Godel-type Universes in String-inspired Charged Gravity

We consider a string-inspired, gravitational theory of scalar and electromagnetic fields and we investigate the existence of axially-symmetric, G\"{o}del-type cosmological solutions. The neutral case is studied first and an "extreme" G\"{o}del-type rotating solution, that respects the causality, is determined. The charged case is considered next and two new configurations for the, minimally-coupled to gravity, electromagnetic field are presented. Another configuration motivated by the expected distribution of currents and charges in a rotating universe is studied and shown to lead to a G\"{o}del-type solution for a space-dependent coupling function. Finally, we investigate the existence of G\"{o}del-type cosmological solutions in the framework of the one-loop corrected superstring effective action and we determine the sole configuration of the electromagnetic field that leads to such a solution. It turns out that, in all the charged cases considered, Closed Timelike Curves do appear and the causality is always violated.Comment: 26 pages, LaTex file, a few comments and references added, version to appear in Physical Review

Identificação e caracterização dos principais solos do município de Coruripe, Alagoas.

RESUMO: A caracterização do solo é fundamental para avaliar o seu comportamento sob diferentes usos. O estudo de solos tem como objetivo subsidiar o planejamento agroambiental do município. Coruripe localiza-se na Mesorregião do Leste Alagoano e tem como atividade o cultivo de cana-de-açúcar. Foram mapeadas as classes Latossolo, Argissolo, Espodossolo, Gleissolo, Neossolo Quartzarênico, Organossolo e solos de mangue. Nas unidades de mapeamento LAd (Latossolos) e PAd (Argissolos Amarelos), os solos ocorrem em relevo plano e suave ondulado, são profundos e apresentam baixa fertilidade. Corrigindo a fertilidade, esses ambientes podem se tornar mais produtivos. A unidade PACd (Argissolo Acinzentado) apresenta fertilidade muito baixa, pouca retenção de umidade e drenagem deficiente. Portanto, é um ambiente com baixo potencial agrícola. Na unidade PVAd, os Argissolos Vermelho-Amarelos apresentam relevo forte ondulado, alto risco de erosão e devem ser destinados à preservação. Fazendo-se a drenagem e corrigindo-se a fertilidade, a unidade GXd (Gleissolo) torna-se um ambiente com alto potencial para produção de hortaliças, cana-de-açúcar e pastagens. As unidades ESKo (Espodossolo), OJs (Organossolo) e SM (Solos de Mangue) apresentam baixa fertilidade, drenagem deficiente e baixo potencial agrícola

Aptidão agrícola das terras do município de Nazaré da Mata (PE).

Este trabalho teve como objetivo determinar a aptidão agrícola das terras do município de Nazaré da Mata, na escala 1:25.000). Na avaliação consideraram-se os fatores de limitação: fertilidade, água, oxigênio, erosão e mecanização e as classes de aptidão boa, regular, restrita e inapta. Os resultados obtidos permitiram verificar que, as terras do subgrupo 1?(a)BC são boas para lavouras nos níveis de manejo B e C e restritas no manejo A, ocupando 10,37% da área municipal; as terras do subgrupo 1?(a)Bc são restritas para lavouras no nível de manejo A, boas no B e regulares no C, com 4,19%. Para o subgrupo 2ab, com 21,70% da área, as terras são regulares nos manejos A e B. Os subgrupos 3(abc) e 3(ab) ocupam no total 12,05 km2 (7,87% da área total) e apresentam aptidão restrita para lavouras nos três sistemas de manejo. No município há predominância de terras com aptidão boa (4P) e regular (4p) para pastagens plantadas, totalizando 83,43 km2

Potencial pedológico para o cultivo de cana-de-açúcar com manejo de alta tecnologia no município de Buenos Aires (PE).

O município de Buenos Aires (PE) está situado na Microrregião da Mata Setentrional e ocupa uma área de 111 km2. Inicialmente foram adquiridas cartas plani-altimétricas, escala 1:25.000 e com curvas de nível equidistantes em 10 metros, utilizadas como material básico para o levantamento dos solos. O presente estudo foi realizado com o objetivo de avaliar o potencial pedológico para o cultivo da cana-de-açúcar com uso de alta tecnologia ? manejo C. As terras do município possuem 14,5 km2 de potencial pedológico Alto e Muito Alto para o cultivo de cana-de-açúcar Essas áreas estão localizadas, nos topos planos das elevações e dispersas, principalmente, nas partes norte, oeste e sudeste do município. Para o potencial pedológico Médio essa classe ocupa 21,6 km2 e localiza-se principalmente nas várzeas. Quanto aos potenciais pedológicos Baixo e Muito Baixo, observa-se que no total eles abrangem 74,9 km2 e estão localizados em todas as partes do município

Causal Anomalies in Kaluza-Klein Gravity Theories

Causal anomalies in two Kaluza-Klein gravity theories are examined, particularly as to whether these theories permit solutions in which the causality principle is violated. It is found that similarly to general relativity the field equations of the space-time-mass Kaluza-Klein (STM-KK) gravity theory do not exclude violation of causality of G\"odel type, whereas the induced matter Kaluza-Klein (IM-KK) gravity rules out noncausal G\"odel-type models. The induced matter version of general relativity is shown to be an efficient therapy for causal anomalies that occurs in a wide class of noncausal geometries. Perfect fluid and dust G\"odel-type solutions of the STM-KK field equations are studied. It is shown that every G\"odel-type perfect fluid solution is isometric to the unique dust solution of the STM-KK field equations. The question as to whether 5-D G\"odel-type non-causal geometries induce any physically acceptable 4-D energy-momentum tensor is also addressed.Comment: 16 page. LaTex file. To appear in Int. J. Mod. Phys. A (1998

Hamilton Operator and the Semiclassical Limit for Scalar Particles in an Electromagnetic Field

We successively apply the generalized Case-Foldy-Feshbach-Villars (CFFV) and the Foldy-Wouthuysen (FW) transformation to derive the Hamiltonian for relativistic scalar particles in an electromagnetic field. In contrast to the original transformation, the generalized CFFV transformation contains an arbitrary parameter and can be performed for massless particles, which allows solving the problem of massless particles in an electromagnetic field. We show that the form of the Hamiltonian in the FW representation is independent of the arbitrarily chosen parameter. Compared with the classical Hamiltonian for point particles, this Hamiltonian contains quantum terms characterizing the quadrupole coupling of moving particles to the electric field and the electric and mixed polarizabilities. We obtain the quantum mechanical and semiclassical equations of motion of massive and massless particles in an electromagnetic field.Comment: 17 page

Superenergy and Supermomentum of Goedel Universes

We review the canonical superenergy tensor and the canonical angular supermomentum tensors in general relativity and calculate them for space-time homogeneous G\"odel universes to show that both of these tensors do not, in general, vanish. We consider both an original dust-filled pressureless acausal G\"odel model of 1949 and a scalar-field-filled causal G\"odel model of Rebou\c cas and Tiomno. For the acausal model, the non-vanishing components of superenergy of matter are different from those of gravitation. The angular supermomentum tensors of matter and gravitation do not vanish either which simply reflects the fact that G\"odel universe rotates. However, the axial (totally antisymmetric) and vectorial parts of supermomentum tensors vanish. It is interesting that superenergetic quantities are {\it sensitive} to causality in a way that superenergy density $_g S_{00}$ of gravitation in the acausal model is {\it positive}, while superenergy density $_g S_{00}$ in the causal model is {\it negative}. That means superenergetic quantities might serve as criterion of causality in cosmology and prove useful.Comment: an amended version, REVTEX, 26 pages, no figures, to appear in Classical and Quantum Gravit

Comparison of QG-Induced Dispersion with Standard Physics Effects

One of the predictions of quantum gravity phenomenology is that, in situations where Planck-scale physics and the notion of a quantum spacetime are relevant, field propagation will be described by a modified set of laws. Descriptions of the underlying mechanism differ from model to model, but a general feature is that electromagnetic waves will have non-trivial dispersion relations. A physical phenomenon that offers the possibility of experimentally testing these ideas in the foreseeable future is the propagation of high-energy gamma rays from GRB's at cosmological distances. With the observation of non-standard dispersion relations within experimental reach, it is thus important to find out whether there are competing effects that could either mask or be mistaken for this one. In this letter, we consider possible effects from standard physics, due to electromagnetic interactions, classical as well as quantum, and coupling to classical geometry. Our results indicate that, for currently observed gamma-ray energies and estimates of cosmological parameter values, those effects are much smaller than the quantum gravity one if the latter is first-order in the energy; some corrections are comparable in magnitude with the second-order quantum gravity ones, but they have a very different energy dependence.Comment: 8 pages; Version to be published in CQG as a letter; Includes some new comments and references, but no changes in the result

Non-minimal coupling for the gravitational and electromagnetic fields: A general system of equations

We establish a new self-consistent system of equations for the gravitational and electromagnetic fields. The procedure is based on a non-minimal non-linear extension of the standard Einstein-Hilbert-Maxwell action. General properties of a three-parameter family of non-minimal linear models are discussed. In addition, we show explicitly, that a static spherically symmetric charged object can be described by a non-minimal model, second order in the derivatives of the metric, when the susceptibility tensor is proportional to the double-dual Riemann tensorComment: 15 page

Godel brane

We consider the brane-world generalisation of the Godel universe and analyse its dynamical interaction with the bulk. The exact homogeneity of the standard Godel spacetime no longer holds, unless the bulk is also static. We show how the anisotropy of the Godel-type brane is dictated by that of the bulk and find that the converse is also true. This determines the precise evolution of the nonlocal anisotropic stresses, without any phenomenological assumptions, and leads to a self-consistent closed set of equations for the evolution of the Godel brane. We also examine the causality of the Godel brane and show that the presence of the bulk cannot prevent the appearance of closed timelike curves.Comment: Revised version, to match paper published in Phys. Rev.