597 research outputs found
Paranotis halfordii (Rubiaceae: Spermacoceae), a new species from the Kimberley region of Western Australia, in a recently described Australian genus
Paranotis halfordii K.L.Gibbons & S.J.Dillon, a new species from the Dampierland and Central Kimberley IBRA bioregions of Western Australia, is here described. Paranotis Pedley ex K.L.Gibbons was recently described to include some Australian species previously included in Oldenlandia L. A key to the species of Paranotis is provided
The effects of room design on computer-supported collaborative learning in a multi-touch classroom.
While research indicates that technology can be useful for supporting learning and collaboration, there is still relatively little uptake or widespread implementation of these technologies in classrooms. In this paper, we explore one aspect of the development of a multi-touch classroom, looking at two different designs of the classroom environment to explore how classroom layout may influence group interaction and learning. Three classes of students working in groups of four were taught in the traditional forward-facing room condition, while three classes worked in a centered room condition. Our results indicate that while the outcomes on tasks were similar across conditions, groups engaged in more talk (but not more off-task talk) in a centered room layout, than in a traditional forward-facing room. These results suggest that the use of technology in the classroom may be influenced by the location of the technology, both in terms of the learning outcomes and the interaction behaviors of students. The findings highlight the importance of considering the learning environment when designing technology to support learning, and ensuring that integration of technology into formal learning environments is done with attention to how the technology may disrupt, or contribute to, the classroom interaction practices
Periodic Solutions of the Einstein Equations for Binary Systems
This revision includes clarified exposition and simplified analysis.
Solutions of the Einstein equations which are periodic and have standing
gravitational waves are valuable approximations to more physically realistic
solutions with outgoing waves. A variational principle is found which has the
power to provide an accurate estimate of the relationship between the mass and
angular momentum of the system, the masses and angular momenta of the
components, the rotational frequency of the frame of reference in which the
system is periodic, the frequency of the periodicity of the system, and the
amplitude and phase of each multipole component of gravitational radiation.
Examination of the boundary terms of the variational principle leads to
definitions of the effective mass and effective angular momentum of a periodic
geometry which capture the concepts of mass and angular momentum of the source
alone with no contribution from the gravitational radiation. These effective
quantities are surface integrals in the weak-field zone which are independent
of the surface over which they are evaluated, through second order in the
deviations of the metric from flat space.Comment: 18 pages, RevTeX 3.0, UF-RAP-93-1
Dynamics of Extremal Black Holes
Particle scattering and radiation by a magnetically charged, dilatonic black
hole is investigated near the extremal limit at which the mass is a constant
times the charge. Near this limit a neighborhood of the horizon of the black
hole is closely approximated by a trivial product of a two-dimensional black
hole with a sphere. This is shown to imply that the scattering of
long-wavelength particles can be described by a (previously analyzed)
two-dimensional effective field theory, and is related to the
formation/evaporation of two-dimensional black holes. The scattering proceeds
via particle capture followed by Hawking re-emission, and naively appears to
violate unitarity. However this conclusion can be altered when the effects of
backreaction are included. Particle-hole scattering is discussed in the light
of a recent analysis of the two-dimensional backreaction problem. It is argued
that the quantum mechanical possibility of scattering off of extremal black
holes implies the potential existence of additional quantum numbers - referred
to as ``quantum whiskers'' - characterizing the black hole.Comment: 31 page
Statistical Mechanics of Black Holes
We analyze the statistical mechanics of a gas of neutral and charged black
holes. The microcanonical ensemble is the only possible approach to this
system, and the equilibrium configuration is the one for which most of the
energy is carried by a single black hole. Schwarzschild black holes are found
to obey the statistical bootstrap condition. In all cases, the microcanonical
temperature is identical to the Hawking temperature of the most massive black
hole in the gas. U(1) charges in general break the bootstrap property. The
problems of black hole decay and of quantum coherence are also addressed.Comment: 21 page
Records of ctenophores from South Africa
Although ctenophores can be conspicuous components of the plankton in coastal marine ecosystems, only six species have been formally described from around South Africa. Using photographs from local community scientists, we add a further three species (Cestum veneris, Beroe forskalii?, Ocyropsis maculata?) and six morphospecies to the regional fauna. These additions suggest that South Africa has a ctenophore fauna that is amongst the most diverse, globally; an observation in agreement with information from other taxa. Tips on how community scientists can improve their photographic contributions to understanding ctenophore diversity are provided
Entropy in Black Hole Pair Production
Pair production of Reissner-Nordstrom black holes in a magnetic field can be
described by a euclidean instanton. It is shown that the instanton amplitude
contains an explicit factor of , where is the area of the event
horizon. This is consistent with the hypothesis that measures the
number of black hole states.Comment: 24 pages (harvmac l mode
Four Dimensional Black Holes in String Theory
Exact solutions of heterotic string theory corresponding to four-dimensional
charge Q magnetic black holes are constructed as tensor products of an
SU(2)/Z(2Q+2) WZW orbifold with a (0,1) supersymmetric SU(1,1)/U(1) WZW coset
model. The spectrum is analyzed in some detail. ``Bad'' marginal operators are
found which are argued to deform these theories to asymptotically flat black
holes. Surprising behaviour is found for small values of Q, where low-energy
field theory is inapplicable. At the minimal value Q=1, the theory degenerates.
Renormalization group arguments are given that suggest the potential
gravitational singularity of the low-energy field theory is resolved by a
massive two-dimensional field theory. At Q=0, a stable, neutral ``remnant,'' of
potential relevance to the black hole information paradox, is found.Comment: 37 pages + 1 figure (tar compressed and uuencoded
Three Dimensional Distorted Black Holes
We present three-dimensional, {\it non-axisymmetric} distorted black hole
initial data which generalizes the axisymmetric, distorted, non-rotating
[Bernstein93a] and rotating [Brandt94a] single black hole data developed by
Bernstein, Brandt, and Seidel. These initial data should be useful for studying
the dynamics of fully 3D, distorted black holes, such as those created by the
spiraling coalescence of two black holes. We describe the mathematical
construction of several families of such data sets, and show how to construct
numerical solutions. We survey quantities associated with the numerically
constructed solutions, such as ADM masses, apparent horizons, measurements of
the horizon distortion, and the maximum possible radiation loss ().Comment: 23 pages, 12 figures, accepted for publication in Classical and
Quantum Gravit
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