1,539 research outputs found
An Introduction to Hyperbolic Barycentric Coordinates and their Applications
Barycentric coordinates are commonly used in Euclidean geometry. The
adaptation of barycentric coordinates for use in hyperbolic geometry gives rise
to hyperbolic barycentric coordinates, known as gyrobarycentric coordinates.
The aim of this article is to present the road from Einstein's velocity
addition law of relativistically admissible velocities to hyperbolic
barycentric coordinates along with applications.Comment: 66 pages, 3 figure
Comment on "Minimal size of a barchan dune"
It is now an accepted fact that the size at which dunes form from a flat sand
bed as well as their `minimal size' scales on the flux saturation length. This
length is by definition the relaxation length of the slowest mode toward
equilibrium transport. The model presented by Parteli, Duran and Herrmann
[Phys. Rev. E 75, 011301 (2007)] predicts that the saturation length decreases
to zero as the inverse of the wind shear stress far from the threshold. We
first show that their model is not self-consistent: even under large wind, the
relaxation rate is limited by grain inertia and thus can not decrease to zero.
A key argument presented by these authors comes from the discussion of the
typical dune wavelength on Mars (650 m) on the basis of which they refute the
scaling of the dune size with the drag length evidenced by Claudin and
Andreotti [Earth Pla. Sci. Lett. 252, 30 (2006)]. They instead propose that
Martian dunes, composed of large grains (500 micrometers), were formed in the
past under very strong winds. We show that this saltating grain size, estimated
from thermal diffusion measurements, is not reliable. Moreover, the microscopic
photographs taken by the rovers on Martian aeolian bedforms show a grain size
of 87 plus or minus 25 micrometers together with hematite spherules at
millimetre scale. As those so-called ``blueberries'' can not be entrained by
reasonable winds, we conclude that the saltating grains on Mars are the small
ones, which gives a second strong argument against the model of Parteli et al.Comment: A six page comment on ``Minimal size of a barchan dune'' by Parteli,
Duran and Herrmann [Phys. Rev. E 75, 011301 (2007) arXiv:0705.1778
Coherent Quantum Engineering of Free-Space Laser Cooling
We perform a quantitative analysis of the cooling dynamics of three-level
atomic systems interacting with two distinct lasers. Employing sparse-matrix
techniques, we find numerical solutions to the fully quantized master equation
in steady state. Our method allows straightforward determination of
laser-cooling temperatures without the ambiguity often accompanied by
semiclassical calculations, and more quickly than non-sparse techniques. Our
calculations allow us to develop an understanding of the regimes of cooling, as
well as a qualitative picture of the mechanism, related to the phenomenon of
electromagnetically induced transparency. Effects of the induced asymmetric
Fano-type lineshapes affect the detunings required for optimum cooling, as well
as the predicted minimum temperatures which can be lower than the Doppler limit
for either transition.Comment: 5 pages, 3 figure
Evidence of strong stabilizing effects on the evolution of boreoeutherian (Mammalia) dental proportions.
The dentition is an extremely important organ in mammals with variation in timing and sequence of eruption, crown morphology, and tooth size enabling a range of behavioral, dietary, and functional adaptations across the class. Within this suite of variable mammalian dental phenotypes, relative sizes of teeth reflect variation in the underlying genetic and developmental mechanisms. Two ratios of postcanine tooth lengths capture the relative size of premolars to molars (premolar-molar module, PMM), and among the three molars (molar module component, MMC), and are known to be heritable, independent of body size, and to vary significantly across primates. Here, we explore how these dental traits vary across mammals more broadly, focusing on terrestrial taxa in the clade of Boreoeutheria (Euarchontoglires and Laurasiatheria). We measured the postcanine teeth of NÂ =Â 1,523 boreoeutherian mammals spanning six orders, 14 families, 36 genera, and 49 species to test hypotheses about associations between dental proportions and phylogenetic relatedness, diet, and life history in mammals. Boreoeutherian postcanine dental proportions sampled in this study carry conserved phylogenetic signal and are not associated with variation in diet. The incorporation of paleontological data provides further evidence that dental proportions may be slower to change than is dietary specialization. These results have implications for our understanding of dental variation and dietary adaptation in mammals
Cold collisions between atoms in optical lattices
We have simulated binary collisions between atoms in optical lattices during
Sisyphus cooling. Our Monte Carlo Wave Function simulations show that the
collisions selectively accelerate mainly the hotter atoms in the thermal
ensemble, and thus affect the steady state which one would normally expect to
reach in Sisyphus cooling without collisions.Comment: 4 pages, 1 figur
Listening and learning : the reciprocal relationship between worker and client
The relationship between worker and client has for the best part of 100 years been the mainstay of probation, and yet has recently been eroded by an increased emphasis on punishment, blame and managerialism. The views of offenders are in direct contradiction to these developments within the criminal justice system and this article argues that only by taking account of the views of those at the 'coal face' will criminologists, policy makers and practitioners be able to effect real change in crime rates. The article thus focuses on the views of a sample of previously persistent offenders in Scotland about offending, desistance and how the system can help them. It explores not only their need for friendship and support in youth but also the close association between relationships and the likelihood of offending. It also demonstrates the views of offenders themselves about the importance of the working relationship with supervising officers in helping them desist from crime. The article concludes that the most effective way of reducing offending is to re-engage with the message of the Probation Act of 100 years ago, namely, to 'advise, assist and befriend' offenders rather than to 'confront, challenge and change' offending behaviour
Methoden zur Analyse der vokalen Gestaltung populÀrer Musik
Although voice and singing play a crucial role in many genres of popular music, to date there are only few approaches to an in-depth exploration of vocal expression. The paper aims at presenting new ways for describing, analysing and visualizing several aspects of singing using computer-based tools. After outlining a theoretical framework for the study of voice and singing in popular music, some of those tools are introduced and exemplified by vocal recordings from various genres (blues, gospel music, country music, jazz). Firstly, pitch gliding (slurs, slides, bends, melismas) and vibrato are discussed referring to a computer-based visualization of pitch contour. Secondly, vocal timbre and phonation (e.g. vocal roughness) are explored and visualized using spectrograms
Dislocation density and graphitization of diamond crystals
Two sets of diamond specimens compressed at 2 GPa at temperatures varying between 1060 K and 1760 K were prepared; one in which graphitization was promoted by the presence of water and another in which graphitization of diamond was practically absent. X-ray diffraction peak profiles of both sets were analyzed for the microstructure by using the modified Williamson-Hall method and by fitting the Fourier coefficients of the measured profiles by theoretical functions for crystallite size and lattice strain. The procedures determined mean size and size distribution of crystallites as well as the density and the character of the dislocations. The same experimental conditions resulted in different microstructures for the two sets of samples. They were explained in terms of hydrostatic conditions present in the graphitized samples
Thomas rotation and Thomas precession
Exact and simple calculation of Thomas rotation and Thomas precessions along
a circular world line is presented in an absolute (coordinate-free) formulation
of special relativity. Besides the simplicity of calculations the absolute
treatment of spacetime allows us to gain a deeper insight into the phenomena of
Thomas rotation and Thomas precession.Comment: 20 pages, to appear in Int. J. Theo. Phy
Traditional vs Gesture Based UAV Control
Abstract. The purpose of this investigation was to assess user preferences for controlling an autonomous system. A comparison using a virtual environment (VE) was made between a joystick based, game controller and a gesture-based system using the leap motion controller. Command functions included basic flight maneuvers and switching between the operator and drone view. Comparisons were made between the control approaches using a representative quadcopter drone. The VE was designed to minimize the cognitive loading and focus on the flight control. It is a physics-based flight simulator built in Unity3D. Participants first spend time familiarizing themselves with the basic controls and vehicle response to command inputs. They then engaged in search missions. Data was gathered on time spent performing tasks, and post test interviews were conducted to uncover user preferences. Results indicate that while th
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