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Peyresq Physics 20th

“Micro and macro structure of spacetime”

June 13 – 19, 2015

Participants and conributions

Charis Anastopoulos

Department of Physics, University of Patras, 26500 Patras, Greece.

“How does gravity couple to quantum systems? Coherence vs decoherence”

We examine the action of gravity on quantum systems. We argue that a conservative application of the principles of quantum field theory and general relativity suggests that (i) in the Newtonian regime, the gravitational field is slaved to the quantum dynamics of matter, (ii) gravitational decoherence, if present, is due to transverse, traceless fluctuations. Alternative quantum theories that postulate non-trivial decoherence or localization effects in the Newtonian regime require radical changes in fundamental physics. Assuming that (i) is correct, gravitational forces fluctuate because they are slaved to the quantum fluctuations of the stress-energy tensor of the matter source, even if the latter consists of a single particle. We examine the effect of such fluctuations for a particle in a cat state and study how they may be recorded.

Eugenio Bianchi

Institute for Gravitation and the Cosmos and Physics Department, Penn State, University Park, PA 16802, USA.

“Entanglement entropy of squeezed vacua on a lattice”

We derive a formula for the entanglement entropy S_A of a squeezed vacuum |J> restricted to a subspace H_A of the Hilbert space. The formula is expressed in the language of symplectic geometry and involves only the projection of the complex structure J to the symplectic subspace A. The new expression is related and generalizes the classical vacuum entropy formula derived by Rafael Sorkin in 1983 and the covariant expression he presented at a previous Peyresq meeting. I will discuss some of the applications of this formula that we have been investigating: area law for the ground state of local Hamiltonians, entanglement entropy in momentum space, entanglement entropy production in the presence of instabilities, and squeezed spin-network states and entanglement in loop quantum gravity.

Steve Carlip

Department of Physics, University of California at Davis, Davis, California (USA)

“Four-dimensional entropy from three-dimensional gravity”

In loop quantum gravity, the boundary term at a black hole horizon is formally equivalent to an action for three-dimensional gravity. I show how to use this equivalence to obtain the four-dimensional Bekenstein-Hawking entropy from well understood computations of the entropy of the three-dimensional black hole.

Larry Ford

Institute of Cosmology, Department of Physics and Astronomy, Tufts University, Medford, Massachusetts (USA)

“Switching of quantum stress tensor fluctuations”

Quantum field operators require some averaging over a spacetime region before the details of their fluctuations can be meaningfully discussed. The fluctuations of linear fields are not especially sensitive to the details of the switching function, but stress tensor fluctuations do depend crucially upon these details. In particular, the probability of large fluctuations can depend greatly on the switching. Some examples will be discussed where the specific physical situation may dictate a particular form for the switching function. This dependence upon switching is a special problem in cosmology, and indicates the need for a better theory of initial conditions.

Valeri Frolov

Theoretical Physics Institute, University of Alberta, Edmonton, Alberta, (Canada)

“Non-singular models of black holes”

At first I discuss the problem of the gravitational collapse of small mass in the higher derivative and ghost free theories of gravity. It will be demonstrated how higher derivative and non-local modifications of gravity equations regularizes static and dynamical solutions. Boosting a static solution of the linearized equations for the gravitational potential of a point mass we obtain a solution for the field of the ultra relativistic source (gyraton). Using the latter we construct solutions for the collapsing spherical (thin and thick) null shell. By analyzing the obtained solutions we demonstrate that for small enough value of the mass M an apparent horizon is not formed for the gravitational collapse of small mass in the higher-derivative and ghost free theories of gravity. We demonstrate that this “mass gap” property is connected with the presence of the UV cut-off in such theories. In second part of the talk I shall discuss some properties of black hole models with regular interior.

Edgard Gunzig

Université Libre de Bruxelles, Bruxelles (Belgium)

Bei-Lok Hu

Joint Quantum Institute and Maryland Center for Fundamental Physics, University of Maryland, College Park, Maryland (USA)

Eugene Lim

Theoretical Particle Physics and Cosmology Group, Physics Department, King’s College London, London (UK)

“How quantum are the cosmological correlations?”

Cosmological perturbations are sourced by quantum fluctuations of the vacuum during inflation. In contrast, our observations of the Cosmic Microwave Background are classical. Can we test for the quantum origins of the perturbations? How much quantum information is lost when we make these observations? Have we totally screwed up by building PLANCK, and measured the correlations in the wrong basis and hence losing the primordial quantum information for good?

Don Page

Department of Physics, University of Alberta, Edmonton, Alberta (Canada)

“Hawking radiation energy and entropy from a Bianchi-Smerlak semiclassical black hole”

Eugenio Bianchi and Matteo Smerlak have found a relationship between the Hawking radiation energy and von Neumann entropy in a conformal field emitted by a semiclassical two-dimensional black hole. We compare this relationship with what might be expected for unitary evolution of a quantum black hole in four and higher dimensions. If one neglects the expected increase in the radiation entropy over the decrease in the black hole Bekenstein-Hawking A/4 entropy that arises from the scattering of the radiation by the barrier near the black hole, the relation works very well, except near the peak of the radiation von Neumann entropy and near the final evaporation. These discrepancies are calculated and discussed as tiny differences between a semiclassical treatment and a quantum gravity treatment.

Renaud Parentani

Laboratoire de Physique Théorique, C6RS-UMR, Université Paris-Sud 11, Orsay Cedex (France)

“Probing the thermal character of analogue Hawking radiation for shallow water waves?”

We study the scattering coefficients of shallow water waves blocked by a stationary counterflow. When the flow is transcritical, the coefficients closely follow Hawking’s thermal prediction. We study how the spectrum deviates from thermality when working with subcritical flows since these have been used to test Hawking’s prediction. For such flows, we show that the emission spectrum is strongly suppressed, and that its Planckian character is lost. Yet, our numerical results reproduce rather well the observations made by S. Weinfurtner et al. in the Vancouver experiment. Our results deliver a new interpretation of what has been observed, and indicate what should be modified to improve the experimental tests.

Alejandro Perez

Centre de Physique Théorique, C6RS-UMR, Luminy Case, Marseille (France)

“Black hole information loss and discreteness of quantum geometry”

In an approach to quantum gravity where space-time arises from coarse graining of fundamentally discrete structures, black hole formation and subsequent evaporation could be described by a unitary evolution without the problems encountered by standard remnant scenarios or the schemes where information is assumed to come out with the radiation while semiclassical evaporation (firewalls and complementarity). I point out the possibility that the final state is purified by correlations with the fundamental pre geometric structures (in the sense of Wheeler) which are available in such approaches, and, like defects in the underlying space-time weave, can carry zero energy.

Albert Roura

Institut für Quantenphysik, Universität Ulm, Ulm (Germany)

“Circumventing the uncertainty principle in atom interferometry tests of the equivalence principle”

The most precise absolute accelerometers and gravimeters are currently based on atom interferometry. This can be exploited to conduct highly sensitive tests of the universality of free fall (UFF) by performing differential measurements of two different atomic species. Gravity gradients, however, pose serious challenges. They lead to loss of contrast in the interference signal and mimic violations of UFF unless the initial colocation of the two species is controlled very precisely (below the nanometer scale). Furthermore, usual methods for minimizing these effects are ultimately limited by Heisenberg’s uncertainty principle. In this talk I will present two effective and easily implementable mitigation strategies to tackle these challenges.

Rafael Sorkin

Perimeter Institute for Theoretical Physics, Waterloo, Ontario (Canada) and Department of Physics, Syracuse University, Syracuse, 6ew York (USA)

“The null-surface boundary term in the variation of the gravitational action-functional”

The variation of the integrated Ricci scalar over a spacetime region R receives four kinds of contributions from the lightlike portion of the boundary of R, of which only two can be expressed covariantly. I will describe these contributions and comment on their non-covariance in relation to the possibility of a gravitational path integral of Schwinger-Kel’dysh type (what Bei Lok calls CTP path integral).

Simone Speziale

Centre de Physique Théorique, C6RS-UMR, Luminy Case, Marseille (France)

“First-order gravity on the light-front”

Null hypersurfaces play a pivotal role in the physical understanding of general relativity. I will first review some known aspects of the associated canonical analysis, such as the second class nature of the Hamiltonian constraint, a different counting of degrees of freedom, and the identification of constraint-free data. I will then extend the analysis to a first order formulation with independent connection and tetrad variables, discussing in particular the presence of tertiary constraints and of zero modes of the fields and Lagrange multipliers. I will comment on possible relations between the first  class part of the Dirac algebra and the BMS group. The resuts are relevant to the understanding of GR dynamics in terms of constraint-free data, and in order to adapt loop quantum gravity techniques to null hypersurfaces.

Enric Verdaguer

Departament de Física Fonamental and Institut de Ciències del Cosmos, Universitat de Barcelona, Barcelona (Spain)

Mikael von Strauss

UPMC-C6RS, UMR7095, Institut d’Astrophysique de Paris, GReCO, 98bis boulevard Arago, F-75014 Paris, France.

“Constraints in massive and bimetric gravity”

In this talk I will first give an overview of the recently developed classical theory of interacting spin-2 fields, mainly ‘massive gravity’ and ‘bimetric gravity’. These are nonlinear extensions of the classical theory of a massive spin-2 field propagating in flat space, discovered by Fierz and Pauli in 1939. I will then discuss some recent work, and work in preparation, where we obtained the linearizations of these theories and demonstrate why this is a non-trivial problem. As an application of this we also uncovered the structure of the Lagrangian versions of the constraints necessary to prove that the theories propagate the correct number of degrees of freedom.