Showing papers by "Center for Theoretical Physics published in 2015"

Non-signaling boxes and quantum logics

Abstract: Using a quantum logic approach we analyze the structure of the so-called non-signaling theories respecting relativistic causality, but allowing correlations violating bounds imposed by quantum mechanics such as CHSH inequality We discuss the relations among such theories, quantum mechanics, and classical physics Our main result is the construction of a probability theory adequate for the simplest instance of a non-signaling theory—the two non-signaling boxes world—in which we exhibit its differences in comparison with classical and quantum probabilities We show that the question of whether such a theory can be treated as a kind of 'generalization' of the quantum theory of the two-qubit system cannot be answered positively Some of its features put it closer to the quantum world—on the one hand, for example, the measurements are destructive, though on the other hand the Heisenberg uncertainty relations are not satisfied Another interesting property contrasting it from quantum mechanics is that the subset of 'classically correlated states', ie the states with only classical correlations, does not reproduce the classical world of the two two-state systems Our results establish a new link between quantum information theory and the well-developed theory of quantum logics

Quantum fluctuations of geometry in a hot Universe

Abstract: The fluctuations of spacetime geometries at finite temperature are evaluated within the linearized theory of gravity. These fluctuations are described by the probability distribution of various configurations of the gravitational field. The field configurations are described by the linearized Riemann–Weyl tensor without any reference to the metric. The probability distribution of various configurations is described by the Wigner functional of the gravitational field. It has a foam-like structure; dominant configurations are those with large changes of geometry at nearby points. In the high-temperature limit one obtains the Bolzmann distribution that enables one to identify the expression for the total energy of the gravitational field. The appearance of the same expression for the total energy when the gravitational field is treated as a collection of gravitons and as the high-temperature limit of the Wigner functional proves the consistency of the whole procedure. Striking differences are found between the fluctuations of the electromagnetic field and the gravitational field; among them is the divergence in the gravitational case of the probability distribution at zero temperature. This divergence is of the 'infrared type' because it occurs in integrals over the wave vector at small k.

Online localization of zooniverse citizen science projects – on the use of translation platforms as tools for translator education

Abstract: This paper aims at describing the way in which online translation platforms can facilitate the process of training translators. Zooniverse, a website hosting a variety of citizen science projects in which everyone can take part, was used as an example of such a concept. The first section of this paper is focused on the history, idea and general description of the website. In section two the online translation platform, which is connected with Zooniverse, has been presented in detail with emphasis put on advantages and weak points. Ideas for practical application of Zooniverse’s localization platform have been outlined in section three. Results have shown that localization platforms hold a great potential in terms of providing training for trainee translators. This is mostly because they offer basic experience in terms of simple computer assisted translation technologies and access to translations into other languages that have been already submitted. They are also characterized by simplicity and accessibility, as the platform can be used from any place all over the world. Zo niverse can, therefore, be the basis for further research on how the potential of such websites can be harnessed for more effective translator training.

Analog of selfduality in dimension nine

Abstract: We introduce a type of Riemannian geometry in nine dimen- sions, which can be viewed as the counterpart of selfduality in four dimen- sions. This geometry is related to a 9-dimensional irreducible representation of SO(3) SO(3) and it turns out to be defined by a dierential 4-form. Structures admitting a metric connection with totally antisymmetric torsion and preserving the 4-form are studied in detail, producing locally homogeneous examples which can be viewed as analogs of self-dual 4-manifolds in dimension nine.

Density fluctuations in a quasi-one-dimensional Bose gas as observed in free expansion

Abstract: We study, within a framework of the classical fields approximation, the density correlations of a weakly interacting expanding Bose gas for the whole range of temperatures across the Bose-Einstein condensation threshold. We focus on elongated quasi-one-dimensional systems where there is a huge discrepancy between the existing theory and experimental results (A. Perrin et al., Nature Phys. 8, 195 (2012)). We find that the density correlation function is not reduced for temperatures below the critical one as it is predicted for the ideal gas or for a weakly interacting system within the Bogoliubov approximation. This behavior of the density correlations agrees with the above mentioned experiment with the elongated system. Although the system was much larger then studied here we believe that the behavior of the density correlation function found there is quite generic. Our theoretical studies indicate also large density fluctuations in the trap in the quasicondensate regime where only phase fluctuations were expected. We argue that the enhanced density fluctuations can originate in the presence of interactions in the system, or more precisely in the existence of spontaneous dark solitons in the elongated gas at thermal equilibrium.

On the existence of rigid spheres in four-dimensional spacetime manifolds

Abstract: This paper deals with the generalization of usual round spheres in the flat Minkowski spacetime to the case of a generic four-dimensional spacetime manifold $M$. We consider geometric properties of sphere-like submanifolds in $M$ and introduce conditions on external curvature and torsion, which lead to a definition of a {\em rigid sphere}. The main result is a local existence theorem concernig such spheres. For this purpose we apply the surjective implicit function theorem. The proof is based on a detailed analysis of the linearized problem and leads to an eight-parameter family of solutions in case when the metric tensor $g$ of $M$ is from a certain neighbourhood of the flat Minkowski metric. This contribution continues the study of rigid spheres in (Class. Quantum Grav. \textbf{30} (2013), 175010, doi:10.1088/0264-9381/30/17/175010, 18 pp.).