbert space structure in the limit where the matter fields are turned off. Remaining areas of research in progress include the illumination of the Hilbert space structure of the GKodS, analysis of various models for which the SQC can produce tractable solutions, in the full theory and in minisuperspace, and the addressal of any issues of interest regarding the mathematical rigor of the states. Eyo Ita http://streamer.perimeterinstitute.ca/mp3/fc17bdf0-658b-412e-9c1c-ae330ae0164d.mp3 Science http://streamer.perimeterinstitute.ca/mp3/fc17bdf0-658b-412e-9c1c-ae330ae0164d.mp3 Thu, 13 Nov 2008 14:00:00 -0500 I discuss a class of compact objects ('monsters') with more entropy than a black hole of the same ADM mass. Such objects are problematic for AdS/CFT duality and the conventional interpretation of black hole entropy as counting of microstates. Nevertheless, monster initial data can be constructed in semi-classical general relativity without requiring large curvatures or energy densities. Stephen Hsu http://streamer.perimeterinstitute.ca/mp3/62b550ca-6ae4-467f-b0b5-c98833171a51.mp3 Science http://streamer.perimeterinstitute.ca/mp3/62b550ca-6ae4-467f-b0b5-c98833171a51.mp3 Thu, 20 Nov 2008 14:00:00 -0500 Motivated by the analogy proposed by Witten between Chern-Simons theories and CFT-Wess-Zumino-Witten models, we explore a new way of computing the entropy of a black hole starting from the isolated horizon framework in Loop Quantum Gravity. The results seem to indicate that this analogy can work in this particular case. This could be a good starting point for the search of a deeper connection between the description of black holes in LQG and a conformal field theory. Ivan Agullo' http://streamer.perimeterinstitute.ca/mp3/e8d05f1d-77df-4a20-adeb-168c8e542937.mp3 Science http://streamer.perimeterinstitute.ca/mp3/e8d05f1d-77df-4a20-adeb-168c8e542937.mp3 Wed, 26 Nov 2008 11:00:00 -0500 As well known, cosmic ray experiments can put strong constraints on possible Lorentz Invariance Violations. In particular, the presence of the so called GZK 'cut-off' may indicate that protons do propagate in the Universe as expected from relativistic invariance. The presence of this feature in the spectrum has been convincingly indicated by the HiRes and Auger experiments, while the Auger Observatory has given indication on the correlation of Ultra High Energy Cosmic particles with nearby sources, as predicted by the GZK feature. I review the experimental results and discuss in particular both the theoretical and experimental intricacies and the limits on LIV parameters that can be deduced. Aurelio Grillo http://streamer.perimeterinstitute.ca/mp3/2ae046f6-f2e1-49e3-b036-489f11c38fff.mp3 Science http://streamer.perimeterinstitute.ca/mp3/2ae046f6-f2e1-49e3-b036-489f11c38fff.mp3 Thu, 27 Nov 2008 14:00:00 -0500 Using 2-dimensional CGHS black holes, I will argue that information is not lost in the Hawking evaporation because the quantum space-time is significantly larger than the classical one. I will begin with a discussion of the conceptual underpinnings of problem and then introduce a general, non-perturbative framework to describe quantum CGHS black holes. I will show that the Hawking effect emerges from it in the first approximation. Finally, I will introduce a mean field approximation to argue that, when the back reaction is included, future null infinity is `long enough' to capture full information contained in pure states at past null infinity and that the S-matrix is unitary. There are no macroscopic remnants. Abhay Ashtekar http://streamer.perimeterinstitute.ca/mp3/984ef64d-a8d9-4e90-a48c-1099acf9199b.mp3 Science http://streamer.perimeterinstitute.ca/mp3/984ef64d-a8d9-4e90-a48c-1099acf9199b.mp3 Thu, 04 Dec 2008 11:00:00 -0500