The Experts below are selected from a list of 2314449 Experts worldwide ranked by ideXlab platform
Rafael D Sorkin - One of the best experts on this subject based on the ideXlab platform.
-
Discreteness without symmetry breaking a theorem
Modern Physics Letters A, 2009Co-Authors: Luca Bombelli, Joe Henson, Rafael D SorkinAbstract:This paper concerns random sprinklings of points into Minkowski spacetime (Poisson processes). It proves that there exists no equivariant measurable map from sprinklings to spacetime directions (even locally). Therefore, if a discrete structure is associated to a sprinkling in an intrinsic manner, then the structure will not pick out a preferred frame, locally or globally. This implies that the Discreteness of a sprinkled causal set will not give rise to "Lorentz breaking" effects like modified dispersion relations. Another consequence is that there is no way to associate a finite-valency graph to a sprinkling consistently with Lorentz invariance.
-
energy momentum diffusion from spacetime Discreteness
Physical Review D, 2009Co-Authors: Lydia Philpott, Fay Dowker, Rafael D SorkinAbstract:We study potentially observable consequences of spatiotemporal Discreteness for the motion of massive and massless particles. First we describe some simple models for the motion of a massive point particle in a fixed causal set background. If the causal set is faithfully embeddable in Minkoswki spacetime, the models give rise to particle motion in the continuum spacetime. At large scales, the microscopic swerves induced by the underlying atomicity manifest themselves as a Lorentz invariant diffusion in energy-momentum governed by a single phenomenological parameter, and we derive in full the corresponding diffusion equation. Inspired by the simplicity of the result, we then derive the most general Lorentz invariant diffusion equation for a massless particle, which turns out to contain two phenomenological parameters describing, respectively, diffusion and drift in the particle's energy. The particles do not leave the light cone however: their worldlines continue to be null geodesics. Finally, we deduce bounds on the drift and diffusion constants for photons from the blackbody nature of the spectrum of the cosmic microwave background radiation.
-
Discreteness without symmetry breaking a theorem
arXiv: General Relativity and Quantum Cosmology, 2006Co-Authors: Luca Bombelli, Joe Henson, Rafael D SorkinAbstract:This paper concerns sprinklings into Minkowski space (Poisson processes). It proves that there exists no equivariant measurable map from sprinklings to spacetime directions (even locally). Therefore, if a discrete structure is associated to a sprinkling in an intrinsic manner, then the structure will not pick out a preferred frame, locally or globally. This implies that the Discreteness of a sprinkled causal set will not give rise to ``Lorentz breaking'' effects like modified dispersion relations. Another consequence is that there is no way to associate a finite-valency graph to a sprinkling consistently with Lorentz invariance.
-
quantum gravity phenomenology lorentz invariance and Discreteness
Modern Physics Letters A, 2004Co-Authors: Fay Dowker, Joe Henson, Rafael D SorkinAbstract:Contrary to what is often stated, a fundamental spacetime Discreteness need not contradict Lorentz invariance. A causal set's Discreteness is in fact locally Lorentz invariant, and we recall the re...
-
quantum gravity phenomenology lorentz invariance and Discreteness
arXiv: General Relativity and Quantum Cosmology, 2003Co-Authors: Fay Dowker, Joe Henson, Rafael D SorkinAbstract:Contrary to what is often stated, a fundamental spacetime Discreteness need not contradict Lorentz invariance. A causal set's Discreteness is in fact locally Lorentz invariant, and we recall the reasons why. For illustration, we introduce a phenomenological model of massive particles propagating in a Minkowski spacetime which arises from an underlying causal set. The particles undergo a Lorentz invariant diffusion in phase space, and we speculate on whether this could have any bearing on the origin of high energy cosmic rays.
Andrew A Fingelkurts - One of the best experts on this subject based on the ideXlab platform.
-
Timing in cognition and EEG brain dynamics: Discreteness versus continuity
Cognitive Processing, 2006Co-Authors: Andrew A FingelkurtsAbstract:This article provides an overview of recent developments in solving the timing problem (Discreteness vs. continuity) in cognitive neuroscience. Both theoretical and empirical studies have been considered, with an emphasis on the framework of operational architectonics (OA) of brain functioning (Fingelkurts and Fingelkurts in Brain Mind 2:291–29, 2001 ; Neurosci Biobehav Rev 28:827–836, 2005 ). This framework explores the temporal structure of information flow and interarea interactions within the network of functional neuronal populations by examining topographic sharp transition processes in the scalp EEG, on the millisecond scale. We conclude, based on the OA framework, that brain functioning is best conceptualized in terms of continuity–Discreteness unity which is also the characteristic property of cognition. At the end we emphasize where one might productively proceed for the future research.
-
timing in cognition and eeg brain dynamics Discreteness versus continuity
Cognitive Processing, 2006Co-Authors: Andrew A FingelkurtsAbstract:This article provides an overview of recent developments in solving the timing problem (Discreteness vs. continuity) in cognitive neuroscience. Both theoretical and empirical studies have been considered, with an emphasis on the framework of Operational Architectonics (OA) of brain functioning (Fingelkurts and Fingelkurts, 2001, 2005). This framework explores the temporal structure of information flow and interarea interactions within the network of functional neuronal populations by examining topographic sharp transition processes in the scalp EEG, on the millisecond scale. We conclude, based on the OA framework, that brain functioning is best conceptualized in terms of continuity-Discreteness unity which is also the characteristic property of cognition. At the end we emphasize where one might productively proceed for the future research.
Fay Dowker - One of the best experts on this subject based on the ideXlab platform.
-
The random discrete action for two-dimensional spacetime
Classical and Quantum Gravity, 2011Co-Authors: Dionigi M T Benincasa, Fay Dowker, Bernhard SchmitzerAbstract:A one-parameter family of random variables, called the Discrete Action, is defined for a 2-dimensional Lorentzian spacetime of finite volume. The single parameter is a Discreteness scale. The expectation value of this Discrete Action is calculated for various regions of 2D Minkowski spacetime, M 2. When a causally convex region of M 2 is divided into subregions using null lines the mean of the Discrete Action is equal to the alternating sum of the numbers of vertices, edges and faces of the null tiling, up to corrections that tend to zero as the Discreteness scale is taken to zero. This result is used to predict that the mean of the Discrete Action of the flat Lorentzian cylinder is zero up to corrections, which is verified. The " topological " character of the Discrete Action breaks down for causally convex regions of the flat trousers spacetime that contain the singularity and for non-causally convex rectangles.
-
energy momentum diffusion from spacetime Discreteness
Physical Review D, 2009Co-Authors: Lydia Philpott, Fay Dowker, Rafael D SorkinAbstract:We study potentially observable consequences of spatiotemporal Discreteness for the motion of massive and massless particles. First we describe some simple models for the motion of a massive point particle in a fixed causal set background. If the causal set is faithfully embeddable in Minkoswki spacetime, the models give rise to particle motion in the continuum spacetime. At large scales, the microscopic swerves induced by the underlying atomicity manifest themselves as a Lorentz invariant diffusion in energy-momentum governed by a single phenomenological parameter, and we derive in full the corresponding diffusion equation. Inspired by the simplicity of the result, we then derive the most general Lorentz invariant diffusion equation for a massless particle, which turns out to contain two phenomenological parameters describing, respectively, diffusion and drift in the particle's energy. The particles do not leave the light cone however: their worldlines continue to be null geodesics. Finally, we deduce bounds on the drift and diffusion constants for photons from the blackbody nature of the spectrum of the cosmic microwave background radiation.
-
quantum gravity phenomenology lorentz invariance and Discreteness
Modern Physics Letters A, 2004Co-Authors: Fay Dowker, Joe Henson, Rafael D SorkinAbstract:Contrary to what is often stated, a fundamental spacetime Discreteness need not contradict Lorentz invariance. A causal set's Discreteness is in fact locally Lorentz invariant, and we recall the re...
-
quantum gravity phenomenology lorentz invariance and Discreteness
arXiv: General Relativity and Quantum Cosmology, 2003Co-Authors: Fay Dowker, Joe Henson, Rafael D SorkinAbstract:Contrary to what is often stated, a fundamental spacetime Discreteness need not contradict Lorentz invariance. A causal set's Discreteness is in fact locally Lorentz invariant, and we recall the reasons why. For illustration, we introduce a phenomenological model of massive particles propagating in a Minkowski spacetime which arises from an underlying causal set. The particles undergo a Lorentz invariant diffusion in phase space, and we speculate on whether this could have any bearing on the origin of high energy cosmic rays.
Daniel Sudarsky - One of the best experts on this subject based on the ideXlab platform.
-
dark energy from quantum gravity Discreteness
Physical Review Letters, 2019Co-Authors: Alejandro Perez, Daniel SudarskyAbstract:We argue that Discreteness at the Planck scale (naturally expected to arise from quantum gravity) might manifest in the form of minute violations of energy-momentum conservation of the matter degrees of freedom when described in terms of (idealized) smooth fields on a smooth spacetime. In the context of applications to cosmology, such "energy diffusion" from the low energy matter degrees of freedom to the discrete structures underlying spacetime would lead to the emergence of an effective dark energy term in Einstein's equations. We estimate this effect using a (relational) hypothesis about the materialization of Discreteness in quantum gravity which is motivated by the strict observational constraints supporting the validity of Lorentz invariance at low energies. Arguments based on a simple dimensional analysis lead to an estimate of an effective cosmological constant agreeing in order of magnitude with its observed value. If correct, this would constitute remarkable empirical evidence for a Planckian granular aspect of spacetime.
-
Dark energy from quantum gravity Discreteness
Physical Review Letters, 2019Co-Authors: Alejandro Perez, Daniel SudarskyAbstract:We argue that Discreteness at the Planck scale (naturally expected to arise from quantum gravity) might manifest in the form of minute violations of energy-momentum conservation of the matter degrees of freedom when described in terms of (idealized) smooth fields on a smooth spacetime. In the context of applications to cosmology such 'energy diffusion' from the low energy matter degrees of freedom to the discrete structures underlying spacetime leads to the emergence of an effective dark energy term in Einstein's equations. We estimate this effect using a (relational) hypothesis about the materialization of Discreteness in quantum gravity which is motivated by the strict observational constraints supporting the validity of Lorentz invariance at low energies. The predictions coming from simple dimensional analysis yield a cosmological constant of the order of magnitude of the observed value without fine tuning.
Alejandro Perez - One of the best experts on this subject based on the ideXlab platform.
-
dark energy from quantum gravity Discreteness
Physical Review Letters, 2019Co-Authors: Alejandro Perez, Daniel SudarskyAbstract:We argue that Discreteness at the Planck scale (naturally expected to arise from quantum gravity) might manifest in the form of minute violations of energy-momentum conservation of the matter degrees of freedom when described in terms of (idealized) smooth fields on a smooth spacetime. In the context of applications to cosmology, such "energy diffusion" from the low energy matter degrees of freedom to the discrete structures underlying spacetime would lead to the emergence of an effective dark energy term in Einstein's equations. We estimate this effect using a (relational) hypothesis about the materialization of Discreteness in quantum gravity which is motivated by the strict observational constraints supporting the validity of Lorentz invariance at low energies. Arguments based on a simple dimensional analysis lead to an estimate of an effective cosmological constant agreeing in order of magnitude with its observed value. If correct, this would constitute remarkable empirical evidence for a Planckian granular aspect of spacetime.
-
Dark energy from quantum gravity Discreteness
Physical Review Letters, 2019Co-Authors: Alejandro Perez, Daniel SudarskyAbstract:We argue that Discreteness at the Planck scale (naturally expected to arise from quantum gravity) might manifest in the form of minute violations of energy-momentum conservation of the matter degrees of freedom when described in terms of (idealized) smooth fields on a smooth spacetime. In the context of applications to cosmology such 'energy diffusion' from the low energy matter degrees of freedom to the discrete structures underlying spacetime leads to the emergence of an effective dark energy term in Einstein's equations. We estimate this effect using a (relational) hypothesis about the materialization of Discreteness in quantum gravity which is motivated by the strict observational constraints supporting the validity of Lorentz invariance at low energies. The predictions coming from simple dimensional analysis yield a cosmological constant of the order of magnitude of the observed value without fine tuning.
-
Black Holes in Loop Quantum Gravity
Rept.Prog.Phys., 2017Co-Authors: Alejandro PerezAbstract:This is a review of results on black hole physics in the context of loop quantum gravity. The key feature underlying these results is the Discreteness of geometric quantities at the Planck scale predicted by this approach to quantum gravity. Quantum Discreteness follows directly from the canonical quantization prescription when applied to the action of general relativity that is suitable for the coupling of gravity with gauge fields, and especially with fermions. Planckian Discreteness and causal considerations provide the basic structure for the understanding of the thermal properties of black holes close to equilibrium. Discreteness also provides a fresh new look at more (at the moment) speculative issues, such as those concerning the fate of information in black hole evaporation. The hypothesis of Discreteness leads, also, to interesting phenomenology with possible observational consequences. The theory of loop quantum gravity is a developing program, this review reports its achievements and open questions in a pedagogical manner, with an emphasis on quantum aspects of black hole physics.