2,889 research outputs found
The Lorentz Force and the Radiation Pressure of Light
In order to make plausible the idea that light exerts a pressure on matter,
some introductory physics texts consider the force exerted by an
electromagnetic wave on an electron. The argument as presented is both
mathematically incorrect and has several serious conceptual difficulties
without obvious resolution at the classical, yet alone introductory, level. We
discuss these difficulties and propose an alternate demonstration.Comment: More or less as in AJ
Harvesting information from the Internet to construct ontologies
The paper evaluates the effectiveness of harvesting information from the internet to aid in the lowcost
construction of an ontology. The paper describes how a proof-of-concept called OntoRanch
was built, to harvest information and its relationships to construct an ontology. A systems
development methodology was adopted which recognises three main stages: concept development,
system building, and system evaluation. The evaluation took an interpretive hybrid approach of
using both a focus group and a questionnaire to evaluate the proof-of-concept OntoRanch. The
findings show that the approach of reusing information by harvesting it from the internet can provide
an effective self-sustaining process that enables ontologies to be constructed in a reduced amount
of time and cost
"Quantum Interference with Slits" Revisited
Marcella [arXiv:quant-ph/0703126] has presented a straightforward technique
employing the Dirac formalism to calculate single- and double-slit interference
patterns. He claims that no reference is made to classical optics or scattering
theory and that his method therefore provides a purely quantum mechanical
description of these experiments. He also presents his calculation as if no
approximations are employed. We show that he implicitly makes the same
approximations found in classical treatments of interference and that no new
physics has been introduced. At the same time, some of the quantum mechanical
arguments Marcella gives are, at best, misleading.Comment: 11 pages, 3 figure
Quantum simulation of multiple-exciton generation in a nanocrystal by a single photon
We have shown theoretically that efficient multiple exciton generation (MEG)
by a single photon can be observed in small nanocrystals (NCs). Our quantum
simulations that include hundreds of thousands of exciton and multi-exciton
states demonstrate that the complex time-dependent dynamics of these states in
a closed electronic system yields a saturated MEG effect on a picosecond
timescale. Including phonon relaxation confirms that efficient MEG requires the
exciton--biexciton coupling time to be faster than exciton relaxation time
Cost-Effectiveness of Antihypertensive Therapy in Patients Older Than 80 Years:Cohort Study and Markov Model
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Assessment of Vessel Density on Non-Contrast Computed Tomography to Detect Basilar Artery Occlusion
Introduction: Basilar artery occlusion (BAO) may be clinically occult due to variable and non-specific symptomatology. We evaluated the qualitative and quantitative determination of a hyperdense basilar artery (HDBA) on non-contrast computed tomography (NCCT) brain for the diagnosis of BAO.Methods: We conducted a case control study of patients with confirmed acute BAO vs a control group of suspected acute stroke patients without BAO. Two EM attending physicians, one third-year EM resident, and one medical student performed qualitative and quantitative assessments for the presence of a HDBA on axial NCCT images. Our primary outcome measures were sensitivity and specificity for BAO. Our secondary outcomes were inter-rater and intra-rater reliability of the qualitative and quantitative assessments.Results: We included 60 BAO and 65 control patients in our analysis. Qualitative assessment of the hyperdense basilar artery sign was poorly sensitive (54%ā72%) and specific (55%ā89%). Quantitative measurement improved the specificity of hyperdense basilar artery assessment for diagnosing BAO, with a threshold of 61.0ā63.8 Hounsfield units demonstrating relatively high specificity of 85%ā94%. There was moderate inter-rater agreement for the qualitative assessment of HDBA (Fleissā kappa statistic 0.508, 95% confidence interval: 0.435ā0.581). Agreement improved for quantitative assessments, but still fell in the moderate range (Shrout-Fleiss intraclass correlation coefficient: 0.635). Intra-rater reliability for the quantitative assessments of the two attending physician reviewers demonstrated substantial consistency.Conclusion: Our results highlight the importance of carefully examining basilar artery density when interpreting the NCCT of patients with altered consciousness or other signs and symptoms concerning for an acute basilar artery occlusion. If the Hounsfield unit density of the basilar artery exceeds 61 Hounsfield units, BAO should be highly suspected
ZNF804A genotype modulates neural activity during working memory for faces
Copyright Ā© 2013 S. Karger AG, Basel.Peer reviewedPublisher PD
Fundamental Behavior of Electric Field Enhancements in the Gaps Between Closely Spaced Nanostructures
We demonstrate that the electric field enhancement that occurs in a gap
between two closely spaced nanostructures, such as metallic nanoparticles, is
the result of a transverse electromagnetic waveguide mode. We derive an
explicit semianalytic equation for the enhancement as a function of gap size,
which we show has a universal qualitative behavior in that it applies
irrespective of the material or geometry of the nanostructures and even in the
presence of surface plasmons. Examples of perfect electrically conducting and
Ag thin-wire antennas and a dimer of Ag spheres are presented and discussed.Comment: 9 pages and 4 figure
tDCS-induced alterations in GABA concentration within primary motor cortex predict motor learning and motor memory: A 7T magnetic resonance spectroscopy study
Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that alters cortical excitability in a polarity specific manner and has been shown to influence learning and memory. tDCS may have both on-line and after-effects on learning and memory, and the latter are thought to be based upon tDCS-induced alterations in neurochemistry and synaptic function. We used ultra-high-field (7 T) magnetic resonance spectroscopy (MRS), together with a robotic force adaptation and de-adaptation task, to investigate whether tDCS-induced alterations in GABA and Glutamate within motor cortex predict motor learning and memory. Note that adaptation to a robot-induced force field has long been considered to be a form of model-based learning that is closely associated with the computation and āsupervisedā learning of internal āforwardā models within the cerebellum. Importantly, previous studies have shown that on-line tDCS to the cerebellum, but not to motor cortex, enhances model-based motor learning. Here we demonstrate that anodal tDCS delivered to the hand area of the left primary motor cortex induces a significant reduction in GABA concentration. This effect was specific to GABA, localised to the left motor cortex, and was polarity specific insofar as it was not observed following either cathodal or sham stimulation. Importantly, we show that the magnitude of tDCS-induced alterations in GABA concentration within motor cortex predicts individual differences in both motor learning and motor memory on the robotic force adaptation and de-adaptation task
Testing the Persistence of Phenotypic Plasticity After Incubation in the Western Fence Lizard, Sceloporus Occidentalis
Hypothesis: Phenotypic variation in traits induced by different incubation temperatures does not persist into the lifetime of young lizards, and therefore contributes little to variation in long-term fitness.
Organism: Western fence lizard (Sceloporus occidentalis).
Methods: Split-clutch laboratory incubation experiment including eggs from two different populations under two different incubation regimes, measurement of morphological traits at hatching, and tracking of morphology and temperature preference behaviour for 7 weeks after hatching.
Results: Several morphological traits, including body mass, hindlimb length, inter-limb length, and tail length, initially differed between the two incubation treatments, but only the difference in tail length persisted to age 7 weeks. Thermal preference was relatively conserved, with juveniles showing no difference in mean selected body temperatures across treatments; however, warm-incubated lizards thermoregulated more precisely than their cool-incubated counterparts.
Conclusion: Studies of incubation effects can reveal changes in animal phenotypes post-hatching, but if these effects do not persist, they may not be subject to natural selection and consequently be of little ecological relevance
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