The unphysical nature of "Warp Drive"

We will apply the quantum inequality type restrictions to Alcubierre's warp drive metric on a scale in which a local region of spacetime can be considered ``flat''. These are inequalities that restrict the magnitude and extent of the negative energy which is needed to form the warp drive metric. From this we are able to place limits on the parameters of the ``Warp Bubble''. It will be shown that the bubble wall thickness is on the order of only a few hundred Planck lengths. Then we will show that the total integrated energy density needed to maintain the warp metric with such thin walls is physically unattainable.Comment: 11 pages, 3 figures, latex. This revision corrects a typographical sign error in Eq. (3

Quantum interest in two dimensions

The quantum interest conjecture of Ford and Roman asserts that any negative-energy pulse must necessarily be followed by an over-compensating positive-energy one within a certain maximum time delay. Furthermore, the minimum amount of over-compensation increases with the separation between the pulses. In this paper, we first study the case of a negative-energy square pulse followed by a positive-energy one for a minimally coupled, massless scalar field in two-dimensional Minkowski space. We obtain explicit expressions for the maximum time delay and the amount of over-compensation needed, using a previously developed eigenvalue approach. These results are then used to give a proof of the quantum interest conjecture for massless scalar fields in two dimensions, valid for general energy distributions.Comment: 17 pages, 4 figures; final version to appear in PR

Peronosclerospora sorghi, o agente etiológico do míldio do sorgo.

O agente etiológico do míldio do sorgo, Peronosclerospora sorghi, infecta as culturas do sorgo (Sorghum spp.) e do milho (Zea mays). Esse patógeno encontra-se disseminado em muitas regiões tropicais e subtropicais do mundo e pode ocasionar danos significativos na produção de sorgo quando as condições climáticas são favoráveis à sua ocorrência e em cultivares de alta susceptibilidade. No Brasil, antes restrito aos estados da região Sul, o míldio foi registrado também nos estados da região Sudeste e Centro-Oeste, causando prejuízos principalmente em áreas de produção de sementes. O cultivo de genótipos resistentes é o método mais eficiente para o controle da doença. Entretanto, essa estratégia é dificultada pela alta variabilidade genética apresentada pelo patógeno. Essa revisão aborda aspectos da taxonomia, biologia e distribuição geográfica do míldio do sorgo e discute questões relacionadas com a sua epidemiologia e controle, enfatizando estratégias que utilizam resistência genética

Nominal Logic Programming

Nominal logic is an extension of first-order logic which provides a simple foundation for formalizing and reasoning about abstract syntax modulo consistent renaming of bound names (that is, alpha-equivalence). This article investigates logic programming based on nominal logic. We describe some typical nominal logic programs, and develop the model-theoretic, proof-theoretic, and operational semantics of such programs. Besides being of interest for ensuring the correct behavior of implementations, these results provide a rigorous foundation for techniques for analysis and reasoning about nominal logic programs, as we illustrate via examples.Comment: 46 pages; 19 page appendix; 13 figures. Revised journal submission as of July 23, 200

Using constrained intuitionistic linear logic for hybrid robotic planning problems

Synthesis of robot behaviors towards nontrivial goals often requires reasoning about both discrete and continuous aspects of the underlying domain. Existing approaches in building automated tools for such synthesis problems attempt to augment methods from either discrete planning or continuous control with hybrid elements, but largely fail to ensure a uniform treatment of both aspects of the domain. In this paper, we present a new formalism, Constrained Intuitionistic Linear Logic (CILL), merging continuous constraint solvers with linear logic to yield a single language in which hybrid properties of robotic behaviors can be expressed and reasoned with. Following a gentle introduction to linear logic, we describe the two new connectives of CILL, introduced to interface the constraint domain with the logical fragment of the language. We then illustrate the application of CILL for robotic planning problems within the Balanced Blocks World, a "physically realistic" extension of the Blocks World domain. Even though some of the formal proofs for the semantic foundations of the language as well as an efficient implementation of a theorem prover are yet to be completed, CILL promises to be a powerful formalism in reasoning within hybrid domains. © 2007 IEEE

Interplay between excitation kinetics and reaction-center dynamics in purple bacteria

Photosynthesis is arguably the fundamental process of Life, since it enables energy from the Sun to enter the food-chain on Earth. It is a remarkable non-equilibrium process in which photons are converted to many-body excitations which traverse a complex biomolecular membrane, getting captured and fueling chemical reactions within a reaction-center in order to produce nutrients. The precise nature of these dynamical processes -- which lie at the interface between quantum and classical behaviour, and involve both noise and coordination -- are still being explored. Here we focus on a striking recent empirical finding concerning an illumination-driven transition in the biomolecular membrane architecture of {\it Rsp. Photometricum} purple bacteria. Using stochastic realisations to describe a hopping rate model for excitation transfer, we show numerically and analytically that this surprising shift in preferred architectures can be traced to the interplay between the excitation kinetics and the reaction center dynamics. The net effect is that the bacteria profit from efficient metabolism at low illumination intensities while using dissipation to avoid an oversupply of energy at high illumination intensities.Comment: 21 pages, 13 figures, accepted for publication in New Journal of Physic

Superluminal travel requires negative energies

I investigate the relationship between faster-than-light travel and weak-energy-condition violation, i.e., negative energy densities. In a general spacetime it is difficult to define faster-than-light travel, and I give an example of a metric which appears to allow superluminal travel, but in fact is just flat space. To avoid such difficulties, I propose a definition of superluminal travel which requires that the path to be traveled reach a destination surface at an earlier time than any neighboring path. With this definition (and assuming the generic condition) I prove that superluminal travel requires weak-energy-condition violation.Comment: 5 pages, RevTeX, 2 figures with epsf. This paper now contains all the material of gr-qc/6805003 and gr-qc/9806091 since these became a single article in Phys. Rev. Let

Identification of sources of resistance in sorghum to Peronosclerospora sorghi.

Este trabalho foi realizado com o objetivo de se identificar fontes de resistência ao míldio do sorgo (Sorghum bicolor), causado pelo patógeno Peronosclerospora sorghi, pela avaliação de 42 genótipos de sorgo sob condições de infecção natural. Os genótipos foram semeados em parcelas de uma fileira, entre duas fileiras da cultivar suscetível SC283, semeada com 30 dias de antecedência para atuar como fonte de inóculo. Foram conduzidos dois ensaios no delineamento experimental de blocos ao acaso com três repetições. Os genótipos CMSXS156, CMSXS157, CMSXS243, TxARG-1, 8902, 9902054, 9910032, 9910296, Tx430, QL-3, SC170-6-17, CMSXS762 e BR304 foram classificados como altamente resistente nos dois ensaios. Entre estes as linhagens SC170-6-17 e 9910296 apresentam-se 0% de infecção sistêmica. Os resultados indicaram a provável ocorrência de diferentes patótipos de P. sorghi nos dois ensaios. The main objective of this work was to identify sources of resistance in sorghum (Sorghum bicolor) to Peronosclerospora sorghi, the causal agent of downy mildew, through the evaluation of 42 sorghum genotypes under natural infection in the field. Genotypes were planted in single row plots between two rows of the susceptible line SC283, planted 30 days before, to act as spreader rows, in two separate nurseries. The experimental design was a completely randomized block design with three replications. Sorghum genotypes CMSXS156, CMSXS157, CMSXS243, TxARG-1, 8902, 9902054, 9910032, 9910296, Tx430, QL-3, SC170-6-17, CMSXS762 and BR304 were classified as highly resistant in both nurseries. Among these, SC170-6-17 and 9910296 showed 0% systemic infection. Results indicated the possible occurrence of different pathotypes of P. sorghi in the two nurseries