Editor’s Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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30 pages, 701 KiB  
Review
Dynamics of Fluids in the Cavity of a Rotating Body: A Review of Analytical Solutions
by Anatoly A. Gurchenkov and Ivan A. Matveev
Physics 2024, 6(1), 426-455; https://doi.org/10.3390/physics6010029 - 19 Mar 2024
Viewed by 879
Abstract
Since the middle of the 20th century, an understanding of the diversity of the natural magnetohydrodynamic phenomena surrounding us has begun to emerge. Magnetohydrodynamic nature manifests itself in such seemingly heterogeneous processes as the flow of water in the world’s oceans, the movements [...] Read more.
Since the middle of the 20th century, an understanding of the diversity of the natural magnetohydrodynamic phenomena surrounding us has begun to emerge. Magnetohydrodynamic nature manifests itself in such seemingly heterogeneous processes as the flow of water in the world’s oceans, the movements of Earth’s liquid core, the dynamics of the solar magnetosphere and galactic electromagnetic fields. Their close relationship and multifaceted influence on human life are becoming more and more clearly revealed. The study of these phenomena requires the development of theory both fundamental and analytical, unifying a wide range of phenomena, and specialized areas that describe specific processes. The theory of translational fluid motion is well developed, but for most natural phenomena, this condition leads to a rather limited model. The fluid motion in the cavity of a rotating body such that the Coriolis forces are significant has been studied much less. A distinctive feature of the problems under consideration is their significant nonlinearity, (i.e., the absence of a linear approximation that allows one to obtain nontrivial useful results). From this point of view, the studies presented here were selected. This review presents studies on the movements of ideal and viscous fluids without taking into account electromagnetic phenomena (non-conducting, non-magnetic fluid) and while taking them into account (conducting fluid). Much attention is payed to the macroscopic movements of sea water (conducting liquid) located in Earth’s magnetic field, which spawns electric currents and, as a result, an induced magnetic field. Exploring the processes of generating magnetic fields in the moving turbulent flows of conducting fluid in the frame of dynamic systems with distributed parameters allows better understanding of the origin of cosmic magnetic fields (those of planets, stars, and galaxies). Various approaches are presented for rotational and librational movements. In particular, an analytical solution of three-dimensional unsteady magnetohydrodynamic equations for problems in a plane-parallel configuration is presented. Full article
(This article belongs to the Section Classical Physics)
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26 pages, 1906 KiB  
Article
Emergent Flow Signal and the Colour String Fusion
by Daria Prokhorova and Evgeny Andronov
Physics 2024, 6(1), 264-289; https://doi.org/10.3390/physics6010019 - 20 Feb 2024
Viewed by 567
Abstract
In this study, we develop the colour string model of particle production, based on the multi-pomeron exchange scenario, to address the controversial origin of the flow signal measured in proton–proton inelastic interactions. Our approach takes into account the string–string interactions but does not [...] Read more.
In this study, we develop the colour string model of particle production, based on the multi-pomeron exchange scenario, to address the controversial origin of the flow signal measured in proton–proton inelastic interactions. Our approach takes into account the string–string interactions but does not include a hydrodynamic phase. We consider a comprehensive three-dimensional dynamics of strings that leads to the formation of strongly heterogeneous string density in an event. The latter serves as a source of particle creation. The string fusion mechanism, which is a major feature of the model, modifies the particle production and creates azimuthal anisotropy. Model parameters are fixed by comparing the model distributions with the ATLAS experiment proton–proton data at the centre-of-mass energy s=13 TeV. The results obtained for the two-particle angular correlation function, C(Δη,Δϕ), with Δη and Δϕ differences in, respectively, pseudorapidities and azimuthal angles between two particles, reveal the resonance contributions and the near-side ridge. Model calculations of the two-particle cumulants, c2{2}, and second order flow harmonic, v2{2}, also performed using the two-subevent method, are in qualitative agreement with the data. The observed absence of the away-side ridge in the model results is interpreted as an imperfection in the definition of the time for the transverse evolution of the string system. Full article
(This article belongs to the Special Issue Jean Cleymans A Life for Physics)
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13 pages, 375 KiB  
Article
Alternative Origin of Galactic Positrons Generated by Ultraperipheral Collisions of Cosmic Rays
by Dmitry Chernyshov, Vladimir Dogiel and Igor Dremin
Physics 2024, 6(1), 251-263; https://doi.org/10.3390/physics6010018 - 19 Feb 2024
Viewed by 575
Abstract
We suggest a new alternative model of positron origin in the Galaxy. It is shown in our model that interactions of the electromagnetic fields of colliding ions (ultraperipheral ion collisions) can contribute to the total production of Galactic positrons. The corresponding cross-section is [...] Read more.
We suggest a new alternative model of positron origin in the Galaxy. It is shown in our model that interactions of the electromagnetic fields of colliding ions (ultraperipheral ion collisions) can contribute to the total production of Galactic positrons. The corresponding cross-section is estimated by using the Born approximation and the equivalent photon method. This process of ion collisions dominates in the range of subrelativistic energies and produces positrons with energies of several MeV. Despite its low efficiency, as it requires more than 0.1 erg to produce a single positron, this process may be an effective source of positrons in the Galactic medium. Full article
(This article belongs to the Section High Energy Physics)
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14 pages, 360 KiB  
Article
Mass and Magnetic Moment of the Electron and the Stability of QED—A Critical Review
by Michael Bordag and Irina G. Pirozhenko
Physics 2024, 6(1), 237-250; https://doi.org/10.3390/physics6010017 - 18 Feb 2024
Viewed by 582
Abstract
The anomalous magnetic moment of the electron, first calculated by Schwinger, lowers the ground state energy of the electron in a weak magnetic field. It is a function of the field and changes signs for large fields, ensuring the stability of the ground [...] Read more.
The anomalous magnetic moment of the electron, first calculated by Schwinger, lowers the ground state energy of the electron in a weak magnetic field. It is a function of the field and changes signs for large fields, ensuring the stability of the ground state. This has been shown in the past 50 years in numerous papers. The corresponding corrections to the mass of the electron have also been investigated in strong fields using semiclassical methods. We critically review these developments and point out that the calculation for low-lying excited states raises questions. Also, we calculate the contribution from the tadpole diagram, the relevance of which was observed only quite recently. Full article
(This article belongs to the Special Issue 75 Years of the Casimir Effect: Advances and Prospects)
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10 pages, 305 KiB  
Communication
Towards Precision Muonic X-ray Measurements of Charge Radii of Light Nuclei
by Ben Ohayon, Andreas Abeln, Silvia Bara, Thomas Elias Cocolios, Ofir Eizenberg, Andreas Fleischmann, Loredana Gastaldo, César Godinho, Michael Heines, Daniel Hengstler, Guillaume Hupin, Paul Indelicato, Klaus Kirch, Andreas Knecht, Daniel Kreuzberger, Jorge Machado, Petr Navratil, Nancy Paul, Randolf Pohl, Daniel Unger, Stergiani Marina Vogiatzi, Katharina von Schoeler and Frederik Wautersadd Show full author list remove Hide full author list
Physics 2024, 6(1), 206-215; https://doi.org/10.3390/physics6010015 - 17 Feb 2024
Cited by 5 | Viewed by 911
Abstract
We, the QUARTET Collaboration, propose an experiment to measure the nuclear charge radii of light elements with up to 20 times higher accuracy. These are essential both for understanding nuclear physics at low energies, and for experimental and theoretical applications in simple atomic [...] Read more.
We, the QUARTET Collaboration, propose an experiment to measure the nuclear charge radii of light elements with up to 20 times higher accuracy. These are essential both for understanding nuclear physics at low energies, and for experimental and theoretical applications in simple atomic systems. Such comparisons advance the understanding of bound-state quantum electrodynamics and are useful for searching for new physics beyond the Standard Model. The energy levels of muonic atoms are highly susceptible to nuclear structure, especially to the mean square charge radius. The radii of the lightest nuclei (with the atomic number, Z=1,2) have been determined with high accuracy using laser spectroscopy in muonic atoms, while those of medium mass and above were determined using X-ray spectroscopy with semiconductor detectors. In this communication, we present a new experiment, aiming to obtain precision measurements of the radii of light nuclei 3Z10 using single-photon energy measurements with cryogenic microcalorimeters; a quantum-sensing technology capable of high efficiency with outstanding resolution for low-energy X-rays. Full article
(This article belongs to the Special Issue Precision Physics and Fundamental Physical Constants (FFK 2023))
13 pages, 3174 KiB  
Article
Dynamical Sensitivity of Three-Layer Micro Electromechanical Systems to the Optical Properties of the Intervening Liquid Layer
by Fatemeh Tajik and George Palasantzas
Physics 2023, 5(4), 1081-1093; https://doi.org/10.3390/physics5040070 - 21 Nov 2023
Viewed by 833
Abstract
Here, we investigate the actuation dynamics of a micro device with different intervening liquids between the actuating components under the influence of Casimir and dissipative hydrodynamic forces. This is enabled via phase space portraits, which demonstrate that by increasing the dielectric response of [...] Read more.
Here, we investigate the actuation dynamics of a micro device with different intervening liquids between the actuating components under the influence of Casimir and dissipative hydrodynamic forces. This is enabled via phase space portraits, which demonstrate that by increasing the dielectric response of the intervening layer the autonomous device may not come into stiction due to the decreasing in magnitude Casmir force. Unlike the micro devices that are placed in vacuum with an intervening liquid, the phase portraits show only a spiral trajectory which eventually stops at a rest position due to the strong energy dissipation by the position dependent hydrodynamic drag forces, even when considering sufficiently strong restoring forces. Moreover, it is feasible to expand the area of motion using intervening liquids with lower dynamic viscosity or increasing the slip length of the intervening fluid. Finally, under the influence of an external driven force, which is the realistic case for possible applications, the system can reach stable oscillation at larger separations with an amplitude higher for the liquid that led to lower Casimir and hydrodynamic forces. Hence, the results presented in this study are essential for studying the dynamical behavior of MEMS and their design in liquid environments. Full article
(This article belongs to the Special Issue 75 Years of the Casimir Effect: Advances and Prospects)
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20 pages, 1072 KiB  
Article
Optical Properties of Two Complementary Samples of Intermediate Seyfert Galaxies
by Benedetta Dalla Barba, Marco Berton, Luigi Foschini, Giovanni La Mura, Amelia Vietri and Stefano Ciroi
Physics 2023, 5(4), 1061-1080; https://doi.org/10.3390/physics5040069 - 14 Nov 2023
Cited by 1 | Viewed by 864
Abstract
We present first results of the analysis of optical spectra of two complementary samples of Seyfert galaxies (Seyferts). The first sample was extracted from a selection of the 4th Fermi Gamma-ray Large Area Telescope (4FGL) catalog and consists of 11 γ-ray-emitting jetted [...] Read more.
We present first results of the analysis of optical spectra of two complementary samples of Seyfert galaxies (Seyferts). The first sample was extracted from a selection of the 4th Fermi Gamma-ray Large Area Telescope (4FGL) catalog and consists of 11 γ-ray-emitting jetted Seyfert galaxies. The second one was extracted from the Swift-BAT AGN Spectroscopic Survey (BASS) and is composed of 38 hard-X-ray-selected active galactic nuclei (AGN). These two samples are complementary, with the former being expected to have smaller viewing angles, while the latter may include objects with larger viewing angles. We measured emission-line ratios to investigate whether the behavior of these Seyferts can be explained in terms of obscuration, as suggested by the Unified Model (UM) of AGN, or if there are intrinsic differences due to the presence of jets or outflows, or due to evolution. We found no indications of intrinsic differences. The UM remains the most plausible interpretation for these classes of objects, even if some results can be challenging for this model. Full article
(This article belongs to the Special Issue Spectral Line Shapes in Astrophysical and Laboratory Plasma 2023)
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13 pages, 379 KiB  
Article
Single-Spin Asymmetry of Neutrons in Polarized pA Collisions
by Boris Z. Kopeliovich, Irina K. Potashnikova and Iván Schmidt
Physics 2023, 5(4), 1048-1060; https://doi.org/10.3390/physics5040068 - 7 Nov 2023
Viewed by 715
Abstract
Absorptive corrections, which are known to suppress proton-neutron transitions with a large fractional momentum z1 in pp collisions, become dramatically strong on a nuclear target, and they push the partial cross sections of leading neutron production to the very periphery [...] Read more.
Absorptive corrections, which are known to suppress proton-neutron transitions with a large fractional momentum z1 in pp collisions, become dramatically strong on a nuclear target, and they push the partial cross sections of leading neutron production to the very periphery of the nucleus. The mechanism of the pion π and axial vector meson a1 interference, which successfully explains the observed single-spin asymmetry in a polarized ppnX, is extended to the collisions of polarized protons with nuclei. When corrected for nuclear effects, it explains the observed single-spin azimuthal asymmetry of neutrons that is produced in inelastic events, which is where the nucleus violently breaks up. This single-spin asymmetry is found to be negative and nearly atomic mass number A-independent. Full article
(This article belongs to the Section High Energy Physics)
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16 pages, 482 KiB  
Article
Casimir Effect Invalidates the Drude Model for Transverse Electric Evanescent Waves
by Galina L. Klimchitskaya and Vladimir M. Mostepanenko
Physics 2023, 5(4), 952-967; https://doi.org/10.3390/physics5040062 - 27 Sep 2023
Cited by 6 | Viewed by 1050
Abstract
We consider the Casimir pressure between two metallic plates and calculate the four contributions to it determined by the propagating and evanescent waves and by the transverse magnetic and transverse electric polarizations of the electromagnetic field. The range of interplate separations is considered [...] Read more.
We consider the Casimir pressure between two metallic plates and calculate the four contributions to it determined by the propagating and evanescent waves and by the transverse magnetic and transverse electric polarizations of the electromagnetic field. The range of interplate separations is considered where nearly the whole pressure has its origin in the electromagnetic response of conduction electrons. In the Casimir physics, this response is described either by the dissipative Drude model resulting in contradictions with the measurement data or by the experimentally consistent but dissipationless plasma model. It is shown that the total transverse magnetic contribution to the Casimir pressure due to both the propagating and evanescent waves and the transverse electric contribution due to only the propagating waves, computed by means of the Drude model, correlate well with the corresponding results obtained using the plasma model. We conclude that the disagreement between the theoretical predictions obtained using the Drude model and precision measurements of the Casimir force is not caused by the account of dissipation in itself, but arises from an incorrect description of the response of metals to the low-frequency transverse electric evanescent waves by this model. It is demonstrated that the Drude model has no supporting experimental evidence in the range of transverse electric evanescent waves, so that the above conclusion is consistent with all available information. The alternative test of the Drude model for the transverse electric evanescent waves suggested in the framework of classical electrodynamics is discussed. Full article
(This article belongs to the Special Issue 75 Years of the Casimir Effect: Advances and Prospects)
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32 pages, 2753 KiB  
Article
Is the Non-Pointness of the Electron Observable in e+e Annihilation at Center-of-Mass Energies 55–207 GeV?
by Yutao Chen, Chih-Hsun Lin, Minghui Liu, Alexander S. Sakharov, Jürgen Ulbricht and Jiawei Zhao
Physics 2023, 5(3), 752-783; https://doi.org/10.3390/physics5030048 - 10 Jul 2023
Cited by 1 | Viewed by 2018
Abstract
The experimental data from VENUS, TOPAS, OPAL, DELPHI, ALEPH and L3 Collaborations collected from 1989 to 2003 are applied to study the quantum electrodynamics (QED) framework through the direct contact interaction term approach, using the annihilation reaction [...] Read more.
The experimental data from VENUS, TOPAS, OPAL, DELPHI, ALEPH and L3 Collaborations collected from 1989 to 2003 are applied to study the quantum electrodynamics (QED) framework through the direct contact interaction term approach, using the annihilation reaction e+eγγ(γ). The analysis involves performing a χ2-test to detect the presence of an excited electron e*, and and evidence of non-point like behavior in the e+e annihilation zone. The analysis yields compelling results, showing a significant signal at a confidence level of approximately 5 standard deviations. These findings suggest the existence of an excited electron with a mass of 308 ± 14 GeV and indicate the presence of a contact interaction characterized by a cutoff scale of 1253.53 ± 226 GeV. Furthermore, the interpretation of the cutoff scale result in terms of a radius of (1.57 ± 0.07) × 10−17 cm raises an intriguing possibility regarding the electron’s non-pointness. Full article
(This article belongs to the Section High Energy Physics)
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48 pages, 511 KiB  
Review
Selected Topics of Social Physics: Nonequilibrium Systems
by Vyacheslav I. Yukalov
Physics 2023, 5(3), 704-751; https://doi.org/10.3390/physics5030047 - 6 Jul 2023
Cited by 1 | Viewed by 1712
Abstract
This paper is devoted to nonequilibrium systems in the physics approach to social systems. Equilibrium systems have been considered in the recenly published first part of the review. The style of the paper combines the features of a tutorial and a review, which, [...] Read more.
This paper is devoted to nonequilibrium systems in the physics approach to social systems. Equilibrium systems have been considered in the recenly published first part of the review. The style of the paper combines the features of a tutorial and a review, which, from one side, makes it simpler to read for nonspecialists aiming at grasping the basics of social physics, and from the other side, describes several rather recent original models containing new ideas that could be of interest to experienced researchers in the field. Full article
(This article belongs to the Section Statistical Physics and Nonlinear Phenomena)
46 pages, 498 KiB  
Review
Selected Topics of Social Physics: Equilibrium Systems
by Vyacheslav I. Yukalov
Physics 2023, 5(2), 590-635; https://doi.org/10.3390/physics5020041 - 2 Jun 2023
Cited by 3 | Viewed by 1950
Abstract
The paper gives an introduction to the physics approach to social systems providing the main definitions and notions used in the modeling of these systems. The behavior of social systems is illustrated by several quite simple, typical models. The present part considers equilibrium [...] Read more.
The paper gives an introduction to the physics approach to social systems providing the main definitions and notions used in the modeling of these systems. The behavior of social systems is illustrated by several quite simple, typical models. The present part considers equilibrium systems. Nonequilibrium systems will be presented in the second part of the review. The style of the paper combines the features of a tutorial and a survey, which, from one side, makes it simpler to read for nonspecialists aiming to grasp the basics of social physics, and from the other side, describes several rather recent original models containing new ideas that could be of interest to experienced researchers in the field. The selection of the material is limited and motivated by the author’s research interests. Full article
(This article belongs to the Section Statistical Physics and Nonlinear Phenomena)
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36 pages, 4465 KiB  
Article
Fluctuations-Induced Quantum Radiation and Reaction from an Atom in a Squeezed Quantum Field
by Matthew Bravo, Jen-Tsung Hsiang and Bei-Lok Hu
Physics 2023, 5(2), 554-589; https://doi.org/10.3390/physics5020040 - 24 May 2023
Viewed by 1452
Abstract
In this third of a series on quantum radiation, we further explore the feasibility of using the memories (non-Markovianity) kept in a quantum field to decipher certain information about the early universe. As a model study, we let a massless quantum field be [...] Read more.
In this third of a series on quantum radiation, we further explore the feasibility of using the memories (non-Markovianity) kept in a quantum field to decipher certain information about the early universe. As a model study, we let a massless quantum field be subjected to a parametric process for a finite time interval such that the mode frequency of the field transits from one constant value to another. This configuration thus mimics a statically-bounded universe, where there is an ‘in’ and an ‘out’ state with the scale factor approaching constants, not a continuously evolving one. The field subjected to squeezing by this process should contain some information of the process itself. If an atom is coupled to the field after the parametric process, its response will depend on the squeezing, and any quantum radiation emitted by the atom will carry this information away so that an observer at a much later time may still identify it. Our analyses show that (1) a remote observer cannot measure the generated squeezing via the radiation energy flux from the atom because the net radiation energy flux is canceled due to the correlation between the radiation field from the atom and the free field at the observer’s location. However, (2) there is a chance to identify squeezing by measuring the constant radiation energy density at late times. The only restriction is that this energy density is of the near-field nature and only an observer close to the atom can use it to unravel the information of squeezing. The second part of this paper focuses on (3) the dependence of squeezing on the functional form of the parametric process. By explicitly working out several examples, we demonstrate that the behavior of squeezing does reflect essential properties of the parametric process. Actually, striking features may show up in more complicated processes involving various scales. These analyses allow us to establish the connection between properties of a squeezed quantum field and details of the parametric process which performs the squeezing. Therefore, (4) one can construct templates to reconstitute the unknown parametric processes from the data of measurable quantities subjected to squeezing. In a sequel paper these results will be applied to a study of quantum radiations in cosmology. Full article
(This article belongs to the Special Issue Vacuum Fluctuations)
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0 pages, 714 KiB  
Article
The Asymmetric Dynamical Casimir Effect
by Matthew J. Gorban, William D. Julius, Patrick M. Brown, Jacob A. Matulevich and Gerald B. Cleaver
Physics 2023, 5(2), 398-422; https://doi.org/10.3390/physics5020029 - 11 Apr 2023
Cited by 3 | Viewed by 2328
Abstract
A mirror with time-dependent boundary conditions will interact with the quantum vacuum to produce real particles via a phenomenon called the dynamical Casimir effect (DCE). When asymmetric boundary conditions are imposed on the fluctuating mirror, the DCE produces an asymmetric spectrum of particles. [...] Read more.
A mirror with time-dependent boundary conditions will interact with the quantum vacuum to produce real particles via a phenomenon called the dynamical Casimir effect (DCE). When asymmetric boundary conditions are imposed on the fluctuating mirror, the DCE produces an asymmetric spectrum of particles. We call this the asymmetric dynamical Casimir effect (ADCE). Here, we investigate the necessary conditions and general structure of the ADCE through both a waves-based and a particles-based perspective. We review the current state of the ADCE literature and expand upon previous studies to generate new asymmetric solutions. The physical consequences of the ADCE are examined, as the imbalance of particles produced must be balanced with the subsequent motion of the mirror. The transfer of momentum from the vacuum to macroscopic objects is discussed. Full article
(This article belongs to the Special Issue Vacuum Fluctuations)
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29 pages, 721 KiB  
Review
Isospin-Symmetry Breaking within the Nuclear Shell Model: Present Status and Developments
by Nadezda A. Smirnova
Physics 2023, 5(2), 352-380; https://doi.org/10.3390/physics5020026 - 31 Mar 2023
Cited by 4 | Viewed by 2964
Abstract
The paper reviews the recent progress in the description of isospin-symmetry breaking within the nuclear shell model and applications to actual problems related to the structure and decay of exotic neutron-deficient nuclei and nuclei along the N=Z line, where N is [...] Read more.
The paper reviews the recent progress in the description of isospin-symmetry breaking within the nuclear shell model and applications to actual problems related to the structure and decay of exotic neutron-deficient nuclei and nuclei along the N=Z line, where N is the neutron number and Z the atomic number. The review recalls the fundamentals of the isospin formalism for two-nucleon and many-nucleon systems, including quantum numbers, the spectrum’s structure and selection rules for weak and electromagnetic transitions; and at the end, summarizes experimental signatures of isospin-symmetry breaking effects, which motivated efforts towards the creation of a relevant theoretical framework to describe those phenomena. The main approaches to construct accurate isospin-nonconserving Hamiltonians within the shell model are briefly described and recent advances in the description of the structure and (isospin-forbidden) decay modes of neutron-deficient nuclei are highlighted. The paper reviews major implications of the developed theoretical tools to (i) the fundamental interaction studies on nuclear decays and (ii) the estimation of the rates of nuclear reactions that are important for nuclear astrophysics. The shell model is shown to be one of the most suitable approaches to describing isospin-symmetry breaking in nuclear states at low energies. Further efforts in extending and refining the description to larger model spaces, and in developing first-principle theories to deal with isospin-symmetry breaking in many-nucleon systems, seem to be indispensable steps towards our better understanding of nuclear properties in the precision era. Full article
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11 pages, 790 KiB  
Article
Diffusion in Phase Space as a Tool to Assess Variability of Vertical Centre-of-Mass Motion during Long-Range Walking
by Nicolas Boulanger, Fabien Buisseret, Victor Dehouck, Frédéric Dierick and Olivier White
Physics 2023, 5(1), 168-178; https://doi.org/10.3390/physics5010013 - 5 Feb 2023
Viewed by 1496
Abstract
When a Hamiltonian system undergoes a stochastic, time-dependent anharmonic perturbation, the values of its adiabatic invariants as a function of time follow a distribution whose shape obeys a Fokker–Planck equation. The effective dynamics of the body’s centre-of-mass during human walking is expected to [...] Read more.
When a Hamiltonian system undergoes a stochastic, time-dependent anharmonic perturbation, the values of its adiabatic invariants as a function of time follow a distribution whose shape obeys a Fokker–Planck equation. The effective dynamics of the body’s centre-of-mass during human walking is expected to represent such a stochastically perturbed dynamical system. By studying, in phase space, the vertical motion of the body’s centre-of-mass of 25 healthy participants walking for 10 min at spontaneous speed, we show that the distribution of the adiabatic invariant is compatible with the solution of a Fokker–Planck equation with a constant diffusion coefficient. The latter distribution appears to be a promising new tool for studying the long-range kinematic variability of walking. Full article
(This article belongs to the Section Classical Physics)
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9 pages, 560 KiB  
Communication
Electron as a Tiny Mirror: Radiation from a Worldline with Asymptotic Inertia
by Michael R. R. Good and Yen Chin Ong
Physics 2023, 5(1), 131-139; https://doi.org/10.3390/physics5010010 - 28 Jan 2023
Cited by 6 | Viewed by 1703
Abstract
We present a moving mirror analog of the electron, whose worldline possesses asymptotic constant velocity with corresponding Bogoliubov β coefficients that are consistent with finite total emitted energy. Furthermore, the quantum analog model is in agreement with the total energy obtained by integrating [...] Read more.
We present a moving mirror analog of the electron, whose worldline possesses asymptotic constant velocity with corresponding Bogoliubov β coefficients that are consistent with finite total emitted energy. Furthermore, the quantum analog model is in agreement with the total energy obtained by integrating the classical Larmor power. Full article
(This article belongs to the Special Issue Vacuum Fluctuations)
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10 pages, 427 KiB  
Article
BFSS Matrix Model Cosmology: Progress and Challenges
by Suddhasattwa Brahma, Robert Brandenberger and Samuel Laliberte
Physics 2023, 5(1), 1-10; https://doi.org/10.3390/physics5010001 - 22 Dec 2022
Cited by 11 | Viewed by 1913
Abstract
We review a proposal to obtain an emergent metric space-time and an emergent early universe cosmology from the Banks–Fischler–Shenker–Susskind (BFSS) matrix model. Some challenges and directions for future research are outlined. Full article
(This article belongs to the Special Issue New Advances in Quantum Geometry)
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