Closed timelike curves and geodesics of Godel-type metrics

It is shown explicitly that when the characteristic vector field that defines a Godel-type metric is also a Killing vector, there always exist closed timelike or null curves in spacetimes described by such a metric. For these geometries, the geodesic curves are also shown to be characterized by a lower dimensional Lorentz force equation for a charged point particle in the relevant Riemannian background. Moreover, two explicit examples are given for which timelike and null geodesics can never be closed.Comment: REVTeX 4, 12 pages, no figures; the Introduction has been rewritten, some minor mistakes corrected, many references adde

Accelerated Levi-Civita-Bertotti-Robinson Metric in D-Dimensions

A conformally flat accelerated charge metric is found in an arbitrary dimension $D$. It is a solution of the Einstein-Maxwell-null fluid with a cosmological constant in $D \ge 4$ dimensions. When the acceleration is zero our solution reduces to the Levi-Civita-Bertotti-Robinson metric. We show that the charge loses its energy, for all dimensions, due to the acceleration.Comment: Latex File, 12 page

G\"odel Type Metrics in Three Dimensions

We show that the G{\" o}del type Metrics in three dimensions with arbitrary two dimensional background space satisfy the Einstein-perfect fluid field equations. There exists only one first order partial differential equation satisfied by the components of fluid's velocity vector field. We then show that the same metrics solve the field equations of the topologically massive gravity where the two dimensional background geometry is a space of constant negative Gaussian curvature. We discuss the possibility that the G{\" o}del Type Metrics to solve the Ricci and Cotton flow equations. When the vector field $u^{\mu}$ is a Killing vector field we finally show that the stationary G{\" o}del Type Metrics solve the field equations of the most possible gravitational field equations where the interaction lagrangian is an arbitrary function of the electromagnetic field and the curvature tensors.Comment: 17 page

Photon rockets moving arbitrarily in any dimension

A family of explicit exact solutions of Einstein's equations in four and higher dimensions is studied which describes photon rockets accelerating due to an anisotropic emission of photons. It is possible to prescribe an arbitrary motion, so that the acceleration of the rocket need not be uniform - both its magnitude and direction may vary with time. Except at location of the point-like rocket the spacetimes have no curvature singularities, and topological defects like cosmic strings are also absent. Any value of a cosmological constant is allowed. We investigate some particular examples of motion, namely a straight flight and a circular trajectory, and we derive the corresponding radiation patterns and the mass loss of the rockets. We also demonstrate the absence of "gravitational aberration" in such spacetimes. This interesting member of the higher-dimensional Robinson-Trautman class of pure radiation spacetimes of algebraic type D generalises the class of Kinnersley's solutions that has long been known in four-dimensional general relativity.Comment: Text and figures modified (22 pages, 8 figures). To appear in the International Journal of Modern Physics D, Vol. 20, No..

Yarı Kristal Polimer Malzemelerin Çok Ölçekli Modellenmesi

Konferans Bildirisi -- Teorik ve Uygulamalı Mekanik Türk Milli Komitesi, 2013Conference Paper -- Theoretical and Applied Mechanical Turkish National Committee, 2013Bu çalışmada iki fazlı yarı kristal polimerik malzemeler için geometrik olarak doğrusal olmayan, mikromekaniksel motivasyonlu ve çok ölçekli bir malzeme modeli geliştirilmiştir. Bu amaç doğrultusunda, amorf ve kristal fazlar için en önemli şekil değiştirme mekanizmaları belirlenmiş ve bu bilgi ışığında her iki faz için ayrı ayrı mikromekaniksel motivasyonu bulunan malzeme modelleri kullanılmıştır. Ardından, iki fazlı yapıyı homojenleştirerek yarı kristal polimer malzemenin makroskopik davranışını betimleyecek bir model geliştirilmiştir.In this paper a geometrically non-linear micromechanically-motivated multi-scale model is developed for two phase semi-crystalline polymeric materials. To this end, most important deformation mechanisms of amorphous and crystalline phases are determined; and in the light of this information, micromechanically-motivated material models are employed separately for both phases. Afterwards, by homogenization of the two-phase structure, , a model that would render the macroscopic response of the semi crystalline polymeric material is developed

The extractive infrastructures of contact tracing apps

The COVID-19 pandemic will go down in history as a major crisis, with calls for debt moratoriums that are expected to have gruesome effects in the Global South. Another tale of this crisis that would come to dominate COVID-19 news across the world was a new technological application: the contact tracing apps. In this article, we argue that both accounts ‐ economic implications for the Global South and the ideology of techno-solutionism ‐ are closely related. We map the phenomenon of the tracing app onto past and present wealth accumulations. To understand these exploitative realities, we focus on the implications of contact tracing apps and their relation with extractive technologies as we build on the notion racial capitalism. By presenting themselves in isolation of capitalism and extractivism, contact tracing apps hide raw realities, concealing the supply chains that allow the production of these technologies and the exploitative conditions of labour that make their computational magic manifest itself. As a result of this artificial separation, the technological solutionism of contract tracing apps is ultimately presented as a moral choice between life and death. We regard our work as requiring continuous undoing ‐ a necessary but unfinished formal dismantling of colonial structures through decolonial resistance

No New Symmetries of the Vacuum Einstein Equations

In this note we examine some recently proposed solutions of the linearized vacuum Einstein equations. We show that such solutions are {\it not} symmetries of the Einstein equations, because of a crucial integrability condition.Comment: 9 pages, Te

Gauss-Bonnet Gravity with Scalar Field in Four Dimensions

We give all exact solutions of the Einstein-Gauss-Bonnet Field Equations coupled with a scalar field in four dimensions under certain assumptions.Comment: Latex file, 7 page

Relationship Between Solitonic Solutions of Five-Dimensional Einstein Equations

We give the relation between the solutions generated by the inverse scattering method and the B\"acklund transformation applied to the vacuum five-dimensional Einstein equations. In particular, we show that the two-solitonic solutions generated from an arbitrary diagonal seed by the B\"acklund transformation are contained within those generated from the same seed by the inverse scattering method.Comment: 17 pages, Some references are added, to be published in Phys.Rev.