Virtual Black Holes Research Articles

Virtual acoustic black holes as a means to achieve low-frequency vibration reduction

Acoustic black holes (ABH) offer exceptional broadband vibration capabilities but have the inherent drawback of being only effective beyond a specific cut-off frequency. The most straightforward approach to set this threshold to a low value is to increase the ABH length. However, such a strategy leads to a very thin tip of the ABH, making it very complicated if not impossible to manufacture. Improving this ABH property has been for the past decade an active field of research. Recently, the concept of a virtual acoustic black (VABH) has been proposed by the authors. The key idea of this approach is to implement digitally the mechanical impedance of the ABH. The virtual feature of this strategy enables the implementation of any kind of ABH (any dimensions, material). Setting the cut-off frequency to low values thus becomes possible. The VABH is implemented through the use of piezoelectric patches, making it very compact and autonomous. Experimental demonstration shows that the proposed idea is able to mitigate vibration below 100 Hz.

Acoustic pressure modulation driven by spatially non-uniform flow

The recent identification of a modulation of acoustic waves that is driven by spatial velocity gradients, using acoustic black and white hole analogues [see Schenke, Sewerin, van Wachem, and Denner, J. Acoust. Soc. Am. 154, 781–791 (2023)], has shed new light on the complex interplay of acoustic waves and non-uniform flows. According to the virtual acoustic black hole hypothesis, these findings should be applicable to acoustic waves propagating in non-uniform flows of arbitrary velocity. In this study, the propagation of acoustic waves in non-uniform flows is investigated by incorporating a leading-order model of acoustic pressure modulation into a Lagrangian wave tracking algorithm. Using this numerical method, the acoustic pressure modulation is recovered accurately in non-uniform subsonic flows. This suggests that spatial velocity gradients drive acoustic pressure modulations in any non-uniform flow, which can, as shown here, be readily quantified.

Realization of an autonomous virtual acoustic black hole with piezoelectric patches

Acoustic black holes (ABHs) offer new opportunities for designing mechanical devices that can trap and reduce the vibrational energy of a system. This paper proposes the digital realization of the ABH effect, also called virtual ABH (VABH), through piezoelectric patches. A self-contained and autonomous reduction vibration device is thus developed. However, piezoelectric VABHs raise theoretical and experimental difficulties which are discussed herein. An improved pseudo-collocated approach is proposed, and the synthetic impedance is theoretically derived. Experiments are conducted using a cantilever beam where the VABH is implemented with few piezoelectric patches. It is shown to provide excellent vibration reduction over a large frequency range. The herein presented original concept solves the two long-lasting challenges of mechanical ABHs, i.e, its manufacturing and inability to operate at low frequencies, making it highly attractive for applications on real-life structures.

A virtual acoustic black hole on a cantilever beam

An acoustic black hole (ABH) consists of a tapered structure whose thickness follows a power-law profile. When attached to a host structure, an ABH localizes and traps the vibrational energy, which can then be dissipated through, e.g., a damping layer. However, effective vibration mitigation is known to occur only above a cut-on frequency which is inversely proportional to the length of the tapered structure. In this context, the main thrust of this paper is to replace a mechanical ABH by a digital controller so as to create a so-called virtual acoustic black hole (VABH), thus, freeing the ABH from possible mechanical constraints (e.g., compactness, manufacturing and fatigue issues). The proposed VABH is first detailed theoretically. The salient features and performance of the VABH are then demonstrated both numerically and experimentally using a cantilever beam as a host structure. Eventually, it is shown that the VABH significantly enlarges the applicability of the concept of an ABH.

Numerical realization of a semi-active virtual acoustic black hole effect

Noise mitigation by means of the acoustic black hole (ABH) effect is a well-known engineering solution. However, the conventional method of applying ABH effect which requires modification of the structure geometry has various limitations which encourage the research of virtual ABH concept. In this study, the effect of ABH was applied through introducing virtual stiffness by a shunt circuit. According to the force-voltage electric analogy, stiffness has an inverse relationship with capacitance. So that the ABH effect can be virtually realized by following a power law profile using an array of independent capacitive shunts. The concept is studied through finite element simulation developing a macro code in ANSYS Parametric Design Language (APDL). To evaluate the influence of capacitance profile on the acoustic radiated power, parametric studies are conducted. Based on the results of the parametric studies, the capacitance profile is tuned for minimum radiated power. It is revealed that the virtual acoustic black hole (ABH) effect can offer 10.29%, 6.37%, and 7.47% reduction in the radiated power from the first to the last targeted mode, respectively. The virtual ABH effect introduced in this study can be used for semi-active structural noise isolation without any weight or manufacturing penalty.

Gamma ray bursters and black holes in gravity’s rainbow

In this paper, we analyze the modification to the thermodynamics of a Schwarzschild black hole and a Kerr black hole due to gravity’s rainbow. The metric for these black holes will be made energy dependent. This will be done by using rainbow functions motivated from the hard spectra from gamma-ray bursters at cosmological distances. This modification of the metric by these rainbow functions will in turn modify the temperature and entropy of these black holes. We will also discuss how this affects the formation of virtual black holes.

Sterile neutrinos, black hole vacuum and holographic principle

We construct an effective field theory (EFT) model that describes matter field interactions with Schwarzschild mini-black-holes (SBH’s), treated as a scalar field, B_0(x). Fermion interactions with SBH’s require a complex spurion field, theta _{ij}, which we interpret as the EFT description of “holographic information,” which is correlated with the SBH as a composite system. We consider Hawking’s virtual black hole vacuum (VBH) as a Higgs phase, langle B_0 rangle =V. Integrating sterile neutrino loops, the information field theta _{ij} is promoted to a dynamical field, necessarily developing a tachyonic instability and acquiring a VEV of order the Planck scale. For N sterile neutrinos this breaks the vacuum to SU(N)times U(1)/SO(N) with N degenerate Majorana masses, and frac{1}{2}N(N+1) Nambu-Goldstone neutrino-Majorons. The model suggests many scalars fields, corresponding to all fermion bilinears, may exist bound nonperturbatively by gravity.

Inter-Legality and Surveillance Technologies: Looking at the Demands of Justice Beyond Borders

On the 19th of May, the German Federal Constitutional Court ruled that telecommunication surveillance of non-German individuals outside German territory violates the German Constitution. The reasoning of the Court entails a number of crucial questions both from the international and European human rights law perspective. The most important one being whether the German Federal Government is bound by the provisions of the German Constitution when it interferes with the rights of non-German individuals in a non-German territory. Relying on international human rights law, the Court answered affirmatively. The reasoning of the judgment has demonstrated a successful example of Inter-legality. Therefore, this paper aims at analyzing the judgment from such a perspective through a three-step analysis: Taking the vantage point of the affair – the case at hand - under scrutiny, understanding the relevant normativities controlling the case, looking at the demands of justice stemming from the case. It concludes that such an inter-legal reasoning provided the Court to close ‘virtual legal black holes’, and avoid injustice.

Neutrino decoherence from quantum gravitational stochastic perturbations

Neutrinos undergoing stochastic perturbations as they propagate experience decoherence, damping neutrino oscillations over distance. Such perturbations may result from fluctuations in space-time itself if gravity is a quantum force, including interactions between neutrinos and virtual black holes. In this work we model the influence of heuristic neutrino-virtual black hole interaction scenarios on neutrino propagation and evaluate the resulting signals in astrophysical and atmospheric neutrinos. We demonstrate how these effects can be represented in the framework of open quantum systems, allowing experimental constraints on such systems to be connected to quantum gravitational effects. Finally, we consider the energy-dependence of such Planck scale physics at energies observed in current neutrino experiments, and show that sensitivity to Planck scale physics well below the `natural' expectation is achievable in certain scenarios.

Novel dual relation and constant in Hawking-Page phase transitions

Universal relations and constants have important applications in understanding a physical theory. In this article, we explore this issue for Hawking-Page phase transitions in Schwarzschild anti-de Sitter black holes. We find a novel exact dual relation between the minimum temperature of the ($d$+1)-dimensional black hole and the Hawking-Page phase transition temperature in $d$ dimensions, reminiscent of the holographic principle. Furthermore, we find that the normalized Ruppeiner scalar curvature is a universal constant at the Hawking-Page transition point. Since the Ruppeiner curvature can be treated as an indicator of the intensity of the interactions amongst black hole microstructures, we conjecture that this universal constant denotes an interaction threshold, beyond which a virtual black hole becomes a real one. This new dual relation and universal constant are fundamental in understanding Hawking-Page phase transitions, and might have new important applications in the black hole physics in the near future.

Proton decay and the quantum structure of space–time

Virtual black holes in noncommutative space–time are investigated using coordinate coherent state formalism such that the event horizon of a black hole is manipulated by smearing it with a Gaussian of width [Formula: see text], where θ is the noncommutativity parameter. Proton lifetime, the main associated phenomenology of the noncommutative virtual black holes, has been studied, first in four-dimensional space–time and then generalized to D dimensions. The lifetime depends on θ and the number of space–time dimensions such that it emphasizes on the measurement of proton lifetime as a potential probe for the microstructure of space–time.

Virtual Black Holes and Space\u2013Time Structure

In the standard formalism of quantum gravity, black holes appear to form statistical distributions of quantum states. Now, however, we can present a theory that yields pure quantum states. It shows how particles entering a black hole can generate firewalls, which however can be removed, replacing them by the ‘footprints’ they produce in the out-going particles. This procedure can preserve the quantum information stored inside and around the black hole. We then focus on a subtle but unavoidable modification of the topology of the Schwarzschild metric: antipodal identification of points on the horizon. If it is true that vacuum fluctuations include virtual black holes, then the structure of space-time is radically different from what is usually thought.

ER = EPR and non-perturbative action integrals for quantum gravity

In this paper, we construct and calculate non-perturbative path integrals in a multiply-connected spacetime. This is done by summing over homotopy classes of paths. The topology of the spacetime is defined by Einstein–Rosen bridges (ERB) forming from the entanglement of quantum foam described by virtual black holes. As these “bubbles” are entangled, they are connected by Planckian ERBs because of the [Formula: see text] conjecture. Hence, the spacetime will possess a large first Betti number [Formula: see text]. For any compact 2-surface in the spacetime, the topology (in particular the homotopy) of that surface is non-trivial due to the large number of Planckian ERBs that define homotopy through this surface. The quantization of spacetime with this topology — along with the proper choice of the 2-surfaces — is conjectured to allow non-perturbative path integrals of quantum gravity theory over the spacetime manifold.

Virtual black holes in a third quantized formalism

In this paper, we will analyse virtual black holes using the third quantization formalism. As the virtual black hole model depends critically on the assumption that the quantum fluctuations dominate the geometry of spacetime at Planck scale, we will analyse the quantum fluctuations for a black hole using third quantization. We will demonstrate that these quantum fluctuations depend on the factor ordering chosen. So, we will show that only certain values of the factor ordering parameter are consistent with virtual black holes model of spacetime foam.

Virtual black holes from the generalized uncertainty principle and proton decay

We investigate the formation of virtual black holes in the context of the generalized uncertainty principle (GUP), as a mediator for a proton decay process which is forbidden by the standard model. Then, we calculate the lower bounds of the GUP deformation parameter by the experimental bound on the half-life of the proton.

∞-∞: Vacuum energy and virtual black holes

We discuss other contributions to the vacuum energy of quantum field theories and quantum gravity, which have not been considered in the literature. As is well known, the presence of virtual particles in vacuum provides the so famous and puzzling contributions to the vacuum energy. In fact, these mainly come from loop integrations over the four-momenta space. However, we argue that these also imply the presence of a mass density of virtual particles in every volume cell of space-time. The most important contribution comes from quantum gravity bubbles, corresponding to virtual black hole pairs. The presence of virtual masses could lead to another paradox: the space-time itself would have an intrinsic virtual mass density contribution leading to a disastrous contraction —as is known, no negative masses exist in general relativity. We dub this effect the cosmological problem of second type: if not other counter-terms existed, the vacuum energy would be inevitably destabilized by virtual-mass contributions. It would be conceivable that the cosmological problem of second type could solve the first one. Virtual masses renormalize the vacuum energy to an unpredicted parameter, as in the renormalization procedure of the Standard Model charges. In the limit of (Pauli-Villars limit), virtual black holes have a mass density providing an infinite counter-term to the vacuum energy divergent contribution (assuming ). Therefore, in the same Schwinger-Feynman-Tomonaga attitude, the problem of a divergent vacuum energy could be analogous to the put-by-hand procedure used for Standard Model parameters.

Black Hole Unitarity and Antipodal Entanglement

Hawking particles emitted by a black hole are usually found to have thermal spectra, if not exactly, then by a very good approximation. Here, we argue differently. It was discovered that spherical partial waves of in-going and out-going matter can be described by unitary evolution operators independently, which allows for studies of space-time properties that were not possible before. Unitarity dictates space-time, as seen by a distant observer, to be topologically non-trivial. Consequently, Hawking particles are only locally thermal, but globally not: we explain why Hawking particles emerging from one hemisphere of a black hole must be 100 % entangled with the Hawking particles emerging from the other hemisphere. This produces exclusively pure quantum states evolving in a unitary manner, and removes the interior region for the outside observer, while it still completely agrees locally with the laws of general relativity. Unitarity is a starting point; no other assumptions are made. Region I and the diametrically opposite region II of the Penrose diagram represent antipodal points in a PT or CPT relation, as was suggested before. On the horizon itself, antipodal points are identified. A candidate instanton is proposed to describe the formation and evaporation of virtual black holes of the type described here. Some important explanations and discussion points are added. In the latest of the paper, again some minor inaccuracies are corrected.

The Black Hole Information Paradox and the Collapse of the Wave Function

The black hole information paradox arises from an apparent conflict between the Hawking black hole radiation and the fact that time evolution in quantum mechanics is unitary. The trouble is that while the former suggests that information of a system falling into a black hole disappears, the latter implies that information must be conserved. In this work we discuss the current divergence in views regarding the paradox, we evaluate the role that objective collapse theories could play in its resolution and we propose a link between spontaneous collapse events and microscopic virtual black holes.

Benefits of Objective Collapse Models for Cosmology and Quantum Gravity

We display a number of advantages of objective collapse theories for the resolution of long-standing problems in cosmology and quantum gravity. In particular, we examine applications of objective reduction models to three important issues: the origin of the seeds of cosmic structure, the problem of time in quantum gravity and the information loss paradox; we show how reduction models contain the necessary tools to provide solutions for these issues. We wrap up with an adventurous proposal, which relates the spontaneous collapse events of objective collapse models to microscopic virtual black holes.

Some aspects of virtual black holes

We first consider consistently third-quantize modified gravity. We then analyze certain aspects of virtual black holes in this third-quantized modified gravity. We see how a statistical mechanical origin for the Bekenstein-Hawking entropy naturally arises in this model. Furthermore, the area and hence the entropy of a real macroscopic black hole is quantized in this model. Virtual black holes cause a loss of quantum coherence, which gives an intrinsic entropy to all physical systems that can be used to define a direction of time and hence provide a solution to the problem of time.