Physics

13 pages, 315 KB

Open AccessArticle

On the Role of Chapman’s Hydrostatic Corona Model in Parker’s Hydrodynamic Solar Wind Theory

by Bhimsen K. Shivamoggi

Physics 2026, 8(3), 60; https://doi.org/10.3390/physics8030060 - 4 Aug 2026

Viewed by 203

The role of Chapman’s hydrostatic solar wind model (based on a hydrostatic force balance condition) in Parker’s hydrodynamic solar wind theory is investigated by invoking the de Laval nozzle analogy for the production of flow acceleration in the latter model. The action of [...] Read more.

The role of Chapman’s hydrostatic solar wind model (based on a hydrostatic force balance condition) in Parker’s hydrodynamic solar wind theory is investigated by invoking the de Laval nozzle analogy for the production of flow acceleration in the latter model. The action of solar gravity in Parker’s hydrodynamic solar wind model is shown to be geometrically equivalent to a renormalization of the actual wind channel area and the renormalization factor is exactly Chapman’s hydrostatic radial density profile, which is totally predicated on the hydrostatic force balance condition. This result appears to be traceable to the encapsulation of the solar gravity effects in Parker’s hydrodynamic solar wind model by Chapman’s hydrostatic corona model, even beyond the coronal base. Furthermore, this result is shown to be robust by considering both isothermal gas and polytropic gas models as well as an n-dimensional (

n =

1, 2, or 3) underlying space for the solar wind. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Lennart Stenflo on the Occasion of His 85th Birthday: Plasma Physics and Nonlinear Science)

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14 pages, 1145 KB

Open AccessArticle

Machine Learning for Dynamic Critical Exponent Estimation in the Three-Dimensional Ising Model with Limited Computational Resources

by Gerardo Alfonso Perez and Jaime Virgilio Colchero Paetz

Physics 2026, 8(3), 59; https://doi.org/10.3390/physics8030059 - 19 Jul 2026

Viewed by 335

Abstract

The dynamic critical exponent z describes the divergence of correlation times near a continuous phase transition. Obtaining accurate estimates of z from noisy time series is complex, especially when computational resources are limited. We compare machine learning approaches for estimating z in the [...] Read more.

The dynamic critical exponent z describes the divergence of correlation times near a continuous phase transition. Obtaining accurate estimates of z from noisy time series is complex, especially when computational resources are limited. We compare machine learning approaches for estimating z in the three-dimensional Ising model. Data are generated from Monte Carlo simulations on a standard laptop computer, requiring 15 to 20 min of runtime. Twelve time-series features are extracted from the simulation output. We compare linear regression, simple scaling, and ensemble methods. Linear regression achieves a root mean square error of 0.0164 on the validation set with a coefficient of determination

R^{2} = 0.974

. The integrated autocorrelation time

log τ

shows a near-perfect correlation with z (the correlation value

r = 0.992

), consistent with the system linear size scaling relation

τ \sim L^{z}

. Autocorrelation features at three lags considered show correlations with z between 0.72 and 0.87. Standardised coefficient analysis indicates that the standard deviation of successive differences and the standard deviation of magnetisation are the dominant predictors in the standardised model, with coefficients of

- 0.246

and

0.239

, respectively. Accurate exponent estimation is possible with minimal computational resources, and fluctuation-based features capture critical slowing down effectively. Full article

(This article belongs to the Section Statistical Physics and Nonlinear Phenomena)

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16 pages, 306 KB

Open AccessArticle

Rethinking the Hierarchy: On the Structural Relation Between Quantum and Classical Theories

by Alessandro Sergi, Agostino Migliore and Antonino Messina

Physics 2026, 8(3), 58; https://doi.org/10.3390/physics8030058 - 7 Jul 2026

Viewed by 876

Abstract

Quantum mechanics is among the most successful physical theories, yet its formulation and empirical testing rely on classical structures. Following Lev Landau and Niels Bohr, this reliance is not merely pragmatic: quantum observables acquire empirical meaning only relative to classical reference frames, and, [...] Read more.

Quantum mechanics is among the most successful physical theories, yet its formulation and empirical testing rely on classical structures. Following Lev Landau and Niels Bohr, this reliance is not merely pragmatic: quantum observables acquire empirical meaning only relative to classical reference frames, and, in practice, quantization starts from classical models. At the same time, the two domains display forms of mutual irreducibility: intrinsically quantum features (that is, spin and exchange statistics) have no counterpart in the phase-space ontology of classical point-particle mechanics, while classical trajectory chaos does not arise straightforwardly from unitary quantum evolution in closed systems. A hierarchy is commonly established between classical and quantum theories, namely, a claim of ontological and explanatory priority according to which quantum mechanics is fundamental and classical mechanics is only a limiting case. This claim is less secure than is often assumed; therefore, the traditional hierarchy deserves to be examined. In this paper, we argue that a quantum–classical framework provides an effective and structurally faithful representation of empirically accessible physical systems in regimes where quantum and classical degrees of freedom coexist within a single, consistent effective dynamical description. To give this point of view a firm theoretical basis, we discuss the quasi-Lie formal structure underlying quantum–classical hybrid dynamics, with applications ranging from gravity and condensed matter to open, driven systems in biology and complex media. Full article

(This article belongs to the Special Issue Quantum Theory 100 Years Later: Advances on Foundations and Applications)

16 pages, 312 KB

Open AccessArticle

Group SU(2) Irreducible Representations and Probability Distributions Describing the Density Matrices of Qubit States

by Margarita A. Man’ko and Vladimir I. Man’ko

Physics 2026, 8(3), 57; https://doi.org/10.3390/physics8030057 - 3 Jul 2026

Viewed by 369

Abstract

The general problem of describing quantum states not only by wave functions and density operators but also by probability distribution functions is discussed in this paper. The qubit-state density-matrix elements expressed in terms of probability distributions are connected with the irreducible representation of [...] Read more.

The general problem of describing quantum states not only by wave functions and density operators but also by probability distribution functions is discussed in this paper. The qubit-state density-matrix elements expressed in terms of probability distributions are connected with the irreducible representation of group

S U (2)

. The transform of the probability distributions corresponding to the unitary transform of the qubit state of the spin-

1 / 2

system is obtained and expressed in terms of matrix elements of the group

S U (2)

. The possibility to extend the introduced formalism to other quantum states is suggested. For qubit systems, the notion of density operators algebra and the relation with the probability representation of quantum states are discussed. The Schrödinger equation for qubit states is obtained as an equation for probabilities. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Viktor Dodonov—55 Years in Quantum Physics)

22 pages, 7455 KB

Open AccessArticle

Piezoelectric and Thermoelectric Analysis of a Multilayer Structure for a Hybrid Energy-Harvesting Application

by Imane Salhi, Yassine Tabbai, Abdelhadi Mortadi, Hajar Rejdali, Fouad Belhora and Abdelowahed Hajjaji

Physics 2026, 8(3), 56; https://doi.org/10.3390/physics8030056 - 3 Jul 2026

Viewed by 575

Abstract

A significant amount of mechanical and thermal energy is lost when typing on a laptop keyboard. To address this, hybrid energy harvesters must increase the generated power density and mitigate energy fluctuation issues. This paper explores the potential enhancement of energy harvesting by [...] Read more.

A significant amount of mechanical and thermal energy is lost when typing on a laptop keyboard. To address this, hybrid energy harvesters must increase the generated power density and mitigate energy fluctuation issues. This paper explores the potential enhancement of energy harvesting by combining thermoelectric and piezoelectric effects within a multilayered structure integrated into a laptop keyboard button. Through numerical simulation, the study assesses how these two behaviors can synergistically increase the power density generated by the hybrid device. The focus is on optimizing energy efficiency by harnessing the heat losses from integrated circuits and the mechanical stresses due to the act of typing. The point is to refine the design of such a system to maximize the conversion of ambient energy into electricity. The findings indicate that the hybrid structure combining both piezoelectric and thermoelectric effects, effectively captures energy from a laptop keyboard, producing a substantial amount of electricity. This investigation shows that the generator can produce up to 2.07 mW of power using PU-40%PZT as piezoelectric material and an additional 71.93 μW through the PEDOT: PSS as thermoelectric material from a single keystroke when pressed and heated. This study underscores the potential for improving energy-harvesting efficiency in laptop keyboards, contributing to more sustainable and energy-efficient electronic devices. Full article

(This article belongs to the Section Applied Physics)

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56 pages, 660 KB

Open AccessReview

Accelerated Expansion of the Universe as a Quantum Gravity Phenomenon

by Jan Novák and Oem Trivedi

Physics 2026, 8(3), 55; https://doi.org/10.3390/physics8030055 - 1 Jul 2026

Viewed by 569

Abstract

It has been known for about hundred years that the universe is expanding, but an impressive discovery came at the end of 20th century, when it was found that the universe is expanding with acceleration. Since then, many models have been developed in [...] Read more.

It has been known for about hundred years that the universe is expanding, but an impressive discovery came at the end of 20th century, when it was found that the universe is expanding with acceleration. Since then, many models have been developed in cosmology to explain this phenomenon. One of the possible elucidations is that the theory of gravity must be modified at the classical level. However, many such models have already been excluded by gravitational wave experiments. Therefore, one must put a more critical question: could accelerated expansion of the universe be a phenomenon of quantum gravity? Here, we review the basic models of how one explains the origin of dark energy or the cosmological constant in metastring theory, discrete approaches to quantum gravity, group field theory, non-commutative geometry, causal dynamical triangulation, asymptotic safety, and models based on holography and entropic gravity. At the end of a newly formed approach to the quantization of gravity, we mention the ring paradigm, which may, after application to cosmology, naturally model the late-time accelerated expansion epoch in the universe. What is most remarkable, though, is that the final formulation of this theory may ultimately solve the old problem of the cosmological constant. Full article

(This article belongs to the Special Issue Beyond the Standard Models of Physics and Cosmology: 2nd Edition)

20 pages, 1138 KB

Open AccessArticle

Bose–Fermi Mapping in Hubbard Models at Imaginary Chemical Potential and Phase-Induced Fermionization

by Evangelos Georgios Filothodoros

Physics 2026, 8(3), 54; https://doi.org/10.3390/physics8030054 - 1 Jul 2026

Viewed by 384

Abstract

A formal thermodynamic mapping is established between the attractive Fermi–Hubbard model and the repulsive Bose–Hubbard model at finite temperature and at imaginary chemical potential

μ = i θ

. By utilizing a large N-expansion, it is shown that the partition functions of [...] Read more.

A formal thermodynamic mapping is established between the attractive Fermi–Hubbard model and the repulsive Bose–Hubbard model at finite temperature and at imaginary chemical potential

μ = i θ

. By utilizing a large N-expansion, it is shown that the partition functions of the two models are related by a plain shift

θ \to θ + π

. This condition maps the BCS–BEC crossover of attractive fermions to a Bose–Fermi crossover (fermion-like occupation) of repulsive bosons. A central feature of this correspondence is the thermal kernel

g (β E, ϕ)

(with

β

the inverse absolute temperature, E the energy scale, and

ϕ

the phase angle), whose analytic continuation

g_{B} (β E, ϕ) = g_{F} (β E, ϕ + π)

governs the bosonic (B) and fermionic (F) sectors. Interestingly, the particular angles

ϕ = 2 π / 3

and

4 π / 3

for fermions correspond to

ϕ = π / 3

and

5 π / 3

for bosons, marking the boundaries of an universal thermal window. It is further argued that the present mechanism shows how an emergent, fermionization-like phenomenon can occur at finite interaction strength through a thermodynamic effect induced by the imaginary chemical potential. It is emphasized that this does not imply a transmutation of quantum statistics at the operator level, but rather a thermodynamic exclusion-like behavior driven by the imaginary chemical potential, unlike the Tonks–Girardeau limit, where fermionization arises from an infinite repulsive interaction and anyonic or Floquet-engineered systems where transmutation emerges from modified statistics or dynamics. Effectively, the phase

ϕ

is a statistical parameter; by twisting the thermal phase, it generates fermion-like behavior without hard-core constraints or infinite repulsion through purely thermodynamic mechanisms. The gap equation and number equation for the bosonic model are derived, highlighting the role of the imaginary chemical potential as a statistical regulator. The results obtained here provide a unified framework for understanding crossovers in interacting lattice systems. Full article

(This article belongs to the Section Statistical Physics and Nonlinear Phenomena)

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