Searcharxiv⌕ Search

arXiv subjects

Gyula Bene

Publications and source records attributed to Gyula Bene.

At least 19 recordsLinked to original sources
arXiv2023

Nonlinear viscous gravity-capillary surface waves at arbitrary wavelengths

This paper presents the second-order perturbation theory of the Navier-Stokes equations for free surface flows, with the wave amplitude considered as the perturbation parameter. Gravity-capillary surface waves in incompressible viscous fluids are subjected. The results provide a systematic derivation of a nonlinear surface wave equation that fully takes into account dispersion, while nonlinearity is included in the leading order. However, the presence of infinitely many overdamped modes has been neglected, and only the two least damped modes are considered. Finally, to describe the elevations and evolution of the surface wave, we introduce a differential equation.

physics.flu-dyn↗
arXiv2023

Bifurcation Analysis and Propagation Conditions of Free-Surface Waves in Incompressible Viscous Fluids in Finite Depth

Viscous linear surface waves are studied at arbitrary wavelength, layer thickness, viscosity and surface tension. We find that in shallow enough fluids no surface waves can propagate. This layer thickness is determined for some fluids, water, glycerin and mercury. Even in any thicker fluid layers, propagation of very short and very long waves is forbidden. When wave propagation is possible, only a single propagating mode exists for a given horizontal wave number. In contrast, there are two types of non-propagating modes. One kind of them exists at all wavelength and material parameters, and there are infinitely many such modes for a given wave number, distinguished by their decay rates. The other kind of non-propagating mode that is less attenuated may appear in zero, one or two specimens. We notice the presence of two length scales as material parameters, one related to viscosity and the other to surface tension. We consider possible modes for a given material on the parameter plane layer thickness versus wave number and discuss bifurcations among different mode types. Motion of surface particles and time evolution of surface elevation is also studied at various parameters in glycerin, and a great variety of behaviour is found, including counterclockwise surface particle motion and negative group velocity in wave propagation.

physics.flu-dyn↗
arXiv2019

Investigating viscous surface wave propagation modes and study of nonlinearities in a finite depth fluid

The object of this study is to investigate the effect of viscosity on propagation of free-surface waves in an incompressible viscous fluid layer of arbitrary depth. While we provide a more detailed study of properties of linear surface waves, the description of fully nonlinear waves in terms of KdV-like equations is discussed. In the linear case, we find that in shallow enough fluids, no surface waves can propagate. Even in any thicker fluid layers, propagation of very short and very long waves is forbidden. When wave propagation is possible, only a single propagating mode exists for any given horizontal wave number. The numerical results show that there can be two types of non-propagating modes. One type is always present, and there exist always infinitely many of such modes at the same parameters. In contrast, there can be zero, one or two modes belonging to the other type. Another significant feature is that KdV-like equations describing propagating nonlinear viscous surface waves may not exist since viscosity gives rise to a new wave number that cannot be small at the same time as the original one. Nonetheless, we propose a reasonable nonlinear description in terms of 1+1 variate functions that makes possible successive approximations.

physics.flu-dyn↗
arXiv2010

Accelerating expansion in the swisscheese model

A version of the Swiss-cheese model is investigated. The flat Friedmann-Robertson-Walker (FRW) universe is modified by the addition of several spherical regions with Lemaitre-Tolman-Bondi metric. We discuss light propagation in this model in detail to pave the way for a detailed numerical study of the Hubble diagram.

astro-ph.CO↗
arXiv2010

Calculating the Hubble diagram by perturbation theory

The effect of density fluctuations upon light propagation is calculated perturbatively in a matter dominated irrotational universe. The starting point is the perturbed metric (second order in the perturbation strength), while the output is the Hubble diagram. Density fluctuations cause this diagram to broaden to a strip. Moreover, the shift of the diagram mimics accelerated expansion.

astro-ph.CO↗
arXiv2003

Accelerating expansion of the universe may be caused by inhomogeneities

We point out that, due to the nonlinearity of the Einstein equations, a homogeneous approximation in cosmology leads to the appearance of an additional term in the Friedmann equation. This new term is associated with the spatial inhomogeneities of the metric and can be expressed in terms of density fluctuations. Although it is not constant, it decays much slower (as t^(-2/3)) than the other terms (like density) which decrease as t^(-2). The presence of the new term leads to a correction in the scale factor that is proportional to t^2 and may give account of the recently observed accelerating expansion of the universe without introducing a cosmological constant.

astro-ph↗
arXiv2003

Breakdown of supersymmetry in homogeneous cosmologies in N=1 supergravity

A condition of supersymmetric cosmological solutions of simple (N=1) supergravity is formulated in the classical case. As an application we prove that supersymmetry is spontaneously broken in Friedmann-Robertson-Walker type cosmologies as well as in the Kasner universe, except for the Minkowski space.

gr-qc↗
arXiv2003

A numerical study of spectral properties of the area operator in loop quantum gravity

The lowest 37000 eigenvalues of the area operator in loop quantum gravity is calculated and studied numerically. We obtain an asymptotical formula for the eigenvalues as a function of their sequential number. The multiplicity of the lowest few hundred eigenvalues is also determined and the smoothed spectral density is calculated. The spectral density is presented for various number of vertices, edges and SU(2) representations. A scaling form of spectral density is found, being a power law for one vertex, while following an exponential for several vertices. The latter case is explained on the basis of the one vertex spectral density.

gr-qc↗
arXiv2001

A perspectival version of the modal interpretation of quantum mechanics and the origin of macroscopic behavior

We study the process of observation (measurement), within the framework of a `perspectival' (`relational', `relative state') version of the modal interpretation of quantum mechanics. We show that if we assume certain features of discreteness and determinism in the operation of the measuring device (which could be a part of the observer's nerve system), this gives rise to classical characteristics of the observed properties, in the first place to spatial localization. We investigate to what extent semi-classical behavior of the object system itself (as opposed to the observational system) is needed for the emergence of classicality. Decoherence is an essential element in the mechanism of observation that we assume, but it turns out that in our approach no environment-induced decoherence on the level of the object system is required for the emergence of classical properties.

quant-ph↗
arXiv2001

Relational modal interpretation for relativistic quantum field theories

The relational version of the modal interpretation offers both a consistent quantum ontology and solution for quantum paradoxes within the framework of nonrelativistic quantum mechanics. In the present paper this approach is generalized for the case of relativistic quantum field theories. Physical systems are defined as Hilbert spaces. The concept of the reduced density matrix is also generalized so that its trace may become smaller than one, expressing the possibility of annihilation. Superselection rules are shown to follow if the whole Universe has a definite electric charge, barionic number and leptonic number.

quant-ph↗
arXiv2001

Quantum origin of classical properties within the modal interpretations

An example is presented when decoherence and quantum interference gives rise to narrow eigenstates (in coordinate representation) for the reduced density matrix of macroscopic quantum systems. On the basis of modal interpretations this means the emergence of classical properties.

quant-ph↗
arXiv2001

Lowest threshold visibility for testing local realistic theories

Analytical and numerical arguments are presented in case of a pair of two-state systems in a singlet state that the threshold visibility for testing Bell's theorem on the entire range of measurement settings is 33.3%. It is also shown that no lower treshold exists.

quant-ph↗
arXiv2000

The Kohn mode for trapped Bose gases within the dielectric formalism

The presence of undamped harmonic center of mass oscillations of a weakly interacting Bose gas in a harmonic trap is demonstrated within the dielectric formalism for a previously introduced finite temperature approximation including exchange. The consistency of the approximation with the Kohn theorem is thereby demonstrated. The Kohn modes are found explicitly, generalizing an earlier zero-temperature result found in the literature. It is shown how the Kohn mode disappears from the single-particle spectrum, while remaining in the density oscillation spectrum, when the temperature increases from below to above the condensation temperature.

cond-mat.stat-mech↗
arXiv2000

Quantum reference systems: reconciling locality with quantum mechanics

The status of locality in quantum mechanics is analyzed from a nonstandard point of view. It is assumed that quantum states are relative, they depend on and are defined with respect to some bigger physical system which contains the former system as a subsystem. Hence, the bigger system acts as a reference system. It is shown that quantum mechanics can be reformulated in accordance with this new physical assumption. There is an important consequence of this dependence: states may not be comparable, i.e., they cannot be checked by suitable measurements simultaneously. This special circumstance is fully reflected mathematically by the theory. Especially, it is shown that certain joint probabilities (or the corresponding combined events) which play a vital role in any proof of Bell's theorem do not exist. The conclusion is that the principle of locality is fully valid in quantum mechanics, and one has to give up instead of locality an intuitively natural-looking feature of realism, namely, the comparability of existing states.

quant-ph↗
arXiv2000

Decoherence within a single atom

An "almost diagonal" reduced density matrix (in coordinate representation) is usually a result of environment induced decoherence and is considered the sign of classical behavior. We point out that the proton of a ground state hydrogen atom can possess such a density matrix. We demonstrate on this example that an "almost diagonal" reduced density matrix may derive from an interaction with a low number of degrees of freedom (in our case with a single electron) which play the role of the environment. We also show that decoherence effects in our example can only be observed if the interaction with the measuring device is significantly faster than the interaction with the environment (the electron). In the opposite case, when the interaction with the environment is significant during the measurement process, coherence is restored. Finally, we propose a neutron scattering experiment on cold He atoms to observe decoherence which shows up as an additional contribution to the differential scattering cross section.

quant-ph↗
arXiv2000

On the Thomas-Fermi approximation in the bulk of trapped Bose-Einstein condensed gases

Corrections to the Thomas-Fermi-type solution of the Gross-Pitaevskii equation are inevitable in order to get correctly the frequencies of the low lying modes out of the Bogolyubov equations. These corrections are important in the bulk, too, thus the failure of the Thomas-Fermi approximation is not confined to the surface. We discuss this effect quantitatively and consider similar phenomena of spin fluctuations in Bose-Einstein condensed gases in an optical trap.

cond-mat↗
arXiv1997

On the nature of the quantum states of macroscopic systems

It is assumed that the quantum state that may describe a macroscopic system at a given instant of time is one of the eigenstates of the reduced density matrix calculated from the wave function of the system plus its environment. This implies that the above quantum state is a member of a special orthonormed set of states. Using a suitable Monte-Carlo simulation, this property is shown to be consistent with the extremely small standard deviation for the coordinates and the momenta of macroscopic systems. Consequences for statistical mechanics and possible observable effects are discussed.

quant-ph↗