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288

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Introduction

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August 2001 - present

August 2001 - present

## Publications

Publications (288)

We introduced with coauthors some years ago a solution to the problem of time in quantum gravity which consists in formulating the quantum theory in terms of real clocks. It combines Page and Wootters' relational proposal with Rovelli's evolving constants of the motion. Time is associated with an operator and not a classical parameter. We show here...

We introduced with coauthors some years ago a solution to the problem of time in quantum gravity which consists in formulating the quantum theory in terms of real clocks. It combines Page and Wootters' relational proposal with Rovelli's evolving constants of the motion. Time is associated with an operator and not a classical parameter. We show here...

The covariance of loop quantum gravity studies of spherically symmetric space-times has recently been questioned. This is a reasonable worry, given that they are formulated in terms of slicing-dependent variables. We show explicitly that the resulting space-times, obtained from Dirac observables of the quantum theory, are covariant in the usual sen...

The covariance of loop quantum gravity studies of spherically symmetric space-times has recently been questioned. This is a reasonable worry, given that they are formulated in terms of slicing-dependent variables. We show explicitly that the resulting space-times, obtained from Dirac observables of the quantum theory, are covariant in the usual sen...

In a recent paper we showed that the collapse to a black hole in one-parameter families of initial data for massless, minimally coupled scalar fields in spherically symmetric semiclassical loop quantum gravity exhibited a universal mass scaling similar to the one in classical general relativity. In particular, no evidence of a mass gap appeared as...

We continue our investigation of an improved quantization scheme for spherically symmetric loop quantum gravity. We find that in the region where the black hole singularity appears in the classical theory, the quantum theory contains semi-classical states that approximate general relativity coupled to an effective anisotropic fluid. The singularity...

In a recent paper we showed that the collapse to a black hole in one-parameter families of initial data for massless, minimally coupled scalar fields in spherically symmetric semi-classical loop quantum gravity exhibited a universal mass scaling similar to the one in classical general relativity. In particular, no evidence of a mass gap appeared as...

We propose a new polymerization scheme for scalar fields coupled to gravity. It has the advantage of being a (non-bijective) canonical transformation of the fields and therefore ensures the covariance of the theory. We study it in detail in spherically symmetric situations and compare to other approaches.

It has been pointed out that the holonomy of generic extended loops is not gauge covariant. We show how to define a family of extended loops for which previous criticism does not apply. We also give sufficient conditions that extended loops must satisfy in order to yield covariant holonomies. This makes a quantum representation for Yang–Mills theor...

We continue our investigation of an improved quantization scheme for spherically symmetric loop quantum gravity. We find that in the region where the black hole singularity appears in the classical theory, the quantum theory contains semi-classical states that approximate general relativity coupled to an effective anisotropic fluid. The singularity...

We review the Montevideo Interpretation of quantum mechanics, which is based on the use of real clocks to describe physics, using the framework that was recently introduced by Höhn, Smith, and Lock to treat the problem of time in generally covariant systems. These new methods, which solve several problems in the introduction of a notion of time in...

We review the Montevideo Interpretation of quantum mechanics, which is based on the use of real clocks to describe physics, using the framework recently introduced by Hoehn, Smith and Lock to treat the problem of time in generally covariant systems. The use of the new formalism makes the whole construction more accessible to readers without familia...

We study the ‘improved dynamics’ for the treatment of spherically symmetric space-times in loop quantum gravity introduced by Chiou et al in analogy with the one that has been constructed by Ashtekar, Pawlowski and Singh for the homogeneous space-times. In this dynamics the polymerization parameter is a well motivated function of the dynamical vari...

In a previous paper, we showed how to use the techniques of the group of loops to formulate the loop approach to gravity proposed by Mandelstam in the 1960’s. Those techniques allow to overcome some of the difficulties that had been encountered in the earlier treatment. In this approach, gravity is formulated entirely in terms of Dirac observables...

We present a simple argument leading to a fundamental minimum uncertainty in the determination of times. It only relies in the uncertainty principle and time dilation in a gravitational field. It implies any attempt to measure times will have a fundamental level of uncertainty. Implications are briefly outlined.

We present a simple argument leading to a fundamental minimum uncertainty in the determination of times. It only relies in the uncertainty principle and time dilation in a gravitational field. It implies any attempt to measure times will have a fundamental level of uncertainty. Implications are briefly outlined.

In a previous paper we formulated axisymmetric general relativity in terms of real Ashtekar–Barbero variables. Here we proceed to quantize the theory. We are able to implement Thiemann’s version of the Hamiltonian constraint. This provides a 2 + 1 dimensional arena to test ideas for the dynamics of quantum gravity and opens the possibility of quant...

In a previous paper we discussed corrections to Hawking radiation from a collapsing shell due to quantum fluctuations of the shell and the resulting horizon. For the computation of the quantum corrections we used several approximations. In this paper we take into account effects that were neglected in the previous one. We find important corrections...

In a previous paper, we showed how to use the techniques of the group of loops to formulate the loop approach to gravity proposed by Mandelstam in the 1960's. Those techniques allow to overcome some of the difficulties that had been encountered in the earlier treatment. In this approach, gravity is formulated entirely in terms of Dirac observables...

We study the collapse in spherical symmetry of a massless scalar field minimally coupled to gravity using the semiclassical equations that are expected from loop quantum gravity. We find the critical behavior of the mass as a function of the parameters of the initial data similar to that found by Choptuik in classical general relativity for a large...

We study the collapse in spherical symmetry of a massless scalar field minimally coupled to gravity using the semiclassical equations that are expected from loop quantum gravity. We find critical behavior of the mass as a function of the parameters of the initial data similar to that found by Choptuik in classical general relativity for a large set...

In a previous paper we formulated axisymmetric general relativity in terms of real Ashtekar--Barbero variables. Here we proceed to quantize the theory. We are able to implement Thiemann's version of the Hamiltonian constraint. We discuss its action including the matrix elements and its solutions. This provides a 2+1 dimensional arena to test ideas...

Consciousness presents a series of characteristics that have been observed through- out the years: unity, continuity, richness and robustness are some of them. It manifests itself in regions of the brain capable of processing a huge quantity of integrated information with a level of neural activity close to criticality. We argue that the physics of...

In a previous paper we discussed corrections to Hawking radiation from a collapsing shell due to quantum fluctuations of the shell and the resulting horizon. For the computation of the quantum corrections we used several approximations. In this paper we take into account effects that were neglected in the previous one. We find important corrections...

It has been pointed out that the holonomy of generic extended loops is not gauge covariant. We show how to define a family of extended loops for which previous criticism does not apply. We also give sufficient conditions that extended loops must satisfy in order to yield covariant holonomies. This makes a quantum representation for Yang--Mills theo...

The production of events in quantum theory, that is, of outcomes to which a probability can be associated, is inconsistent with a unitary evolution for systems in arbitrary initial states. In particular, within unitary quantum mechanics, there exist global protocols that allow verifying that no definite event occurs. Instead, states that start in a...

We formulate axisymmetric general relativity in terms of real Ashtekar–Barbero variables. We study the constraints and equations of motion and show how the Kerr, Schwarzschild and Minkowski solutions arise. We also discuss boundary conditions. This opens the possibility of a midisuperspace quantization using loop quantum gravity techniques for spac...

We formulate axisymmetric general relativity in terms of real Ashtekar--Barbero variables. We study the constraints and equations of motion and show how the Kerr, Schwarzschild and Minkowski solutions arise. We also discuss boundary conditions. This opens the possibility of a midisuperspace quantization using loop quantum gravity techniques for spa...

In the 1960's, Mandelstam proposed a new approach to gauge theories and gravity based on loops. The program for gauge theories was completed for Yang--Mills theories by Gambini and Trias in the 1980's. Gauge theories could be understood as representations of certain group: the group of loops. The same formalism could not be implemented at that time...

Within ordinary ---unitary--- quantum mechanics there exist global protocols that allow to verify that no definite event ---an outcome to which a probability can be associated--- occurs. Instead, states that start in a coherent superposition over possible outcomes always remain as a superposition. We show that, when taking into account fundamental...

We study Hawking radiation on the quantum space-time of a collapsing null shell. We use the geometric optics approximation as in Hawking's original papers to treat the radiation. The quantum space-time is constructed by superposing the classical geometries associated with collapsing shells with uncertainty in their position and mass. We show that t...

We argue that a conformally invariant extension of general relativity coupled to the Standard Model is the fundamental theory that needs to be quantized. We show that it can be treated by loop quantum gravity techniques. Through a gauge fixing and a modified Higgs mechanism particles acquire mass and one recovers general relativity coupled to the S...

The problem of how space-time responds to gravitating quantum matter in full
quantum gravity has been one of the main questions that any program of
quantization of gravity should address. Here we analyze this issue by
considering the quantization of a collapsing null shell coupled to spherically
symmetric loop quantum gravity. We show that the cons...

We introduce an ontology of objects and events that is particularly
well suited for several interpretations of quantum mechanics. It
leads to an important revision of the notion of
matter and its implications. Within this context one can show that
systems in entangled states present emergent new properties and
downward causation where certain behav...

We show that several interpretations of quantum mechanics admit an ontology of objects and events. This ontology reduces the breach between mind and matter. When humans act, their actions do not appear explainable in mechanical terms but through mental activity: motives, desires or needs that propel them to action. These are examples of what in the...

We show, following a previous quantization of a vacuum spherically symmetric spacetime carried out in Ref. [1], that this setting admits a Schr\"odinger-like picture. More precisely, the technique adopted there for the definition of parametrized Dirac observables (that codify local information of the quantum theory) can be extended in order to acco...

We consider a massive scalar field living on the recently found exact quantum
space-time corresponding to vacuum spherically symmetric loop quantum gravity.
The discreteness of the quantum space time naturally regularizes the scalar
field, eliminating divergences. However, the resulting finite theory depends on
the details of the micro physics. We...

In loop quantum gravity the discrete nature of quantum geometry acts as a
natural regulator for matter theories. Studies of quantum field theory in
quantum space-times in spherical symmetry in the canonical approach have shown
that the main effect of the quantum geometry is to discretize the equations of
matter fields. This raises the possibility t...

We present a summary for non-specialists of loop quantum gravity as part of
the modern legacy of the series of papers by Arnowitt, Deser and Misner circa
1960.

We apply quantum field theory in quantum space-time techniques to study the
Casimir effect for large spherical shells. As background we use the recently
constructed exact quantum solution for spherically symmetric vacuum space-time
in loop quantum gravity. All calculations are finite and one recovers the usual
results without the need of regulariza...

We show that a new interpretation of quantum mechanics, in which the notion
of event is defined without reference to measurement or observers, allows to
construct a quantum general ontology based on systems, states and events.
Unlike the Copenhagen interpretation, it does not resort to elements of a
classical ontology. The quantum ontology in turn...

The Montevideo interpretation of quantum mechanics, which consists in
supplementing environmental decoherence with fundamental limitations in
measurement stemming from gravity, has been described in several publications.
However, some of them appeared before the full picture provided by the
interpretation was developed. As such it can be difficult...

We study the quantum motion of null shells in the quantum space-time of a
black hole in loop quantum gravity. We treat the shells as test fields and use
an effective dynamics for the propagation equations. The shells propagate
through the region where the singularity was present in the classical black
hole space-time, but is absent in the quantum s...

We quantize spherically symmetric electrovacuum gravity. The algebra of
Hamiltonian constraints can be made Abelian via a rescaling and linear
combination with the diffeomorphism constraint. As a result the constraint
algebra is a true Lie algebra. We complete the Dirac quantization procedure
using loop quantum gravity techniques. We present explic...

We incorporate elements of the recently discovered exact solutions of the
quantum constraints of loop quantum gravity for vacuum spherically symmetric
space-times into the paradigm of black hole evaporation due to Ashtekar and
Bojowald. The quantization of the area of the surfaces of symmetry of the
solutions implies that the number of nice slices...

We consider a quantum field theory on a spherically symmetric quantum space
time described by loop quantum gravity. The spin network description of space
time in such a theory leads to equations for the quantum field that are
discrete. We show that to avoid significant violations of Lorentz invariance
one needs to consider specific non-local intera...

We introduce quantum field theory on quantum space-times techniques to
characterize the quantum vacua as a first step towards studying black hole
evaporation in spherical symmetry in loop quantum gravity and compute the
Hawking radiation. We use as quantum space time the recently introduced exact
solution of the quantum Einstein equations in vacuum...

We study the quantization of spherically symmetric vacuum spacetimes within loop quantum gravity. In particular, we give additional details about our previous work in which we showed that one could complete the quantization of the model and that the singularity inside black holes is resolved. Moreover, we consider an alternative quantization based...

Loop representations (and the related topic of knot theory) are of considerable current interest because they provide a unified arena for the study of the gauge invariant quantization of Yang-Mills theories and gravity, and suggest a promising approach to the eventual unification of the four fundamental forces. This text provides a self-contained i...

We review recent developments in the treatment of spherically symmetric black
holes in loop quantum gravity. In particular, we discuss an exact solution to
the quantum constraints that represents a black hole and is free of
singularities. We show that new observables that are not present in the
classical theory arise in the quantum theory. We also...

We quantize spherically symmetric vacuum gravity without gauge fixing the diffeomorphism constraint. Through a rescaling, we make the algebra of Hamiltonian constraints Abelian, and therefore the constraint algebra is a true Lie algebra. This allows the completion of the Dirac quantization procedure using loop quantum gravity techniques. We can con...

We study the quantization of vacuum spherically symmetric space-times.
We use variables adapted to spherical symmetry but do not fix the gauge
further. One is left with a diffeomorphism constraint and a Hamiltonian
constraint. Rescaling the latter turns the constraint algebra into a
true Lie algebra and allows to implement the Dirac quantization
pr...

We summarize the talks presented at the QG4 session (loop quantum gravity:
cosmology and black holes) of the 13th Marcel Grossmann Meeting held in
Stockholm, Sweden.

We discuss a gauge fixing of gravity coupled to a scalar field in spherical
symmetry such that the Hamiltonian is an integral over space of a local
density. In a previous paper we had presented it using Ashtekar's new
variables. Here we study it in metric variables. We specify completely the
initial-boundary value problem for ingoing Gaussian pulse...

We outline three principles that should guide us in the construction of a theory of canonical quantum gravity: (1) diffeomorphism invariance, (2) implementing the proper dynamics and related constraint algebra, (3) local Lorentz invariance. We illustrate each of them with its role in model calculations in loop quantum gravity.

Students who are interested in quantum gravity usually face the
difficulty of working through a large amount of prerequisite material
before being able to deal with actual quantum gravity. A First Course in
Loop Quantum Gravity by Rodolfo Gambini and Jorge Pullin, aimed at
undergraduate students, marvellously succeeds in starting from the
basics of...

Several proposals to deal with the dynamics of general relativity involve
gauge fixings or the introduction matter fields in terms of which the theory is
deparameterized. The resulting theories have true Hamiltonians for their
evolution that usually involve square roots, and this poses certain challenges
for their implementation as self-adjoint qua...

This is a summary of the talk presented by one of us in Loops 2011. We discuss the application of the uniform discretization approach to spherically symmetric gravity coupled to a spherically symmetric scalar field.

We respond to a comment by Joseph Polchinski on: 'Small Lorentz violations in quantum gravity: do they lead to unacceptably large effects?'

We present a gauge fixing of gravity coupled to a scalar field in spherical symmetry such that the Hamiltonian is an integral over space of a local density. Such a formulation had proved elusive over the years. As in any gauge fixing, it works for a restricted set of initial data. We argue that the set could be large enough to attempt a quantizatio...

We discuss the fundamental loss of unitarity that appears in quantum mechanics when one uses physically realistic devices to measure time and space. The effect is independent of any interaction with the environment and appears in addition to any usual environmental decoherence. We discuss the conceptual and potential experimental implications of th...

In this review we discuss the interplay between discretization, constraint implementation, and diffeomorphism symmetry in Loop Quantum Gravity and Spin Foam models. To this end we review the Consistent Discretizations approach, which is an application of the master constraint program to construct the physical Hilbert space of the canonical theory,...

We discuss the applicability of the argument of Collins, P\'erez, Sudarsky,
Urrutia and Vucetich to loop quantum gravity. This argument suggests that
Lorentz violations, even ones that only manifest themselves at energies close
to the Planck scale, have significant observational consequences at low
energies when one considers perturbative quantum f...

We recently studied gravity coupled to a scalar field in spherical symmetry
using loop quantum gravity techniques. Since there are local degrees of freedom
one faces the "problem of dynamics". We attack it using the "uniform
discretization technique". We find the quantum state that minimizes the value
of the master constraint for the case of weak f...

We reconsider the argument of Collins, Perez, Sudarsky, Urrutia and
Vucetich concerning violations of Lorentz invariance in the context of
loop quantum gravity. We show that even if one introduces a lattice that
violates Lorentz invariance at the Planck scale, this does not translate
itself into large violations that would conflict with experiment.

We summarize several recent developments suggesting that solving the problem
of time in quantum gravity leads to a solution of the measurement problem in
quantum mechanics. This approach has been informally called "the Montevideo
interpretation". In particular we discuss why definitions in this approach are
not "for all practical purposes" (fapp) a...

We present progress of our first step towards theoretical and numerical study of the quantum evolution of mass(less) scalar field in gravity. We utilise the framework of loop quantum gravity and calculate the Classical Hamilton's equation of motion for scaler field phi and gravitational variables (connection variable K and triads E^), equations are...

In recent papers we put forth a new interpretation of quantum mechanics,
colloquially known as ``the Montevideo interpretation''. This interpretation is
based on taking into account fundamental limits that gravity imposes on the
measurement process. As a consequence one has that situations develop where a
reduction process is undecidable from an ev...

We make a first attempt to axiomatically formulate the Montevideo
interpretation of quantum mechanics. In this interpretation environmental
decoherence is supplemented with loss of coherence due to the use of realistic
clocks to measure time to solve the measurement problem. The resulting
formulation is framed entirely in terms of quantum objects w...

We show that the use of evolving Dirac observables in conjunction with
the conditional probabilities of Page and Wootters correctly predicts
the physical propagators in model systems. This eliminates the main
objection to that treatment and opens possibilities for correctly
handling the problem of time in quantum gravity in terms of observable
quan...

We show that the canonical formulation of a generic action for (1 + 1)-dimensional models of gravity coupled to matter admits a description in terms of Ashtekar-type variables. This opens the possibility of discussing models of black hole evaporation using loop representation techniques and verifying which paradigm emerges for the possible eliminat...

We summarize recent results concerning the quantization of the complete extension of the Schwarzschild space-time using spherically symmetric loop quantum gravity. We find an exact solution of the polymerized theory, that is expected to capture features of the semi-classical limit. The singularity is eliminated but the space-time still contains a h...

We study gravity coupled to a scalar field in spherical symmetry using loop quantum gravity techniques. Since this model has local degrees of freedom, one has to face ``the problem of dynamics'', that is, diffeomorphism and Hamiltonian constraints that do not form a Lie algebra. We tackle the problem using the ``uniform discretization'' technique....

In a series of recent papers we have introduced a new interpretation of quantum mechanics, which for brevity we will call the Montevideo interpretation. In it, the quantum to classical transition is achieved via a phenomenon called "undecidability" which stems from environmental decoherence supplemented with a fundamental mechanism of loss of coher...