258 research outputs found
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
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
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
Variable Coefficient Third Order KdV Type of Equations
We show that the integrable subclassess of a class of third order
non-autonomous equations are identical with the integrable subclassess of the
autonomous ones.Comment: Latex file , 15 page
Godel-type Metrics in Various Dimensions II: Inclusion of a Dilaton Field
This is the continuation of an earlier work where Godel-type metrics were
defined and used for producing new solutions in various dimensions. Here a
simplifying technical assumption is relaxed which, among other things,
basically amounts to introducing a dilaton field to the models considered. It
is explicitly shown that the conformally transformed Godel-type metrics can be
used in solving a rather general class of Einstein-Maxwell-dilaton-3-form field
theories in D >= 6 dimensions. All field equations can be reduced to a simple
"Maxwell equation" in the relevant (D-1)-dimensional Riemannian background due
to a neat construction that relates the matter fields. These tools are then
used in obtaining exact solutions to the bosonic parts of various supergravity
theories. It is shown that there is a wide range of suitable backgrounds that
can be used in producing solutions. For the specific case of (D-1)-dimensional
trivially flat Riemannian backgrounds, the D-dimensional generalizations of the
well known Majumdar-Papapetrou metrics of general relativity arise naturally.Comment: REVTeX4, 17 pp., no figures, a few clarifying remarks added and
grammatical errors correcte
A Note on Stress-Tensors, Conservation and Equations of Motion
Some unusual relations between stress tensors, conservation and equations of
motion are briefly reviewed.Comment: 4 pages. Invited contribution, A. Peres Festschrift, to be published
in Found. Phy
2+1 KdV(N) Equations
We present some nonlinear partial differential equations in 2+1-dimensions
derived from the KdV Equation and its symmetries. We show that all these
equations have the same 3-soliton structures. The only difference in these
solutions are the dispersion relations. We also showed that they pass the
Painlev\'e test.Comment: 15 page
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
Killing Vector Fields in Three Dimensions: A Method to Solve Massive Gravity Field Equations
Killing vector fields in three dimensions play important role in the
construction of the related spacetime geometry. In this work we show that when
a three dimensional geometry admits a Killing vector field then the Ricci
tensor of the geometry is determined in terms of the Killing vector field and
its scalars. In this way we can generate all products and covariant derivatives
at any order of the ricci tensor. Using this property we give ways of solving
the field equations of Topologically Massive Gravity (TMG) and New Massive
Gravity (NMG) introduced recently. In particular when the scalars of the
Killing vector field (timelike, spacelike and null cases) are constants then
all three dimensional symmetric tensors of the geometry, the ricci and einstein
tensors, their covariant derivatives at all orders, their products of all
orders are completely determined by the Killing vector field and the metric.
Hence the corresponding three dimensional metrics are strong candidates of
solving all higher derivative gravitational field equations in three
dimensions.Comment: 25 pages, some changes made and some references added, to be
published in Classical and Quantum Gravit
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
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