238 research outputs found
Probabilistic approach for analysis of strength of ceramics with different porous structure based on movable cellular automaton modeling
Movable cellular automaton method which is a computational method of particle mechanics is applied to simulating uniaxial compression of 3D porous ceramic specimens. Pores were considered explicitly by removing automata selected randomly from the original fcc packing. Distribution of pores in space, their size and the total fraction were varied. For each values of porosity there were generated several represented specimens with individual pore position in space. The resulting values of elastic modulus and strength of the specimens were scattered and well described by the Weibull distribution. We showed that to reveal dependence of the elastic and strength properties on porosity it is much better to consider not average of the values for the specimens of the same porosity, but the mathematical expectation of the corresponding Weibull distribution. It is shown that relation between mechanical properties of the material and its porosity depends significantly on pore structure. Namely, percolation transition from closed porosity to interconnected pores strongly manifests itself on strength dependence on porosity. Thus, the curve of strength versus porosity fits different equations for different kind of pore structure. Composite ceramics which pores are filled by plastic filler shows the similar behavior
Emergence of Spacetime
Starting from a background Zero Point Field (or Dark Energy) we show how an
array of oscillators at the Planck scale leads to the formation of elementary
particles and spacetime and also to a cosmology consistent with latest
observations.Comment: Latex, 39 page
Lorentz gauge theory as a model of emergent gravity
We consider a class of Lorentz gauge gravity theories within Riemann-Cartan
geometry which admits a topological phase in the gravitational sector. The
dynamic content of such theories is determined only by the contortion part of
the Lorentz gauge connection. We demonstrate that there is a unique Lagrangian
that admits propagating spin one mode in correspondence with gauge theories of
other fundamental interactions. Remarkably, despite the R^2 type of the
Lagrangian and non-compact structure of the Lorentz gauge group, the model
possesses rather a positive-definite Hamiltonian. This has been proved in the
lowest order of perturbation theory. This implies further consistent
quantization and leads to renormalizable quantum theory. It is assumed that the
proposed model describes possible mechanism of emergent Einstein gravity at
very early stages of the Universe due to quantum dynamics of contortion.Comment: 11 pages, final version, minor correction
СТРУКТУРА ФРОНТА ДЕФОРМАЦИОННЫХ АВТОСОЛИТОНОВ В ГОРНЫХ ПОРОДАХ И ГЕОСРЕДАХ
The paper describes numerical modeling of the generation and propagation of the fronts of moving deformation autosolitons in a loaded nonlinear strong medium. It presents solving a system of dynamic equations for solid mechanics, using an equation of state written in a relaxation form that takes into account both an overload of the solid medium and subsequent stress relaxation. The structure of a deformation autosoliton front is investigated in detail. It is shown that the front of a deformation autosoliton that is moving in an elastoplastic medium is a shear band (i.e. a narrow zone of intense shearing strain), which is oriented in the direction of maximum shear stress. Consecutive formation of such shear bands can be viewed as deformation autosoliton perturbations propagating along the axis of loading (compression or extension). A fine structure of a deformation autosoliton front is revealed. It is shown that slow autosoliton dynamics is an integral component of any deformation process, including the seismic process, in any solid medium. In contrast to fast autosoliton dynamics (when the velocities of stress waves are equal to the speed of sound), slow deformation autosoliton perturbations propagate at velocities 5–7 orders of magnitude lower than the velocities of sound. Considering the geomedium, it should be noted that slow dynamics plays a significant role in creating deformation patterns of the crust elements.Численно изучен процесс генерации и распространения фронтов бегущих деформационных автосолитонов в нелинейной прочной нагружаемой среде. Решалась система динамических уравнений механики деформируемого твердого тела с уравнением состояния, записанным в релаксационной форме, обеспечивающим как перегрузку прочной среды, так и последующую релаксацию напряжений. Подробно исследована структура фронта деформационного автосолитона. Показано, что фронт бегущего в упругопластической среде деформационного автосолитона представляет собой полосу локализованного сдвига, которая ориентирована по направлению максимальных касательных напряжений. Процесс последовательного формирования таких полос локализованных сдвигов и представляет собой деформационное автосолитонное возмущение, которое распространяется вдоль оси нагружения (сжатия либо растяжения). Выявлена тонкая структура фронтов деформационных автосолитонов. Показано, что медленная автосолитонная динамика является неотъемлемой частью любого процесса деформирования любой прочной среды, в том числе и сейсмического. В отличие от быстрой динамики, для которой скорости волн напряжений равны скоростям звука, медленные деформационные автосолитонные возмущения распространяются со скоростями на 5–7 порядков ниже скорости звука. Для случая деформирования геосреды именно медленная динамика играет заметную роль в формировании наблюдаемой деформационной картины элементов земной коры
Structure and magnetic properties of the Ho2Ge2O7 pyrogermanate
We report the anisotropic magnetic properties of Ho2Ge2O7 determined from dc
and ac magnetization, specific heat and powder neutron diffraction experiments.
The magnetic lanthanide sublattice, seen in our refinement of the tetragonal
pyrogermanate crystal structure, is a right-handed spiral of edge-sharing and
corner-sharing triangles; the local Ho-O coordination indicates that the
crystal field is anisotropic. Susceptibility and magnetization data indeed show
that the magnetism is highly anisotropic, and the magnetic structure has the Ho
moments confined to the plane perpendicular to the structural spiral. The
ordered moment of Ho3+, as determined from refinement of the neutron
diffraction data, is 9.0 mu_B. Magnetic ordering occurs around 1.6 K.
Temperature and field dependent ac susceptibility measurements show that this
compound displays spin relaxation phenomena analogous to what is seen in the
spin ice pyrochlore system Ho2Ti2O7
Many-worlds interpretation of quantum theory and mesoscopic anthropic principle
We suggest to combine the Anthropic Principle with Many-Worlds Interpretation
of Quantum Theory. Realizing the multiplicity of worlds it provides an
opportunity of explanation of some important events which are assumed to be
extremely improbable. The Mesoscopic Anthropic Principle suggested here is
aimed to explain appearance of such events which are necessary for emergence of
Life and Mind. It is complementary to Cosmological Anthropic Principle
explaining the fine tuning of fundamental constants. We briefly discuss various
possible applications of Mesoscopic Anthropic Principle including the Solar
Eclipses and assembling of complex molecules. Besides, we address the problem
of Time's Arrow in the framework of Many-World Interpretation. We suggest the
recipe for disentangling of quantities defined by fundamental physical laws and
by an anthropic selection.Comment: 11 page
Symmetries and observables in topological gravity
After a brief review of topological gravity, we present a superspace approach
to this theory. This formulation allows us to recover in a natural manner
various known results and to gain some insight into the precise relationship
between different approaches to topological gravity. Though the main focus of
our work is on the vielbein formalism, we also discuss the metric approach and
its relationship with the former formalism.Comment: 34 pages; a few explanations added in subsection 2.2.1, published
version of pape
Semiclassical Mechanics of the Wigner 6j-Symbol
The semiclassical mechanics of the Wigner 6j-symbol is examined from the
standpoint of WKB theory for multidimensional, integrable systems, to explore
the geometrical issues surrounding the Ponzano-Regge formula. The relations
among the methods of Roberts and others for deriving the Ponzano-Regge formula
are discussed, and a new approach, based on the recoupling of four angular
momenta, is presented. A generalization of the Yutsis-type of spin network is
developed for this purpose. Special attention is devoted to symplectic
reduction, the reduced phase space of the 6j-symbol (the 2-sphere of Kapovich
and Millson), and the reduction of Poisson bracket expressions for
semiclassical amplitudes. General principles for the semiclassical study of
arbitrary spin networks are laid down; some of these were used in our recent
derivation of the asymptotic formula for the Wigner 9j-symbol.Comment: 64 pages, 50 figure
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