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6 pages, 241 KiB

Open AccessCommunication

Bekenstein Bound and Non-Commutative Canonical Variables

by Fabio Scardigli

Universe 2022, 8(12), 645; https://doi.org/10.3390/universe8120645 - 05 Dec 2022

Viewed by 997

Abstract

A universal upper limit on the entropy contained in a localized quantum system of a given size and total energy is expressed by the so-called Bekenstein bound. In a previous paper [Buoninfante, L. et al. 2022], on the basis of general thermodynamic arguments, [...] Read more.

A universal upper limit on the entropy contained in a localized quantum system of a given size and total energy is expressed by the so-called Bekenstein bound. In a previous paper [Buoninfante, L. et al. 2022], on the basis of general thermodynamic arguments, and in regimes where the equipartition theorem still holds, the Bekenstein bound has been proved practically equivalent to the Heisenberg uncertainty relation. The smooth transition between the Bekenstein bound and the holographic bound suggests a new pair of canonical non-commutative variables, which could be thought to hold in strong gravity regimes. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

12 pages, 329 KiB

Open AccessArticle

Unruh Effect for Mixed Neutrinos and the KMS Condition

by Massimo Blasone, Gaetano Lambiase, Giuseppe Gaetano Luciano and Luciano Petruzziello

Universe 2022, 8(6), 306; https://doi.org/10.3390/universe8060306 - 28 May 2022

Cited by 1 | Viewed by 1431

Abstract

The quantization of mixed (neutrino) fields in an accelerated background reveals a non-thermal nature for Unruh radiation, which can be fitted by a Tsallis-like distribution function. However, for relativistic flavor neutrinos, which are represented by the standard Pontecorvo states, such a correction turns [...] Read more.

The quantization of mixed (neutrino) fields in an accelerated background reveals a non-thermal nature for Unruh radiation, which can be fitted by a Tsallis-like distribution function. However, for relativistic flavor neutrinos, which are represented by the standard Pontecorvo states, such a correction turns out to be negligible and thermality is restored. We show that the usage of Pontecorvo states for the calculation of the decay rate of an accelerated proton in the laboratory and comoving frames leads to consistent results and correctly implements the KMS thermal condition. Thus, the employment of these states in the above framework is not at odds with the principle of general covariance, in contrast to recent claims in the literature. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

10 pages, 603 KiB

Open AccessArticle

Are Quantum-Classical Hybrids Compatible with Ontological Cellular Automata?

by Hans-Thomas Elze

Universe 2022, 8(4), 207; https://doi.org/10.3390/universe8040207 - 26 Mar 2022

Cited by 1 | Viewed by 1348

Abstract

Based on the concept of ontological states and their dynamical evolution by permutations, as assumed in the Cellular Automaton Interpretation (CAI) of quantum mechanics, we address the issue of whether quantum-classical hybrids can be described consistently in this framework. We consider chains of [...] Read more.

Based on the concept of ontological states and their dynamical evolution by permutations, as assumed in the Cellular Automaton Interpretation (CAI) of quantum mechanics, we address the issue of whether quantum-classical hybrids can be described consistently in this framework. We consider chains of ‘classical’ two-state Ising spins and their discrete deterministic dynamics as an ontological model with an unitary evolution operator generated by pair-exchange interactions. A simple error mechanism is identified, which turns them into quantum mechanical objects: chains of qubits. Consequently, an interaction between a quantum mechanical and a ‘classical’ chain can be introduced and its consequences for this quantum-classical hybrid can be studied. We found that such hybrid character of composites, generally, does not persist under interactions and, therefore, cannot be upheld consistently, or even as a fundamental notion à la Kopenhagen interpretation, within CAI. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

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9 pages, 372 KiB

Open AccessEditor’s ChoiceArticle

On the Inner Horizon Instability of Non-Singular Black Holes

by Francesco Di Filippo, Raúl Carballo-Rubio, Stefano Liberati, Costantino Pacilio and Matt Visser

Universe 2022, 8(4), 204; https://doi.org/10.3390/universe8040204 - 25 Mar 2022

Cited by 14 | Viewed by 1832

Abstract

Regular black holes represent a conservative model in which the classical singularity is replaced by a non-singular core without necessarily modifying the spacetime outside the trapping horizon. Given the possible lack of phenomenological signatures, it is crucial to study the consistency of the [...] Read more.

Regular black holes represent a conservative model in which the classical singularity is replaced by a non-singular core without necessarily modifying the spacetime outside the trapping horizon. Given the possible lack of phenomenological signatures, it is crucial to study the consistency of the model. In this short work, we review the physical mechanism leading to the instability of the central core, arguing that that non-perturbative backreation is non-negligible and must be taken into account to provide a meaningful description of physical black holes. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

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19 pages, 350 KiB

Open AccessArticle

Gravitational Waves: The Theorist’s Swiss Knife

by Mairi Sakellariadou

Universe 2022, 8(2), 132; https://doi.org/10.3390/universe8020132 - 19 Feb 2022

Cited by 4 | Viewed by 1393

Abstract

Gravitational waves provide a novel and powerful way to test astrophysical models of compact objects, early universe processes, beyond the Standard Model particle physics, dark matter candidates, Einstein’s theory of General Relativity and extended gravity models, and even quantum gravity candidate theories. A [...] Read more.

Gravitational waves provide a novel and powerful way to test astrophysical models of compact objects, early universe processes, beyond the Standard Model particle physics, dark matter candidates, Einstein’s theory of General Relativity and extended gravity models, and even quantum gravity candidate theories. A short introduction to the gravitational-wave background and the method we are using to detect it will be presented. Constraints on various astrophysical/cosmological models from the non-detectability of the gravitational-wave background will be discussed. Gravitational waves from transients will be highlighted and their physical implications will be summarised. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

11 pages, 587 KiB

Open AccessEditor’s ChoiceArticle

On the Significance of Interferometric Revivals for the Fundamental Description of Gravity

by Kirill Streltsov, Julen Simon Pedernales and Martin Bodo Plenio

Universe 2022, 8(2), 58; https://doi.org/10.3390/universe8020058 - 18 Jan 2022

Cited by 24 | Viewed by 3107

Abstract

We show that an interaction between a harmonic oscillator and a two-level test mass (TLTM) mediated by a local operations and classical communication (LOCC) channel produces a signature that in (D. Carney et al., PRX Quantum 2, 030330 (2021)) is claimed to be [...] Read more.

We show that an interaction between a harmonic oscillator and a two-level test mass (TLTM) mediated by a local operations and classical communication (LOCC) channel produces a signature that in (D. Carney et al., PRX Quantum 2, 030330 (2021)) is claimed to be exclusively reserved for channels that can transmit quantum information. We provide an explicit example based on a measurement-and-feedback channel, explain where the proof of Carney et al. fails, discuss to what degree setups of this type can test the nature of the gravitational interaction and remark on some fundamental implications that an LOCC model of gravity may have in black hole physics. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

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11 pages, 282 KiB

Open AccessArticle

Quantum Spacetime, Noncommutative Geometry and Observers

by Fedele Lizzi

Universe 2022, 8(1), 24; https://doi.org/10.3390/universe8010024 - 31 Dec 2021

Viewed by 1037

Abstract

I discuss some issues related to the noncommutative spaces

κ

and its angular variant

ρ

-Minkowski with particular emphasis on the role of observers. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

9 pages, 292 KiB

Open AccessCommunication

Position in Minimal Length Quantum Mechanics

by Pasquale Bosso

Universe 2022, 8(1), 17; https://doi.org/10.3390/universe8010017 - 29 Dec 2021

Cited by 2 | Viewed by 1286

Abstract

Several approaches to quantum gravity imply the presence of a minimal measurable length at high energies. This is in tension with the Heisenberg Uncertainty Principle. Such a contrast is then considered in phenomenological approaches to quantum gravity by introducing a minimal length in [...] Read more.

Several approaches to quantum gravity imply the presence of a minimal measurable length at high energies. This is in tension with the Heisenberg Uncertainty Principle. Such a contrast is then considered in phenomenological approaches to quantum gravity by introducing a minimal length in quantum mechanics via the Generalized Uncertainty Principle. Several features of the standard theory are affected by such a modification. For example, position eigenstates are no longer included in models of quantum mechanics with a minimal length. Furthermore, while the momentum-space description can still be realized in a relatively straightforward way, the (quasi-)position representation acquires numerous issues. Here, we will review such issues, clarifying aspects regarding models with a minimal length. Finally, we will consider the effects of such models on simple quantum mechanical systems. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

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10 pages, 264 KiB

Open AccessCommunication

Jordan and Einstein Frames Hamiltonian Analysis for FLRW Brans-Dicke Theory

by Matteo Galaverni and Gabriele Gionti S. J.

Universe 2022, 8(1), 14; https://doi.org/10.3390/universe8010014 - 27 Dec 2021

Cited by 4 | Viewed by 1631

Abstract

We analyze the Hamiltonian equivalence between Jordan and Einstein frames considering a mini-superspace model of the flat Friedmann–Lemaître–Robertson–Walker (FLRW) Universe in the Brans–Dicke theory. Hamiltonian equations of motion are derived in the Jordan, Einstein, and anti-gravity (or anti-Newtonian) frames. We show that, when [...] Read more.

We analyze the Hamiltonian equivalence between Jordan and Einstein frames considering a mini-superspace model of the flat Friedmann–Lemaître–Robertson–Walker (FLRW) Universe in the Brans–Dicke theory. Hamiltonian equations of motion are derived in the Jordan, Einstein, and anti-gravity (or anti-Newtonian) frames. We show that, when applying the Weyl (conformal) transformations to the equations of motion in the Einstein frame, we did not obtain the equations of motion in the Jordan frame. Vice-versa, we re-obtain the equations of motion in the Jordan frame by applying the anti-gravity inverse transformation to the equations of motion in the anti-gravity frame. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

6 pages, 329 KiB

Open AccessCommunication

Quantum Black Holes and (Re)Solution of the Singularity

by Roberto Casadio

Universe 2021, 7(12), 478; https://doi.org/10.3390/universe7120478 - 04 Dec 2021

Cited by 11 | Viewed by 1835

Abstract

Classical general relativity predicts the occurrence of spacetime singularities under very general conditions. Starting from the idea that the spacetime geometry must be described by suitable states in the complete quantum theory of matter and gravity, we shall argue that this scenario cannot [...] Read more.

Classical general relativity predicts the occurrence of spacetime singularities under very general conditions. Starting from the idea that the spacetime geometry must be described by suitable states in the complete quantum theory of matter and gravity, we shall argue that this scenario cannot be realised physically since no proper quantum state may contain the infinite momentum modes required to resolve the singularity. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

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Review

Jump to: Research

50 pages, 545 KiB

Open AccessReview

Introduction to Quantization of Conformal Gravity

by Lesław Rachwał

Universe 2022, 8(4), 225; https://doi.org/10.3390/universe8040225 - 06 Apr 2022

Cited by 4 | Viewed by 1591

Abstract

A method for consistent quantization of conformal gravity treating conformal symmetry in a very controllable way is presented. First, we discuss local conformal symmetry in the framework of gravitational interactions, where we view it as an example of a general gauge theory. We [...] Read more.

A method for consistent quantization of conformal gravity treating conformal symmetry in a very controllable way is presented. First, we discuss local conformal symmetry in the framework of gravitational interactions, where we view it as an example of a general gauge theory. We also present some early attempts at quantization of conformal gravity and use the generalized framework of covariant quantization due to Faddeev and Popov. Some salient issues such as the need for conformal gauge-fixing, an issue with conformal third ghosts, and discontinuities in conformal gravity are studied as well. Finally, we provide some explanations of the original ad hoc methods of computation valid at the first quantum loop level in conformal gravity. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

13 pages, 312 KiB

Open AccessReview

Quantum Gravity Phenomenology from the Thermodynamics of Spacetime

by Ana Alonso-Serrano and Marek Liška

Universe 2022, 8(1), 50; https://doi.org/10.3390/universe8010050 - 13 Jan 2022

Cited by 1 | Viewed by 1958

Abstract

This work is based on the formalism developed in the study of the thermodynamics of spacetime used to derive Einstein equations from the proportionality of entropy within an area. When low-energy quantum gravity effects are considered, an extra logarithmic term in the area [...] Read more.

This work is based on the formalism developed in the study of the thermodynamics of spacetime used to derive Einstein equations from the proportionality of entropy within an area. When low-energy quantum gravity effects are considered, an extra logarithmic term in the area is added to the entropy expression. Here, we present the derivation of the quantum modified gravitational dynamics from this modified entropy expression and discuss its main features. Furthermore, we outline the application of the modified dynamics to cosmology, suggesting the replacement of the Big Bang singularity with a regular bounce. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

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15 pages, 3299 KiB

Open AccessReview

From Quantum Codes to Gravity: A Journey of Gravitizing Quantum Mechanics

by Chun-Jun Cao

Universe 2022, 8(1), 1; https://doi.org/10.3390/universe8010001 - 21 Dec 2021

Viewed by 2422

Abstract

In this note, I review a recent approach to quantum gravity that “gravitizes” quantum mechanics by emerging geometry and gravity from complex quantum states. Drawing further insights from tensor network toy models in AdS/CFT, I propose that approximate quantum error correction codes, when [...] Read more.

In this note, I review a recent approach to quantum gravity that “gravitizes” quantum mechanics by emerging geometry and gravity from complex quantum states. Drawing further insights from tensor network toy models in AdS/CFT, I propose that approximate quantum error correction codes, when re-adapted into the aforementioned framework, also have promise in emerging gravity in near-flat geometries. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

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7 pages, 250 KiB

Open AccessReview

Spacetime Foam, Midisuperspace, and the Cosmological Constant

by Steven Carlip

Universe 2021, 7(12), 495; https://doi.org/10.3390/universe7120495 - 15 Dec 2021

Cited by 1 | Viewed by 1771

Abstract

Perhaps the cosmological constant really is huge at the Planck scale, but is “hidden” by Planck scale quantum fluctuations of spacetime. I briefly review this proposal and provide some evidence, coming from a simplified midisuperspace model, that an appropriate “foamy” structure can do [...] Read more.

Perhaps the cosmological constant really is huge at the Planck scale, but is “hidden” by Planck scale quantum fluctuations of spacetime. I briefly review this proposal and provide some evidence, coming from a simplified midisuperspace model, that an appropriate “foamy” structure can do the job of hiding a large cosmological constant, and can persist under time evolution. Full article

(This article belongs to the Special Issue The Quantum & The Gravity)

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