String-Like Lagrangians from a Generalized Geometry

This note will use Hitchin's generalized geometry and a model of axionic gravity developed by Warren Siegel in the mid-nineties to show that the construction of Lagrangians based on the inner product arising from the pairing of a vector and its dual can lead naturally to the low-energy Lagrangian of the bosonic string.Comment: Conclusions basically unchanged, but presentation streamlined significantly. Published versio

Maintaining a Wormhole with a Scalar Field

It is well known that it takes matter that violates the averaged weak energy condition to hold the throat of a wormhole open. The production of such ``exotic'' matter is usually discussed within the context of quantum field theory. In this paper I show that it is possible to produce the exotic matter required to hold a wormhole open classically. This is accomplished by coupling a scalar field to matter that satisfies the weak energy condition. The energy-momentum tensor of the scalar field and the matter separately satisfy the weak energy condition, but there exists an interaction energy-momentum tensor that does not. It is this interaction energy-momentum tensor that allows the wormhole to be maintained.Comment: 12 pages, LaTe

Psychological Issues in Online Adaptive Task Allocation

Adaptive aiding is an idea that offers potential for improvement over many current approaches to aiding in human-computer systems. The expected return of tailoring the system to fit the user could be in the form of improved system performance and/or increased user satisfaction. Issues such as the manner in which information is shared between human and computer, the appropriate division of labor between them, and the level of autonomy of the aid are explored. A simulated visual search task was developed. Subjects are required to identify targets in a moving display while performing a compensatory sub-critical tracking task. By manipulating characteristics of the situation such as imposed task-related workload and effort required to communicate with the computer, it is possible to create conditions in which interaction with the computer would be more or less desirable. The results of preliminary research using this experimental scenario are presented, and future directions for this research effort are discussed

Exact solutions of Brans-Dicke wormholes in the presence of matter

A fundamental ingredient in wormhole physics is the presence of exotic matter, which involves the violation of the null energy condition. Although a plethora of wormhole solutions have been explored in the literature, it is useful to find geometries that minimize the usage of exotic matter. In this work, we find exact wormhole solutions in Brans-Dicke theory where the normal matter threading the wormhole satisfies the null energy condition throughout the geometry. Thus, the latter implies that it is the effective stress-energy tensor containing the scalar field, that plays the role of exotic matter, that is responsible for sustaining the wormhole geometry. More specifically, we consider a zero redshift function and a particular choice for the scalar field and determine the remaining quantities, namely, the stress-energy tensor components and the shape function. The solution found is not asymptotically flat, so that this interior wormhole spacetime needs to be matched to an exterior vacuum solution.Comment: 7 pages, 3 figure

Magnetic resonance imaging study of the sciatic nerve variation in the pediatric gluteal region: Implications for the posterior approach of the sciatic nerve blockade

Introduction: In pediatric patients, the sciatic nerve is one of the most commonly blocked peripheral nerves during orthopedic procedures of the lower limb. Ultrasound guidance is the current standard for a successful localization of the sciatic nerve in the gluteal region. Relevant anatomical landmarks are also used to determine the nerve location when ultrasound is not available or inadequate. However, reports have demonstrated paucity of information regarding the sciatic nerve location and variation in the hip throughout pediatric development. This imaging study aimed to document and analyze the relative morphometric relationship of the sciatic nerve in the pediatric gluteal region throughout development. Methods: The location of the sciatic nerve in relation to bony landmarks was measured in 84 pediatric magnetic resonance imaging of patients aged 0.7–15.8 years. Results: The sciatic nerve was identified medial to the most lateral point of greater trochanter at the level of ischial spine and the tip of coccyx. The strong positive correlation between sciatic nerve to landmark distances and age and stature demonstrated linear variation between sciatic nerve location with age and growth of children. To predict the nerve location in the gluteal region, regression equations using patient age were created, having implications for the posterior approach of the sciatic nerve blockade in children. Clinically significant differences were found between sexes, specifically in the older age group. Conclusion: Despite the small sample size of younger age group, this study is the first to document the morphometric changes of the sciatic nerve in the gluteal region across pediatric development and may be useful for providing confirmatory guidelines for nerve location when ultrasound is not accessible or cannot be utilized for practice

Exact solutions of charged wormhole

In this paper, the backreaction to the traversable Lorentzian wormhole spacetime by the scalar field or electric charge is considered to find the exact solutions. The charges play the role of the additional matter to the static wormhole which is already constructed by the exotic matter. The stability conditions for the wormhole with scalar field and electric charge are found from the positiveness and flareness for the wormhole shape function.Comment: 9 pages, Revtex, no figures, to appear in Phys. Rev. D(2001

Negative Energy Density States for the Dirac Field in Flat Spacetime

Negative energy densities in the Dirac field produced by state vectors that are the superposition of two single particle electron states are examined. I show that for such states the energy density of the field is not bounded from below and that the quantum inequalities derived for scalar fields are satisfied. I also show that it is not possible to produce negative energy densities in a scalar field using state vectors that are arbitrary superpositions of single particle states.Comment: 11 pages, LaTe